Purpose: MLN2704 is an immunoconjugate designed to deliver the maytansinoid antimicrotubule agent drug maytansinoid-1 directly to prostate-specific membrane antigen (PSMA) –expressing cells via the PSMA-targeted monoclonal antibody MLN591. This novel immunoconjugate has shown cytotoxic anti–prostate cancer activity. This study investigated the safety profile, pharmacokinetics, immunogenicity, and preliminary antitumor activity of MLN2704.

Patients and Methods: Patients with progressive, metastatic, castration-resistant prostate cancer received MLN2704 intravenously over 2.5 hours. Dose-limiting toxicity (DLT), maximum-tolerated dose (MTD), pharmacokinetics, immunogenicity, and antitumor activity were assessed.

Results: Twenty-three patients received MLN2704 at doses of 18 to 343 mg/m². Eighteen of these patients received three doses at 4-week intervals. Pharmacokinetics of conjugate levels were dose proportional. There was no correlation between clearance and body-surface area. MLN2704 was nonimmunogenic. Study drug–related grade 3 toxicities occurred in three (13%) of 23 patients, including uncomplicated febrile neutropenia (the only DLT) in one patient, reversible elevations in hepatic transaminases, leukopenia, and lymphopenia. No grade 4 toxicities were observed. The most frequent grade 1 or 2 toxicities included fatigue, nausea, and diarrhea. Neuropathy occurred in eight (35%) of 23 patients, including five of six patients treated at 343 mg/m². Two (22%) of the nine patients treated at 264 or 343 mg/m² had sustained a more than 50% decrease in prostate-specific antigen versus baseline, accompanied by measurable tumor regression in the patient treated at 264 mg/m².

Conclusion: Therapeutic doses of MLN2704 can be administered safely on a repetitive basis. An MTD was not defined. MLN2704 is being administered at more frequent intervals in ongoing trials to determine an optimal dosing schedule.