Background: In patients (pts) with muscle invasive bladder (MIBC) suitable for curative definitive chemoradiotherapy (CRT), we hypothesise that the addition of pembrolizumab may be safe and improve efficacy. A pre-planned safety analysis was performed after the first 10 of planned 30 pts were enrolled and completed treatment. Methods: Patients with maximally resected non-metastatic MIBC and ECOG 0-1, who desire bladder preservation or are ineligible for cystectomy were treated with 64Gy in 32 daily radiation fractions to the whole bladder alone over 6.5 weeks in combination with 6 concurrent doses of weekly cisplatin at 35mg/m2 IV. Pembrolizumab was commenced concurrently with radiation and given flat-dose 200mg IV q21 days for 7 doses. Surveillance cystoscopy, urine cytology and CT chest-abdomen-pelvis were performed 12 & 24 weeks post CRT. The primary endpoint is feasibility, defined by a satisfactory low rate of unacceptable toxicity of a) G3-4 non-urinary adverse events (AE) or b) failure of completion of planned CRT according to defined parameters. Secondary endpoints include complete cystoscopic response without metastatic disease at 12 & 24 weeks, loco-regional PFS, metastatic DFS, and overall survival. A 2-stage design was planned, with accrual to be halted if >5 of the first 10 pts experienced unacceptable toxicity up to 12 weeks post treatment. Results: All 10 pts completed the course of CRT and pembrolizumab without alteration in radiation dose or schedule. 1 patient had a dose of cisplatin withheld. 4/10 pts experienced G3-4 non-urinary adverse events within 12 weeks of completing treatment. One immune related AE interrupted pembrolizumab delivery (G2 nephritis). By week 24, 9/10 pts achieved a complete cystoscopic response to treatment post CRT and were free of distant metastatic disease. Conclusions: Interim results indicate that pembrolizumab and CRT shows satisfactory safety, and promising efficacy. There were no unexpected safety signals. Follow up of these and additional pts will better define the efficacy and safety of the combination. Enrolment is ongoing with 20 pts recruited out of a planned total of 30. Clinical trial information: NCT02662062