Circulating IFNγ-associated protein signatures predict response to neoadjuvant immunotherapy in patients with stage III melanoma

Background: Immune checkpoint inhibitors (ICIs), such as anti-programmed cell death protein 1 (PD-1) and anti-cytotoxic T-lymphocyte associated protein 4 (CTLA-4), have transformed the management of stage III melanoma in the neoadjuvant setting. However, a substantial proportion of patients do not derive benefit from ICI therapy. To improve clinical outcomes, there remains a critical unmet need to identify early biomarkers of response to neoadjuvant immunotherapy in stage III melanoma. Methods: In this study, we performed longitudinal serum proteomic profiling in 39 patients undergoing neoadjuvant combination anti-PD-1 and anti-CTLA-4 therapy. Using a multiplex proximity extension assay, we measured 702 proteins at three timepoints: baseline, early on-treatment (3–4 weeks after treatment initiation), and pre-surgery (4–8 weeks post-treatment). Results: The most pronounced differences between major pathological responders (MPR) and non-MPR patients were detected at baseline and were linked to interferon gamma (IFNγ) signalling, but these differences diminish at later timepoints. A 10-protein IFNγ-associated signature derived from baseline serum profile achieved an AUC of 0.68 for predicting pathological response, comparable to a previously reported tumour-based IFNγ gene signature (AUC = 0.67). Conclusions: These findings support the use of circulating protein signatures as minimally invasive, scalable biomarkers to inform early treatment decisions in the neoadjuvant setting.