Alpha-Emitting Radionuclides: Current Status and Future Perspectives

The use of radionuclides for targeted endoradiotherapy is a rapidly growing field in oncology. In particular, the focus on the biological effects of different radiation qualities is an important factor in understanding and implementing new therapies. Together with the combined approach of imaging and therapy, therapeutic nuclear medicine has recently made great progress. A particular area of research is the use of alpha-emitting radionuclides, which have unique physical properties associated with outstanding advantages, e.g., for single tumor cell targeting. Here, recent results and open questions regarding the production of alpha-emitting isotopes as well as their chemical combination with carrier molecules and clinical experience from compassionate use reports and clinical trials are discussed.

Engineering CAR-T cells for radiohapten capture in imaging and radioimmunotherapy applications

Rationale: The in vivo dynamics of CAR-T cells remain incompletely understood. Novel methods are urgently needed to longitudinally monitor transferred cells non-invasively for biodistribution, functionality, proliferation, and persistence in vivo and for improving their cytotoxic potency in case of treatment failure. Methods: Here we engineered CD19 CAR-T cells ("Thor"-cells) to express a membrane-bound scFv, huC825, that binds DOTA-haptens with picomolar affinity suitable for labeling with imaging or therapeutic radionuclides. We assess its versatile utility for serial tracking studies with PET and delivery of α-radionuclides to enhance anti-tumor killing efficacy in sub-optimal adoptive cell transfer in vivo using Thor-cells in lymphoma models. Results: We show that this reporter gene/probe platform enables repeated, sensitive, and specific assessment of the infused Thor-cells in the whole-body using PET/CT imaging with exceptionally high contrast. The uptake on PET correlates with the Thor-cells on a cellular and functional level. Furthermore, we report the ability of Thor-cells to accumulate cytotoxic alpha-emitting radionuclides preferentially at tumor sites, thus increasing therapeutic potency. Conclusion: Thor-cells are a new theranostic agent that may provide crucial information for better and safer clinical protocols of adoptive T cell therapies, as well as accelerated development strategies.

Efficacy of HER2-Targeted Intraperitoneal 225Ac α-Pretargeted Radioimmunotherapy for Small-Volume Ovarian Peritoneal Carcinomatosis

Epithelial ovarian cancer (EOC) is often asymptomatic and presents clinically in an advanced stage as widespread peritoneal microscopic disease that is generally considered to be surgically incurable. Targeted α-therapy with the α-particle-emitting radionuclide 225Ac (half-life, 9.92 d) is a high-linear-energy-transfer treatment approach effective for small-volume disease and even single cells. Here, we report the use of human epidermal growth factor receptor 2 (HER2) 225Ac-pretargeted radioimmunotherapy (PRIT) to treat a mouse model of human EOC SKOV3 xenografts growing as peritoneal carcinomatosis (PC). Methods: On day 0, 105 SKOV3 cells transduced with a luciferase reporter gene were implanted intraperitoneally in nude mice, and tumor engraftment was verified by bioluminescent imaging (BLI). On day 15, treatment was started using 1 or 2 cycles of 3-step anti-HER2 225Ac-PRIT (37 kBq/cycle as 225Ac-Proteus DOTA), separated by a 1-wk interval. Efficacy and toxicity were monitored for up to 154 d. Results: Untreated PC-tumor-bearing nude mice showed a median survival of 112 d. We used 2 independent measures of response to evaluate the efficacy of 225Ac-PRIT. First, a greater proportion of the treated mice (9/10 1-cycle and 8/10 2-cycle; total, 17/20; 85%) survived long-term compared with controls (9/27, 33%), and significantly prolonged survival was documented (log-rank [Mantel-Cox] P = 0.0042). Second, using BLI, a significant difference in the integrated BLI signal area to 98 d was noted between controls and treated groups (P = 0.0354). Of a total of 8 mice from the 2-cycle treatment group (74 kBq total) that were evaluated by necropsy, kidney radiotoxicity was mild and did not manifest itself clinically (normal serum blood urea nitrogen and creatinine). Dosimetry estimates (relative biological effectiveness-weighted dose, where relative biological effectiveness = 5) per 37 kBq administered for tumors and kidneys were 56.9 and 16.1 Gy, respectively. One-cycle and 2-cycle treatments were equally effective. With immunohistology, mild tubular changes attributable to α-toxicity were observed in both therapeutic groups. Conclusion: Treatment of EOC PC-tumor-bearing mice with anti-HER2 225Ac-PRIT resulted in histologic cures and prolonged survival with minimal toxicity. Targeted α-therapy using the anti-HER2 225Ac-PRIT system is a potential treatment for otherwise incurable EOC.

Quantitative In Vivo Imaging of the Androgen Receptor Axis Reveals Degree of Prostate Cancer Radiotherapy Response

Noninvasive biomarkers for androgen receptor (AR) pathway activation are urgently needed to better monitor patient response to prostate cancer therapies. AR is a critical driver and mediator of resistance of prostate cancer but currently available noninvasive prostate cancer biomarkers to monitor AR activity are discordant with downstream AR pathway activity. External beam radiotherapy (EBRT) remains a common treatment for all stages of prostate cancer, and DNA damage induced by EBRT upregulates AR pathway activity to promote therapeutic resistance. [89Zr]11B6-PET is a novel modality targeting prostate-specific protein human kallikrein 2 (hK2), which is a surrogate biomarker for AR activity. Here, we studied whether [89Zr]11B6-PET can accurately assess EBRT-induced AR activity.Genetic and human prostate cancer mouse models received EBRT (2-50 Gy) and treatment response was monitored by [89Zr]11B6-PET/CT. Radiotracer uptake and expression of AR and AR target genes was quantified in resected tissue.EBRT increased AR pathway activity and [89Zr]11B6 uptake in LNCaP-AR and 22RV1 tumors. EBRT increased prostate-specific [89Zr]11B6 uptake in prostate cancer-bearing mice (Hi-Myc x Pb_KLK2) with no significant changes in uptake in healthy (Pb_KLK2) mice, and this correlated with hK2 protein levels.

IMPLICATIONS

hK2 expression in prostate cancer tissue is a proxy of EBRT-induced AR activity that can noninvasively be detected using [89Zr]11B6-PET; further clinical evaluation of hK2-PET for monitoring response and development of resistance to EBRT in real time is warranted.

[18F]-Labeled PARP-1 PET imaging of PSMA targeted alpha particle radiotherapy response

The growing interest and clinical translation of alpha particle (α) therapies brings with it new challenges to assess target cell engagement and to monitor therapeutic effect. Noninvasive imaging has great potential to guide α-treatment and to harness the potential of these agents in the complex environment of disseminated disease. Poly(ADP) ribose polymerase 1 (PARP-1) is among the most abundantly expressed DNA repair enzymes with key roles in multiple repair pathways-such as those induced by irradiation. Here, we used a third-generation PARP1-specific radiotracer, [18F]-PARPZ, to delineate castrate resistant prostate cancer xenografts. Following treatment with the clinically applied [225Ac]-PSMA-617, positron emission tomography was performed and correlative autoradiography and histology acquired. [18F]-PARPZ was able to distinguish treated from control (saline) xenografts by increased uptake. Kinetic analysis of tracer accumulation also suggests that the localization of the agent to sites of increased PARP-1 expression is a consequence of DNA damage response. Together, these data support expanded investigation of [18F]-PARPZ to facilitate clinical translation in the ⍺-therapy space.

Treatment of Patients with Acute Myeloid Leukemia with the Targeted Alpha-Particle Nano-Generator Actinium-225-Lintuzumab

PURPOSE

The anti-CD33 antibody lintuzumab has modest activity against acute myeloid leukemia (AML). To increase its potency, lintuzumab was conjugated to actinium-225 (225Ac), a radionuclide yielding 4 α-particles. This first-in-human, phase I trial was conducted to determine the safety, pharmacology, and biological activity of 225Ac-lintuzumab.

PATIENTS AND METHODS

Eighteen patients (median age, 64 years; range, 45-80) with relapsed or refractory AML received a single infusion of 225Ac-lintuzumab at activities of 18.5-148 kBq/kg.

RESULTS

The maximum tolerated dose was 111 kBq/kg. Dose-limiting toxicities included myelosuppression lasting > 35 days in one patient receiving 148 kBq/kg and death from sepsis in two patients treated with 111 and 148 kBq/kg. Myelosuppression was the most common toxicity. Significant extramedullary toxicities were limited to transient grade 3 liver function abnormalities. Pharmacokinetics were determined by gamma counting serial whole blood, plasma, and urine samples at energy windows for the 225Ac daughters, francium-221 and bismuth-213. Two-phase elimination kinetics were seen with mean plasma t1/2-α and t1/2-β of 1.9 and 38 hours, respectively. Peripheral blood blasts were eliminated in 10 of 16 evaluable patients (63%) but only at doses of {greater than or equal to} 37 kBq/kg. Bone marrow blasts were reduced in 10 of 15 evaluable patients (67%), including 3 patients with marrow blasts {less than or equal to} 5% and 1 patient with a morphologic leukemia-free state.

