A phase I/II dose-escalation study of fractionated 225Ac-J591 for progressive metastatic castration-resistant prostate cancer (mCRPC) in patients with prior treatment with 177Lu-PSMA

TPS288

Background: As prostate-specific membrane antigen targeted radiotherapy (PSMA-TRT) is now an active standard-of-care treatment in mCRPC, ongoing studies with alternative approaches to targeting PSMA will increasingly need to consider the consequences of sequential PSMA-TRT exposure. Our past and ongoing investigations into antibody-based targeting (e.g., J591) and potent alpha emitting payloads (e.g., 225Ac) impact drug kinetics, biodistribution, and resultant clinical toxicities. In a first-in-human phase I dose-escalation study of 225Ac-J591, patients with mCRPC were treated with a single dose of 225Ac-J591 on seven dose levels, up to 93.3 KBq/kg without achievement of maximal tolerated dose (MTD). One patient treated at 80 KBq/kg developed dose-limiting toxicity (DLT) of Gr 4 anemia and thrombocytopenia, but 0 of 6 at 93.3 KBq/Kg had Gr > 3 heme toxicity or Gr > 2 non-heme toxicity. Although not intentionally preselected for prior exposure, 55% (12/22) of patients had 177Lu-PSMA previously. With approval of 177Lu vipivotide tetraxetan, we amended an ongoing phase I dose-escalation study to include a post-177-Lu-PSMA cohort. Methods: Entry criteria include progressive mCRPC by PCWG3 criteria, ECOG PS 0-2, intact organ function, and prior receipt of AR pathway inhibitor and chemotherapy (or refused/ineligible). There is no limit to prior lines of therapy except alpha-emitting therapies (i.e., PSMA-TRT, 223Ra) and in this amended dose-escalation cohort, all patients must have had prior treatment with 177Lu-PSMA. Treatment will be given in a single fractionated cycle of 225Ac-J591 administered on D1 and D15. The phase I component is a 3+3 dose-escalation study design with up to 18 patients, with the goal of identifying MTD. The phase II component will include up to 16-19 patients in a Simon 2-stage design with 90% power to exclude the null hypothesis (35% or fewer patients with PSA50). Eligible men with negative PSMA PET scans will be offered treatment with informed consent in an exploratory subgroup but will not be counted towards phase II efficacy. Secondary outcomes include radiographic response by PCWG3-modified RECIST 1.1 criteria and PSMA PET, biochemical and radiographic progression-free survival, circulating tumor cell counts, and overall survival. Patient reported outcomes, genomic, and immune analyses are exploratory. Enrollment to the post-177Lu-PSMA cohort began in August 2022. Clinical trial information: NCT04506567 .

Increased utilization of prostate-specific membrane antigen (PSMA)-targeted radionuclide therapy (PSMA-TRT) in African American (AA) patients at an academic medical center

36

Background: At Weill Cornell Medicine (WCM), we have had a research program utilizing anti-PSMA mAb J591 since the year 2000. With the addition of PSMA ligand-based therapies in 2017, we have enrolled around 300 patients on investigational PSMA-TRT clinical trials. Since the approval of 177Lu-PSMA-617 (Pluvicto; Lu-177 vipivotide tetraxetan), we began standard-of-care (SOC) treatment, co-enrolling willing patients into a research registry. Recognizing the low numbers of AA patients enrolled on therapeutic clinical trials in the U.S., we made a concerted effort to increase the number of AA patients with prostate cancer (PC) enrolled on clinical trials at WCM. We retrospectively assessed the demographic data of patients enrolled on PSMA-TRT clinical trials to determine changing patterns of enrollment, and to determine if our efforts have improved access to these novel treatments for this under-represented, but high-risk population of patients. Methods: We collected demographic data (namely race and ethnicity) on patients with PC from WCM who were included on our PSMA-TRT investigational clinical trial databases or enrolled on our SOC 177Lu-PSMA-617 research registry. We used self-reported race and, when not available (due to patient death or loss of follow-up), demographic information documented in the medical record. Patients were grouped into 5-year cohorts based on either the date of consent for trial enrollment, or the start date of PSMA-TRT treatment (based on available information). Institutional tumor registry data was used as a comparator to assess the total percentage of AA patients with PC seen at WCM. Results: The percentages of patients included on PSMA-TRT clinical trials at WCM who were identified as AA were as follows: 2000-2004: 3.1% (2/65), 2005-2009: 5.1% (3/59), 2010-2014: 6.1% (2/33), and 2015-2019: 5.9% (5/85). The percentage of AA patients on PSMA-TRT studies from 2020 through July 2022 was 18.2% (16/88, inclusive of 8/72 investigational TRT subjects and 8/16 SOC registry participants). The total percentages of AA patients seen at Cornell based upon analysis of our tumor registry data were as follows: 2000-2004: 10.5% (182/1728), 2005-2009: 6.9% (250/3607), 2010-2014: 10% (326/3255), 2015-2019: 11.5% (278/2413), and 2020: 14.1%. Tumor registry data for 2021-present were not yet available. Conclusions: The percentage of AA patients on investigational PSMA-TRT trials at our institution notably increased from 2000-2019 (3.1%-6.1%) to 2020-2022 (11.1%). Moreover, 50% of those treated with 177Lu-PSMA-617 since its FDA approval and co-enrolled on our research registry identified as AA. These data suggest that outreach and increasing access to AA patients for novel PC treatment such as PSMA-TRT can result in increased numbers of underrepresented patients enrolling on clinical trials and receiving the most modern standards of care (i.e., PSMA-TRT).

Phase I results of a phase I/II study of pembrolizumab and AR signaling inhibitor (ARSI) with 225Ac-J591

181

Background: Only a small fraction of patients with PC respond to immunotherapy; radiation increases responses. PSMA targeted radionuclide therapy (PSMA-TRT) radiates multiple sites of disease simultaneously. ARSI can lead to radiosensitization and upregulate PSMA and PD-L1 expression. We hypothesize that the addition of potent alpha-PSMA-TRT (225Ac-J591) will lead to double-stranded DNA breaks, cell death, and subsequent release of neoantigens, thereby increasing the response proportion and duration to pembrolizumab plus ARSI. Here, we present preliminary phase I results of a phase I/II study, including an unexpected cytokine release syndrome (CRS). Methods: Eligibility: Progressive mCRPC by PCWG3 criteria (at least one ARSI, no chemotherapy in the mCRPC setting). Patients received ARSI of physician’s choice, pembrolizumab (400mg every 6 weeks), and single infusion of 225Ac-J591 at two different dose levels (65 or 80 KBq/kg). The primary endpoint for phase I is determination of 225Ac-J591 dose for phase II in a pick-the-winner design. Results: 12 patients were treated (6 at 65 KBq/kg, 6 at 80 KBq/kg). 7 (58%) received enzalutamide, 3 (25%) apalutamide, and 2 (17%) darolutamide. Median age 66.5, median PSA 7.75 ng/mL, 6 (50%) CALGB intermediate risk, 4 (33%) low risk, 11 (92%) with bone metastases, 3 (25%) with nodal metastases. 7 (58%) with prior abiraterone, 6 (50%) prior enzalutamide, 5 (42%) sip-T. All patients experienced PSA decline following therapy, 6 (50%) with >50% decline. With >6 months follow-up, 4 (33%; all at 80 KBq/kg) remain progression-free and on study. Of note, 7 (58%) developed an unexpected CRS 7-14 days following treatment characterized by morbilliform rash, fever >101F, and low blood counts. Inflammatory markers were elevated: IL-6 (2.1-7.7 pg/mL), D-Dimer (306-2378 ng/mL), ferritin (361.3-513.4 ng/mL), fibrinogen (386-461 mg/dL), ESR (16-42 mm/hr). After pausing the ARSI, this reaction improved within 1 week. Thrombocytopenia and/or neutropenia occurred during this syndrome, then typically improved before counts fell again at expected nadir 4 weeks after 225Ac-J591. Overall AEs on study include: 9 (75%) thrombocytopenia (3 with g≥3; 1 g4 with PC marrow infiltration), 7 (58%) neutropenia (none g≥3), 6 (50%) nausea, 7 (58%) g1-2 fatigue, 9 (75%) g1-2 xerostomia, 7 (58%) g1 AST. In contrast to heme AEs with unexpected initial occurrence during CRS followed by typical nadir from TRT, the described non-hematologic AEs were generally independent of CRS. 4 (33%) developed typical irAEs: 2 with rash (D60, D62) and 2 hypothyroidism (D77, D82). Conclusions: Combination therapy with alpha-PSMA-TRT, ARSI, and pembrolizumab demonstrates efficacy in the phase I run-in. A key safety signal that has emerged with triplet therapy is CRS that can be managed supportively. The randomized phase II component is accruing with additional safety visits and sample collection for cytokine analysis. Clinical trial information: NCT04946370 .

Phase I/II study of 225Ac-J591 plus 177Lu-PSMA-I&T for progressive metastatic castration-resistant prostate cancer

TPS5100

Background: PSMA is overexpressed by most prostate cancers and can be successfully targeted by both antibodies (mAb) and small molecule ligands (SML), each with overlapping and distinct binding sites, kinetics, and biodistributions [Kratochwil Sem Nuc Med 2019]. mAbs are larger, with longer circulating times resulting in greater exposure to bone marrow, but lesser access to PSMA expression on luminal tissue ( e.g. salivary glands, small bowel, and kidney). In contrast, SMLs are rapidly excreted via kidneys and readily diffuse to all PSMA-expressing sites. Toxicities of 177Lu vary with these differences in biodistribution ( e.g. more hematologic toxicity with mAb, more xerostomia and nausea with SML, p < 0.001)[Niaz AUA 2020]. Alpha emitting isotopes have shorter ranges but high potency compared to beta emitters which have longer ranges, but lower linear energy transfer. In preclinical models, the combination of mAb plus SML has demonstrated additive binding in LNCaP, CWR22Rv1, and PC3/PSMA PC cell lines, and synergistic uptake of 177Lu-mAb plus 177Lu-SML in xenograft models. We developed a phase I/II study to test our hypothesis that concomitant mAb and SML targeting, plus the combination of alpha (225Ac) and beta emitters (177Lu), may offer complementary benefits in a safe and effective manner. Methods: Key eligibility criteria include progressive mCRPC (PCWG3), at least 1 prior AR pathway inhibitor and taxane chemotherapy (or ineligible/refused), and adequate organ function and performance status. PSMA PET/CT must have at least 1 lesion with SUVmax > liver SUV. Prior PSMA-based therapy with radioisotopes is not allowed. 177Lu-PSMA-I&T (PNT2002) will be administered as in the phase III SPLASH study (6.8 GBq q8w for up to 2 doses). The phase I includes up to two dose-escalation cohorts of concurrent 225Ac-J591 (30 & 40 KBq/Kg q8w x2) in a modified 3+3 schema. All subjects undergo 177Lu SPECT on Day 8 after each dose. The primary objective of the Phase I study is to determine the dose-limiting toxicity and recommended phase II dose (RP2D) for this combination. Primary objective of the Phase II study is to assess the proportion of patients with > 50% PSA decline after treatment. Secondary objectives include radiographic response rate (PCWG3-modified RECIST 1.1), biochemical and radiographic progression-free survival, overall survival, safety (CTCAE 5.0), CTC count changes and conversions, and patient-reported outcomes (FACT-P, BPI-SF, EQ-5D). Exploratory objectives include pre- and post-treatment PSMA-based imaging changes, effects of PSMA radionuclides on the microbiome, relationship between genomic alterations and response, and relationship between PSMA PET/CT results and outcome. The phase I was activated at Weill Cornell Medicine in May 2021. Following determination of the RP2D, a multicenter phase II is planned at Prostate Cancer Clinical Trials Consortium (PCCTC) in 2022. Clinical trial information: 04886986.

