KEYNOTE-199: Pembrolizumab (pembro) for docetaxel-refractory metastatic castration-resistant prostate cancer (mCRPC).

5007Background: Efficacy of PD-1 inhibition has not been demonstrated in large-scale mCRPC trials. Pembro showed preliminary antitumor activity in PD-L1+ mCRPC in KEYNOTE-028 (n = 23). Here, we present results from cohorts 1-3 (n = 258) of the phase 2 KEYNOTE-199 study of pembro monotherapy in docetaxel-refractory mCRPC (NCT02787005). Methods: Cohorts 1 (C1) and 2 (C2) enrolled patients (pts) with RECIST-measurable PD-L1+ and PD-L1– disease, respectively. C3 enrolled pts with nonmeasurable, bone-predominant disease. All pts had ECOG PS 0-2 and received ≥1 novel endocrine therapy (eg, abiraterone, enzalutamide) and 1-2 prior chemotherapies including docetaxel. Pts received pembro 200 mg Q3W until PD or intolerable toxicity. Response was assessed Q9W in yr 1, then Q12W. Primary end point was ORR per RECIST v1.1 by central review in C1 and C2, separately and combined. Key secondary end points included DCR (CR + PR + SD) per PCWG3-modifed RECIST and safety in all 3 cohorts. Results: 131 pts enrolled in C1, 67...