CONCLUSIONS

Therapy for AML with the targeted α-particle generator 225Ac-lintuzumab was feasible with an acceptable safety profile. Elimination of circulating blasts or reductions in marrow blasts were observed across all dose levels.

Abstract P168: Pretargeted radioimmunotherapy using 225Ac for intraperitoneal Her2-expressing epithelial ovarian carcinoma xenografts

Objectives Epithelial ovarian carcinoma (EOC) is a common and lethal gynecologic malignancy that frequently presents as advanced staged disease, such as peritoneal carcinomatosis (PC). We previously reported cures in BT-474 murine xenografts with Pretargeted Radioimmunotherapy (PRIT) using 177Lu radiohaptens targeting Her2 via a bispecific antibody (BsAb). Here-in we report the use of PRIT with alpha emitter 225Ac labelled PrDOTA to treat a PC model of SKOV3, a Her2+ cell line of EOC. Methods 6 wk old female athymic nude mice inoculated IP with 1E5 luciferase/GFP-transfected SKOV3 cells were separated into 5 groups (n=10). Treatment mice received 1 or 2 cycles of Anti-Her2-C825 BsAb + [225Ac]PrDOTA (Her2-Targeted), at 14 and 21 days after inoculation, respectively. Control groups received Anti-Her2 BsAb only, Anti-GPA33 BsAb + [225Ac]PrDOTA (Off-Targeted) or no treatment. On cycle day 1, the mice were injected IP with 0.25mg (1.19nmol) BsAb. On cycle day 2, 25µg (2.76nmol) CCA16-DOTAY clearing agent (CA) was given IV 22h from BsAb. Mice in therapy groups were injected IP with 1µCi (0.74-0.79nmol) [225Ac]PrDOTA-Bn 4h after CA. Weekly weights and BLIs with IP cavity ROIs were obtained and normalized to the respective values for each mouse at week 0 pre-treatment. End points: weight loss >20% baseline, moribund, or severe abdominal distension. At 154 days, 15 surviving treatment and 1 untreated control mice were submitted for hematology and histopathology. Results Histologic cures and prolonged survival were demonstrated in treatment mice (17/20 at 133 days) as compared to control mice (12/27 at 133 d, Logrank p<0.04). 3 mice from control groups were excluded due to BLI values <50% background in the first 3 weeks, suggesting no tumor burden. Tumors, as measured by normalized BLI values (nBLI), regressed in treatment mice when compared to control mice (2-way ANOVA p<0.01). nBLI values between treatment mice (1 and 2 cycles) and control mice (BsAb only, Off-target, no treatment) diverged at week 10 (Tukey’s test p<0.01). There was no difference in nBLI values between mice treated with 1 or 2 cycles of targeted PRIT (Tukey’s test p>0.05; all weeks). BLI of mice treated with 1 and 2 cycle of Her2 PRIT decreased 47% when compared to baseline within 1 week (T test p=0.04), suggesting treatment effects as early as 1 week. There were no differences in weights when compared to baseline (2-way ANOVA p>0.05). While the untreated mouse had high peritoneal adenocarcinoma tumor burden, there was no histologic evidence of viable neoplasia in 15/15 submitted treatment mice. Treatment mice had moderate renal tubular degenerative lesions on histology, but this did not affect renal function based on serum BUN or Cr. All hematologic parameters were within normal limits for treated mice. Conclusions 1 and 2 cycles of [225Ac]PrDOTA-PRIT against Her2 resulted in histologic cures and prolonged survival in IP SKOV3 xenografts with minimal toxicity. The anti-Her2 PrDOTA-PRIT system is a promising theranostic approach for otherwise incurable PC.

Citation Format: Sebastian K. Chung, Christopher S. Chandler, Daniela Burnes Vargas, Shin H. Seo, Michael R. McDevitt, Darren Veach, Blesida Punzalan, Xu Hong, Hong-fen Guo, Garrett M. Nash, Andrea Cercek, Nai-Kong V. Cheung, Steven M. Larson, Sarah M. Cheal. Pretargeted radioimmunotherapy using 225Ac for intraperitoneal Her2-expressing epithelial ovarian carcinoma xenografts [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P168.

Radiation Safety Considerations and Clinical Advantages of Alpha-Emitting Therapy Radionuclides

Alpha-emitting radionuclides provide an effective means of delivering large radiation doses to targeted treatment locations. Radium-223 dichloride (²²³RaCl₂) is FDA approved for treatment of metastatic castration-resistant prostate cancer (mCRPC) and Actinium-225 (²²⁵Ac-Lintuzumab) radiolabeled antibodies have been shown to be beneficial for patients with acute myeloid leukemia. In recent years, there is an increasing use of alpha emitters in theranostic agents with both small and large molecule constructs. The proper precautionary means for their use and surveying documentation of these isotopes in a clinical setting are an essential accompaniment to these treatments. Methods: Patient treatment data collected over a three-year period, as well as regulatory requirements and safety practices, are described. Commonly used radiation instrumentation was evaluated for their ability to identify potential radioactive material spills and contamination events during a clinical administration of ²²⁵Ac. These instruments were placed at 0.32 cm from a 1.0 cm ²²⁵Ac disk source for measurement purposes. Radiation background values, efficiencies, and minimal detectable activities were measured and calculated for each type of detector. Results: The median external measured patient dose rate from ²²³RaCl₂ patients (n = 611) was 2.5 µSv hr-1 on contact and 0.2 µSv hr-1 at 1 meter immediately after administration. Similarly, ²²⁵Ac-Lintuzumab (n = 19) patients had median external dose rates of 2.0 µSv hr-1 on contact and 0.3 µSv hr-1 at 1 meter. For the measurement of ²²⁵Ac samples, a liquid scintillation counter was found to have the highest overall efficiency (97%), while a zinc sulfide (ZnS) alpha probe offered the lowest minimal detectable activity at 3 counts per minute. Conclusion: In this study, we report data from 630 patients who were undergoing treatment with alpha-emitting isotopes ²²³Ra and ²²⁵Ac. While alpha emitters have ability to deliver higher internal radiation dose to the tissues exposed as compared with other unsealed radionuclides, they typically present minimal external dose rate concerns. Additionally, alpha radiation can be efficiently detected with appropriate radiation instrumentation, such as a liquid scintillation counter or ZnS probe, that should be prioritized when surveying for spills of alpha-emitters.

Engineered Cells as a Test Platform for Radiohaptens in Pretargeted Imaging and Radioimmunotherapy Applications

Pretargeted imaging and radioimmunotherapy approaches are designed to have superior targeting properties over directly targeted antibodies but impose more complex pharmacology, which hinders efforts to optimize the ligands prior to human applications. Human embryonic kidney 293T cells expressing the humanized single-chain variable fragment (scFv) C825 (huC825) with high-affinity for DOTA-haptens (293T-huC825) in a transmembrane-anchored format eliminated the requirement to use other pretargeting reagents and provided a simplified, accelerated assay of radiohapten capture while offering normalized cell surface expression of the molecular target of interest. Using binding assays, ex vivo biodistribution, and in vivo imaging, we demonstrated that radiohaptens based on benzyl-DOTA and a second generation "Proteus" DOTA-platform effectively and specifically engaged membrane-bound huC825, achieving favorable tumor-to-normal tissue uptake ratios in mice. Furthermore, [⁸⁶Y]Y-DOTA-Bn predicted absorbed dose to critical organs with reasonable accuracy for both [¹⁷⁷Lu]Lu-DOTA-Bn and [²²⁵Ac]Ac-Pr, which highlights the benefit of a dosimetry-based treatment approach.

PSA-Targeted Alpha-, Beta-, and Positron-Emitting Immunotheranostics in Murine Prostate Cancer Models and Nonhuman Primates

PURPOSE

Most patients with prostate cancer treated with androgen receptor (AR) signaling inhibitors develop therapeutic resistance due to restoration of AR functionality. Thus, there is a critical need for novel treatment approaches. Here we investigate the theranostic potential of hu5A10, a humanized mAb specifically targeting free PSA (KLK3).

