STC-15, a novel METTL3 inhibitor, and its combination with Venetoclax confer anti-tumour activity in AML models

Lina Vasiliauskaitė¹, Yaara Ofir-Rosenfeld¹, Mark Albertella¹, Coralie Hoareau-Aveilla², Jerry McMahon¹, Oliver Rausch¹

¹Storm Therapeutics Ltd., Cambridge, UK

²Evotec SAS, Toulouse, FR

N6-methyladenosine (m⁶A) is one of the most abundant RNA modifications, which influences mRNA and lncRNA localization, half-life, translation, and splicing. The majority of m⁶A modifications on cellular mRNAs are deposited by the RNA methyltransferase METTL3. To date, METTL3 has been implicated in the initiation and progression of multiple cancer types, with the highest expression of METTL3 mRNA observed in acute myeloid leukemia (AML). Currently, one line of standard of care therapy for AML patients is Venetoclax, which targets the anti-apoptotic protein BCL2. It was shown that m⁶A, deposited by METTL3 on BCL2 transcript, affects BCL2 mRNA stability and translation. Storm Therapeutics has developed potent and selective METTL3 inhibitors, including the clinical candidate STC-15. Here, we explore pharmacological inhibition of METTL3 as monotherapy or in combination with Venetoclax in AML models in vitro and in vivo.

Sulforhodamine B and CellTiterGlo^TM assays were used to assess the viability of AML cell lines and patient-derived xenografts (PDXs), respectively, following METTL3 inhibition in vitro. BCL2 protein level was evaluated by Western blotting. SynergyFinder software was used to assess the degree of synergy between METTL3 inhibitors and Venetoclax. Intra-tibial implantation of human-derived AML cells (AML-PDXs) in NSG mice was used to determine single agent and combination therapy efficacy.

Multiple AML cells lines and AML-PDXs were sensitive to pharmacological inhibition of METTL3 in vitro, as assessed by loss of viability. Treatment with METTL3 inhibitors led to downregulation of BCL2 protein level in several AML cell lines, as previously suggested by literature. Based on these results, the synergy between METTL3 inhibition and Venetoclax was assessed. Matrix-combination experiments have shown a high degree of synergy between the two drugs (defined by a synergy score >10) in THP-1 and MOLM-13 cell lines.

To test METTL3 inhibition as a monotherapy and in combination with Venetoclax in vivo, three AML-PDX studies were initiated. Significantly lower spleen weight was observed in all animals treated with STC-15 or STC-15 + Venetoclax, and reduced number of circulating hCD45⁺ cells was observed in 2 out of the 3 models. In one of the models, STC-15 monotherapy outperformed Venetoclax (median survival 68 days vs 58 days, respectively), while the combination therapy extended median group survival to 85 days in comparison to 51.5 days in the vehicle group.

In conclusion, we demonstrated that METTL3 inhibition results in anti-tumour effects across different AML models. Moreover, we demonstrated a synergistic effect between the novel METTL3 inhibitor STC-15 and Venetoclax, both in vitro and in vivo. These studies provide evidence for the utility of METTL3 inhibitors as a new therapeutic agent to treat AML. Currently, STC-15 is under clinical development (NCT05584111).

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