Metabolism of terfenadine associated with CYP3A(4) activity in human hepatic microsomes.

Terfenadine (Seldane) undergoes extensive metabolism to form azacyclonol and terfenadine alcohol. Terfenadine alcohol is subsequently metabolized to azacyclonol and terfenadine acid. Although testosterone 6 beta-hydroxylation [CYP3A(4)] has been shown to be the principal enzyme involved in the first step in terfenadine's biotransformation (formation of azacyclonol and terfenadine alcohol), the enzymes catalyzing the subsequent metabolic steps in the conversion of terfenadine alcohol to azacyclonol and terfenadine acid have not been identified. The purpose of these studies was to determine the role of cytochrome P450 isoforms in the biotransformation of terfenadine and terfenadine alcohol. To this end, both terfenadine and its alcohol were incubated with 10 individual human liver microsomal samples that have been characterized for major isozyme activities. The metabolites and parent drugs were quantified by HPLC. The formation of azacyclonol and terfenadine alcohol from terfenadine is confirmed to be catalyzed predominantly by CYP3A(4) isozyme, and the ratio of the rate of terfenadine alcohol formation to that of azacyclonol is 3:1. Involvement of the CYP3A(4) in terfenadine metabolism was further confirmed by the following studies: a) inhibition of terfenadine alcohol formation by ketoconazole and troleandomycin, two specific inhibitors of CYP3A(4), and b) time course of terfenadine alcohol formation by cloned human CYP3A(4). When terfenadine alcohol was used as substrate, both the terfenadine acid and azacyclonol formation were also catalyzed by CYP3A(4) isozyme. However, the rate of formation of the terfenadine acid metabolite is almost 9 times faster than that of azacyclonol. The net ratio of terfenadine acid to azacyclonol is 2:1.

Regioselectivity of metabolic activation of acetylenic steroids by hepatic cytochrome P450 isozymes.

Liver cytochrome P450 monooxygenases (P450), a group of isozymes that catalyze the reductive cleavage of molecular oxygen, dominate hepatic metabolism of xenobiotic lipophilic substances. These P450 enzymes exhibit broad and overlapping substrate specificities, in contrast to the P450 isozymes of the steroid biosynthetic pathways, which are highly substrate specific. Hepatic heme pigments, N-alkylated porphyrins, accumulate following the self-catalyzed destruction of P450 by the metabolic activation of 17 alpha-ethynyl steroids. Acetylenic substituted steroidal aromatase inactivators, norethisterone (NET), and 10-(2-propynyl)estr-4-ene-3,17-dione (MDL 18,962) were administered to rats to determine if the acetylenic substituent was activated by hepatic P450 mixed-function oxidases. This metabolism could result in the formation of a reactive species that would alkylate a pyrrole nitrogen atom of heme. Male Sprague-Dawley rats were treated with 0, 10, 30, or 100 mg/kg NET or MDL 18,962 intraperitoneally. Four hours later, these animals received 40 mg/kg sodium pentobarbital and their sleeping times were recorded. On arousal, the rats were killed and their livers were taken for determination of P450 content and formation of N-alkylated porphyrins (green pigments). Norethisterone inhibited hepatic P450 isozymes, resulting in a dose-related increased sleeping time (89.2 +/- 3.5 to 156.3 +/- 7.6 minutes) and decreased P450 levels (maximum 25% decrease at 100 mg/kg), and the amount of green pigments increased with doses of 10 to 100 mg/kg. In contrast, MDL 18,962 treatment did not increase sleeping time and caused only a 15% decrease in hepatic P450 content at 100 mg/kg, with no detectable green pigments.(ABSTRACT TRUNCATED AT 250 WORDS)

Dose related induction of the drug metabolizing enzymes of rat liver by cilobamine.

Cilobamine , an antidepressant, was investigated for its influence on the hepatic drug metabolizing enzymes ( DME ) of male Charles River CD rats. Cilobamine doses (3, 10, 30, 100, and 300 mg/kg po, as free base) were compared to sodium phenobarbital (PB) doses (3, 10, 30, 100, and 200 mg/kg po, as free acid). Compounds were given daily for 4 days and all tests were done on Day 5. Ethylmorphine n-demethylase, aniline hydroxylase, microsomal cytochrome P-450 content, relative liver weight, and recoverable microsomal protein were quantitated. The results indicated that cilobamine was an inducer of the DME but not as potent as PB. Cilobamine did not exert any inductive responses at 3 mg/kg. At 10 and 30 mg/kg some but not all test systems were increased. However, at 100 and 300 mg/kg all were increased. PB increased all systems at all doses studied. Electron micrographs of livers of rats given 100 mg/kg of cilobamine or PB revealed hypertrophy of the smooth endoplasmic reticulum. The time course of induction in rats given 100 mg/kg po showed that responses in the cilobamine rats peaked after the second dose and plateaued with later doses. Responses in PB rats increased markedly after one dose and showed a continual increase with later doses. Induction of the DME was also demonstrated in female rats.

Metabolic disposition of terfenadine in laboratory animals.

