Application of Individualized PBPK Modeling of Rate Data to Evaluate the Effect of Hemodialysis on Nonrenal Clearance Pathways.

The aim of this study was to apply individualized, physiologically based pharmacokinetic modeling of 14 CO2 production rates (iPBPK-R) measured by the erythromycin breath test to characterize the effect of hemodialysis on the function of nonrenal clearance pathways in patients with end-stage renal disease. Twelve patients previously received 14 C-erythromycin intravenously pre- and post-hemodialysis. Serial breath samples were collected after each dose over 2 hours. Eight PBPK parameters were co-estimated across periods, whereas activity of cytochrome P450 (CYP) 3A4 clearance was independently estimated for each period. Inhibition coefficients for organic anion transporting polypeptide (OATP), P-glycoprotein, and multidrug resistance-associated protein 2 activities were also estimated. Nonrenal clearance parameter estimates were explored regarding sex differences and correlations with uremic toxins and were used in hierarchical cluster analysis (HCA). Relationships between the function of nonrenal clearance pathways and uremic toxin concentrations were explored. Mean CYP 3A4 clearance increased by 2.2% post-hemodialysis. Uptake transporter activity was highly intervariable across hemodialysis. Females had 22% and 30% higher median CYP3A4 activity than males pre- and post-hemodialysis, respectively. Exploratory HCA indicated that using both CYP3A4 and OATP activity parameters at pre- and post-hemodialysis best identifies heterogeneous patients. This is the first study to use the iPBPK-R approach to simultaneously estimate multiple in vivo nonrenal elimination pathways in individual patients with kidney disease and to assess the effect of hemodialysis.

In vitro interactions of abiraterone, erythromycin, and CYP3A4: implications for drug-drug interactions.

Potential drug-drug interactions of the antitumor drug abiraterone and the macrolide antibiotic erythromycin were studied at the stage of cytochrome P450 3A4 (CYP3A4) biotransformation. Using differential spectroscopy, we have shown that abiraterone is a type II ligand of CYP3A4. The dependence of CYP3A4 spectral changes on the concentration of abiraterone is sigmoidal, which indicates cooperative interactions of CYP3A4 with abiraterone; these interactions were confirmed by molecular docking. The dissociation constant (Kd ) and Hill coefficient (h) values for the CYP3A4-abiraterone complex were calculated as 3.8 ± 0.1 µM and 2.3 ± 0.2, respectively. An electrochemical enzymatic system based on CYP3A4 immobilized on a screen-printed electrode was used to show that abiraterone acts as a competitive inhibitor toward erythromycin N-demethylase activity of CYP3A4 (apparent Ki  = 8.1 ± 1.2 µM), while erythromycin and its products of enzymatic metabolism do not affect abiraterone N-oxidation by CYP3A4. In conclusion, the inhibition properties of abiraterone toward CYP3A4-dependent N-demethylation of erythromycin and the biologically inert behavior of erythromycin toward abiraterone hydroxylation were demonstrated.

Cost-effectiveness analysis and safety of erythromycin 4% gel and 4% chlorhexidine scrub for pitted keratolysis treatment.

Background: Studies of Pitted keratolysis (PK) treatment are limited. Objectives: To study cost-effectiveness and to compare the safety of 4% chlorhexidine scrub with 4% erythromycin gel, for PK infections. Materials and methods: This cohort study was conducted on naval rating cadets with a clinical diagnosis of PK at Chumpol Naval Rating School, Thailand in 2016. Participants were randomly treated with either 4% erythromycin gel or 4% chlorhexidine scrub for 4 weeks. The clinical examinations were evaluated at the baseline and at 1 and 2 months after treatment. A decision-tree model was used to evaluate the costs, resource utilization and outcomes as quality-adjusted life-years (QALYs). Results: Of 344 naval rating cadets, 125 (36.3%) were diagnosed with PK. Sixty-four were treated with erythromycin. Approximately 80% of participants had complete resolution Foot odor were significantly improved at 2 months (p < .001) for both groups. No adverse effects were reported. Total cost for 4 weeks' treatment with the erythromycin gel and chlorhexidine scrub was US$77.34, US$51.9, respectively. Chlorhexidine treatment and erythromycin gel had 0.1526 and 0.1425 QALYs, respectively. Conclusions: treatment of PK with either 4% chlorhexidine scrub or 4% erythromycin gel had similar outcomes. However, using chlorhexidine scrub was more cost-effective.