Self-reported race and zip code by men with prostate cancer in New York City and association with access to PSMA PET scans

e17007

Background: Prostate-specific membrane antigen (PSMA) is overexpressed on most prostate cancers (PCs), a fact that has been exploited to perform both targeted imaging and treatment. PSMA-based PET imaging allows for more sensitive detection of PC and can be useful even in the context of negative CT and bone scans. Prior to 2021, research protocols using PSMA-PET (PET) were common at large academic centers, but will likely be more commonplace as standard of care (SOC) after recent FDA approval. With a change in SOC imaging, the impact of cost to patients must be examined. We hypothesize that socioeconomic and demographic-based care disparities may widen and access may narrow in the face of these changes. Methods: We collected demographic data on patients with PC from Weill Cornell Medicine (WCM), Brooklyn Methodist Hospital (BMH), and Columbia University Medical Center (CUMC) who received a PSMA PET scan between 1 January 2018 and 31 January 2022. Data collected included age, zip code, primary language, self-reported race and ethnicity, and type of insurance coverage. Zip codes were used to estimate income strata of patients using publicly-available 2018 IRS adjusted gross income (AGIs) as follows ($): < 25k, 25k-50k, 50k-75k, 75k-100k, 100k-200k, and > 200k. Any AGI strata comprising > 20% of population was counted. Patients were divided into four cohorts: those who had a scan (1) as part of therapeutic-based studies for mCRPC, (2) non-therapeutic imaging studies, (3) as a part of a cost-recovery study with an out-of-pocket cost of $1054, and (4) commercial use under post-approval standards of care (SOC) billed to insurance. Proportion of self-reported race in each cohort was compared against overall race distribution for prostate cancer registry from WCM and CUIMC (Table). Results: 896 patients underwent PSMA PET. Approximately half of all scans were in Cohort 4 (49%, n = 368) including all BMH scans. CUMC scans were 90% in Cohort 2 (122/136). In Cohort 1, 2, and 3, > 90% of subjects resided in zip codes with AGI > $100k. In Cohort 4, 84% resided in zip codes with AGI > $100k. Overall 70.4% of subjects identified as White, 8.3% Black or African American, 3.5% Asian, 4.4% Other, and 13.5% were unknown/or declined. African American/Black was most represented on Cohort 4 (11%), and least represented in Cohort 1 (5%). Conclusions: White patients comprise the largest proportion or PET scans whereas non-white groups are underrepresented across all cohorts. Access to PETs has appeared to improve for lower AGI, non-white patients following FDA approval and insurance coverage. It will be critical to assure equitable access across all demographic groups as deployment of PET scanning becomes the new standard of care.[Table: see text]

Phase I/II trial of pembrolizumab and AR signaling inhibitor +/- 225Ac-J591 for chemo-naive metastatic castration-resistant prostate cancer (mCRPC)

TPS216

Background: The role of immune checkpoint inhibition (ICI) in prostate cancer remains undefined outside of the subset with mismatch repair. Several studies have suggested that ICI combined with androgen receptor signaling inhibitors (ARSI) or kinase inhibitors may result in improved and/or more durable response in a proportion of men with mCRPC. While not yet proven, addition of external beam radiation to ICI may improve outcomes (for instance Kwon et al Lancet Oncol 2014, Fizazi K et al, Eur Urol 2020). PSMA targeted therapy with 177Lu-PSMA-617 improves survival in men with CRPC, has been combined with ICI in early phase studies, and we have previously demonstrated the benefit of PSMA targeted therapy using radiolabeled mAb J591. PSMA-targeted alpha-emitters have a very high potency and the potential to generate immune response. Based upon i) ARSI may increase PSMA expression, ii) ARSI may radiosensitize tumors, iii) ARSI resistance may lead to increased PD-L1 expression, and iv) alpha emitters may generate an immune response, we hypothesize that the addition of an alpha-emitting radionuclide (225Ac) targeting prostate cancer (i.e PSMA+ tumors targeted with J591) will lead to double-stranded DNA breaks, cell death, and subsequent release of neoantigens, and thus will increase the response proportion to pembrolizumab plus ARSI resulting in more durable response. Methods: Key eligibility criteria include progressive mCRPC by PCWG3 on at least 1 prior AR pathway inhibitor and no prior chemotherapy for mCRPC. A phase I dose-finding study will first test safety of the triplet combination of pembrolizumab, an ARSI of physician choice, and 2 different doses of 225Ac-J591 (one with minimal and one with moderate single-agent toxicity). Following determination of the optimal dose, a randomized phase II trial will treat subjects with a fixed dose of pembrolizumab 400 mg every 6 weeks (for up to 2 years) plus a standard ARSI (until progression or intolerance) with or without 225Ac-J591. The primary endpoint of the study will test the hypothesis that the addition of a PSMA-targeted alpha emitter increases the composite of RECIST measurable disease, PSA, and CTC count response to immuno-hormonal therapy with 90% power. Key secondary clinical endpoints include 1-year progression-free survival, duration of response, and overall survival. Exploratory objectives include assessment of immunogenic cell death, immune serologic and host microbiome changes, plasma ctDNA, serial PSMA PET, and patient reported outcomes (FACT-P, BPI, EQ-5D-5L). The phase I portion of this DOD-funded study was activated in summer 2021 with the randomized phase II portion expected to open at PCCTC sites in 2022. Clinical trial information: NCT04946370.

Assessment of patient-reported outcomes (PROs) and longer-term adverse events (AEs) in phase I study of 225Ac-J591-PSMA for metastatic castration-resistant prostate cancer (mCRPC)

77

Background: Prostate Specific Membrane Antigen (PSMA) is a conserved cell surface protein in PC and is used for targeted imaging and therapeutics. Antibodies circulate longer than small molecules and are less likely to reach luminal PSMA on normal organs. Here we report PROs and longer-term AEs from the dose-escalation and expansion cohorts of a first-in-human study of combined monoclonal antibody and potent alpha emitter (225Ac-J591). Methods: Eligible subjects with mCPRC were administered 225Ac-J591. Initial to maximum doses were 13.3 to 93.3 KBq/kg). AEs are reporting using CTCAE v5 and PROs, including pain (BPI-SF) and quality of life (QOL, FACT-P), and associations with PSA response were also examined. Results: A total of 32 subjects (one enrolled in both dose-escalation and expansion) were treated with a single dose of 225Ac-J591 across 7 dose levels with expansion at the level (93.3 KBq/kg, n = 16). Median age 69.5 (52-89) and PSA 149.1 (4.8-7168.4). All subjects had at least 1 AE of any grade. Most common were fatigue (31/32, 1 Gr > 2), anorexia (25/32, all Gr 1-2), and thrombocytopenia (25/32, 3 Gr 3, 2 Gr 4). Xerostomia was observed in 14/32 subjects (all Gr 1), 7 of whom had prior 177Lu-PSMA. Pain flare was reported in 43% (17/32) subjects (11 Gr 1, 6 Gr 2). 19 had evaluable PROs at baseline and efficacy visit (week 12). Pain severity (p = 0.8) and interference from pain (p = 0.4) were unchanged from baseline to 12 weeks, yet better PSA response (by percent) was associated with reduced pain severity (r = 0.7, p = 0.0023). Despite at least one AE in each subject, total FACT-P was not significantly changed after treatment (p = 0.2), but emotional well-being declined over time (15 [10.0, 18.0] v 10.0 [7.5, 13.0], p = 0.011). Reduction in median emotional well-being reached clinically important score differences. When stratified by AE, subjects with xerostomia had lower FACT-P total scores, but no difference was observed between those with and without pain. PSA response was not associated with change in QOL or subscales. Conclusions: Pain and quality of life in subjects with mCRPC did not change, on average, from baseline to 12 weeks after treatment with 225Ac-J591. This is despite preliminary evidence of clinical efficacy being accompanied by frequent, treatment emergent AEs. A promising trend toward improved pain in those with PSA response warrants further analysis. Small numbers limited statistical power for testing other subgroup associations. Additional correlations with pretreatment sites of disease, performance status, and adverse event distribution are ongoing. Assessment of changes in PROs in the follow up studies [NCT04506567] are underway. Clinical trial information: NCT03276572.

Quantitative assessment of PSMA imaging before and after 177Lu-PSMA-617 treatment in a Ph I/II trial

37

Background: We have previously reported a dose-intense single-cycle of 177Lu-PSMA-617 was effective in pretreated patients with mCRPC without requiring PSMA-positive imaging for enrollment. Prior post-hoc analyses of these data using approximate quantification of exclusively the most PSMA-positive disease sites have demonstrated associations between PSA response and PFS with pre-treatment 68Ga-PSMA11-PET signal. Greater sophistication in pre- and post-treatment evaluation of PSMA-expression in tumor and normal organs may allow for better patient selection and prediction of toxicities. Methods: A total of 50 patients were treated on a phase I/II study of fractionated-dose (D1, 15) 177Lu-617-PSMA. Quantification using artificial intelligence (AI) were used to measure pre- and post-treatment PSMA signal intensity. Scoring cutoffs with confidence intervals around scan variation were empirically established from a subset of test/re-test of subjects within 24h without intervening therapy. A variety of measurements were performed including SUVmean across all detectable tumor lesions, volume of lesions, and SUVtotal (Total + Volume), as well as select normal organs and changes after treatment. Associations with survival were tested via Cox proportional hazard models in univariate analyses and associations with adverse events (AEs) and PSA responses were via assessed via Wilcoxon rank sum tests. Results: 13 subjects were selected to complete AI-based quantification and associated survival analyses. Among these, 10 (77%) experienced any PSA decline, with 8 (62%) with PSA50 and 3 (23%)with PSA90. Median overall survival (OS) was 17.0 mos (10, NA) via Kaplan-Meier estimates. In univariate analysis, pretreatment SUVmean was associated with improved PFS (HR 0.66, 95% CI 0.49-0.90, p = 0.009) and OS (HR 0.81, 95% CI 0.65-1.00, p = 0.048). The metrics most strongly associated with PSA50 were pretreatment SUVmean (median [IQR]: 7.66 [6.52, 10.54] v 3.50 [3.02, 3.56], p = 0.019) and SUV Total (14982 [11110, 20595] v 1303 [576, 1512], p = 0.019), and change in Volume (-27 [-44, -20] v 145 [38, 154], p = 0.006) and SUVtotal (-57 [-67, -35] v 132 [9, 269], p = 0.030). Subjects with xerostomia had higher salivary gland SUVmax (pretreatment and change in after treatment). Those with pain flare had lower pretreatment SUV scores (Mean, Max, Total) in unaffected portions of bony skeleton. Conclusions: Sophisticated AI-based quantification analysis of PSMA expression on pre- and post-treatment 68Ga-PSMA11-PETs demonstrate associations with treatment efficacy (PSA response, OS), and associations between patient experience (AEs) and PSMA expression in non-tumor tissues. Expansion of this algorithm to a larger patient cohort may improve our ability to anticipate toxicity by body-wide PSMA detection and predict treatment response. Clinical trial information: NCT03042468.

Folate hydrolase-1 (FOLH1) is a novel target for antibody-based brachytherapy in Merkel cell carcinoma

Backgrounds

Folate Hydrolase‐1 (FOLH1; PSMA) is a type II transmembrane protein, luminally expressed by solid tumour neo‐vasculature. Monoclonal antibody (mAb), J591, is a vehicle for mAb‐based brachytherapy in FOLH1+ cancers. Brachytherapy is a form of radiotherapy that involves placing a radioactive material a short distance from the target tissue (e.g., on the skin or internally); brachytherapy is commonly accomplished with the use of catheters, needles, metal seeds and antibody or small peptide conjugates. Herein, FOLH1 expression in primary (p) and metastatic (m) Merkel cell carcinoma (MCC) is characterized to determine its targeting potential for J591‐brachytherapy.

Materials & Methods

Paraffin sections from pMCC and mMCC were evaluated by immunohistochemistry for FOLH1. Monte Carlo simulation was performed using the physical properties of conjugated radioisotope lutetium‐177. Kaplan–Meier survival curves were calculated based on patient outcome data and FOLH1 expression.

Results

Eighty‐one MCC tumours were evaluated. 67% (54/81) of all cases, 77% (24/31) pMCC and 60% (30/50) mMCC tumours were FOLH1+. Monte Carlo simulation showed highly localized ionizing tracks of electrons emitted from the targeted neo‐vessel. 42% (34/81) of patients with FOLH1+/− MCC had available survival data for analysis. No significant differences in our limited data set were detected based on FOLH1 status (p = 0.4718; p = 0.6470), staining intensity score (p = 0.6966; p = 0.9841) or by grouping staining intensity scores (− and + vs. ++, +++, +++) (p = 0.8022; p = 0.8496) for MCC‐specific survival or recurrence free survival, respectively.

Conclusions

We report the first evidence of prevalent FOLH1 expression within MCC‐associated neo‐vessels, in 60‐77% of patients in a large MCC cohort. Given this data, and the need for alternatives to immune therapies it is appropriate to explore the safety and efficacy of FOLH1‐targeted brachytherapy for MCC.

What's already known about this topic?

We report the first evidence of prevalent folate hydrolase‐1 (FOLH1; also known as prostate‐specific membrane antigen) expression within MCC‐associated neovessels.

What does this study add?

Herein, FOLH1 expression in Merkel cell carcinoma neovasculature is validated, and the therapeutic mechanism of specific, systemic targeting of disseminated disease with antibody‐based brachytherapy, is defined.

