Modeling Developmental Changes in Caffeine Clearance Considering Differences between Pre- and Postnatal Period.

Taguchi et al. reported that postmenstrual age (PMA) is a promising factor in describing and understanding the developmental change of caffeine (CAF) clearance. The aim of the present study was to quantify how developmental changes occur and to determine the effect of the length of the gestational period on CAF clearance. We performed a nonlinear mixed effect model (NONMEM) analysis and evaluated the fit of six models. A total of 115 samples were obtained from 52 patients with a mean age of 34.3 ± 18.2 d. The median values of gestational age (GA) and postnatal age (PNA) were 196 and 31 d, respectively. Serum CAF levels corrected for dose per body surface area (BSA) (C/D ratioBSA) were dependent on PMA rather than PNA, which supports the findings of a previous study. NONMEM analysis provided the following final model of oral clearance: CL/F = 0.00603∙WT∙∙0.877GA ≤ 196 L/h. This model takes into account developmental changes during prenatal and postnatal periods separately. The model successfully described the variation in clearance of CAF. Our findings suggest that the dosage of CAF in preterm infants should be determined based not only on body weight (WT) but also on both PNA and GA.

Effects of Concomitant Administration of PXR Ligand Drugs on the Anticoagulant Effects of Warfarin.

We encountered cases in which the anticoagulant effects of warfarin (CYP2C9 substrate) were reversibly attenuated by the concomitant administration of rifampicin or bosentan, which are potent pregnane X receptor (PXR) ligands. The purpose of the present study is to report the previous case with rifampicin, and to evaluate the changes in the warfarin anticoagulant effects when withdrawing or switching bosentan treatment. The former is a case study of a 4-year-old girl undergoing warfarin treatment. The latter is a longitudinal study of 20 pediatric patients receiving stable warfarin treatment. The prothrombin time and international normalized ratio (PT-INR) values were extracted from the medical records and normalized by the daily-dose per body size as an index for the warfarin anticoagulant effects. Rifampicin treatment resulted in a 52.0% decrease in the anticoagulant index. On the other hand, 10 of 20 patients started bosentan and their anticoagulant index was reduced by a median of 2.00. Bosentan was withdrawn in 4 of 20 patients and their anticoagulant index increased by a median of 3.67. Six of 20 patients switched from bosentan to macitentan, which is considered not to activate PXR in clinical settings. However, switching from bosentan to macitentan resulted in a median of 2.25 reduction of the anticoagulant index rather than recovery of the response to warfarin. This study suggests not only the possibility of heterogeneity in the response to PXR activation and deactivation, but also the importance of long-term monitoring of drug-drug interactions when switching from bosentan to macitentan.

Voriconazole injection may induce delayed methotrexate excretion: a case report and experimental study.

BACKGROUND: We report a case of delayed excretion of methotrexate (MTX) in a pediatric patient on high-dose MTX therapy in response to a change in the concomitant dosage of voriconazole from oral to intravenous. As the intravenous, but not the oral formulation of voriconazole includes sulfobutylether-ß-cyclodextrin (SBECD), which has an anionic residue, we hypothesized that SBECD inhibits the renal excretion of anionic compounds. METHODS: We evaluated the inhibitory effects of SBECD on renal excretion of phenolsulfonphthalein (PSP), which is eliminated in urine via organic anion transport systems. PSP was administered intravenously to rats at 2.5 and 25 mg/kg with or without SBECD pretreatment (320 mg/kg). RESULTS: The plasma concentration of PSP at the dosage of 2.5 mg/kg were comparable between control and SBECD groups. On the other hand, at 25 mg/kg the elimination of PSP was delayed. The clearance of PSP at the dosage of 25 mg/kg was 9.71 ± 1.65 and 4.13 ± 0.76 mL/min/kg in control and SBECD groups, respectively (p < 0.05). This suggested that SBECD partly inhibits the renal excretion of anionic drugs. CONCLUSION: The present case report discusses the delayed elimination of MTX in high dose therapy and possible mechanism involving SBECD as an excipient in concomitant drugs. It seems better to avoid choosing injection containing SBECD for patients undergoing HD-MTX treatment. Further studies are needed to confirm the inhibitory effects of SBECD on the renal excretion of MTX, especially in high-dose regimens.