Effect of erlotinib on CYP3A activity, evaluated in vitro and by dual probes in patients with cancer.

In vitro, erlotinib (0-30 µmol/l) and C-labelled midazolam (MDZ) (5 µmol/l) were incubated with human liver microsomes; separately, microsomes were preincubated with erlotinib (10 µmol/l) before the addition of MDZ. Results showed a time-dependent inhibition of MDZ metabolism by erlotinib, with a Ki of 7.5 µmol/l and an inactivation rate constant of 0.009/min. Patients with cancer (n=24) received a single oral dose of 7.5 mg MDZ and a single intravenous dose of 3 µCi [C-N-methyl] erythromycin on days 1, 8, 14 and 21. Patients also received 150 mg oral erlotinib daily from day 8 to day 14. Plasma concentrations of erlotinib and OSI-420 were determined on days 8 and 14; MDZ and 1'-hydroxymidazolam were determined on days 1, 8, 14 and 21. Coadministration of erlotinib resulted in a 4 and a 16% increase in CO2 on days 8 and 14, respectively, after the administration of erythromycin. The mean AUC0-last of MDZ decreased 17 and 34% after erlotinib treatment on day 8 and day 14, respectively. The half-life of MDZ and the AUC ratio of 1'-hydroxymidazolam to MDZ were not significantly changed. Although erlotinib may be a weak mechanism-based irreversible inhibitor of CYP3A4 in vitro, in vivo, erlotinib did not inhibit CYP3A-mediated metabolism, as determined by the erythromycin breath test and the MDZ pharmacokinetics. The mechanism for reduced exposure of MDZ is unclear, but may be because of an increase in intestinal metabolism or decreased absorption. These findings suggest that coadministration of erlotinib may not result in clinically relevant increases in exposure of CYP3A substrates.

Breath tests to phenotype drug disposition in oncology.

Breath tests (BTs) have been investigated as diagnostic tools to phenotype drug disposition in cancer patients in the pursuit to individualize drug treatment. The choice of the right phenotype probe is crucial and depends on the metabolic pathway of the anticancer agent of interest. BTs using orally or intravenously administered selective non-radioactive (13)C-labeled probes to non-invasively evaluate dihydropyrimidine dehydrogenase, cytochrome P450 (CYP) 3A4, and CYP2D6 enzyme activity have been published. Clinically, a (13)C-dextromethorphan BT to predict endoxifen levels in breast cancer patients and a (13)C-uracil BT to predict fluoropyrimidine toxicity in colorectal cancer patients are most promising. However, the clinical benefit and cost effectiveness of these phenotype BTs need to be determined in order to make the transition from an experimental setting to clinical practice as companion diagnostic tests.

Effect of netupitant, a highly selective NK1 receptor antagonist, on the pharmacokinetics of midazolam, erythromycin, and dexamethasone.

