Ben A. Hitt

Metabolism and Renal Effects of Enflurane in Man

The metabolism and renal effects of enflurane were studied during and after anesthesia in ten surgical patients without renal disease; ten control patients received halothane. Enflurane was metabolized to inorganic fluoride with a mean peak serum level of 22.2 ± 2.8 µM four hours after anesthesia. Urinary inorganic and organic fluoride excretions were increased but oxalic acid excretion was not. suggesting that the latter is not an enflurane metabolite. Postanesthetic renal function, including the response to vasopressin, was normal in both groups. During enflurane anesthesia renal blood flow, glomerular filtration rate, and urinary flow rate were 77, 79, and 67 per cent of control values, respectively. In this study of patients without renal disease, metabolism of enflurane to inorganic fluoride was insufficient to cause clinically significant renal dysfunction.

Renal Effects and Metabolism of Sevoflurane in Fischer 344 RatsAn In-vivo and In-vitro Comparison with Methoxyflurane

Sevoflurane, 1.4 per cent (MAC), was administered to groups of Fischer 344 rats for 10 hour, 4 hours, or 1 hour; additional rats received 0.5 per cent methoxyflurane for 3 hour or 1 hour. Urinary inorganic fluoride excretion of sevoflurane in cico was a third to a fourth that of methoxyflurane. However, using hepatic microsomes, sevoflurane and methoxyflurane were defluorinated in citro at essentially the same rate. The discrepancy between defluorination of sevoflurane and methoxy-flurane in cico and in citro 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.

Mutagenicity of Halogenated Ether Anesthetics

An in vitro microbial assay system employing two histidinedependent strains of Salmonella typhimurium, TA1535 and TA100, was used to test the mutagenicities of enflurane, methoxyflurane, isoflurane and fluroxcne. Enflurane, isoflurane and fluroxene in concentrations ranging from 0.01 to 30 per cent and methoxyflurane in concentrations ranging from 0.01 to 7 per cent were incubated with bacteria in the presence or absence of homogenates of liver prepared from rats pretreated with the enzyme inducer, Aroclor 1254. Enflurane, methoxyflurane, isoflurane, and urines from patients anesthetized with these agents were not mutagenic. Fluroxene, however, was highly mutagenic, and therefore poses a possible hazard for operating room personnel and patients.

Mutagenicity of Volatile Anesthetics: Halothane

The mutagenicity of halothane was tested In an < in-vitro microbial assay system employing two histidine-dependent mutants of Salmonella typhimurium, TA9S and TA100. Halothane in concentrations ranging from 0.1 to 30 per cent was incubated with bacteria in the presence or absence of a metabolic activation system prepared from either nit liver treated with Aroclur 1254 or human liver. Trifluoroacelic acid, a major metabolite of halothane, and urine from patients anesthetized with halothane also were tested. Halothane, trifluoroacetic acid, and patients* urines were not mutagenic.

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 percent 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 abncirmalities 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.

Metabolism of Isoflurane in Fischer 344 Rats and Man

The metabolism of isoflurane was studied in Fischer 344 rats and man. In both species, ionic fluoride and non-ionic fluoride were detected in urine, with ionic fluoride the principal metabolite in rats and non-ionic fluoride the predominant end-product in man. Thin-layer chromatography indicated that the non-ionic fluoride metabolite was trifluoroacetic acid (TFA).

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.

Thermoregulatory defect in rats during anesthesia.

