Phase I, First-in-Human Study of FOR46 (FG-3246), an Immune-Modulating Antibody-Drug Conjugate Targeting CD46, in Patients With Metastatic Castration-Resistant Prostate Cancer

PURPOSE FOR46, a fully human antibody conjugated to monomethyl auristatin E, targets a tumor-selective epitope of CD46, which is overexpressed in metastatic castration-resistant prostate cancer (mCRPC). FOR46 demonstrates potent nonclinical activity in enzalutamide-resistant CRPC models. PATIENTS AND METHODS This was a phase I, first-in-human, dose escalation/expansion study in patients with progressive mCRPC after treatment with ≥one androgen signaling inhibitors (ClinicalTrials.gov identifier: NCT03575819 ). The starting dose of FOR46 was 0.1 mg/kg given intravenously every 3 weeks. The primary objective was to determine the maximally tolerated dose (MTD). Whole-blood mass cytometry (cytometry by time of flight) was used to characterize peripheral immune response and CD46 expression in CRPC tissue that underwent central pathology review. RESULTS Fifty-six patients were enrolled. Dose-limiting toxicities included neutropenia (n = 4), febrile neutropenia (n = 1), and fatigue (n = 1). The MTD was 2.7 mg/kg using adjusted body weight. The most common grade ≥3 adverse events across all dose levels were neutropenia (59%), leukopenia (27%), lymphopenia (7%), anemia (7%), and fatigue (5%). One grade 3 febrile neutropenia event was observed. There were no treatment-related deaths. In the efficacy evaluable subset (patients with adenocarcinoma treated with a starting dose ≥1.2 mg/kg, n = 40), the median radiographic progression-free survival was 8.7 months (range, 0.1-33.9). Fourteen of 39 evaluable patients (36%) achieved a PSA50 response. The confirmed objective response rate was 20% (5 of 25 RECIST-evaluable patients). The median duration of response was 7.5 months. Responders had a significantly higher on-treatment frequency of circulating effector CD8 + T cells. CONCLUSION FOR46 demonstrated encouraging preliminary clinical activity with a manageable safety profile. Targeting CD46 elicited an immune priming effect that was associated with clinical outcomes.