Current state on tryptophan 2,3-dioxygenase inhibitors: a patent review

Introduction: Tryptophan-2, 3-dioxygenase (TDO2) is a tryptophan-degrading enzyme constitutively expressed in the liver and to a lesser extend in the brain. Before its link to cancer immunotherapy was discovered in 2011, the search for TDO2 inhibitors was initially driven by depression therapy. In the recent years, TDO2 has drawn an increasing attention as a promising target in both cancer and neuropsychiatric diseases.Areas covered: Patent literature regarding Tryptophan-2, 3-dioxygenase inhibitors is reviewed. Compounds are categorized by chemical structure. Representative examples of each category are presented with their inhibitory activity and, when available, structure-activity relationships. Data from patent literature is deepened with relevant peer-reviewed literature.Expert opinion: Very few selective and potent inhibitors were to this day reported and there are currently no TDO2 inhibitors in clinical trials. Despite the challenges in their discovery, the search for TDO2 inhibitors is a very active area of research, as such molecules may prove to be of great interest in not only cancer immunotherapy drug arsenal, but also in neurodegenerative diseases.