Pharmacokinetic Variability of Caffeine in Routinely Treated Preterm Infants: Preliminary Considerations on Developmental Changes of Systemic Clearance.

The purpose of this study was to clarify the variability of serum concentrations of caffeine (CAF) in preterm infants, and to deliberate on a better explanation for developmental changes of systemic clearance during the neonatal period. Forty-nine serum samples were obtained from 23 preterm neonates (age, 34.1 ± 18.8 d), and additive blood sampling was conducted periodically for 10 of the 23 patients after discontinuation of CAF treatment. The concentrations of CAF and its major metabolites were determined by liquid chromatography-tandem mass spectrometory. The serum concentrations of CAF were within therapeutic levels (5-25 µg/mL) in 37 samples and exceeded 25 µg/mL in the rest of the 12 samples, although no sample was in the toxic range (> 50 µg/mL). The inter- and intra-individual variability of the concentration to dose (C/D) ratio corrected for body surface area (BSA) was more negatively associated with postmenstrual age (PMA) rather than postnatal age (PNA). The serum concentrations of major metabolites were much smaller than those of CAF throughout the study, suggesting that the contribution of hepatic metabolism to drug elimination was small in the preterm infants under 241 d of PMA. The mean values for elimination half-life and oral clearance estimated in the 10 patients were 124.6 ± 44.6 h and 2.26 ± 0.73 mL/min/1.73 m2, respectively. Consequently, we confirmed that the exposure to CAF was considerably variable and provided additive insight that the C/D ratio corrected for patient's BSA and PMA are promising for describing and understanding the developmental change of clearance in preterm infants.

The Pharmacokinetics of Sildenafil May Be Affected by Intestinal Absorption Rate in Children Admitted to the Intensive Care Unit.

This study was performed for a better understanding of the pharmacokinetics of sildenafil (SIL) and N-desmethyl sildenafil (DMS) in 13 children treated in the intensive care unit (ICU). Blood samples were taken periodically after the first oral administration of SIL (0.5 mg/kg). Plasma concentrations were analyzed by tandem LC/MS. Of the 13 patients, apparent peaks in the plasma concentration of SIL were observed in four patients, with the other nine patients showing reduced or delayed drug absorption of SIL. The maximum plasma concentrations of SIL after administration varied in range from 7.8 to 101.0 ng/mL. The parent drug-to-metabolite (SIL/DMS) ratios of the nine patients with reduced or delayed drug absorption of SIL were relatively lower than those in the four patients with rapid absorption of the drug. These observations suggested that the inter-individual variability of intestinal absorption and/or first-pass extraction of SIL was involved in the pharmacokinetic variability of the drug. Next, we evaluated the impact of changes in the gastrointestinal absorption rate on the pharmacokinetics of the drug. That is, SIL (2.5 mg/body) was administered at two different rates in the duodenum of rats. When SIL was administered for 10 min, the Cmax and bioavailability were 3.46 ± 1.65 µg/mL and 23.2 ± 11.1%, respectively. When SIL was administered for 60 min, the Cmax and bioavailability were 0.990 ± 0.352 µg/mL and 9.91 ± 3.79%, respectively. These findings suggest that the drug absorption rate was at least partly responsible for the pharmacokinetic variability of SIL in the ICU children.

CYP2C9, VKORC1, and CYP4F2 polymorphisms and pediatric warfarin maintenance dose: a systematic review and meta-analysis.

Studies on the effect of cytochrome P450 2C9 (CYP2C9), vitamin K epoxide reductase complex subunit 1 (VKORC1), and cytochrome P450 4F2 (CYP4F2) polymorphisms on warfarin maintenance dose in children are conflicting. We conducted a systematic review and meta-analysis to evaluate the effect of these polymorphisms on warfarin maintenance dose in children. We searched relevant literature using the MEDLINE, EMBASE, and Cochrane Central Register of Controlled Trial libraries without any language restrictions from their inception to 23 July 2017. Dose differences are expressed as standardized mean difference (SMD) or mean difference (MD) with 95% confidence intervals (CI). This review was registered in the PROSPERO prospective register of systematic reviews (CRD42015016172). We included a total of nine studies (745 participants) in the meta-analysis. Patients with CYP2C9 *1/*2, *1/*3, *2/*2, *2/*3, or *3/*3 required a lower warfarin maintenance dose compared with patients with CYP2C9 *1/*1 (SMD = -0.610, 95% CI: -0.802 to -0.419, I2 = 0%). Patients with VKORC1-1639GA or AA required a lower warfarin maintenance dose compared with patients with VKORC1-1639GG (SMD = -0.666, 95% CI: -0.887 to -0.445, I2 = 33%). However, no associations were observed between CYP4F2 polymorphisms and warfarin maintenance dose (MD = 0.005 mg/kg/day, 95% CI: -0.006 to 0.015, I2 = 0%). These results were not affected by a sensitivity analysis. Our meta-analysis provides evidence that CYP2C9 and VKORC1 variant statuses affect warfarin maintenance dose in children, but not CYP4F2.

