L-Kynurenine ((S)-Kynurenine) is a key intermediate in the breakdown pathway of tryptophan. L-Kynurenine is a substrate of kynureninase, KMO, and KAT associated with the suppression of antitumor immune responses. It has been shown to inhibit allogeneic T-cell proliferation and to increase malignant U87 glioma cell invasion into a collagen matrix.

Kynurenine and its further breakdown products carry out diverse biological functions, including dilating blood vessels during inflammation and regulating the immune response. Some cancers increase kynurenine production thereby increasing tumor growth. L-kynurenine (Kyn) is an aryl hydrocarbon receptor (AHR) agonist that activates AHR-directed, naive T cell polarization to the anti-inflammatory Treg phenotype. Kynurenine activates AHR signaling at physiological concentrations in H1L7.5c3 cells and acts as an AHR agonist after a 24-hr exposure by inducing the AHR-regulated luciferase gene in H1L7.5c3 mouse hepatocyte cells.

Kynurenine expands rat and human arteries by activating Kv7 channels in the vascular smooth muscle, a process that leads to rat hypotension, partially reversible by inhibiting these channels. When L-kynurenine is given one hour before hypoxia-ischemia, it significantly prevents neurological damage in a dose-dependent manner, achieving full protection at 300 mg/kg. Additionally, this dosage inhibits the activation of c-fos in the cerebral cortex.