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.

Human serum and plasma protein binding of enoximone and its sulfoxide metabolite.

Experimental factors and determinants of the protein binding of enoximone (a new cardiotonic agent) were investigated in human serum from healthy, drug-free subjects using a rapid ultrafiltration method; these factors and determinants included nonspecific binding to the apparatus, ultrafiltrate volume, temperature, serum pH, enoximone serum concentration, and enoximone sulfoxide (metabolite) concentration. It was demonstrated from mass balance experiments that nonspecific binding to the apparatus did not occur. Within the range investigated, ultrafiltrate volume did not affect the binding result. However, serum pH and temperature were critical variables. At pH 7.4 and 37 degrees C, enoximone serum binding occurred to the extent of approximately 70%; over the therapeutic serum concentration range, this binding was concentration independent. Experiments with purified albumin solutions indicated that much of the serum binding could be accounted for by albumin. At concentrations exceeding those observed clinically, enoximone sulfoxide did not affect enoximone serum binding. In another experiment, enoximone binding to serum was compared with that from plasma containing either heparin or disodium EDTA. There were essentially no differences. Enoximone sulfoxide serum protein binding was also investigated in serum from healthy, drug-free human subjects; binding occurred to the extent of approximately 5%.

Pharmacokinetics and biotransformation studies of terfenadine in man.

Comparison of plasma level data obtained from a dose-response study to that of a 14C material balance study indicates that 99.48% of the alpha-[4-(1,1-dimethylethyl)phenyl] 4-(hydroxydiphenylmethyl)-1-piperidinebutanol (terfenadine, RMI 9918, Triludan, Teldane, resp.) related material absorbed undergoes biotransformation. From the balance study it has been determined that fecal excretion accounts for ca. 60% of the dose administered. Thin-layer chromatographic analysis of urine and feces revealed the presence of two major metabolic products. These compounds have been isolated and their structures determined by GC-MS. One of the metabolic products, a carboxylic acid analog of terfenadine has been shown to have antihistaminic activity and may play a role in the activity of the parent drug in vivo.

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.

Toxicity and teratogenicity studies with the hypolipidemic drug RMI 14,514 in rats.

The hypolipidemic drug RMI 14,514 (5-tetradecyloxy-2-furoic acid) has an oral LD50 of over 5000 mg/kg in rats. In a chronic toxicity study (6 months drug diet) doses of 30, 100, or 300 mg/kg/day produced no obvious signs of toxicity or abnormal clinical pathology parameters, other than prominent growth retardation at 300 mg/kg, which was somewhat alleviated when the dose was reduced to 200 mg/kg after 6 weeks. Hepatic change in the form of mild lipid accumulation was noted histopathologically after 6 months of treatment at 100 or 300 mg/kg/day, but was not present at 3 months or after 4 weeks off drug. The administration of RMI 14,514 in the diet to pregnant rats at 30, 100, or 150 mg/kg/day on Days 7 thru 21 of pregnancy (day 1 = day sperm detected) did not induce any teratogenic effects. When rats were exposed to the drug from implantation thru sexual maturity (126 days of age) at the same dosage, it produced no adverse developmental or behavioral effects, except for slight reduction in weight gain from birth to sexual maturity at 150 mg/kg/day. The drug caused reductions in plasma cholesterol and total fatty acids, but no distinct changes in various tissue lipids, except in the erythrocyte where fatty acids and phospholipids were reduced. These differences did not affect membrane integrity of the erythrocyte as far as osmotic or mechanical fragility tests could determine. The drug, which bears a structural resemblance to long-chain fatty acids, was incorporated into tissue lipids in detectable amounts, but tended to disappear from tissues at a rate similar to that of expected lipid turnover after treatment was stopped.

The objective and timing of drug disposition studies, appendix II. Plasma concentrations of oxyphenbutazone in dogs given oxyphenbutazone or the calcium or sodium salts of its phosphate ester.

Plasma levels of oxyphenbutazone were measured in beagle dogs following oral administration of oxyphenbutazone and salts of oxyphenbutazone phosphate. Dosage with sodium and calcium salts of oxyphenbutazone phosphate produced higher oxyphenbutazone plasma levels than dosage with oxyphenbutazone itself. Oxyphenbutazone phosphate was not detected in plasma following oral dosing with salts of oxyphenbutazone phosphate but was found in plasma following intramuscular administration of oxyphenbutazone phosphate sodium salt. A metabolite, oxyphenbutazone glucuronide, was found in plasma of dogs following administration of either oxyphenbutazone or salts of oxyphenbutazone phosphate.