Measuring oncogenic signaling pathways in cancer with PET: an emerging paradigm from studies in castration-resistant prostate cancer

The Relationship Between PSA and Total Testosterone Levels in Men With Prostate Cancer

BACKGROUND

Prostate-specific antigen (PSA) secretion is a testosterone (T) dependent process. Published data suggest that a low T level is an independent predictor of higher-grade prostate cancer (PC).

AIM

To evaluate the relationship between T and PSA in patients with PC.

METHODS

All men diagnosed with PC with a recorded pre-treatment total T level measurement were included in this analysis. We analyzed demographic, clinical, and pathological data. Patients were stratified according to pretreatment PSA levels: <2 ng/mL, 2-4 ng/mL, >4 ng/mL. Low T was defined as total T < 10.4 nmol/L (300 ng/dL), very low T < 6.9 nmol/L (200 ng/dL).

OUTCOMES

T levels by PSA groups according to the PC pathology.

RESULTS

In this retrospective study, mean patient age was 61 years among 646 men. The distribution by PSA group was: 8% (<2), 17% (2-4), and 76% (>4). The mean T level across the entire cohort was 13 nmol/L (374 ng/dL). Overall, 30% had a T level < 10.4 nmol/L (300 ng/dL). The mean total T level by PSA group was: <2 ng/mL, 7 nmol/L (206 ng/dL); 2-4 ng/mL, 13 nmol/L (362 ng/dL); >4 ng/mL, 14 nmol/L (393 ng/dL), P < .001. PSA <4 ng/mL was a significant predictor of low T in men with PC GS ≥8. PSA <2 ng/mL was a significant predictor of very low T independent of the PC pathology.

CLINICAL IMPLICATIONS

These findings suggest that clinicians should consider measuring T levels when a patient diagnosed with PC GS ≥8 and PSA level <4 ng/mL, and for each patient with PSA level <2 ng/mL independent of the PC pathology.

STRENGTHS & LIMITATIONS

Our study has several strengths including (i) inclusion of a large population of men, (ii) use of a database which is audited and reviewed for accuracy annually, and (iii) use of an accurate T assay (LCMS). Nonetheless, there are limitations: (i) the subjects of the study are from a single institution, and (ii) we did not measure free T levels.

CONCLUSION

In men with PC with GS ≥8, PSA level <4 ng/mL predicts low T. PSA <2 ng/mL predicts very low T independent of the PC pathology. Flores JM, Bernie HL, Miranda E, et al. The Relationship Between PSA and Total Testosterone Levels in Men With Prostate Cancer. J Sex Med 2022;19:471-478.

Preparation of Radiolabeled Antibodies for Nuclear Medicine Applications in Immuno-Oncology

The mixed patient responses to antibodies targeting immune checkpoint proteins (e.g., CTLA-4, PD-1, PD-L1) have generated tremendous interest in discovering biomarkers that predict which patients will best respond to these treatments. To complement molecular biomarkers obtained from biopsies, the nuclear medicine community has begun developing radiopharmaceuticals that may provide a more holistic assessment of the biological character of all disease sites in patients. On the leading edge of clinical translation are a spectrum of radiolabeled antibodies targeting immune checkpoint proteins or T cell-specific antigens. The adoption of these reagents requires development of efficient and versatile methods for antibody bioconjugation and radiochemistry. We report herein protocols for the preparation of an anti-PD-L1 IgG1 (termed C4) labeled with zirconium-89. The approach is time and cost economical, high yielding, and adaptable to numerous antibody clones and platforms of interest to the immune-oncology community. Included also are representative methods for characterizing the pharmacology of the antibody post bioconjugation, and conducting an in vivo assessment of radiotracer biodistribution in tumor bearing mouse models.

