Increased amino acid catabolism is essential for tumor growth and immune regulation in cancer. This is especially true for the kynurenine metabolites of tryptophan, which exert diverse biological effects and have been investigated as markers of tumor progression and therapeutic effect. The kynurenine pathway has been associated mostly with immune tolerance and tumor escape [1]. Approximately 95% of the free L-Trp in the body is metabolised down the Kyn pathway, generating several biologically active metabolites, including kynurenine (Kyn), kynurenic acid, 3-hydroxykynurenine (3-HK), 3-hydroxyanthranilic acid (3-HAA) [2].
In this research the main tasks were to review low molecular weight (LMW) biomarkers of inflammation and cancer, which are derivatives of amino acids, e.g., derivatives of histidine, tryptophan, arginine, etc; to evaluate methods enabling transdermal detection of amino-acid derived LMW biomarkers of inflammation and cancer; to experimentally assess the relevance of some of these methods for detection of skin inflammation and cancer, e.g., skin penetration assays and cell culture methods.
Our results showed that the 3-HKYN and 3-HAA are relatively stable in the pH range 5.5-7.4. If we are going to make extraction of these compounds from skin at basic solutions we might need to study stability at pH 10 and higher. It is possible to measure penetration of these compounds through skin after lipid extraction from skin using electrochemical methods. Detection limits by electrochemistry are $1~\mu M$ or higher.