Are all measures of liver Kpuu a function of FH, as determined following oral dosing, or have we made a critical error in defining hepatic drug clearance?

Here we present, utilizing universally accepted relationships for hepatic clearance at steady state, that for all models of hepatic elimination the ratio of unbound liver drug concentration to unbound systemic blood concentration, Kpuu, is a function of or related to the hepatic bioavailability for that drug, FH. According to the derivation for the well-stirred model, Kpuu can never exceed unity, can frequently be a function of hepatic blood flow, and is equivalent to the value of FH as determined following oral dosing. For the parallel tube model, Kpuu will not equal FH but will be a function of FH and will also never be a value greater than 1. When hepatic clearance is rate limited by basolateral transporters, Kpuu will be less than 1, and less than FH. We believe that such outcomes are highly unlikely, and that the error arises from a basic assumption concerning hepatic clearance that leads to the mechanistic models of hepatic elimination, the well-stirred, parallel tube and dispersion models. That basic assumption is that the steady-state systemic concentration multiplied by the hepatic systemic clearance is equal to the product of the average unbound liver steady-state concentration and the intrinsic hepatic clearance (Css · CL = CH,u · CLint). Calculations of Kpuu and FH based on present methods of analysis provide a strong argument as to why this universally accepted relationship is not correct. Alternatively, we have shown in recent publications that hepatic clearance may be adequately determined based on Kirchhoff's Laws where no assumption of the above equality concerning hepatic intrinsic clearance is required, and where Kpuu is independent of hepatic extraction ratio and FH.

HIV-infected liver and kidney transplant recipients: 1- and 3-year outcomes.

Improvements in human immunodeficiency virus (HIV)-associated mortality make it difficult to deny transplantation based upon futility. Outcomes in the current management era are unknown. This is a prospective series of liver or kidney transplant recipients with stable HIV disease. Eleven liver and 18 kidney transplant recipients were followed for a median of 3.4 years (IQR [interquartile range] 2.9-4.9). One- and 3-year liver recipients' survival was 91% and 64%, respectively; kidney recipients' survival was 94%. One- and 3-year liver graft survival was 82% and 64%, respectively; kidney graft survival was 83%. Kidney patient and graft survival were similar to the general transplant population, while liver survival was similar to the older population, based on 1999-2004 transplants in the national database. CD4+ T-cell counts and HIV RNA levels were stable; and there were two opportunistic infections (OI). The 1- and 3-year cumulative incidence (95% confidence intervals [CI]) of rejection episodes for kidney recipients was 52% (28-75%) and 70% (48-92%), respectively. Two-thirds of hepatitis C virus (HCV)-infected patients, but no patient with hepatitis B virus (HBV) infection, recurred. Good transplant and HIV-related outcomes among kidney transplant recipients, and reasonable outcomes among liver recipients suggest that transplantation is an option for selected HIV-infected patients cared for at centers with adequate expertise.

An examination of protein binding and protein-facilitated uptake relating to in vitro-in vivo extrapolation.

As explained by the free drug theory, the unbound fraction of drug has long been thought to drive the efficacy of a molecule. Thus, the fraction unbound term, or fu, appears in equations for fundamental pharmacokinetic parameters such as clearance, and is used when attempting in vitro to in vivo extrapolation (IVIVE). In recent years though, it has been noted that IVIVE does not always yield accurate predictions, and that some highly protein bound ligands have more efficient uptake than can be explained by their unbound fractions. This review explores the evolution of fu terms included when implementing IVIVE, the concept of protein-facilitated uptake, and the mechanisms that have been proposed to account for facilitated uptake.

The Universally Unrecognized Assumption in Predicting Drug Clearance and Organ Extraction Ratio.

For almost a half-century clearance concepts have been utilized in pharmacokinetics to understand the relationship between the dose administered and the time course of systemic concentrations to predict efficacy and safety, as well as how dosing should be modified in disease states. Various models of organ clearance/elimination have been proposed and tested. Surprisingly, however, the theoretical basis for the appropriate data collection to test these models has never been evaluated. Here we show that in vivo data collection limitations and the extraction ratio concept itself are only consistent with the well-stirred model of hepatic elimination. Evaluating measures of drug concentrations entering and leaving an organ will appear to best fit the well-stirred model, since driving force concentrations within the organ of elimination cannot be measured.

