Repurposing Masitinib Mesylate as a Novel FOXM1 Inhibitor for the Treatment of Aggressive Solid Tumors: Preclinical Validation in Human Breast and Oral Cancer Cells and Organotypic Tumor Slice Culture.

The Forkhead box M1 (FOXM1) transcription factor, recognized as a prominent oncogene, facilitates cell cycle progression and mitotic fidelity in aggressive cancers. Overexpression of FOXM1 is correlated with uncontrolled cell proliferation, metastasis, and poor prognosis. Despite its pivotal role in promoting genomic instability, cancer stemness, and metastasis, FOXM1 remains a clinically undruggable target. In this study, we identified masitinib mesylate, a small-molecule inhibitor known for its multitargeted tyrosine kinase inhibition, targeting c-Kit, PDGFRα, and PDGFRβ, as well as Lyn, which demonstrates anticancer activity in preclinical models. Masitinib mesylate emerges as a potent, novel first-in-class small-molecule inhibitor of FOXM1. In silico prediction using pharmacophore-based virtual screening revealed that masitinib mesylate binds with high affinity to the FOXM1 DNA-binding domain (DBD) through hydrogen bonding with residues at His287 and Leu291, as well as a salt bridge with Asp293, effectively stabilizing the FOXM1-DNA interface and perturbing its activity. In vitro validation in the triple-negative breast cancer cell line MDA-MB-231 and the highly aggressive oral cancer cell line HSC-3 demonstrated that masitinib mesylate significantly reduces FOXM1 protein expression, thereby contributing to reduced cancer cell proliferation, stemness, and migration. Importantly, we validated these findings using ex vivo human primary tumor slice cultures from patients with breast cancer and oral squamous cell cancer. Treatment with masitinib mesylate significantly induced apoptosis within the native tumor microenvironment, confirming its efficacy in heterogeneous, patient-derived tissues. Our findings establish a novel mechanistic link between masitinib mesylate and FOXM1 inhibition by DNA binding, providing a compelling rationale for drug repurposing, offering a targeted therapeutic strategy to disrupt the FOXM1-driven regulatory network in aggressive, therapy-resistant cancers. In silico analyses predict a potential interaction of masitinib with the FOXM1 DNA-binding domain (DBD); however, further experimental validation is required to confirm direct binding.