Background: Immunotherapy with anti-PD-1 + CTLA-4 Abs improves response rates over anti-PD-1 Ab alone; however, the utility of this combination after first line anti-PD-1 is unknown. We report the first prospective data evaluating pembro + low dose ipi immediately following progression on anti-PD-1 (NCT02743819). Methods: Patients (pts) with mel and measurable disease, no autoimmunity, and no prior anti-CTLA-4 who had progressed immediately prior on an anti-PD-1 (or non-CTLA4 combination) were eligible. Prior BRAF inhibitor was allowed (none received it). Pts received pembro 200 mg + ipi 1 mg/kg Q3W for 4 doses, then pembro alone. The primary endpoint was response rate (RR) as assessed by irRECIST. An optimal Simon two-stage design was employed to test the null hypothesis of a 10% RR vs 30% alternative (1-sided alpha 0.10, 90% power, ≥2/12 RR to continue to total of ≥6/35). The data analysis cutoff date was January 2, 2018. Results: 22 patients have been accrued with 17 evaluable for the primary endpoint (4 have not yet had their first imaging evaluation and 1 was not enrolled). Prior treatment included 21 on anti-PD-1 alone and 1 on combination with IDO inhibitor. Median length of treatment on prior anti-PD-1 was 5.6 months among all 22 pts. The study met its interim efficacy analysis with 5/12 responses to move to stage 2. Among the 17 response-evaluable pts there were 2 CR, 6 PR (47% RR), and 5 SD for disease control rate (DCR) of 76% and rejection of the null hypothesis. Progression-free survival at 6 months was 75% (CI 47%-90%). All responses are ongoing. At last follow-up, 8 pts have gone off treatment with 14/22 (64%) having any drug-related and 3/22 (14%) ≥ grade 3-4 drug-related AE (hyperglycemia, acute kidney injury and skin tissue disorder, diarrhea and rash acneiform). Among 11 response-evaluable pts with staining results currently available, RR and DCR were 67% and 100% in PD-L1+ (n = 3) and 50% and 88% in PD-L1 negative (n = 8) tumors. Further biomarker analysis is underway. Conclusions: Pembro + 1 mg/kg ipi is tolerable and has antitumor activity in pts with mel who have progressed immediately prior on an anti-PD-1 Ab. The trial sample size has been expanded to further explore this regimen. Clinical trial information: NCT02743819