Long-term adverse events (AE) in patients with metastatic castration-resistant prostate cancer (mCRPC) receiving prostate-specific membrane antigen (PSMA)-based targeted radionuclide therapy (TRT)

5055

Background: PSMA-TRT is a promising investigational treatment for patients with mCRPC. Expected short-term toxicities associated with PSMA-TRT include dose-dependent myelosuppression and xerostomia. However, there is a lack of information regarding long-term effects of PSMA-TRT on marrow, renal, and liver function. Additionally, potential organ dose limits for radiation are derived from studies of external beam radiation, which may not be applicable to TRT. Methods: Men treated on prospective clinical trials of PSMA-TRT, from 2003 through 2020 with at least six months of follow-up were included. Variables included treatment, co-morbidities, baseline and most recent renal, liver, and marrow function, along with respective short-term ( < 6 months) and long-term toxicities. AEs were graded using CTCAE version 5 and attribution was assessed with most recent clinical follow up. Multivariable logistic regression was used to control for type of TRT, comorbidities, and subsequent therapies. Results: 71 (59.7%) patients who received 177Lu-J591, 30 (25.2%) 177Lu-PSMA-617, 11 (9.2%) 225Ac-J591, and 7 (5.9%) 90Y-J591 were included, with median follow up 18 months (range 6-133). Long-term (most recent) laboratory values and AEs are summarized in the table. A majority of AEs were attributed to alternate etiologies. 5 of 14 cases of grade (Gr) ≥2 creatinine increase, 3 of 36 cases of Gr ≥2 platelets, 2 of 14 cases of Gr ≥2 bilirubin, 1 of 15 cases of Gr ≥2 AST increase, and 1 of 5 cases of Gr ≥2 ALT increase were deemed possibly related to PSMA-TRT. Only two Gr ≥3 AEs were attributed to possibly being related to PSMA-TRT: one case of Gr 4 creatinine elevation and one case of Gr 3 ALT elevation. On multivariable analysis, alpha-TRT was associated with hepatic AEs (OR 4.38, p = 0.047), and there was a trend towards higher Charlson Comorbidity scores associating with hematologic AEs (OR 1.27, p = 0.095). Conclusions: This is the largest analysis to-date of long-term AEs in patients who have received PSMA-TRT. Long-term effects on renal, liver, and marrow function are infrequent.[Table: see text]

Phase I study of 225Ac-J591 for men with metastatic castration-resistant prostate cancer (mCRPC)

5015

Background: Antibodies and small molecule ligands target PSMA with different kinetics and biodistribution, with certain sites of PSMA expression such as salivary/lacrimal glands, kidneys, and small bowel less accessible to large antibodies. Alpha emitters such as 225Ac have high potency, but short range. We report dose-escalation plus expansion cohort results of a first in human study of 225Ac-J591. Methods: Men with progressive mCRPC following at least 1 potent AR-pathway inhibitor (ARPI, e.g. abi/enza) and chemo (or unfit/refuse chemo) without limit of # prior therapies (including Ra-223 or prior 177Lu-PSMA) provided adequate organ function were eligible. Baseline 68Ga-PSMA11 PET was performed, but not used for eligibility. Dose-escalation was in single-subjects x4 followed by 3+3 with a single infusion of 225Ac-J591 (13.3 KBq/kg with planned escalation up to 93.3 KBq/kg). Dose-limiting toxicity (DLT) was defined as attributable grade (Gr) 4 heme toxicity or Gr 3/4 non-heme tox. Imaging, genomic, patient-reported outcomes (PRO), and immune correlates embedded. Results: 32 men were treated with a single dose of 225Ac-J591 on 7 dose levels with expansion at the highest dose level (n = 16). Median age 69.5 (range 52-89), PSA 149.1 (4.8-7168.4); 75% with >2 prior ARPI, 62.5% chemo, 28% Ra-223, 43.7% 177Lu-PSMA. One (3.1%) CALGB (Halabi) good prognostic risk, 8 (25%) intermediate, and 23 (71.9%) poor risk. While PSMA uptake was not a prerequisite for treatment, of 28 with pre-treatment PSMA PET, none had tumor SUVmax < liver, 5 (17.8%) with tumor SUVmax 1-2.5x liver, 2 (7.2%) with tumor SUVmax 2.5-5x liver, and 21 (75%) with tumor SUVmax > 5x liver SUVmean. 1 of 6 in cohort 6 (80 KBq/kg) had DLT (Gr 4 anemia and platelets) with 0 of 6 at the highest dose level (93.3 KBq/Kg) and this dose was expanded. High Gr AEs were restricted to hematologic: In addition to DLT, 4 (12.5%) Gr 3 platelets and 2 (6.2%) with Gr 3 neutropenia. Non-heme AE’s were restricted to Gr 1/2 and included: 10 (31.2%) fatigue, 5 (15.6%) pain flare, 14 (43.7%) nausea, 8 (25%) with Gr 1 xerostomia (of which 5 received prior 177Lu-PSMA), 12 (37.5%) AST elevation. Despite prior treatment including 177Lu-PSMA and no selection for PSMA expression, 22 (68.7%) with any PSA decline, 12 (37.5%) with > 50% PSA decline. Of 21 with paired baseline and 12-wk CTC counts, 12 declined (5 converting from unfavorable to favorable and 5 converting detectable to 0), 5 remained 0, 4 increased. In the subset with PRO data, pain scores by BPI-SF tended to improve by wk 12. Following a single dose of 225Ac-J591, median PFS 7.2 months [95% CI 4.6-NR], median OS 10.9 months [7.6-21.1]. Conclusions: PSMA-targeted alpha-emitter 225Ac utilizing intact antibody J591 is tolerable with early evidence of clinical activity. Based upon these results, a follow up study [NCT04506567] testing multiple and fractionated dosing of 225Ac-J591 is underway. Clinical trial information: NCT03276572.

Imaging expression of prostate-specific membrane antigen and response to PSMA-targeted β-emitting radionuclide therapies in metastatic castration-resistant prostate cancer

BACKGROUND

Prostate-specific membrane antigen (PSMA)-targeted radionuclide therapy (TRT) has demonstrated efficacy and tolerability with a dose-response effect in phase I/II trials in men with metastatic castration-resistant prostate cancer (mCRPC). The need for positive PSMA imaging before PSMA-TRT to select patients is largely practiced, but its utility is not proven. Given target heterogeneity, developing a biomarker to identify the optimal patient population remains an unmet need. The aim of this study was to assess PSMA uptake by imaging and response to PSMA-TRT.

METHODS

We performed an analysis of men with mCRPC enrolled in sequential prospective phase I/II trials of PSMA-TRT. Each patient had baseline PSMA imaging by planar ¹¹¹ In and/or ¹⁷⁷ Lu SPECT (N = 171) or ⁶⁸ Ga-PSMA-11 PET/CT (N = 44), but the results were not used to include/exclude treatment. Semiquantitative imaging scores (IS) on a 0-4 scale were assigned based on PSMA uptake in tumors compared to liver uptake. We compared the ≥50% PSA decline response proportions between low (0-1) and high (2-4) PSMA IS using the χ² -test. We used multivariable logistic regression analysis to understand the relationship between independent and dependent variables, including IS, radionuclide activity (dose) administered, CALGB (Halabi) prognostic risk score, prior taxane use.

RESULTS

215 men with progressive mCRPC received PSMA-TRT as follows: ¹⁷⁷ Lu-J591 (n = 137), ¹⁷⁷ Lu-PSMA-617 (n = 44), ⁹⁰ Y-J591 (n = 28), ¹⁷⁷ Lu-J591 + ¹⁷⁷ Lu-PSMA-617 (n = 6). High PSMA expression (IS 2-4) was found in 160 (74.4%) patients and was significantly associated with more frequent ≥ 50% PSA reduction (26.2 vs. 7.3%, p = .006). On multivariate logistic regression analysis, higher IS was associated with a ≥50% decrease in PSA, even after accounting for CALGB (Halabi) prognostic score, the dose administered, and previous taxane use (OR, 4.72; 95% CI, 1.71-16.85; p = .006). Patients with low PSMA expression (N = 55, 24.7%) were less likely to respond. Thirteen of 26 (50%) with no PSMA uptake (IS = 0) had post-PSMA-TRT PSA decline with 2 (7.7%) having ≥ 50% PSA declines.

CONCLUSION

Collectively, the data provide evidence in favor of the hypothesis that patients with high PSMA uptake and high administered radionuclide dose correlate with a higher chance of response.

Baseline and post-treatment circulating tumor cell (CTC) counts with prostate-specific membrane antigen (PSMA)-targeted radionuclide therapy (TRT) in men with metastatic castration-resistant prostate cancer (mCRPC)

158

Background: CTC counts are an independent prognostic factor in men with mCRPC; certain changes following treatment (conversion from detectable to undetectable or unfavorable to favorable) are associated with improved overall survival. Most PSMA-TRT efficacy data have focused on PSA or imaging changes. Here, we describe baseline and post-treatment CTC counts from subjects receiving PSMA-TRT. Methods: Men with mCRPC treated on prospective clinical trials of PSMA-TRT and with available CTC counts (CellSearch) were included in our analysis. Depending upon the era of the trial, post-treatment counts were performed at 4-6 (initial era) or 12 weeks (recent era) after a single cycle of PSMA-TRT (individual trial data reported elsewhere). We describe CTC counts at baseline and compare pre-treatment counts to those after PSMA-TRT. Results: 116 men treated with PSMA-TRT had baseline CTC count (90 with both pre- and post-treatment CTC). Forty-four patients (37.9%) received 177Lu-J951, 46 (39.7%) received 177Lu-PSMA-617, and 26 (22.4%) received 225Ac-J591. Median age was 71.5. Fifty-eight patients (50%) had previously received taxane chemotherapy, median PSA was 82.98 ng/mL, and 66 (56.9%) were in the high-risk Halabi (CALGB) prognostic group. Eighty-nine out of one hundred sixteen (76.7%) had detectable baseline CTC and 58/116 (50%) had unfavorable baseline CTC count. Forty-nine out of seventy (70%) had post-treatment CTC count decline, 23/70 (32.9%) converted from detectable to undetectable, and 17/47 (36.2%) converted from unfavorable to favorable. CTC changes stratified by type of PSMA-TRT are reported in the table. Conclusions: This is the largest analysis of CTC changes in patients who have received PSMA-TRT. In addition to PSA changes and other previously reported outcomes, even when low doses of radionuclide therapy as part of dose-escalation studies are included, the majority with detectable CTC counts have post-treatment CTC count decline. A significant portion of patients experience favorable CTC changes. [Table: see text]

Pilot study of anti-prostate-specific membrane antigen (PSMA) antibody J591 for men with metastatic castration-resistant prostate cancer (mCRPC) and unfavorable circulating tumor cell (CTC) count

120

Background: Elevated CTC counts are associated with a poor prognosis in men with mCRPC. PSMA targeted radionuclide therapy has been associated with decline in CTC count, but it remains unclear whether this effect results from radionuclide-induced cytotoxicity. J591 was engineered to have antibody-dependent cytotoxicity. A subset of patients was observed to have CTC count decline following imaging with 111In-J591, so a prospective study was launched to test the hypothesis that “naked” J591 leads to CTC count decline. Methods: In a Simon 2-stage dose de-escalation study, men with progressive mCRPC and unfavorable CTC count (CellSearch > 4) received a single dose of J591. Initial dose cohort 300 mg with de-escalation to 20 mg. CTC count was re-assessed 4, 8, and 12 weeks following therapy along with PSA and standard imaging. An optional PSMA PET was included prior to treatment. The primary endpoint was proportion of subjects with conversion to favorable CTC count ( < 5 CTCs/7.5 mL blood) and/or > 30% decline from baseline within 12 weeks post-treatment. Results: 10 men were enrolled, 9 of whom were evaluable (1 died of progressive mCRPC prior to post-treatment CTC count). Median age was 71.5 years (range 60-81), 78% had prior chemo, ECOG PS 1 in 45% and 2 in 55%. 7 of 9 (78%) evaluable subjects were Halabi CALGB prognostic poor risk category and 2 (22%) intermediate. 6 of 9 had pre-treatment PSMA PET/CT (three 89Zr-J591 and three 68Ga-PSMA11). Though not required, all scans showed > 1 lesion with SUVmax > liver SUV (range 9.12-70.15). 2 of 6 in the 300 mg cohort had CTC count decline; 1 of 6 converted to favorable count (9 to 0 with decrease of 35 to 12 in other). 3 were treated with 20 mg; 1 had CTC count decline of 316 to 112, but 0 converted to favorable count. Across both cohorts, 3 of 9 had a CTC count decline at any point in time, ranging from 65-100% decline. With the pre-specified 2-stage design, enrollment was halted for futility based upon the primary endpoint of 12-week CTC count. PSA values post-treatment increased in 8 (89%) patients and remained unchanged in 1 (11%) patient. Conclusions: Single-agent anti-PSMA antibody J591 may lead to decline in CTC count, though the study did not meet its primary endpoint. A combination or maintenance approach might be preferable and is worthy of exploration.