EXPERIMENTAL DESIGN

LNCaP-AR (LNCaP with overexpression of wildtype AR) xenografts (NSG mice) and KLK3_Hi-Myc transgenic mice were imaged with ⁸⁹Zr- or treated with ⁹⁰Y- or ²²⁵Ac-labeled hu5A10; biodistribution and subcellular localization were analyzed by gamma counting, PET, autoradiography, and microscopy. Therapeutic efficacy of [²²⁵Ac]hu5A10 and [⁹⁰Y]hu5A10 in LNCaP-AR tumors was assessed by tumor volume measurements, time to nadir (TTN), time to progression (TTP), and survival. Pharmacokinetics of [⁸⁹Zr]hu5A10 in nonhuman primates (NHP) were determined using PET.

RESULTS

Biodistribution of radiolabeled hu5A10 constructs was comparable in different mouse models. Specific tumor uptake increased over time and correlated with PSA expression. Treatment with [⁹⁰Y]/[²²⁵Ac]hu5A10 effectively reduced tumor burden and prolonged survival (P ≤ 0.0054). Effects of [⁹⁰Y]hu5A10 were more immediate than [²²⁵Ac]hu5A10 (TTN, P < 0.0001) but less sustained (TTP, P < 0.0001). Complete responses were observed in 7 of 18 [²²⁵Ac]hu5A10 and 1 of 9 mice [⁹⁰Y]hu5A10. Pharmacokinetics of [⁸⁹Zr]hu5A10 were consistent between NHPs and comparable with those in mice. [⁸⁹Zr]hu5A10-PET visualized the NHP-prostate over the 2-week observation period.

CONCLUSIONS

We present a complete preclinical evaluation of radiolabeled hu5A10 in mouse prostate cancer models and NHPs, and establish hu5A10 as a new theranostic agent that allows highly specific and effective downstream targeting of AR in PSA-expressing tissue. Our data support the clinical translation of radiolabeled hu5A10 for treating prostate cancer.

Author Correction: Radiopharmaceutical therapy in cancer: clinical advances and challenges

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

A Self-Assembling and Disassembling (SADA) Bispecific Antibody (BsAb) Platform for Curative Two-step Pretargeted Radioimmunotherapy

PURPOSE

Many cancer treatments suffer from dose-limiting toxicities to vital organs due to poor therapeutic indices. To overcome these challenges we developed a novel multimerization platform that rapidly removes tumor-targeting proteins from the blood to substantially improve therapeutic index.

EXPERIMENTAL DESIGN

The platform was designed as a fusion of a self-assembling and disassembling (SADA) domain to a tandem single-chain bispecific antibody (BsAb, anti-ganglioside GD2 × anti-DOTA). SADA-BsAbs were assessed with multiple in vivo tumor models using two-step pretargeted radioimmunotherapy (PRIT) to evaluate tumor uptake, dosimetry, and antitumor responses.

RESULTS

SADA-BsAbs self-assembled into stable tetramers (220 kDa), but could also disassemble into dimers or monomers (55 kDa) that rapidly cleared via renal filtration and substantially reduced immunogenicity in mice. When used with rapidly clearing DOTA-caged PET isotopes, SADA-BsAbs demonstrated accurate tumor localization, dosimetry, and improved imaging contrast by PET/CT. When combined with therapeutic isotopes, two-step SADA-PRIT safely delivered massive doses of alpha-emitting (²²⁵Ac, 1.48 MBq/kg) or beta-emitting (¹⁷⁷Lu, 6,660 MBq/kg) S-2-(4-aminobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (DOTA) payloads to tumors, ablating them without any short-term or long-term toxicities to the bone marrow, kidneys, or liver.

CONCLUSIONS

The SADA-BsAb platform safely delivered large doses of radioisotopes to tumors and demonstrated no toxicities to the bone marrow, kidneys, or liver. Because of its modularity, SADA-BsAbs can be easily adapted to most tumor antigens, tumor types, or drug delivery approaches to improve therapeutic index and maximize the delivered dose.See related commentary by Capala and Kunos, p. 377.

Alpha radioimmunotherapy using 225Ac-proteus-DOTA for solid tumors - safety at curative doses

This is the initial report of an α-based pre-targeted radioimmunotherapy (PRIT) using ²²⁵Ac and its theranostic pair, ¹¹¹In. We call our novel tumor-targeting DOTA-hapten PRIT system "proteus-DOTA" or "Pr." Herein we report the first results of radiochemistry development, radiopharmacology, and stoichiometry of tumor antigen binding, including the role of specific activity, anti-tumor efficacy, and normal tissue toxicity with the Pr-PRIT approach (as α-DOTA-PRIT). A series of α-DOTA-PRIT therapy studies were performed in three solid human cancer xenograft models of colorectal cancer (GPA33), breast cancer (HER2), and neuroblastoma (GD2), including evaluation of chronic toxicity at ~20 weeks of select survivors. Methods: Preliminary biodistribution experiments in SW1222 tumor-bearing mice revealed that ²²⁵Ac could not be efficiently pretargeted with current DOTA-Bn hapten utilized for ¹⁷⁷Lu or ⁹⁰Y, leading to poor tumor uptake in vivo. Therefore, we synthesized Pr consisting of an empty DOTA-chelate for ²²⁵Ac, tethered via a short polyethylene glycol linker to a lutetium-complexed DOTA for picomolar anti-DOTA chelate single-chain variable fragment (scFv) binding. Pr was radiolabeled with ²²⁵Ac and its imaging surrogate, ¹¹¹In. In vitro studies verified anti-DOTA scFv recognition of [²²⁵Ac]Pr, and in vivo biodistribution and clearance studies were performed to evaluate hapten suitability and in vivo targeting efficiency. Results: Intravenously (i.v.) administered ²²⁵Ac- or ¹¹¹In-radiolabeled Pr in mice showed rapid renal clearance and minimal normal tissue retention. In vivo pretargeting studies show high tumor accumulation of Pr (16.71 ± 5.11 %IA/g or 13.19 ± 3.88 %IA/g at 24 h p.i. for [²²⁵Ac]Pr and [¹¹¹In]Pr, respectively) and relatively low uptake in normal tissues (all average ≤ 1.4 %IA/g at 24 h p.i.). Maximum tolerated dose (MTD) was not reached for either [²²⁵Ac]Pr alone or pretargeted [²²⁵Ac]Pr at administered activities up to 296 kBq/mouse. Single-cycle treatment consisting of α-DOTA-PRIT with either huA33-C825 bispecific anti-tumor/anti-DOTA-hapten antibody (BsAb), anti-HER2-C825 BsAb, or hu3F8-C825 BsAb for targeting GPA33, HER2, or GD2, respectively, was highly effective. In the GPA33 model, no complete responses (CRs) were observed but prolonged overall survival of treated animals was 42 d for α-DOTA-PRIT vs. 25 d for [²²⁵Ac]Pr only (P < 0.0001); for GD2, CRs (7/7, 100%) and histologic cures (4/7, 57%); and for HER2, CRs (7/19, 37%) and histologic cures (10/19, 56%) with no acute or chronic toxicity. Conclusions: [²²⁵Ac]Pr and its imaging biomarker [¹¹¹In]Pr demonstrate optimal radiopharmacologic behavior for theranostic applications of α-DOTA-PRIT. For this initial evaluation of efficacy and toxicity, single-cycle treatment regimens were performed in all three systems. Histologic toxicity was not observed, so MTD was not observed. Prolonged overall survival, CRs, and histologic cures were observed in treated animals. In comparison to RIT with anti-tumor IgG antibodies, [²²⁵Ac]Pr has a much improved safety profile. Ultimately, these data will be used to guide clinical development of toxicity and efficacy studies of [²²⁵Ac]Pr, with the goal of delivering massive lethal doses of radiation to achieve a high probability of cure without toxicity.

A Genomic Profile of Local Immunity in the Melanoma Microenvironment Following Treatment with α Particle-Emitting Ultrasmall Silica Nanoparticles

An α particle-emitting nanodrug that is a potent and specific antitumor agent and also prompts significant remodeling of local immunity in the tumor microenvironment (TME) has been developed and may impact the treatment of melanoma. Biocompatible ultrasmall fluorescent core-shell silica nanoparticles (C' dots, diameter ∼6.0 nm) have been engineered to target the melanocortin-1 receptor expressed on melanoma through α melanocyte-stimulating hormone peptides attached to the C' dot surface. Actinium-225 is also bound to the nanoparticle to deliver a densely ionizing dose of high-energy α particles to cancer. Nanodrug pharmacokinetic properties are optimal for targeted radionuclide therapy as they exhibit rapid blood clearance, tumor-specific accumulation, minimal off-target localization, and renal elimination. Potent and specific tumor control, arising from the α particles, was observed in a syngeneic animal model of melanoma. Surprisingly, the C' dot component of this drug initiates a favorable pseudopathogenic response in the TME generating distinct changes in the fractions of naive and activated CD8 T cells, Th1 and regulatory T cells, immature dendritic cells, monocytes, MΦ and M1 macrophages, and activated natural killer cells. Concomitant upregulation of the inflammatory cytokine genome and adaptive immune pathways each describes a macrophage-initiated pseudoresponse to a viral-shaped pathogen. This study suggests that therapeutic α-particle irradiation of melanoma using ultrasmall functionalized core-shell silica nanoparticles potently kills tumor cells, and at the same time initiates a distinct immune response in the TME.

