Clinical and Translational Results from PORTER, a Multicohort Phase I Platform Trial of Combination Immunotherapy in Metastatic Castration-Resistant Prostate Cancer

Abstract Purpose: Current immune checkpoint therapies offer limited benefits for metastatic castration-resistant prostate cancer. Novel combinations may enhance immunotherapy efficacy. Patients and Methods: We conducted an open-label, noncomparative platform trial (NCT03835533) in metastatic castration-resistant prostate cancer to assess nivolumab-based combinations. The cohorts were as follows: (A) bempegaldesleukin 0.006 mg/kg and nivolumab 360 mg i.v. every 3 weeks; (B) stereotactic body radiotherapy 30 to 50 Gy, CDX-301 75 μg/kg s.c. for 5 days, poly-ICLC 1 mg intramuscularly weekly twice for 3 weeks, and nivolumab 480 mg every 4 weeks; and (C) CDX-301 75 μg/kg for 10 days, INO-5151 3 mg intramuscularly on lead-in day 8, day 1 of cycles 1 to 3, and then every 12 weeks, and nivolumab 480 mg every 4 weeks. The primary endpoint was safety; secondary endpoints included composite response rate (radiographic, PSA, or circulating tumor cell responses), 6-month disease control rate, progression-free survival, and overall survival. Serial blood and tissue samples were analyzed for pharmacodynamics and association with disease control. Results: A total of 43 patients were enrolled (n = 14, 15, and 14 in cohorts A, B, and C, respectively). Grade 3 to 4 treatment-related adverse events occurred in 10 (71%), 2 (13%), and 2 (14%) patients, respectively, with one grade 5 treatment-related adverse event in cohort A. Composite response rates were 7% (1/14), 33% (5/15), and 7% (1/14). Across cohorts, 6-month disease control was associated with preexisting memory/regulatory T cells, TNFα, and other inflammatory pathways. Conclusions: Cohort B, which combined radiotherapy with CDX-301, poly-ICLC, and nivolumab, demonstrated encouraging clinical activity. Preexisting rather than treatment-induced immune activation was associated with clinical benefit across cohorts, highlighting the importance of baseline immune fitness.