Inhibition of human liver microsomal CYP by nateglinide.

OBJECTIVES: Nateglinide is metabolized by CYP2C9 and CYP3A4, therefore drug-drug interactions through cytochrome P450 (CYP) inhibition may occur. In this study, we examined the inhibitory effects of nateglinide and its major metabolite N-[trans-4-(1-hydroxy-1-methylethyl)-cyclohexanecarbonyl]-D-phenylalanine (M1) on various CYP isoforms in human liver microsomes. METHODS: We used typical substrates (7-ethoxyresorufin for CYP1A1/2, tolbutamide for CYP2C9, S-mephenytoin for CYP2C19, bufuralol for CYP2D6, chlorzoxazone for CYP2E1 and midazolam for CYP3A4) in the evaluation of the inhibitory effects, and examined the possibility of mechanism-based inhibition (MBI) by evaluating the influence of pre-incubation in the inhibition. KEY FINDINGS: The results showed that nateglinide inhibited CYP2C9 and CYP2C19 with an IC50(app) (apparent value of the 50% inhibitory concentration) of 125 micromol/l and 946 micromol/l, respectively, while M1 did not inhibit any of the CYP isoforms. The inhibition constant (K(i)) value of the inhibitory effect of nateglinide on CYP2C9 and the 1 + I(in,max,u)/K(i) value were estimated (where I(in,max,u)= the maximum unbound concentration of nateglinide). The 1 + I(in,max,u)/K(i) value was 1.02 (close to 1), suggesting a low risk of drug-drug interactions. The influence of pre-incubation on the inhibition by nateglinide of CYP3A4, CYP2C9 and CYP2C19 was examined. The results revealed that the inhibition of CYP by nateglinide was not influenced by pre-incubation, and that the possibility of MBI is very low. CONCLUSIONS: The possibility of drug-drug interactions involving nateglinide that might be attributable to CYP inhibition is low.

Studies of the toxicological potential of capsinoids, XI: pharmacokinetic and tissue distribution study of 14C-dihydrocapsiate and metabolites in rats.

Pharmacokinetics of a single gavage dose of (14)C-labeled dihydrocapsiate (10 mg/kg) were investigated in male rats. Maximal plasma concentration was achieved in 40 minutes and exhibited an apparent half-life of 2.4 hours. Excretion of radioactivity in the urine, feces, and expired air was 78.2%, 19.4%, and 0.5% of the dose, respectively. Highest tissue concentrations were achieved in the kidney, liver, and blood; the data indicate that radioactivity accumulation following daily exposure at a dose of 10 mg/kg body weight is unlikely. Radioactivity in the plasma was associated with metabolites and their conjugates, probably vanillyl alcohol, vanillic acid, glucuronide of vanillyl alcohol, sulphate of vanillyl alcohol, and sulphate of vanillic acid. These results suggest dihydrocapsiate is metabolized by hydrolysis in the gut, or esterase or other enzymes in the blood, and the metabolites were rapidly absorbed and converted to their conjugates in the liver and eliminated by the kidneys into the urine.

Studies of the toxicological potential of capsinoids, XII: pharmacokinetic study of capsinoid-containing CH-19 Sweet extract in rats.

Pharmacokinetics of the main capsinoid components of CH-19 Sweet extract (capsiate, dihydrocapsiate, and nordihydrocapsiate) were investigated in rats receiving a single gavage dose of extract containing 10 or 100 mg of capsinoids per kilogram in medium-chain triglyceride. Resultant blood levels of these capsinoids and a capsinoid metabolite, vanillyl alcohol, were measured in portal vein and systemic blood. Capsinoids were never detected. Portal compartment vanillyl alcohol concentrations and area under the plasma concentration versus time curve increased approximately with dose, whereas the time to maximum concentration of vanillyl alcohol was independent of dose (30 minutes post dosing), suggesting that precipitation in the stomach or intestines was unlikely. Vanillyl alcohol levels were just barely detectable in systemic plasma (5 minutes post dosing). Significant levels of vanillyl alcohol conjugates, sulfate, and glucuronide were detected in the systemic blood. Given that the orally administered capsinoids were never detected in the portal vein or systemic circulation, these compounds must be broken down (chemically or enzymatically) to vanillyl alcohol.