Genetic signature of prostate cancer mouse models resistant to optimized hK2 targeted α-particle therapy

Hu11B6 is a monoclonal antibody that internalizes in cells expressing androgen receptor (AR)-regulated prostate-specific enzyme human kallikrein-related peptidase 2 (hK2; KLK2). In multiple rodent models, Actinium-225-labeled hu11B6-IgG1 ([225Ac]hu11B6-IgG1) has shown promising treatment efficacy. In the present study, we investigated options to enhance and optimize [225Ac]hu11B6 treatment. First, we evaluated the possibility of exploiting IgG3, the IgG subclass with superior activation of complement and ability to mediate FC-γ-receptor binding, for immunotherapeutically enhanced hK2 targeted α-radioimmunotherapy. Second, we compared the therapeutic efficacy of a single high activity vs. fractionated activity. Finally, we used RNA sequencing to analyze the genomic signatures of prostate cancer that progressed after targeted α-therapy. [225Ac]hu11B6-IgG3 was a functionally enhanced alternative to [225Ac]hu11B6-IgG1 but offered no improvement of therapeutic efficacy. Progression-free survival was slightly increased with a single high activity compared to fractionated activity. Tumor-free animals succumbing after treatment revealed no evidence of treatment-associated toxicity. In addition to up-regulation of canonical aggressive prostate cancer genes, such as MMP7, ETV1, NTS, and SCHLAP1, we also noted a significant decrease in both KLK3 (prostate-specific antigen ) and FOLH1 (prostate-specific membrane antigen) but not in AR and KLK2, demonstrating efficacy of sequential [225Ac]hu11B6 in a mouse model.

Targeted melanoma radiotherapy using ultrasmall 177Lu-labeled α-melanocyte stimulating hormone-functionalized core-shell silica nanoparticles

Lutetium-177 (¹⁷⁷Lu) radiolabeled ultrasmall (~6 nm dia.) fluorescent core-shell silica nanoparticles (Cornell prime dots or C' dots) were developed for improving efficacy of targeted radiotherapy in melanoma models. PEGylated C' dots were surface engineered to display 10-15 alpha melanocyte stimulating hormone (αMSH) cyclic peptide analogs for targeting the melanocortin-1 receptor (MC1-R) over-expressed on melanoma tumor cells. The ¹⁷⁷Lu-DOTA-αMSH-PEG-C' dot product was radiochemically stable, biologically active, and exhibited high affinity cellular binding properties and internalization. Selective tumor uptake and favorable biodistribution properties were also demonstrated, in addition to bulk renal clearance, in syngeneic B16F10 and human M21 xenografted models. Prolonged survival was observed in the treated cohorts relative to controls. Dosimetric analysis showed no excessively high absorbed dose among normal organs. Correlative histopathology of ex vivo treated tumor specimens revealed expected necrotic changes; no acute pathologic findings were noted in the liver or kidneys. Collectively, these results demonstrated that ¹⁷⁷Lu-DOTA-αMSH-PEG-C' dot targeted melanoma therapy overcame the unfavorable biological properties and dose-limiting toxicities associated with existing mono-molecular treatments. The unique and tunable surface chemistries of this targeted ultrasmall radiotherapeutic, coupled with its favorable pharmacokinetic properties, substantially improved treatment efficacy and demonstrated a clear survival benefit in melanoma models, which supports its further clinical translation.

Harnessing Androgen Receptor Pathway Activation for Targeted Alpha Particle Radioimmunotherapy of Breast Cancer

PURPOSE

The impact of androgen receptor (AR) activity in breast cancer biology is unclear. We characterized and tested a novel therapy to an AR-governed target in breast cancer.Experimental Design: We evaluated the expression of prototypical AR gene products human kallikrein 2 (hK2) and PSA in breast cancer models. We screened 13 well-characterized breast cancer cell lines for hK2 and PSA production upon in vitro hormone stimulation by testosterone [dihydrotestosterone (DHT)]. AR-positive lines were further evaluated by exposure to estrogen (17β-Estradiol) and the synthetic progestin D-Norgestrel. We then evaluated an anti-hK2-targeted radiotherapy platform (hu11B6), labeled with alpha (α)-particle emitting Actinium-225, to specifically treat AR-expressing breast cancer xenografts under hormone stimulation.

RESULTS

D-Norgestrel and DHT activated the AR pathway, while 17β-Estradiol did not. Competitive binding for AR protein showed similar affinity between DHT and D-Norgestrel, indicating direct AR-ligand interaction. In vivo production of hK2 was sufficient to achieve site-specific delivery of therapeutic radionuclide to tumor tissue at >20-fold over background muscle uptake; effecting long-term local tumor control.

CONCLUSIONS

[²²⁵Ac]hu11B6 targeted radiotherapy was potentiated by DHT and by D-Norgestrel in murine xenograft models of breast cancer. AR activity in breast cancer correlates with kallikrein-related peptidase-2 and can be activated by D-Norgestrel, a common contraceptive, and AR induction can be harnessed for hK2-targeted breast cancer α-emitter radiotherapy.

The effects of amine-modified single-walled carbon nanotubes on the mouse microbiota

BACKGROUND

Amine-modified carbon nanotubes are drug delivery platforms with great potential that have not yet been applied in human clinical trials. Although modified nanotube vectors have the ability to carry multiple effectors, targeting agents, and even wrapped RNA, reports on unmodified, insoluble carbon nanotubes have highlighted inflammation in organs, including the intestine, with disruption of its resident microbiota. Disruption of the microbiota may allow for colonization by pathogenic bacteria, such as Clostridoidies difficile, stimulate immunoinfiltrates into the lamina propria or alter the absorption of therapeutics. Most proposed nanotube drugs are soluble, modified structures that are administered parenterally, and the majority of these soluble macromolecules are renally excreted; however, some are released into the bile, gaining access to the gastrointestinal tract.

METHODS

Using environmentally isolated BALB/C mice in oral and intraperitoneal dosing models, high dose (3.80 or 4.25 mg/week), we administered amine-modified, soluble carbon nanotubes for 7 or 8 weeks. The general health and weight of the mice were monitored weekly, and upon killing, the diversity and content of their colonic, cecal, and ileal microbiota were assessed using shotgun 16S DNA sequencing.

RESULTS AND CONCLUSION

We show that while oral administration at suprapharmacological doses modestly altered the α- and β-diversity of the mouse microbiome, these changes did not result in observed changes in clinical end points. Intraperitoneally-dosed mice exhibited none of the toxicities assessed.

Targeted and Nontargeted α-Particle Therapies

α-Particle irradiation of cancerous tissue is increasingly recognized as a potent therapeutic option. We briefly review the physics, radiobiology, and dosimetry of α-particle emitters, as well as the distinguishing features that make them unique for radiopharmaceutical therapy. We also review the emerging clinical role of α-particle therapy in managing cancer and recent studies on in vitro and preclinical α-particle therapy delivered by antibodies, other small molecules, and nanometer-sized particles. In addition to their unique radiopharmaceutical characteristics, the increased availability and improved radiochemistry of α-particle radionuclides have contributed to the growing recent interest in α-particle radiotherapy. Targeted therapy strategies have presented novel possibilities for the use of α-particles in the treatment of cancer. Clinical experience has already demonstrated the safe and effective use of α-particle emitters as potent tumor-selective drugs for the treatment of leukemia and metastatic disease.

Feed-forward alpha particle radiotherapy ablates androgen receptor-addicted prostate cancer

Human kallikrein peptidase 2 (hK2) is a prostate specific enzyme whose expression is governed by the androgen receptor (AR). AR is the central oncogenic driver of prostate cancer (PCa) and is also a key regulator of DNA repair in cancer. We report an innovative therapeutic strategy that exploits the hormone-DNA repair circuit to enable molecularly-specific alpha particle irradiation of PCa. Alpha-particle irradiation of PCa is prompted by molecularly specific-targeting and internalization of the humanized monoclonal antibody hu11B6 targeting hK2 and further accelerated by inherent DNA-repair that up-regulate hK2 (KLK2) expression in vivo. hu11B6 demonstrates exquisite targeting specificity for KLK2. A single administration of actinium-225 labeled hu11B6 eradicates disease and significantly prolongs survival in animal models. DNA damage arising from alpha particle irradiation induces AR and subsequently KLK2, generating a unique feed-forward mechanism, which increases binding of hu11B6. Imaging data in nonhuman primates support the possibility of utilizing hu11B6 in man.