Male rats of the Fischer 344, Sprague-Dawley, Brattleboro, and Wistar strains, balb/C mice, and Hartley guinea pigs were divided into 2 treatment groups. One group drank tap water while the other group drank water containing 1 mg/ml of phenobarbital. Animals were exposed to sevoflurane, enflurane, methoxyflurane, isoflurane, or halothane in a closed chamber. In some of the experiments, soda lime was included and in others the chamber was heated to 39° C with a water blanket. Eighty-six percent (43/50) of Fischer 344 rats treated with phenobarbital and exposed to either sevoflurane or enflurane, in the presence of either soda lime or exogenous heat, died within a few hours after exposure. Fischer 344 rats and rats of other strains drinking phenobarbital water and exposed to methoxyflurane were affected, but to a lesser degree. Rats drinking ordinary tap water and phenobarbital-treated rats not exposed to either soda lime or exogenous heat were unaffected. Guinea pigs and mice also were unaffected. We postulate that the toxic response represents a species-specific thermoregulatory defect, precipitated by heat and occurring in rats treated with phenobarbital in combination with sevoflurane, enflurane, or methoxyflurane.

Kinetics of local anesthetic esters and the effects of adjuvant drugs on 2-chloroprocaine hydrolysis.

A rapid, reliable method for the determination of 2-chloroprocaine in serum was developed. The method, using double-beam ultraviolet spectroscopy, provides rapid, accurate analysis of 2-chloroprocaine in the range of 5.5 to 111 µM (1.5µ30 µg/ml), as documented by comparison with the accepted gas chromatographic procedure. The contribution of 4-amino-2-chlorobenzoic acid, the principal metabolite of 2-chloroprocaine, to the total absorbance at 300 nm was examined and found to be negligible. Using the ultraviolet spectrophotometric method, values of the Michaelis-Menton constant (Km) and maximal reaction velocity (Vmax) for hydrolysis of procaine and 2-chloroprocaine by homozygous typical, heterozygous, and homozygous atypical plasma cholinesterases were determined. The Kms for the three genotypes were 5.0, 6.2, and 14.7 µM, respectively, for procaine, and 8.2, 17, and 103 µM, respectively for 2-chloroprocaine. The Vmaxs for the three genotypes were similar for all esters. Vmax for procaine was 18.6 ± 0.9 nmol/min/ml serum, while Vmax for 2-chloroprocaine was 98.4 ±2.1 nmol/min/ml serum. At high concentrations, 2-chloroprocaine acts as an inhibitor of its hydrolysis. The inhibitory effects of lidocaine, bupivacaine, neostigmine, and succinyldicholine on 2-chloroprocaine hydrolysis for homozygous typical and atypical variants, respectively, were studied. Competitive inhibition was demonstrated for all four drugs. However, at clinically significant concentrations, only neostigmine and bupivacaine produced high degrees of inhibition. The competitive inhibition constants (KI) for the typical and atypical variants, respectively, were 3.3 ± 0.3 µM and 15.1 ± 4.8 µM for neostigmine, and 4.2 ± 0.3 µM and 36.9 ± 9.8 µM for bupivacaine.

Enflurane and methoxyflurane metabolism at anesthetic and at subanesthetic concentrations.

In an attempt to determine the importance of concentration of an anesthetic agent as a determinant of the extent of its biotransformation, we measured fluoride excretion in groups of Fischer 344 rats treated with one of several subanesthetic or an anesthetic concentration (1 MAC) of either enflurane or methoxyflurane. Anesthetic administrations (2.0% enflurane or 0.26% methoxyflurane) ranged from 0.15 hours (9 minutes) to 4.8 hours. Subanesthetic exposures, all of 48 hours duration, ranged in concentration from 0.2% enflurane to 0.0016% methoxyflurane. Greatest metabolism occurred at the lowest concentration time (MAC-hours) of subanesthetic administrations and at the shortest duration of anesthetic exposure. Increasing time in the case of anesthetizing exposures, or concentration in subanesthetic exposures, increased the amount of metabolite produced. However, the increased production of metabolite was not proportional to the increase of concentration or duration of exposure. Enzyme induction was ruled out as an important factor in the larger amount of metabolism seen during the subanesthetic exposures. Therefore, the exposure of a patient to the metabolites of an anesthetic is actually low although the anesthetic is administered at a high concentration.