Studies of the toxicological potential of capsinoids, XIII: inhibitory effects of capsaicin and capsinoids on cytochrome P450 3A4 in human liver microsomes.

This study evaluated potential effects of a number of capsinoids (ie, capsiate, dihydrocapsiate, nordihydrocapsiate) and a single capsaicinoid (ie, capsaicin) on liver microsomal cytochrome P450 3A4-mediated midazolam 1'-hydroxylase activity. Where possible, an inhibition curve was prepared; the concentration at which enzyme activity dropped to 50% was calculated. Capsaicin clearly inhibited cytochrome P450 3A4 activity, losing 50% of the activity at 21.5 micromol/L. No enzyme inhibition was observed in the presence of capsiate, dihydrocapsiate, or nordihydrocapsiate (<100 micromol/L). Preincubation increased the capsaicin inhibitory activity against cytochrome P450 3A4 in a time-dependent manner. Enzyme activity was slightly reduced by capsiate, dihydrocapsiate, and nordihydrocapsiate to the same level as that attained with tolbutamide, the negative control compound. Capsaicin was shown to inhibit cytochrome P450 3A4, probably through a mechanism-based inhibition. In contrast, capsiate, dihydrocapsiate, and nordihydrocapsiate did not inhibit cytochrome P450 3A4 activity and were unlikely to be mechanism-based inhibitors of CYP3A4.

Studies of the toxicological potential of capsinoids: X. Safety assessment and pharmacokinetics of capsinoids in healthy male volunteers after a single oral ingestion of CH-19 Sweet extract.

The safety and pharmacokinetics of capsinoids, physiologically active ingredients of CH-19 Sweet extract, were investigated in 16 healthy male volunteers following a single oral ingestion of CH-19 Sweet extract. The study subjects consumed soft gel capsules containing either capsinoids (15 or 30 mg/person) or placebo. Capsinoids were well tolerated, and no clinically significant changes in physical examinations, blood pressure, heart rate, body temperature, electrocardiogram, hematology, blood chemistry, and urinalysis were observed at either the 15 or 30 mg dose. Body temperature tended to increase after the ingestion of capsinoids, but remained within the normal range. Plasma levels of capsinoids and their metabolite, vanillyl alcohol, were below the lower limit of quantitation. In addition, some study subjects showed increases in urinary excretion of 3-methoxy-4-hydroxyphenylglycol that, when compared to the group receiving the placebo, did not achieve statistical significance.

Prediction of the metabolic interaction of nateglinide with other drugs based on in vitro studies.

Nateglinide is an antidiabetic agent metabolized by CYP2C9 and CYP3A4; hence inhibitors of these CYP isozymes may interact with nateglinide. There are, however, only limited in vitro data on how to predict drug-drug interactions in vivo. We examined the effects of 18 drugs that may be prescribed together with nateglinide (metformin, buformin, aspirin, gemfibrozil, simvastatin, pioglitazone, rosiglitazone, carbamazepine, clarithromycin, gliclazide, clofibrate, fluconazole, bezafibrate, phenylbutazone, nifedipine, famotidine, ibuprofen and miconazole) on the conversion of nateglinide to its major metabolite (N-[trans-4-(1-hydroxy-1-methylethyl)-cyclohexanecarbonyl]-D-phenylalanine) using human liver microsomes. Eight compounds showed a<50% inhibitory effect and we estimated the K(i) values for the remaining 10 compounds. Except for fluconazole and miconazole, 1+I(in, max, u)/K(i) calculated from the K(i) values, was approximately 1 and thus the possibility of a drug-drug interaction was considered low. The value for fluconazole suggested the risk of interaction and agreed with the results of clinical studies in which the AUC of nateglinide increased by 48% when it was co-administered with fluconazole. The present study showed that nateglinide metabolism would hardly be affected by the drugs used in this study, except for miconazole and fluconazole that are potent inhibitors of multiple isoforms of CYPs.