α-Emitters for Radiotherapy: From Basic Radiochemistry to Clinical Studies-Part 1

With a short particle range and high linear energy transfer, α-emitting radionuclides demonstrate high cell-killing efficiencies. Even with the existence of numerous radionuclides that decay by α-particle emission, only a few of these can reasonably be exploited for therapeutic purposes. Factors including radioisotope availability and physical characteristics (e.g., half-life) can limit their widespread dissemination. The first part of this review will explore the diversity, basic radiochemistry, restrictions, and hurdles of α-emitters.

α-Emitters for Radiotherapy: From Basic Radiochemistry to Clinical Studies-Part 2

The use of radioactive sources to deliver cytotoxic ionizing radiation to disease sites dates back to the early 20th century, with the discovery of radium and its physiologic effects. α-emitters are of particular interest in the field of clinical oncology for radiotherapy applications. The first part of this review explored the basic radiochemistry, high cell-killing potency, and availability of α-emitting radionuclides, together with hurdles such as radiolabeling methods and daughter redistribution. The second part of this review will give an overview of the most promising and current uses of α-emitters in preclinical and clinical studies.

Carbon nanotubes exhibit fibrillar pharmacology in primates

Nanomedicine rests at the nexus of medicine, bioengineering, and biology with great potential for improving health through innovation and development of new drugs and devices. Carbon nanotubes are an example of a fibrillar nanomaterial poised to translate into medical practice. The leading candidate material in this class is ammonium-functionalized carbon nanotubes (fCNT) that exhibits unexpected pharmacological behavior in vivo with important biotechnology applications. Here, we provide a multi-organ evaluation of the distribution, uptake and processing of fCNT in nonhuman primates using quantitative whole body positron emission tomography (PET), compartmental modeling of pharmacokinetic data, serum biomarkers and ex vivo pathology investigation. Kidney and liver are the two major organ systems that accumulate and excrete [⁸⁶Y]fCNT in nonhuman primates and accumulation is cell specific as described by compartmental modeling analyses of the quantitative PET data. A serial two-compartment model explains renal processing of tracer-labeled fCNT; hepatic data fits a parallel two-compartment model. These modeling data also reveal significant elimination of the injected activity (>99.8%) from the primate within 3 days (t_1/2 = 11.9 hours). These favorable results in nonhuman primates provide important insight to the fate of fCNT in vivo and pave the way to further engineering design considerations for sophisticated nanomedicines to aid late stage development and clinical use in man.

In vivo immuno-targeting of an extracellular epitope of membrane bound preferentially expressed antigen in melanoma (PRAME)

Preferentially Expressed Antigen in Melanoma (PRAME) is a cancer/testis antigen that is overexpressed in a broad range of malignancies, while absent in most healthy human tissues, making it an attractive diagnostic cancer biomarker and therapeutic target. Although commonly viewed as an intracellular protein, we have demonstrated that PRAME has a membrane bound form with an external epitope targetable with conventional antibodies. We generated a polyclonal antibody (Membrane associated PRAME Antibody 1, MPA1) against an extracellular peptide sequence of PRAME. Binding of MPA1 to recombinant PRAME was evaluated by Enzyme-Linked Immunosorbent Assay (ELISA). Flow cytometry and confocal immunofluorescence microscopy of MPA1 was performed on multiple tumor cell lines. Reverse Transcription Polymerase Chain Reaction (RT-PCR) for PRAME was conducted to compare protein and transcriptional expression levels. We demonstrated a robust proof-of-concept for PRAME targeting in vivo by radiolabeling MPA1 with zirconium-89 (⁸⁹Zr-DFO-MPA1) and demonstrating high specific uptake in PRAME expressing tumors. To our knowledge, this is the first time a cancer testis antigen has been targeted using conventional antibody technologies. Thus, PRAME can be exploited for multiple clinical applications, including targeted therapy, diagnostic imaging and treatment guidance in a wide-range of malignancies, with minimal off-target toxicity.

Advances in the clinical translation of nanotechnology

The use of novel materials in the nano-scale size range for applications in devices, drugs and diagnostic agents comes with a number of new opportunities, and also serious challenges to human applications. The larger size of particulate-based agents, as compared to traditional drugs, allows for the significant advantages of multivalency and multi-functionality. However, the human use of nanomaterials requires a thorough understanding of the biocompatibility of the synthetic molecules and their complex pharmacology. Possible toxicities created by the unusual properties of the nanoparticles are neither well-understood, nor predictable yet. A key to the successful use of the burgeoning field of nanomaterials as diagnostic and therapeutic agents will be to appropriately match the biophysical features of the particle to the disease system to be evaluated or treated.

Targeted fibrillar nanocarbon RNAi treatment of acute kidney injury

RNA interference has tremendous yet unrealized potential to treat a wide range of illnesses. Innovative solutions are needed to protect and selectively deliver small interfering RNA (siRNA) cargo to and within a target cell to fully exploit siRNA as a therapeutic tool in vivo. Herein, we describe ammonium-functionalized carbon nanotube (fCNT)-mediated transport of siRNA selectively and with high efficiency to renal proximal tubule cells in animal models of acute kidney injury (AKI). fCNT enhanced siRNA delivery to tubule cells compared to siRNA alone and effectively knocked down the expression of several target genes, includingTrp53,Mep1b,Ctr1, andEGFP A clinically relevant cisplatin-induced murine model of AKI was used to evaluate the therapeutic potential of fCNT-targeted siRNA to effectively halt the pathogenesis of renal injury. Prophylactic treatment with a combination of fCNT/siMep1band fCNT/siTrp53significantly improved progression-free survival compared to controls via a mechanism that required concurrent reduction of meprin-1β and p53 expression. The fCNT/siRNA was well tolerated, and no toxicological consequences were observed in murine models. Toward clinical application of this platform, fCNTs were evaluated for the first time in nonhuman primates. The rapid and kidney-specific pharmacokinetic profile of fCNT in primates was comparable to what was observed in mice and suggests that this approach is amenable for use in humans. The nanocarbon-mediated delivery of siRNA provides a therapeutic means for the prevention of AKI to safely overcome the persistent barrier of nephrotoxicity during medical intervention.

Deconvoluting hepatic processing of carbon nanotubes

Single-wall carbon nanotubes present unique opportunities for drug delivery, but have not advanced into the clinic. Differential nanotube accretion and clearance from critical organs have been observed, but the mechanism not fully elucidated. The liver has a complex cellular composition that regulates a range of metabolic functions and coincidently accumulates most particulate drugs. Here we provide the unexpected details of hepatic processing of covalently functionalized nanotubes including receptor-mediated endocytosis, cellular trafficking and biliary elimination. Ammonium-functionalized fibrillar nanocarbon is found to preferentially localize in the fenestrated sinusoidal endothelium of the liver but not resident macrophages. Stabilin receptors mediate the endocytic clearance of nanotubes. Biocompatibility is evidenced by the absence of cell death and no immune cell infiltration. Towards clinical application of this platform, nanotubes were evaluated for the first time in non-human primates. The pharmacologic profile in cynomolgus monkeys is equivalent to what was reported in mice and suggests that nanotubes should behave similarly in humans.

Application of carbon nanotubes as drug delivery carriers is stalled by uncertainties over their distribution and toxicity in vivo. Here, the authors use animal models to show that, while the bulk of nanotubes is renally cleared, a fraction can be eliminated through an alternative hepatobiliary pathway.

Remodeling the Vascular Microenvironment of Glioblastoma with α-Particles

Tumors escape antiangiogenic therapy by activation of proangiogenic signaling pathways. Bevacizumab is approved for the treatment of recurrent glioblastoma, but patients inevitably develop resistance to this angiogenic inhibitor. We previously investigated targeted α-particle therapy with ²²⁵Ac-E4G10 as an antivascular approach and showed increased survival and tumor control in a high-grade transgenic orthotopic glioblastoma model. Here, we investigated changes in tumor vascular morphology and functionality caused by ²²⁵Ac-E4G10.

METHODS

We investigated remodeling of the tumor microenvironment in transgenic Ntva glioblastoma mice using a therapeutic 7.4-kBq dose of ²²⁵Ac-E4G10. Immunofluorescence and immunohistochemical analyses imaged morphologic changes in the tumor blood-brain barrier microenvironment. Multicolor flow cytometry quantified the endothelial progenitor cell population in the bone marrow. Diffusion-weighted MR imaged functional changes in the tumor vascular network.