Imaging Hepatocellular Carcinoma With 68Ga-Citrate PET: First Clinical Experience

While cross-sectional imaging with computed tomography (CT) and magnetic resonance imaging is the primary method for diagnosing hepatocellular carcinoma (HCC), they provide little biological insight into this molecularly heterogeneous disease. Nuclear imaging tools that can detect molecular subsets of tumors could greatly improve diagnosis and management of HCC. To this end, we conducted a patient study to determine whether HCC can be resolved using ⁶⁸Ga-citrate positron emission tomography (PET). One patient with recurrent HCC was injected with 300 MBq of ⁶⁸Ga-citrate and imaged with PET/CT 249 minutes post injection. Four (28%) of 14 hepatic lesions were avid for ⁶⁸Ga-citrate. One extrahepatic lesion was not PET avid. The average maximum standardized uptake value (SUV_max) for the lesions was 7.2 (range: 6.2-8.4), while the SUV_max of the normal liver parenchyma was 4.7 and blood pool was 5.7. The avid lesions were not significantly larger than the quiescent lesions, and a prior contrast CT showed uniform enhancement among the lesions, suggesting that tumor signals are due to specific binding of the radiotracer to the transferrin receptor, rather than enhanced vascularity in the tumor microenvironment. Further studies are required in a larger patient cohort to verify the molecular basis of radiotracer uptake and the clinical utility of this tool.

A Feasibility Study Showing [68Ga]Citrate PET Detects Prostate Cancer

PURPOSE

The management of advanced or recurrent prostate cancer is limited in part by the lack of effective imaging agents. Metabolic changes in prostate cancer have previously been exploited for imaging, culminating in the recent US FDA approval of [¹¹C]choline for the detection of subclinical recurrent disease after definitive local therapy. Despite this milestone, production of [¹¹C]choline requires an on-site cyclotron, limiting the scope of medical centers at which this scan can be offered. In this pilot study, we tested whether prostate cancer could be imaged with positron emission tomography (PET) using [⁶⁸Ga]citrate, a radiotracer that targets iron metabolism but is produced without a cyclotron.

PROCEDURES

Eight patients with castrate-resistant prostate cancer were enrolled in this single-center feasibility study. All patients had evidence of metastatic disease by standard of care imaging [X-ray computed tomography (CT), bone scan, or magnetic resonance imaging (MRI)] prior to PET with [⁶⁸Ga]citrate. Patients were intravenously injected with increasing doses of [⁶⁸Ga]citrate (136.9 to a maximum of 259 MBq). Uptake time was steadily increased from 1 h to approximately 3.5 h for the final 4 patients, and all patients were imaged with a PET/MRI. Qualitative and semi-quantitative (maximum standardized uptake value (SUV_max)) assessment of the metastatic lesions was performed and compared to the standard of care imaging.

RESULTS

At 1- and 2-h imaging times post injection, there were no detectable lesions with [⁶⁸Ga]citrate PET. At 3- to 4-h uptake time, there were a total of 71 [⁶⁸Ga]citrate-positive lesions (67 osseous, 1 liver, and 3 lymph node). Of these, 65 lesions were visible on the standard of care imaging (CT and/or bone scan). One PET-avid osseous vertebral body metastasis was not apparent on either CT or bone scan. Twenty-five lesions were not PET-avid but seen on CT and bone scan (17 bone, 6 lymph node, 1 pleural, and 1 liver). The average of the maximum SUVs for bone or soft tissue metastases for patients treated at higher doses and uptake time was statistically higher than the corresponding parameter in normal liver, muscle, and bone. Visually obvious blood pool activity was observed even 3-4 h post injection, suggesting that further optimization of the [⁶⁸Ga]citrate imaging protocol is required to maximize signal-to-background ratios.

CONCLUSIONS

Our preliminary results support that PET with [⁶⁸Ga]citrate may be a novel tool for imaging prostate cancer. Future studies are needed to determine the optimal imaging protocol, the clinical significance of [⁶⁸Ga]citrate uptake, and its role in therapeutic decisions.

