Transport vs. Metabolism: What Determines the Pharmacokinetics and Pharmacodynamics of Drugs? Insights From the Extended Clearance Model.

The well-stirred hepatic clearance model (WSHM) has been expanded to include drug transporters (i.e., extended clearance model [ECM]). However, the consequences of this expansion in understanding when transporters vs. metabolic enzymes will affect the pharmacokinetic (PK) and pharmacodynamic (PD) of drugs remains opaque. Identifying the rate-determining step(s) in systemic or tissue drug PK/PD will allow accurate predictions of drug PK/PD and drug-drug interactions (DDIs). Here, we clarify the implications of the ECM on PK/PD of drugs.

Ontogeny of Hepatic Drug Transporters as Quantified by LC-MS/MS Proteomics.

Protein expression of major hepatic uptake and efflux drug transporters in human pediatric (n = 69) and adult (n = 41) livers was quantified by liquid chromatography / tandem mass spectroscopy (LC-MS/MS). Transporter protein expression of OCT1, OATP1B3, P-gp, and MRP3 was age-dependent. Particularly, significant differences were observed in transporter expression (P < 0.05) between the following age groups: neonates vs. adults (OCT1, OATP1B3, P-gp), neonates or infants vs. adolescents and/or adults (OCT1, OATP1B3, and P-gp), infants vs. children (OATP1B3 and P-gp), and adolescents vs. adults (MRP3). OCT1 showed the largest increase, of almost 5-fold, in protein expression with age. Ontogenic expression of OATP1B1 was confounded by genotype and was revealed only in livers harboring SLCO1B1*1A/*1A. In livers >1 year, tissues harboring SLCO1B1*14/*1A showed 2.5-fold higher (P < 0.05) protein expression than SLCO1B1*15/*1A. Integration of these ontogeny data in physiologically based pharmacokinetic (PBPK) models will be a crucial step in predicting hepatic drug disposition in children.

Interindividual variability of CYP2C19-catalyzed drug metabolism due to differences in gene diplotypes and cytochrome P450 oxidoreductase content.

Large interindividual variability has been observed in the metabolism of CYP2C19 substrates in vivo. The study aimed to evaluate sources of this variability in CYP2C19 activity, focusing on CYP2C19 diplotypes and the cytochrome P450 oxidoreductase (POR). CYP2C19 gene analysis was carried out on 347 human liver samples. CYP2C19 activity assayed using human liver microsomes confirmed a significant a priori predicted rank order for (S)-mephenytoin hydroxylase activity of CYP2C19*17/*17 > *1B/*17 > *1B/*1B > *2A/*17 > *1B/*2A > *2A/*2A diplotypes. In a multivariate analysis, the CYP2C19*2A allele and POR protein content were associated with CYP2C19 activity. Further analysis indicated a strong effect of the CYP2C19*2A, but not the *17, allele on both metabolic steps in the conversion of clopidogrel to its active metabolite. The present study demonstrates that interindividual variability in CYP2C19 activity is due to differences in both CYP2C19 protein content associated with gene diplotypes and the POR concentration.The Pharmacogenomics Journal advance online publication, 1 September 2015; doi:10.1038/tpj.2015.58.

The concept of fraction of drug transported (ft ) with special emphasis on BBB efflux of CNS and antiretroviral drugs.

The fraction of drug transported (ft) into or out of a tissue is a concept useful to understand the impact of transporters on absorption, distribution, metabolism, and excretion (ADME) and tissue distribution of a drug. Here, ft is utilized to explain the impact of transporters on central nervous system (CNS) distribution of drugs, drug interactions (DDI), and to predict the unbound brain concentration (Cu,b) of the drug. The latter is important to ascertain if Cu,b is sufficient for efficacy.

Evaluation of various static and dynamic modeling methods to predict clinical CYP3A induction using in vitro CYP3A4 mRNA induction data.