RESULTS

The mechanism of drug action is a combination of remodeling of the glioblastoma vascular microenvironment, relief of edema, and depletion of regulatory T and endothelial progenitor cells. The primary remodeling event is the reduction of both endothelial and perivascular cell populations. Tumor-associated edema and necrosis were lessened, resulting in increased perfusion and reduced diffusion. Pharmacologic uptake of dasatinib into tumor was enhanced after α-particle therapy.

CONCLUSION

Targeted antivascular α-particle radiation remodels the glioblastoma vascular microenvironment via a multimodal mechanism of action and provides insight into the vascular architecture of platelet-derived growth factor-driven glioblastoma.

Vascular Targeted Radioimmunotherapy for the Treatment of Glioblastoma

Glioblastoma is characterized by an aggressive and aberrant vascular network that promotes tumor progression and hinders effective treatment; the median survival is 16 mo despite standard-of-care therapies. There is a need to improve therapeutic options for this disease. We hypothesized that antibody targeting of the vascular endothelium of glioblastoma with cytotoxic short-range, high-energy α-particles would be an effective therapeutic approach.

METHODS

E4G10, an antibody directed at an epitope of monomeric vascular endothelium cadherin that is expressed in tumor neovasculature and on endothelial progenitor cells in the bone marrow, was labeled with α-particle-emitting ²²⁵Ac. Pharmacokinetic studies investigated the tissue distribution and blood clearance of the ²²⁵Ac-E4G10 radioimmunoconstruct in a transgenic Nestin-tumor virus A (Ntva) mouse model of high-grade glioblastoma. Histologic analysis was used to demonstrate local therapeutic effects in treated brain tumor sections. Radioimmunotherapy with ²²⁵Ac-E4G10 was performed in Ntva mice to assess overall survival alone and in combination with temozolomide, the standard-of-care chemotherapeutic agent.

RESULTS

²²⁵Ac-E4G10 was found to accumulate in tissues expressing the target antigen. Antivascular α-particle therapy of glioblastoma in the transgenic Ntva model resulted in significantly improved survival compared with controls and potent control of tumor growth. Adding the chemotherapeutic temozolomide to the treatment increased survival to 30 d (vs. 9 d for vehicle-treated animals). Histologic analyses showed a remodeled glioblastoma vascular microenvironment.

CONCLUSION

Targeted α-particle antivascular therapy is shown for the first time to be effective in increasing overall survival in a solid tumor in a clinically relevant transgenic glioblastoma mouse model.

Abstract LB-095: Distribution and clearance of single-walled carbon nanotubes in mouse tissues: in situ detection, imaging and analysis

Single-walled carbon nanotubes (SWCNT) are the subject of expanding research in the fields of targeted drug delivery and biosensors for disease treatment and monitoring. Functionalization of the SWCNT has proven to enhance the efficiency of distribution in the organism. The goal of this project was to establish and standardize methods of detection and characterization of the local tissue distribution of functionalized SWCNT in mouse models. We investigated the distribution and clearance of SWCNT in several tissues at various times after injection, up to one month. We also try to standardize methods of tracking SWCNT in both live and fixed tissues and image analysis protocols to quantify the distribution of the SWCNT in tissues. Another aim is to evaluate local immune response to SWCNT. We have analyzed liver, kidney, spleen, lung, brain, ovary, colon and small intestine from mice sacrificed 24 hours, 3 days, 7 days and 30 days following the injection of SWCNT. Our evaluation revealed that some organs, like kidney and small intestine, retain SWCNTs for at least a month. Slow rates of SWCNT removal from these organs do not appear to affect the overall health of the animals. We have made multiple attempts to image anesthetized live animals and detect SWCNTs in situ, however, the fluorescent signal emitted from SWCNT is too low to be reliably detected. The tissues must be fixed and signals amplified through immunodetection. Detection of immune markers, especially in immunologically active tissues such as spleen is a challenge for histological experiments, but is extremely important and such experiments is underway. Our findings of SWCNT persistence in certain organs 30 days post-injection is surprising and should be studied further. This observation opens further questions about the effect of long-term presence of SWCNT in some tissues and shows that careful investigation into the advantages and disadvantages of SWCNT retention is necessary.

Citation Format: Afsar Barlas, Ke Xu, Yevgeniy Romin, Simone Alidori, Dmitry Yarilin, Ning Fan, Mesruh Turkekul, Sho Fujisawa, David A. Scheinberg, Michael R. McDevitt, Katia Manova-Todorova. Distribution and clearance of single-walled carbon nanotubes in mouse tissues: in situ detection, imaging and analysis. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-095. doi:10.1158/1538-7445.AM2015-LB-095

Dialytic Separation of Bundled, Functionalized Carbon Nanotubes from Carbonaceous Impurities

Separating functionalized single-wall carbon nanotubes (SWCNTs) from functionalized amorphous carbon is challenging, due to their polydispersity and similar physicochemical properties. We describe a single-step, dialytic separation method that takes advantage of the ability of heavily functionalized SWCNTs to bundle in a polar environment while maintaining their solubility. Experiments on functionalized SWCNTs were compared with functionalized, C60 fullerenes (buckyballs) to probe the general applicability of the method and further characterize the bundling process. This approach may simultaneously be used to purify a functionalization reaction mixture of unreacted small molecules and of residual solvents, such as dimethylformamide.

Fibrillous carbon nanotube: an unexpected journey

The emergence of nanomedicine, a discipline at the nexus of materials engineering, chemistry, biology, and pharmacology, has generated much excitement in the field of translational medical research and provided some unexpected results. Nanomedicine seeks to introduce nanoscale technology to the practice of medicine via the design and development of nanomaterials possessing therapeutic or diagnostic functions. However, as expected, any modification of the base nanomaterial platform to decorate it with solublizing, targeting, therapeutic, or diagnostic modalities yields a material with a very different pharmacological profile than the original platform. Clearly, the goal of nanotechnology is to put into practice a novel synthetic substance in which the function of the complex is greater than the sum of its components. These new compositions must be thoroughly evaluated in vivo. Therefore, reliance on pharmacokinetic predictions based solely on the baseline profile of the original platform can confuse the field and delay progress. Carbon nanotube pharmacokinetic profiles provide an interesting example of this situation. Covalently functionalized nanotubes exhibit fibrillar pharmacology while those nanotubes that are not covalently functionalized transiently behave as fibers and then tend toward an overall colloidal profile in vivo.

Synthesis, pharmacokinetics, and biological use of lysine-modified single-walled carbon nanotubes

We aimed to create a more robust and more accessible standard for amine-modifying single-walled carbon nanotubes (SWCNTs). A 1,3-cycloaddition was developed using an azomethine ylide, generated by reacting paraformaldehyde and a side-chain-Boc (tert-Butyloxycarbonyl)-protected, lysine-derived alpha-amino acid, H-Lys(Boc)-OH, with purified SWCNT or C60. This cycloaddition and its lysine adduct provides the benefits of dense, covalent modification, ease of purification, commercial availability of reagents, and pH-dependent solubility of the product. Subsequently, SWCNTs functionalized with lysine amine handles were covalently conjugated to a radiometalated chelator, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). The (111)In-labeled construct showed rapid renal clearance in a murine model and a favorable biodistribution, permitting utility in biomedical applications. Functionalized SWCNTs strongly wrapped small interfering RNA (siRNA). In the first disclosed deployment of thermophoresis with carbon nanotubes, the lysine-modified tubes showed a desirable, weak SWCNT-albumin binding constant. Thus, lysine-modified nanotubes are a favorable candidate for medicinal work.

Efficient 1-step radiolabeling of monoclonal antibodies to high specific activity with 225Ac for α-particle radioimmunotherapy of cancer

Targeted α-particle radiation using the radioisotope (225)Ac is a promising form of therapy for various types of cancer. Historic obstacles to the use of (225)Ac have been the difficulty in finding suitable chelators to stably attach it to targeting vehicles such as peptides and monoclonal antibodies, the low specific activities of the products, and the lack of cost-effective radiolabeling procedures. We initially solved the first problem with a procedure involving 2 chemical steps that has been used as a standard in preclinical and clinical studies. However, this procedure involves the loss of 90% of the input (225)Ac. A more efficient, economical process is needed to facilitate the more widespread use of (225)Ac.

METHODS

We conjugated representative antibodies with 2 forms of DOTA as well as other chelators as controls. We developed conditions to radiolabel these constructs in 1 chemical step and characterized their stability, immunoreactivity, biodistribution, and therapeutic efficacy in healthy and tumor-bearing mice.