Review and critique of the Institute of Medicine report "the future of drug safety".

In September 2006 an Institute of Medicine (IOM) ad hoc committee on the Assessment of the US Drug Safety System released its report entitled "The Future of Drug Safety: Promoting and Protecting the Health of the Public". The committee's report includes 25 recommendations that "will bring the strengths of the preapproval process (data, regulatory authority, organizational function and capabilities, and resources) to the postapproval phase in order to fulfill a lifecycle approach to the study, regulation, and communication about the risks and benefits of drugs." Copies of the report are available from the National Academies Press (800-624-6242), and the full text is available at http://www.nap.edu.

Effects of uptake and efflux transporter inhibition on erythromycin breath test results.

The erythromycin breath test (EBT) is a standard test used to evaluate the extent of CYP3A4 activity. This study examines whether presumed changes in CYP3A4 activity are in fact related to inhibition of an uptake organic anion transporter using rifampin and inhibition of the efflux hepatic P-glycoprotein transporter using lansoprazole. Three EBT tests in healthy adults were conducted: EBT alone, with lansoprazole, and with rifampin. For all subjects, lansoprazole treatment increased respiratory (14)C excretion by +0.25+/-0.51 met/h (P=0.07) and rifampin decreased (14)C excretion by -0.44+/-0.40 met/h (P<0.001) compared with baseline. Comparing lansoprazole to rifampin, (14)C excretion increased by +0.69+/-0.50 met/h (P<0.001). Only women had significant changes after drug infusion: (14)C excretion after rifampin -0.40+/-0.36 met/h (P=0.018) and +0.47+/-0.44 met/h (P=0.018) after lansoprazole. Relying on EBT without considering transporter interactions can lead to errors in interpreting the degree of CYP3A4 metabolism.

effect of OATP1B transporter inhibition on the pharmacokinetics of atorvastatin in healthy volunteers.

The inhibition of hepatic uptake transporters, such as OATP1B1, on the pharmacokinetics of atorvastatin is unknown. Here, we investigate the effect of a model hepatic transporter inhibitor, rifampin, on the kinetics of atorvastatin and its metabolites in humans. The inhibitory effect of a single rifampin dose on atorvastatin kinetics was studied in 11 healthy volunteers in a randomized, crossover study. Each subject received two 40-mg doses of atorvastatin, one on study day 1 and one on study day 8, separated by 1 week. One intravenous 30-min infusion of 600 mg rifampin was administered to each subject on either study day 1 or study day 8. Plasma concentrations of atorvastatin and metabolites were above the limits of quantitation for up to 24 h after dosing. Rifampin significantly increased the total area under the plasma concentration-time curve (AUC) of atorvastatin acid by 6.8+/-2.4-fold and that of 2-hydroxy-atorvastatin acid and 4-hydroxy-atorvastatin acid by 6.8+/-2.5- and 3.9+/-2.4-fold, respectively. The AUC values of the lactone forms of atorvastatin, 2-hydroxy-atorvastatin and 4-hydroxy-atorvastatin, were also significantly increased, but to a lower extent. An intravenous dose of rifampin substantially increased the plasma concentrations of atorvastatin and its acid and lactone metabolites. The data confirm that OATP1B transporters represent the major hepatic uptake systems for atorvastatin and its active metabolites. Inhibition of hepatic uptake may have consequences for efficacy and toxicity of drugs like atorvastatin that are mainly eliminated by the hepatobiliary system.

Irreversible binding of zomepirac to plasma protein in vitro and in vivo.

