848PDPhase II study of pembrolizumab with enzalutamide (Enz) in metastatic, castration-resistant prostate cancer (mCRPC): 30 patient expansion with examination of tumour-infiltrating immune cells and fecal microbiota

Abstract Background PD-1 inhibition (α-PD-1) can lead to deep, durable responses in mCRPC, but only in a minority of patients (pts). We previously reported 5 (18%) responders among 28 pts. No marker has been shown to universally predict response. Analysis of fecal microbiota from pts with epithelial cancers showed an association between elevated Akkermansia muciniphila (A. muc) levels and response to α-PD-1. To evaluate predictive markers, we treated an additional 30 pts obtaining tumor biopsies, blood, and fecal specimens collected for analysis. Methods 30 pts with mCRPC progressing on Enz received α-PD-1 pembrolizumab 200 mg IV every 3 weeks for 4 doses with continued Enz. Prior chemotherapy for mCRPC was prohibited. The primary endpoint was the proportion of pts with a prostate-specific antigen (PSA) decline ≥ 50%. The secondary endpoints were objective response, PSA progression-free survival (PFS), radiographic PFS, overall survival (OS) and correlative studies. Results Four of 30 (13%) achieved a PSA reduction ≥ 50%. Six of 24 (25%) pts with measurable disease responded radiographically. After median follow up of 17.4 months (mos), median PSA PFS was 5.57 mos (95%CI: 3.48-8.06). Median OS was 17.25 mos (95% CI: 7.71-17.68). Of 27 pts evaluable for radiographic PFS, 17 (63%) have progressed; median radiographic PFS was 5.26 mos (95% CI: 2.6-11.2). Eight pts had immune related adverse events. Single cell analysis revealed that immune cells represent an average 0.5% of cells in tumor biopsies. Single cell transcriptomic analysis prior to α-PD-1 in 12 pts revealed enrichment in a CD8+ exhausted T cell population (p