Abstract 2368: Domatinostat increases apoptosis, G2M cell-cycle arrest and immunogenicity of Merkel cell carcinoma

Merkel cell carcinoma (MCC) is a rare, highly aggressive skin cancer prevalent in elderly and immunocompromised patients. MCC is highly immunogenic as it is either associated with Merkel cell polyoma virus (MCPyV) integration or UV-associated mutations in non-viral cases. Indeed, immune checkpoint inhibitors (CPI) like PD-1/PD-L1 blocking antibodies exert a strong clinical activity; however, primary or secondary resistance often occurs. Domatinostat (4SC-202) is an orally available small molecule inhibitor targeting histone deacetylases (HDAC) class I currently in clinical evaluation to improve response to CPI (SENSITIZE, NCT03278665). In syngeneic tumor mouse models domatinostat treatment demonstrated immune-modulatory effects by increasing the intra-tumoral infiltration of cytotoxic CD8+ T cells (CTLs) and enhancing gene expression of a CPI response signature. Immune escape mechanisms described for MCC include low intra-tumoral levels of CTLs and reduced expression of antigen-presenting MHC class I molecules. Our previous data suggest that low MHC-I levels are reversible and involve epigenetic silencing of genes encoding the antigen-processing machinery (APM). Here, we present novel preclinical data about the efficacy and mode of action of domatinostat in MCC. Global gene expression by single cell RNA-seq revealed regulation of genes involved, among others, in apoptosis and antigen presentation. Importantly, domatinostat additionally inhibited proliferation of MCC cell lines by induction of a G2M cell-cycle arrest and apoptosis. Moreover, expression of MCPyV-encoded transforming early genes, particularly during the G1-phase, was inhibited by domatinostat. Thus, it exerts also a direct anti-tumoral effect. In viable cells, domatinostat increases their susceptibility to immune responses, as qPCR and immunoblot analysis confirmed the induction of APM and MHC-I expression by domatinostat. APM and MHC-I expression has been reported to be restored by an epigenetic drug combination (vorinostat plus mithramycin). In contrast, we now provide evidence that the single agent treatment with domatinostat alone is sufficient for increasing immunogenicity of MCC cells. In summary, domatinostat counteracts immune escape of MCC in many aspects suggesting a combination treatment of the HDACi domatinostat with CPI as a promising therapeutic strategy. Prospective clinical trials are needed to confirm this hypothesis. Citation Format: Lina Song, Anne Catherine Bretz, Jan Gravemeyer, Ivelina Spassova, Svetlana Hamm, Shakhlo Muminova, Rene Bartz, Juergen C. Becker. Domatinostat increases apoptosis, G2M cell-cycle arrest and immunogenicity of Merkel cell carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2368.