Effects of gemfibrozil, itraconazole, and their combination on the pharmacokinetics of pioglitazone

BACKGROUND AND OBJECTIVE: The thiazolidinedione antidiabetic drug pioglitazone is metabolized mainly by cytochrome P450 (CYP) 2C8 and CYP3A4 in vitro. Our objective was to study the effects of gemfibrozil, itraconazole, and their combination on the pharmacokinetics of pioglitazone to determine the role of these enzymes in the fate of pioglitazone in humans. METHODS: In a randomized, double-blind, 4-phase crossover study, 12 healthy volunteers took either 600 mg gemfibrozil or 100 mg itraconazole (first dose, 200 mg), both gemfibrozil and itraconazole, or placebo twice daily for 4 days. On day 3, they received a single dose of 15 mg pioglitazone. Plasma drug concentrations and the cumulative excretion of pioglitazone and its metabolites into urine were measured for up to 48 hours. RESULTS: Gemfibrozil alone raised the mean total area under the plasma concentration-time curve from time 0 to infinity [AUC(0-infinity)] of pioglitazone 3.2-fold (range, 2.3-fold to 6.5-fold; P < .001) and prolonged its elimination half-life (t (1/2) ) from 8.3 to 22.7 hours ( P < .001) but had no significant effect on its peak concentration (C max ) compared with placebo (control). Gemfibrozil increased the 48-hour excretion of pioglitazone into urine by 2.5-fold ( P < .001) and reduced the ratios of the active metabolites M-III and M-IV to pioglitazone in plasma and urine. Gemfibrozil decreased the area under the plasma concentration-time curve from time 0 to 48 hours [AUC(0-48)] of the metabolites M-III and M-IV by 42% ( P < .05) and 45% ( P < .001), respectively, but their total AUC(0-infinity) values were reduced by less or not at all. Itraconazole had no significant effect on the pharmacokinetics of pioglitazone and did not alter the effect of gemfibrozil on pioglitazone pharmacokinetics. The mean area under the concentration versus time curve to 49 hours [AUC(0-49)] of itraconazole was 46% lower ( P < .001) during the gemfibrozil-itraconazole phase than during the itraconazole phase. CONCLUSIONS: Gemfibrozil elevates the plasma concentrations of pioglitazone, probably by inhibition of its CYP2C8-mediated metabolism. CYP2C8 appears to be of major importance and CYP3A4 of minor importance in pioglitazone metabolism in vivo in humans. Concomitant use of gemfibrozil with pioglitazone may increase the effects and risk of dose-related adverse effects of pioglitazone. However, studies in diabetic patients are needed to determine the clinical significance of the gemfibrozil-pioglitazone interaction.     

Plasma concentrations of active simvastatin acid are increased by gemfibrozil

BACKGROUND: Concomitant treatment with simvastatin and gemfibrozil, two lipid-lowering drugs, has been associated with occurrence of myopathy in case reports. The aim of this study was to determine whether gemfibrozil affects the pharmacokinetics of simvastatin and whether it affects CYP3A4 activity in vitro. METHODS: A double-blind, randomized crossover study with two phases (placebo and gemfibrozil) was carried out. Ten healthy volunteers were given gemfibrozil (600 mg twice daily) or placebo orally for 3 days. On day 3 they ingested a single 40-mg dose of simvastatin. Plasma concentrations of simvastatin and simvastatin acid were measured up to 12 hours. In addition, the effect of gemfibrozil (0 to 1,200 micromol/L) on midazolam 1'-hydroxylation, a CYP3A4 model reaction, was investigated in human liver microsomes in vitro. RESULTS: Gemfibrozil increased the mean total area under the plasma concentration-time curve of simvastatin [AUC(0-infinity)] by 35% (P < .01) and the AUC(0-infinity) of simvastatin acid by 185% (P < .001). The elimination half-life of simvastatin was increased by 74% (P < .05), and that of simvastatin acid was increased by 51% (P < .01) by gemfibrozil. The peak concentration of simvastatin acid was increased by 112%, from 3.20 +/- 2.73 ng/mL to 6.78 +/- 4.67 ng/mL (mean +/- SD; P < .01). In vitro, gemfibrozil showed no inhibition of midazolam 1'-hydroxylation. CONCLUSIONS: Gemfibrozil increases plasma concentrations of simvastatin and, in particular, its active form, simvastatin acid, suggesting that the increased risk of myopathy in combination treatment is, at least partially, of a pharmacokinetic origin. Because gemfibrozil does not inhibit CYP3A4 in vitro, the mechanism of the pharmacokinetic interaction is probably inhibition of non-CYP3A4-mediated metabolism of simvastatin acid.     

