Background: KEYNOTE-029 cohort 1B showed substantial efficacy for standard-dose pembro + 4 doses of ipi 1 mg/kg with lower rate of grade 3-5 toxicity than reported for standard-dose nivolumab + ipi in patients (pts) with advanced melanoma. In KEYNOTE-029 cohort 1C, we assessed the toxicity and antitumor activity of standard-dose pembro plus 2 alternate ipi doses (NCT02089685). Methods: Eligible pts with previously untreated stage III/IV melanoma, ECOG PS 0-1, and no active CNS metastases were randomized 1:1 to pembro 200 mg Q3W for 24 mo + 4 doses of ipi 50 mg Q6W (arm A) or pembro 200 mg Q3W for 24 mo + 4 doses of ipi 100 mg Q12W (arm B). Primary endpoints were the grade 3-5 treatment-related AE (TRAE) rate and ORR (RECIST v1.1, central review). With 50 pts per arm and compared with other anti–PD-1 + ipi regimens, a grade 3-5 TRAE rate ≤26% would suggest a meaningful reduction in toxicity and an ORR ≥48% would suggest no decrease in efficacy. Data cutoff date was Jul 17, 2018, and will be updated. Results: 102 pts were randomized: 51 to each arm. Median age was 63.5 y, 70% were male, 15% had ECOG PS 1, 34% had BRAF mutation, and 30% had elevated LDH at baseline. With 9.4 mo median follow-up, 69% of pts in arm A and 71% in arm B remained on treatment. All pts in arm A and 96.1% in arm B had ≥1 TRAE; grade 3-5 TRAE rates were 22% in arm A and 33% in arm B. 1 pt had a grade 5 TRAE (autoimmune myocarditis; arm A). ORR was 49% (95% CI 35-63) in arm A, including 7 CRs and 18 PRs, and 53% (95% CI 39-67) in arm B, including 6 CRs and 21 PRs. An additional 16 pts in each arm had SD or non-CR/non-PD, leading to a DCR of 80% and 84%, respectively. Median response duration was not reached in either arm (range 1.4+ to 9.6+ in arm A, 1.4+ to 9.8+ in arm B). Updated data based on longer follow-up will be presented. Conclusions: Standard-dose pembro + ipi 50 mg Q6W and standard-dose pembro + ipi 100 mg Q12W showed robust antitumor activity in this initial analysis. Both regimens appeared to have a lower rate of grade 3-5 TRAEs than previously observed. Longer follow-up and randomized studies are needed to confirm that these regimens reduce toxicity without compromising efficacy compared with other anti–PD-1 and ipi combinations. Clinical trial information: NCT02089685