A preclinical study of minibody 177Lu-DOTA-IAB2 targeting PSMA for Radiopharmaceutical Therapy of Prostate Cancer

Introduction: Prostate cancer is the second cause of cancer related death in men in the US, with an estimate of 250,000 new cases and 34,000 deaths in 2021. During the past three decades novel therapeutic options and improvement in preventative diagnosis, reduced mortality by a factor of 2. Nevertheless, the 5-years survival for metastatic disease is still low, at 7%.

Prostate Specific Membrane Antigen (PSMA) is a cell-surface biomarker expressed on prostate cancer cells, and the neovasculature of solid tumors, and presents limited expression in normal tissues. PSMA expression in prostate cancer is associated with increased tumor aggressiveness and is higher in high-grade tumors, metastatic lesions, and androgen-independent disease.

Recently, the successful completion of the Phase 3 VISION trial with ¹⁷⁷Lu-PSMA-617 in patients with metastatic castration-resistant prostate cancer, proved that radiopharmaceutical therapy with b-emitters can be successful at prolonging survival in this advanced malignancy.

Minibodies have optimal pharmacokinetics for the delivery of toxic payloads, allowing rapid tumor uptake in less than 24h, combined with faster clearance from circulation, thus offering the potential to reduce off-target toxicity. These constructs present deeper tumor penetration relative to full-length antibodies and higher target specificity, and retention at the tumor site, with respect to small molecules.

Clinical imaging in prostate cancer patients with the PSMA PET probe ⁸⁹Zr-desferoxamine-IAB2 proved that this minibody has exquisite specificity for PSMA+ lesions, with minimal background in non-cancerous tissue. To test the hypothesis that this minibody is also amenable for systemic cancer radiopharmaceutical therapy, we tested ¹⁷⁷Lu labeled IAB2 (¹⁷⁷Lu-DTPA-IAB2) in a preclinical model of prostate cancer.

Methods: We collected time-course biodistribution in LNCaP xenograft mice measuring probe uptake in tumor and major organs at 1, 3, 6, 24 and 48 hours. Forty BALB/c nude mice were implanted with LNCaP cells subcutaneously in the right shoulder, and tumor grown to a volume of 150-300mm³. To this end, DTPA-IAB2 was radiolabeled with ¹⁷⁷Lu and dosed at 60mCi/animal. As comparator, a separate cohort was treated with ¹⁷⁷Lu-PSMA-617.

Results: The IAB2 minibody displayed a maximum tumor uptake of 35%ID/gr at 24h, with negligible decline at 48h. The small molecule ¹⁷⁷Lu-PSMA-617 reached maximum tumor uptake of 30%ID/gr at 1h post injection, but rapidly declined to less than 10%ID/gr at the 24h timepoint.

The major clearance organ for both molecules was the kidney, with ¹⁷⁷Lu-DTPA-IAB2 showing an uptake of 25%ID/gr at 24h. Conversely, ¹⁷⁷Lu-PSMA-617 demonstrated rapid kidney clearance, with an initial high uptake at 1h, which decreased to 20% and 10%ID/gr at the 3h and 6h timepoints. At 24h, less than 1%ID/gr activity was detectable in the kidney.

Poised by the high tumor uptake and retention for IAB2, we completed a radiopharmaceutical therapy efficacy study with ¹⁷⁷Lu-DOTA-IAB2. Twenty-seven Balb/c nude mice were implanted with 22Rv1 prostate carcinoma xenografts and divided in 3 cohorts. A fourth group was used as control and treated with vehicle. Nine mice in each treatment cohort were treated with 7.4MBq, 11.1MBq and 18.5MBq, through lateral tail-vain injection. Periodic measurement of tumor volume revealed a rapid therapeutic response at a single dose, with all treated animals, including the low dose cohort, showing signs of tumor volume regression as early as 8 days post injection.

Conclusions: Overall, this preliminary study suggests that the IAB2 minibody is highly effective in a therapeutic model of prostate cancer with b-emitter. The extended retention at the tumor, spanning several days post injection, is an advantage with respect to current radiopharmaceutical therapeutic options and warrants clinical translation.