STC-15, an oral small molecule inhibitor of the RNA methyltransferase METTL3, inhibits tumour growth through activation of anti-cancer immune responses associated with increased interferon signalling, and synergises with T cell checkpoint blockade

Background: METTL3 is an RNA methyltransferase responsible for the deposition of N-6-methyladenosine (m6A) modification on mRNA and long non-coding RNA (lncRNA) targets, to regulate their stability, splicing, transport and translation. Small molecule inhibitors of METTL3 catalytic activity have previously demonstrated direct anti-tumour efficacy in models of acute myeloid leukemia (AML). Here we present pre-clinical data showing that the orally bioavailable small molecule METTL3 inhibitor STC-15 inhibits cancer growth and induces anti-cancer immunity. Materials & Methods: To characterise transcriptomic changes following METTL3 inhibition, RNA sequencing studies were performed across a panel of cancer cell lines treated with STC-15. Induction of specific genes was validated by qPCR and Western Blots. The functional consequence of the upregulation of innate immune pathways was investigated in vitro using a coculture system of SKOV3 ovarian cancer cells and human peripheral blood mononuclear cells (PBMC), and animal studies using subcutaneous A20 and MC38 syngeneic tumour models. Results: Inhibition of METTL3 by STC-15 in cancer cell lines leads to prominent upregulation of genes associated with innate immunity, such as those in the interferon (IFN) signalling pathway. Transcription of type-I and type-III IFNs was activated following STC-15 treatment, in agreement with the expression of many Interferon Stimulated Genes (ISG). Cells treated with STC-15 accumulated double-stranded RNA (dsRNA), suggesting that activation of IFN signalling is triggered by innate pattern recognition sensors. In an in vitro co-culture system, STC-15 demonstrated strong and dose-dependent enhancement of PBMC-mediated killing of cancer cells that occurred at concentrations where STC-15 caused little or no direct killing of cancer cells in the absence of PBMCs. In MC38 colorectal and A20 lymphoma syngeneic models, oral treatment of immune-competent tumour bearing mice with STC-15 significantly inhibited tumour growth. Combination of STC-15 with anti-PD1 antibody resulted in significant tumour regression in both models, with mice remaining tumour-free until the end of study, long after treatment ceased. Even when regressed mice from the A20 model were re-challenged with a new batch of A20 cells, no new tumour growth was observed, further demonstrating the induction of durable anti-tumour immunity. Conclusions: In pre-clinical cancer models, STC-15 treatment results in activation of innate immune pathways, inhibits tumour growth and enhances the anti-tumour properties of anti-PD1 therapy to generate a durable antitumour immune response. These data provide the rationale for the development of STC-15 both as monotherapy and in combination with checkpoint inhibition for the treatment of solid tumour malignancies. A Phase I, First-in-Human clinical trial is planned to begin in 2022. Conflict of interest: Ownership: MA is a stockholder of Storm Therapeutics. Other Substantive Relationships: YOR, LV, CS, AS, GT, MA, JBH and OR are current/former employees/consultants of Storm Therapeutics. MC and JSF are employees of Charles River.