PURPOSE: Netupitant is a new highly selective neurokinin-1 receptor antagonist being studied for the prevention of nausea and vomiting in patients undergoing chemotherapy. In vitro studies suggest that netupitant inhibits the cytochrome P-450 isoenzyme 3A4 (CYP3A4). Because netupitant may be used with a variety of drugs, which may be substrates of CYP3A4, two studies were designed to establish the potential risk for drug-drug interaction with three different CYP3A4 substrates: midazolam, erythromycin, and dexamethasone. METHODS: Both trials were three-period crossover studies performed in healthy subjects. In the first study, 20 subjects received netupitant and either midazolam or erythromycin. In the second study, 25 subjects received netupitant and dexamethasone. Serial blood samples were collected over the course of the two studies and pharmacokinetic parameters were determined for all analytes. RESULTS: Netupitant, by inhibiting the CYP3A4, increased the C max and AUCinf of midazolam by 40 and 144 %, respectively, and the C max and AUCinf of erythromycin by 30 %. Netupitant was shown to increase the exposure to dexamethasone in a dose-dependent manner with the mean increase in AUC and C max by 72 and 11 %, respectively, on day 1 and by 138 and 75 %, respectively, on day 4 when co-administered with 300 mg of netupitant. CONCLUSIONS: The results of these studies suggest that netupitant is a moderate inhibitor of CYP3A4 and therefore, co-administration with drugs that are substrates of CYP3A4 may require dose adjustments. Treatments were well tolerated in both studies.

Co-administration of rivaroxaban with drugs that share its elimination pathways: pharmacokinetic effects in healthy subjects.

AIMS: The anticoagulant rivaroxaban is an oral, direct Factor Xa inhibitor for the management of thromboembolic disorders. Metabolism and excretion involve cytochrome P450 3A4 (CYP3A4) and 2J2 (CYP2J2), CYP-independent mechanisms, and P-glycoprotein (P-gp) and breast cancer resistance protein (Bcrp) (ABCG2). METHODS: The pharmacokinetic effects of substrates or inhibitors of CYP3A4, P-gp and Bcrp (ABCG2) on rivaroxaban were studied in healthy volunteers. RESULTS: Rivaroxaban did not interact with midazolam (CYP3A4 probe substrate). Exposure to rivaroxaban when co-administered with midazolam was slightly decreased by 11% (95% confidence interval [CI] -28%, 7%) compared with rivaroxaban alone. The following drugs moderately affected rivaroxaban exposure, but not to a clinically relevant extent: erythromycin (moderate CYP3A4/P-gp inhibitor; 34% increase [95% CI 23%, 46%]), clarithromycin (strong CYP3A4/moderate P-gp inhibitor; 54% increase [95% CI 44%, 64%]) and fluconazole (moderate CYP3A4, possible Bcrp [ABCG2] inhibitor; 42% increase [95% CI 29%, 56%]). A significant increase in rivaroxaban exposure was demonstrated with the strong CYP3A4, P-gp/Bcrp (ABCG2) inhibitors (and potential CYP2J2 inhibitors) ketoconazole (158% increase [95% CI 136%, 182%] for a 400 mg once daily dose) and ritonavir (153% increase [95% CI 134%, 174%]). CONCLUSIONS: Results suggest that rivaroxaban may be co-administered with CYP3A4 and/or P-gp substrates/moderate inhibitors, but not with strong combined CYP3A4, P-gp and Bcrp (ABCG2) inhibitors (mainly comprising azole-antimycotics, apart from fluconazole, and HIV protease inhibitors), which are multi-pathway inhibitors of rivaroxaban clearance and elimination.

CYP3A4 intron 6 C>T SNP (CYP3A4*22) encodes lower CYP3A4 activity in cancer patients, as measured with probes midazolam and erythromycin.