A phase I/II dose-escalation study of fractionated and multiple dose 225Ac-J591 for progressive metastatic castration-resistant prostate cancer (mCRPC)

TPS188

Background: Prostate-specific membrane antigen (PSMA)-based targeted radionuclide therapy (TRT) is a promising treatment. PSMA-targeting via large antibodies vs small molecules has different kinetics, biodistribution, and resulting clinical toxicities. Using beta-TRT, 177Lu-J591 has more heme toxicity and 177Lu-PSMA-617 more non-heme toxicity (xerostomia and nausea) [Niaz AUA 2020]. Alpha-emitters are more potent than beta radionuclides, and alpha-PSMA-TRT has efficacy even after beta-PSMA-TRT. In a first-in-human phase I dose-escalation study of 225Ac-J591, patients with mCRPC were treated with a single dose of 225Ac-J591 on seven dose levels, up to 93.3 KBq/kg [Tagawa ASCO 2020]. No maximal tolerated dose (MTD) was achieved. One patient treated at 80 KBq/kg developed dose-limiting toxicity (DLT) of grade 4 anemia and thrombocytopenia, but 0 of 6 at 93.3 KBq/Kg had grade > 3 heme toxicity or grade > 2 non-heme toxicity. Preliminary results indicate 64% had any PSA decline and 41% had > 50% PSA decline (PSA50) across all doses, despite lack of selection for PSMA expression and the majority having been previously treated with 177Lu-PSMA. Methods: Entry criteria include progressive mCRPC by PCWG3 criteria, ECOG PS 0-2, intact organ function (including normal neutrophil and platelet counts), and prior receipt of AR pathway inhibitor and chemotherapy (or refuse/ineligible for chemotherapy). There is no limit to prior lines of therapy except alpha-PSMA-TRT. Phase I includes 2 separate parallel dose-escalation cohorts. In the fractionated-dose cohort, men will receive a single cycle of 225Ac-J591 administered on D1 and D15. In the multiple-dose cohort, 225Ac-J591 will be given every 6 weeks for up to 4 cycles. The phase I component is a 3+3 dose-escalation study design, with the goal of identifying MTD. Each phase II component will treat up to 27 men with PSMA+ PET scans in a Simon 2-stage design with 90% power to exclude the null hypothesis (35% or fewer patients with PSA50). Eligible men with negative PSMA PET scans will be offered treatment with informed consent in an exploratory subgroup, but will not be counted towards phase II efficacy. Secondary outcomes include radiographic response by PCWG modified RECIST 1.1 criteria and PSMA PET, biochemical and radiographic progression-free survival, circulating tumor cell counts, and overall survival. Patient reported outcomes, genomic, and immune analyses are exploratory. Enrollment began in August 2020 (NCT04506567). Clinical trial information: NCT04506567.

Phase I dose-escalation study of PSMA-targeted alpha emitter 225Ac-J591 in men with metastatic castration-resistant prostate cancer (mCRPC)

5560

Background: Antibodies (Abs) or small molecules can target PSMA with different biodistribution. Certain sites of PSMA expression (e.g. salivary/lacrimal glands, kidneys, small bowel) are not accessible to Abs. Given radiosensitivity of PC and potency of alpha emitters plus the ability to minimize targeting off tumor PSMA+ sites with J591, we performed a 1st in human study of 225Ac-J591. Methods: Men with progressive mCRPC following at least 1 potent AR-pathway inhibitor (ARPI) and chemo (or unfit/refuse) without limit of # prior therapies (Ra-223 and 177Lu-PSMA allowed) with ECOG PS 0-2 and adequate organ function were eligible. Baseline 68Ga-PSMA11 PET was performed, but not used for eligibility. Initially 1-subject cohorts treated until transition to 3+3 at dose level 5 (predicted by dosimetry to have moderate toxicity) with a single infusion of 225Ac-J591 (13.3 KBq/kg with planned escalation up to 93.3 KBq/kg). Dose-limiting toxicity (DLT) defined as attributable grade (Gr) 4 heme toxicity or Gr 3/4 non-heme tox. Imaging, genomic, patient-reported outcomes (PRO), and immune correlates embedded. Results: 22 men treated on 7 dose levels; median age 72.5 (range 58-89), PSA 147 (5-7168); 82% with >2 prior ARPI, 64% chemo, 23% Ra-223, 55% 177Lu-PSMA. 1 (5%) CALGB (Halabi) good prognostic risk, 10 (45%) intermed, 11 (50%) poor risk. 1 of 6 in cohort 6 (80 KBq/kg) had DLT (Gr 4 anemia and platelets); no others had attributable Gr > 2 non-heme or Gr > 3 heme AE (including 0 of 6 at the highest dose level 93.3 KBq/Kg). Gr 1/2 AE’s: 17 (77%) fatigue, 11 (50%) pain flare, 10 (45%) anemia (+1 Gr 3), 10 (45%) platelets, 6 (27%) nausea, 6 (27%) xerostomia (5 of 6 with prior 177Lu-PSMA), 5 (23%) neutropenia, 4 (18%) AST elevation. Despite prior treatment including 177Lu-PSMA and no selection for PSMA expression, 14 (64%) with any PSA decline, 9 (41%) with > 50% PSA decline. 15 (68%) had initial PSA rise followed by decline from peak (delayed effect). 2 with response > 1 year despite prior 177Lu-PSMA. Of 15 with paired baseline and 12-wk CTC counts, 8 declined, 4 remained undetectable, 3 increased. In subset with complete PRO data (baseline to 12 wks), pain was improved or absent by BPI-SF in 63% and by FACT-P in 89%. Social and emotional well-being domains of FACT-P improved or stabilized in majority; physical well-being improved in most responders. Conclusions: Alpha-emitter 225Ac targeting PSMA via J591 Ab is tolerable with early evidence of clinical activity in a pre-treated population with favorable PRO’s. Enrollment to expansion cohort being completed. Clinical trial information: NCT03276572 .

Dose-escalation results of a phase I study of 225Ac-J591 for progressive metastatic castration resistant prostate cancer (mCRPC)

114

Background: Prostate-specific membrane antigen (PSMA) can be targeted by antibodies (Ab) or small molecule ligands labeled with potent α emitters (e.g. 225Ac). Unlike ligands, Ab biodistribution does not include non-tumor organs such as the salivary glands, kidneys and small bowel. We performed a phase I single ascending dose Ab study. Methods: Eligibility: progressive mCRPC following at least 1 potent AR pathway inhibitor (ARPI; e.g. abi/enza) and docetaxel (or unfit/refuse chemo) without limit of # prior therapies provided adequate organ function. Baseline 68Ga-PSMA11 PET was performed but not used for eligibility. Single-subject cohorts received a single infusion of 225Ac-J591 until grade (Gr) > 1 attributable toxicity or dose level 5, then transition to 3+3 design. Cohort 1 = 13.3 KBq/kg with planned escalation up to dose level 7 (93.3 KBq/kg). Dose-limiting toxicity (DLT) defined as attributable Gr 4 heme or Gr 3/4 non-heme toxicity. Results: 22 men were treated on 7 dose levels; median age 72.5 (range 58-89), PSA 146.5 (4.8-7168.4); 82% with >2 prior ARPI, 64% chemo, 45% sipuleucel-T, 23% radium-223, 55% prior 177Lu-PSMA. By standard imaging 82% bone, 36% lymph node, 9% liver mets. At the time of data cutoff, 1 of 6 men in cohort 6 (80 KBq/kg) had DLT (Gr 4 anemia and platelets); he had 4 prior cycles of 177Lu-PSMA. No other attributable Gr >2 non-hematologic or Gr >3 heme AE (including 0 of 6 at the highest dose level). Low Gr temporary AE’s include: 16 (73%) with fatigue, 11 (50%) pain, 11 (50%) nausea, 6 (27%) with xerostomia (5 of 6 with prior 177Lu-PSMA), 3 (14%) AST elevation. With follow-up ongoing, 60% with any PSA decline, 35% with >50% PSA decline. Of 10 with detectable baseline and 12-week CTC counts (CellSearch), 8 declined (45-100% decline); 5 (50%) with CTC count conversion. While PSMA uptake was not a prerequisite for treatment, all had some PSMA uptake on 68Ga-PSMA11 PET/CT; 64% with SUVmax >5x liver SUV, 14% 2.5x – 5x liver, and 23% with SUVmax 0-2.5x liver SUV. Conclusions: PSMA-targeted alpha-emitter 225Ac utilizing intact Ab J591 is well tolerated with early evidence of clinical activity in a pre-treated population, including the majority with prior 177Lu-PSMA. Clinical trial information: NCT03276572.

Patient-reported outcomes (PRO) from a phase I/II dose-escalation study of fractionated dose 177Lu-PSMA-617 for progressive metastatic castration-resistant prostate cancer (mCRPC)

45

Background: We performed the 1st dose-escalation study of PSMA-targeted radionuclide therapy with 177Lu-PSMA-617. Using dose-fractionation, we intended to deliver a dose-intense regimen designed to minimize radioresistance due to repopulation. Radionuclide therapy may be able to treat symptoms due to tumor and therefore may be associated with improvement in PRO. Methods: Inclusion: progressive mCRPC following potent AR-pathway inhibitor (ARPI, e.g. abi/enza) and taxane (or unfit/refuse chemo) without limit of # prior therapies, adequate organ function, ECOG performance status 0-2, without preselection for PSMA expression. Treatment was a single cycle of fractionated dose 177Lu-PSMA-617 on D1 and D15 (7.4 to 22 GBq in phase 1; 22.2 GBq in phase 2). PRO tools included FACT-P and BPI-SF at baseline and follow up. Results: 44 men with median age 69 (range 55-91), median PSA 182.97 (range 0.89-5541) were treated. 93% with bone, 45% nodal, 18% lung, 9% liver, 9% other visceral metastases. 55% with at least 1 prior chemo regimen, 52% >2 prior ARPI, 27% with Ra223, 30% sip-T, 5% 177Lu-J591. 59.1% with >50% PSA decline (66.7% at 22.2 GBq, n=21), median overall survival 16 months (95% CI 11-NR). High grade (Gr) toxicity was rare with 6.8% Gr 3 anemia and 2.3% Gr 3 platelets. Gr 1/2 treatment-emergent AE’s include 81.8% with pain flare, 61.4% xerostomia, 29.5% fatigue, 25% platelets, 25% anemia, 25% pain, 15.5% nausea. FACT-P scores tended to improve in all categories by D22 (1 week later), with overall FACT-P scores improving by mean of 8.9 points (p=0.07) at D22 and remaining improved at 12 wks. All BPI scores also improved, with BPI overall severity score improving by mean of 3.0 at D22 (p=0.008) and remained better than baseline at 12 wks. There was no clear association with any AE and PRO changes, but those with PSA decline tended to have improved pain scores (p=0.1). Conclusions: A single cycle of up to 22.2 GBq of 177Lu-PSMA-617 is safe with fractionated (D1 & D15) dosing, with encouraging early efficacy signals in a population unselected for PSMA expression and improved quality of life and pain scores by validated PRO instruments. Clinical trial information: NCT03042468.

Association of noninvasive, radiographic measurement of prostate-specific membrane antigen (PSMA) expression with response to PSMA-targeted radionuclide therapy (TRT)

5013

Background: Prostate surface membrane antigen (PSMA) is usually overexpressed in PC and is enriched in castration-resistant tumors. PSMA-TRT is of increasing interest to the field. Many in the field of theranostics have assumed that PSMA uptake on imaging is a pre-requisite for response. We have conducted a number of trials which have incorporated PSMA imaging, but have not selected patients for treatment based upon imaging results and performed an analysis examining the relationship between imaging and response. Methods: Men with mCRPC had either planar radiolabeled J591 imaging (111In-J591 and/or 177Lu-J591) or 68Ga-PSMA11 PET/CT. Visual scores were assigned based upon PSMA uptake in tumors compared to liver uptake and scored on a 0-4 scale. Imaging scores were associated with PSA decline (≥30%, ≥50%) using Cox regression analysis. As several studies were dose-escalation in nature with prior demonstration of dose-response, we controlled for dose administered. Results: 216 men with metastatic CRPC, median PSA 72.45ng/dl, were treated with PSMA-TRT as follows: 177Lu-J591 (n=136), 177Lu-PSMA-617 (n=38), Lu-J591 + Lu-PSMA-617 combination (n=6), 225Ac-J591 (n=7), 90Y-J591 (n=29). 116 (53.7%) pts received low dose and 100 (46.3%) high dose as previously defined in the individual studies. 55 (25.5%) pts had low PSMA expression by imaging (VS 0-1) whereas 161 (74.5%) had high PSMA (VS 2-4). High PSMA expression was associated with more frequent PSA decline (≥30%: 39.2 vs 17.9% p=0.003; ≥50%: 27.5 vs 8.9% p=0.004). When controlling for dose level, this association remained significant for low (≥30%: 25 vs 8.6% p=0.04) and high doses of radionuclide therapy (≥50%: 34.9 vs 9.5% p=0.02). 13 (6%) pts with no PSMA uptake (VS=0) had PSA declines. Conclusions: This is the first study to formally analyze response to PSMA-TRT by PSMA imaging expression in an unselected patient population. The level of PSMA expression measured by imaging is associated with the chance of response. However, a subset of patients without any significant PSMA uptake on imaging did demonstrate response to PSMA-TRT, indicating that imaging cannot exclude all patients that might benefit. Clinical trial information: NCT03545165, NCT03276572, NCT03042468, NCT02552394.

