The species-dependent metabolism of efavirenz produces a nephrotoxic glutathione conjugate in rats.

Efavirenz, a potent nonnucleoside reverse transcriptase inhibitor widely prescribed for the treatment of HIV infection, produces renal tubular epithelial cell necrosis in rats but not in cynomolgus monkeys or humans. This species selectivity in nephrotoxicity could result from differences in the production or processing of reactive metabolites, or both. A detailed comparison of the metabolites produced by rats, monkeys, and humans revealed that rats produce a unique glutathione adduct. The mechanism of formation and role of this glutathione adduct in the renal toxicity were investigated using both chemical and biochemical probes. Efavirenz was labeled at the methine position on the cyclopropyl ring with the stable isotope deuterium, effectively reducing the formation of the cyclopropanol metabolite, an obligate precursor to the glutathione adduct. This substitution markedly reduced both the incidence and severity of nephrotoxicity as measured histologically. Further processing of this glutathione adduct was also important in producing the lesion and was demonstrated by inhibiting gamma-glutamyltranspeptidase with acivicin pretreatment (10 mg/kg, IV) prior to dosing with efavirenz. Again, both the incidence and severity of the nephrotoxicity were reduced, such that four of nine rats given acivicin were without detectable lesions. These studies provide compelling evidence that a species-specific formation of glutathione conjugate(s) from efavirenz is involved in producing nephrotoxicity in rats. Mechanisms are proposed for the formation of reactive metabolites that could be responsible for the renal toxicity observed in rats.

Developmental neurotoxicity evaluation of the avermectin pesticide, emamectin benzoate, in Sprague-Dawley rats.

The potential of emamectin benzoate (EB) to cause developmental neurotoxicity in Sprague-Dawley rats was assessed using a study design by the US EPA. Dosages of 0 (deionized water), 0.1, 0.6, or 3.6 mg/kg/day were administered at 5 ml/kg by oral gavage from gestational day (GD) 6 to lactational day (LD) 20 to groups of 25 mated females each. Between GD 17 and 20 the high dose was reduced to 2.5 mg/kg/day because of pup tremors observed at this dose level in a concurrent two-generation study. Females were allowed to deliver and the young were evaluated for survival, growth, development, behavior, and histological changes to brain, spinal cord, peripheral nerve, and skeletal muscle. Behavioral assessment of the offspring consisted of open field motor activity, auditory startle habituation, and passive avoidance tests; each was conducted on weanling and adult animals (one animal/sex/litter). Histopathological examination of the CNS and PNS was conducted on one animal/sex/litter on postnatal days (PND) 11 and 60. There were significant increases in average F0 maternal body weight gains during gestation in the 0.6 and 3.6/2.5 mg/kg/day groups, but no other effects were observed in pregnant females of these or the low-dose groups during the study. Beginning on PND 6, tremors were observed in high-dose pups, and this was followed by hindlimb splay in all high-dose pups by PND 15-26. Both of these physical signs disappeared by PND 34 (i.e., 10-11 days after weaning). There were no compound-related deaths in F1 offspring. Beginning on PND 11, progressive decreases in preweaning average weights were observed in the high-dose group (to 42% below control in females on PND 21). Average weight gain during the postweaning period was significantly decreased in the 3.6/2.5 mg/kg/day group. There were EB-related effects in behavioral tests only in the high-dose group. A significant increase in PND 13 average horizontal motor activity was due to stereotypical movements. Average horizontal activity was decreased on PND 17 and in adult females, but there was no effects on PND 21. Average peak auditory startle response amplitude was decreased on PND 22 and in adults. There were no EB-related effects in the passive avoidance test, relative brain weights, or in the histological examination (including morphometry) of the nervous system. These results demonstrate that the high-dose EB exposure during gestation and lactation to rats produced evidence of neurotoxicity in the F1 offspring, and a clear No Observed Adverse Effect Level (NOAEL) for developmental neurotoxicity of EB was determined to be 0.6 mg/kg/day.

The toxicity of a fluorinated-biphenyl HMG-CoA reductase inhibitor in beagle dogs.