AIM: The CYP3A4*22 allele was recently reported to be associated with reduced CYP3A4 activity. We investigated the impact of this allele on the metabolism of the CYP3A-phenotyping probes, midazolam (MDZ) and erythromycin. PATIENTS & METHODS: Genomic DNA from 108 cancer patients receiving intravenous MDZ and 45 undergoing the erythromycin breath test was analyzed for CYP3A4*22 (rs35599367 C>T) and CYP3A5*3. RESULTS: The MDZ metabolic ratio (1´-OH-MDZ:MDZ) was 20.7% (95% CI: -36.2 to -6.2) lower for CYP3A4*22 carriers compared with CYP3A4*1/*1 patients (p = 0.01). Combining CYP3A4*22 and CYP3A5*3 genotypes showed a 38.7% decrease (95% CI: -50.0 to -27.4; p < 0.001) in 1´-OH-MDZ:MDZ for poor (CYP3A4*22-CYP3A5*3/*3) and 28.0% (95% CI: -33.3 to -22.6; p < 0.001) for intermediate (CYP3A4*1/*1-CYP3A5*3/*3) metabolizers, compared with extensive (CYP3A4*1/*1-CYP3A5*1) CYP3A metabolizers. CYP3A4 erythromycin N-demethylation activity was 40% lower in CYP3A4*22 carriers compared with CYP3A4*1/*1 patients (p = 0.032). CONCLUSION: The CYP3A4*22 allele is associated with decreased CYP3A4-mediated metabolism, as verified by CYP3A-phenotyping probes.

Impact of POR*28 on the clinical pharmacokinetics of CYP3A phenotyping probes midazolam and erythromycin.

OBJECTIVE: P450 oxidoreductase (POR) is essential for cytochrome P450 (CYP) activity in humans. The POR*28 allele (A503V) has been shown to impact on in-vitro CYP-mediated metabolism, including CYP3A isoenzymes. The aim of the present study was to determine the in vivo impact of the POR*28 allele on the pharmacokinetics of the classic CYP3A phenotyping probes midazolam and erythromycin. Whereas midazolam is metabolized by both CYP3A4 and CYP3A5, erythromycin is exclusively oxidized by CYP3A4. MATERIALS AND METHODS: To assess CYP3A activity, 108 cancer patients received midazolam and 45 others underwent the erythromycin breath test. Patients were genotyped for POR*28, CYP3A4*22 and CYP3A5*3. RESULTS: In patients expressing CYP3A5, POR*28 carriers showed 45% lower midazolam metabolic ratios compared with POR*1/*1 patients (P<0.001). This is in line with a lower CYP3A5 activity toward midazolam for POR*28 carriers. In CYP3A5 nonexpressers, POR*28 had no influence on midazolam pharmacokinetics. For erythromycin, POR*28 carriership did not influence its metabolism. CONCLUSION: Our data show that the POR*28 allele is associated with a lower in vivo CYP3A5 activity, but has no effects on CYP3A4-mediated erythromycin and midazolam metabolism.

A step closer to personalized chemotherapy: consideration of the influence of genetic variation in hepatic uptake transporters on the metabolism of CYP3A substrates.

Understanding the metabolic, transporter, and genetic influences on the disposition of drugs used in chemotherapy is critical to individualization of drug therapy. Recognition of the importance of transporter-enzyme interplay, in which genetic variants of drug uptake transporters can change drug metabolism when the enzymes are unchanged, is an important advance in predicting appropriate drug dosage regimens for the individual patient.

OATP1B1 polymorphism as a determinant of erythromycin disposition.

Previous studies have demonstrated that the pharmacokinetic profile of erythromycin, a probe for CYP3A4 activity, is affected by inhibitors or inducers of hepatic solute carriers. We hypothesized that these interactions are mediated by OATP1B1 (gene symbol, SLCO1B1), a polypeptide expressed on the basolateral surface of hepatocytes. Using stably transfected Flp-In T-Rex293 cells, erythromycin was found to be a substrate for OATP1B1*1A (wild type) with a Michaelis-Menten constant of ~13 µmol/l, and that its transport was reduced by ~50% in cells expressing OATP1B1*5 (V174A). Deficiency of the ortholog transporter Oatp1b2 in mice was associated with a 52% decrease in the metabolic rate of erythromycin (P = 0.000043). In line with these observations, in humans the c.521T>C variant in SLCO1B1 (rs4149056), encoding OATP1B1*5, was associated with a decline in erythromycin metabolism (P = 0.0072). These results suggest that impairment of OATP1B1 function can alter erythromycin metabolism, independent of changes in CYP3A4 activity.