Real-Time Transferrin-Based PET Detects MYC-Positive Prostate Cancer

Noninvasive biomarkers that detect the activity of important oncogenic drivers could significantly improve cancer diagnosis and management of treatment. The goal of this study was to determine whether 68Ga-citrate (which avidly binds to circulating transferrin) can detect MYC-positive prostate cancer tumors, as the transferrin receptor is a direct MYC target gene. PET imaging paired with 68Ga-citrate and molecular analysis of preclinical models, human cell-free DNA (cfDNA), and clinical biopsies were conducted to determine whether 68Ga-citrate can detect MYC-positive prostate cancer. Importantly, 68Ga-citrate detected human prostate cancer models in a MYC-dependent fashion. In patients with castration-resistant prostate cancer, analysis of cfDNA revealed that all patients with 68Ga-citrate avid tumors had a gain of at least one MYC copy number. Moreover, biopsy of two PET avid metastases showed molecular or histologic features characteristic of MYC hyperactivity. These data demonstrate that 68Ga-citrate targets prostate cancer tumors with MYC hyperactivity. A larger prospective study is ongoing to demonstrate the specificity of 68Ga-citrate for tumors with hyperactive MYC.Implications: Noninvasive measurement of MYC activity with quantitative imaging modalities could substantially increase our understanding of the role of MYC signaling in clinical settings for which invasive techniques are challenging to implement or do not characterize the biology of all tumors in a patient. Moreover, measuring MYC activity noninvasively opens the opportunity to study changes in MYC signaling in patients under targeted therapeutic conditions thought to indirectly inhibit MYC. Mol Cancer Res; 15(9); 1221-9. ©2017 AACR.

Noninvasive Measurement of mTORC1 Signaling with 89Zr-Transferrin

Purpose: mTOR regulates many normal physiological processes and when hyperactive can drive numerous cancers and human diseases. However, it is very challenging to detect and quantify mTOR signaling noninvasively in clinically relevant animal models of disease or man. We hypothesized that a nuclear imaging tool measuring intracellular mTOR activity could address this unmet need.Experimental Design: Although the biochemical activity of mTOR is not directly amenable to nuclear imaging probe development, we show that the transferrin receptor can be used to indirectly measure intracellular changes in mTOR activity.Results: After verifying that the uptake of radiolabeled transferrin (the soluble ligand of the transferrin receptor) is stimulated by active mTORC1 in vitro, we showed that ⁸⁹Zr-labeled transferrin (Tf) can measure mTORC1 signaling dynamics in normal and cancerous mouse tissues with PET. Finally, we show that ⁸⁹Zr-Tf can detect the upregulation of mTORC1 by tumor cells to escape the antitumor effects of a standard-of-care antiandrogen, which is to our knowledge the first example of applying PET to interrogate the biology of treatment resistant cancer.Conclusions: In summary, we have developed the first quantitative assay to provide a comprehensive measurement of mTOR signaling dynamics in vivo, in specific normal tissues, and during tumor development in genetically engineered animal models using a nuclear imaging tool that is readily translatable to man. Clin Cancer Res; 23(12); 3045-52. ©2016 AACR.

68Ga-PSMA-11 PET Imaging of Response to Androgen Receptor Inhibition: First Human Experience

The purpose of this work was to evaluate the effect of androgen receptor (AR) inhibition on prostate-specific membrane antigen (PSMA) uptake imaged using ⁶⁸Ga-PSMA-11 PET in a mouse xenograft model and in a patient with castration-sensitive prostate cancer.

METHODS

We imaged 3 groups of 4 mice bearing LNCaP-AR xenografts before and 7 d after treatment with ARN-509, orchiectomy, or control vehicle. Additionally, we imaged one patient with castration-sensitive prostate cancer before and 4 wk after treatment with androgen deprivation therapy (ADT). Uptake on pre- and posttreatment imaging was measured and compared.

RESULTS

PSMA uptake increased 1.5- to 2.0-fold in the xenograft mouse model after treatment with both orchiectomy and ARN-509 but not with vehicle. Patient imaging demonstrated a 7-fold increase in PSMA uptake after the initiation of ADT. Thirteen of 22 lesions in the imaged patient were visualized on PSMA PET only after treatment with ADT.