Several drug-drug interaction (DDI) prediction models were evaluated for their ability to identify drugs with cytochrome P450 (CYP)3A induction liability based on in vitro mRNA data. The drug interaction magnitudes of CYP3A substrates from 28 clinical trials were predicted using (i) correlation approaches (ratio of the in vivo peak plasma concentration (Cmax) to in vitro half-maximal effective concentration (EC50); and relative induction score), (ii) a basic static model (calculated R3 value), (iii) a mechanistic static model (net effect), and (iv) mechanistic dynamic (physiologically based pharmacokinetic) modeling. All models performed with high fidelity and predicted few false negatives or false positives. The correlation approaches and basic static model resulted in no false negatives when total Cmax was incorporated; these models may be sufficient to conservatively identify clinical CYP3A induction liability. Mechanistic models that include CYP inactivation in addition to induction resulted in DDI predictions with less accuracy, likely due to an overprediction of the inactivation effect.

A PBPK Model to Predict Disposition of CYP3A-Metabolized Drugs in Pregnant Women: Verification and Discerning the Site of CYP3A Induction.

Besides logistical and ethical concerns, evaluation of safety and efficacy of medications in pregnant women is complicated by marked changes in pharmacokinetics (PK) of drugs. For example, CYP3A activity is induced during the third trimester (T3). We explored whether a previously published physiologically based pharmacokinetic (PBPK) model could quantitatively predict PK profiles of CYP3A-metabolized drugs during T3, and discern the site of CYP3A induction (i.e., liver, intestine, or both). The model accounted for gestational age-dependent changes in maternal physiological function and hepatic CYP3A activity. For model verification, mean plasma area under the curve (AUC), peak plasma concentration (Cmax), and trough plasma concentration (Cmin) of midazolam (MDZ), nifedipine (NIF), and indinavir (IDV) were predicted and compared with published studies. The PBPK model successfully predicted MDZ, NIF, and IDV disposition during T3. A sensitivity analysis suggested that CYP3A induction in T3 is most likely hepatic and not intestinal. Our PBPK model is a useful tool to evaluate different dosing regimens during T3 for drugs cleared primarily via CYP3A metabolism.CPT: Pharmacometrics & Systems Pharmacology (2012) 1, e3; doi:10.1038/psp.2012.2; advance online publication 26 September 2012.

Regional P-glycoprotein activity and inhibition at the human blood-brain barrier as imaged by positron emission tomography.

We used positron emission tomography (PET) to evaluate the contribution of P-glycoprotein (P-gp), present at the human blood-brain barrier (BBB), to regional drug distribution in the brain. Eleven healthy volunteers underwent PET imaging with [(11)C]-verapamil before and during cyclosporine A infusion. Regional P-gp inhibition was expressed as cyclosporine A-induced percentage change in the distributional clearance of verapamil (K(1)) in the brain, normalized to the regional blood flow (rCBF). K(1) estimates were similar across gray-matter regions of the brain and lower in the white matter regions, but all these estimates were considerably lower than rCBF. Normalization of K(1) by rCBF diminished the differences in estimates related to gray matter and white matter. In contrast, the K(1) for the pituitary, which is situated outside the BBB, approximated the rCBF. The magnitude of P-gp inhibition was comparable across BBB-protected brain structures. Our results indicate that P-gp and its inhibition equally affect the distribution of drugs (and therefore their neuro-efficacy and toxicity) in the various brain regions protected by the BBB.

Positron emission tomography imaging of tissue P-glycoprotein activity during pregnancy in the non-human primate.

BACKGROUND AND PURPOSE: Changes in tissue P-glycoprotein (P-gp) activity during pregnancy could affect the pharmacokinetics and thus the efficacy and toxicity of many drugs. Therefore, using positron emission tomography (PET) imaging, we tested whether gestational age affects tissue P-gp activity in the pregnant non-human primate, Macaca nemestrina. EXPERIMENTAL APPROACH: Mid-gestational (day 75 +/- 13, n= 7) and late-gestational (day 150 +/- 10, n= 5) age macaques were imaged after administration of a prototypic P-gp substrate, (11)C-verapamil (13.7-75.4 MBq.kg(-1)), before and during intravenous infusion of a P-gp inhibitor, cyclosporin A (CsA) (12 or 24 mg.kg(-1).h(-1)). Accumulation of radioactivity in the fetal liver served as a reporter of placental P-gp activity. P-gp activity was expressed as CsA-induced percent change in the ratio of the area (0-9 min) under the (11)C-radioactivity concentration-time curve in the tissue (AUC(tissue)) to that in the maternal plasma (AUC(plasma)). KEY RESULTS: The CsA-induced change in AUC(fetal liver)/AUC(maternal)(plasma) of (11)C-radioactivity significantly increased from mid- (35 +/- 25%) to late gestation (125 +/- 66%). Likewise, the CsA-induced change in AUC(maternal brain)/AUC(plasma) increased from mid- (172 +/- 80%) to late gestation (337 +/- 148%). The AUC ratio for the other maternal tissues was not significantly affected. Neither the CsA blood concentrations nor the level of circulating (11)C-verapamil metabolites were significantly affected by gestational age. CONCLUSIONS AND IMPLICATIONS: P-gp activity at the blood-brain barrier and the placental barrier in the macaque increased with gestational age. If replicated in humans, the exposure of the fetus and maternal brain to P-gp substrate drugs, and therefore their efficacy and toxicity, will change during pregnancy.

