Putting bioactivation reactions to work: Targeting antioxidants to mitochondria

The goal of this research was to test the hypothesis that bioactivation reactions could be exploited to deliver and activate mitochondria-targeted antioxidant prodrugs. The concept that bioactivation reactions could be used for prodrug delivery and activation has received little attention. Most bioactivation reactions result in the conversion of the parent drug to a reactive electrophilic metabolite, but bioactivating enzymes that catalyze elimination or hydrolytic reactions may offer potential for targeted drug delivery. Because mitochondria are the major cellular source of reactive oxygen species, there is much interest in targeting antioxidants to mitochondria. Previous studies showed that the mitochondrial fatty acid β-oxidation pathway biotransforms a range of xenobiotic alkanoates, including ω-(phenyl)alkanoates and ω-(phenoxy)alkanoates. 5,6-Dichloro-4-thia-5-hexenoate, the desamino analog of S-(1,2-dichlorovinyl)-l-cysteine, is biotransformed by the fatty acid β-oxidation pathway. Hence, the prodrugs ω-(phenoxy)alkanoates, 3-(phenoxy)acrylates, and ω-(1-methyl-1H-imidazol-2-ylthio)alkanoates were expected to undergo biotransformation by the mitochondrial β-oxidation pathway to release phenolic antioxidants and the antioxidant methimazole (Roser et al., Bioorg. Med. Chem. 18 (2010) 1441-1448). The rates of biotransformation of ω-(phenoxy)alkanoates varied with the structure, and bulky substituents on the phenoxy moiety reduced rates of biotransformation; this was attributed to substrate limitations imposed by the medium-chain acyl-CoA dehydrogenase. Hence, 3-(2,6-dimethylphenoxy)acrylate was prepared; it was expected that, after conversion to its CoA thioester, 3-(2,6-dimethylphenoxy)acryloyl-CoA would be a substrate for enoyl-CoA hydratase. This expectation was correct: 3-(2,6-dimethylphenoxy)acrylate was an excellent substrate. ω-(1-Methyl-1H-imidazol-2-ylthio)alkanoates were also good substrates for the β-oxidation pathway. Significantly, 3-(2,6-dimethylphenoxy)propanoate, 3-(2,6-dimethylphenoxy)acrylate, and 3-(1-methyl-1H-imidazol-2-ylthio)propanoate were cytoprotective in a hypoxia-reoxygenation model in rat cardiomyocytes. These results demonstrate the feasibility of exploiting bioactivation reactions for targeted drug delivery.