Gemfibrozil greatly increases plasma concentrations of cerivastatin

BACKGROUND: Concomitant use of gemfibrozil with statins, particularly with cerivastatin, increases the risk of rhabdomyolysis, but the mechanism of this potentially fatal drug interaction remains unclear. Our aim was to study the effect of gemfibrozil on cerivastatin pharmacokinetics. METHODS: In a randomized, double-blind crossover study, 10 healthy volunteers took 600 mg gemfibrozil or placebo twice daily for 3 days. On day 3, each subject ingested a single 0.3-mg dose of cerivastatin. Plasma concentrations of cerivastatin, its metabolites, and gemfibrozil were measured up to 24 hours. RESULTS: During gemfibrozil treatment, the area under the plasma concentration-time curve [AUC(0-infinity)] of parent cerivastatin was on average 559% (range, 138% to 995%; P =.0002) and the peak concentration in plasma was 307% (138% to 809%; P =.0019) of the corresponding values in the placebo phase. Gemfibrozil increased the AUC(0-infinity) of cerivastatin lactone, on average, to 440% (94% to 594%; P =.0024) and that of metabolite M-1 to 435% (216% to 802%; P =.0002) of the control (placebo) values, whereas the AUC(0-24) of metabolite M-23 was decreased to 22% (11% to 74%; P =.0017). CONCLUSIONS: Gemfibrozil greatly increases plasma concentrations of cerivastatin, cerivastatin lactone, and metabolite M-1, whereas the level of metabolite M-23 is markedly reduced by gemfibrozil. Gemfibrozil therefore inhibits the formation of M-23, which is thought to be dependent on CYP2C8. The increased exposure to cerivastatin in the presence of gemfibrozil may explain the high incidence of myopathy observed with this combination, although the role of pharmacodynamic interactions between these 2 agents cannot be excluded.     

The cytochrome P4503A4 inhibitor clarithromycin increases the plasma concentrations and effects of repaglinide.

OBJECTIVE: Our objective was to study the effects of the macrolide antibiotic clarithromycin on the pharmacokinetics and pharmacodynamics of repaglinide, a novel short-acting antidiabetic drug. METHODS: In a randomized, double-blind, 2-phase crossover study, 9 healthy volunteers were treated for 4 days with 250 mg oral clarithromycin or placebo twice daily. On day 5 they received a single dose of 250 mg clarithromycin or placebo, and 1 hour later a single dose of 0.25 mg repaglinide was given orally. Plasma repaglinide, serum insulin, and blood glucose concentrations were measured up to 7 hours. RESULTS: Clarithromycin increased the mean total area under the concentration-time curve of repaglinide by 40% (P <.0001) and the peak plasma concentration by 67% (P <.005) compared with placebo. The mean elimination half-life of repaglinide was prolonged from 1.4 to 1.7 hours (P <.05) by clarithromycin. Clarithromycin increased the mean incremental area under the concentration-time curve from 0 to 3 hours of serum insulin by 51% (P <.05) and the maximum increase in the serum insulin concentration by 61% (P <.01) compared with placebo. No statistically significant differences were found in the blood glucose concentrations between the placebo and clarithromycin phases. CONCLUSIONS: Even low doses of the cytochrome P4503A4 (CYP3A4) inhibitor clarithromycin increase the plasma concentrations and effects of repaglinide. Concomitant use of clarithromycin or other potent inhibitors of CYP3A4 with repaglinide may enhance its blood glucose-lowering effect and increase the risk of hypoglycemia.     

Effects of fluconazole and fluvoxamine on the pharmacokinetics and pharmacodynamics of glimepiride

OBJECTIVE: Our objective was to study the effects of fluconazole and fluvoxamine on the pharmacokinetics and pharmacodynamics of glimepiride, a new sulfonylurea antidiabetic drug. METHODS: In this randomized, double-blind, three-phase crossover study, 12 healthy volunteers took 200 mg of fluconazole once daily (400 mg on day 1), 100 mg of fluvoxamine once daily, or placebo once daily for 4 days. On day 4, a single oral dose of 0.5 mg of glimepiride was administered. Plasma glimepiride and blood glucose concentrations were measured up to 12 hours. RESULTS: In the fluconazole phase, the mean total area under the plasma concentration-time curve of glimepiride was 238% (P <.0001) and the peak plasma concentration was 151% (P <.0001) of the respective control value. The mean elimination half-life of glimepiride was prolonged from 2.0 to 3.3 hours (P <.0001) by fluconazole. In the fluvoxamine phase, the mean area under the plasma concentration-time curve of glimepiride was not significantly different from that in the placebo phase. However, the mean peak plasma concentration of glimepiride was 143% (P <.05) of the control and the elimination half-life was prolonged from 2.0 to 2.3 hours (P <.01) by fluvoxamine. Fluconazole and fluvoxamine did not cause statistically significant changes in the effects of glimepiride on blood glucose concentrations. CONCLUSIONS: Fluconazole considerably increased the area under the plasma concentration-time curve of glimepiride and prolonged its elimination half-life. This was probably caused by inhibition of the cytochrome P-450 2C9-mediated biotransformation of glimepiride by fluconazole. Concomitant use of fluconazole with glimepiride may increase the risk of hypoglycemia as much as would a 2- to 3-fold increase in the dose of glimepiride. Fluvoxamine moderately increased the plasma concentrations and slightly prolonged the elimination half-life of glimepiride.     

