The primary objectives of this portion of the study were to assess safety and tolerability of PF-06863135 administered subcutaneously, to determine the maximum tolerated dose, and to select the recommended Phase 2 dose. In the study, no dose-limiting toxicities were observed across any of the subcutaneous dose levels evaluated (80 to 1,000 μg/kg weekly) during dose escalation. Cytokine release syndrome (CRS) was reported in 73.3% of patients and was limited exclusively to grade 1 (56.7%) or grade 2 (16.7%). Grade 3 or higher adverse events (AEs) occurring in more than 10% of patients included lymphopenia (53.3%), neutropenia (26.7%), thrombocytopenia (16.7%) and anemia (16.7%).

The overall response rate (ORR) was 80% among the 20 patients treated in cohorts across the efficacious dose range of 215 to 1,000 μg/kg weekly. Among these 20 patients, six achieved stringent complete response or complete response, three achieved very good partial response, and six achieved partial response. Three responding patients had received at least one prior BCMA-targeted therapy. At the highest dose level of 1,000 μg/kg, the ORR was 83% (5/6 patients). Based on these data, 1,000 μg/kg weekly is the recommended Phase 2 dose.