Metabolism of Compound A by Renal Cysteine-S-Conjugate beta-Lyase Is Not the Mechanism of Compound A-Induced Renal Injury in the Rat

Metabolic and Pharmaceutical Aspects of Fluorinated Compounds

The applications of fluorine in drug design continue to expand, facilitated by an improved understanding of its effects on physicochemical properties and the development of synthetic methodologies that are providing access to new fluorinated motifs. In turn, studies of fluorinated molecules are providing deeper insights into the effects of fluorine on metabolic pathways, distribution, and disposition. Despite the high strength of the C-F bond, the departure of fluoride from metabolic intermediates can be facile. This reactivity has been leveraged in the design of mechanism-based enzyme inhibitors and has influenced the metabolic fate of fluorinated compounds. In this Perspective, we summarize the literature associated with the metabolism of fluorinated molecules, focusing on examples where the presence of fluorine influences the metabolic profile. These studies have revealed potentially problematic outcomes with some fluorinated motifs and are enhancing our understanding of how fluorine should be deployed.

Cysteine conjugate beta-lyase-dependent biotransformation of the cysteine S-conjugates of the sevoflurane degradation product 2-(fluoromethoxy)-1,1,3,3,3-pentafluoro-1-propene (compound A)

2-(Fluoromethoxy)-1,1,3,3,3-pentafluoro-1-propene (1, Compound A) is a fluoroalkene formed by the base-catalyzed degradation of sevoflurane that is nephrotoxic in rats. Fluoroalkene 1 is a structural analog of other nephrotoxic haloalkenes that undergo glutathione S-conjugate formation and cysteine S-conjugate beta-lyase-dependent bioactivation to reactive intermediates. The present experiments were designed to study the beta-lyase-dependent biotransformation of S-[2-(fluoromethoxy)-1,1,3,3,3-pentafluoropropyl]-L-cysteine (4) and S-[2-(fluoromethoxy)-1,3,3,3-tetrafluoro-1-propenyl]-L-cysteine (5) by 19F NMR and UV spectroscopy and GC/MS. Incubation of cysteine S-conjugate 4 with rat kidney cytosol or a pyridoxal model system showed the formation of inorganic fluoride, pyruvate, and 2-(fluoromethoxy)-3,3,3-trifluoropropanoic acid (9), the expected products of a beta-lyase-catalyzed reaction. The ratio of fluoride to pyruvate ranged from 2.3 to 2.5. The amount of acid 9 formed in the rat kidney cytosol and the pyridoxal model system was, however, less than 5% of the amount of pyruvate formed. Incubation of conjugate 4 with rat kidney cytosol and analysis by 19F NMR spectroscopy showed resonances that were assigned to 3,3,3-trifluorolactic acid (10); the formation of acid 10 was observed in the pyridoxal model only after prolonged incubation (> 18 h). Lactic acid 10 was identified as a degradation product of acid 9. Cysteine S-conjugate 5 was not stable in pH 7.4 buffer and underwent a rapid cyclisation reaction (t1/2 approximately 5 min) to form 2-[1-(fluoromethoxy)-2,2,2-trifluoroethyl]-4,5-dihydro-1,3-thiazol e-4 -carboxylic acid (14). These data show that fluoroalkene 1-derived cysteine S-conjugates are substrates for renal beta-lyase and that acid 9 is formed as a terminal product. Acid 9 is, however, unstable and affords lactic acid 10 as a degradation product.