The effect of gemfibrozil on repaglinide pharmacokinetics persists for at least 12 h after the dose: evidence for mechanism-based inhibition of CYP2C8 in vivo

Repaglinide is metabolized by cytochrome P450 (CYP) 2C8 and 3A4. Gemfibrozil has the effect of increasing the area under the concentration-time curve (AUC) of repaglinide eightfold. We studied the effect of dosing interval on the extent of the gemfibrozil-repaglinide interaction. In a randomized five-phase crossover study, 10 healthy volunteers ingested 0.25 mg repaglinide, with or without gemfibrozil pretreatment. Plasma repaglinide, gemfibrozil, their metabolites, and blood glucose were measured. When the last dose of 600 mg gemfibrozil was ingested simultaneously with repaglinide, or 3, 6, or 12 h before, it increased the AUC(0-infinity) of repaglinide 7.0-, 6.5-, 6.2- and 5.0-fold, respectively (P < 0.001). The peak repaglinide concentration increased approximately twofold (P < 0.001), and the half-life was prolonged from 1.2 h to 2-3 h (P < 0.001) during all the gemfibrozil phases. The drug interaction effects persisted at least 12 h after gemfibrozil was administered, although plasma gemfibrozil and gemfibrozil 1-O-beta-glucuronide concentrations were only 5 and 10% of their peak values, respectively. The long-lasting interaction is likely caused by mechanism-based inhibition of CYP2C8 by gemfibrozil glucuronide.     

Plasma concentrations of active lovastatin acid are markedly increased by gemfibrozil but not by bezafibrate

BACKGROUND: Concomitant use of fibrates with statins has been associated with an increased risk of myopathy, but the underlying mechanism of this adverse reaction remains unclear. Our aim was to study the effects of bezafibrate and gemfibrozil on the pharmacokinetics of lovastatin. METHODS: This was a randomized, double-blind, 3-phase crossover study. Eleven healthy volunteers took 400 mg/day bezafibrate, 1200 mg/day gemfibrozil, or placebo for 3 days. On day 3, each subject ingested a single 40 mg dose of lovastatin. Plasma concentrations of lovastatin, lovastatin acid, gemfibrozil, and bezafibrate were measured up to 24 hours. RESULTS: Gemfibrozil markedly increased the plasma concentrations of lovastatin acid, without affecting those of the parent lovastatin compared with placebo. During the gemfibrozil phase, the mean area under the plasma concentration-time curve from 0 to 24 hours [AUC(0-24)] of lovastatin acid was 280% (range, 131% to 1184%; P < .001) and the peak plasma concentration (Cmax) was 280% (range, 123% to 1042%; P < .05) of the corresponding value during the placebo phase. Bezafibrate had no statistically significant effect on the AUC(0-24) or Cmax of lovastatin or lovastatin acid compared with placebo. CONCLUSIONS: Gemfibrozil markedly increases plasma concentrations of lovastatin acid, but bezafibrate does not. The increased risk of myopathy observed during concomitant treatment with statins and fibrates may be partially of a pharmacokinetic origin. The risk of developing myopathy during concomitant therapy with lovastatin and a fibrate may be smaller with bezafibrate than with gemfibrozil.     

Gemfibrozil increases plasma pravastatin concentrations and reduces pravastatin renal clearance