Evaluation of the effects of ontogenetic or maturation functions and chronic heart failure on the model analysis for the dose-response relationship of warfarin in Japanese children.

PURPOSE: We previously demonstrated that the rational pediatric dosage of warfarin can be well-described by a SIZE parameter that includes an allometry exponent of weight. On the other hand, allometry alone is considered to be insufficient to predict drug clearance in neonates and infants. The primary purpose of the present study was to evaluate the effects of incorporation of the maturation process into the analysis model for the dose-response relationship of warfarin in Japanese children. In addition, we evaluated the effect of chronic heart failure (CHF) on the response to warfarin as an independent risk factor for increased anticoagulant effects. METHODS: Thirty-eight patients with stable anticoagulation by warfarin were enrolled. During a mean follow-up period of 4.74 ± 3.51 years, 1092 data points including prothrombin time-international normalized ratio (PT-INR) were obtained. The data were subjected to multiple regression analysis to identify covariates related to the anticoagulant effects. RESULTS: Two different models describing the maturation process did not improve the predictive performance for the dose-response relationship in pediatric patients. In addition to the SIZE-normalized daily dose, the vitamin K epoxide reductase complex 1 (VKORC1) genotype, and concomitant use of bosentan, CHF was identified as a covariate increasing the anticoagulant effects of warfarin to 118%. CONCLUSION: The SIZE parameter was useful even without incorporation of maturation models to describe the response to warfarin in pediatric patients, and our longitudinal follow-up study design with multiple observations was beneficial to detect changes within individual subjects.

Determination of the Serum Unbound Fraction of Tadalafil in Children with Protein-Losing Enteropathy and Its Specific Binding to Human Serum Proteins in Vitro.

The purpose of this study was to determine the serum protein binding of tadalafil in children with protein-losing enteropathy (PLE) and to evaluate the specific binding of the drug to human serum-derived proteins in vitro. Seventeen serum samples from two PLE patients used after biochemical tests were collected, and the unbound fraction of tadalafil was determined by an ultrafiltration method. The serum albumin concentrations observed in patients #1 and #2 were 2.4-4.2 and 2.9-3.5 g/dL, respectively. The ranges of unbound fraction of tadalafil in patients #1 and #2 were 3.9-13 and 5.0-7.0%, respectively. This suggested that serum albumin was at least a binding carrier for tadalafil because the unbound fraction of tadalafil and serum albumin were slightly correlated. The unbound fraction of tadalafil at the total concentration of 300 ng/mL was negatively dependent on the serum albumin concentration (range: 1.0-5.0 g/dL) in vitro. In the presence of albumin, the additive effect of γ-globulin on the unbound fraction of tadalafil was marginal, but the addition of α1-acid glycoprotein to test samples decreased the unbound fraction of the drug. The decrease in the unbound fraction of tadalafil was greater at low albumin levels (2 g/dL). The addition of lipoprotein to test samples also decreased the unbound fraction of tadalafil, suggesting that lipoprotein was also a binding carrier of the drug. These results suggested that the disposition and/or response to tadalafil in PLE patients was altered by the change in protein bindings of the drug.

The possible clinical impact of risperidone on P-glycoprotein-mediated transport of tacrolimus: A case report and in vitro study.

