1373 STC-15, an oral small molecule inhibitor of the RNA methyltransferase METTL3, inhibits tumour growth through activation of anti-cancer immune responses and synergises with immune checkpoint blockade

BackgroundMETTL3 is an RNA methyltransferase responsible for the deposition of N-6-methyladenosine (m6A) modification on mRNA and long non-coding RNA (lncRNA) transcripts, 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 STC-15, an orally bioavailable small molecule inhibitor of METTL3, restrains cancer growth and induces anti-cancer immunityMethodsTo characterise transcriptomic changes following METTL3 inhibition, RNA sequencing studies were performed on several 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 co-culture system of SKOV3 ovarian cancer cells and human peripheral blood mononuclear cells (PBMC) or purified primary CD8+ T-cells, and animal studies using subcutaneous A20 and MC38 mouse syngeneic tumour modelsResultsInhibition of METTL3 by STC-15 in cancer cell lines leads to prominent upregulation of genes associated with innate immunity, including type-I and type-III IFNs, as well as many interferon stimulated genes. Cells treated with STC-15 accumulated double-stranded RNA 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. Similar results were obtained when replacing PBMC with purified CD8+ T-cells. In MC38 colorectal and A20 lymphoma syngeneic models, oral treatment of immune-competent tumour bearing mice with STC-15 significantly inhibited tumour growth. In vivo depletion of CD8+ T-cells abrogated the response to STC-15. Combination of STC-15 with anti-PD1 antibody resulted in tumour regression in both models, with mice remaining tumour-free long after treatment ceased. 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 immunityConclusionsIn pre-clinical cancer models, STC-15 treatment results in activation of innate immune pathways, inhibits tumour growth via activation of CD8+ T-cell mediated tumour cell killing, and enhances the anti-tumour properties of anti-PD1 therapy to generate a durable anti-tumour 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 2022Ethics ApprovalAnimal welfare for this study complies with the UK Animals Scientific Procedures Act 1986 (ASPA) in line with Directive 2010/63/EU of the European Parliament and the Council of 22 September 2010 on the protection of animals used for scientific purposes.