Are we optimizing gestational diabetes treatment with glyburide? The pharmacologic basis for better clinical practice.

Glyburide's pharmacokinetics (PK) and pharmacodynamics have not been studied in women with gestational diabetes mellitus (GDM). The objective of this study was to assess steady-state PK of glyburide, as well as insulin sensitivity, beta-cell responsivity, and overall disposition indices after a mixed-meal tolerance test (MMTT) in women with GDM (n = 40), nonpregnant women with type 2 diabetes mellitus (T2DM) (n = 26), and healthy pregnant women (n = 40, MMTT only). At equivalent doses, glyburide plasma concentrations were approximately 50% lower in pregnant women than in nonpregnant subjects. The average umbilical cord/maternal plasma glyburide concentration ratio at the time of delivery was 0.7 +/- 0.4. Insulin sensitivity was approximately fivefold lower in women with GDM as compared with healthy pregnant women. Despite comparable beta-cell responsivity indices, the average beta-cell function corrected for insulin resistance was more than 3.5-fold lower in women with glyburide-treated GDM than in healthy pregnant women. Women with GDM in whom glyburide treatment has failed may benefit from alternative medication or dosage escalation; however, fetal safety should be kept in mind.

Effects of pregnancy on CYP3A and P-glycoprotein activities as measured by disposition of midazolam and digoxin: a University of Washington specialized center of research study.

The objectives of the study were to evaluate the effects of pregnancy on CYP3A and P-glycoprotein (P-gp) activities, as measured by disposition of midazolam and digoxin, respectively. Thirteen women received digoxin (0.25 mg p.o.) and midazolam (2 mg p.o.) in random order, separated by 1-2 weeks at 28-32 weeks gestation, and the same order was repeated at 6-10 weeks postpartum. Plasma and urine concentrations were determined by liquid chromatography-mass spectrometry and analyzed by noncompartmental methods. Midazolam CL/F(unbound) (593 +/- 237 l/min vs. 345 +/- 103 l/min; P = 0.007), digoxin CL(Renal, unbound) (272 +/- 45 ml/min vs. 183 +/- 37 ml/min; P < 0.002) and digoxin CL(secretion,) (unbound) (109 +/- 34 ml/min vs. 58 +/- 22 ml/min; P < 0.002) were higher during pregnancy than postpartum. These data are consistent with increased hepatic and/or intestinal CYP3A and renal P-gp activities during pregnancy.

Grapefruit juice, a glass full of drug interactions?

Numerous reports have documented drug interactions with grapefruit juice (GFJ) that occur via inhibition of CYP3A enzymes. As reported by Glaeser et al. in the March 2007 issue of this journal, there is increasing recognition that GFJ may also affect the activity of influx (e.g., OATPs) and efflux (e.g., P-glycoprotein) transporters. This commentary focuses on these interactions between GFJ and drug transporters.

Molecular requirements of the human nucleoside transporters hCNT1, hCNT2, and hENT1.

