Immune checkpoint inhibitor responses in KIT-mutated metastatic melanoma

199 Background: KIT-mutated MM is a rare melanoma entity. Response rates of KIT-mutated MM to anti CTLA-4 and anti PD-1 have not previously been reported. Methods: A single-institution retrospective review identified patients (pts) with KIT-mutated MM treated with at least two doses of anti CTLA-4 or anti PD-1 between 2008-2017 with response determined by immune-related response criteria (irRC). Overall survival (OS) was from ICI start to death or last follow-up date, and progression-free-survival (PFS) was from ICI start to date of progression or death if pts deceased without disease progression. Results: Thirty-five pts treated with ipilimumab were identified. Median follow-up was 63.7 months. Median age at MM diagnosis was 65.4 years (range 26-81) and 54.3% were male. Subtypes were 51.4% mucosal, 22.9% acral-lentiginous, 22.9% cutaneous, and 2.9% unknown primary. KIT mutations were 57.1% exon 11, 25.7% exon 17, 11.4% exon 13, and 5.7% exon 2. Anatomical M stage at treatment initiation was 11.4% M1a, 34.3% M1b, and 54.3% M1c. Median OS was 11.8 months (8.4-35.6) and PFS was 3.0 months (2.8-5.8). Responses to ipilimumab were 8.6% complete response (CR), 11.4% partial response (PR), 28.6% stable disease (SD), and 51.4% progressive disease (PD) for a 48.6% clinical response rate (CRR). Twenty pts treated with anti PD-1 were identified. Median follow-up was 7.9 months. Median age at MM diagnosis was 66.7 years (range 42.6-86.8) and 70% were male. Subtypes were 40% mucosal, 30% cutaneous, 20% acral-lentiginous, and 10% unknown primary. KIT mutations were 45% exon 11, 35% exon 17, 10% exon 2, 5% exon 13 and and 5% exon 10. Anatomical M stage at treatment initiation was 10% M1a, 15% M1b, and 75% M1c. Median OS was 22.5 months (7.4-NA ) and PFS was 3.2 months (2.7-NA). Responses to anti PD-1 were 10.0% CR, 25.0% PR, 20.0% SD, and 45.0% PD for a 55.0% CRR. For both anti CTLA-4 and anti PD-1 treated cohorts, univariate analysis demonstrated no statistical significance between tumor response to ICI and clinical pt characteristics or KIT mutation exon. Conclusions: KIT-mutated MM demonstrated CRR of 48.6-55.0% to ICI. There was no correlation between pt characteristics or KIT exon mutation and response to therapy; however, analysis was limited by sample size.