James DeCaprio1,2,3 1 . 2 . 3 . 4 . 5

Merkel cell carcinoma (MCC) is an aggressive neuroendocrine carcinoma of the skin with 2 etiologies. Merkel cell polyomavirus (MCPyV) integration is present in about 80% of all MCC. Viruspositive MCC (MCCP) tumors have few somatic mutations and usually express WT p53 (TP53). By contrast, virus-negative MCC (MCCN) tumors present with a high tumor mutational burden and predominantly UV mutational signature. MCCN tumors typically contain mutated TP53. MCCP tumors express 2 viral proteins: MCPyV small T antigen and a truncated form of large T antigen. MCPyV ST specifically activates expression of MDM2, an E3 ubiquitin ligase of p53, to inhibit p53mediated tumor suppression. In this study, we assessed the efficacy of milademetan, a potent, selective, and orally available MDM2 inhibitor in several MCC models. Milademetan reduced cell viability of WT p53 MCC cell lines and triggered a rapid and sustained p53 response. Milademetan showed a dose-dependent inhibition of tumor growth in MKL-1 xenograft and patient-derived xenograft models. Here, along with preclinical data for the efficacy of milademetan in WT p53 MCC tumors, we report several in vitro and in vivo models useful for future MCC studies.

Automated summary

In this study, the efficacy of milademetan, an MDM2 inhibitor, was assessed in various MCC models, showing significant inhibition on WT p53 MCC tumors. Useful in vitro and in vivo models for future MCC studies were also reported.