Concentrative nucleoside transporters (CNTs) and equilibrative nucleoside transporters (ENTs) are important in physiological and pharmacological activity and disposition of nucleosides and nucleoside drugs. A better understanding of the structural requirements of inhibitors for these transporters will aid in designing therapeutic agents. To define the relative and unified structural requirements of nucleoside analogs for interaction with hCNT1, hCNT2, and hENT1, we applied an array of structure-activity techniques. Unique pharmacophore models for each respective nucleoside transporter were generated. These models reveal that hCNT2 affinity is dominated by hydrogen bonding features, whereas hCNT1 and hENT1 displayed mainly electrostatic and steric features. Hydrogen bond formation over 3'-OH is essential for all nucleoside transporters. Inhibition of nucleoside transporters by a series of uridine and adenosine analogs and a variety of drugs was analyzed by comparative molecular field analysis. Cross-validated r2 (q2) values were 0.65, 0.52, and 0.74 for hCNT1, hCNT2, and hENT1, respectively. The predictive quality of the models was further validated by successful prediction of the inhibition of a set of test compounds. Addition of a hydroxyl group around the 2-position of purine (or 3-position of pyrimidine) may increase inhibition to hCNT2 transporter; addition of hydroxyl group around the 2,7-position of purine (or the 3,5-position of pyrimidine) would increase the inhibition to hENT1 transporter. Utilization of these models should assist the design of high-affinity nucleoside transporter inhibitors and substrates for both anticancer and antiviral therapy.

CYP3A inductive potential of the rifamycins, rifabutin and rifampin, in the rabbit.

Rifabutin is effective in the treatment and prevention of Mycobacterium avium infection in people with HIV infection. Rifabutin is structurally related to another rifamycin, rifampin, a well-known inducer of the human P-450 isoform 3A. The rabbit isoform CYP3A6 and the human isoform CYP3A4 have similar P-450 predominance and substrate specificity and are both induced by rifampin. Our goal was to predict the CYP3A induction capacity of rifabutin and to determine if ex vivo CYP3A induction potential of rifamycins is predictive of that obtained in vivo. We determined the in vivo and ex vivo CYP3A6 induction by 4 days of treatment with rifabutin (100 mg/kg), rifampin (100 mg/kg), or vehicle (DMSO) in the rabbit. The ex vivo measures were CYP3A6 activity (N-demethylation of erythromycin and hydroxylation of triazolam) and CYP3A content in rabbit hepatic microsomes preparations. The in vivo measures were oral clearance of triazolam and its formation clearance to its hydroxylated metabolites, alpha-hydroxytriazolam and 4-hydroxytriazolam. Rifampin increased CYP3A6 activity by 2- to 3-fold in hepatic microsomes compared to vehicle. Rifabutin increased CYP3A content 1.7-fold, but did not significantly increase microsomal CYP3A6 activity. Oral triazolam clearance and formation clearances to the two hydroxylated metabolites were 2- to 3-fold greater in rabbits treated with rifampin. These clearances were unaffected by rifabutin administration. Ex vivo enzyme activities correlated with in vivo changes in clearance of triazolam and the formation clearance to its hydroxylated metabolites. Rifabutin is a weaker inducer of CYP3A6 than rifampin. These data suggest that ex vivo enzyme activity is a viable approach to predict in vivo inductive potential of CYP3A inducers.

Characterization of a maternal-fetal HIV transmission model using pregnant macaques infected with HIV-2(287).

To study mechanisms involved in mother-to-fetus transmission of human immunodeficiency virus (HIV) in utero, we have developed a chronically catheterized pregnant macaque model that permits simultaneous and sequential determination of virus in maternal and fetal blood and amniotic fluid during pregnancy. In this report, we have characterized this model using three groups of pregnant macaques designed to sample: (1) maternal blood, fetal blood, and amniotic fluid (n = 6); (2) maternal blood and amniotic fluid (n = 6); or (3) maternal blood only (n = 2). After inoculation with the highly pathogenic HIV-2(287), all pregnant macaques developed brief but intense viremias followed by precipitous CD4+ T-cell declines within 2-3 weeks. While all the infants born to dams of the three groups were HIV positive, the degree of infection and outcome of HIV infection varied. All infants were shown to be HIV-RNA-positive by reverse transcriptase-polymerase chain reaction (RT-PCR). However, HIV-infected cells were detected only in the blood of those born to dams enrolled in groups 1 and 2: most of these infants progressed to CD4+ T-cell depletion. The infants in group 3 exhibited HIV-RNA in plasma, although neither HIV-infected cells nor CD4+ T-cell depletion was detectable. However, all infants developed HIV-2-specific antibody at various levels by 2 months of age. Together, the data suggest that, while the degree of instrumentation may modulate intensity of virus transmission to fetus, the highly pathogenic HIV-2(287) exhibited a high frequency of virus transmission from the mother to fetus.