Docetaxel pharmacokinetics and its correlation with two in vivo probes for cytochrome P450 enzymes: the C(14)-erythromycin breath test and the antipyrine clearance test.

BACKGROUND: Docetaxel has marked inter-patient PK variability, and metabolic phenotypic probes may enable individualised dosing. This is the first report directly comparing the erythromycin breath test (EBT) (a CYP3A4 probe) with the antipyrine clearance test (ACT), (a general CYP-P450/predominant CYP3A4 probe) for the correlation with docetaxel PK and toxicity. METHODS: Patients pretherapy underwent: (A) EBT: IV C(14)[N-methyl]-erythromycin was administered and breath samples analysed for (14)CO(2), derived parameters included (1) (14)CO(2) flux at 10-min (CO(2)f(10)), (2) 20-min (CO(2)f(20)), (3) terminal rate constant k(CO2) and (4) AUC(CO2,(0-∞)) and AUC(CO2,(0-60).) (B) ACL test: patients were given oral antipyrine 10 mg/kg, blood samples were taken for PK, and the clearance (CL(Ant)) was derived. Docetaxel was then given at 75 mg/m(2)/3-weekly or 35 mg/m(2)/weekly. Samples taken for docetaxel PK in first course on day 1 and PK parameters included clearance (CL(Doc)). RESULTS: Twenty patients accrued, docetaxel: 3-weekly/weekly = 13:7. EBT parameters (N = 19) (mean, [CV%]): CO(2)f(10) (%/min) 0.051 (106), CO(2)f(20) 0.052 (82), k(CO2) (min(-1)) 0.007 (22), AUC(CO2,(0-∞)) 7.9 (85), AUC(CO2,(0-60)) 2.64 (81). CL(Ant) (N = 19) (ml/min); 35.8 (37). Docetaxel PK parameters (N = 19): CL(Doc) (l/h) = 57.2 (36), t(Doc1/2) (h) = 12.7 (33). No correlations were observed between the docetaxel PK and EBT parameters. For docetaxel weekly patients, a significant linear relationship was observed between CL(Doc) and CL(Ant) (P = 0.007, R (2) = 79.47%). CONCLUSIONS: The utility of EBT for the prediction of docetaxel PK was not confirmed in this study. The antipyrine clearance test may be superior in this regard for docetaxel, but regimen dependent and hence warrants further evaluation.

Antimalarial activity of 9a-N substituted 15-membered azalides with improved in vitro and in vivo activity over azithromycin.

Novel classes of antimalarial drugs are needed due to emerging drug resistance. Azithromycin, the first macrolide investigated for malaria treatment and prophylaxis, failed as a single agent and thus novel analogues were envisaged as the next generation with improved activity. We synthesized 42 new 9a-N substituted 15-membered azalides with amide and amine functionalities via simple and inexpensive chemical procedures using easily available building blocks. These compounds exhibited marked advances over azithromycin in vitro in terms of potency against Plasmodium falciparum (over 100-fold) and high selectivity for the parasite and were characterized by moderate oral bioavailability in vivo. Two amines and one amide derivative showed improved in vivo potency in comparison to azithromycin when tested in a mouse efficacy model. Results obtained for compound 6u, including improved in vitro potency, good pharmacokinetic parameters, and in vivo efficacy higher than azithromycin and comparable to chloroquine, warrant its further development for malaria treatment and prophylaxis.

[Mechanisms of pharmacokinetic drug-drug interactions]. / Mécanismes des interactions médicamenteuses d'origine pharmacocinétique.

