Abstract / Summary
Abstract Harmine and its derivatives, a class of β-carboline alkaloids, are promising natural antitumor agents with drug resistance–modulating activities. Emerging evidence suggests that these small molecules can reshape the tumor microenvironment (TME) and modulate antitumor immunity. Immune checkpoint blockade (ICB) therapies provide durable benefit in a subset of colorectal cancers but remain largely ineffective in microsatellite-stable (MSS) subtype, which accounts for approximately 85% of cases. This lack of response is driven by an immune-desert TME characterized by defective antigen presentation and poor cytotoxic T cell infiltration. Whether harmine derivatives can enhance ICB efficacy in MSS colorectal cancer remains unknown, and the underlying molecular mechanisms are poorly defined. We therefore evaluated how ACB1801, a synthetic derivative of the natural harmine, reprograms the TME to restore sensitivity to anti-Programmed cell death protein 1 (PD-1) therapy in MSS CRC. In murine MSS CRC models, ACB1801 markedly enhanced the antitumor efficacy of anti–PD-1, resulting in increased intratumoral CD8⁺ T cell infiltration and reduced regulatory T cell accumulation. Integrated transcriptomic analyses of sorted CD45⁺ immune and CD45⁻ tumor compartments revealed that combination therapy induced coordinated metabolic reprogramming, characterized by suppression of glycolysis and activation of ferroptosis-related pathways. These findings were validated in murine and human MSS CRC cell lines, where ACB1801 reduced glycolytic flux and increased ferroptotic susceptibility. Mechanistically, ACB1801 enhanced C-X-C motif chemokine ligand 10 (CXCL10) secretion in a STAT1-dependent manner and upregulated NOD-like receptor family CARD domain containing 5 (NLRC5)-driven MHC class I antigen presentation, thereby increasing tumor immunogenicity. Together, these effects convert immunologically “cold” MSS tumors into an immune-responsive state. Collectively, our findings position ACB1801 as a clinically relevant metabolic-immunomodulatory agent and provide strong preclinical justification for combining harmine derivatives with anti–PD-1 therapy to overcome resistance in MSS CRC patients.