The article "Tryptophan metabolism as a common therapeutic target in cancer, neurodegeneration and beyond" by Platten et al. (2019) reviews the role of tryptophan (Trp) metabolism through the kynurenine pathway (KP) in various physiological and pathological processes. The KP, which converts Trp into kynurenine (Kyn) and its metabolites, plays a crucial role in immunity, neuronal function, and intestinal homeostasis. Imbalances in Trp metabolism have been linked to disorders such as cancer, neurodegenerative diseases, and autoimmune diseases. Key enzymes in the KP include indoleamine-2,3-dioxygenase 1 (IDO1), tryptophan-2,3-dioxygenase (TDO), kynurenine monooxygenase (KMO), and kynurenine aminotransferases (KATs). These enzymes are potential therapeutic targets due to their involvement in immune regulation, neuroprotection, and disease progression. For instance, IDO1 inhibitors have shown promise in early-stage cancer immunotherapy, but phase III trials have been disappointing. The review highlights the physiological and pathophysiological roles of Trp metabolism, emphasizing the therapeutic opportunities and challenges. It discusses the involvement of Trp metabolites in neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD), and neuropsychiatric disorders like major depressive disorder and schizophrenia. The role of Trp metabolism in infectious diseases and autoimmune diseases is also explored, including the potential of targeting KP enzymes to modulate immune responses and pathogen load. The article concludes by summarizing the current understanding of the KP's role in aging and its potential as a therapeutic target, while acknowledging the need for further research to optimize drug development and clinical translation.The article "Tryptophan metabolism as a common therapeutic target in cancer, neurodegeneration and beyond" by Platten et al. (2019) reviews the role of tryptophan (Trp) metabolism through the kynurenine pathway (KP) in various physiological and pathological processes. The KP, which converts Trp into kynurenine (Kyn) and its metabolites, plays a crucial role in immunity, neuronal function, and intestinal homeostasis. Imbalances in Trp metabolism have been linked to disorders such as cancer, neurodegenerative diseases, and autoimmune diseases. Key enzymes in the KP include indoleamine-2,3-dioxygenase 1 (IDO1), tryptophan-2,3-dioxygenase (TDO), kynurenine monooxygenase (KMO), and kynurenine aminotransferases (KATs). These enzymes are potential therapeutic targets due to their involvement in immune regulation, neuroprotection, and disease progression. For instance, IDO1 inhibitors have shown promise in early-stage cancer immunotherapy, but phase III trials have been disappointing. The review highlights the physiological and pathophysiological roles of Trp metabolism, emphasizing the therapeutic opportunities and challenges. It discusses the involvement of Trp metabolites in neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD), and neuropsychiatric disorders like major depressive disorder and schizophrenia. The role of Trp metabolism in infectious diseases and autoimmune diseases is also explored, including the potential of targeting KP enzymes to modulate immune responses and pathogen load. The article concludes by summarizing the current understanding of the KP's role in aging and its potential as a therapeutic target, while acknowledging the need for further research to optimize drug development and clinical translation.