In vitro studies on DNA-photosensitization by different drug stereoisomers

The possible stereoselectivity in DNA-photosensitization by carprofen (a NSAID drug) and ofloxacin (a fluoroquinolone agent) was investigated. The different drug stereoisomers or racemic mixtures were UVA-irradiated and the relaxation of the supercoiled circular pBR322 quantified by electrophoresis. Formation of single strand breaks was compared for each group of compounds. Moreover a mechanistic study by means of repair enzymes: T4 endonuclease V (specific of cyclobutane pyrimidine dimers), E. coli endonuclease III (revealing oxidized pyrimidines) and E. coli Formamidopyrimidine-DNA glycosylase (revealing oxidized purines) provided further insights into a possible stereoselectivity of the different reaction pathways in drug photosensitized-DNA damage. Ofloxacin and levofloxacin (its S stereoisomer) were responsible of single strand breaks formation as well as oxidation of pyrimidine and purine bases. No pyrimidine dimers were observed. Racemic, R and S stereoisomers of carprofen were less efficient than ofloxacin in DNA single strand breaks formation and did not induce enzyme-sensitive sites. The photoproducts distribution of drug-photosensitized reactions of 2'-deoxyguanosine and thymidine were established by HPLC as fingerprints for assignment of the DNA-photosensitization mechanism. Both Type I and Type II mechanisms were assigned to nucleoside-photosensitization by ofloxacin and levofloxacin. In the case of carprofen, a weak nucleoside degradation was obtained. The data suggest that levofloxacin, the (S) stereoisomer, might be slightly more efficient than racemic ofloxacin. In the case of carprofen the (S) isomer appears to be somewhat less active than its (R) enantiomer. However, due to the small differences found, the possible stereoselectivity has to be confirmed by future studies.