HUMAN KIDNEY METHOXYFLURANE AND SEVOFLURANE METABOLISM - INTRARENAL FLUORIDE PRODUCTION AS A POSSIBLE MECHANISM OF METHOXYFLURANE NEPHROTOXICITY

Background: Methoxyflurane nephrotoxicity is mediated by cytochrome P4 50-catalyzed metabolism to toxic metabolites. It is historically accep ted that one of the metabolites, fluoride, is the nephrotoxin, and tha t methoxyflurane nephrotoxicity is caused by plasma fluoride concentra tions in excess of 50 mu M. Ssvoflurane also is metabolized to fluorid e ion, and plasma concentrations may exceed 50 mu M, yet sevoflurane n ephrotoxicity has not been observed. It is possible that in situ renal metabolism of methoxyflurane, rather than hepatic metabolism, is a cr itical event leading to nephrotoxicity. We tested whether there was a metabolic basis for this hypothesis by examining the relative rates of methoxyflurane and sevoflurane defluorination by human kidney microso mes. Methods: Microsomes and cytosol were prepared from kidneys of org an donors. Methoxyflurane and sevoflurane metabolism were measured wit h a fluoride-selective electrode. Human cytochrome P450 isoforms contr ibuting to renal anesthetic metabolism were identified by using isofor m-selective inhibitors and by Western blot analysis of renal P450s in conjunction with,metabolism by individual P450s expressed from a human hepatic complementary deoxyribonucleic acid library.Results: Swvoflur ane and methoxyflurane did undergo defluorination by human kidney micr osomes. Fluoride production was dependent an time, reduced nicotinamid e adenine dinucleotide phosphate, protein concentration, and anestheti c concentration. In seven human kidneys studied, enzymatic sevoflurane defluorination was minimal, whereas methoxyflurane defluorination rat es were substantially greater and exhibited large interindividual vari ability. Kidney cytosol did not catalyze anesthetic defluorination. Ch emical inhibitors of the P450 isoforms 2E1, 2A6, and 3A diminished met hoxyflurane and sevoflurane defluorination. Complementary deoxyribonuc leic acid-expressed P450s 2E1, 2A6, and 3A4 catalyzed methoxyflurane a nd sevoflurane metabolism, in diminishing order of activity. These thr ee P450s catalyzed the defluorination of methoxyflurane three to ten t imes faster than they did that of sevoflurane. Expressed P450 2B6 also catalyzed methoxyflurane defluorination, but 2B6 appeared not to cont ribute to renal microsomal methoxyflurane defluorination because the P 450 2B6-selective inhibitor had no effect. Conclusions: Human kidney m icrosomes metabolize methoxyflurane, and to a much lesser extent sevof lurane, to fluoride ion. P450s 2E1 and/or 2A6 and P450 3A are implicat ed in the defluorination. If intrarenally generated fluoride or other metabolites are nephrotoxic, then renal metabolism may contribute to m ethoxyflurane nephrotoxicity. The relative paucity of renal sevofluran e defluorination may explain the absence of clinical sevoflurane nephr otoxicity to date, despite plasma fluoride concentrations that may exc eed 50 mu M.