CONCLUSION

Inhibition of the AR can increase PSMA expression in prostate cancer metastases and increase the number of lesions visualized using PSMA PET. The effect seen in cell and animal models can be recapitulated in humans. A better understanding of the temporal changes in PSMA expression is needed to leverage this effect for both improved diagnosis and improved therapy.

Influence of Androgen Deprivation Therapy on the Uptake of PSMA-Targeted Agents: Emerging Opportunities and Challenges

Prostate-specific membrane antigen (PSMA) is an attractive target for both diagnosis and therapy because of its high expression in the vast majority of prostate cancers. Development of small molecules for targeting PSMA is important for molecular imaging and radionuclide therapy of prostate cancer. Recent evidence implies that androgen-deprivation therapy increase PSMA-ligand uptake in some cases. The reported upregulations in PSMA-ligand uptake after exposure to second-generation antiandrogens such as enzalutamide and abiraterone might disturb PSMA-targeted imaging for staging and response monitoring of patients undergoing treatment with antiandrogen-based drugs. On the other hand, second-generation antiandrogens are emerging as potential endoradio-/chemosensitizers. Therefore, the enhancement of the therapeutic efficiency of PSMA-targeted theranostic methods can be listed as a new capability of antiandrogens. In this manuscript, we will present what is currently known about the mechanism of increasing PSMA uptake following exposure to antiandrogens. In addition, we will discuss whether these above-mentioned antiandrogens could play the role of endoradio-/chemosensitizers in combination with the well-established PSMA-targeted methods for pre-targeting of prostate cancer.

Synthesis and Characterization of (89)Zr-Labeled Ultrasmall Nanoparticles

The ultrasmall nanoparticle AGuIX is a versatile platform that tolerates a range of chemical diversity for theranostic applications. Our previous work showed that AGuIX clears rapidly from normal tissues, while durably accumulating within the tumor microenvironment. On this basis, AGuIX was used to detect tumor tissue with Gd(3+) enhanced MRI and can sensitize tumors to radiation therapy. As we begin the translation of AGuIX, we appreciated that coupling AGuIX to a long-lived radioisotope would help to more completely measure the magnitude and duration of its retention within the tumor microenvironment. Therefore, we developed (89)Zr-DFO-AGuIX. AGuIX was coupled to DFO and then to (89)Zr in ∼99% radiochemical yield. Stability studies showed that (89)Zr-DFO-AGuIX did not dissociate after 72 h. In animals bearing U87MG xenografts, it was detectable at levels above background for 72 h. Lastly, (89)Zr-DFO-AGuIX did not accumulate in inflammatory abscesses in vivo, highlighting its specificity for well vascularized tumors.

Androgen receptor targeted therapies in castration-resistant prostate cancer: Bench to clinic

The androgen receptor is a transcription factor and validated therapeutic target for prostate cancer. Androgen deprivation therapy remains the gold standard treatment, but it is not curative, and eventually the disease will return as lethal castration‐resistant prostate cancer. There have been improvements in the therapeutic landscape with new agents approved, such as abiraterone acetate, enzalutamide, sipuleucel‐T, cabazitaxel and Ra‐223, in the past 5 years. New insight into the mechanisms of resistance to treatments in advanced disease is being and has been elucidated. All current androgen receptor‐targeting therapies inhibit the growth of prostate cancer by blocking the ligand‐binding domain, where androgen binds to activate the receptor. Persuasive evidence supports the concept that constitutively active androgen receptor splice variants lacking the ligand‐binding domain are one of the resistant mechanisms underlying advanced disease. Transcriptional activity of the androgen receptor requires a functional AF‐1 region in its N‐terminal domain. Preclinical evidence proved that this domain is a druggable target to forecast a potential paradigm shift in the management of advanced prostate cancer. This review presents an overview of androgen receptor‐related mechanisms of resistance as well as novel therapeutic agents to overcome resistance that is linked to the expression of androgen receptor splice variants in castration‐resistant prostate cancer.

