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

Abstract

TPS188 Background: Prostate-specific membrane antigen (PSMA)-based targeted radionuclide therapy (TRT) is a promising treatment. PSMA-targeting via large antibodies vs small molecules has different kinetics, biodistribution, and resulting clinical toxicities. Using beta-TRT, 177Lu-J591 has more heme toxicity and 177Lu-PSMA-617 more non-heme toxicity (xerostomia and nausea) [Niaz AUA 2020]. Alpha-emitters are more potent than beta radionuclides, and alpha-PSMA-TRT has efficacy even after beta-PSMA-TRT. In a first-in-human phase I dose-escalation study of 225Ac-J591, patients with mCRPC were treated with a single dose of 225Ac-J591 on seven dose levels, up to 93.3 KBq/kg [Tagawa ASCO 2020]. No maximal tolerated dose (MTD) was achieved. One patient treated at 80 KBq/kg developed dose-limiting toxicity (DLT) of grade 4 anemia and thrombocytopenia, but 0 of 6 at 93.3 KBq/Kg had grade > 3 heme toxicity or grade > 2 non-heme toxicity. Preliminary results indicate 64% had any PSA decline and 41% had > 50% PSA decline (PSA50) across all doses, despite lack of selection for PSMA expression and the majority having been previously treated with 177Lu-PSMA. Methods: Entry criteria include progressive mCRPC by PCWG3 criteria, ECOG PS 0-2, intact organ function (including normal neutrophil and platelet counts), and prior receipt of AR pathway inhibitor and chemotherapy (or refuse/ineligible for chemotherapy). There is no limit to prior lines of therapy except alpha-PSMA-TRT. Phase I includes 2 separate parallel dose-escalation cohorts. In the fractionated-dose cohort, men will receive a single cycle of 225Ac-J591 administered on D1 and D15. In the multiple-dose cohort, 225Ac-J591 will be given every 6 weeks for up to 4 cycles. The phase I component is a 3+3 dose-escalation study design, with the goal of identifying MTD. Each phase II component will treat up to 27 men with PSMA+ PET scans in a Simon 2-stage design with 90% power to exclude the null hypothesis (35% or fewer patients with PSA50). Eligible men with negative PSMA PET scans will be offered treatment with informed consent in an exploratory subgroup, but will not be counted towards phase II efficacy. Secondary outcomes include radiographic response by PCWG modified RECIST 1.1 criteria and PSMA PET, biochemical and radiographic progression-free survival, circulating tumor cell counts, and overall survival. Patient reported outcomes, genomic, and immune analyses are exploratory. Enrollment began in August 2020 (NCT04506567). Clinical trial information: NCT04506567.