Pilot study of dual imaging with 89Zr-IAB2M and 68Ga-PSMA-11 PET/CT prior to prostatectomy

90

Background: Due to high expression of Prostate Specific Membrane Antigen (PSMA), it’s an excellent candidate for targeted molecular imaging. 89Zr-df-IAB2M is a J591 antibody derived minibody and 68Ga-PSMA-11 is a PSMA small molecule. Methods: Patients with clinically significant (defined as: ≥ 0.5 cm3 with Gleason pattern ≥ 4) localized prostate cancer (PCa) on conventional imaging modalities who planned to undergo surgery were imaged by PET/CT 1-3 hours after 5±2mCi of 68Ga-PSMA-11 injection and 89Zr-IAB2M PET 2-4 days after 10mg IAB2M labeled with 2.5 mCi of 89Zr injection. Image results were interpreted by a central reader without knowledge of the surgical pathology, mapped and the mapped findings later compared with surgical pathology map. Results: 9 patients having clinically localized PCa with median age 65 (46-79) and PSA 8.42 (1.6-12.2) were enrolled. All dominant PCa lesions, which had a median Gleason score 8 (6-9), were detected by both IAB2M and PSMA-11. The smallest of these dominant lesions was 8mm in size. Median SUVmax for dominant lesions was 3.3 (2.3-7.5) on IAB2M and 7.1 (1.6-23.2) on PSMA-11; median MR PIRADS was 4 (4-5). Of 9 Gleason 6 lesions identified on pathology, PSMA-11 detected 3 and IAB2M detected 2. Four extra-prostatic lesions in one patient were detected by both PET agents, 3 were confirmed by surgical pathology and 1 confirmed on bone scan 2 months post-op. A total of 8 lesions reported on scans were not reported by path: 3 were false positive on both scans, 3 were false positive on IAB2M scan only and 2 were false positive on PSMA-11 scan only. 5 Gleason 6 and 2 Gleason 3+4 lesions on pathology were missed by both PET agents and could be considered false negatives. Conclusions: In this small pilot series, performance of the both PET agents was very similar. An advantage of IAB2M is its hepatobiliary clearance rather than renal/urinary, which makes it potentially easier to visualize the PCa and/or pelvic disease; an advantage of PSMA-11 is the ability to image within 1 hour. Clinical trial information: NCT03675451.

Phase I/II dose-escalation trial of fractionated dose 177Lu-J591 plus 177Lu-PSMA-617 for metastatic castration-resistant prostate cancer (mCRPC)

TPS339

Background: PC is a radiosensitive disease. PSMA is selectively overexpressed in advanced PC with upregulation by androgen receptor (AR) pathway dysregulation; limited expression exists in other organs. Prior studies of beta-emitting radiolabeled anti-PSMA antibody J591 demonstrated accurate targeting, efficacy with dose-response effect, and safety with predictable dose-limiting myelosuppression. Recent prospective trials have shown efficacy and safety of 177Lu-PSMA-617 in mCRPC. Studies demonstrate different binding sites of J591 and PSMA-617 and co-administration leads to non-competitive additive binding and delivery of payloads to PSMA+ cells. As the pharmacokinetics and biodistribution of the 2 agents is mostly non-overlapping (other than tumor) and given our prior dose-response data, we hypothesize that delivery of the combination will yield higher tumor delivery with less off-target toxicity (i.e. better responses without increased toxicity). Methods: Men with progressive mCRPC following at least 1 potent AR-targeted agent (e.g. abi/enza) and docetaxel (or unfit/refuse chemo) with metastatic disease on CT/MRI or bone scan without limit of # prior therapies (excluding bone-targeted beta-emitting radioisotopes) provided adequate organ function will be enrolled. Pre-treatment 88Ga-PSMA-11 will be performed, but results are not used for eligibility. Treatment includes fractionated 177Lu-J591 at a fixed moderate dose with escalating doses of 177Lu-PSMA-617 (7.4 – 18.5 GBq per cycle, fractionated D1 and D15) in a 3+3 dose-escalation study. Dose-limiting toxicity (DLT) is defined as attributable grade > 3 heme toxicity or grade > 2 non-heme toxicity. Following determination of recommended phase 2 dose (RP2D), the phase 2 portion will enroll in a 2-stage design. Primary endpoints: DLT and RP2D (ph 1) and PSA decline proportion (ph 2). Secondary endpoints include toxicity, radiographic response, PFS, rPFS, OS, CTC count changes. Correlatives include baseline/follow up PSMA imaging, whole body distribution of 177Lu, tissue and circulating genomic assessment, immunologic assessment, and patient reported outcomes (FACT-P and BPI-SF). Clinical trial information: NCT03545165.

Phase I dose-escalation study of 225Ac-J591 for progressive metastatic castration resistant prostate cancer (mCRPC)

TPS399

Background: PC is a radiosensitive disease. PSMA is overexpressed in advanced PC with upregulation by androgen receptor (AR) pathway dysregulation; limited expression exists in other organs. A series of sequential studies of beta-emitting radiolabeled anti-PSMA monoclonal antibody (mAb) J591 have demonstrated accurate targeting, efficacy with dose-response effect, and safety with predictable, reversible dose-limiting myelosuppression. Alpha emitters are significantly more potent with a shorter range than beta emitters. Though there is no direct tumor-targeting, the bone-targeting alpha emitter Ra223 is approved. Anecdotal reports of PSMA small molecule targeted alpha emitters have hinted at efficacy but are limited by xerostomia and in mouse models may lead to long-term renal damage. Intact mAb J591 has comparatively no to minimal distribution in salivary glands and kidneys. Preclinical studies demonstrated purity, immunoreactivity, and stability with efficacy in a xenograft model [AACR 2017]. Methods: Men with progressive mCRPC following at least 1 potent AR-targeted agent (e.g. abi/enza) and docetaxel (or unfit/refuse chemo) without limit of # prior therapies (excluding beta-emitting bone-targeted radioisotopes) provided adequate organ function will undergo imaging with 68Ga-PSMA-11 PET/CT followed by a single dose of 225Ac-J591. Single-subject cohorts will be enrolled until grade > 1 attributable toxicity, then transition to 3+3 design. Cohort 1 = 13.3 KBq/kg with planned escalation up to 93.3 KBq/kg of 225Ac with fixed 20 mg J591. Dose-limiting toxicity (DLT) is defined as attributable grade 4 heme toxicity or grade 3/4 non-heme toxicity. Planned cohort expansion will occur at recommended phase 2 dose (RP2D) in a 2-stage design. The primary endpoint is determination of DLT and RP2D. Secondary endpoints include toxicity, PSA decline rate, RECIST response, PFS, rPFS, OS, and patient reported outcomes (FACT-P and BPI-SF). Correlatives include baseline/follow up PSMA imaging, CTC count (CellSearch) changes, tissue and circulating genomic assessment, and immune studies. Enrollment began in October, 2017. Clinical trial information: NCT03276572.

Phase I dose-escalation study of fractionated-dose 177Lu-PSMA-617 for progressive metastatic castration resistant prostate cancer (mCRPC)

TPS5093

Background: PC is a radiosensitive disease. PSMA is selectively overexpressed in advanced PC with upregulation by androgen receptor (AR) pathway dysregulation; limited expression exists in other organs. A series of sequential studies of radiolabeled anti-PSMA antibody J591 revealed 1) targeting and safety [Bander 2003]; 2) safety and prelim efficacy [Milowsky 2004, Bander 2005]; 3) efficacy and initial dose-response [Tagawa 2013]; 4) dose-fractionation allows higher doses, ability to combine with docetaxel, confirmation of dose-response (PSA and overall survival) [ASCO 2010, 2014, 2016]; 5) predictable, reversible myelosuppression is dose-limiting [Tagawa 2013]. Small molecule PSMA inhibitor ligands can be successfully radiolabeled and are widely used for imaging and treatment in Europe. 177Lu-PSMA-617 is the most commonly used, but experience is mostly anecdotal/retrospective and no formal dose-escalation studies have been performed. Methods: Men with progressive mCRPC following at least 1 potent AR-targeted agent (e.g. abi/enza) and docetaxel (or unfit/refuse chemo) without limit of # prior therapies provided adequate organ function will undergo imaging with 68Ga-PSMA-HBED-CC PET/CT followed by escalating fractionated doses of 177Lu-PSMA-617. Cohort 1 = 3.7 GBq x2 two weeks apart up to 11.1 GBq x2 in a 3+3 dose-escalation study. Dose-limiting toxicity (DLT) is defined as attributable grade 4 heme toxicity or grade 3/4 non-heme toxicity. Planned cohort expansion will occur at recommended phase 2 dose (RP2D) in a 2-stage design. The primary endpoint is determination of DLT and RP2D. Secondary endpoints include toxicity, PSA decline rate, RECIST response, PFS, rPFS, OS. Correlatives include baseline/follow up PSMA imaging, whole body distribution of 177Lu-PSMA-617, CTC count (CellSearch) changes, tissue and circulating genomic assessment of DNA repair pathways, patient reported outcomes (FACT-P and BPI-SF). Clinical trial information: NCT03042468.

A phase 1/1b multicenter, open-label, dose escalation and dose expansion study to evaluate the safety, pharmacokinetics, immunogenicity, and antitumor activity of MEDI3726 in patients with metastatic, castration-resistant prostate cancer who have received prior treatment with abiraterone or enzalutamide

TPS5088

Background: Therapeutic advances have recently been achieved for patients with metastatic, castration-resistant prostate cancer (mCRPC) due to abiraterone acetate (ABI) and enzalutamide (ENZ). However, virtually all patients with mCRPC eventually progress in their disease, and further treatment options are limited. Prostate-specific membrane antigen (PSMA) is highly expressed in nearly all prostate cancers, and its expression is highest in mCRPC. MEDI3726 is an antibody-drug conjugate composed of anti-PSMA antibody derived from J591, site-specifically conjugated to the cytotoxic, DNA cross-linking, pyrrolobenzodiazepine dimer. MEDI3726 has demonstrated potent and specific in vitro and in vivo antitumor activity in human prostate cancer-derived preclinical models with different expression levels of PSMA. Methods: This is a first-in-human, phase 1/1b, multicenter, open-label, dose escalation and dose expansion study in patients who have received prior treatment with ABI or ENZ, with or without prior taxane-based chemotherapy in the mCRPC setting (NCT02991911). The primary objectives are to assess safety and tolerability, describe dose-limiting toxicities, and determine the maximum tolerated dose or maximum administered dose of MEDI3726. The secondary objectives are to evaluate MEDI3726 for its antitumor activity (based on a composite response according to RECIST Version 1.1, a reduction in prostate-specific antigen level of 50% or more compared to baseline, or a conversion in the circulating tumor cell count [defined as a reduction from ≥5 cells/7.5 mL blood to < 5 cells/7.5 mL blood]), safety and tolerability in combination with ENZ, pharmacokinetics alone and in combination with ENZ, and immunogenicity. Recruitment is ongoing for this study, which has an estimated total target enrollment of 224 patients. Clinical trial information: NCT02991911.

Fractionated dose radiolabeled anti−prostate specific membrane antigen (PSMA) radioimmunotherapy (177Lu−J591) for progressive metastatic castration−resistant prostate cancer (mCRPC)

205

Background: Phase I and II single dose 177Lu−J591 studies have been published. Based on the theoretical advantages of dose fractionation on safety and efficacy, we initiated a phase I dose−escalation study of fractionated−dose 177Lu−J591. Methods: Men with progressive mCRPC with normal neutrophil and platelet counts were enrolled. The initial portion was 3+3 dose−escalation, with 6 cohorts of 3−6 patients receiving 2 doses 177Lu−J591, 2 weeks apart starting with 20 mCi/m2, escalating to 45 mCi/m2. After determining maximum tolerated dose (MTD), patients enrolled in 2 expansion cohorts at recommended phase II doses (RP2D). Planar imaging of 177Lu−J591 at 6−8 days following the initial dose was performed. Pre− and post−treatment PSA was measured for all patients and the highest dose−level cohorts had CTC counts (CellSearch) measured before and after treatment. Results: 48 patients enrolled with median age 74 (55−95) ), median baseline PSA 45.38 (1.93−766.5), 37.5% with prior chemo. The RP2D’s of fractionated 177Lu−J591 were 40 mCi/m2 or 45 mCi/m2 x2 with option for GCSF. Overall PSA decline ≥ 50% in 9 (18.8%), ≥ 30% in 14 (29.2%) and any PSA decline in 26 (54.2%); at RP2D doses (n = 32), 8 (25%) with ≥ 50%, 12 (37.5%) with ≥ 30%, and 20 (62.5%) with any PSA decline. Of 25 with available CTC counts, 14 declined, 8 remained stably favorable, and 3 increased. Of 12 with unfavorable baseline CTC counts, 8 (66.7%) became favorable at follow up, 2 decreased by 30% and 88% but remained ≥ 5, and 2 increased; 1 converted from < 5 to ≥ 5. Thirty-five (72.9%) had grade 3/4 hematological toxicities; 19 (59.4%) with Gr 4 heme toxicity in RP2D cohorts, with 15 (16.9%) receiving at least 1 platelet transfusion, 6 receiving GCSF, and 0 with febrile neutropenia. Overall 4 had Gr 1 transaminitis and 14 (29.2%) had grade 1 infusion reactions (without pre−medication). Accurate targeting of 177Lu−J591 at known sites of disease seen in 84.4%. Conclusions: Fractionated 177Lu−J591 is well tolerated with predictable, reversible myelosuppression, achieving a higher cumulative dose than possible with a single dose. Both PSA and CTC count control was achieved. Clinical trial information: NCT00538668.

