Kynurenine:tryptophan ratio (K/T ratio)

The kynurenine:tryptophan ratio (K/T ratio) is a biomarker widely used to reflect the activity of the tryptophan-degrading enzymes, notably indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO), which mediate the catabolism of tryptophan along the kynurenine pathway[1][3][6][7][8]. This ratio increases in conditions of immune activation or inflammation, as both cytokines and other inflammatory mediators stimulate IDO expression, which then accelerates the conversion of tryptophan to kynurenine[3][8]. The K/T ratio is not a protein, enzyme, receptor, or direct molecular target but a metabolic indicator or biomarker for cellular immune activation, and is clinically studied in a wide range of conditions including infections, cancer, cardiovascular and neurodegenerative diseases, and psychiatric disorders[1][5][6][7][8]. The specificity of the ratio for IDO activity is limited, as it can be influenced by hepatic TDO and other metabolic processes, and its interpretation should consider these factors[1]. Direct interactions with drugs do not apply to the ratio itself; rather, drugs may target underlying enzymes (IDO, TDO), whose activity is then monitored by changes in the K/T ratio.

Other names

Kynurenine to tryptophan ratioKyn/Trp ratioK/T ratio