Background: Standard of care for noncurative adenocarcinoma mCRPC is docetaxel or the next-generation hormonal agents (NHAs) abiraterone and enzalutamide. However, approximately 20% of patients develop t-NE after treatment for adenocarcinoma mCRPC; t-NE is associated with shorter survival, and it has no standard of care beyond combination platinum chemotherapy. The vascular endothelial growth factor (VEGF)/fibroblast growth factor receptor (FGFR) inhibitor lenvatinib inhibits proliferation and angiogenesis in preclinical models of adenocarcinoma prostate cancer. The phase 2 KEYNOTE-199 trial showed some antitumor activity with pembrolizumab monotherapy in docetaxel-pretreated patients with adenocarcinoma mCRPC. VEGF/FGFR inhibition combined with PD-1 inhibition may have enhanced benefit in mCRPC, for adenocarcinoma and possibly as a new treatment option for t-NE. Methods: KEYNOTE-365 (NCT02861573) is a nonrandomized, open-label, multicohort, phase 1b/2 trial designed to evaluate different pembrolizumab combination therapies in several patient populations with mCRPC. Cohort E will enroll patients with confirmed adenocarcinoma of the prostate without small cell histology, per investigator. Cohort F will enroll patients with t-NE (≥1% neuroendocrine cells in a recent biopsy specimen confirmed by central histology review). Each cohort will include patients who previously received docetaxel treatment for mCRPC. Prior treatment with ≤2 NHAs (for hormone-sensitive metastatic prostate cancer [mHSPC] or mCRPC) and 1 other chemotherapy for mCRPC is permitted. Additionally, enrollment in cohort F requires prostate cancer progression within 6 months of starting an NHA (for mHSPC or mCRPC) and within < 6 cycles of docetaxel for mCRPC. Each cohort will enroll 40-100 patients with Eastern Cooperative Oncology Group performance status score of 0 or 1. Both cohorts will receive pembrolizumab 200 mg IV every 3 weeks + oral lenvatinib 20 mg once daily until disease progression, withdrawal of consent, prespecified alanine aminotransaminase or aspartate aminotransaminase level increase, or other discontinuation event. Primary end points are safety and tolerability, prostate-specific antigen (PSA) response rate, and objective response rate (ORR) per RECIST v1.1 by blinded independent central review (BICR). Secondary end points include time to PSA progression; ORR and radiographic progression-free survival per Prostate Cancer Working Group 3 (PCWG3)–modified RECIST v1.1 by BICR; duration of response and disease control rate per RECIST v1.1 and PCWG3-modified RECIST v1.1 by BICR; and overall survival. These cohorts of KEYNOTE-365 are enrolling in Australia, Germany, New Zealand, Spain, and the United States. Clinical trial information: NCT02861573.