Methoxyflurane - Biodegradation and Toxicity

Biodegradation and Toxicity

Further information: Drug metabolism and Fluoride poisoning

The first report of nephrotoxicity appeared in 1964, when Paddock and colleagues reported three cases of acute renal insufficiency, two of whom were found to have calcium oxalate crystals in the renal tubules at autopsy. In 1966, Crandell and colleagues reported a series in which 17/95 (18%) of patients developed an unusual type of nephropathy after operations in which methoxyflurane was used as a general anesthetic. This particular type of renal insufficiency was characterized by vasopressin-resistant high-output renal failure (production of large volumes of poorly concentrated urine) with a negative fluid balance, pronounced weight loss, elevation of serum sodium, chloride, osmolality and blood urea nitrogen. The urine of these patients was of a relatively fixed specific gravity and an osmolality very similar to that of the serum. Furthermore, the high urine output persisted a challenge test of fluid deprivation. Most cases resolved within 2–3 weeks, but evidence of renal dysfunction persisted for more than one year in 3 of these 17 cases (18%), and more than two years in one case (6%).

Reports of severe and even fatal hepatotoxicity related to the use of methoxyflurane began to appear in 1966. These reports prompted anesthesiologists to subject this agent to intense and systematic scrutiny. A study published in 1973 by Cousins and Mazze demonstrated that compared with halothane, methoxyflurane produces dose-dependent and deleterious abnormalities in renal function. The authors showed that subclinical nephrotoxicity occurred following methoxyflurane at minimum alveolar concentration (MAC) for 2.5 to 3 hours (2.5 to 3 MAC hours), while overt toxicity was present in all patients at dosages greater than five MAC hours. This landmark study provided a model that would be used for the assessment of the nephrotoxicity of volatile anesthetics for the next two decades.

The biodegradation of methoxyflurane begins immediately after the onset of exposure. The kidney and liver toxicity observed after anesthetic doses is attributable to one or more metabolites produced by O-demethylation of methoxyflurane. Significant products of this catabolic process include methoxyfluoroacetic acid (MFAA), dichloroacetic acid (DCAA), and inorganic fluoride. Methoxyflurane nephrotoxicity is dose dependent and irreversible, resulting from O-demethylation of methoxyflurane to fluoride and DCAA. This effect is so predictable and reproducible that methoxyflurane now serves as a pharmacologic model of fluoride-related nephrotoxicity, one with which newer drugs are compared. It is not entirely clear whether the fluoride itself is toxic—it may simply be a surrogate measure for some other toxic metabolite. The concurrent formation of inorganic fluoride and DCAA is unique to methoxyflurane biotransformation compared with other volatile anesthetics, and this combination is more toxic than fluoride alone. This may explain why fluoride formation from methoxyflurane is associated with nephrotoxicity, while fluoride formation from other volatile anesthetics (such as enflurane and sevoflurane) is not. Furthermore, the concurrent use of tetracyclines and methoxyflurane has been reported to result in fatal renal toxicity.

Based on the findings of these and other studies in the early 1970s, the current consensus is that the use of methoxyflurane should be restricted only to healthy individuals, in situations where it offers specific advantages and even then, only at dosages less than 2.5 MAC hours. Partly because of these warnings, but also because of the development of newer volatile anesthetics such as enflurane, isoflurane, desflurane and sevoflurane, the clinical use of methoxyflurane as a general anesthetic in humans was largely abandoned in the mid-1970s.