Pharmacokinetic drug-drug interactions occur when a drug alters the disposition (absorption, distribution, elimination) of a coadministered agent. Pharmacokinetic interactions may result in the increase or the decrease of plasma drug concentrations. These modifications are variable in intensity but can lead to contraindications of the association. The mechanisms of pharmacokinetic interactions involve drug metabolizing enzymes, drug transporters and orphan nuclear receptors that regulate at the transcriptional level the expression of enzymes and transporters. The increase of drug plasma concentrations is generally related to the inhibition of enzymes and/or drug transport. The decrease of drug concentrations reflects the activation of orphan nuclear receptors by inducers that lead to the increase of the expression of enzymes and drug transporters. Inhibition of drug metabolism or transport is quite immediate (24-48h) while induction is a slower process (7-10 days). Complex situations may be observed with drugs that are both inducers and inhibitors (rifampin, ritonavir). They can cause the decrease and the increase of the exposure of the combined agent depending on the duration of the association.

Influence of solute carriers on the pharmacokinetics of CYP3A4 probes.

We hypothesized that the assessment of baseline CYP3A4 activity is influenced by probe-specific differences in hepatocellular uptake mechanisms. There was no significant correlation between the erythromycin breath test (ERMBT) parameters and midazolam clearance in 30 cancer patients (R(2) < 0.01), regardless of their CYP3A5 genotype status. In cellular models overexpressing 10 different solute carriers, erythromycin uptake was significantly increased by OATP1A2 (P < 0.005) and OATP1B3 (P < 0.01). Midazolam was not a substrate for any of the tested transporters. In a separate cohort of 119 patients, 6 nonsynonymous variants in the OATP1B3 gene SLCO1B3 were identified. Individuals carrying two copies of the T allele at the 334 locus had a 2.4-fold lower value for ERMBT 1/T(max) (P = 0.001), a measure reflecting more rapid hepatic uptake. These findings suggest that differential affinities for solute carriers should be considered when selecting an appropriate phenotypic probe to allow tailored dosing of pharmaceuticals that are CYP3A4 substrates.

Effect of preoperative erythromycin or dexamethasone/vitamin C on postoperative abomasal emptying rate in dairy cows undergoing surgical correction of abomasal volvulus.

OBJECTIVE: To determine the effects of preoperative erythromycin or combined dexamethasone/vitamin C treatment on postoperative abomasal emptying rate in cows undergoing surgical correction of abomasal volvulus (AV). STUDY DESIGN: Prospective, controlled, clinical study using a convenience sample. ANIMALS: Lactating Holstein-Friesian cows (n=45) with AV were alternately assigned to 3 groups (n=15): group C: untreated (control); group E: erythromycin (10 mg/kg intramuscularly [IM]); group D: dexamethasone (0.02 mg/kg intravenously [IV]) and vitamin C (10 mg/kg IV). METHODS: Drugs were administered 1 hour before surgical correction of AV. D-xylose solution (50%, 0.5 g/kg body weight) was injected into the abomasal lumen during surgery. Jugular venous blood samples for determination of serum d-xylose concentration were periodically obtained. Time to maximal serum D-xylose concentration (T(max-model)) was pharmacokinetically determined. RESULTS: Abomasal emptying rate was significantly (P<0.05) faster in group E (T(max-model)=182+/-69 min; mean+/-SD) than in group C cows (T(max-model)=237+/-64 min). Abomasal emptying rate was similar in group D (T(max-model)=196+/-47 min) and group C. Both treatments improved postoperative milk yield within 1 day after surgery. CONCLUSION: Preoperative injection of erythromycin (10 mg/kg IM) is an effective method for ameliorating postoperative abomasal hypomotility in cows with AV. CLINICAL RELEVANCE: Parenteral erythromycin can be recommended for preoperative treatment of cows with AV.

Use of microdosing to predict pharmacokinetics at the therapeutic dose: experience with 5 drugs.

