Erlotinib and Leflunomide Disrupt 6‐Formylindolo[3,2‐ <i>b</i> ]Carbazole Metabolism and Sensitize Keratinocytes to <scp>UVA</scp> Radiation‐Induced Apoptosis

BACKGROUND/PURPOSE: Phototoxicity is a common adverse effect triggered by systemic or topical drug treatments. It is mainly caused by drug-induced sensitization to UVA radiation, arising from either the drug's inherent photosensitizing potential or its interference with the metabolism of endogenous photosensitizers. A potent endogenous UVA sensitizer is 6-formylindolo[3,2-b]carbazole (FICZ), a tryptophan photoproduct formed in UVB-irradiated epidermal cells. By sequentially activating the aryl hydrocarbon receptor signaling pathway and inducing cytochrome P450 (CYP) 1A1 expression, FICZ induces its own degradation. Recently, we reported that the BRAF inhibitor vemurafenib interferes with CYP1A1 activity and sensitizes keratinocytes to FICZ/UVA-induced phototoxicity. Herein, we screened 12 clinical drugs, known to exhibit phototoxicity in patients, for their potential to interfere with the metabolism of (exogenous) FICZ and sensitize HaCaT keratinocytes to UVA-induced phototoxicity. METHODS: The UV-VIS absorption of the drugs was determined, and their effect on CYP1A1 activity and FICZ/UVA-triggered apoptosis was assessed in immortalized and primary human keratinocytes using 7-ethoxyresorufin-O-deethylase (EROD) and caspase-3 activity assays. Moreover, the impact of the candidate drugs on the metabolic degradation of FICZ in cells (LC analysis) as well as on the generation of oxidative stress (MitoSOX assay, qPCR analyses) was investigated. RESULTS: We identified two drugs, erlotinib and leflunomide, to sensitize human keratinocytes to FICZ/UVA-induced apoptosis by inhibiting CYP1A1 activity. Moreover, both drugs attenuated the metabolic breakdown of FICZ, enhanced the FICZ/UVA-triggered formation of mitochondrial superoxide anions, and increased heme oxygenase-1 (HMOX1) transcript levels, indicative of antioxidant defense activation. CONCLUSION: Disruption of FICZ metabolism may contribute to the phototoxicity of drugs.