AHR and melanoma: In conclusion, the selected tryptophan-derived AhR ligands, KYN, KYNA and FICZ, produced endogenously in the skin and present in herbs and plant extracts used in skin care treatments, did not promote melanoma cell growth in vitro, and even in higher concentrations they may exhibit antiproliferative and cytotoxic activity and promote cell death in melanoma A375 and RPMI7951 cells.