Association of circulating tumor DNA kinetics with disease recurrence in patients with stage IIIB/C/IV melanoma treated with adjuvant immunotherapy in Checkmate 238

9577 Background: In patients (pts) with resected, stage III/IV melanoma receiving adjuvant immunotherapy, pre-treatment (pre-Tx) and on-treatment (on-Tx) circulating tumor DNA (ctDNA) combined with tumor features were evaluated to predict recurrence risk. Methods: We used analytically validated, mutation-specific droplet digital PCR (ddPCR) assays to measure ctDNA in pre- and on-Tx plasma samples from pts with resected stage IIIB/C/IV melanoma enrolled in CheckMate 238 (NCT02388906) receiving either adjuvant Nivolumab (NIVO) or Ipilimumab (IPI). Assay choice was based on detection of one of the 7 melanoma hot-spot mutations in pt tumors: BRAF V600E/K, NRAS Q61R/L/K, or TERT C228T/C250T. We evaluated associations between ctDNA detection, recurrence free survival (RFS), distant metastasis free survival (DMFS), and known immunotherapy tumor biomarkers including PD-L1, IFNg gene signature, CD8, and tumor mutational burden. Associations between ctDNA kinetics and RFS/DMFS were analyzed using Kaplan-Meier and Cox regression models. Results: Mutations were identified in 87% of tumors. ctDNA was detected in pre-Tx samples from 94/753 (12.5%) pts. Pre-Tx ctDNA was significantly associated with higher stage of disease, LDH ≥ ULN, PD-L1 < 5%, and tumor IFNg-RNA signature expression below the median. Detection of pre-Tx ctDNA was associated with shorter RFS and DMFS in both NIVO and IPI arms compared to undetectable ctDNA (Table). ctDNA detection on-Tx (week (w) 3, 7, 13, 25, 37, 49) was also associated with shorter RFS. Refined subgroup predictions that potentially impact therapy were achieved by incorporating pre- and on-Tx time points. Of those with baseline and w7 ctDNA results, pts in the NIVO arm with undetectable pre-Tx ctDNA (n=308) that became positive at w7 (n=22) had shorter RFS (median RFS 20.68 mos [95% CI, 2.79, NR]) than pts in whom on-Tx ctDNA remained undetectable (n=286) (median RFS, NR [95% CI, 61.1, NR]). Pts with positive pre-Tx ctDNA (n=42) that remained positive at w7 (n=23) had markedly shorter RFS (median RFS, 3.35 mos [95% CI, 2.73, 20.44]) vs pts in whom on-Tx ctDNA became undetectable (n=19) (median RFS, NR [95% CI, 14, NR]). Similar results were observed in the IPI arm. Conclusions: Pre- and on-Tx ctDNA measurements are associated with melanoma recurrence risk during adjuvant checkpoint inhibitor therapy. Incorporation of ctDNA with other tumor molecular features may improve prediction of RFS and DMFS in this setting. Clinical trial information: NCT02388906 . [Table: see text]