Alpha-[4-(1,1-Dimethylethyl)phenyl]-4-(hydroxydiphenylmethyl)-1- piperidinebutanol (terfenadine, RMI 9918, Triludan, Teldane, resp.), a clinically effective antihistamine, with little or no central nervous system effects, was investigated in order to understand its metabolic disposition. These 14C-terfenadine studies were performed in laboratory animals, principally the rat, and also in beagle dog and monkey (Macaca mulatta). In all three species the fecal pathway of excretion was predominant with nearly all the elimination occurring within 24 h of administration. The data suggest that biliary excretion plays a prominent role. Tissue distribution studies indicated that the liver and lung, relative to other tissues, exhibited the highest concentrations of 14C-terfenadine equivalents. However, brain had low amounts of 14C-terfenadine which was in general agreement with the whole-body autoradiographic studies which did not show radioactivity in brain or spinal cord. The evidence suggests that terfenadine was metabolized rapidly with no terfenadine in urine or bile and small amounts in feces. Tissue concentrations of terfenadine were in the low nanogram range and peaked at 0.5-1.0 h with lung exhibiting the greatest and the brain sub-quantifiable amounts. The ratios of 14C-terfenadine eq/terfenadine were high and greatest in the liver and kidney. These studies demonstrated that terfenadine was readily absorbed and eliminated, virtually had undergone complete biotransformation and exhibited very low tissue concentrations per se.

Bioavailability of terfenadine in man.

Fourteen normal male subjects were given either 60mg or 180mg of terfenadine suspension in a randomized two-way crossover study. Peak plasma concentrations of 1.544 +/- 0.726 (mean +/- S.D.) ng ml-1 were obtained in 0.786 h following the 60 mg dose and displayed an AUC or 11.864 +/- 3.369 ng h ml-1. Whereas peak plasma concentrations of 4.519 +/- 2.002 ng ml-1 in 1.071 +/- 0.514 h were obtained following the 180 mg dose. The AUC following the 180 mg dose was 44.341 +/- 22.041 ng h ml-1. When 60 mg of 14C terfenadine was given to six additional subjects, the peak plasma concentrations of 351 +/- 43 ng equivalents per ml were obtained in 1.67 +/- 0.41 h and the AUC was 2297.71 +/- 310.85 ng-equivalents h ml-1. This indicates that approximately 99.5 per cent of the terfenadine related material that is absorbed undergoes biotransformation. Urinary excretion of 14C accounted for 39.89 +/- 5.29 per cent of the dose while 60.58 +/- 2.44 per cent of the dose was recovered in the feces in twelve days. Thin-layer chromatographic (TLC) examination of fecal extracts showed only a trace of material chromatographing with terfenadine. This may indicate that the 14C present in the feces is not due to lack of absorption.

Radioimmunoassay for terfenadine in human plasma.

A radioimmunoassay procedure was developed for the antihistamine terfenadine (alpha[4-(1,2-dimethylethyl)phenyl]-4-(hydroxydiphenylmethyl)-1-piperidinebutanol). The keto analog of terfenadine was converted to its O-carboxymethyloxime derivative, which was conjugated to bovine thyroglobulin by a mixed anhydride technique. Rabbits were immunized with the resulting conjugate, and antiserums capable of binding radiolabeled terfenadine were obtained. Tritium-labeled terfenadine was prepared by a combination of exchange and reduction with platinum oxide in the presence of tritium gas, and the procedure yielded a specific activity of 48 Ci/mmole. Plasma containing terfenadine was diluted with sodium carbonate solution and extracted with hexane, and the hexane extracts were evaporated and analyzed. The between-assay coefficient of variation on control samples ranged from 8% at 10 ng/ml to 14% at 1 ng/ml. The lower practical sensitivity limit was at least as low as 0.25 ng/ml (25 pg measured). Two metabolites of terfenadine cross-reacted 16-30% with the antiserum used. However, extraction eliminated essentially all of these compounds. Analysis of plasma samples from human subjects given terfenadine showed marked intersubject variability and low plasma levels.

Identification of basic metabolites of 4-[4-(p-chlo- robenzoyl)piperidino]-4'-fluorobutyrophenone, an experimental neuroleptic agent.

Two metabolites of 4-[4-(p-(chlorobenzoyl)piperidino]-4'-fluorobutyrophenone (RMI 9901) as well as unchanged drug, have been identified in the urine of rats following oral administration of the drug. Analysis of basic urine extracts by combined gas chromatography-mass spectrometry was employed for metabolite identification. N-dealkylation appears to be a major metabolic pathway and results in formation of 4-(p-chlorobenzoyl)piperidine (I). Subsequent oxidation of this metabolite results in the formation of 4-(p-chlorobenzoyl)-2-piperidone (II), and represents rather unusual metabolic pathway for compounds of this chemical class.

Decrease in the activity of the drug-metabolizing enzymes of rat liver following the administration of tilorone hydrochloride.

Tilorone hydrochloride, 2,7-bias(2-(diethylamino)ethoxy(fluoren-9-one dihydrochloride, has been studied to determine its effect on the drug-metabolizing enzymes of the liver of male Charles River CD strain rats. Single and multiple doses of tilorone-HCl, 100 mg/kg/day po, were used. Most experiments were performed 24 hr after the last dose, except for a study 5 hr after dosing, and those in which the duration of effects of tilorone hydrochloride were determined. The hexobarbital sleeping time was prolonged after both single doses and four doses of tilorone hydrochloride. The 4-dose regimen prolonged the zoxazolamine paralysis time but the single dose did not. A decrease in microsomal protein was observed after the single- and 4-dose regimens but not after 21 daily doses of tilorone-HCl. Cytochrome P-450 content of microsomes was decreased by the single doses, 100 and 250 mg/kg po, and by 4 and 21 doses of 100 mg/kg/day po. Activities of aminopyrine demethylase and hexobarbital oxidase also were decreased by the above regimens, but the activity of hexobarbital oxidase was affected more markedly. Electron micrographs of rat liver, after treatment with tilorone-HCl, 100 mg/kg/day for 21 days, revealed many membranous structures in the form of whorls.