RESULTS

DOTA-antibody constructs were labeled to a wide range of specific activities in 1 chemical step at 37°C. Radiochemical yields were approximately 10-fold higher, and specific activities were up to 30-fold higher than with the previous approach. The products retained immunoreactivity and were stable to serum challenge in vitro and in mice. Labeling kinetics of DOTA-antibody constructs linked through a benzyl isothiocyanate linkage were more favorable than those linked through an N-hydroxysuccinimide linkage. Tissue distribution was similar but not identical between the constructs. The constructs produced specific therapeutic responses in a mouse model of acute myeloid leukemia.

CONCLUSION

We have characterized an efficient, 1-step radiolabeling method that produces stable, therapeutically active conjugates of antibodies with (225)Ac at high specific activity. We propose that this technology greatly expands the possible clinical applications of (225)Ac monoclonal antibodies.

Carbon nanotubes as vaccine scaffolds

Carbon nanotubes display characteristics that are potentially useful in their development as scaffolds for vaccine compositions. These features include stability in vivo, lack of intrinsic immunogenicity, low toxicity, and the ability to be appended with multiple copies of antigens. In addition, the particulate nature of carbon nanotubes and their unusual properties of rapid entry into antigen-presenting cells, such as dendritic cells, make them especially useful as carriers of antigens. Early attempts demonstrating carbon nanotube-based vaccines can be used in both infectious disease settings and cancer are promising.

Self-assembly of carbon nanotubes and antibodies on tumours for targeted amplified delivery

Single-walled carbon nanotubes (SWNTs) can deliver imaging agents or drugs to tumours and offer significant advantages over approaches based on antibodies or other nanomaterials. In particular, the nanotubes can carry a substantial amount of cargo (100 times more than a monoclonal antibody), but can still be rapidly eliminated from the circulation by renal filtration, like a small molecule, due to their high aspect ratio. Here we show that SWNTs can target tumours in a two-step approach in which nanotubes modified with morpholino oligonucleotide sequences bind to cancer cells that have been pretargeted with antibodies modified with oligonucleotide strands complementary to those on the nanotubes. The nanotubes can carry fluorophores or radioisotopes, and are shown to selectively bind to cancer cells in vitro and in tumour-bearing xenografted mice. The binding process is also found to lead to antigen capping and internalization of the antibody-nanotube complexes. The nanotube conjugates were labelled with both alpha-particle and gamma-ray emitting isotopes, at high specific activities. Conjugates labelled with alpha-particle-generating (225)Ac were found to clear rapidly, thus mitigating radioisotope toxicity, and were shown to be therapeutically effective in vivo.

A self-assembling short oligonucleotide duplex suitable for pretargeting

Monoclonal antibodies (mAbs) have naturally evolved as suitable, high affinity and specificity targeting molecules. However, the large size of full-length mAbs yields poor pharmacokinetic properties. A solution to this issue is the use of a multistep administration approach, in which the slower clearing mAb is administered first and allowed to reach the target site selectively, followed by administration of a rapidly clearing small molecule carrier of the cytotoxic or imaging ligand, which bears a cognate receptor for the mAb. Here, we introduce a novel pretargetable RNA based system comprised of locked nucleic acids (LNA) and 2'O-Methyloligoribonucleotides (2'OMe-RNA). The duplex shows fast hybridization, high melting temperatures, excellent affinity, and high nuclease stability in plasma. Using a prototype model system with rituximab conjugated to 2'OMe-RNA (oligo), we demonstrate that LNA-based complementary strand (c-oligo) effectively hybridizes with rituximab-oligo, which is slowly circulating in vivo, despite the high clearance rates of c-oligo.

Deploying RNA and DNA with Functionalized Carbon Nanotubes

Carbon nanotubes internalize into cells and are potential molecular platforms for siRNA and DNA delivery. A comprehensive understanding of the identity and stability of ammoniumfunctionalized carbon nanotube (f-CNT)-based nucleic acid constructs is critical to deploying them in vivo as gene delivery vehicles. This work explored the capability of f-CNT to bind single- and double-strand oligonucleotides by determining the thermodynamics and kinetics of assembly and the stoichiometric composition in aqueous solution. Surprisingly, the binding affinity of f-CNT and short oligonucleotide sequences was in the nanomolar range, kinetics of complexation were extremely rapid, and from one to five sequences were loaded per nanotube platform. Mechanistic evidence for an assembly process that involved electrostatic, hydrogen-bonding and π-stacking bonding interactions was obtained by varying nanotube functionalities, oligonucleotides, and reaction conditions. ³¹P-NMR and spectrophotometric fluorescence emission data described the conditions required to assemble and stably bind a DNA or RNA cargo for delivery in vivo and the amount of oligonucleotide that could be transported. The soluble oligonucleic acid-f-CNT supramolecular assemblies were suitable for use in vivo. Importantly, key evidence in support of an elegant mechanism by which the bound nucleic acid material can be 'off-loaded' from the f-CNT was discovered.

Actinium-225 in targeted alpha-particle therapeutic applications

Alpha particle-emitting isotopes are being investigated in radioimmunotherapeutic applications because of their unparalleled cytotoxicity when targeted to cancer and their relative lack of toxicity towards untargeted normal tissue. Actinium- 225 has been developed into potent targeting drug constructs and is in clinical use against acute myelogenous leukemia. The key properties of the alpha particles generated by 225Ac are the following: i) limited range in tissue of a few cell diameters; ii) high linear energy transfer leading to dense radiation damage along each alpha track; iii) a 10 day halflife; and iv) four net alpha particles emitted per decay. Targeting 225Ac-drug constructs have potential in the treatment of cancer.

Alpha particles induce apoptosis through the sphingomyelin pathway

The sphingomyelin pathway involves the enzymatic cleavage of sphingomyelin to produce ceramide, a second messenger that serves as a key mediator in the rapid apoptotic response to various cell stressors. Low-linear energy transfer (LET) γ radiation can initiate this pathway, independent of DNA damage, via the cell membrane. Whether short-ranged, high-LET α particles, which are of interest as potent environmental carcinogens, radiotherapies and potential components of dirty bombs, can act through this mechanism to signal apoptosis is unknown. Here we show that irradiation of Jurkat cells with α particles emitted by the ²²⁵Ac-DOTA-anti-CD3 IgG antibody construct results in dose-dependent apoptosis. This apoptosis was significantly reduced by pretreating cells with cholesterol-depleting nystatin, a reagent known to inhibit ceramide signaling by interfering with membrane raft coalescence and ceramide-rich platform generation. The effects of nystatin on α-particle-induced apoptosis were related to disruption of the ceramide pathway and not to microdosimetry alterations, because similar results were obtained after external irradiation of the cells with a broad beam of collimated α particles using a planar ²⁴¹Am source. External irradiation allowed for more precise control of the dosimetry and geometry of the irradiation, independent of antibody binding or cell internalization kinetics. Mechanistically consistent with these findings, Jurkat cells rapidly increased membrane concentrations of ceramide after external irradiation with an average of five α-particle traversals per cell. These data indicate that α particles can activate the sphingomyelin pathway to induce apoptosis.

Selective killing of tumor neovasculature paradoxically improves chemotherapy delivery to tumors

Antiangiogenic therapies are frequently used with concomitantly administered cancer chemotherapy to improve outcomes, but the mechanism for the benefit of the combination is uncertain. We describe a mechanism by which a specific, cytotoxic antivascular agent causes vascular remodeling and improved chemotherapy results. By selectively killing tumor neovasculature using short-ranged α-particles targeted to vascular endothelial (VE)-cadherin on vascular endothelial cells (by use of 225Ac-labeled E4G10 antibody) we were able both to reduce tumor growth and to increase the efficacy of chemotherapy, an effect seen only when the chemotherapy was administered several days after the vascular targeting agent, but not if the order of administration was reversed. Immunohistochemical and immunofluorescence studies showed that the vasculature of 225Ac-E4G10-treated tumors was substantially depleted; the remaining vessels appeared more mature morphologically and displayed increased pericyte density and coverage. Tumor uptake and microdistribution studies with radioactive and fluorescent small molecule drugs showed better accumulation and more homogenous distribution of the drugs within 225Ac-E4G10-treated tumors. These results show that 225Ac-E4G10 treatment leads to ablation and improvement of the tumor vascular architecture, and also show that the resulting vascular remodeling can increase tumor delivery of small molecules, thus providing a process for the improved outcomes observed after combining antivascular therapy and chemotherapy. This study directly shows evidence for what has long been a speculated mechanism for antiangiogenic therapies. Moreover, targeting the vessel for killing provides an alternative mode of improving chemotherapy delivery and efficacy, potentially avoiding some of the drawbacks of targeting a highly redundant angiogenic pathway.