BACKGROUND: Gemfibrozil increases the plasma concentrations of active acid forms of cerivastatin, lovastatin, and simvastatin. Pravastatin pharmacokinetics differs from those of these 3 statins, which are extensively metabolized. Our aim was to study the effects of gemfibrozil on the pharmacokinetics of pravastatin. METHODS: A randomized, placebo-controlled, 2-phase crossover study was carried out. Ten healthy volunteers took gemfibrozil (1200 mg/d) or placebo for 3 days. On day 3, each subject ingested a single 40-mg dose of pravastatin. The concentrations of pravastatin and gemfibrozil in plasma and the cumulative excretion of pravastatin into urine were measured up to 24 hours. RESULTS: During the gemfibrozil phase, the mean total area under the plasma concentration-time curve (AUC) of pravastatin from 0 hours to infinity was 202% (range, 40%-412%) of the corresponding value during the placebo phase (P <.05), but there was no difference in the half-life between the phases. The renal clearance of pravastatin was reduced from 25 L/h to 14 L/h by gemfibrozil (P <.0001), but the cumulative excretion of pravastatin into urine did not change significantly. The increase in the AUC of pravastatin from 0 to 24 hours correlated significantly with the decrease in the renal clearance of pravastatin (r = 0.72, P =.02). However, the change in renal clearance was only a minor contributor to the increase in pravastatin AUC. CONCLUSIONS: Gemfibrozil increases plasma concentrations of pravastatin. This is partly but not solely the result of the reduced renal clearance of pravastatin. The increase in pravastatin AUC from 0 hours to infinity by gemfibrozil may represent an interference with a transport protein.     

The effect of gemfibrozil on the pharmacokinetics of rosuvastatin

BACKGROUND: Coadministration of statins and gemfibrozil is associated with an increased risk for myopathy, which may be due in part to a pharmacokinetic interaction. Therefore the effect of gemfibrozil on rosuvastatin pharmacokinetics was assessed in healthy volunteers. Rosuvastatin has been shown to be a substrate for the human hepatic uptake transporter organic anion transporter 2 (OATP2). Inhibition of this transporter could increase plasma concentrations of rosuvastatin. The effect of gemfibrozil on rosuvastatin uptake by cells expressing OATP2 was also examined. METHODS: In a randomized, double-blind, 2-period crossover trial, 20 healthy volunteers were given oral doses of gemfibrozil, 600 mg, or placebo twice daily for 7 days. On the fourth morning of each dosing period, a single oral dose of rosuvastatin, 80 mg, was coadministered. Plasma concentrations of rosuvastatin, N-desmethyl rosuvastatin, and rosuvastatin-lactone were measured. In addition, the effect of gemfibrozil on the uptake of radiolabeled rosuvastatin by OATP2-transfected Xenopus oocytes was studied. RESULTS: Gemfibrozil increased the rosuvastatin area under the plasma concentration-time curve from time 0 to the time of the last quantifiable concentration [AUC(0-t)] 1.88-fold (90% confidence interval, 1.60-2.21) and the maximum observed rosuvastatin plasma concentration (C(max)) 2.21-fold (90% confidence interval, 1.81-2.69) compared with placebo. N-desmethyl rosuvastatin AUC(0-t) and C(max) decreased by 48% and 39%, respectively. Pharmacokinetics of rosuvastatin-lactone was unchanged. The in vitro results indicate that the maximum gemfibrozil inhibition of rosuvastatin OATP2-mediated uptake was 50%; the inhibition constant for the inhibitory process was 4.0 +/- 1.3 micromol/L. CONCLUSIONS: Gemfibrozil increased rosuvastatin plasma concentrations approximately 2-fold, which is similar to the effect of gemfibrozil on pravastatin, simvastatin acid, and lovastatin acid plasma concentrations and substantially less than the effect observed for cerivastatin. Gemfibrozil inhibition of OATP2-mediated rosuvastatin hepatic uptake may contribute to the mechanism of the drug-drug interaction. Care is warranted when gemfibrozil is coadministered with rosuvastatin and other statins.     

Impact of CYP2C9 amino acid polymorphisms on glyburide kinetics and on the insulin and glucose response in healthy volunteers

BACKGROUND: Glyburide (INN, glibenclamide) is a second-generation sulfonylurea antidiabetic agent with high potency. We hypothesized that glyburide may be a substrate of cytochrome P450 2C9 (CYP2C9), an enzyme that has two low-activity amino acid variants-Arg144Cys (CYP2C9*2) and Ile359Leu (CYP2C9*3). We explored the impact of these polymorphisms on glyburide pharmacokinetics and the effects on insulin and glucose concentrations. METHODS: Twenty-one healthy volunteers who represented all possible combinations of the two variant alleles were studied (genotypes CYP2C9*1/*1, *1/*2, *2/*2, *1/*3, *2/*3, and *3/*3 ). They received a single oral dose of 3.5 mg glyburide followed by 75 g glucose at 1, 4.5, and 8 hours after administration of glyburide. Glyburide was quantified in plasma by reversed-phase HPLC. Venous blood concentrations of glyburide, insulin, and glucose were analyzed with a population pharmacokinetic-pharmacodynamic model by use of NONMEM statistical software. RESULTS: Pharmacokinetics of glyburide depended significantly on CYP2C9 genotypes. In homozygous carriers of the genotype *3/*3, total oral clearance was less than half of that of the wild-type genotype *1/*1 (P <.001). Correspondingly, insulin secretion measured within 12 hours after glyburide ingestion was higher in carriers of the genotype *3/*3 compared with the other genotypes (P =.028), whereas the differences in glucose concentrations were not significant. CONCLUSIONS: Carriers of the CYP2C9 variant *3 had decreased oral clearances of glyburide. This confirms that glyburide is metabolized by CYP2C9. Corresponding differences in insulin plasma levels indicated that dose adjustment based on CYP2C9 genotype may improve antidiabetic treatment.     