The authors encountered the case of an 8-fold increase in the concentration/dose (C/D) ratio of tacrolimus (TAC) following the coadministration of voriconazole (VRCZ) in a hematopoietic stem cell transplantation (HSCT) recipient. The interaction observed was much greater than expected and the patient had also been treated with oral risperidone (RSP). It was hypothesized that cytochrome P450 (CYP)3A inhibition of the small intestine by voriconazole and P-glycoprotein (P-gp) inhibition of the small intestine by risperidone exerted a synergistic effect on the bioavailability of tacrolimus. The aim of the present study was to evaluate the effect of risperidone on the P-gp-mediated transport of tacrolimus. The transcellular transport of P-gp substrates was examined in Caco-2 and P-gp-expressing renal epithelial LLC-GA5-COL150 cells. In Caco-2 cells, the apical-basal (A-B) transport of rhodamine123 (Rh123) after a 120 min incubation was increased by 47.1%, whereas that in the B-A direction was decreased by 61.7% in the presence of risperidone (100 µm). These results indicate that risperidone showed an inhibitory effect on the P-gp-mediated transport of Rh123. In LLC-GA5-COL150 cells, the A-B transport of tacrolimus after 120 min incubation was increased by 21.7% in the presence of risperidone (100 µm), whereas that in the B-A direction was decreased by 10.7%. These results suggest that risperidone was at least partly involved in the mechanism of the marked increase in the C/D ratio of tacrolimus. This case report provides new insights into the diversity of drug interactions of tacrolimus triggered by the combination of two concomitant drugs.

In Vitro P-Glycoprotein-Mediated Transport of Tadalafil: A Comparison with Sildenafil.

Tadalafil and sildenafil are selective inhibitors of phosphodiesterase type 5, showing marked pharmacokinetic variability in patients with pulmonary arterial hypertension. It has been reported that sildenafil is a substrate for P-glycoprotein (P-gp), but whether tadalafil is a substrate for P-gp remains to be determined. The objective of the present study was to elucidate whether tadalafil is a substrate for P-gp. Transcellular transport of sildenafil and tadalafil (5 µM each) was examined using renal epithelial LLC-PK1 and P-gp-expressing LLC-GA5-COL150 cell monolayers. The efflux ratio of the basal to apical (B to A) transport of sildenafil to the A to B transport after 120-min incubation in LLC-GA5-COL150 cells (1.52) was significantly higher than that in LLC-PK1 cells (0.711). The efflux ratio of the B to A transport of tadalafil to the A to B transport after 120-min incubation in LLC-GA5-COL150 cells (10.4) was significantly higher than that in LLC-PK1 cells (1.23). In LLC-GA5-COL150 cell monolayers, the Vmax and Km values of sildenafil transport calculated from a modified Michaelis-Menten equation were 101±64 pmol/min/cm2 and 112±47 µM, respectively. On the other hand, those of tadalafil transport were 13.6±4.8 pmol/min/cm2 and 22.7±9.3 µM, respectively. In the presence of a P-gp inhibitor (PSC833), the B to A transport of tadalafil was decreased by 28.6% in LLC-GA5-COL150 cells, and the A to B transport of tadalafil was 6.59-fold greater than that in its absence. These results indicate that tadalafil is a substrate for P-gp.

In Vitro Enhancement of Carvedilol Glucuronidation by Amiodarone-Mediated Altered Protein Binding in Incubation Mixture of Human Liver Microsomes with Bovine Serum Albumin.

Carvedilol is mainly metabolized in the liver to O-glucuronide (O-Glu). We previously found that the glucuronidation activity of racemic carvedilol in pooled human liver microsomes (HLM) was increased, R-selectively, in the presence of amiodarone. The aim of this study was to clarify the mechanisms for the enhancing effect of amiodarone on R- and S-carvedilol glucuronidation. We evaluated O-Glu formation of R- and S-carvedilol enantiomers in a reaction mixture of HLM including 0.2% bovine serum albumin (BSA). In the absence of amiodarone, glucuronidation activity of R- and S-carvedilol for 25 min was 0.026, and 0.51 pmol/min/mg protein, and that was increased by 6.15 and 1.60-fold in the presence of 50 µM amiodarone, respectively. On the other hand, in the absence of BSA, or when BSA was replaced with human serum albumin, no enhancing effect of amiodarone on glucuronidation activity was observed, suggesting that BSA played a role in the mechanisms for the enhancement of glucuronidation activity. Unbound fraction of S-carvedilol in the reaction mixture was greater than that of R-carvedilol in the absence of amiodarone. Also, the addition of amiodarone caused a greater increase of unbound fraction of R-carvedilol than that of S-carvedilol. These results suggest that the altered protein binding by amiodarone is a key mechanism for R-selective stimulation of carvedilol glucuronidation.

