Clinical and FDG-PET markers of immune checkpoint inhibitor (ICI) response in patients with metastatic Merkel cell carcinoma (mMCC).

9540 Background: mMCC is a rare, highly aggressive neuroendocrine cancer with a poor prognosis. ICIs have favourable efficacy and safety in clinical trials. We outline single centre experience utilising ICIs in mMCC. Methods: Medical records of patients (pts) with mMCC treated with ICIs from Aug 2015 to Dec 2018 at Peter MacCallum Cancer Centre in Australia were retrospectively analysed. RNA sequencing and immunohistochemistry for PD-L1, CD3 and Merkel cell polyomavirus (MCPyV) on tumor samples were performed. Baseline tumor volumes and responses were assessed with FDG-PET scans using the Hicks criteria. Results: 23 pts with mMCC were treated with ICIs. Pt characteristics are summarised in Table. A median of 8 cycles (range 1 to 47) were administered, with treatment ongoing in 7 pts. Objective responses (OR) were observed in 14 pts (61%); 10 (44%) complete metabolic responses (CMR) and 4 (17%) partial metabolic responses (PMR). Median time to response was 9 weeks (range 4 to 11) and 12-month progression-free survival (PFS) rate was 32%. Increased OR were seen in pts aged less than 75 (OR 8/10, 80% vs 46%), no prior history of chemotherapy (OR 10/14, 71% vs 44%), pts with an immune-related adverse event (irAE) (OR 6/6, 100% vs 47%) and in MCPyV negative pts (OR 9/11, 82% vs 50%). Pts with a CMR had lower mean-tumor volume on baseline FDG-PET scan (CMR: 35.7mL, no CMR: 187.8mL, p value 0.05). 10 pts received radiation (RT) during ICI: 4 pts started RT concurrently (OR 75%, CMR 50%), 3 pts had isolated ICI-resistant lesions successfully treated with RT and 3 pts with multisite progression continued to progress despite RT. 6 pts (26%) had a Grade 1-2 irAE. Conclusions: ICIs showed efficacy and safety consistent with trial data. Younger age, negative MCPyV status, no prior chemotherapy, lower baseline FDG-PET tumor volume and irAEs are potentially associated with better responses. [Table: see text]