A phase I study of the safety and pharmacokinetics of DSTP3086S, an anti-STEAP1 antibody-drug conjugate (ADC), in patients (pts) with metastatic castration-resistant prostate cancer (CRPC)

5020 Background: Six-transmembrane epithelial antigen of the prostate-1 (STEAP1) protein is a cell-surface antigen overexpressed in human epithelial prostate cancers. The ADC DSTP3086S contains the humanized IgG1 anti-STEAP1 monoclonal antibody linked to the potent anti-mitotic agent MMAE. Methods: This study evaluated safety, pharmacokinetics, and pharmacodynamic activity of intravenous DSTP3086S (0.3-2.8 mg/kg) given every 3 weeks (q3w) to pts with CRPC. A traditional 3+3 design was used to determine maximum-tolerated dose, followed by cohort expansion at the recommended Phase II dose (RP2D). Clinical activity was evaluated per PCWG criteria. Dose escalation results are presented. Results: Twenty-eight pts were enrolled with a median age of 67 (43-76), all ECOG PS 0-1, and with a median of 7 prior systemic regimens (including a median of 4 hormonal and 3 non-hormonal regimens). Pts received a median of 3 doses (range 1-10) of DSTP3086S. Reversible Grade 3 transaminitis DLTs occurred in one pt each in the 2.25 mg/kg and 2.8 mg/kg cohorts. Serious AEs (SAE) related to study drug (3 total) included one DVT (Grade 3) in the 1.5 mg/kg cohort, as well as one GI hemorrhage (Grade 3) and one sepsis event (Grade 5) in the 2.25 mg/kg cohort. The most common related AEs across all doses were fatigue (36%), nausea (32%), constipation (25%), decreased appetite and diarrhea (each 21%), and musculoskeletal pain and vomiting (each 18%). Exposure for total antibody, free MMAE, and conjugated MMAE was dose proportional. Approximately 60% of the tumor samples assessed showed high STEAP1 expression. CTC reductions were most robust at 2.8 mg/kg; 4/4 patients with unfavorable CTCs at baseline (median of 99, range: 21-205) exhibited CTC conversions from unfavorable to favorable (