[CLINICAL UTILITY OF T-SPOT.TB ASSAY WITH T-Cell Xtend REAGENT FOR ACTIVE TUBERCULOSIS DIAGNOSIS IN THE FIELD TEST AT OUR HOSPITAL].

BACKGROUND: T-SPOT.TB (T-SPOT), an interferon-gamma release assay, has shown promise as a diagnostic tool for active tuberculosis (TB), and its use is expanding. Addition of the T-Cell Xtend (TCX) reagent may allow delayed processing, and this characteristic is important for using this test in the field. However, limited data is available on the usefulness of T-SPOT with TCX as a field test for diagnosing active TB. PURPOSE: To investigate the clinical utility of T-SPOT with TCX and the risk factors for a false-negative result in patients with active TB. METHODS: A total of 57 patients with active TB who underwent the T-SPOT test with TCX prior to treatment were enrolled between May 2013 and May 2015. One patient with an indeterminate result for T-SPOT was excluded; therefore, the data of 56 patients were eventually included in the final analysis. The basic characteristics and clinical findings were compared between the true-positive and false-negative T-SPOT groups. RESULTS: Of the 56 patients, 40 (71.4%), 13 (23.2%), 3 (5.4%) had true-positive, false-negative, and borderline T-SPOT results, respectively. This study did not reveal any significant risk factors for a false-negative T-SPOT result. CONCLUSION: In this clinical study, the proportion of patients with a false-negative result for T-SPOT with TCX for active TB was higher than that reported previously. Therefore, careful interpretation of a negative result for T-SPOT with TCX is necessary, regardless of the patient's background.

A model analysis for dose-response relationship of warfarin in Japanese children: An introduction of the SIZE parameter.

The objective of the present study was to develop an optimal equation for the pediatric dose-response relationship of warfarin using a size parameter with an exponent of body weight (SIZE) which has been proposed for scaling drug clearance. Twenty patients with stable anticoagulation by warfarin were enrolled in the present study. During a mean follow-up period of 7.36 years, 857 data points were obtained. The average patient age and body weight were 8.49 years and 24.5 kg, respectively. The relative response index to warfarin with PT-INR values normalized by daily-dose per SIZE showed fewer systematic changes than those per body weight. The anticoagulant effect of warfarin in patients with the VKORC1 1173CT or 1173CC genotype was 47.3% of that with the 1173TT genotype. Concomitant use of bosentan attenuated the anticoagulant effect of warfarin to 84.1%. In conclusion, the SIZE parameter appeared to be an effective way to describe the pediatric dose-response relationship of warfarin, and consequently, a longitudinal follow-up study design with multiple measurements was useful to detect changes within individual subjects.

Contribution of CYP3A isoforms to dealkylation of PDE5 inhibitors: a comparison between sildenafil N-demethylation and tadalafil demethylenation.

The aim of this study was to characterize the kinetics of metabolite formation of the phosphodiesterase type-5 (PDE5) inhibitors sildenafil and tadalafil by CYP3A4, CYP3A5, and CYP3A7 isoforms. The formations of N-desmethyl sildenafil and desmethylene tadalafil were examined using CYP3A supersomes co-expressing human P450 oxidoreductase and cytochrome b5. Both sildenafil N-demethylation and tadalafil demethylenation were catalyzed by CYP3A4, CYP3A5, and to a lesser extent by CYP3A7. The kinetics of desalkyl metabolite formation of the two drugs were well fitted to the Hill equation; however, the Hill coefficients (n) suggested CYP3A-mediated negative cooperativity. Next, we analyzed the kinetics with a two binding sites model assuming two reaction steps: reaction 1 with high-affinity and low-capacity metabolism and reaction 2 with low-affinity and high-capacity metabolism. The kinetics of desalkyl metabolite formation were also fitted to the two binding sites model. The intrinsic clearance (CLint) values of reactions 1 and 2 for sildenafil N-demethylation were 0.733 and 0.033 µL/min/pmol P450 for CYP3A4, 0.788 and 0.019 µL/min/pmol P450 for CYP3A5, and 0.079 and 0.004 µL/min/pmol P450 for CYP3A7, respectively. The CLint values of reactions 1 and 2 for tadalafil demethylenation were 0.187 and 0.014 µL/min/pmol P450 for CYP3A4, 0.050 and <0.001 µL/min/pmol P450 for CYP3A5, and 0.004 and <0.001 µL/min/pmol P450 for CYP3A7, respectively. These results may provide the basis not only for understanding the metabolic properties of the two PDE5 inhibitors, but also for one possible explanation of the mechanisms of CYP3A-mediated negative cooperativity.