Ontogenic and longitudinal activity of Na(+)-nucleoside transporters in the human intestine.

The objectives of our study were to identify the types of nucleoside transporters present in the human fetal small intestine and to characterize their developmental activity, longitudinal distribution, and transport kinetics compared with those present in the adult intestine. Nucleoside uptake by intestinal brush-border membrane vesicles was measured by an inhibitor-stop rapid filtration technique. Only the purine-specific (N1; hCNT2) and the pyrimidine-specific (N2; hCNT1) Na(+)-dependent nucleoside transporters were found to be present on the brush-border membranes of the enterocytes along the entire length of the fetal and adult small intestines. The activity of these transporters was higher in the proximal than in the distal small intestine. Both the N1 and N2 transporters found in the fetal intestine shared similar kinetic properties (Michaelis-Menten constant and Na(+)-nucleoside stoichiometry) to those in the adult intestine. During the period of rapid morphogenesis (11-15 wk gestation), no temporal differences were apparent in the activity of the N1 and N2 transporters in the fetal small intestine. These findings have implications for the absorption of drugs from the amniotic fluid by the fetus after maternal drug administration of nucleoside drugs such as the antivirals zidovudine and didanosine.

Suppression of maternal virus load with zidovudine, didanosine, and indinavir combination therapy prevents mother-to-fetus HIV transmission in macaques.

Recently, we developed a maternal-fetal macaque model using a highly pathogenic HIV-2 strain, HIV-2287, to study the time course of HIV transmission in utero. Most pregnant macaques (Macaca nemestrina) infected with HIV-2287 (10-103 infective doses) transmitted HIV to their fetuses, as verified by positive identification of virus-infected mononuclear cells and free viral RNA in fetal blood. To determine whether an antiretroviral drug combination therapy composed of two dideoxynucleosides, azidothymidine (15 mg/kg) and dideoxyinosine (15 mg/kg), and a protease inhibitor, indinavir (25 mg/kg), could completely inhibit mother-to-fetus HIV transmission, we administered these drugs orally through gastric catheters to five pregnant macaques infected with 10 infective doses of HIV-2287. Beginning 30 minutes after HIV inoculation, the dams were given the combination antiviral therapy three times daily until delivery by cesarean section. Drug treatment reduced the maternal virus load to a minimally detectable level but did not prevent primary HIV-2287 infection. All fetal and infant blood samples were virus negative by internally controlled RNA polymerase chain reaction (QC-RNA-PCR) and virus coculture assays. Fetal and infant CD4+ T-cell levels remained normal throughout the experiment. These findings strongly suggest that combination chemotherapy with azidothymidine, dideoxyinosine, and indinavir can suppress maternal viral load enough to prevent mother-to-fetus transmission of HIV.

Structure-inhibitory profiles of nucleosides for the human intestinal N1 and N2 Na+-nucleoside transporters.

PURPOSE: To determine the structure-inhibitory profiles of nucleosides for the N1 and N2 Na+-nucleoside transporters of the human intestine. METHODS: The uptake of 3H-labeled prototypic substrates of the N1 (inosine) and N2 (thymidine) transporters into human intestinal brush border membrane vesicles was measured by a rapid filtration technique in the presence and absence of various uridine and adenosine analogs and antiviral and anticancer nucleoside drugs (100 and 1000 microM). RESULTS: In the ribose ring, the 3'-oxygen is required for inhibition of uptake of nucleosides by both the N1 and N2 transporters. The structural requirements for such inhibition differ with respect to modifications on the 5' position of the sugar ring or on the base. The N2 transporter is more tolerant to these substitutions than is the N1 transporter. The 6 position on uracil and the 8 position on adenine are critical for inhibition of uptake of nucleosides by both the N1 and N2 nucleoside transporters. CONCLUSIONS: These data are the first evidence that the binding site(s) of the human N1 and N2 transporters differ in their interaction with analogs of their common substrates, uridine and adenosine. Such studies can provide insight into the critical structural determinants of the substrate necessary for recognition by the Na+-nucleoside transporters of the human intestine.

CYP2C8/9 mediate dapsone N-hydroxylation at clinical concentrations of dapsone.

