Background: Prostate-specific membrane antigen (PSMA) is a well-characterized target that is overexpressed selectively on prostate cancer cells. PSMA antibodydrug conjugate (ADC) is a fully human IgG1 monoclonal antibody conjugated to the microtubule disrupting agent monomethyl auristatin E (MMAE), which is designed to specifically bind PSMA-positive cells, internalize, and then release its cytotoxic payload into the cells. PSMA ADC has demonstrated potent and selective antitumor activity in preclinical models of advanced prostate cancer. A Phase 1 study was conducted to assess the safety, pharmacokinetics, and preliminary antitumor effects of PSMA ADC in subjects with treatment-refractory prostate cancer.
Methods:In this first-in-man dose-escalation study, PSMA ADC was administered by intravenous infusion every three weeks to subjects with progressive metastatic Present address: William C. Olson, Regeneron Pharmaceuticals, Inc., 777 Old Saw Mill River Rd, Tarrytown, NY 10591. Present address: Robert J. Israel, Salix Pharmaceuticals, 400 Somerset Corporate Blvd, Bridgewater, NJ 08807. † Deceased PETRYLAK ET AL. | 3 605castration-resistant prostate cancer (mCRPC) who were previously treated with docetaxel chemotherapy. The primary endpoint was to establish a maximum tolerated dose (MTD). The study also examined the pharmacokinetics of the study drug, total antibody, and free MMAE. Antitumor effects were assessed by measuring changes in serum prostate-specific antigen (PSA), circulating tumor cells (CTCs), and radiologic imaging.Results: Fifty-two subjects were administered doses ranging from 0.4 to 2.8 mg/kg. Subjects had a median of two prior chemotherapy regimens and prior treatment with abiraterone and/or enzalutamide. Neutropenia and peripheral neuropathy were identified as important first-cycle and late dose-limiting toxicities, respectively. The dose of 2.5 mg/kg was determined to be the MTD. Pharmacokinetics were approximately dose-proportional with minimal drug accumulation. Reductions in PSA and CTCs in subjects treated with doses of ≥1.8 mg/kg were durable and often concurrent.
Conclusions:In an extensively pretreated mCRPC population, PSMA ADC demonstrated acceptable toxicity. Antitumor activity was observed over dose ranges up to and including 2.5 mg/kg. The observed anti-tumor activity supported further evaluation of this novel agent for the treatment of advanced metastatic prostate cancer.
K E Y W O R D Santibody-drug conjugate, Prostate cancer, prostate-specific membrane antigen