Computational Studies of the Photodegradation Mechanism of the Highly Phototoxic Agent Benoxaprofen

Computational quantum chemistry within the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) framework is used to investigate the photodegradation mechanism as well as the photochemical and photophysical properties of benoxaprofen (BP), a non steroid anti-inflammatory molecule (2-[2-(4-chlorophenyl)-1,3-benzoxazol-5-yl] propanoic acid). BP is a highly phototoxic agent that causes cutaneous phototoxicity shortly after its administration. On the grounds of concern about serious side effects, especially hepatotoxicity, it was withdrawn from the world market after only 2 years of its release. Our study shows that the drug has the capability to absorb radiation in the UV region, mainly between 300 and 340 nm, and undergoes spontaneous photoinduced decarboxylation from the triplet state. It shows very similar photochemical properties to the highly photolabile non-steroidal anti-inflammatory drugs (NSAIDs) ketoprofen, suprofen, and tiaprofenic acid. Like ketoprofen, BP can also decarboxylate from excited singlet states by overcoming low energy barriers. The differences in molecular orbital (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) distributions between the neutral and deprotonated BP, their absorption spectra, and the energetics and fate of various photoproducts produced throughout the photodegradation are discussed. Initiation and termination of decarboxylated BP radical species and initiation of propagating lipid peroxidation reactions due to the addition of molecular oxygen giving rise to the corresponding peroxyl radical are also explored in detail.