OBJECTIVES: A volunteer trial was performed to compare the pharmacokinetics of 5 drugs--warfarin, ZK253 (Schering), diazepam, midazolam, and erythromycin--when administered at a microdose or pharmacologic dose. Each compound was chosen to represent a situation in which prediction of pharmacokinetics from either animal or in vitro studies (or both) was or is likely to be problematic. METHODS: In a crossover design volunteers received (1) 1 of the 5 compounds as a microdose labeled with radioactive carbon (carbon 14) (100 microg), (2) the corresponding (14)C-labeled therapeutic dose on a separate occasion, and (3) simultaneous administration of an intravenous (14)C-labeled microdose and an oral therapeutic dose for ZK253, midazolam, and erythromycin. Analysis of (14)C-labeled drugs in plasma was done by use of HPLC followed by accelerator mass spectrometry. Liquid chromatography-tandem mass spectrometry was used to measure plasma concentrations of ZK253, midazolam, and erythromycin at therapeutic concentrations, whereas HPLC-accelerator mass spectrometry was used to measure warfarin and diazepam concentrations. RESULTS: Good concordance between microdose and therapeutic dose pharmacokinetics was observed for diazepam (half-life [t((1/2))] of 45.1 hours, clearance [CL] of 1.38 L/h, and volume of distribution [V] of 90.1 L for 100 microg and t((1/2)) of 35.7 hours, CL of 1.3 L/h, and V of 123 L for 10 mg), midazolam (t((1/2)) of 4.87 hours, CL of 21.2 L/h, V of 145 L, and oral bioavailability [F] of 0.23 for 100 microg and t((1/2)) of 3.31 hours, CL of 20.4 L/h, V of 75 L, and F of 0.22 for 7.5 mg), and development compound ZK253 (F = <1% for both 100 microg and 50 mg). For warfarin, clearance was reasonably well predicted (0.17 L/h for 100 microg and 0.26 L/h for 5 mg), but the discrepancy observed in distribution (67 L for 100 microg and 17.9 L for 5 mg) was probably a result of high-affinity, low-capacity tissue binding. The oral microdose of erythromycin failed to provide detectable plasma levels as a result of possible acid lability in the stomach. Absolute bioavailability for the 3 compounds examined yielded excellent concordance with data from the literature or data generated in house. CONCLUSION: Overall, when used appropriately, microdosing offers the potential to aid in early drug candidate selection.

In vitro and in vivo correlation of hepatic transporter effects on erythromycin metabolism: characterizing the importance of transporter-enzyme interplay.

The effects of hepatic uptake and efflux transporters on erythromycin (ERY) disposition and metabolism were examined by comparing results from rat hepatic microsomes, freshly isolated hepatocytes, and in vivo studies. Uptake studies carried out in freshly isolated rat hepatocytes showed that ERY and its metabolite (N-demethyl-ERY) are substrates of Oatp1a4 and Oatp1b2. Whereas rifampin and GG918 [GF120918: N-{4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)-ethyl]-phenyl}-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamine] exerted minimal effects on metabolism in microsomes, rifampin (2.5 microM) and GG918 (0.5 microM) significantly decreased and increased ERY metabolism in hepatocytes, respectively. Concentration-time course studies further demonstrated that, compared with the intracellular N-demethyl-ERY control area under the curve (AUC) (0.795 +/- 0.057 microM . min), a decreased AUC (0.513 +/- 0.028 microM . min, p < 0.005) was observed when ERY was coincubated with rifampin, and an increased AUC (2.14 +/- 0.21 microM . min, p < 0.05) was found when GG918 was present. The results of the i.v. bolus studies showed that, compared with the ERY clearance of the controls (47.2 +/- 12.5 ml/min/kg for the rifampin group and 42.1 +/- 5.7 for the GG918 group), a decreased blood clearance, 29.8 +/- 6.1 ml/min/kg (p < 0.05) and 21.7 +/- 9.0 ml/min/kg (p < 0.01), was observed when rifampin or GG918, respectively, was coadministered. When either inhibitor was codosed with ERY, volume of distribution at steady state was unchanged, but t1/2 and mean residence time significantly increased compared with the controls. Hepatic uptake and efflux transporters modulate intracellular concentrations of ERY, thereby affecting metabolism. The interplay of transporters and enzymes must be considered in evaluating potential drug-drug interactions.