Optimization of chromatography to overcome matrix effect for reliable estimation of four small molecular drugs from biological fluids using LC-MS/MS.

The article describes a systematic study to overcome the matrix effect during chromatographic analysis of gemfibrozil, rivastigmine, telmisartan and tacrolimus from biological fluids using LC-ESI-MS/MS. All four methods were thoroughly developed by the appropriate choice of analytical column, elution mode and pH of mobile phase for improved chromatography and overall method performance. Matrix effect was assessed by post-column analyte infusion, slope of calibration line approach and post-extraction spiking. The best chromatographic conditions established were: Acquity BEH C18 (50 × 2.1 mm, 1.7 µm) column with 5.0 mm ammonium acetate, pH 6.0-methanol as the mobile phase under gradient program for gemfibrozil; Luna CN (50 × 2.0 mm, 3 µm) column with a mobile phase consisting of acetonitrile-10 mm ammonium acetate, pH 7.0 (90:10, v/v) for rivastigmine; Inertsustain C18 (100 × 2.0 mm, 5 µm) column using methanol-2.0 mm ammonium formate, pH 5.5 (80: 20, v/v) as the mobile phase for isocratic elution of telmisartan; and Acquity BEH C18 (50 × 2.1 mm, 1.7 µm) with methanol-10 mm ammonium acetate, pH 6.0 (95:5, v/v) as mobile phase for tacrolimus. The methods were thoroughly validated as per European Medicines Agency and US Food and Drug Administration guidance and were successfully applied for pharmacokinetic studies in healthy subjects.

UPLC-MS/MS method for gemfibrozil determination in plasma with application to a pharmacokinetic study in healthy Brazilian volunteers.

Gemfibrozil (GFZ) is a derivative of fibric acid and is used in the treatment of dyslipidemia. GFZ may affect the metabolism of various drugs, including statins, by inhibiting the sinusoidal influx transporter OATP1B1 and also CYP2C9 and CYP2C8 enzymes. This study presents the development and validation of a rapid, simple, sensitive and reproducible method of GFZ analysis in human plasma using UPLC-MS/MS. The method was applied in a pharmacokinetic study following administration of multiple doses of 600 mg GFZ every 12 h in healthy volunteers (n = 15). GFZ was separated on a C18 column using a mixture of 0.01% formic acid and acetonitrile (40:60, v/v) as the mobile phase at a flow rate of 0.4 mL/min. The method showed linearity in the range from 0.01 µg/mL to 100 µg/mL plasma. The coefficients of variation and the relative standard errors of the accuracy and precision analyses were <15%. The method allowed quantification of plasma concentrations of GFZ in the dose interval of the sixth day of administration of multiple oral doses of GFZ every 12 h. The pharmacokinetic parameters are presented as mean (95% CI): area under the plasma concentration versus time curve 88.84 (72.72-104.96) µg·h/mL, steady state mean plasma concentration 7.40 (6.06-8.75) µg/mL, minimum plasma concentration 1.24 (0.87-1.61) µg/mL, maximum plasma concentration 26.73 (21.31-32.15) µg/mL, time to reach maximum plasma concentration 2.28 (1.42-3.13) h, elimination half-life 2.81 (2.22-3.40) h, apparent total clearance 7.72 (5.85-9.58) L/h, apparent distribution volume 33.97 (18.41-49.53) L. In conclusion, the method for analysis of GFZ in human plasma showed sensitivity, linearity, precision and accuracy compatible with application in pharmacokinetic studies of multiple oral dose of 600 mg GFZ every 12 h.

Application of a physiologically based pharmacokinetic model for the prediction of mirabegron plasma concentrations in a population with severe renal impairment.

