A Phase I Dose Escalation and Cohort Expansion Study of CB307, a Trispecific Humabody against PSMA, CD137, and Albumin in Patients with PSMA-Positive Solid Tumors

Abstract Purpose: CB307 is a trispecific variable heavy-chain antibody fragment against prostate-specific membrane antigen (PSMA), CD137, and human serum albumin. It is designed to mitigate hepatotoxicity by activating T cells only in PSMA-positive tumors and to increase drug half-life through albumin binding. This phase I study investigated the safety and tolerability of CB307 as monotherapy or in combination with pembrolizumab. Patients and Methods: Patients who were heavily pretreated with PSMA-positive solid tumors were enrolled in the dose-escalation phase of CB307 monotherapy. The additional safety and efficacy of CB307 were assessed in the CB307 monotherapy expansion cohort and in combination with pembrolizumab. Results: CB307 was administered to 75 patients. CB307 was given once every 7 days as monotherapy (N = 50) or in combination with pembrolizumab (N = 25). Two dose-limiting toxicities (grade 3 transient transaminitis) were observed. A total of three grade 3 transaminitis events (one in the monotherapy cohort and two in the combination cohort) were observed, and none involved bilirubin elevation. Durable RECIST responses were observed in two patients with metastatic castration-resistant prostate cancer enrolled in the 800 mg CB307 monotherapy and in one patient in the combination cohort (overall response rates: 11.1% and 7.1%, respectively). A disease control rate (DCR) of 50% was observed in patients enrolled in the 800 mg CB307 monotherapy cohort and 42.9% in the combination cohort. In a post hoc analysis, the response was numerically better in patients who had not received chemotherapy in the 6 months prior to starting CB307 (ORR = 20%, DCR = 60% vs. ORR = 0%, DCR = 37.5%). CB307 induces cytotoxic cell expansion in tumors and PD-L1 expression. Conclusions: CB307 was well tolerated as a monotherapy and in combination with pembrolizumab, and tumor responses were observed in patients with metastatic castration-resistant prostate cancer. See related commentary by Nguyen and Luke, p. 3356