L-645, 164, a potent inhibitor of hydroxymethylglutarylcoenzyme A (HMG-CoA) reductase, is a structurally unique, synthetic monofluorinated-biphenyl that was administered to beagle dogs at dosages of 2, 10, or 50 mg/kg/day for 14 weeks to evaluate its toxic potential. Previously tested HMG-CoA reductase inhibitors from this laboratory have either been semisynthetic or fermentation-derived products containing a hexahydronaphthalene ring structure (i.e., lovastatin and simvastatin). Administration of L-645, 164 produced a significant spectrum of lesions, some of which have been previously associated with compounds of this pharmacological class, while others were unique to this monofluorinated-biphenyl inhibitor. Subcapsular lenticular opacities were produced in six of eight of the dogs receiving 50 mg/kg/day of L-645, 164 within 8 weeks of dosing. One dog receiving this dosage level experienced increases in serum alanine aminotransferase activity to levels 10 times those in concurrent control dogs. Light and electron microscopy of a wedge biopsy obtained within 3 days of this transaminase elevation failed to reveal any significant changes and the elevation resolved spontaneously despite continued drug administration. Lesions of the optic nerve and acoustic-vestibular tract and trapezoid decussation were observed in several dogs receiving 50 mg/kg/day. In addition, similar changes were observed in the optic tract in several of the dogs receiving 50 mg/kg/day and in one dog receiving 2 mg/kg/day of L-645,164. These were unique to L-645,164 and have not been observed after the administration of other HMG-CoA reductase inhibitors in this laboratory. Optic tract changes were generally mild, consisting of small to medium vacuoles without apparent myelin loss. Lesions in the other areas ranged from very slight to prominent vacuolation. No clinical signs were observed. Peak plasma drug levels of L-645,164 at 50 mg/kg were greater than 5 micrograms/ml, about one order of magnitude greater than those attained after administration of pharmacologically equipotent doses of lovastatin and simvastatin. These findings support previous observations that HMG-CoA reductase inhibitors producing high plasma drug levels are associated with a significant degree of systemic toxicity. In addition, the drug-induced CNS lesions attributed to L-645,164 appear also to be related to its chemical structure since similar lesions have not been observed after the administration of other structurally unrelated HMG-CoA reductase inhibitors that produce high plasma drug concentrations and comparable degrees of serum cholesterol lowering.

Studies on the mechanism of simvastatin-induced thyroid hypertrophy and follicular cell adenoma in the rat.

Female Sprague-Dawley rats were treated with either simvastatin (a novel competitive inhibitor of HMG CoA reductase) or phenobarbital (positive control) to ascertain the possible relationship between the effects of simvastatin on hepatic metabolism and the thyroid hypertrophy and follicular cell adenomas which it produces in this strain of rat. The test compounds were administered orally at doses of 100 mg/kg (divided doses at 50 mg/kg, b.i.d.). (This dose of simvastatin represents approximately 250 times the human dose.) After 5 weeks of treatment, either simvastatin or phenobarbital produced significant increases (35% and 39% above control, respectively) in serum thyroid stimulating hormone (TSH), a significant increase (39% and 120% above control, respectively) in the systemic clearance of 125I-thyroxine, and slight decreases in serum thyroxine levels. Statistically significant increases in liver and thyroid weights were associated with phenobarbital treatment. With simvastatin, increased liver weights occurred. At the microscopic level, thyroid hypertrophy was observed in all phenobarbital-treated rats and to a lesser degree in most simvastatin-treated animals. Simvastatin did not markedly alter liver microsomal enzyme activities with the exception of the anticipated induction of HMG CoA reductase (which increased approximately 4.4-fold). Conversely, phenobarbital produced large increases in liver microsomal enzymes, including glucuronosyl transferase, but did not affect the activity of HMG CoA reductase. Therefore, the increased clearance of thyroxine in simvastatin-treated female rats was not associated with enzyme induction but may have been related to the increase in functional liver mass produced by this compound at this dose.(ABSTRACT TRUNCATED AT 250 WORDS)

On the etiology of subcapsular lenticular opacities produced in dogs receiving HMG-CoA reductase inhibitors.

The administration of high dosages of various hydroxymethylglutaryl-CoA (HMG-CoA) reductase inhibitors has resulted in the development of subcapsular lenticular opacities in dogs. While dogs receiving cataractogenic doses of HMG-CoA reductase inhibitors experienced profound decreases in circulating serum cholesterol concentrations (40-60% reductions in total serum cholesterol), a causal relationship between serum cholesterol lowering and cataractogenesis was not established. A strong relationship was demonstrated, however, between the systemic exposure to inhibitor (plasma drug levels) and the cataractogenic potential of the various compounds studied. Analysis of lenses from dogs chronically dosed with various HMG-CoA reductase inhibitors revealed the presence of low drug levels in the lens (less than 500 ng equivalents g-1), but no correlation was observed between the amount of drug associated with the lens after chronic treatment and cataract development. In addition, no abnormalities in cholesterol content or sterol composition were observed in clear and/or cataract containing lenses from dogs chronically dosed with HMG-CoA reductase inhibitors. The kinetics of drug appearance in the aqueous and lens cortex was assessed after doses of various HMG-CoA reductase inhibitors, and suggested somewhat higher but not statistically significant peak concentrations of inhibitor were achieved by compounds which produced a higher incidence of cataracts. These data have suggested that high doses of HMG-CoA reductase inhibitors may increase lenticular exposure to drug via the aqueous humor by producing a substantial systemic exposure to drug substance. This may result in an increased concentration of inhibitor in the outer cortical region of the lens where cholesterol synthesis is critical, thereby resulting in the development of opacities. The production of lenticular changes by a HMG-CoA reductase inhibitor of diverse chemical structure establishes, with reasonable assurance, that these lens changes are mechanism based (i.e. a product of the biochemical mechanism of action of this class of compounds). An extrapolation of these findings to patients receiving therapeutic dosages enables a favorable risk evaluation since the doses to be employed clinically are much lower and result in a far lower systemic exposure to drug substance.