We previously verified a physiologically based pharmacokinetic (PBPK) model for mirabegron in healthy subjects using the Simcyp Simulator by incorporating data on the inhibitory effect on cytochrome P450 (CYP) 2D6 and a multi-elimination pathway mediated by CYP3A4, uridine 5'-diphosphate-glucuronosyltransferase (UGT) 2B7 and butyrylcholinesterase (BChE). The aim of this study was to use this PBPK model to assess the magnitude of drug-drug interactions (DDIs) in an elderly population with severe renal impairment (sRI), which has not been evaluated in clinical trials. We first determined the system parameters, and meta-analyses of literature data suggested that the abundance of UGT2B7 and the BChE activity in an elderly population with sRI was almost equivalent to and 20% lower than that in healthy young subjects, respectively. Other parameters, such as the CYP3A4 abundance, for an sRI population were used according to those built into the Simcyp Simulator. Second, we confirmed that the PBPK model reproduced the plasma concentration-time profile for mirabegron in an sRI population (simulated area under the plasma concentration-time curve (AUC) was within 1.5-times that of the observed value). Finally, we applied the PBPK model to simulate DDIs in an sRI population. The PBPK model predicted that the AUC for mirabegron with itraconazole (a CYP3A4 inhibitor) was 4.12-times that in healthy elderly subjects administered mirabegron alone, and predicted that the proportional change in AUC for desipramine (a CYP2D6 substrate) with mirabegron was greater than that in healthy subjects. In conclusion, the PBPK model was verified for the purpose of DDI assessment in an elderly population with sRI.

ß-Cyclodextrin anchoring onto pericarpium granati-derived magnetic mesoporous carbon for selective capture of lopid in human serum and pharmaceutical wastewater samples.

Functionalized magnetic carbonaceous nanomaterials, which are important materials with many practical and research applications in biomedical, pharmaceutical and biological fields, have recently attracted much attention. In this study, a magnetic mesoporous carbon coated with ß-cyclodextrin (MMC@ß-CD) was synthesized for the first time from natural pericarpium granati (PG). The as-obtained MMC@ß-CD has high surface areas (203 m(2)g(-1)), large pore volumes (0.16 cm(3)g(-1)), relatively broad mesoporous sizes (6.8 nm) and a high saturation magnetization of 26.2 emu g(-1), which is sufficient for magnetic separation by an external magnetic field. The MMC@ß-CD was used as an innovative adsorbent for magnetic solid-phase extraction of lopid via host-guest interaction prior to spectrofluorometric analysis. The proposed method was successfully applied to analyze lopid in human serum and pharmaceutical wastewater samples with recoveries in the range of 85.0-103.5% for the spiked samples. Overall, this work not only provides an inexpensive and eco-friendly method to fabricate MMC@ß-CD (or MMC) from PG, but also develops a highly selective approach for capture of lopid in biological samples and environmental substances.

Evaluation and Optimization of Blood Micro-Sampling Methods: Serial Sampling in a Cross-Over Design from an Individual Mouse.

PURPOSE: Current practices applied to mouse pharmacokinetic (PK) studies often use large numbers of animals with sporadic or composite sampling that inadequately describe PK profiles.  The purpose of this work was to evaluate and optimize blood microsampling techniques coupled with dried blood spot (DBS) and LC-MS/MS analysis to generate reliable PK data in mice.  In addition, the feasibility of cross-over designs was assessed and recommendations are presented. METHODS: The work describes a comprehensive evaluation of five blood microsampling techniques (tail clip, tail vein with needle hub, submandibular, retro-orbital, and saphenous bleeding) in CD-1 mice.  The feasibility of blood sampling was evaluated based on animal observations, ease of bleeding, and ability to collect serial samples.  Methotrexate, gemfibrozil and glipizide were used as test compounds and were dosed either orally or intravenously, followed by DBS collection and LC-MS/MS analysis to compare PK with various bleeding methods. RESULTS: Submandibular and retro-orbital methods that required non-serial blood collections did not allow for inter-animal variability assessments and resulted in poorly described absorption and distribution kinetics.  The submandibular and tail vein with needle-hub methods were the least favorable from a technical feasibility perspective.  Serial bleeding was possible with cannulated animals or saphenous bleeding in non-cannulated animals. CONCLUSIONS:   Of the methods that allowed serial sampling, the saphenous method when executed as described in this report, was most practical, reproducible and provided for assessment of inter-animal variability.  It enabled the collection of complete exposure profiles from a single mouse and the conduct of an intravenous/oral cross-over study design.  This methodology can be used routinely, it promotes the 3Rs principles by achieving reductions in the number of animals used, decreased restraints and animal stress, and improved the quality of data obtained in mouse PK studies. This article is open to POST-PUBLICATION REVIEW. Registered readers (see "For Readers") may comment by clicking on ABSTRACT on the issue's contents page.

Alteration of the intravenous and oral pharmacokinetics of valsartan via the concurrent use of gemfibrozil in rats.