Effect of rifampin on the pharmacokinetics and pharmacodynamics of gliclazide

OBJECTIVE: Our objective was to investigate the effect of rifampin (INN, rifampicin) on the pharmacokinetics and pharmacodynamics of gliclazide, a sulfonylurea antidiabetic drug. METHOD: In a randomized 2-way crossover study with a 4-week washout period, 9 healthy Korean subjects were treated once daily for 6 days with 600 mg rifampin or with placebo. On day 7, a single dose of 80 mg gliclazide was administered orally. Plasma gliclazide, blood glucose, and insulin concentrations were measured. RESULTS: Rifampin decreased the mean area under the plasma concentration-time curve for gliclazide by 70% (P <.001) and the mean elimination half-life from 9.5 to 3.3 hours (P <.05). The apparent oral clearance of gliclazide increased about 4-fold after rifampin treatment (P <.001). A significant difference in the blood glucose response to gliclazide was observed between the placebo and rifampin phases. CONCLUSION: The effect of rifampin on the pharmacokinetics and pharmacodynamics of gliclazide suggests that rifampin affects the disposition of gliclazide in humans, possibly by the induction of cytochrome P450 2C9. Concomitant use of rifampin with gliclazide can considerably reduce the glucose-lowering effects of gliclazide.     

Effects of rifampin on the pharmacokinetics and pharmacodynamics of glyburide and glipizide.

OBJECTIVE: To study the effects of rifampin (INN, rifampicin) on the pharmacokinetics and pharmacodynamics of glyburide (INN, glibenclamide) and glipizide, 2 sulfonylurea antidiabetic drugs. METHODS: Two separate, randomized, 2-phase, crossover studies with an identical design were conducted. In each study, 10 healthy volunteers received 600 mg rifampin or placebo once daily for 5 days. On day 6, a single dose of 1.75 mg glyburide (study I) or 2.5 mg glipizide (study II) was administered orally. Plasma glyburide and glipizide and blood glucose concentrations were measured for 12 hours. RESULTS: In study I, rifampin decreased the area under the plasma concentration--time curve [AUC(0-infinity)] of glyburide by 39% (P <.001) and the peak plasma concentration by 22% (P =.01). The elimination half-life of glyburide was shortened from 2.0 to 1.7 hours (P <.05) by rifampin. The blood glucose decremental AUC(0-7) (net area below baseline) and the maximum decrease in the blood glucose concentration were decreased by 44% (P =.05) and 36% (P <.001), respectively, by rifampin. In study II, rifampin decreased the AUC(0-infinity) of glipizide by 22% (P <.05) and shortened its half-life from 3.0 to 1.9 hours (P =.01). No statistically significant differences in the blood glucose concentrations were found between the phases; however, 4 subjects had moderate hypoglycemia during the placebo phase but only 1 subject had moderate hypoglycemia during the rifampin phase. CONCLUSIONS: Rifampin moderately decreased the plasma concentrations and effects of glyburide but had only a slight effect on glipizide. The mechanism underlying the interaction between rifampin and glyburide is probably induction of either CYP2C9 or P-glycoprotein or both. Induction of CYP2C9 would explain the increased systemic elimination of glipizide. It is probable that the blood glucose--lowering effect of glyburide is reduced during concomitant treatment with rifampin. In some patients, the effects of glipizide may also be reduced by rifampin.     

Telithromycin, but not montelukast, increases the plasma concentrations and effects of the cytochrome P450 3A4 and 2C8 substrate repaglinide

