High Incidence Of Brain Metastases (Brm) In Patients With Metastatic Cutaneous Melanoma (Mcm) And Mutations In The Apc/Beta-Catenin (Ctnnb1)

9527 Background: Activation of the WNT/β-catenin pathway is associated with low/absent tumor-infiltrating lymphocytes (TILs) and resistance to anti-PD-L1/anti-CTLA-4 agents in mouse melanoma models. We aimed to investigate if APC/CTNNB1 mutations in melanoma pts are associated with TIL status, prediction of response to immunotherapy (IT), and overall survival (OS). Methods: Pts with CM and APC or CTNNB1 mutations identified in melanoma tumors using the TruSight Tumor 26 Illumina assay were enrolled. Demographics, clinical (stage, treatment response, follow-up), pathologic (TIL status), and molecular (BRAF/NRAS, C→T (i.e. UV signature) nucleotide transition, functional significance by IMPACT, mutant allele frequency (MAF) corrected from the % tumor) characteristics were investigated. Results: We identified a total of 25 pts (13 males; age at original diagnosis (median 61 yrs, range 22-78 yrs). CTNNB1 and APC mutations were mutually exclusive. 48% (12/25) had APC mutations and 52% (13/25) had CTNNB1 mutations; of which (i.e. CTNNB1 mutations) 69% (9/13) had absent TILs. 88% (22/25) of APC/CTNNB1 mutations had moderate functional significance, 64% (16/25) of the mutations had a C→T nucleotide change, 36% (9/25) had BRAFV600, and 20% (5/25) NRASQ61 mutations. 64% (14/22) of pts with stage II-III progressed to stage IV; of these 14 pts, 8 (57%) developed parenchymal BrM. 13 of the stage II-III 22 pts who progressed to stage IV received IT; of these 13 pts, 7 (53%) had absent TILs. APC/CTNNB1 mutations did not influence response to IT, irrespective of the MAF of the mutations. Of the 12 pts with MCM and measurable disease who received IT 9/12 had absent TILs and 7/12 responded. The median OS from the time of diagnosis of distant MCM (N = 17; 14 pts who progressed from initial stage II-III and 3 pts who were originally diagnosed with stage IV) was 18.8 months (range, 2.4-48.0 months). Conclusions: APC & CTNNB1 mutations are mutually exclusive. CTNNB1 mutations are more frequently associated with absent TILs. Pts with MCM had relatively shorter OS (18.8 months) in part due to development of BrM. In this small cohort APC/CTNNB1 mutations did not seem to impair response to immunotherapy.