The present study aimed to examine the potential pharmacokinetic drug interaction between valsartan and gemfibrozil. Compared with the control given valsartan (10 mg/kg) alone, the concurrent use of gemfibrozil (10 mg/kg) significantly (p < 0.05) increased the oral exposure of valsartan in rats. In the presence of gemfibrozil, the Cmax and AUC of oral valsartan increased by 1.7- and 2.5-fold, respectively. Consequently, the oral bioavailability of valsartan was significantly higher (p < 0.05) in the presence of gemfibrozil compared with that of the control group. Furthermore, the intravenous pharmacokinetics of valsartan (1 mg/kg) was also altered by pretreatment with oral gemfibrozil (10 mg/kg). The plasma clearance of valsartan was decreased by two-fold in the presence of gemfibrozil, while the plasma half-life was not altered. In contrast, both the oral and intravenous pharmacokinetics of gemfibrozil were not affected by the concurrent use of valsartan. The cellular uptake of valsartan and gemfibrozil was also investigated by using cells overexpressing OATP1B1 or OATP1B3. Gemfibrozil and gemfibrozil 1-O-ß glucuronide inhibited the cellular uptake of valsartan with IC50 values (µm) of 39.3 and 20.4, respectively, in MDCK/OATP1B1, while they were less interactive with OATP1B3. The cellular uptake of gemfibrozil was not affected by co-incubation with valsartan in both cells. Taken together, the present study suggests the potential drug interaction between valsartan and gemfibrozil, at least in part, via the OATP1B1-mediated transport pathways during hepatic uptake. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2015 John Wiley & Sons, Ltd.

Bio-generation of stable isotope-labeled internal standards for absolute and relative quantitation of phase II drug metabolites in plasma samples using LC-MS/MS.

Quantification of drug metabolites in biological samples has been of great interest in current pharmaceutical research, since metabolite concentrations and pharmacokinetics can contribute to a better understanding of the toxicity of drug candidates. Two major categories of Phase II metabolites, glucuronide conjugates and glutathione conjugates, may cause significant drug toxicity and therefore require close monitoring at early stages of drug development. In order to achieve high precision, accuracy, and robustness, stable isotope-labeled (SIL) internal standards (IS) are widely used in quantitative bioanalytical methods using liquid chromatography and tandem mass spectrometry (LC-MS/MS), due to their capability of compensating for matrix effects, extraction variations and instrument response fluctuations. However, chemical synthesis of SIL analogues of Phase II metabolites can often be very difficult and require extensive exploratory research, leading to higher cost and significant delays in drug research and development. To overcome these challenges, we have developed a generic method which can synthesize SIL analogues of Phase II metabolites from more available SIL parent drugs or SIL conjugation co-factors, using in vitro biotransformation. This methodology was successfully applied to the bio-generation of SIL glucuronide conjugates and glutathione conjugates. The method demonstrated satisfactory performance in both absolute quantitation and assessment of relative exposure coverage across species in safety tests of drug metabolites (MIST). This generic technique can be utilized as an alternative to chemical synthesis and potentially save time and cost for drug research and development.

Magnetic solid phase extraction of gemfibrozil from human serum and pharmaceutical wastewater samples utilizing a ß-cyclodextrin grafted graphene oxide-magnetite nano-hybrid.

A magnetic solid phase extraction method based on ß-cyclodextrin (ß-CD) grafted graphene oxide (GO)/magnetite (Fe3O4) nano-hybrid as an innovative adsorbent was developed for the separation and pre-concentration of gemfibrozil prior to its determination by spectrofluorometry. The as-prepared ß-CD/GO/Fe3O4 nano-hybrid possesses the magnetism property of Fe3O4 nano-particles that makes it easily manipulated by an external magnetic field. On the other hand, the surface modification of GO by ß-CD leads to selective separation of the target analyte from sample matrices. The structure and morphology of the synthesized adsorbent were characterized using powder X-ray diffraction, Fourier transform infrared spectroscopy, and field emission scanning electron microscopy. The experimental factors affecting the extraction/pre-concentration and determination of the analyte were investigated and optimized. Under the optimized experimental conditions, the calibration graph was linear in the range between 10 and 5000 pg mL(-1) with a correlation coefficient of 0.9989. The limit of detection and enrichment factor for gemfibrozil were 3 pg mL(-1) and 100, respectively. The maximum sorption capacity of the adsorbent for gemfibrozil was 49.8 mg g(-1). The method was successfully applied to monitoring gemfibrozil in human serum and pharmaceutical wastewaters samples with recoveries in the range of 96.0-104.0% for the spiked samples.