Effect of prostate-specific membrane antigen (PSMA) radioimmunotherapy on circulating tumor cell (CTC) count

199

Background: CTC counts with CellSearch methodology are prognostic; changes following therapy may be associated with improved outcomes. Since FDA clearance, baseline and follow up CTC counts were included in studies of radiolabeled J591. The effect of J591 alone (without an effector molecule) is unknown. Methods: Our phase II single-dose 177Lu-J591 (Cohort 3), phase I fractionated dose 177Lu-J591 (expansion cohorts), and phase I fractionated 177Lu-J591 + docetaxel (all pts) trials were analyzed. Any pt with baseline and at least 1 follow-up CTC count (CellSearch) was included in the analysis. A retrospective subset of patients undergoing J591-based imaging without an effector molecule attached was also analyzed. Results: 48 pts received 177Lu-J591 with prospectively measured CTC counts with median age of 73.7 years. 26 of 48 (54.2%) had unfavorable (≥5 CTCs/7.5 mL blood) baseline CTCs (median 29, range 5-449 CTCs/7.5 mL). Of the 23 with ≥5 baseline CTCs and available 4-6 wk post-treatment counts, 22 (95.6%) had declines of 15-100%, with 12 (52.2%) converting to favorable counts. Of the 18 with ≥5 baseline CTCs and available 12-14 wk post-treatment counts, 10 (55.5%) had declines of 8-100%, with 7 (38.9%) converting to favorable counts. Of the 22 of 48 (45.8%) with baseline CTC counts <5, 1 became unfavorable at 4-6 wks, later re-converting to favorable (early control rate of 95.5%) and 1 became unfavorable at 12-14 wks (sustained control rate of 95.5%). 7 men with unfavorable counts underwent imaging with 20 mg of J591 without an attached effector molecule; 4 (57%) demonstrated decline in CTCs following the imaging dose, with 1 stable and 2 continued to increase prior to subsequent therapy. Conclusions: In addition to the favorable PSA declines previously reported, radiolabeled J591 results in frequent favorable changes in CTC counts, with >90% of men with baseline unfavorable counts receiving 177Lu-J591 demonstrating CTC count decline or control in pts with baseline favorable counts. A small retrospective subset suggests that CTC counts may also be controlled with naked (unlabeled) anti-PSMA monoclonal antibody and a prospective study investigating this phenomenon will begin enrollment. Clinical trial information: NCT00195039 , NCT00538668 , NCT00916123 .

Fractionated dose radiolabeled antiprostate specific membrane antigen (PSMA) radioimmunotherapy (177Lu-J591) with or without docetaxel for metastatic castration-resistant prostate cancer (mCRPC)

194

Background: A phase II trial single-dose 177Lu-J591 was published. Dose fractionation offers theoretic advantages and 2 phase I dose-escalation studies of 177Lu-J591 have been performed to test the hypothesis that higher cumulative radiation doses can be administered with an acceptable toxicity profile. Methods: Pts with mCRPC and normal neutrophil and platelet counts were enrolled in 2 sequential phase I dose-escalation studies: (1) 177Lu-J591 administered as a single-agent 2 doses 2-weeks apart to determine the maximum tolerated dose (MTD) with expansion cohorts at the recommend phase II doses (RP2D) and (2) fractionated dose 177Lu-J591 in combination with docetaxel 75 mg/m2q3 weeks to determine the MTD. Results: 44 men with median age 75.4 (range 55.1-95.2) were enrolled in the single-agent fractionated dose escalation 177Lu-J591 trial and 15 men with median age 69.1 (49.3-80.8) were enrolled in 177Lu-J591 + docetaxel trial. The RP2D’s of fractionated 177Lu-J591 were 40 mCi/m2 x2 or 45 mCi/m2 x2 with the option for GCSF; at these doses (n=28), 8 (28.6%) with >50%, 11 (39.3%) >30%, and 16 (57%) of 28 pts with any PSA decline; pts at RP2D lived longer (p<0.0001). Predictable, reversible myelosuppression was seen. 18 (40.9%) received chemo prior to RIT and 24 (54.5%) received chemo post 177Lu-J591 [median of 7 cycles (range 1-20)]. In combination with docetaxel, the MTD/RP2D of 177Lu-J591 is 40 mCi/m2x2 doses (delivered with cycle 3), with 73.3% >50%, 80.0% >30%, and 86.7% with any PSA decline. Conclusions: Fractionated 177Lu-J591 is tolerated with subsequent PSA declines and reversible myelosuppression. The apparent dose-response of single-dose 177Lu-J591 appears confirmed with fractionated dosing (improved PSA declines and OS at higher doses). Despite predictable myelosuppression, chemotherapy is able to be delivered prior to 177Lu-J591, concurrently, or following radioimmuotherapy. Clinical trial information: NCT00538668 , NCT00916123 .[Table: see text]

Tumor-directed PET imaging of metastases in metastatic castration-resistant prostate cancer (mCRPC) using Zr-89 labeled antiprostate-specific membrane antigen (PSMA) antibody J591

164

Background: There is no standard imaging (SI) modality that specifically and accurately images prostate cancer (PC) metastases, hampering prognostication and response assessment. J591 is a humanized antibody that targets the external domain of PSMA. We have previously reported on the feasibility, PK and biodistribution properties of 89Zr-J591 in 10 patients (pts) (Pandit-Taskar et al, Eur J Nuc Med Img 2014). We now report on the targeting/accuracy in 50 pts with mCRPC. Methods: Following standard CT/MRI, bone scan (BS), and FDG PET imaging, 5 mCi of 89Zr-J591 was administered IV. 89Zr-J591 was imaged 6-8 days after injection. Positive (pos) scan findings were confirmed, where possible, with biopsies (bxs) in the following preference: concordant 89Zr-J591 and FDG pos, 89Zr-J591 and FDG mismatch, and a mismatch between SI and any PET. Results: Imaging: A total of 703 lesions in 50 pts were identified using all imaging modalities. Bone:538 total bone lesions were detected. 491(91%) lesions were present on J591 of which 99 were only evident by J591. BS identified 339 (63%), CT 301 (56%), and FDG 207 (38%). Soft Tissue: 165 total soft tissue lesions were detected. 90 (55%) were seen on J591 of which 17 were only evident by J591. CT identified 124 (75%) and FDG 88 (53%). Pathology:46 bx’s were evaluable (21 bone, 25 soft tissue) in 34 pts. Of the unique J591 lesions biopsied (bx’d), 5/7 were pos for PC. Bone: We bx’d 19 J591 pos lesions and 2 J591 neg sites. Overall, path concordance with J591 was: 89% true pos, 100% true neg, 11% false pos, and 0% false neg. Soft tissue: We bx’d 16 J591 pos lesions and 9 J591 neg sites. Of these, we found 88% true pos, 11% true neg, 13% false pos, and 89% false neg. Conclusions: J591 PET identifies additional disease in bone not seen using other imaging modalities. These lesions are highly likely to correspond to disease by bx. However, the tracer performed less well in soft tissue, with a high false neg rate. Clinical trial information: NCT01543659.

Tumor-directed PET imaging of bone metastases in metastatic castration-resistant prostate cancer (mCRPC) using Zr-89 labeled anti-prostate specific membrane antigen (PSMA) antibody J591

25

Background: There is no standard imaging modality that specifically and accurately images prostate cancer metastases, hampering assessments of tumor distribution, prognostication, and response. J591 is a humanized antibody that targets the external domain of PSMA, which is copiously expressed in metastatic disease and upregulated in mCRPC. We have previously reported on the feasibility, and PK and biodistribution properties of Zr-89-J591 in 10 patients (Morris et al, GU ASCO 2013). We now report on the targeting/accuracy in 49 patients with mCRPC. Methods: Following standard CT/MRI, bone scintigraphy, and FDG PET imaging, 5 mCi of Zr-89-J591 was administered intravenously. Zr-89-J591 was imaged 6-8 days after injection. Positive scan findings were confirmed, where possible, with biopsies in the following preference: Zr-89-J591 and FDG positivity, then Zr-89-J591 and FDG mismatch, and lastly standard imaging and any PET mismatch. Results: A total of 535 unique bone lesions in 49 patients were identified using all imaging modalities. 339 (63%) of these were evident on bone scans, 302 (56%) on CT, 202 (38%) on FDG PET, and 490 (92%) on Zr-89-J591 PET. The concordance between bone scans and Zr-89-J591 was 303/490 (62%). 102 sites were seen only on Zr-89-J591 and not on any other imaging modality. 44 biopsies were performed from 33 patients; 21 of those biopsies were bone. The histopathologic correlation for bone is provided in the table below. All positive biopsies were J591 positive (16/16); sensitivity of J591 was =100%, and positive predictive value (16/18) was 89%. Conclusions: Imaging using J591 PET/CT detects bone disease at greater rates than any other imaging modality tested. Biopsy data of J591 positive lesions suggest that most J591 positive lesions represent biopsy confirmed prostate metastasis. We are now testing minibodies based on J591 that may target faster, offering a more favorable imaging schedule. Clinical trial information: NCT01543659. [Table: see text]

Noninvasive measurement of prostate-specific membrane antigen (PSMA) expression with radiolabeled J591 imaging: A prognostic tool for metastatic castration-resistant prostate cancer (CRPC)

11081

Background: PSMA is nearly universally expressed by PC, upregulated with increased grade and castration-resistance. Evidence points towards PSMA expression as a downstream cellular biomarker of androgen receptor (AR) activity and non-invasive measurement of PSMA expression has recently been demonstrated to be a novel biomarker of AR activity. Methods: Planar gamma camera images following radiolabeled J591 (111In-J591 or 177Lu-J591) were semi-quantitatively scored using 2 methods by 2 independent radiologists blinded to outcome.A 5-point visual score (VS) of 0 - 4+ was assigned. Tissue Targeting Index (TTI), a novel metric designed to semi-quantitatively score images was calculated using the ratio of lesion count density (corrected for background) to whole body count density, with maximum (TTImax) and mean (TTIave) scores recorded. Follow up tabulating subsequent therapies and overall survival (OS) was recorded and imaging scores were associated with OS using Cox regression analysis. Results: 130 men with metastatic CRPC underwent radiolabeled J591 imaging. 86.2% had bone metastases, 51.5% lymph node, 16.9% lung, 9.2% liver. 87.7% had accurate targeting of known sites of disease by planar imaging. CALGB (Halabi) nomogram scores were prognostic for the population. As continuous variables, TTImax (p=0.013) and TTIave (p=0.002) were associated with worse survival. VS demonstrated a trend for worse survival (p=0.09). In multivariate analysis, TTI maintained independent prognostic value when controlling for Halabi score: TTImax HR 1.05 [95% CI 1.01, 1.10; p=0.02], TTIaveHR 1.09 [1.03, 1.16; p=0.004]. Conclusions: Level of PSMA expression measured by planar gamma-camera imaging following radiolabeled J591 is associated with OS in men with metastatic CRPC. High PSMA expression may indicate more aggressive tumor biology with increased AR pathway dysfunction. Improvements in quantitative molecular imaging techniques such as PSMA PET/CT with 89Zr-J591 may prove to be a valuable prognostic and predictive biomarker, particularly in the setting of AR- and PSMA-targeted therapy. Clinical trial information: NCT00195039, NCT00538668.