Zomepirac is a nonsteroidal anti-inflammatory drug recently withdrawn from use because of an unexplained high incidence of immunological reactions. It is metabolized in humans to a reactive, unstable acyl glucuronide which accumulates in plasma. Because of the similarity of zomepirac glucuronide to bilirubin glucuronide in structure and stability and the documented irreversible binding of bilirubin to albumin through its acyl glucuronide, we studied the reaction of zomepirac acyl glucuronide with albumin in vitro from pH 5 to 9 and in vivo in six healthy human volunteers who had received a single 100-mg oral dose of zomepirac. Irreversible binding of zomepirac to protein was determined by exhaustive washing of protein, followed by hydrolysis of bound zomepirac-protein adduct with base, extraction of the liberated drug, and chromatographic measurement. Irreversible binding was observed both in vitro and in vivo. The extent of binding in vitro was time- and pH-dependent. In vitro drug binding was also observed for the isomers of zomepirac glucuronide which were formed by intramolecular acyl migration. Irreversible binding in vivo correlated with overall exposure to zomepirac glucuronide when exposure was expressed as the area under the plasma concentration vs. time curve. When probenecid (500 mg, twice daily), which decreases the plasma clearance of zomepirac glucuronide, was administered concurrently with zomepirac, irreversible binding of zomepirac was increased. The nature of the zomepirac protein binding is probably covalent. Formation of irreversibly protein-bound zomepirac occurs via the acyl glucuronide as previously shown for bilirubin glucuronide, and the reaction may be general for other drugs that are metabolized to acyl glucuronides.

Between-Batch Pharmacokinetic Variability Inflates Type I Error Rate in Conventional Bioequivalence Trials: A Randomized Advair Diskus Clinical Trial.

We previously demonstrated pharmacokinetic differences among manufacturing batches of a US Food and Drug Administration (FDA)-approved dry powder inhalation product (Advair Diskus 100/50) large enough to establish between-batch bio-inequivalence. Here, we provide independent confirmation of pharmacokinetic bio-inequivalence among Advair Diskus 100/50 batches, and quantify residual and between-batch variance component magnitudes. These variance estimates are used to consider the type I error rate of the FDA's current two-way crossover design recommendation. When between-batch pharmacokinetic variability is substantial, the conventional two-way crossover design cannot accomplish the objectives of FDA's statistical bioequivalence test (i.e., cannot accurately estimate the test/reference ratio and associated confidence interval). The two-way crossover, which ignores between-batch pharmacokinetic variability, yields an artificially narrow confidence interval on the product comparison. The unavoidable consequence is type I error rate inflation, to ∼25%, when between-batch pharmacokinetic variability is nonzero. This risk of a false bioequivalence conclusion is substantially higher than asserted by regulators as acceptable consumer risk (5%).

Batch-to-batch pharmacokinetic variability confounds current bioequivalence regulations: A dry powder inhaler randomized clinical trial.

Current pharmacokinetic (PK) bioequivalence guidelines do not account for batch-to-batch variability in study design or analysis. Here we evaluate the magnitude of batch-to-batch PK variability for Advair Diskus 100/50. Single doses of fluticasone propionate and salmeterol combinations were administered by oral inhalation to healthy subjects in a randomized clinical crossover study comparing three different batches purchased from the market, with one batch replicated across two treatment periods. All pairwise comparisons between different batches failed the PK bioequivalence statistical test, demonstrating substantial PK differences between batches that were large enough to demonstrate bio-inequivalence in some cases. In contrast, between-replicate PK bioequivalence was demonstrated for the replicated batch. Between-batch variance was ∼40-70% of the estimated residual error. This large additional source of variability necessitates re-evaluation of bioequivalence assessment criteria to yield a result that is both generalizable and consistent with the principles of type I and type II error rate control.

Isoniazid hair concentrations in children with tuberculosis: a proof of concept study.

Assessing treatment adherence and quantifying exposure to anti-tuberculosis drugs among children is challenging. We undertook a 'proof of concept' study to assess the drug concentrations of isoniazid (INH) in hair as a therapeutic drug monitoring tool. Children aged <12 years initiated on a thrice-weekly treatment regimen including INH (10 mg/kg) for newly diagnosed tuberculosis were enrolled. INH concentrations in hair were measured using liquid chromatography-tandem mass spectrometry at 1, 2, 4 and 6 months after initiating anti-tuberculosis treatment. We found that INH hair concentrations in all children on thrice-weekly INH were detectable and displayed variability across a dynamic range.

The drug efflux-metabolism alliance: biochemical aspects.