Analysis of the repaglinide concentration increase produced by gemfibrozil and itraconazole based on the inhibition of the hepatic uptake transporter and metabolic enzymes.

The plasma concentration of repaglinide is reported to increase greatly when given after repeated oral administration of itraconazole and gemfibrozil. The present study analyzed this interaction based on a physiologically based pharmacokinetic (PBPK) model incorporating inhibition of the hepatic uptake transporter and metabolic enzymes involved in repaglinide disposition. Firstly, the plasma concentration profiles of inhibitors (itraconazole, gemfibrozil, and gemfibrozil glucuronide) were reproduced by a PBPK model to obtain their pharmacokinetic parameters. The plasma concentration profiles of repaglinide were then analyzed by a PBPK model, together with those of the inhibitors, assuming a competitive inhibition of CYP3A4 by itraconazole, mechanism-based inhibition of CYP2C8 by gemfibrozil glucuronide, and inhibition of organic anion transporting polypeptide (OATP) 1B1 by gemfibrozil and its glucuronide. The plasma concentration profiles of repaglinide were well reproduced by the PBPK model based on the above assumptions, and the optimized values for the inhibition constants (0.0676 nM for itraconazole against CYP3A4; 14.2 µM for gemfibrozil against OATP1B1; and 5.48 µM for gemfibrozil glucuronide against OATP1B1) and the fraction of repaglinide metabolized by CYP2C8 (0.801) were consistent with the reported values. The validity of the obtained parameters was further confirmed by sensitivity analyses and by reproducing the repaglinide concentration increase produced by concomitant gemfibrozil administration at various timings/doses. The present findings suggested that the reported concentration increase of repaglinide, suggestive of synergistic effects of the coadministered inhibitors, can be quantitatively explained by the simultaneous inhibition of the multiple clearance pathways of repaglinide.

Pharmacokinetic drug interaction between gemfibrozil and sitagliptin in healthy Indian male volunteers.

PURPOSE: To study the impact of gemfibrozil co-administration on the pharmacokinetics of sitagliptin in healthy Indian male volunteers. METHODS: A randomized open label two-period crossover study involving 12 healthy Indian male volunteers was conducted at a single center. In each phase, the volunteers were administered sitagliptin as 100 mg tablets, either alone or co-administered with gemfibrozil as 600 mg tablets twice daily for 3 days. There was a 2-week washout period between phases. The venous blood samples were serially collected at 0-12 h post-dose, and plasma concentrations of the study drugs were estimated by a validated high-performance liquid chromatography-ultraviolet method. RESULTS: Relative to the administration of sitagliptin alone, co-administration with gemfibrozil increased the AUC0₋12 (2,167 ± 82.9 vs. 2,970 ± 76.4 ng h/ml; p < 0.0001), AUC(0-∞) (3,621 ± 222.5 vs. 5,574 ± 249.6 ng h/ml; p < 0.0002), C(max) (282.9 ± 7.7 vs. 344.1 ± 5.9 ng/ml; p < 0.0001), and t(½) (7.4 ± 0.6 vs. 10 ± 0.6 h; p = 0.0076) to statistically significant levels. The interindividual differences in the pharmacokinetic parameters of sitagliptin were found to be within acceptable limits (coefficient of variation <20%). No adverse drug events associated with sitagliptin occurred in the subjects during the study period. CONCLUSION: Although the bioavailability of sitagliptin was increased by 54% when co-administered with gemfibrozil, this interaction may not have any clinical significance as sitagliptin has a wide therapeutic index. Hence, in clinical practice, sitagliptin as 100 mg tablets and gemfibrozil as 600 mg tablets may be co-prescribed without much threat of sitagliptin toxicity. However, these results may not hold if the dose of sitagliptin is increased or if is co-prescribed with other antidiabetic drugs and/or cytochrome P450 2C8/human organic anion transporter-3 inhibitors. Further studies are needed to confirm these results in patients.

Dose-dependent interaction between gemfibrozil and repaglinide in humans: strong inhibition of CYP2C8 with subtherapeutic gemfibrozil doses.