Using selective cytochrome P450 (CYP) inhibitors and clinical concentrations (4 microM) of dapsone (DDS), we found a major contribution of CYP2C9 and little or no contribution (< or = 10%) of CYP3A4 and CYP2E1 to dapsone N-hydroxylation (DDS-NHY) in human liver microsomes. Sulfaphenazole (2.16 microM) and tolbutamide (500 microM), selective inhibitors of CYP2C9 (or 2C8/9), inhibited DDS-NHY by 48 +/- 14 and 41 +/- 15%, respectively. The apparent Michaelis-Menten Km values for DDS-NHY by cloned CYP2C8, CYP2C9, CYP2C18, and CYP2C19 were 75 microM, 31 microM, 25 microM, and greater than 1 mM, respectively. CYP3A4 and CYP2E1 were incapable of DDS-NHY at 4 microM DDS. S-mephenytoin (360 microM) activated DDS-NHY by human liver microsomes and by CYP2C8 by 43 +/- 36 and 193 +/- 16%, respectively. This activation was cytochrome b5-dependent. In contrast, S-mephenytoin inhibited DDS-NHY by CYP2C9, CYP2C18, and CYP2C19 by 27 +/- 2, 49 +/- 1, and 32 +/- 4%, respectively. Because CYP2C18 and CYP19 are expressed at low concentrations in the human liver, these observations indicate that at clinical DDS concentrations, CYP2C9 is a major and CYP2C8 is a likely minor contributor to DDS-NHY in human liver microsomes.

Human intestinal es nucleoside transporter: molecular characterization and nucleoside inhibitory profiles.

PURPOSE: To clone and sequence the equilibrative nitrobenzylthioinosine (NBMPR)-sensitive nucleoside transporter (es) from the human small intestine and to examine the capacities of nucleosides and nucleoside analogs to inhibit the uptake of uridine by this transporter. METHODS: Using PCR, es was cloned from a cDNA library of the human small intestine. The uptake of 3H-uridine (10 microM) by the recombinant es, expressed in Xenopus oocytes, was measured in the presence (2 mM) and absence of nucleosides and nucleoside analogs. RESULTS: The amino acid sequence of this es transporter was identical to that of the human placental es transporter. Uptake of 3H-uridine by this es transporter was inhibitable by 1 microM NBMPR. Removal of the oxygen from the 3' position or from both the 2' and 3' positions, but not from 2' or 5' position, resulted in a partial or total loss of the capacity of the nucleosides to inhibit 3H-uridine uptake. No modifications of the adenosine base or of the uridine base (except for 3 and 6 positions on uracil) affected nucleoside inhibitory capacity. CONCLUSION: The es transporters of the human intestine and placenta are identical in their amino acid sequences. Moreover, the inhibitory profiles of various nucleoside analogs in inhibiting the uptake of uridine by the intestinal es transporter are similar to those obtained with the as-yet-uncloned human erythrocyte es transporter. Collectively, these findings suggest that the es transporter does not appear to be functionally variant in the human placenta, small intestine or erythrocytes.

Pharmacokinetics of didanosine in antepartum and postpartum human immunodeficiency virus--infected pregnant women and their neonates: an AIDS clinical trials group study.

Didanosine (ddI) pharmacokinetics in antepartum and postpartum human immunodeficiency virus (HIV)-infected women and their neonates were studied. HIV-infected pregnant women received an intravenous (iv) ddI infusion (1.6 mg/kg/h) or an oral dose (200 mg bid or 125 mg bid) at 31 weeks antepartum and 6 weeks postpartum. Blood samples were obtained regularly up to 6 or 8 h after drug administration. The same oral dose of ddI (bid) was administered until labor began. Then, ddI was infused iv until delivery. An oral pharmacokinetic study (60 mg/m2) was conducted in infants at day 1 and at week 6 after birth. Plasma concentrations of ddI were measured by radioimmunoassay. After iv ddI administration, only the maternal plasma clearance was found to be significantly increased antepartum (1028+/-231 mL/min) versus postpartum (707+/-213 mL/min). No pharmacokinetic parameters after oral administration were significantly affected by pregnancy. The pharmacokinetics of ddI in the neonates were highly variable. We conclude that the oral ddI dose need not be adjusted during pregnancy.