Background: Uveal melanoma (UM) is the most frequent intraocular malignant tumor in adults. Approximately 50% of all patients (pts) will develop metastatic disease in the liver. Until now, there is no systemic therapy that has been shown to improve overall survival (OS), apart from tebentafusp. M-PHP is frequently applied for liver-only UM. However, the majority of pts eventually develops extrahepatic disease after M-PHP. IPI+NIVO has been shown to induce up to 20% response rates in mUM. Our observations that checkpoint inhibition was most effective on extrahepatic UM disease has led to the CHOPIN trial testing the combination of M-PHP and IPI+NIVO. Here we present the safety and efficacy data of the phase 1b part of CHOPIN. Methods: Adult pts with confirmed measurable hepatic mUM and WHO PS 0-1 were included. Two courses of 6 weekly M-PHPs (melphalan 3mg/kg, max 220mg) were combined with four courses IPI+NIVO three-weekly escalating the dosing from 1mg/kg each IPI+NIVO (cohort 1) to IPI 1mg/kg + NIVO 3mg/kg (cohort 2). Primary endpoint was safety of IPI+NIVO plus M-PHP. Secondary endpoints were best overall response (BOR) according to RECIST 1.1, progression-free survival (PFS), and OS. Results: 7 pts were included (4 male, median age 63.6 years (range 50-74)). Both cohorts were tolerated with no dose-limiting toxicities or deaths. Grade III/IV adverse events (AE) were observed in 2/3 pts in cohort 1 and in 3/4 pts in cohort 2 consisting of SIRS, febrile neutropenia, cholecystitis, neutropenia, thrombopenia, leukopenia, increased transaminases and fever. Grade I/II immune-related AEs occurred in all pts (myositis, hypothyroidism, hepatitis and dermatitis). BOR was 1 complete response, 5 partial responses and 1 stable disease accounting for an objective response rate (ORR) of 85.7%. At a median FU time of 20.2 months, 4 pts have an ongoing response. Currently the median PFS is 22.4 months, and all pts are still alive. Conclusions: Combining M-PHP with IPI+NIVO is safe at a dosing of IPI 1 mg/kg and NIVO 3 mg/kg and very promising ORR, PFS and OS have been observed. The randomized phase II part comparing M-PHP versus M-PHP+IPI+NIVO is currently recruiting. Clinical trial information: NCT04283890.