First-in-human, phase I study of PF-06753512, a vaccine-based immunotherapy regimen (PrCa VBIR), in biochemical relapse (BCR) and metastatic castration-resistant prostate cancer (mCRPC).

Abstract

2612 Background: Therapeutic vaccines targeting PC-associated antigens represent attractive approaches in combination with immune checkpoint inhibitors (ICI). Safety/antitumor activity of PF-06753512 (PrCa VBIR) was evaluated in a phase I, dose-escalation and expansion study in patients (pts) with BCR prior to ADT and in pts with mCRPC either prior to or after failure of novel hormone therapy. PrCa VBIR consists of: 1) priming immunization with a replication-deficient adenoviral vector (AdC68) expressing PSA, prostate-specific membrane antigen and prostate stem cell antigen; 2) boosts with plasmid DNA (pDNA) encoding the same antigens by IM electroporation; 3) ICI given subcutaneously, including anti CTLA-4 antibody tremelimumab (TRM) and anti PD-1 antibody sasanlimab (SSL). Methods: AdC68 ± ICI(s) were given on months (mos) 1 and 5 and pDNA ± ICI(s) on mos 2–4 and 6–8. After 8 mos, maintenance pDNA + ICI(s) were given every 1 or 2 mos. In Part A (6 escalation cohorts), pts with mCRPC received AdC68 (4 or 6x10e11 viral particles) + pDNA 5 mg ± ICIs (TRM alone 80 mg; TRM 40 or 80 mg + SSL 130 or 300 mg). In Part B (3 expansion cohorts), pts with mCRPC received AdC68 6x10e11 + pDNA 5 mg + TRM 80 mg + SSL 300 mg; pts with BCR received similar vector and pDNA + TRM 80 mg ± SSL 130 mg. Primary objectives: Assess overall safety (CTCAE v4.03), determine expansion dose. Secondary objectives: Anti-tumor activity (RECIST v1.1, Prostate Cancer Working Group 3, PSA 50 response) and immune response. (Note: Database remains open, some queries pending). Results: As of Sept 15, 2020, 91 pts were treated in dose-escalation (n=38) and expansion (n=53; BCR=35, mCRPC=18). Immune responses (ELISpot) were positive in some pts. Grade (G) 3 or 4 treatment-related adverse events (TRAEs) developed in 38.5% (35/91) of pts. G5 TRAEs occurred in 2 pts (n=1 G4 myasthenia gravis + G5 pulmonary embolism; n=1 G5 myocarditis). irAEs were more frequent in BCR compared to mCRPC. See the table for efficacy data. Conclusions: Vaccination with PrCa VBIR had a manageable safety profile. TRAEs increased when 2 ICIs were given. Some pts with BCR experienced durable PSA-50 responses without ADT; patients with mCRPC had few objective tumor responses, but had prolonged median rPFS. PrCa VBIR appears to stimulate antigen-specific immunity and results in noticeable antitumor activity, particularly in androgen sensitive disease. Clinical trial information: NCT02616185. [Table: see text]