Development of skin inflammation and cancer in viable epidermis and dermis involve slow molecular weight (LMW) metabolites. We hypothesize that these LMW compounds can be collected on the surface of the skin and used for non-invasive diagnostics of skin disorders. Keeping in mind that substantial transdermal penetrationis achieved only for molecules of < 500 Da, we focused on topical monitoring of LMW biomarkers. In this thesis we investigated non-invasive, topical methods for monitoring LMW biomarkers by relevant in vitro and in vivo experiments. The LMW biomarkers were:

- reactive oxygen species (ROS), specifically, hydrogen peroxide, H2O2

- amino acids and their derivatives, i.e., tryptophan (Trp), kynurenine (Kyn; a Trpderivative), phenylalanine (Phe), and tyrosine (Tyr; a Phe derivative).

Initially, we have carried out in vitro experiments using dermatomed porcine skin and cell cultures. We characterized permeability of the biomarkers through skin and assessed methods of their monitoring. By using Prussian white particles, deposited on porcine skin, we demonstrated that hydrophilic biomarkers, such as H2O2, permeate the skin mainly through hair follicle pathways (Paper I). In paper II, we have showed that the enzymes transforming Trp to the inflammation and cancer biomarker Kyn, are expressed in the basal layer of epidermis. The magnitude of changes of the Trp/Kyn ratio in the cell culture model was assessed. In paper III, we have characterized Trp and Kyn permeability through skin in vitro, concluding that their permeabilities through stratum corneum are comparable. By in vivo experiments outlined in Paper IV, we have demonstrated the feasibility of topical, non-invasive sampling of Trp and Kyn, in relation to other amino acids. Kyn detection was compromised by its low abundance on the skin. In paper V, we performed a proof-of-concept study in vivo and confirmed that non-invasive sampling of Trp and amino acids of similar abundance, such as Phe and Tyr, is more robust. We concluded that Phe/Trp ratio might be equally good biomarker of skin disorders as a predicted Trp/Kyn ratio. Summarizing, the results of this thesis provide basic knowledge for deeper clinical studies of non-invasive, topical sampling of hydrophilic LMW biomarkers of skin inflammation and cancer.