Animal safety and toxicology of simvastatin and related hydroxy-methylglutaryl-coenzyme A reductase inhibitors.

Simvastatin, a hydroxy-methylglutaryl-coenzyme A reductase inhibitor intended for use as a hypocholesterolemic agent, has undergone a thorough preclinical toxicology evaluation. This review describes preclinical toxicology findings associated with simvastatin administration in animals and provides the rationale for our conclusion that these changes are not indicative of potential human toxicity. Although it was not surprising to find that a potent inhibitor of this key biochemical pathway produces toxicity at high dosages in animals, none of the observed changes poses a significant risk to humans at clinical dosages. Many of the toxicities produced by high dosage levels of simvastatin in animals are directly related to the drug's biochemical mechanism of action and are the result of a profound, sustained inhibition of the target enzyme that is not anticipated at clinical dosages. Furthermore, several of the simvastatin-induced changes are species-specific responses to this agent and are not relevant to human risk assessment. Of the treatment-related changes reported for simvastatin, the development of cataracts in dogs has received considerable attention. The available data demonstrate a wide margin of safety in terms of dosage levels required to elicit this response as well as the plasma concentrations associated with the development of these ocular lesions. The data suggest that the development of lenticular opacities at clinical doses of simvastatin is highly improbable. Overall, simvastatin is highly improbable. Overall, simvastatin was well-tolerated by animals in preclinical toxicology studies, and no findings contraindicating its use in humans were identified.

Preclinical evaluation of lovastatin.

Administration of lovastatin to animals at high dosage levels produces a broad spectrum of toxicity. This toxicity is expected based on the critical nature of the target enzyme (HMG CoA reductase) and the magnitude of the dosage levels used. The information reviewed in this paper demonstrates that these adverse findings in animals do not predict significant risk in humans. The reason for this derives from the fact that all the available evidence suggests that the adverse effects observed are produced by an exaggeration of the desired biochemical effect of the drug at high dosage levels. The presence of clear and high no-effect doses for these toxic effects along with the fact that most of the changes observed are clearly mechanism-based (directly attributable to inhibition of mevalonate synthesis) indicate that it is unlikely that similar changes will be observed at the therapeutic dosage levels in humans. This hypothesis is supported by the extensive human safety experience described by Tobert in the following report.

Killing of cultured hepatocytes by the mixed-function oxidation of ethoxycoumarin.

Ethoxycoumarin is metabolized by mixed-function oxidation to give 7-hydroxycoumarin (umbelliferone) and acetaldehyde, without formation of an intermediate electrophile. Ethoxycoumarin was found, nevertheless, to injure cultured rat hepatocytes. Male hepatocytes were more sensitive than female to ethoxycoumarin. Phenobarbital increased cell killing, and SKF 525A, an inhibitor of ethoxycoumarin metabolism, prevented it. Neither umbelliferone nor acetaldehyde were toxic. Cellular glutathione decreased and oxidized glutathione (GSSG) accumulated in the culture medium. Sulfhydryl reagents prevented the cell killing without inhibiting metabolism. Lipid peroxidation was detected prior to evidence of cell death, and the antioxidant N,N'-diphenyl-phenylenediamine prevented both the lipid peroxidation and cell killing without inhibiting metabolism. Inhibition of glutathione reductase with 1,3-bis(chloroethyl)-1-nitrosourea potentiated the cell killing without increasing metabolism. Pretreatment of the cells with the ferric iron chelator deferoxamine reduced cell killing, again without inhibiting metabolism. Ferric chloride restored the sensitivity of deferoxamine-pretreated hepatocytes to ethoxycoumarin. These data define a new experimental model in which lethal liver cell injury is dependent on the metabolism of ethoxycoumarin but unrelated to its two known metabolites. An oxidative stress accompanying the cytochrome P-450-dependent metabolism of ethoxycoumarin is proposed as the mechanism coupling metabolism to lethal cell injury.