Sequential cytarabine and alpha-particle immunotherapy with bismuth-213-lintuzumab (HuM195) for acute myeloid leukemia

PURPOSE

Lintuzumab (HuM195), a humanized anti-CD33 antibody, targets myeloid leukemia cells and has modest single-agent activity against acute myeloid leukemia (AML). To increase the potency of the antibody without the nonspecific cytotoxicity associated with β-emitters, the α-particle-emitting radionuclide bismuth-213 ((213)Bi) was conjugated to lintuzumab. This phase I/II trial was conducted to determine the maximum tolerated dose (MTD) and antileukemic effects of (213)Bi-lintuzumab, the first targeted α-emitter, after partially cytoreductive chemotherapy.

EXPERIMENTAL DESIGN

Thirty-one patients with newly diagnosed (n = 13) or relapsed/refractory (n = 18) AML (median age, 67 years; range, 37-80) were treated with cytarabine (200 mg/m(2)/d) for 5 days followed by (213)Bi-lintuzumab (18.5-46.25 MBq/kg).

RESULTS

The MTD of (213)Bi-lintuzumab was 37 MB/kg; myelosuppression lasting >35 days was dose limiting. Extramedullary toxicities were primarily limited to grade ≤2 events, including infusion-related reactions. Transient grade 3/4 liver function abnormalities were seen in five patients (16%). Treatment-related deaths occurred in 2 of 21 (10%) patients who received the MTD. Significant reductions in marrow blasts were seen at all dose levels. The median response duration was 6 months (range, 2-12). Biodistribution and pharmacokinetic studies suggested that saturation of available CD33 sites by (213)Bi-lintuzumab was achieved after partial cytoreduction with cytarabine.

CONCLUSIONS

Sequential administration of cytarabine and (213)Bi-lintuzumab is tolerable and can produce remissions in patients with AML.

A multivalent DNA aptamer specific for the B-cell receptor on human lymphoma and leukemia

Long-term survival still eludes most patients with leukemia and non-Hodgkin’s lymphoma. No approved therapies target the hallmark of the B cell, its mIgM, also known as the B-cell receptor (BCR). Aptamers are small oligonucleotides that can specifically bind to a wide range of target molecules and offer some advantages over antibodies as therapeutic agents. Here, we report the rational engineering of aptamer TD05 into multimeric forms reactive with the BCR that may be useful in biomedical applications. Systematic truncation of TD05 coupled with modification with locked nucleic acids (LNA) increased conformational stability and nuclease resistance. Trimeric and tetrameric versions with optimized polyethyleneglycol (PEG) linker lengths exhibited high avidity at physiological temperatures both in vitro and in vivo. Competition and protease studies showed that the multimeric, optimized aptamer bound to membrane-associated human mIgM, but not with soluble IgM in plasma, allowing the possibility of targeting leukemias and lymphomas in vivo. The B-cell specificity of the multivalent aptamer was confirmed on lymphoma cell lines and fresh clinical leukemia samples. The chemically engineered aptamers, with significantly improved kinetic and biochemical features, unique specificity and desirable pharmacological properties, may be useful in biomedical applications.

Imaging and treating tumor vasculature with targeted radiolabeled carbon nanotubes

Single wall carbon nanotube (SWCNT) constructs were covalently appended with radiometal-ion chelates (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid [DOTA] or desferrioxamine B [DFO]) and the tumor neovascular-targeting antibody E4G10. The E4G10 antibody specifically targeted the monomeric vascular endothelial-cadherin (VE-cad) epitope expressed in the tumor angiogenic vessels. The construct specific activity and blood compartment clearance kinetics were significantly improved relative to corresponding antibodyalone constructs. We performed targeted radioimmunotherapy with a SWCNT-([²²⁵Ac]DOTA) (E4G10) construct directed at the tumor vasculature in a murine xenograft model of human colon adenocarcinoma (LS174T). The specific construct reduced tumor volume and improved median survival relative to controls. We also performed positron emission tomographic (PET) radioimmunoimaging of the tumor vessels with a SWCNT-([⁸⁹Zr]DFO)(E4G10) construct in the same murine LS174T xenograft model and compared the results to appropriate controls. Dynamic and longitudinal PET imaging of LS174T tumor-bearing mice demonstrated rapid blood clearance (<1 hour) and specific tumor accumulation of the specific construct. Incorporation of the SWCNT scaffold into the construct design permitted us to amplify the specific activity to improve the signal-to-noise ratio without detrimentally impacting the immunoreactivity of the targeting antibody moiety. Furthermore, we were able to exploit the SWCNT pharmacokinetic (PK) profile to favorably alter the blood clearance and provide an advantage for rapid imaging. Near-infrared three-dimensional fluorescent-mediated tomography was used to image the LS174T tumor model, collect antibody-alone PK data, and calculate the number of copies of VE-cad epitope per cell. All of these studies were performed as a single administration of construct and were found to be safe and well tolerated by the murine model. These data have implications that support further imaging and radiotherapy studies using a SWCNT-based platform and focusing on the tumor vessels as the target.

Conscripts of the infinite armada: systemic cancer therapy using nanomaterials

The field of clinical nanomaterials is enlarging steadily, with more than a billion US dollars of funding allocated to research by US government agencies in the past decade. The first generation of anti-cancer agents using novel nanomaterials has successfully entered widespread use. Newer nanomaterials are garnering increasing interest as potential multifunctional therapeutic agents; these drugs are conferred novel properties, by virtue of their size and shape. The new features of these agents could potentially allow increased cancer selectivity, changes in pharmacokinetics, amplification of cytotoxic effects, and simultaneous imaging capabilities. After attachment to cancer target reactive-ligands, which interact with cell-surface antigens or receptors, these new constructs can deliver cytolytic and imaging payloads. The molecules also introduce new challenges for drug development. While nanoscale molecules are of a similar size to proteins, the paradigms for how cells, tissues and organs of the body react to the non-biological materials are not well understood, because most cellular and metabolic processes have evolved to deal with globular, enzyme degradable molecules. We discuss examples of different materials to illustrate interesting principles for development and future applications of these nanomaterial medicines with emphasis on the possible pharmacologic and safety hurdles for accomplishing therapeutic goals.

MIRD Pamphlet No. 22 (abridged): radiobiology and dosimetry of alpha-particle emitters for targeted radionuclide therapy

The potential of alpha-particle emitters to treat cancer has been recognized since the early 1900s. Advances in the targeted delivery of radionuclides and radionuclide conjugation chemistry, and the increased availability of alpha-emitters appropriate for clinical use, have recently led to patient trials of radiopharmaceuticals labeled with alpha-particle emitters. Although alpha-emitters have been studied for many decades, their current use in humans for targeted therapy is an important milestone. The objective of this work is to review those aspects of the field that are pertinent to targeted alpha-particle emitter therapy and to provide guidance and recommendations for human alpha-particle emitter dosimetry.

Large anti-HER2/neu liposomes for potential targeted intraperitoneal therapy of micrometastatic cancer

Effective targeting and killing of intraperitoneally disseminated micrometastases remains a challenge.

OBJECTIVE/METHODS

 In this work, we evaluated the potential of antibody-labeled PEGylated large liposomes as vehicles for direct intraperitoneal (i.p.) drug delivery with the aim to enhance the tumor-to-normal organ ratio and to improve the bioexposure of cancer cells to the delivered therapeutics while shifting the toxicities toward the spleen. These targeted liposomes are designed to combine: (1) specific targeting to and internalization by cancer cells mediated by liposome-conjugated tumor-specific antibodies, (2) slow clearance from the peritoneal cavity, and (3) shift of normal organ toxicities from the liver to the spleen due to their relatively large size.

RESULTS

 Conjugation of anti-HER2/neu antibodies to the surface of large (approximately 600 nm in diameter) PEGylated liposomes results in fast, specific binding of targeted liposomes to cancer cells in vitro, followed by considerable cellular internalization. In vivo, after i.p. administration, these liposomes exhibit fast, specific binding to i.p. cancerous tumors. Large liposomes are slowly cleared from the peritoneal cavity, and they exhibit increased uptake by the spleen relative to the liver, while targeted large liposomes demonstrate specific tumor uptake at early times. Although tissue and tumor uptake are greater for cationic liposomes, the tumor-to-liver and spleen-to-liver ratios are similar for both membrane compositions, suggesting a primary role for the liposome's size, compared to the liposome's surface charge.

CONCLUSIONS

 The findings of this study suggest that large targeted liposomes administered i.p. could be a potent drug-delivery strategy for locoregional therapy of i.p. micrometastatic tumors.