Six-transmembrane epithelial antigen of the prostate 1 (STEAP1) is highly expressed in prostate cancers. DSTP3086S is a humanized immunoglobulin G1 anti-STEAP1 monoclonal antibody linked to the potent antimitotic agent monomethyl auristatin E. This study evaluated the safety and activity of DSTP3086S in patients with metastatic castration-resistant prostate cancer.
Patients were enrolled in a 3 + 3 dose escalation study to evaluate DSTP3086S (0.3 to 2.8 mg/kg intravenously) given once every 3 weeks followed by cohort expansion at the recommended phase II dose or weekly (0.8 to 1.0 mg/kg).
Seventy-seven patients were given DSTP3086S once every 3 weeks, and seven were treated weekly. Two patients in the once-every-3-weeks dose escalation had dose-limiting grade 3 transaminitis. Grade 3 hyperglycemia and grade 4 hypophosphatemia were dose-limiting toxicities in one patient treated at 1.0 mg/kg weekly. Initial cohort expansion evaluated dosing at 2.8 mg/kg once every 3 weeks (n = 10), but frequent dose reductions led to testing of 2.4 mg/kg (n = 39) in the expansion phase. Common related adverse events (> 20%) across doses (once every 3 weeks) were fatigue, peripheral neuropathy, nausea, constipation, anorexia, diarrhea, and vomiting. DSTP3086S pharmacokinetics were linear. Among 62 patients who received > 2 mg/kg DSTP3086S once every 3 weeks, 11 (18%) demonstrated a ≥ 50% decline in prostate-specific antigen; two (6%) of 36 with measurable disease at baseline achieved a radiographic partial response; and of 27 patients with informative unfavorable baseline circulating tumor cells ≥ 5/7.5 mL of blood, 16 (59%) showed conversions to favorable circulating tumor cells < 5. No prostate-specific antigen or RECIST responses were seen with weekly dosing.
DSTP3086S has acceptable safety at the recommended phase II dose level of 2.4 mg/kg once every 3 weeks. Antitumor activity at doses between 2.25 and 2.8 mg/kg once every 3 weeks supports the potential benefit of treating STEAP1-expressing metastatic castration-resistant prostate cancer with an STEAP1-targeting antibody-drug conjugate.
Journal of clinical oncology : official journal of the American Society of Clinical Oncology. 2019 Nov 05 [Epub]
Daniel C Danila, Russell Z Szmulewitz, Ulka Vaishampayan, Celestia S Higano, Ari D Baron, Houston N Gilbert, Flavia Brunstein, Marija Milojic-Blair, Bei Wang, Omar Kabbarah, Michael Mamounas, Bernard M Fine, Daniel J Maslyar, Alexander Ungewickell, Howard I Scher
Memorial Sloan Kettering Cancer Center and Weill Cornell Medical College, New York, NY., The University of Chicago, Chicago, IL., Karmanos Cancer Institute, Detroit, MI., University of Washington and Fred Hutchinson Cancer Research Center, Seattle, WA., California Pacific Medical Center, San Francisco, CA., Genentech, South San Francisco, CA.