Oxygen-mediated cell injury in the killing of cultured hepatocytes by acetaminophen.

Sensitivity of cultured hepatocytes to acetaminophen was induced by pretreatment of the rat with 3-methylcholanthrene. Under these conditions, 10 uM B-naphthoflavone but not SKF-525A prevented the cell killing, indicating dependence on metabolism. Inhibition of glutathione reductase by 50 uM bis-chloro-nitrosourea, shown previously to increase the sensitivity of hepatocytes to an oxidative stress, potentiated the toxicity of acetaminophen without increasing the covalent binding of acetaminophen metabolites. Pretreatment of the hepatocytes with the ferric iron chelator deferoxamine, known to reduce the sensitivity of hepatocytes to an oxidative stress, prevented the cell killing without reducing covalent binding. Addition of ferric chloride to the culture medium restored the sensitivity of the cells to acetaminophen, again without effect on the extent of covalent binding. These data demonstrate that the toxicity of acetaminophen can be dissociated from the covalent binding of its metabolites and support the conclusion that the hepatocytes were lethally injured by an oxidative stress accompanying the mixed function oxidase-dependent biotransformation of acetaminophen.

Differences in the uptake of cadmium and mercury by rat hepatocyte primary cultures. Role of a sulfhydryl carrier.

Studies measuring the uptake of cadmium or mercury in isolated hepatocytes demonstrated that hepatocytes accumulated more cadmium than mercury in serum-containing medium, serum-free medium, or balanced salt solution. The preferential hepatocellular accumulation of cadmium, independent of medium composition, suggested that the uptake mechanism for cadmium and mercury might be different in hepatocytes. Pretreatment of hepatocytes with 50 microM N-ethylmaleimide decreased cadmium uptake by 23% while having no effect on the uptake of mercury was inhibited in a concentration-dependent manner. The uptake of cadmium was maximally inhibited (80%) with 75 microM parachloromercuribenzenesulfonate or 20 microM mercury respectively. Cadmium had no effect on mercury. Hepatocytes treated with parachloromercuribenzenesulfonate or mercury accumulated cadmium at a rate closely resembling the rate of mercury uptake in untreated hepatocytes. These results suggested that an SH-containing carrier may be operative in the uptake of cadmium by hepatocytes. Mercury can interact with this carrier to inhibit cadmium uptake; however, this carrier does not appear to facilitate mercury uptake.

Uptake and binding of cadmium and mercury to metallothionein in rat hepatocyte primary cultures.

The administration of inorganic Cd and Hg in vivo has been shown to result in markedly different metal concentrations in rat liver. Primary cultures of rat hepatocytes were utilized to gain insight into the dispositional differences between these chemically similar metals. Hepatocyte monolayer cultures were exposed to several concentrations of Cd or Hg (3, 10 and 30mum) in serum-containing medium for 30min. The cells were then washed and incubated in fresh medium for the remainder of the experiment. Hepatocytes exposed to Cd accumulated significantly more metal than hepatocytes exposed to equimolar concentrations of Hg. In cells exposed to 3mum-Cd there was an initial loss of Cd from the hepatocytes when placed in fresh medium, followed by a gradual re-uptake of metal, concomitant with increased binding to metallothionein. In hepatocytes exposed to 3 and 10mum-Cd, 87 and 77% of the intracellular Cd was bound to metallothionein within 24h. Loss of Hg from hepatocytes pulsed with 30mum-Hg was also observed upon the addition of fresh medium and continued for the duration of the experiment. No time-dependent increase in Hg binding to metallothionein was observed. A maximum of about 10% of the intracellular Hg was found associated with metallothionein in hepatocytes exposed to 30mum-Hg. Studies utilizing [(35)S]cysteine incorporation indicated significant increases in the amount of metallothionein synthesized in hepatocytes exposed to 3 and 10mum-Cd (300% of control value) and 30mum-Hg (150% of control value) 24h after metal pulsing. Time-course studies revealed a 6-12h lag in metallothionein synthesis, followed by a significant elevation in [(35)S]cysteine incorporation into metallothionein between 12 and 24h. These studies suggest that (a) isolated hepatocytes differentiate between Cd and Hg and preferentially accumulate the former, and (b) Cd strongly stimulates the induction of, and preferentially binds to, metallothionein, whereas Hg induces weakly, and does not preferentially bind to, metallothionein.