Applying ⁸⁹Zr-Transferrin To Study the Pharmacology of Inhibitors to BET Bromodomain Containing Proteins

Chromatin modifying proteins are attractive drug targets in oncology, given the fundamental reliance of cancer on altered transcriptional activity. Multiple transcription factors can be impacted downstream of primary target inhibition, thus making it challenging to understand the driving mechanism of action of pharmacologic inhibition of chromatin modifying proteins. This in turn makes it difficult to identify biomarkers predictive of response and pharmacodynamic tools to optimize drug dosing. In this report, we show that ⁸⁹Zr-transferrin, an imaging tool we developed to measure MYC activity in cancer, can be used to identify cancer models that respond to broad spectrum inhibitors of transcription primarily due to MYC inhibition. As a proof of concept, we studied inhibitors of BET bromodomain containing proteins, as they can impart antitumor effects in a MYC dependent or independent fashion. In vitro, we show that transferrin receptor biology is inhibited in multiple MYC positive models of prostate cancer and double hit lymphoma when MYC biology is impacted. Moreover, we show that bromodomain inhibition in one lymphoma model results in transferrin receptor expression changes large enough to be quantified with ⁸⁹Zr-transferrin and positron emission tomography (PET) in vivo. Collectively, these data further underscore the diagnostic utility of the relationship between MYC and transferrin in oncology, and provide the rationale to incorporate transferrin-based PET into early clinical trials with bromodomain inhibitors for the treatment of solid tumors.

Molecular Imaging of Post-Src Inhibition Tumor Signatures for Guiding Dasatinib Combination Therapy

Noninvasive, real-time, quantitative measurement of key biomarkers associated with cancer therapeutic interventions could provide a better understanding of cancer biology. We investigated in this study whether incorporating multiple molecular imaging approaches could be used to guide dasatinib anti-Src therapy and aid in the rational design of a combination therapy regimen.

METHODS

Bioluminescence imaging, (18)F-FDG PET, integrin αvβ3-targeted SPECT/CT, and vascular endothelial growth factor-targeted near-infrared fluorescence imaging were performed before and after dasatinib treatment in a tumor mouse model.

RESULTS

There was no significant difference in the bioluminescence imaging signal or (18)F-FDG tumor uptake in dasatinib-treated tumors compared with the control tumors. However, the uptake of (99m)T-3PRGD2 (integrin αvβ3-specific) and DyLight755-ranibizumab (vascular endothelial growth factor-specific) in the dasatinib-treated tumors was significantly lower than that in the control tumors. In vitro studies confirmed the antiangiogenic effects of dasatinib but indicated a lack of cytotoxicity. Dasatinib plus cytotoxic docetaxel elicited marked synergistic tumor growth inhibition in vivo.

CONCLUSION

Visualization of post-Src inhibition tumor signatures through multiple imaging approaches facilitates sensitive and quantitative measurement of cancer biomarkers in vivo, thus aiding in the rational design of dasatinib combination therapy.

Imaging active urokinase plasminogen activator in prostate cancer

The increased proteolytic activity of membrane-bound and secreted proteases on the surface of cancer cells and in the transformed stroma is a common characteristic of aggressive metastatic prostate cancer. We describe here the development of an active site-specific probe for detecting a secreted peritumoral protease expressed by cancer cells and the surrounding tumor microenvironment. Using a human fragment antigen-binding phage display library, we identified a human antibody termed U33 that selectively inhibited the active form of the protease urokinase plasminogen activator (uPA, PLAU). In the full-length immunoglobulin form, U33 IgG labeled with near-infrared fluorophores or radionuclides allowed us to noninvasively detect active uPA in prostate cancer xenograft models using optical and single-photon emission computed tomography imaging modalities. U33 IgG labeled with (111)In had a remarkable tumor uptake of 43.2% injected dose per gram (%ID/g) 72 hours after tail vein injection of the radiolabeled probe in subcutaneous xenografts. In addition, U33 was able to image active uPA in small soft-tissue and osseous metastatic lesions using a cardiac dissemination prostate cancer model that recapitulated metastatic human cancer. The favorable imaging properties were the direct result of U33 IgG internalization through an uPA receptor-mediated mechanism in which U33 mimicked the function of the endogenous inhibitor of uPA to gain entry into the cancer cell. Overall, our imaging probe targets a prostate cancer-associated protease, through a unique mechanism, allowing for the noninvasive preclinical imaging of prostate cancer lesions.

