Ed. Kharasch et al., HUMAN KIDNEY METHOXYFLURANE AND SEVOFLURANE METABOLISM - INTRARENAL FLUORIDE PRODUCTION AS A POSSIBLE MECHANISM OF METHOXYFLURANE NEPHROTOXICITY, Anesthesiology, 82(3), 1995, pp. 689-699
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.