The considerable overlap in the substrate selectivity and tissue localization of CYP3A and P-glycoprotein has led to the hypothesis that this transporter and enzyme pair act as a coordinated absorption barrier against xenobiotics. A historical perspective on the investigation of this interactive alliance is given, starting from the understanding of the role of intestinal metabolism in explaining cyclosporine clinical data. Several animal studies using mdr1a-/- knockout mice have demonstrated P-glycoprotein's importance in limiting drug absorption and decreasing bioavailability. Human clinical studies investigating the importance of intestinal CYP3A and P-glycoprotein through inhibition or induction of these proteins have provided further evidence of this interaction. Recent in vitro studies using CYP3A4-expressing Caco-2 cells are reported. These studies reveal that the role of P-glycoprotein in the intestine extends beyond simply limiting parent drug absorption but also includes increasing the access of drug to metabolism by CYP3A through repeated cycles of absorption and efflux.

Active secretion and enterocytic drug metabolism barriers to drug absorption.

Intestinal phase I metabolism and active extrusion of absorbed drug have only recently been recognized as major determinants of oral drug bioavailability. Both CYP3A4, the major phase I drug metabolizing enzyme in humans, and the multidrug efflux pump, P-glycoprotein (P-gp), are present at high levels in the villus enterocytes of the small intestine, the primary site of absorption for orally administered drugs. Moreover, these proteins are induced by many of the same compounds and demonstrate a broad overlap in substrate and inhibitor specificities, suggesting that they act as a concerted barrier to drug absorption. Clinical studies have demonstrated that inhibition of CYP3A4-mediated intestinal metabolism can significantly improve the oral bioavailability of a wide range of drugs. Intestinal P-gp is a major route of elimination for both orally and intravenously administered anticancer drugs in animal models, and experiments with the Caco-2 cell line have provided strong evidence that inhibition of intestinal P-gp is another means by which oral drug bioavailability could be enhanced.

Prednisolone clearance at steady state in man.

The present study was undertaken to determine whether prednisolone exhibits dose-dependent kinetics in man. Ten normal volunteers were infused to steady state over a 7-h period at a low (5.5 microgram/h . kg) and a high (64 microgram/h . kg) rate with prednisolone. Steady state prednisolone levels differed by a factor of 5 [91 +/- 25 and 437 +/- 116 ng/ml (mean +/- SD)] when the infusion rate was increased 12-fold, indicating a marked increase in the clearance of total prednisolone with increasing dose (the ratio of clearances, high to low dose, was 2.47 +/- 0.29). The fraction of unbound prednisolone increased from 0.12 +/- 0.02 to 0.24 +/- 0.02 with increasing dose. Since the increase in the free fraction (2-fold change) was not as great as the increase in the total prednisolone clearance, there was a slight but significant (P less than 0.05) increase in the apparent clearance of unbound prednisolone (ratio of apparent unbound clearances, high to low dose, was 1.29 +/- 0.24). The interconversion between prednisolone and prednisone appears to approach a maximum prednisone concentration, as was noted previously by us in dogs. In humans, we found this maximum prednisone concentration to be 52 ng/ml when prednisolone is infused. Therefore, the ratio of concentrations, prednisolone to prednisone, also increased with increasing prednisolone dose. These results indicate that prednisolone exhibits dose- and concentration-dependent kinetics and that the great majority of the change in kinetics may be attributed to saturable protein binding of prednisolone. Although there is an increase in the apparent clearance of unbound prednisolone with increasing concentrations, these results are confounded by the interconversion process between prednisone and prednisolone.

Impairment of prednisolone disposition in women taking oral contraceptives or conjugated estrogens.

Companion studies were designed to determine the effects of oral contraceptives and conjugated estrogens on the pharmacokinetics of prednisolone. Twenty-four normal women entered the studies, including six young women taking oral contraceptives and six age-matched control women, and six postmenopausal women receiving conjugated estrogens and six age-matched postmenopausal women. All received 0.53 mg/kg prednisolone phosphate, iv. Significant decreases (P less than 0.05) in the clearance and volume of distribution and significant increases in the half-life were found for both total and unbound prednisolone in the women taking oral contraceptives compared to values in the young control women. A significant decrease in the unbound clearance and increases in the total and unbound half-lives of prednisolone were found in the women receiving conjugated estrogens compared to values in the postmenopausal control women. Total clearance and volume of distribution were unchanged by conjugated estrogen therapy. Administration of prednisolone to women receiving estrogen-containing oral contraceptives or conjugated estrogens results in exposure of these women to increased concentrations of unbound prednisolone for increased periods of time. Increases in the pharmacological and toxic effects of prednisolone might be expected in these women.

