Use of early circulating tumour DNA dynamics in patients with stage III melanoma receiving neoadjuvant combination immunotherapy.

9577 Background: Neoadjuvant therapy in resectable stage III cutaneous melanoma improves relapse-free survival compared to adjuvant therapy alone and is now a standard of care.While pathological response can help predict recurrence risk and the need for further treatment, more biomarkers are required. We investigated circulating tumour DNA (ctDNA) as a predictive biomarker for recurrence in stage IIIB/C melanoma patients following neoadjuvant immunotherapy and surgery. Methods: We retrospectively analysed plasma samples collected at baseline and six weeks post-surgery from 30 patients enrolled in the OpACIN-neo and PRADO clinical trials, who received two cycles of ipilimumab (anti-CTLA-4) and nivolumab (anti-PD-1) before surgery. Cell free DNA (cfDNA) underwent pre-amplification followed by tumour-informed mutation detection analysis using droplet digital polymerase chain reaction (ddPCR) with the Bio-Rad QX600 PCR system. BRAF mutations were identified in 60% of patients through tumour tissue sequencing. Following neoadjuvant therapy, 7/30 (23%) achieved complete pathological response (pCR), 5/30 (17%) near pCR, 4/30 (13%) partial response, and 14/30 (47%) non-response (pNR). Results: Baseline ctDNA was detectable in 15/30 (50%) patients using the pre-amplification ddPCR method. At median follow-up of 47 months, 9/30 (30%) patients recurred, with a median time to recurrence of 8 months (range 6 to 29 months). Post-surgery, ctDNA was detectable in 4/9 patients who recurred; all four were pathological non-responders and recurred with distant, unresectable disease. Among the 15 ctDNA-positive patients at baseline, 13/15 (87%) patients achieved ctDNA clearance while 2/15 (13%) remained persistently ctDNA-positive; with a subsequent relapse rate of 0% and 100% respectively. ctDNA detection post-surgery predicted patients at higher risk of disease recurrence [relapse-free survival (RFS) hazard ratio (HR) 7.83, 95%CI 0.82-74.48; p=0.0002] and poorer melanoma-specific survival (MSS) [HR 6.35, 95% CI 0.36-110.70; p=0.0337]. Conclusions: Post-surgery ctDNA positivity can signal imminent recurrence, thus offering a window for personalised adjuvant therapy modification.