Dual MAO A and HSP90 inhibitors enhance anti-PD-1 therapy and suppress colorectal cancer.

INTRODUCTION: Immune checkpoint inhibitors (ICIs) have revolutionized cancer therapy, yet their efficacy remains limited by tumor resistance, immune-related adverse events, and poor response in microsatellite stable colorectal cancer (CRC). To address these challenges, dual monoamine oxidase A (MAO-A) and heat shock protein 90 (HSP90) inhibitors, MPT1B098 and MPT1B099, were developed and evaluated for their therapeutic potential and immune-modulatory efficacy in CRC. METHODS: Human and murine CRC cell lines were treated with MPT1B098 and MPT1B099 to assess their effects in cytotoxicity, cell migration, apoptosis, cell cycle arrest, and cell-surface PD-L1 expression. Western blotting was performed to evaluate key apoptotic and EMT-related markers. In vivo therapeutic efficacy and safety were evaluated using CT26 and MC38 subcutaneous syngeneic tumor models in BALB/c and C57BL/6 mice treated with MPT1B098, MPT1B099, and/or anti-PD1 antibodies. Tumor infiltration of CD8+ T cells and tissue histopathology were evaluated by immunohistochemistry and H&E staining. RESULTS: In vitro, MPT1B098 and MPT1B099 exhibited potent cytotoxicity against human and murine CRC cell lines with sub-micromolar IC50 values, effectively inhibiting cell growth and migration while inducing apoptosis. Both compounds decreased cell-surface PD-L1 expression and modulated key apoptotic and EMT markers on Western blot. In vivo, both compounds were well tolerated at 10 mg/kg and significantly suppressed tumor growth in microsatellite instable and stable models. Notably, anti-PD1 monotherapy was effective only in MSI models, whereas MPT1B098 and MPT1B099 showed efficacy in both MSI and MSS tumors, with combination therapy producing a modest synergistic effect. Immunohistochemistry revealed increased CD8+ T cell infiltration following treatment, particularly with MPT1B099 in combination with anti-PD1 antibodies. DISCUSSION: These findings highlight the dual anti-tumor and immune-modulatory properties of MPT1B098 and MPT1B099, mediated in part through PD-L1 downregulation and enhanced intratumoral T cell infiltration. Dual targeting of MAO-A and HSP90 represents a promising novel strategy to overcome immunotherapy resistance in MSS CRC and enhance the therapeutic efficacy of immune checkpoint blockade.