BACKGROUND AND OBJECTIVE: The antidiabetic repaglinide is metabolized by cytochrome P450 (CYP) 2C8 and CYP3A4. Telithromycin, an antimicrobial agent, inhibits CYP3A4 in vitro and in vivo. Montelukast, an antiasthmatic drug, is a potent inhibitor of CYP2C8 in vitro. We studied the effects of telithromycin, montelukast, and the combination of telithromycin and montelukast on the pharmacokinetics and pharmacodynamics of repaglinide. METHODS: In a randomized 4-phase crossover study, 12 healthy volunteers received 800 mg telithromycin, 10 mg montelukast, both telithromycin and montelukast, or placebo once daily for 3 days. On day 3, they ingested a single 0.25-mg dose of repaglinide. Plasma and urine concentrations of repaglinide and its metabolites M1, M2, and M4, as well as blood glucose concentrations, were measured for 12 hours. RESULTS: Telithromycin alone raised the mean peak plasma repaglinide concentration to 138% (range, 91%-209%; P = .006) and the total area under the plasma concentration-time curve from 0 hours to infinity [AUC0-infinity] of repaglinide to 177% (range, 125%-257%; P < .001) of control (placebo). Telithromycin reduced the AUC0-infinity ratio of the metabolite M1 to repaglinide by 68% (P < .001) and the urinary excretion ratio of M1 to repaglinide by 77% (P = .001). In contrast to previous estimates based on in vitro CYP2C8 inhibition data, montelukast had no significant effect on the pharmacokinetics of repaglinide or its metabolites and did not significantly alter the effect of telithromycin on repaglinide pharmacokinetics. Telithromycin, unlike montelukast, lowered the maximum blood glucose concentration (P = .002) and mean blood glucose concentration from 0 to 3 hours (P = .008) after repaglinide intake, as compared with placebo. CONCLUSIONS: Telithromycin increases the plasma concentrations and blood glucose-lowering effect of repaglinide by inhibiting its CYP3A4-catalyzed biotransformation and may increase the risk of hypoglycemia. Unexpectedly, montelukast has no significant effect on repaglinide pharmacokinetics, suggesting that it does not significantly inhibit CYP2C8 in vivo. The low free fraction of montelukast in plasma may explain the lack of effect on CYP2C8 in vivo, despite the low in vitro inhibition constant, highlighting the importance of incorporating plasma protein binding to interaction predictions.     

Pharmacokinetic and pharmacodynamic modelling of the effects of glimepiride on insulin secretion and glucose lowering in healthy humans

Glimepiride is an oral sulfonylurea antihyperglycaemic agent. We used pharmacokinetic-pharmacodynamic (PK-PD) modelling to analyse the relationship between plasma glimepiride concentration, insulin secretion and glucose lowering to determine the effects of the drug in healthy volunteers. A single 2-mg oral dose of glimepiride was administered to six healthy volunteers. The control group received a placebo. All subjects consumed 12 g of sugar immediately after drug administration in order to standardize the initial plasma glucose levels. Serial blood sampling was performed for 9 h after oral dosing. Plasma glimepiride, insulin and glucose levels were determined by validated methods (LC/MS/MS assay, hexokinase method and radioimmunoassay respectively). Time courses of plasma glimepiride concentration, insulin secretion, and glucose lowering effects were analysed by means of PK-PD modelling with the ADAPT II program. The time course of the plasma concentrations followed a two-compartmental model with a lag time. The glimepiride concentration peaked at 191.5 ng/mL at approximately 4 h after administration. The maximal increase in insulin secretion was 9.98 mIU/L and the maximal decrease in plasma glucose was 19.33 mg/dL. Both peak effects occurred at approximately 2.5 h after drug intake. The glucose disappearance model was used to analyse glimepiride's insulin secretion and glucose lowering effects. The PK-PD model described well the relationship between plasma glimepiride and its insulin secretion and hypoglycaemic effects in healthy volunteers.     

Mechanism-based inactivation of CYP2C8 by gemfibrozil occurs rapidly in humans

To study the time to onset of mechanism-based inactivation of cytochrome P450 (CYP) 2C8 by gemfibrozil in vivo, we conducted a randomized five-phase crossover study in 10 healthy volunteers. In one phase the volunteers ingested 0.25?mg of repaglinide alone (control), and in the other phases they received 600?mg of gemfibrozil 0-6?h prior to the repaglinide dose. When gemfibrozil was taken 0, 1, 3, or 6?h before repaglinide, the geometric mean ratio relative to control (90% confidence interval (CI)) of repaglinide area under the plasma concentration-time curve (AUC(0-∞)) was 5.0-fold (4.3-5.7-fold), 6.3-fold (5.4-7.5-fold), 6.6-fold (5.6-7.7-fold), and 5.4-fold (4.8-6.1-fold), respectively (P < 0.001 vs. control). The geometric mean ratio relative to control (90% CI) of the maximum plasma concentration (C(max)) of the CYP2C8-mediated metabolite M4 was 1.0-fold (0.8-1.3-fold), 0.10-fold (0.06-0.17-fold, P < 0.001), 0.06-fold (0.04-0.10-fold, P < 0.001), and 0.09-fold (0.05-0.14-fold, P < 0.001), respectively. The strong inactivation of CYP2C8, evident as soon as 1?h after gemfibrozil dosing, has implications in clinical practice and in studies with gemfibrozil as a CYP2C8 model inhibitor.     

