Background: Bacillus Calmette-Guérin (BCG) therapy is the standard of care for high-risk non-muscle invasive bladder cancer (NMIBC) after transurethral resection of bladder tumor. However, disease recurrence or progression is common and patients with BCG-unresponsive disease are unlikely to respond to further BCG therapy. In these patients, the current standard of care is radical cystectomy and bladder-preserving treatment options, limited to intravesical chemotherapy or intravenous pembrolizumab. In a phase 1 study, sasanlimab (PF-06801591), a monoclonal antibody to programmed cell death protein 1 (PD-1), was administered subcutaneously at 300 mg every 4 weeks. Sasanlimab had an acceptable safety profile and showed clinical activity aligned to other anti-PD-1/PD-ligand 1 (PD-L1) agents in patients with advanced urothelial carcinoma and non-small cell lung cancer, while offering the convenience of subcutaneous administration. Therefore, CREST Study Cohort B aims to evaluate sasanlimab administered subcutaneously in patients with BCG-unresponsive NMIBC. Methods: CREST Study Cohort B is a non-randomized, multicenter, multinational, open-label, phase 3 study and will enroll ̃160 patients with histologically confirmed BCG-unresponsive, high-risk, non-muscle invasive transitional cell carcinoma of the bladder urothelium (high-grade Ta or T1 tumor, or carcinoma in situ [CIS]) in 2 separate Cohorts, B1 and B2 (̃110 and ̃50 patients, respectively). Cohort B1 will enroll patients with persistent or recurrent CIS with or without concomitant recurrent high-grade Ta/T1 disease, within 12 months of completing adequate BCG therapy. Cohort B2 will enroll patients with recurrent high-grade Ta/T1 disease within 6 months of completing adequate BCG therapy. All patients will receive subcutaneous sasanlimab as a single agent. Efficacy will be assessed at regular intervals by cystoscopy, urine cytology, biopsy, and imaging. The primary endpoint is complete response (CR) and event-free survival (EFS) for Cohort B1 and B2, respectively. Secondary endpoints include duration of CR (Cohort B1 only), EFS (Cohort B1 only), overall survival, time to cystectomy, safety, health-related quality of life, pharmacokinetic parameters, PD-L1 expression, and incidence of anti-drug antibodies. Recruitment of patients in CREST Study Cohort B will be opened in Canada and the United States of America, with other sites in Asia, Australia, and Europe. Clinical trial information: NCT04165317.