Plasma concentrations of tadalafil in children with pulmonary arterial hypertension.

BACKGROUND: There is no report documenting the plasma concentrations of tadalafil in children. This study was performed to evaluate the variability in the pharmacokinetics of tadalafil in children with pulmonary arterial hypertension (PAH) treated routinely with the drug. METHODS: Plasma samples were taken twice (post- and predose) after repetitive oral administration, and the pharmacokinetic parameters (CL/F and V/F) in individual patients were estimated by the Bayesian method using the nonlinear mixed effects model. We also determined the unbound concentration of tadalafil using ultrafiltration. RESULTS: Tadalafil was administered to 23 children aged between 0.25 and 17.4 years, with a mean age of 3.58 years. The mean (±SD) daily dose of tadalafil was 0.97 ± 0.41 mg/kg. Sixteen of the 23 children received bosentan concomitantly. The mean CL/F and V/F values of tadalafil were 0.149 L·h-1·kg-1 and 1.87 L/kg, respectively, which were higher than those reported in adults. No effects of age, bosentan, or the estimated glomerular filtration rate were observed on the CL/F value, indicating that other residual factors might account for the interindividual variability among children with PAH. The unbound tadalafil concentrations of the postdose samples ranged from 5.9 to 146 (46.9 ± 37.1) nmol/L, higher than the reported IC50 value of this phosphodiesterase-5 drug for humans (2-4 nmol/L, corresponding to 0.8-1.6 ng/mL). CONCLUSIONS: We demonstrated variability in the total and unbound plasma concentrations of tadalafil in children. However, all children received the empirical doses of the drug; a mean dose of 0.97 mg·kg-1·d-1 showed sufficient unbound concentrations needed for half-maximal inhibition of human phosphodiesterase-5 in vitro. These observations may provide information for the proper use of tadalafil to treat children with PAH.

Pharmacokinetic variability of flecainide in younger Japanese patients and mechanisms for renal excretion and intestinal absorption.

The aims of the present study were to evaluate the variability of pharmacokinetics of flecainide in young Japanese patients and to investigate the mechanisms of renal excretion and intestinal absorption of the drug using cultured epithelial cells. First the plasma concentration data of flecainide was analysed in 16 Japanese patients aged between 0.07 and 18.30 years using a one-compartment model. Considerable interindividual variability was observed in the oral clearance (CL/F) and the apparent volume of distribution (V/F) of flecainide in the young patients. Flecainide was transported selectively in the basolateral-to-apical direction in P-glycoprotein-expressing renal epithelial LLC-GA5-COL150 cell monolayers. The uptake of flecainide into intestinal epithelial LS180 cells was decreased significantly by acidification of the extracellular medium, and was inhibited by tertiary amines, such as diphenhydramine and quinidine. These findings in the present study suggest that flecainide is excreted by P-glycoprotein in the renal tubule and is taken up by the postulated H(+)/tertiary amine antiporter in the intestine, and that functional variability of not only the hepatic drug-metabolizing enzymes, but also the transporters in the kidney and intestine, may be responsible for the interindividual variability of systemic clearance (CL) and/or the bioavailability (F) of flecainide.

Variability of bioavailability and intestinal absorption mechanisms of metoprolol.

We previously reported that aging and/or cytochrome P450 2D6 polymorphism are responsible for the interindividual variability in the systemic clearance (CL) and bioavailability (F) of metoprolol. The aim of the present study was to evaluate the residual variability of F of metoprolol in routinely treated Japanese patients and to investigate the intestinal absorption mechanism of the drug using human intestinal epithelial LS180 cells. We first re-analyzed the blood concentration data for metoprolol in 34 Japanese patients using a nonlinear mixed effects model. The oral clearance (CL/F) of metoprolol was positively correlated with the apparent volume of distribution (V/F), suggesting the residual variability of F. The uptake of metoprolol into LS180 cells was significantly decreased by the acidification of extracellular medium pH, and was dependent on temperature and intracellular pH. Furthermore, the cellular uptake of metoprolol was saturable, and was significantly decreased in the presence of hydrophobic cationic drugs such as diphenhydramine, procainamide, bisoprolol, and quinidine. These findings indicate that residual variability of F is one of the causes of the interindividual pharmacokinetic variability of metoprolol, and that the interindividual variability of not only presystemic first-pass metabolism, but also intestinal absorption, may be responsible for the variable F of the drug.