Annotating STEAP1 regulation in prostate cancer with 89Zr immuno-PET

Antibodies and antibody-drug conjugates targeting the cell surface protein 6 transmembrane epithelial antigen of prostate 1 (STEAP1) are in early clinical development for the treatment of castration-resistant prostate cancer (PCa). In general, antigen expression directly affects the bioactivity of therapeutic antibodies, and the biologic regulation of STEAP1 is unusually complicated in PCa. Paradoxically, STEAP1 can be induced or repressed by the androgen receptor (AR) in different human PCa models, while also expressed in AR-null PCa. Consequently, there is an urgent need to translate diagnostic strategies to establish which regulatory mechanism predominates in patients to situate the appropriate therapy within standard of care therapies inhibiting AR.

METHODS

To this end, we prepared and evaluated (89)Zr-labeled MSTP2109A ((89)Zr-2109A), a radiotracer for PET derived from a fully humanized monoclonal antibody to STEAP1 in preclinical PCa models.

RESULTS

(89)Zr-2109A specifically localized to the STEAP1-positive human PCa models CWR22Pc, 22Rv1, and PC3. Moreover, (89)Zr-2109A sensitively measured treatment-induced changes (∼66% decline) in STEAP1 expression in CWR22PC in vitro and in vivo, a model we showed to express STEAP1 in an AR-dependent manner.

CONCLUSION

These findings highlight the ability of immuno-PET with (89)Zr-2109A to detect acute changes in STEAP1 expression and argue for an expansion of ongoing efforts to image PCa patients with (89)Zr-2109A to maximize the clinical benefit associated with antibodies or antibody-drug conjugates to STEAP1.

Molecular imaging reveals trastuzumab-induced epidermal growth factor receptor downregulation in vivo

Previous in vitro studies demonstrated that treating tumors expressing both epidermal growth factor receptor (EGFR) and human epidermal growth factor receptor 2 with trastuzumab resulted in increased EGFR homodimerization and subsequent rapid downregulation of EGFR. We investigated whether molecular imaging using near-infrared fluorescence (NIRF) imaging and PET probes could sensitively detect trastuzumab-induced EGFR downregulation in vivo.

METHODS

The F(ab')2 antibody fragment PaniF(ab')2 was generated by digesting the anti-EGFR monoclonal antibody panitumumab. PaniF(ab')2 was labeled with either a NIRF dye or (68)Ga, and optical imaging and small-animal PET imaging of Dye-PaniF(ab')2 and (68)Ga-PaniF(ab')2, respectively, were performed in HT-29 tumor-bearing nude mice treated with trastuzumab or untreated control.

RESULTS

Longitudinal NIRF imaging studies revealed significantly reduced tumor uptake of Dye-PaniF(ab')2 on days 5 and 7 in trastuzumab-treated HT-29 tumors, compared with control. Western blotting confirmed the downregulation of EGFR after treatment with trastuzumab. Small-animal PET on day 5 after trastuzumab treatment also demonstrated decreased (68)Ga-PaniF(ab')2 uptake in trastuzumab-treated HT-29 tumors. The tumor uptake value of (68)Ga-PaniF(ab')2 obtained from PET imaging had an excellent linear correlation with the uptake value measured using biodistribution.

CONCLUSION

The downregulation of EGFR induced by trastuzumab treatment could be detected noninvasively using optical and PET imaging. This molecular imaging strategy could provide a dynamic readout of changes in the tumor signaling and may facilitate the noninvasive monitoring of the early tumor response to drug treatment.