9543 Background: mMCC is a rare, aggressive neuroendocrine cancer which often occurs in older patients (pts) with multiple comorbidities. While initial response rates to ICI are high, optimal treatment duration, durability of response after treatment cessation and response to retreatment with ICI is unknown. Methods: mMCC pts from 12 international centres who received at least one dose of ICI and subsequently stopped treatment without progression for a minimum of 12 weeks were studied. Demographics, disease characteristics and treatment course were examined. Results: 40 pts with mMCC were included. Pt characteristics are summarised in Table. Median time on treatment was 13.5 months (range 1 to 35). Median time to best response was 4.5 months (range 1 to 17) and median time receiving treatment after best response was 8 months (range 0 to 29). 25 pts (63%) stopped primarily due to being in a complete or partial response (CR or PR), 9 (23%) due to toxicity and 6 (15%) due to other reasons, primarily pt choice or comorbidities. At time of discontinuation, 30 pts (75%) were in a CR, 8 (20%) in a PR and 2 pts (5%) had stable disease (SD). After a median follow up of 12 months from discontinuation, 14 pts (35%) have progressed (PD); 5 (36%) at a previous site, 5 (36%) at a new site and 4 (29%) at both. PD occurred after a median of 5.5 months (range 4 to 29) off treatment. 4 pts (29%) had a CNS recurrence, none of whom previously had CNS involvement. Pts in CR at time of discontinuation were less likely to progress (CR: 26% PD vs non-CR: 67% PD, p=0.044), but still had a considerable rate of PD (CR: 26%, PR: 57%, SD: 100%). Those who progressed had numerically less cycles of ICI prior to treatment cessation (17 vs 32, p>0.05). Baseline disease factors, time to best response and duration of treatment after best response were not associated with PD. ICI was restarted in 8 of 14 pts (57%) with PD, with response rate to retreatment of 75% (4 CR, 2 PR, 1 SD, 1 PD – pt with leptomeningeal disease). Median time to best response at retreatment was 3 months (range 2 to 7), with all responses ongoing after a median of 10 months back on treatment. 3 pts had an isolated site of PD successfully treated with radiation therapy and remain in remission off ICI. Conclusions: ICI responses in mMCC do not appear as durable off treatment as in other cancers, including in patients who achieve a CR. Ongoing treatment should be considered, though initial data on response to retreatment is promising.[Table: see text]