Phase II trial of 177lutetium radiolabeled anti-PSMA antibody J591 (177Lu-J591) for metastatic castrate-resistant prostate cancer (metCRPC): Survival update and expansion cohort with biomarkers

121

Background: A phase II trial in men with progressive metCRPC receiving a single dose of 177Lu-J591 at 65 mCi/m2 (15 pts) or 70 mCi/m2 (phase I MTD, 17 pts) was performed without selection for PSMA expression, suggesting a larger than expected dose-response (13 vs 47% >30% PSA decline respectively), leading to an expansion cohort to validate the response rate at 70 mCi/m2. Methods: Endpoints: to validate the PSA and/or measurable disease response at 70 mCi/m2, evaluate circulating tumor cell counts (CellSearch) and pre-treatment PSMA imaging with 111In-J591 in the expansion cohort, and examine overall survival (OS) for all pts. Results: 15 additional pts were treated. Expansion cohort demographics were similar to the initial cohorts, and PSA responses and toxicity were similar to the initial cohort treated at 70 mCi/m2 (see Table), with myelotoxicity improving in all following nadir at 1 month. More PSA declines and longer OS were seen at 70 mCi/m2. 12 of 15 pts had baseline and follow up CTC counts at 4-6 weeks: 66.7% had >50% decline and 25% were unchanged at 0 or 1 (one declined 27%). Although 93.3% had accurate targeting (imaging) of known sites of disease, as seen in initial analysis, a trend for fewer > 30% PSA declines was seen with less intense PSMA imaging. Conclusions: Single dose 177Lu-J591 at70 mCi/m2 was generally well tolerated, with predictable, reversible myelosuppression, and demonstrates anti-tumor activity in pts with progressive metCRPC. A dose-response relationship was confirmed for both toxicity and activity, with improved response and OS at 70 mCi/m2. CTC declines are demonstrated. Selection of pts based upon non-invasive testing (PSMA imaging) may improve the therapeutic profile. Clinical trial information: NCT00195039. [Table: see text]

Phase I trial of zirconium 89 (Zr89) radiolabeled J591 in metastatic castration-resistant prostate cancer (mCRPC)

31

Background: Presently, there are no means of accurately and reproducibly imaging bone metastases for patients with mCRPC. We are investigating a prostate-specific imaging method that can directly visualize metastases using PSMA as a target and Zr89 radiolabeled anti-PSMA antibody J591 as a tracer. Following the FDA roadmap for biomarker development, we report the first human data on the tracer and its preliminary analytic validation. Methods: 5 mCi of Zr89-J591 was administered intravenously and 4 serial PET/CT scans were obtained within the following time intervals post-injection: 2-4 hours, 18-30 hours, 2-5 days, and 6-7 days. Pharmacokinetic (PK) sampling occurred at 5, 30, and 60 minutes post-injection, and at each PET scan. Patients underwent standard cross-sectional imaging, bone scintigraphy, and FDG PET scanning. Metastatic sites were biopsied in order of preference: Zr89-J591 and FDG positivity, then Zr89-J591 and FDG mismatch, and lastly standard imaging and any PET mismatch. Results: Ten patients were scanned, 132 Zr89-J591 positive sites were detected, and 12 lesions (5 bone, 6 node, 1 lung) in 8 patients were biopsied. Tissue correlation: 12/12 biopsies were positive for cancer. 11/12 biopsied lesions (5 bone, 5 node, 1 lung) were positive on Zr89-J591 imaging. 10/12 were positive by standard scans or FDG. 1 lesion was negative by Zr89-J591 PET, but positive by other modalities. Immunokinetics: Blood clearance T1/2α: 7 +/- 4.5 h (1.1-14 h); T1/2β: 62 +/- 13 h (51-89 h); Whole body clearance T1/2: 219 +/- 48 h (153-317 h). Dosimetry: Maximum retention in liver 8 +/- 1.5 SUV; Kidney 4.1 +/- 0.8 SUV; Tumor 8.2 +/- 6.5 SUV (mean SUV in bone 11.13 +/- 7.57 vs. soft tissue 4.31 +/- 2.59). Optimal time for patient imaging after injection, in terms of tumor to background ratios was 7 +/- 1 days. Conclusions: Zr89-J591 imaging demonstrates excellent tumor localization, pathology correlation, and can demonstrate the presence of tumor even when lesions are negative by standard imaging or FDG PET imaging. PK properties of the tracer have been defined. Use of Zr89-J591 to direct biopsies of metastases provides excellent yield. Further studies to examine reproducibility and post-treatment effects are planned. Clinical trial information: NCT01543659.

Prospective analysis of prostate cancer (PC) circulating tumor cells (CTCs) to predict response to docetaxel (DOC) chemotherapy

100

Background: Taxane chemotherapy induces cellular microtubule stabilization (bundling) leading to mitotic arrest and apoptotic cell death. We recently reported that in castration-resistant PC, stabilizing of microtubules by taxanes inhibits ligand-induced androgen receptor (AR) nuclear translocation and activation of ARE-gene transcription (Cancer Res 2011;71:6019). This suggests the clinical activity of DOC results from inhibition of AR signaling. A pilot analysis of CTCs from 14 CRPC pts receiving taxanes revealed that predominant cytoplasmic AR and decreased total AR intensity each was associated with increased odds of response (P<0.001). We initiated a prospective trial to determine if CTC analysis could predict DOC response.

Methods: Eligibility: Patients (pts) receiving DOC q21 days for CRPC with >5 CTCs determined by CellSearch. On cycle 1 d1 + d8, CTCs are isolated by ficoll tube separation and by CellSearch enrichment. Cells are stained for expression of prostate specific membrane antigen (PSMA), CD45, AR and acetylated tubulin (AcTub) as a marker of microtubule stabilization. AR localization and total fluorescence intensity, and AcTub intensity are quantitated by confocal microscopy and correlated with response to DOC using PCWG2 criteria. 72 pts are planned to provide 80% power to detect an association.

Results: 28 of 72 pts have been prospectively enrolled to assess AR and AcTub as a biomarker of response. 20 pts have discontinued DOC. Day 1 samples were obtained from 27 pts and day 8 on 22 (5 pts on days 9 to 21). Immunofluorescence staining and evaluation of PSMA, CD45, and acetylated tubulin was performed successfully in 8 pts analyzed to date. CTCs can be identified in all pts. Cells are scored for AR and AcTub intensity (scale 1- 4) and AR nuclear vs cytoplasmic localization.

Conclusions: Our results to date indicate that PC CTCs can be readily isolated from men receiving DOC chemotherapy in a multicenter fashion and assayed for AR localization and AcTub. Preliminary results suggest that monitoring AR subcellular localization and microtubule bundling in CTCs may predict clinical responses to DOC chemotherapy, warranting further study, including our prospective trial.

In vitro and preclinical targeted alpha therapy of human prostate cancer with Bi-213 labeled J591 antibody against the prostate specific membrane antigen

Limited options for the treatment of prostate cancer have spurred the search for new therapies. One innovative approach is the use of targeted alpha therapy (TAT) to inhibit cancer growth, using an alpha particle emitting radioisotope such as (213)Bi. Because of its short range and high linear energy transfer (LET), alpha-particles may be particularly effective in the treatment of cancer, especially in inhibiting the development of metastatic tumors from micro-metastases. Prostate-specific membrane antigen (PSMA) is expressed in prostate cancer cells and the neovasculature of a wide variety of malignant neoplasms including lung, colon, breast and others, but not in normal vascular endothelium. The expression is further increased in higher-grade cancers, metastatic disease and hormone-refractory prostate cancer (PCA). J591 is one of several monoclonal antibodies (mabs) to the extracellular domain of PSMA. Chelation of J591 mab with (213)Bi forms the alpha-radioimmunoconjugate (AIC). The objective of this preclinical study was to design an injectable AIC to treat human prostate tumors growing subcutaneously in mice. The anti-proliferative effects of AIC against prostate cancer were tested in vitro using the MTS assay and in vivo with the nude mice model. Apoptosis was documented using terminal deoxynucleotidyl transferase [TdT]-mediated deoxyuridinetriphosphate [dUTP] nick end-labeling (TUNEL) assay, while proliferative index was assessed using the Ki-67 marker. We show that a very high density of PSMA is expressed in an androgen-dependent human PCA cell line (LNCaP-LN3) and in tumor xenografts from nude mice. We also demonstrate that the AIC extensively inhibits the growth of LN3 cells in vitro in a concentration-dependent fashion, causing the cells to undergo apoptosis. Our in vivo studies showed that a local AIC injection of 50 microCi at 2 days post-cell inoculation gave complete inhibition of tumor growth, whereas results for a non-specific AIC were similar to those for untreated mice. Further, after 1 and 3 weeks post-tumor appearance, a single (100 microCi/100 microl) intra-lesional injection of AIC can inhibit the growth of LN3 tumor xenografts (volume<100 mm(3)) in nude mice. Tumors treated with AIC decreased in volume from a mean 46+/-14 mm(3) in the first week or 71+/-15 mm(3) in the third week to non-palpable, while in control mice treated with a non-specific AIC using the same dose, tumor volume increased from 42 to 590 mm(3). There were no observed side effects of the treatment. Because of its in vitro cytotoxicity and these anti-proliferative properties in vivo, the (213)Bi-J591 conjugate has considerable potential as a new therapeutic agent for the treatment of prostate cancer.

Downregulation of Bcl-2 sensitises interferon-resistant renal cancer cells to Fas

Interferon α (IFNα) is used to treat patients with advanced renal cell carcinoma (RCC) despite limited clinical benefit. IFNα can induce Fas receptor-mediated apoptosis by direct activation of pro-caspase-8 followed by activation of caspase-3. Alternative, indirect activation of caspase-3 via mitochondrial release of cytochrome c can occur and may explain the rescue from Fas-activated cell death by the antiapoptotic members of the Bcl-2 family. In this study, we examined G3139, a novel antisense compound targeting Bcl-2, in combination with IFNα. Human RCC lines (SK-RC-44 and SK-RC-07) were treated with IFNα, G3139 or a combination of the two. Fas-mediated cytotoxicity was induced by anti-Fas mAb, CH11. An analysis of Bcl-2, Fas and the cleavage of PARP was performed. IFNα induced Fas and Bcl-2 in SK-RC-44 and SK-RC-07. IFNα sensitised SK-RC-44 to anti-Fas and induced PARP cleavage confirming that IFNα has a cytotoxic effect on RCC lines by induction of the Fas antigen. Cytotoxicity was not evident in SK-RC-07 cells treated with IFNα. G3139 induced a specific downregulation of Bcl-2 in SK-RC-07 cells, which were then sensitised to anti-Fas after treatment with IFNα. Taken together, these results suggest that Fas-dependent pathways as well as alternative pathways, which can be inhibited by Bcl-2, exist in renal cell carcinoma. G3139 in combination with IFNα is a potential therapy in patients with metastatic renal cell carcinoma.

Tumor therapy with targeted atomic nanogenerators

A single, high linear energy transfer alpha particle can kill a target cell. We have developed methods to target molecular-sized generators of alpha-emitting isotope cascades to the inside of cancer cells using actinium-225 coupled to internalizing monoclonal antibodies. In vitro, these constructs specifically killed leukemia, lymphoma, breast, ovarian, neuroblastoma, and prostate cancer cells at becquerel (picocurie) levels. Injection of single doses of the constructs at kilobecquerel (nanocurie) levels into mice bearing solid prostate carcinoma or disseminated human lymphoma induced tumor regression and prolonged survival, without toxicity, in a substantial fraction of animals. Nanogenerators targeting a wide variety of cancers may be possible.

The T cell death knell: immune-mediated tumor death in renal cell carcinoma

The antitumor effect of T cells is executed either through CD95 or Perforin (PFN)/Granzyme B (GrB) pathways. Induction of apoptosis by either mode requires activation of caspase family members. However, recent studies have suggested that cell death can proceed in the absence of caspase induction and apoptotic events. We investigated the contribution of CD95 and PFN/GrB-mediated cytotoxicity to apoptotic and necrotic mechanisms of cell death in human renal cell carcinoma. Although freshly isolated and cultured tumors expressed CD95 on their surface, they were resistant to CD95-mediated apoptosis. CD95 resistance coincided with decreased levels of FADD protein and diminished caspase-3-like activity. In contrast, we demonstrated that tumor cell death mediated by PFN/GrB can be achieved in the absence of functional caspase activity and is accompanied by a dramatic accumulation of nonapoptotic necrotic cells.

Society of Urologic Oncology Biotechnology Forum: new approaches and targets for advanced prostate cancer

PURPOSE

We provide an overview of advances in molecular based therapeutic strategies for prostate cancer and summarize the studies presented at the Society of Urologic Oncology Biotechnology Forum in 2000.

MATERIALS AND METHODS

Three promising new treatment strategies are presented, and a critique of the advantages and limitations of each is offered by a leading expert in the field.

RESULTS

Treatment results and the current state of dendritic cell based immunotherapy, monoclonal antibody therapy and anti-apoptotic treatment approaches are presented.

CONCLUSIONS

Currently patients with advanced prostate carcinoma have expanded therapeutic options available in the form of molecular based phases II and III clinical trials.

