Background: Combination PD1 + CTLA4 antibodies (Abs) shows greater response rate (RR) versus PD1 Ab alone in MEL, but RR after initial PD1 Ab progression awaits robust investigation. CTLA4 Ab alone after PD1 Ab progression has a historical RR of 13%. We report final results of the first prospective clinical trial evaluating IPI 1mg/kg + PEMBRO immediately following progression on PD1 Ab (NCT02743819). Methods: Patients (pts) with advanced MEL, no prior CTLA4 Ab for metastatic disease, and who had progressed on PD1 Ab as immediately prior therapy (or non-CTLA4 Ab combination) were eligible. Pts received PEMBRO 200 mg + IPI 1 mg/kg Q3W for 4 doses, then PEMBRO alone for up to two years. The primary endpoint was RR by irRECIST. After 35 pts, the study met its primary endpoint with 10/22 evaluable pts achieving a response. The trial was expanded to enroll a total of 70 pts in open-label accrual to further describe the RR for this regimen in an exploratory fashion. The data analysis cutoff was January 30, 2020. Results: 67/70 accrued patients were evaluable for treatment response. Prior treatments included 60 on PD1 Ab alone and 10 on PD1 Ab-based combinations. Of these, 10 pts had progressed in the adjuvant setting. Median length of treatment on prior PD1 Ab was 4.8 months. Response assessments included 4 CR, 17 PR and 16 SD for a RR of 31% (21/67) in evaluable pts, and 30% (21/70) in all enrolled pts. 4 pts with a PR and 6 with SD had unconfirmed responses making the irRECIST response rate 25% (17/67) and 24% (17/70) among evaluable and enrolled pts, respectively. Median progression free survival (PFS) was 4.7 mo (95% CI: 2.8-8.3) and PFS at six months was 45% (95% CI: 33%-57%). 15/70 (21%) pts experienced ≥ grade 3-4 drug-related AEs, the most common being diarrhea, rash and transaminase elevation. PD-L1 positive vs negative status from historical tumor specimens did not associate with RR. Conclusions: This is the largest prospective study of IPI 1mg/kg + PEMBRO, demonstrating significant antitumor activity and tolerability in MEL post-PD1 Ab. Clinical trial information: NCT02743819