Background: A trial of neoadjuvant pembrolizumab (P) in combination with high dose interferon-α (HDI) in high-risk patients (pts) with locoregionally advanced melanoma (mel) has completed enrollment. Methods: Primary endpoint: safety of combination P-HDI. Pts were treated with P x 2 doses followed by definitive surgery, then x1 year. HDI was given concurrently, and both agents were per standard regimen. Tumor and blood samples were obtained at baseline and at surgery (wk 6-8), blood at 6 wks, 3,6,12 months (mos). Results: 30 pts were treated (22 male, 8 female, age 26-83). 16 had cutaneous primary, 3 mucosal, 11 unknown. At enrollment, 16 had recurrent disease, 6 received prior adjuvant therapy with ipilimumab (4) or HDI (2). 16 had AJCC 7 stage IIIB, 9 IIIC, 5 IV. 332 P cycles have been delivered (median 13), 496 doses of HDI induction (median 17), 1329 doses of HDI maintenance (median 44). HDI was dose reduced in 20 pts, discontinued in 27, P discontinued in 8. Radiologic preoperative RR was 77% (95% CI, 59-88) (6 CR, 17 PR). 20% (6) had SD and 1 had PD. All pts underwent definitive surgery. The pathologic complete response (pCR) of 26 pts was 32% (95% CI, 18-51). 6 pts recurred and 3 died. No pt with pCR has recurred. Median f/u time is 17.4 mos, median PFS/OS not reached. Most common grade (Gr) 3 toxicities: hypophosphatemia (10; 33%), fatigue (10; 33%), ↑CPK (6; 20%), ↑lipase (4; 13%). 3 Gr 4 events (↑CPK, hyperglycemia, lymphocyte count decreased). 1 suspected grade 5 event occurred 6 months after completion of therapy. PD-L1 expression at baseline did not correlate with clinical outcomes. In 8 pts with pre and post treatment tumor samples, IHC expression of PD-1, PD-L1, CD11b, CD8, Foxp3 and CD25 increased post-treatment (p < 0.05). Conclusions: Neoadjuvant P-HDI has promising clinical activity, although treatment is limited by HDI toxicity. Treatment increases the immune cell infiltrate, and outcomes do not correlate with baseline expression of PD-L1. Longer follow up and further mechanistic studies are underway. Clinical trial information: NCT02339324