Abstract Purpose: Treatment efficacy of nivolumab was evaluated in patients with advanced, treatment-refractory solid dMMR/MSI tumors and in-depth biomarker analyses were performed to inform precision immunotherapy approaches. Patients and methods: Patients with dMMR/MSI tumors who exhausted standard-of-care treatment options were enrolled in the Drug Rediscovery Protocol (DRUP), a pan-cancer clinical trial that treats patients with cancer based on their tumor molecular profile with off-label anticancer drugs (NCT02925234). Patients received nivolumab (four cycles of 240 mg every 2 weeks, thereafter 480 mg every 4 weeks). The primary endpoint was clinical benefit (CB: objective response (OR) or stable disease ≥ 16 weeks). Whole-genome sequencing and RNA-sequencing were performed on pre-treatment tumor biopsies. Results: 130 evaluable patients were enrolled with 16 different cancer types. CB was observed in 62% (95% CI: 53 – 70) with an OR in 45% (95% CI: 36 – 54). After a median follow-up of 14.5 months (95% CI: 13 – 19), median progression-free survival was 18 months (95% CI 9 – not reached) and median overall survival was not reached. While CB was not or only weakly associated with markers of adaptive immune cell infiltration, CB was strongly associated with expression of a broad set of innate immune receptors/ligands. This clearly contrasted findings in melanoma, where markers of adaptive immunity dominated the biomarker landscape. Conclusions: Nivolumab proved highly effective in advanced dMMR/MSI tumors. Expression of key innate immune receptors/ligands was the main predictor of good treatment outcome, contrasting findings in melanoma and strengthening the rationale for tumor-type specific biomarkers for guiding immunotherapy.

The innate immune landscape of dMMR/MSI cancers predicts outcome of nivolumab treatment: results from the Drug Rediscovery Protocol