2376P Pembrolizumab monotherapy following tri-modality treatment for selected patients with muscle-invasive bladder cancer

This is a phase 2, single arm, single-site study of Pembrolizumab as maintenance therapy in muscle-invasive bladder cancer (MIBC) participants who has received maximum Transurethral resection of bladder tumor (TURBT) and tri-modality treatment (TMT) and achieved complete response (CR). This trial includes Patients with cT2-4N0M0 MIBC who declined or were ineligible for cystectomy (RC), and achieved CR after receiving maximal TURBT then Stereotactic ablative radiotherapy (SABR) boost to bladder tumor or tumor bed and concurrent radio-chemotherapy. During SABR, intravesical installation of isovolumetric saline through urinary catheter ensured adequate bladder filling. After TMT, patients who achieved CR will receive Pembrolizumab as maintenance therapy. Pembrolizumab (200 mg q3w) monotherapy will begin on Day 1 of each 3-week cycle and will continue for up to 17 cycles (approximately 1 year). The primary endpoint was progression-free survival (PFS). From January 11, 2022 to April 20, 2023, twenty-three patients with MIBC were enrolled. The median age of included patients was 68 years and clinical stage ranged from cT2 (n=20) to cT3 (n=3). TURBT was visibly complete in twelve cases. After a median follow-up time of 6 months, 5/23 (21.7%) patients reported immune-related AEs, which included Grade 2 myositis, hypothyroidism, pneumonitis and cystitis, and had no ongoing toxicity after administration of steroids. The estimated 1-year PFS, overall survival and local control rate was 78%, 94.4% and 94.7%. One patient experienced local recurrence with MIBC and bone metastasis respectively after 7 months of TMT. One patient died from pulmonary infection related to COVID-19, the others are alive with no evidence of cancer and with intact well-functioning urinary bladder at median follow-up of 6 months. Pembrolizumab maintenance therapy after SABR boost to bladder tumor and concurrent radio-chemotherapy was well-tolerated with promising efficacy in the early analysis. Immune related toxicity was consistent with prior monotherapy trials. Selected correlative analyses from serially collected blood and tissue specimens will be presented.