In the ongoing search for novel immunotherapies that might rival or surpass the efficacy of immune checkpoint inhibitors (ICIs), drugs targeting IDO1—a key enzyme in tryptophan metabolism—have been a major focus in recent years.
These efforts suggest that the IDO pathway story is far from over.
Figure. The Role of IDO1 in the Tumor Microenvironment17 (Click to Enlarge)
Leveraging an Essential Amino Acid
Tryptophan is the rarest of the essential amino acids—those that must be acquired through the diet—and, as such, its metabolism is tightly regulated.13
Several biochemical pathways are involved in tryptophan breakdown, but the kynurenine (KYN) pathway predominates, converting tryptophan into biologically active metabolites, including the eponymous KYN.13-17
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