A comparison of renal effects and metabolism of sevoflurane and methoxyflurane in enzyme-induced rats

Twenty-five 5-month-old male Fischer-344 rats were randomly divided into 5 groups: Group I, no anesthesia; Group II, 1.4 precent sevoflurane for 2 hours; Group III, 0.1 percent phenobarbital, ad lib, in drinking water for 7 days; followed by 1.4 percent sevoflurane for 2 hours; Group IV, 0.25 percent methoxyflurane, 1 hour; Group V, phenobarbital in water as in Group III, followed by methoxyflurane as in group IV. Pre- and postanesthetic serum and urinary osmolality, Na+, K+, urea nitrogen (BUN), inorganic fluoride (F-) levels, and 24-hour urine volume were measured. Kidney tissue was obtained for examination by light and electron microscopy. Sevoflurane was metabolized to F- to a lesser extent than was methoxyflurane; treatment with phenobarbital-sevoflurane doubled urinary F- excretion, resulting in a value similar to that seen after methoxyflurane alone. There was no functional or morphologic evidence of renal abnormalities in either group of rats anesthetized with sevoflurane. Methoxyflurane dosage was sufficiently low that renal abnormalities did not occur except in rats treated also with phenobarbital; these animals developed polyuria and the morphologic lesion typically associated with F--induced nephrotoxicity.

The influence of age on the distribution, metabolism and excretion of methoxyflurane in Fischer 344 rats: a possible relationship to nephrotoxicity

Age as a factor in methoxyflurane nephrotoxicity was evaluated in Fischer 344 rats of various ages by determination of: 1) serum inorganic fluoride and methoxyflurane concentrations, and urinary inorganic fluoride excretion in methoxyflurane-exposed rats; 2) liver microsomal methoxyflurane defluorinase activity; and 3) distribution of injected sodium fluoride. Only rats in the youngest age group (6 weeks) did not develop nephrotoxicity after anesthesia. Older rats had a biphasic rather than a monophasic decay in serum methoxyflurane concentration and also had increased serum inorganic fluoride concentration and urinary inorganic fluoride excretion. Older rats also excreted a greater proportion of an injected dose of sodium fluoride compared to young rats. Microsomal methoxyflurane defluorinase specific activity was similar among rats of all ages. It is likely that increased availability of methoxyflurane due to its greater storage in fat led to more inorganic fluoride production in older compared to younger rats. Bone sequestration of inorganic fluoride in younger rats probably accounts for decreased serum inorganic fluoride levels in that group. Both factors cause significant differences in renal exposure to inorganic fluoride; thus the risk of nephrotoxicity is less in younger animals.

[The biotransformation of inhalation anaesthetics and its relevance to clinical side effects (author's transl)]

The volatile anesthetics are not inert substances, but they are metabolised by means of a non-specific drug metabolizing enzyme system of the liver. Thus, in combined administration of several drugs, the knowledge of the metabolic pathways and the influence of the anesthetics on the transforming system is not only theoretically important, but represents a conditio sine qua non to the anesthesist as regards the handling of modern anesthetics. The occasionally observed liver and kidney injuries indicate a potential formation of toxic metabolites. Renal damage due to methoxyflurane may be caused by F- which is released during its biotransformation while, in the group of halogenated inhalation anesthetics, only chloroform is a true hepatotoxic substance. The explanation of the liver damage which is rarely observed after halothane administration, is an open question till now. For this, the investigator must go back to animal experiments where the quantity and quality of the metabolism differ considerably from one species to the other. In men it is widely unknown to what extent the microsomal enzymatic system is influenced by the disease itself and/or by the pharmacotherapy. Therefore great efforts must still be made in order to establish the formation of side-effects of halogenated inahlation anesthetics and, thus, to improve their therapeutic safety.

[Renal effects and metabolism of sevoflurane in Fisher 3444 rats: an in-vivo and in-vitro comparison with methoxyflurane]

Sevoflurane, 1.4 per cent (MAC), was administered to groups of Fischer 344 rats for 10 hours, 4 hours, or 1 hour; additional rats received 0.5 per cent methoxyflurane for 3 hours or 1 hour. Urinary inorganic fluoride excretion of sevoflurane in vivo was a third to a fourth that of methoxyflurane. However, using hepatic microsomes, sevoflurane and methoxyflurane were defluorinated in vitro at essentially the same rate. The discrepancy between defluorination of sevoflurane and methoxyflurane in vivo and in vitro was probably due to differences in tissue solubility between the drugs. There were no renal functional or morphologic defects following sevoflurane administration. An unexplained adverse effect was significant weight loss, which occurred following all exposures to sevoflurane.

