HDACi combination therapy with IDO1i remodels the tumor microenvironment and boosts antitumor efficacy in colorectal cancer with microsatellite stability.

Background: Immunotherapy for colorectal cancer (CRC) with microsatellite stability (MSS) and mismatch repair proficiency (pMMR) has shown limited success in clinical trials. The combination of immunomodulators and immune checkpoint inhibitors (ICIs) is a potential strategy for treating CRC.

Methods: Histone deacetylase (HDAC) and indoleamine 2,3-dioxygenase 1 (IDO1) expression in CRC tissues and adjacent normal tissues was analyzed via database analysis, immunohistochemistry, and western blotting. A nanodrug designated as NP-I/P was subsequently formulated, encapsulating an IDO1 inhibitor (IDO1i; namely, epacadostat) and an immunomodulatory HDAC inhibitor (HDACi; namely, panobinostat). The antitumor efficacy of the nanoparticles and their effects on tumor microenvironment features were evaluated via in vitro and in vivo experiments.

Results: In the present study, we found that HDAC overexpression and IDO1 expression were attenuated in MSS/pMMR CRC. Thus, a nanodrug designated as NP-I/P was formulated to encapsulate epacadostat and panobinostat. In vitro, NP-I/P treatment promoted the apoptosis of tumor cells and induced the release of damage-associated molecular patterns, thereby leading to cell death-associated immune activation. The in vivo results revealed that NP-I/P treatment reversed the immunosuppressive phenotype of the microenvironment by inducing tumor immunogenic cell death (ICD), promoting CD8⁺ T cell infiltration, and reducing the numbers of Tregs, tumor-associated macrophages, and myeloid-derived suppressor cells. Finally, the results of the patient-derived xenograft and patient-derived organoid models demonstrated that NP-I/P treatment triggered tumor cell death and modulated the immune microenvironment in human CRC.

Conclusion: The combination of IDO1 and HDAC inhibitors represents a promising strategy for CRC treatment, and NP-I/P is a candidate for clinical trials.