Gemfibrozil 1-O-ß-glucuronide inactivates CYP2C8 irreversibly. We investigated the effect of gemfibrozil dose on CYP2C8 activity in humans using repaglinide as a probe drug. In a randomized, five-phase crossover study, 10 healthy volunteers ingested 0.25 mg of repaglinide 1 h after different doses of gemfibrozil or placebo. Concentrations of plasma repaglinide, gemfibrozil, their metabolites, and blood glucose were measured. A single gemfibrozil dose of 30, 100, 300, and 900 mg increased the area under the concentration-time curve of repaglinide 1.8-, 4.5-, 6.7-, and 8.3-fold (P < 0.001), and its peak concentration 1.4-, 1.7-, 2.1-, and 2.4-fold (P < 0.05), compared with placebo, respectively. Gemfibrozil pharmacokinetics was characterized by a slightly more than dose-proportional increase in the area under the curve of gemfibrozil and its glucuronide. The gemfibrozil-repaglinide interaction could be mainly explained by gemfibrozil 1-O-ß-glucuronide concentration-dependent, mechanism-based inhibition of CYP2C8, with a minor contribution by competitive inhibition of organic anion-transporting polypeptide 1B1 at the highest gemfibrozil dose. The findings are consistent with ∼50% inhibition of CYP2C8 already with a single 30-mg dose of gemfibrozil and >95% inhibition with 900 mg. In clinical drug-drug interaction studies, a single 900-mg dose of gemfibrozil can be used to achieve nearly complete inactivation of CYP2C8.

Synthesis, characterization and in vitro hydrolysis of a gemfibrozil-nicotinic acid codrug for improvement of lipid profile.

Combination therapy of fibrates and nicotinic acid has been reported to be synergistic. Herein, we describe a covalent codrug of gemfibrozil (GEM) and nicotinic acid (NA) that was synthesized and characterized by (1)H NMR, (13)C NMR, FT-IR, MS analysis and elemental analysis. A validated HPLC method was developed that allows for the accurate quantitative determination of the codrug and its hydrolytic products that are formed during the in vitro chemical and enzymatic hydrolysis. The physico-chemical properties of codrug were improved compared to its parent drugs in term of water solubility and partition coefficient. The kinetics of hydrolysis of the codrug was studied using accelerated hydrolysis experiments at high temperatures in aqueous phosphate buffer solution in pH 1.2, 6.8 and 7.4. Using the Arrhenius equation, the extrapolated half-life at 37°C were 289 days at pH 1.2 for the codrug and 130 and 20,315 days at pH 6.8 for the codrug and gemfibrozil 2-hydroxyethyl ester (GHEE), respectively. The shortest half-lives were at pH 7.4; 42 days for the codrug and 5837 days for GHEE, respectively. The hydrolysis of the latter was studied, alone, at 80°C and pH 1.2 and compared to its hydrolysis when it is produced from the codrug using similar conditions. The k(obs) was found in both cases to be 1.60×10(-3)h(-1). The half-lives in plasma were 35.24 min and 26.75 h for the codrug and GHEE, respectively. With regard to liver homogenate, the hydrolysis half-lives were 1.96 min and 48.13 min for the codrug and GHEE, respectively. It can be expected that in vivo, the codrug will liberate NA immediately in plasma then GEM will be liberated from its 2-hydroxyethyl ester in the liver.

The CYP2C8 inhibitor gemfibrozil does not affect the pharmacokinetics of zafirlukast.

PURPOSE: Gemfibrozil, a strong inhibitor of cytochrome P450 (CYP) 2C8 in vivo, was recently found to markedly increase the plasma concentrations of montelukast in humans. Like montelukast, zafirlukast is a substrate of CYP2C9 and CYP3A4 and a potent inhibitor of CYP2C8 in vitro. To investigate the contribution of CYP2C8 to the metabolism of zafirlukast in vivo, we studied the effect of gemfibrozil on the pharmacokinetics of zafirlukast. METHODS: Ten healthy subjects in a randomized cross-over study took gemfibrozil 600 mg or placebo twice daily for 5 days, and on day 3, a single oral dose of 20 mg zafirlukast. The plasma concentrations of zafirlukast were measured for 72 h postdose. RESULTS: The mean total area under the plasma concentration-time curve of zafirlukast during the gemfibrozil phase was 102% (geometric mean ratio; 95% confidence interval 89-116%) of that during the placebo phase. Furthermore, there were no statistically significant differences in the peak plasma concentration, time of peak concentration, or elimination half-life of zafirlukast between the phases. CONCLUSIONS: Gemfibrozil has no effect on the pharmacokinetics of zafirlukast, indicating that CYP2C8 does not play a significant role in the elimination of zafirlukast.

