Photosensitization of Thymine Nucleobase by Benzophenone Derivatives as Models for Photoinduced DNA Damage: Paterno-Bu 1 chi vs Energy and Electron Transfer Processes Susana Encinas, † Noureddine Belmadoui, † Maria J. Climent, † Salvador Gil, ‡ and Miguel A. Miranda* ,† Instituto de Tecnologı ´a Quı ´mica UPV-CSIC/Departamento de Quı ´mica, Universidad Polite ´ cnica de Valencia, Avda los Naranjos s/n, 46022 Valencia, Spain, and Departamento de Quı ´mica Orga ´ nica, Universidad de Valencia, 46100 Burjassot, Valencia, Spain Received December 1, 2003 Time-resolved and product studies have shown that there is a strong interaction between drugs containing the benzophenone chromophore and the free thymidine nucleoside. In quantitative terms, such an interaction is stronger for the lowest lying nπ* triplet states (S- ketoprofen) than for mixed nπ*-ππ* triplets (fenofibrate and fenofibric acid), as indicated by the quenching rate constants. This is consistent with a Paterno-Bu ¨ chi photoreaction, where the initial step is the formation of a new bond between the excited carbonyl oxygen and one of the thymine olefinic carbons. Actually, oxetanes are obtained as photoproducts when benzo- phenone is irradiated in the presence of thymidine. Hence, triplet-triplet energy transfer resulting in formation of cyclobutane pyrimidine dimers, which would be thermodynamically disfavored, does not seem to play a major role. However, in DNA, the contribution of energy transfer could be higher, due to the lower energy of the thymine triplet in the biomacromolecule. These results are discussed in connection with the observed DNA damage upon photosensi- tization with ketoprofen, fenofibrate, and fenofibric acid. Introduction Chemical research on the phototoxicity of drugs has received considerable attention in the past decade, prompted by reports on the photosensitivity side effects caused by certain active ingredients in pharmaceutical products. Those related to the BP 1 chromophore have been studied in detail (1, 2). KP and FB are two drugs of similar structure (Chart 1) with antiinflammatory and hypolipidemic activities, respectively; they are derived from BP and display photophysical properties characteristic of this chro- mophore (3-7). Both drugs are photoallergic and photo- toxic (8-19), and their photosensitizing properties toward biological targets have been widely investigated (1, 2, 20- 24). They induce photooxidative DNA damage (including strand breaks and base lesions) and photosensitize formation of cyclobutane thymine (Thy) dimers (25-29). These photobiological properties are attributable to the common BP substructure, as the parent compound BP photosensitizes similar DNA reactions (1). A satisfactory understanding of the photochemical reactivity of phototoxic drugs is essential to anticipate the photobiological risk associated with their use by patients. In this context, the photochemistry of KP and FB, like that of other aromatic ketones with an efficient intersystem crossing, is dominated by their triplet excited states. The lowest triplet of KP and FA, the main metabolite of FB, is of nπ* nature with characteristics similar to those of BP (7). Depending on the excited state energy, triplets may be able to photoinduce the formation of cyclobutane Thy dimers, which are at the origin of biological effects such as mutagenesis and carcinogenesis (29-31). Such dimers can also be obtained by direct UV irradiation, and their chemical structures have been well- established (32-35). Only a limited number of articles have dealt with the mechanistic aspects of ketone photosensitization of thy- midine (dThd). On the basis of product analysis, it has been hypothesized that an electron transfer process takes place between dThd and the triplet state of BP (36). This is followed by nucleophilic trapping or deprotonation of the Thy radical cation. By contrast, in the BP photo- * To whom correspondence should be addressed. E-mail: mmiranda@qim.upv.es. † Universidad Polite ´cnica de Valencia. ‡ Universidad de Valencia. 1 Abbreviations: KP, ketoprofen; FB, fenofibrate; BP, benzophenone; FA, fenofibric acid; PMT, photomultiplier tube; NOE, nuclear Over- hauser effect. Chart 1. Chemical Structures of BP, S-KP, FA, and FB 857 Chem. Res. Toxicol. 2004, 17, 857-862 10.1021/tx034249g CCC: $27.50 © 2004 American Chemical Society Published on Web 06/10/2004