Effect of chronic hypoxic hypoxia on oxidation and glucuronidation of carvedilol in rats.

Heart failure is accompanied with tissue (circulatory) hypoxia, and the metabolism of several drugs has been reported to be reduced in heart failure. The aim of this study was to investigate the effect of another type of respiratory hypoxia, hypoxic hypoxia (FiO2 15 % for 24 h followed by FiO2 10 % for 9 days) on the metabolism of carvedilol enantiomers in rats. Oxidation of carvedilol in rat liver microsomes was evaluated in the presence of reduced nicotinamide adenine dinucleotide phosphate, whereas glucuronidation was evaluated in the presence of UDP-glucuronic acid. Both oxidation and glucuronidation activities for two carvedilol enantiomers in hypoxic rat liver microsomes were similar to those in control rat liver microsomes. We also performed pharmacokinetic analysis of carvedilol enantiomers following intraportal infusion in control and hypoxic rats. The mean (±S.E.) portal clearance value of R- and S-carvedilol in control rats was 72 ± 16 and 156 ± 31 ml/min/kg, respectively, whereas that of the R- and S-enantiomers in hypoxic rats was 68 ± 8 and 113 ± 14 ml/min/kg, respectively. These findings indicated that the metabolism of carvedilol enantiomers was not significantly diminished in rats with chronic hypoxic hypoxia, and that other factor(s) besides hypoxia may be responsible for the reduced drug metabolism in heart failure.

Variability of bioavailability and intestinal absorption characteristics of bisoprolol.

We previously reported that renal function is partly responsible for the interindividual variability of the pharmacokinetics of bisoprolol. The aim of the present study was to examine the variability of bioavailability (F) of bisoprolol in routinely treated Japanese patients and intestinal absorption characteristics of the drug. We first analyzed the plasma concentration data of bisoprolol in 52 Japanese patients using a nonlinear mixed effects model. We also investigated the cellular uptake of bisoprolol using human intestinal epithelial LS180 cells. The oral clearance (CL/F) of bisoprolol in Japanese patients was positively correlated with the apparent volume of distribution (V/F), implying variable F. The uptake of bisoprolol in LS180 cells was temperature-dependent and saturable, and was significantly decreased in the presence of quinidine and diphenhydramine. In addition, the cellular uptake of bisoprolol dissolved in an acidic buffer was markedly less than that dissolved in a neutral buffer. These findings suggest that the rate/extent of the intestinal absorption of bisoprolol is another cause of the interindividual variability of the pharmacokinetics, and that the uptake of bisoprolol in intestinal epithelial cells is highly pH-dependent and also variable.

Effect of salt intake on bioavailability of mizoribine in healthy Japanese males.

Bioavailability of mizoribine in subjects with the concentrative nucleoside transporter 1 (CNT1, SLC28A1) 565-A/A allele is significantly lower than that in subjects with the SLC28A1 565-G/G allele. The aims of the present study were to investigate the cellular uptake of mizoribine in CNT1- and CNT2-expressing Madin-Darby canine kidney type II (MDCKII) cells, and to evaluate the effect of salt intake on bioavailability of mizoribine in healthy Japanese volunteers with SLC28A1 565-A/A and -G/A alleles. Eight healthy males participated in the present study, and took 150 mg mizoribine concomitantly with/without 300 mg salt. Bioavailability of mizoribine was estimated by total cumulative urinary excretion of the drug. Mizoribine was taken up Na(+)-dependently into not only CNT1-expressing but also CNT2-expressing MDCKII cells, indicating that mizoribine is a substrate for both CNT1 and CNT2. Mean bioavailability of mizoribine taken with salt (83.8%) was significantly higher than that taken without salt (73.0%). These findings suggest that the salt intake is expected to improve the bioavailability of mizoribine in patients with insufficient intestinal absorption.