Anesthetic biotransformation and renal function in obese patients during and after methoxyflurane or halothane anesthesia

Anesthetic biotransformation and renal function were studied in obese adult patients (148 plus or minus 8 kg; mean plus or minus SE) anesthetized for three hours with 60 per cent nitrous oxide plus either methoxyflurane or halothane for elective jejunoileal small-bowel-bypass operations. There was no evidence of persistent renal dysfunction in any patient postoperatively, but serum osmolality was elevated 72 hours after methoxyflurane anesthesia. Urine concentrating ability was not determined. Peak serum ionic fluoride concentration was 55.8 plus or minus 5.8 muM/1 two hours after discontinuation of methoxyflurane. Urinary ionic fluoride and oxalate excretions increased postoperatively. Compared with previously reported data from nonobese patients, serum ionic fluoride concentrations in obese patients increased more rapidly during methoxyflurane anesthesia and peaked higher and sooner after discontinuation of methoxyflurane. The peak serum ionic fluoride concentration was 10.4 plus or minus 1.5 muM/1 at the conclusion of halothane anesthesia, significantly more than the corresponding value in nonobese patients. Intraoperative liver biopsies from 23 of 27 patients showed moderate to severe fatty metamorphosis. Fatty liver infiltration may have increased hepatic anesthetic uptake and exposed more methoxyflurane or halothane to hepatic microsomal enzymes. The more rapid elevation and higher peak levels of serum ionic fluoride following methoxyflurane, and to a lesser extent following halothane, may reflect increased anesthetic biotransformation in obese compared with nonobese patients. To avoid excessive serum ionic fluoride elevations the authors recommended limiting low-dose methoxyflurane anesthesia delivered to obese patients with potential fatty liver infiltration to no more than three hours.

Metabolism in vitro of enflurane, isoflurane, and methoxyflurane

Specific activities of enflurane, isoflurane, and methoxyflurane defluorinases were measured in microsomes prepared from the livers of Fischer 344 rats; the ratio of these activities was 23:3:1. Pretreatment with phenobarbital significantly increased the defluorinase activities of all three agents. Factors that influence anesthetic drug metabolism are discussed; tissue solubility is considered to be the most important.

Methyoxuflurane and serum fluoride concentrations during anaesthesia and postoperative renal function

In twenty gynaecological patients scheduled for hysterectomy, arterial blood concentrations of methoxyflurane and fluoride ions were measured during operation. Renal function was assessed by pre and postoperative measurements of BUN, creatinine, osmolality and serum electrolytes. Fluid balances were calculated by the daily intake and output values. During the first five postoperative days, from the 12 hour urine specimens the fluoride concentrations were measured, and from the 24 hour specimens urine electrolytes and creatinine levels were calculated. No clinical or laboratory evidence of renal dysfunction was observed. Fluoride levels were lower than those previously associated with renal toxicity. Methoxyflurane concentrations in arterial blood showed concentrations lower as previously reported.

Methoxyflurane anesthesia in pediatric patients: evaluation of anesthetic metabolism and renal function

Serum ionic fluoride concentrations during and following low-dose (6.0 mg/100 ml, 3 hours) methoxyflurane anesthesia and elective operation were measured in 13 pediatric patients (mean age 10.2 years; mean weight 34.5 kg). Peak measured serum ionic fluoride concentration was 21.6 plus or minus 3.3 mumol/1 24 hours after anesthesia. In a previously reported study of adult patients (47.5 years; 71.9 kg), the peak measured serum ionic fluoride concentration was 43.9 plus or minus 5.7 mumol/1 24 hours after low-does (6.8 mg/100 ml,3 hours) methoxyflurane anesthesia. Possible explanations for lower serum ionic fluoride concentrations in pediatric patients comared with adults include 1) slower metabolism of nethoxyflurand; 2) increased renal clearance of ionic fluoride from the blood; 3) greater sorage of ionic fluride in bone; 4) more rapid methoxyflurane elimiantion in the postoperative period. Serum uric acid increased (4.4 to 6.4 mg/100 ml, not significant) 24 hours after anesthesia and operation, while blood urea nitrogen and serum creatinine and osmolality were unchanged postoperatively.