Effects of trimethoprim and rifampin on the pharmacokinetics of the cytochrome P450 2C8 substrate rosiglitazone

BACKGROUND: Trimethoprim is a relatively selective inhibitor of the cytochrome P450 (CYP) 2C8 enzyme in vitro. Rifampin (INN, rifampicin) is a potent inducer of several CYP enzymes, and in vitro studies have suggested that it also induces CYP2C8. OBJECTIVE: Our aims were to investigate possible effects of trimethoprim and rifampin on CYP2C8 activity by use of rosiglitazone, a thiazolidinedione antidiabetic drug metabolized primarily by CYP2C8, as an in vivo probe. METHODS: Two separate randomized crossover studies with 2 phases were conducted. In study 1, 10 healthy volunteers took 160 mg trimethoprim or placebo orally twice daily for 4 days. On day 3, they ingested a single 4-mg dose of rosiglitazone. In study 2, 10 healthy volunteers took 600 mg rifampin or placebo orally once daily for 5 days. On day 6, they ingested a single 4-mg dose of rosiglitazone. In both studies, plasma rosiglitazone and N -desmethylrosiglitazone concentrations were measured for up to 48 hours. Results In study 1, trimethoprim raised the area under the plasma rosiglitazone concentration-time curve [AUC(0- infinity )] by 37% (range, 16% to 51%; P <.0001) and the peak plasma rosiglitazone concentration (C max ) by 14% (range, -3% to 38%; P =.0014). The elimination half-life (t 1/2 ) of rosiglitazone was prolonged from 3.8 to 4.8 hours ( P =.0013). Trimethoprim reduced the formation of N -desmethylrosiglitazone. In study 2, rifampin reduced the AUC(0- infinity ) and C max of rosiglitazone by 54% (range, 46% to 63%; P <.0001) and 28% (range, 2% to 56%; P =.0003), respectively. The t 1/2 of rosiglitazone was shortened from 3.8 to 1.9 hours ( P <.0001). Rifampin increased the formation of N -desmethylrosiglitazone. CONCLUSIONS: Trimethoprim raises and rifampin reduces the plasma concentrations of rosiglitazone by inhibiting and inducing, respectively, the CYP2C8-catalyzed biotransformation of rosiglitazone.     

Gemfibrozil in familial combined hyperlipidaemia: effect of added low-dose cholestyramine on plasma and biliary lipids

Gemfibrozil is frequently used for lipid-lowering in familial combined hyperlipidaemia (FCHL) and in other forms of combined hyperlipidaemia. This therapy increases biliary cholesterol saturation, enhancing the risk for gallstone formation. Furthermore, in hypertriglyceridaemia, LDL cholesterol levels often tend to rise. We have explored the possibility that addition of a low dose of cholestyramine to gemfibrozil therapy obliterates these phenomena. Eighteen gallstone-free patients with definite (n = 5) or probable (n = 10) FCHL, or combined hyperlipoproteinaemia (n = 3) were randomized to a 6 week treatment with gemfibrozil, 600 mg b.i.d., or gemfibrozil 600 mg b.i.d. plus 4 g cholestyramine o.d. After 6 weeks the patients were crossed over to the alternative treatment. Plasma lipoproteins and biliary lipids were determined at baseline and at the end of each period. Institution of gemfibrozil treatment resulted in a decrease in plasma cholesterol by 15% (P less than 0.05) and in plasma triglycerides by 47% (P less than 0.05); HDL cholesterol increased by 18% (P less than 0.05). Addition of cholestyramine further decreased plasma and LDL total cholesterol by 9% (P less than 0.05). Total triglycerides and HDL cholesterol did not change. Gemfibrozil treatment was associated with a rise in the relative biliary concentration of cholesterol from 5.6 +/- 0.4 to 6.9 +/- 0.5 molar percent (P less than 0.01), and a parallel decrease in the relative concentration of bile acids, resulting in an increased cholesterol saturation of the bile, from 77 +/- 5 to 90 +/- 6% (P less than 0.05). This change was not observed during the combined therapy (mean cholesterol saturation, 82 +/- 4%).(ABSTRACT TRUNCATED AT 250 WORDS)     

