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[EN] Gefitinib (GFT) is a tyrosine kinase inhibitor currently used for the treatment of metastatic non-small cell lung cancer. Although it has been suggested that GFT can be phototoxic, there are no systematic studies on this issue. Here, the photosensitizing potential of GFT has been assessed by means of NRU assays and protein photooxidation. In addition, a thorough photophysical study is presented based on ultrafast transient absorption spectroscopy, fluorescence and laser flash photolysis. Transient species generated after excitation of GFT have been characterized in solution and in biological environments (i.e. HSA and HaCaT cells) to gain insight into the mechanisms involved in photodamage. The photobehavior of GFT was strongly medium-dependent. Excitation of the drug resulted in the formation of locally excited (LE) singlet states ((1)GFT*), which were found to be the main emissive species in non-polar solvents and also within HSA and HaCaT cells. By contrast, in polar solvents, LE states rapidly evolved (similar to 1 ps) towards the formation of longer-lived intramolecular charge transfer (ICT) states. The triplet excited state of GFT ((3)GFT*) can be formed through intersystem crossing from (1)GFT* in non-polar solvents and from ICT states in the polar ones, or in the particular case of ethanol, by photosensitization using 2-methoxyacetophenone as an energy donor. In the HSA environment, (3)GFT* was hardly detected due to quenching of its LE (1)GFT* precursor by Trp through an electron transfer process. Accordingly, HSA photooxidation by GFT was demonstrated using the protein carbonylation method. In summary, a good correlation is established between the photophysical behavior and the photobiological properties of GFT, which provides a mechanistic basis for the observed phototoxicity., Financial support from the Spanish Government (RYC-2015-17737, CTQ2017-89416-R, BEAGAL 18/00211, FPU19/00048 and PID2020-115010RB-I00; the Carlos III Institute (ISCIII) of Health, grants: PI16/01877, and CPII16/00052) and Conselleria d'Educacio, Cultura i Esport (PROMETEO/2017/075) is gratefully acknowledged.