Endogenous hydrocortisone, a possible factor contributing to the genesis of cushingoid habitus in patients on prednisone.

To establish if the cushingoid habitus in patients taking prednisone is associated with relatively high total or free prednisolone and low endogenous hydrocortisone concentrations in plasma, 15 stable renal transplant patients and 12 patients treated with prednisone for oral mucocutaneous vesiculo-erosive diseases were investigated. After the patients' usual prednisone doses, the areas under the plasma concentration time curve of total and free prednisolone were not different when the 14 patients without cushingoid appearance were compared to the 13 patients with cushingoid appearance. Patients with cushingoid habitus more frequently exhibited peak hydrocortisone levels within the normal range (6 of 14 vs. 1 of 13) and had higher areas under the plasma concentration time curve of hydrocortisone (median, range), i.e. 2672 ng/ml.min (0-21, 637 ng/ml.min) vs. 308 ng/ml.min (0-12, 495 ng/ml.min) compared to those without cushingoid appearance (P less than 0.05). These results indicate that pharmacokinetic differences of prednisone do not explain the presence or absence of cushingoid habitus and that there is an association between cushingoid habitus and endogenous hydrocortisone levels.

Variable glyceryl dinitrate formation as a function of route of nitroglycerin administration.

Plasma concentrations of 1,2- and 1,3-glyceryl dinitrates were measured in each of four healthy volunteers who received intravenous infusions (10, 20, and 40 micrograms/min), topical ointment (20 mg/200 cm2), and oral solution (6.5 mg) doses of nitroglycerin and from two subjects who received sublingual (0.4 mg) nitroglycerin. The ratio of 1,2-glyceryl dinitrate to 1,3-glyceryl dinitrate (dinitrate ratio) was determined for each subject after each dose and was found to vary with route of administration. Dinitrate ratios were 7.36, 4.60, 3.86, and 1.99 for intravenous, sublingual, topical, and oral doses, respectively. Nonspecific metabolism of nitroglycerin would result in twice as much of the 1,2-dinitrate as the 1,3-dinitrate (i.e., a dinitrate ratio of 2.0, such as that produced after oral administration). A high ratio (e.g., after intravenous administration) indicates that the metabolism was more specific. These results indicate that metabolite formation depends on route of administration, implying different metabolic specificity of enzymes in the gut, liver, skin, sublingual mucosa, and blood vessels.

Marked alterations in dose-dependent prednisolone kinetics in women taking oral contraceptives.

Six healthy women chronically (greater than 6 months) using oral contraceptives were age- and weight-matched with female controls. Each subject received prednisolone, 0.53 and 0.14 mg/kg iv. In the subjects taking oral contraceptives there were significant decreases in total clearance, unbound clearance, and volume of distribution at steady state for total drug, and significant increases in total and unbound prednisolone t1/2 and hydrocortisone concentrations compared with control subjects at both the high and low prednisolone doses. Both the oral contraceptive group and the control group had significantly higher total clearance, volume of distribution for total drug, and unbound fraction for the high dose compared with the low dose. Data suggest that chronic low-dose contraceptive steroid use results in a marked decrease in prednisolone clearance; however, dose-dependent changes in kinetics are still observed.

Probenecid-induced changes in the clearance of carprofen enantiomers: a preliminary study.

Probenecid inhibits the elimination of several acidic drugs. In this study, the influence of probenecid on the pharmacokinetics of carprofen was investigated in three healthy volunteers after single peroral administration of 150 mg of RS-(+/-)-carprofen. Carprofen enantiomers and their glucuronides (after cleavage with sodium hydroxide) were measured by use of a stereospecific procedure. The plasma concentrations of S-(+)-carprofen were higher than those of R-(-)-carprofen at most of the sampling points. Probenecid reduced apparent total and renal clearances for both enantiomers. It also reduced the clearances of the carprofen enantiomers to their glucuronides and the renal clearances of the glucuronides. The differences caused by probenecid were significant, but few stereoselective effects were observed.