Efficacy of sulfonylureas with insulin in type 2 diabetes mellitus

BACKGROUND: In subjects with type 2 diabetes mellitus, glycemic control deteroriates while patients use sulfonylurea drugs during the course of the disease. Adjunctive therapy with insulin at this stage requires a lesser daily insulin dose in comparison with insulin monotherapy while restoring desirable glycemic control. However, data regarding direct comparison between various sulfonylureas in this regard are lacking. OBJECTIVE: To examine comparative efficacies of adjunctive therapy with insulin in subjects with type 2 diabetes manifesting lapse of glycemic control while receiving various individual sulfonylurea drugs. METHODS: Four groups of 10 subjects, each presenting with glycosylated hemoglobin (HbA(1C)) >8.0% while using either tolazamide, glyburide, glipizide Gastrointestinal Therapeutic System (GITS), or glimepiride, were recruited. Two from each group were randomized to receive placebo; the others continued the same drug. Pre-supper subcutaneous 70 NPH/30 regular insulin was initiated at 10 units and gradually increased and adjusted as necessary to attain fasting blood glucose levels between 80 and 120 mg/dL and maintain the same range for 6 months. Fasting plasma glucose, plasma C-peptide, and HbA(1C) concentrations were determined prior to the addition of insulin and at the end of the study. Daily insulin dose and changes in body weight (BW) were noted at the end of the study, and the number of hypoglycemic events during the last 4 weeks of the study was determined.RESULTS: Daily insulin dose (units/kg BW), weight gain, and number of hypoglycemic events were significantly lower (p < 0.01) in subjects receiving sulfonylureas in comparison with placebo. However, the daily insulin dose alone was significantly lower (p < 0.05) with glimepiride (0.49 +/- 0.10; mean +/- SE) than with other sulfonylureas (tolazamide 0.58 +/- 0.12, glyburide 0.59 +/- 0.12, glipizide GITS 0.59 +/- 0.14). Finally, a significant correlation (r = 0.68; p < 0.001) was noted between suppression of plasma C-peptide level and the daily insulin dose among all participants. CONCLUSIONS: By lowering the daily insulin dose, sulfonylurea drugs appear to improve the sensitivity of exogenous insulin in subjects with type 2 diabetes mellitus manifesting lapse of glycemic control. Moreover, glimepiride appears to possess a greater insulin-sparing property than other sulfonylureas.     

Gemfibrozil is a strong inactivator of CYP2C8 in very small multiple doses

Therapeutic doses of gemfibrozil cause mechanism-based inactivation of CYP2C8 via formation of gemfibrozil 1-O-β-glucuronide. We investigated the extent of CYP2C8 inactivation caused by three different doses of gemfibrozil twice dailyfor 5 days, using repaglinide as a probe drug, in 10 healthy volunteers. At the end of this 5-day regimen, there were dose-dependent increases in the area under the plasma concentration–time curve from 0 to infinity (AUC0–∞) of repaglinide by3.4-, 5.5-, and 7.0-fold corresponding to 30, 100, and 600 mg of gemfibrozil, respectively, as compared with the control phase (P < 0.001). On the basis of a mechanism-based inactivation model involving gemfibrozil 1-O-β-glucuronide, a gemfibrozil dose of 30 mg twice daily was estimated to inhibit CYP2C8 by >70% and 100 mg twice daily was estimated to inhibit it by >90%. Hence, gemfibrozil is a strong inactivator of CYP2C8 even in very small, subtherapeutic, multiple doses. Administration of small gemfibrozil doses may be useful in optimizing the pharmacokinetics of CYP2C8 substrate drugs and in reducing the formation of their potentially toxic metabolites via CYP2C8.     

Effect of rifampin on the pharmacokinetics of rosiglitazone in healthy subjects

BACKGROUND AND OBJECTIVE: Rifampin (INN, rifampicin) causes several drug interactions with coadministered antidiabetic drugs. Rosiglitazone is a novel thiazolidinedione antidiabetic drug, but little is known about the drug interaction between rifampin and rosiglitazone. Our objective was to investigate the effect of rifampin on the pharmacokinetics of rosiglitazone in humans. METHOD: In an open-label, randomized, 2-way crossover study, 10 healthy Korean male subjects were treated once daily for 6 days with 600 mg rifampin or with placebo. On day 7, a single dose of 8 mg rosiglitazone was administered orally. Plasma rosiglitazone concentrations were measured. RESULTS: Rifampin significantly decreased the mean area under the plasma concentration-time curve for rosiglitazone by 65% (2947.9 ng. h/mL versus 991.5 ng. h/mL, P <.001) and the mean elimination half-life from 3.9 to 1.5 hours (P <.001). The peak plasma concentration of rosiglitazone was significantly decreased by rifampin (537.7 ng/mL versus 362.3 ng/mL, P <.01). The apparent oral clearance of rosiglitazone increased about 3-fold after rifampin treatment (2.8 L/h versus 8.5 L/h, P <.001). CONCLUSION: This study showed that rifampin affected the disposition of rosiglitazone in humans, probably by the induction of cytochrome P450 (CYP) 2C8 and, to a lesser extent, CYP2C9. Therefore caution should be exercised during the coadministration of rifampin and rosiglitazone.