Effect of gemfibrozil and fenofibrate on the pharmacokinetics of atorvastatin.

Coadministration of statins and fibrates is beneficial in some patients by allowing simultaneous reduction of triglycerides and low-density lipoprotein cholesterol alongside elevation of high-density lipoprotein cholesterol. However, the potential for drug interactions must be taken into consideration. Gemfibrozil increases systemic exposure to various different statins, whereas similar effects are not observed with fenofibrate, suggesting it may be a more appropriate choice for coadministration with statins. Gemfibrozil is reported to cause a moderate increase in the area under the curve (AUC) of atorvastatin, but the effect of fenofibrate on atorvastatin pharmacokinetics has not been described. This study compared the effects of multiple-dose administration of gemfibrozil and fenofibrate on the single-dose pharmacokinetics of atorvastatin. Gemfibrozil coadministration led to significant increases in the AUC of atorvastatin, 2-hydroxyatorvastatin, 2-hydroxyatorvastatin lactone, and 4-hydroxyatorvastatin lactone. In contrast, fenofibrate administration did not lead to clinically meaningful changes in the AUC for atorvastatin, atorvastatin lactone, 2-hydroxyatorvastatin, or 2-hydroxyatorvastatin lactone. The absence of a significant pharmacokinetic interaction between fenofibrate and atorvastatin is consistent with recent results showing no difference in safety profile between atorvastatin as monotherapy or in combination with fenofibric acid. Together, these data suggest that atorvastatin-fenofibrate combination therapy is unlikely to pose a risk to patients.

Validation of an LC/MS method for the determination of gemfibrozil in human plasma and its application to a pharmacokinetic study.

Gemfibrozil, a fibric acid hypolipidemic agent, is increasingly being used in clinical drug-drug interaction studies as an inhibitor of drug metabolizing enzymes and drug transporters. The validation of a fast, accurate and precise LC/MS method is described for the quantitative determination of gemfibrozil in an EDTA-anticoagulated human plasma matrix. Briefly, gemfibrozil was extracted from human plasma by an acetonitrile protein precipitation method. The assay was reproducible with intra-assay precision between 1.6 and 10.7%, and inter-assay precision ranging from 4.4 to 7.8%. The assay also showed good accuracy, with intra-assay concentrations within 85.6-108.7% of the expected value, and inter-assay concentrations within 89.4-104.0% of the expected value. The linear concentration range was between 0.5 and 50 µg/mL with a lower limit of quantitation of 0.5 µg/mL when 125 µL of plasma were extracted. This LC/MS method yielded a quick, simple and reliable protocol for determining gemfibrozil concentrations in plasma and is applicable to clinical pharmacokinetic studies.

Two different spectrofluorimetric methods for simultaneous determination of gemfibrozil and rosiglitazone in human plasma.

Two accurate, reliable, and highly sensitive spectrofluorimetric methods were developed for simultaneous determination of binary mixture gemfibrozil and rosiglitazone in human plasma without prior separation steps. The first method is based on synchronous fluorescence spectrometry using double scans. At Δλ=27nm, gemfibrozil yields detectable signal that is independent of the presence of rosiglitazone. Similarly, at Δλ=120nm the signal of rosiglitazone is not influenced by the presence of gemfibrozil. Signals at two wavelengths, 301 (Δλ=27nm) and 368nm (Δλ=120nm) vary linearly with gemfibrozil and rosiglitazone concentrations over the range 100-700ngmL(-1) (for gemfibrozil) and 20-140ngmL(-1) (for rosiglitazone), respectively. The limits of detection (LOD) were 2.3 and 2.72ngmL(-1) for gemfibrozil and rosiglitazone, respectively. The second method is based on the technique of simultaneous equations (Vierodt's method), in which 258nm was selected as the excitation wavelength. Two equations are constructed based on the fact that at ( λ(EM)2=302 nm of gemfibrozil) and (λ(EM)2=369 nm of rosiglitazone) the fluorescence of the mixture is the sum of the individual fluorescence of gemfibrozil and rosiglitazone. The limits of detection (LOD) were 28.1 and 23.63ngmL(-1) for gemfibrozil and rosiglitazone, respectively. The proposed methods were successfully applied for the determination of the two compounds in synthetic mixtures and in human plasma with a good recovery.