Response of LNCaP spheroids after treatment with an alpha-particle emitter (213Bi)-labeled anti-prostate-specific membrane antigen antibody (J591)

A theoretical drawback to alpha-particle therapy with 213Bi is the short range of the particle track coupled with the short half-life of the radionuclide, thereby potentially limiting effective cytotoxicity to rapidly accessible, disseminated individual tumor cells (e.g., as in leukemia). In this work, a prostate carcinoma spheroid model was used to evaluate the feasibility of targeting micrometastatic clusters of tumor cells using 213Bi-labeled anti-prostate-specific membrane antigen (PSMA) antibody, J591. In prostate cancer, vascular dissemination of tumor cells or tumor cell clusters to the marrow constitutes an important step in the progression of this disease to widespread skeletal involvement, an incurable state. Such prevascularized clusters are ideal targets for radiolabeled antibodies because the barriers to antibody penetration that are associated with the capillary basal lamina have not yet formed. Beta- and gamma-emitting radionuclides such as 131I, which are widely used in radioimmunotherapy, are not expected to be effective when targeting single cells or small cell clusters. This is because the range of the emissions is one to two orders of magnitude greater than the target size, and the energy deposited per traversal is insufficient to produce any significant radiobiological effect. Spheroids of the prostate cancer cell line, LNCaP-LN3, were used as a model of prevascularized micrometastases; their response to an anti-PSMA antibody, J591, radiolabeled with the alpha-particle emitter 213Bi (T(1/2), 45.6 min.) has been measured. The time course of spheroid volume reductions was found to be sensitive to the initial spheroid volume. J591 labeled with 0.9 MBq/ml 213Bi resulted in a 3-log reduction in spheroid volume on day 33, relative to control, for spheroids with an initial diameter of 130 microm; 1.8 MBq/ml were required to achieve a similar response for spheroids with an initial diameter of 180 microm. Equivalent spheroid responses were observed after 12 Gy of acute external beam photon irradiation. Monte Carlo-based microdosimetric analyses of the 213Bi decay distribution in individual spheroids of 130-microm diameter yielded an average alpha-particle dose of 3.7 Gy to the spheroids, resulting in a relative biological effectiveness factor of 3.2 over photon irradiation. The activity concentrations used in the experiments were clinically relevant, and this work supports the possibility of using 213Bi-labeled antibodies not only for disseminated single tumor cells, as found in patients with leukemia, but also for micrometastatic tumor deposits up to 180 microm in diameter (1200 cells).

Overview of evolving strategies incorporating prostate-specific membrane antigen as target for therapy

Prostate-specific membrane antigen (PSMA) is a potential target in prostate cancer patients because it is very highly expressed and because it has been reported to be upregulated by androgen deprivation. This overview addresses the expression of the PSMA gene in terms of the promoter and enhancer and how that may play a role in gene therapy. We also review PSMA as a target for antibodies for imaging and treatment and the development of a novel hybrid T-cell receptor that combines the specificity of anti-PSMA antibodies with that of T-cell receptor activation when introduced into primary lymphocytes by retroviral-mediated gene transfer. We also discuss our recent findings on the expression of a PSMA-like gene and how that understanding allows specific targeting of PSMA.

Na,K-ATPase beta-subunit is required for epithelial polarization, suppression of invasion, and cell motility

The cell adhesion molecule E-cadherin has been implicated in maintaining the polarized phenotype of epithelial cells and suppression of invasiveness and motility of carcinoma cells. Na,K-ATPase, consisting of an alpha- and beta-subunit, maintains the sodium gradient across the plasma membrane. A functional relationship between E-cadherin and Na,K-ATPase has not previously been described. We present evidence that the Na,K-ATPase plays a crucial role in E-cadherin-mediated development of epithelial polarity, and suppression of invasiveness and motility of carcinoma cells. Moloney sarcoma virus-transformed Madin-Darby canine kidney cells (MSV-MDCK) have highly reduced levels of E-cadherin and beta(1)-subunit of Na,K-ATPase. Forced expression of E-cadherin in MSV-MDCK cells did not reestablish epithelial polarity or inhibit the invasiveness and motility of these cells. In contrast, expression of E-cadherin and Na,K-ATPase beta(1)-subunit induced epithelial polarization, including the formation of tight junctions and desmosomes, abolished invasiveness, and reduced cell motility in MSV-MDCK cells. Our results suggest that E-cadherin-mediated cell-cell adhesion requires the Na,K-ATPase beta-subunit's function to induce epithelial polarization and suppress invasiveness and motility of carcinoma cells. Involvement of the beta(1)-subunit of Na,K-ATPase in the polarized phenotype of epithelial cells reveals a novel link between the structural organization and vectorial ion transport function of epithelial cells.

An alpha-particle emitting antibody ([213Bi]J591) for radioimmunotherapy of prostate cancer

A novel alpha-particle emitting monoclonal antibody construct targeting the external domain of prostate-specific membrane antigen (PSMA) was prepared and evaluated in vitro and in vivo. The chelating agent, N-[2-amino-3-(p-isothiocyanatophen-yl)propyl]-trans-cyclohexane-1, 2-diamine-N,N',N',N'',N''-pentaacetic acid, was appended to J591 monoclonal antibody to stably bind the 213Bi radiometal ion. Bismuth-213 is a short-lived (t 1/2 = 46 min) radionuclide that emits high energy alpha-particles with an effective range of 0.07-0.10 mm that are ideally suited to treating single-celled neoplasms and micrometastatic carcinomas. The LNCaP prostate cancer cell line had an estimated 180,000 molecules of PSMA per cell; J591 bound to PSMA with a 3-nM affinity. After binding, the radiolabeled construct-antigen complex was rapidly internalized into the cell, carrying the radiometal inside. [213Bi]J591 was specifically cytotoxic to LNCaP. The LD50 value of [213Bi]J591 was 220 nCi/ml at a specific activity of 6.4 Ci/g. The potency and specificity of [213Bi]J591 directed against LNCaP spheroids, an in vitro model for micrometastatic cancer, also was investigated. [213Bi]J591 effectively stopped growth of LNCaP spheroids relative to an equivalent dose of the irrelevant control [213Bi]HuM195 or unlabeled J591. Cytotoxicity experiments in vivo were carried out in an athymic nude mouse model with an i.m. xenograft of LNCaP cells. [213Bi]J591 was able to significantly improve (P < 0.0031) median tumor-free survival (54 days) in these experiments relative to treatment with irrelevant control [213Bi]HuM195 (33 days), or no treatment (31 days). Prostate-specific antigen (PSA) was also specifically reduced in treated animals. At day 51, mean PSA values were 104 ng/ml +/- 54 ng/ml (n = 4, untreated animals), 66 ng/ml +/- 16 ng/ml (n = 6, animals treated with [213Bi]HuM195), and 28 ng/ml +/- 22 ng/ml (n = 6, animals treated with [213Bi]J591). The reduction of PSA levels in mice treated with [213Bi]J591 relative to mice treated with [213Bi]HuM195 and untreated control animals was significant with P < 0.007 and P < 0.0136, respectively. In conclusion, a novel [213Bi]-radiolabeled J591 has been constructed that selectively delivers alpha-particles to prostate cancer cells for potent and specific killing in vitro and in vivo.

In vitro characterization of radiolabeled monoclonal antibodies specific for the extracellular domain of prostate-specific membrane antigen

Prostate-specific membrane antigen (PSMA) is a well-characterized cell surface antigen expressed by virtually all prostate cancers (PCas). PSMA has been successfully targeted in vivo with (111)In-labeled 7E11 monoclonal antibody (mAb; ProstaScint; Cytogen, Princeton, NJ), which binds to an intracellular epitope of PSMA. This work reports the in vitro characterization of three recently developed mAbs that bind the extracellular domain of PSMA (PSMAext). Murine mAbs J415, J533, J591, and 7E11 were radiolabeled with 131I and evaluated in competitive and saturation binding studies with substrates derived from LNCaP cells. J415 and J591 were conjugated to 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid labeled with (111)In. The uptake and cellular processing of these antibodies were evaluated in viable LNCaP cells. All four mAbs could be labeled with 131I up to a specific activity of 350 MBq/mg with no or little apparent loss of immunoreactivity. Competition assays revealed that J415 and J591 compete for binding to PSMAext antigen. J533 bound to a region close to the J591 binding epitope, but J533 did not interfere with J415 binding to PSMA. mAb 7E11 did not inhibit the binding of J415, J533, or J591 (or vice versa), consistent with earlier work that these latter mAbs bind PSMAext whereas 7E11 binds the intracellular domain of PSMA. Saturation binding studies demonstrated that J415 and J591 bound with a similar affinity (Kds 1.76 and 1.83 nM), whereas J533 had a lower affinity (Kd, 18 nM). In parallel studies, all four mAbs bound to a similar number of PSMA sites expressed by permeabilized cells (1,000,000-1,300,000 sites/cell). In parallel studies performed with viable LNCaP cells, J415, J533, and J591 bound to a similar number of PSMA sites (i.e., 600,000-800,000 sites/cell), whereas 7E11 bound only to a subpopulation of the available PSMA sites (95,000 sites/cell). This apparent binding of 7E11 to viable cells can be accounted for by a 5-7% subpopulation of permeabilized cells produced when the cells were trypsinized and suspended. Up to five DOTA chelates could be bound to either J415 or J591 without compromising immunoreactivity. A comparison of the cellular uptake and metabolic processing of the 131I- and (111)In-labeled antibodies showed a rapid elimination of 131I from the cell and a high retention of (111)In. All four mAbs recognized and bound to similar numbers of PSMAs expressed by ruptured LNCaP cells (i.e., the exposed intracellular and extracellular domains of PSMA). By comparison to J415 and J591, J533 had a lower binding affinity. Both J415 and J591 recognized and bound to the same high number of PSMAs expressed by intact LNCaP. By contrast, 7E11 bound to fewer sites expressed by intact LNCaP cells (i.e., the exposed extracellular domain of PSMA). Both J415 and J591 are promising mAbs for the targeting of viable PSMA-expressing tissue with diagnostic and therapeutic metallic radionuclides.

Prostate-specific membrane antigen (PSMA)-specific monoclonal antibodies in the treatment of prostate and other cancers

Prostate-specific membrane antigen (PSMA) is a cell surface glycoprotein that is expressed by prostate epithelial cells. PSMA-specific monoclonal antibodies have been utilized to characterize the biologic function and in vivo biodistribution of PSMA. PSMA is an attractive target protein for monoclonal antibody directed imaging or therapeutics for prostate cancer since its expression is relatively restricted to prostate epithelial cells and is over-expressed in prostate cancer, including in advanced stages. Currently, clinical usage of PSMA specific monoclonal antibodies has been limited to diagnostic immunohistochemistry and imaging of patients with prostate cancer. Novel applications for these antibodies will be discussed.

Antigens recognized by autologous antibody in patients with renal-cell carcinoma

The screening of cDNA expression libraries derived from human tumors with autologous antibody (SEREX) is a powerful method for defining the structure of tumor antigens recognized by the humoral immune system. Sixty-five distinct antigens (NY-REN-1 to NY-REN-65) reactive with autologous IgG were identified by SEREX analysis of 4 renal cancer patients and were characterized in terms of cDNA sequence, mRNA expression pattern, and reactivity with allogeneic sera. REN-9, -10, -19, and -26 have a known association with human cancer. REN-9 (LUCA-15) and REN-10 (gene 21) map to the small cell lung cancer tumor suppressor gene locus on chromosome 3p21.3. REN-19 is equivalent to LKB1/STK11, a gene that is defective in Peutz-Jeghers syndrome and cancer. REN-26 is encoded by the bcr gene involved in the [t(9:22)] bcr/abl translocation. Genes encoding 3 of the antigens in the series showed differential mRNA expression; REN-3 displays a pattern of tissue-specific isoforms, and REN-21 and REN-43 are expressed at a high level in testis in comparison to 15 other normal tissues. The other 62 antigens were broadly expressed in normal tissues. With regard to immunogenicity, 20 of the 65 antigens reacted only with autologous sera. Thirty-three antigens reacted with sera from normal donors, indicating that their immunogenicity is not restricted to cancer. The remaining 12 antigens reacted with sera from 5-25% of the cancer patients but not with sera from normal donors. Seventy percent of the renal cancer patients had antibodies directed against one or more of these 12 antigens. Our results demonstrate the potential of the SEREX approach for the analysis of the humoral immune response against human cancer.

Monoclonal antibodies: will they become an integral part of the evaluation and treatment of prostate cancer--focus on prostate-specific membrane antigen?

Over the past two decades, monoclonal antibody technology has had an increasing impact on clinical diagnostic and therapeutic options, and this is true in the realm of managing prostate cancer. Several targets such as prostate-specific antigen and prostatic acid phosphatase as well as, more recently, angiogenic antigens such as vascular endothelial growth factor have been examined for therapy. Prostate-specific membrane antigen, a type II integral membrane glycoprotein initially characterized by the monoclonal antibody 7E11, has shown promise. Recent evidence suggests that prostate-specific membrane antigen is also expressed in tumor-associated neovasculature of a wide variety of malignant neoplasms. With its expression in prostate secretory-acinar epithelium and the prostate and in the neovasculature associated with tumors, prostate-specific membrane antigen represents an excellent antigenic target for monoclonal antibody diagnostic and therapeutic options. As research continues, the role of monoclonal antibody imaging and therapy will become increasingly important in the management of prostate cancer.