Definition

An inhibitor of indoleamine 2,3-dioxygenase 1 (IDO1; IDO-1), with potential immunomodulating and antineoplastic activities. Upon intra-tumoral administration, IDO1 inhibitor RiMO-401 specifically targets and binds to IDO1, a cytosolic enzyme responsible for the oxidation of the amino acid tryptophan into the immunosuppressive metabolite kynurenine. By inhibiting IDO1 and decreasing kynurenine in tumor cells, RiMO-401 restores and promotes the proliferation and activation of various immune cells, including dendritic cells (DCs), natural killer (NK) cells, and T lymphocytes, and causes a reduction in tumor-associated regulatory T cells (Tregs). Re-activation of the immune system, which is suppressed in many cancers, may induce a cytotoxic T-lymphocyte (CTL) response against the IDO1-expressing tumor cells, thereby inhibiting the growth of IDO1-expressing tumor cells. IDO1, overexpressed by multiple tumor cell types, plays an important role in immunosuppression. Tryptophan depletion inhibits T-lymphocyte proliferation and activation, and subsequently suppresses the immune system.