Effect of a single gemfibrozil dose on the pharmacokinetics of rosuvastatin in bile and plasma in healthy volunteers.

The effect of a single intrajejunal dose of gemfibrozil (600 mg) on the plasma pharmacokinetics and biliary excretion of a single intrajejunal dose of rosuvastatin (20 mg) was investigated by using a multichannel catheter positioned in the distal duodenum-proximal jejunum in 8 healthy volunteers. Bile and plasma samples were collected every 20 minutes for 200 minutes, with additional plasma samples being drawn for up to 48 hours. Gemfibrozil did not affect the bioavailability of rosuvastatin, although it increased the apparent absorption phase during the initial 200 minutes (AUC(plasma,200min)) by 1.56-fold (95% confidence interval, 1.14-2.15). The interaction was less pronounced in this single-dose study than in a previous report when gemfibrozil was administered repeatedly; nevertheless, the interaction coincided with the highest exposure to gemfibrozil. The plausible reason why the interaction in this investigation was only minor is the low exposure to gemfibrozil (and its metabolites), suggesting that the total plasma concentration of gemfibrozil needs to be above 20 µM to affect the disposition of rosuvastatin. This study demonstrates the value of monitoring the plasma pharmacokinetics of the inhibitor, and not only the drug under investigation, to improve the mechanistic interpretation.

CYP2C8 activity recovers within 96 hours after gemfibrozil dosing: estimation of CYP2C8 half-life using repaglinide as an in vivo probe.

Gemfibrozil 1-O-beta-glucuronide is a mechanism-based inhibitor of cytochrome P450 2C8. We studied the recovery of CYP2C8 activity after discontinuation of gemfibrozil treatment using repaglinide as a probe drug, to estimate the in vivo turnover half-life of CYP2C8. In a randomized five-phase crossover study, nine healthy volunteers ingested 0.25 mg of repaglinide alone or after different time intervals after a 3-day treatment with 600 mg of gemfibrozil twice daily. The area under the plasma concentration-time curve (AUC) from time 0 to infinity of repaglinide was 7.6-, 2.9-, 1.4- and 1.0-fold compared with the control phase when it was administered 1, 24, 48, or 96 h after the last gemfibrozil dose, respectively (P < 0.001 versus control for 1, 24, and 48 h after gemfibrozil). Thus, a strong CYP2C8 inhibitory effect persisted even after gemfibrozil and gemfibrozil 1-O-beta-glucuronide concentrations had decreased to less than 1% of their maximum (24-h dosing interval). In addition, the metabolite to repaglinide AUC ratios indicated that significant (P < 0.05) inhibition of repaglinide metabolism continued up to 48 h after gemfibrozil administration. Based on the recovery of repaglinide oral clearance, the in vivo turnover half-life of CYP2C8 was estimated to average 22 +/- 6 h (mean +/- S.D.). In summary, CYP2C8 activity is recovered gradually during days 1 to 4 after gemfibrozil discontinuation, which should be considered when CYP2C8 substrate dosing is planned. The estimated CYP2C8 half-life will be useful for in vitro-in vivo extrapolations of drug-drug interactions involving induction or mechanism-based inhibition of CYP2C8.

A high performance liquid chromatography method for simultaneous determination of rosiglitazone and gemfibrozil in human plasma.

A high performance liquid chromatography (HPLC) method was developed for the simultaneous determination of rosiglitazone and gemfibrozil in human plasma using alpha-asarone as an internal standard (IS). Plasma samples were pretreated by protein precipitation. The analytes were separated on a Macherey-Nagel Nucleodur C(18) (250 mm x 4.6 mm, 5 microm) column using acetonitrile and 30 mmol/l ammonium acetate solution (including 0.1% methanoic acid) as mobile phase which was delivered at 1.2 ml/min. A gradient elution program was adopted to adjust proportion of solvent in mobile phase. A time program was used to regulate conditions of fluorescence detection. The method was validated over the concentration range of 5.0-751.3 ng/ml for rosiglitazone and 0.5-75.4 microg/ml for gemfibrozil with acceptable accuracy, precision and extraction recoveries. This method is suitable for routine therapeutic drug monitoring and pharmacokinetic interaction study of the two drugs.

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.