Predicting Drug Disposition via Application of BCS: Transport/Absorption/ Elimination Interplay and Development of a Biopharmaceutics Drug Disposition Classification System

The Biopharmaceutics Classification System (BCS) was developed to allow prediction of in vivo pharmacokinetic performance of drug products from measurements of permeability (determined as the extent of oral absorption) and solubility. Here, we suggest that a modified version of such a classification system may be useful in predicting overall drug disposition, including routes of drug elimination and the effects of efflux and absorptive transporters on oral drug absorption; when transporter-enzyme interplay will yield clinically significant effects (e.g., low bioavailability and drug-drug interactions); the direction, mechanism, and importance of food effects; and transporter effects on postabsorption systemic drug concentrations following oral and intravenous dosing. These predictions are supported by a series of studies from our laboratory during the past few years investigating the effect of transporter inhibition and induction on drug metabolism. We conclude by suggesting that a Biopharmaceutics Drug Disposition Classification System (BDDCS) using elimination criteria may expand the number of Class 1 drugs eligible for a waiver of in vivo bioequivalence studies and provide predictability of drug disposition profiles for Classes 2, 3, and 4 compounds.

Role of intestinal P-glycoprotein (mdr1) in interpatient variation in the oral bioavailability of cyclosporine

Interpatient differences in the oral clearance of cyclosporine (INN, ciclosporin) have been partially attributed to variation in the activity of a single liver enzyme termed CYP3A4. Recently it has been shown that small bowel also contains CYP3A4, as well as P-glycoprotein, a protein able to transport cyclosporine. To assess the importance of these intestinal proteins, the oral pharmacokinetics of cyclosporine were measured in 25 kidney transplant recipients who each had their liver CYP3A4 activity quantitated by the intravenous [14C-N-methyl]-erythromycin breath test and who underwent small bowel biopsy for measurement of CYP3A4 and P-glycoprotein. Forward multiple regression revealed that 56% (i.e., r2 = 0.56) and 17% of the variability in apparent oral clearance [log (dose/area under the curve)] were accounted for by variation in liver CYP3A4 activity (p < 0.0001) and intestinal P-glycoprotein concentration (p = 0.0059), respectively. For peak blood concentration, liver CYP3A4 activity accounted for 32% (p = 0.0002) and P-glycoprotein accounted for an additional 30% (p = 0.0024) of the variability. Intestinal levels of CYP3A4, which varied tenfold, did not appear to influence any cyclosporine pharmacokinetic parameter examined. We conclude that intestinal P-glycoprotein plays a significant role in the first-pass elimination of cyclosporine, presumably by being a rate-limiting step in absorption. Drug interactions with cyclosporine previously ascribed to intestinal CYP3A4 may instead be mediated by interactions with intestinal P-glycoprotein.

BDDCS applied to over 900 drugs

Here, we compile the Biopharmaceutics Drug Disposition Classification System (BDDCS) classification for 927 drugs, which include 30 active metabolites. Of the 897 parent drugs, 78.8% (707) are administered orally. Where the lowest measured solubility is found, this value is reported for 72.7% (513) of these orally administered drugs and a dose number is recorded. The measured values are reported for percent excreted unchanged in urine, LogP, and LogD (7.4) when available. For all 927 compounds, the in silico parameters for predicted Log solubility in water, calculated LogP, polar surface area, and the number of hydrogen bond acceptors and hydrogen bond donors for the active moiety are also provided, thereby allowing comparison analyses for both in silico and experimentally measured values. We discuss the potential use of BDDCS to estimate the disposition characteristics of novel chemicals (new molecular entities) in the early stages of drug discovery and development. Transporter effects in the intestine and the liver are not clinically relevant for BDDCS class 1 drugs, but potentially can have a high impact for class 2 (efflux in the gut, and efflux and uptake in the liver) and class 3 (uptake and efflux in both gut and liver) drugs. A combination of high dose and low solubility is likely to cause BDDCS class 4 to be underpopulated in terms of approved drugs (N = 53 compared with over 200 each in classes 1-3). The influence of several measured and in silico parameters in the process of BDDCS category assignment is discussed in detail.

BDDCS, the Rule of 5 and drugability

The Rule of 5 methodology appears to be as useful today in defining drugability as when it was proposed, but recognizing that the database that we used includes only drugs that successfully reached the market. We do not view additional criteria necessary nor did we find significant deficiencies in the four Rule of 5 criteria originally proposed by Lipinski and coworkers. BDDCS builds upon the Rule of 5 and can quite successfully predict drug disposition characteristics for drugs both meeting and not meeting Rule of 5 criteria. More recent expansions of classification systems have been proposed and do provide useful qualitative and quantitative predictions for clearance relationships. However, the broad range of applicability of BDDCS beyond just clearance predictions gives a great deal of further usefulness for the combined Rule of 5/BDDCS system.

The gut as a barrier to drug absorption: combined role of cytochrome P450 3A and P-glycoprotein

Intestinal phase I metabolism and active extrusion of absorbed drug have recently been recognised as major determinants of oral bioavailability. Cytochrome P450 (CYP) 3A, the major phase I drug metabolising enzyme in humans, and the multidrug efflux pump, P-glycoprotein, are present at high levels in the villus tip of enterocytes in the gastrointestinal tract, the primary site of absorption for orally administered drugs. The importance of CYP3A and P-glycoprotein in limiting oral drug delivery is suggested to us by their joint presence in small intestinal enterocytes, by the significant overlap in their substrate specificities, and by the poor oral bioavailability of joint substrates for these 2 proteins. These proteins are induced or inhibited by many of the same compounds. A growing number of preclinical and clinical studies have demonstrated that the oral bioavailability of many CYP3A and/or P-glycoprotein substrate drugs can be increased by concomitant administration of CYP3A inhibitors and/or P-glycoprotein inhibitors. We believe that further understanding the physiology and biochemistry of the interactive nature of intestinal CYP3A and P-glycoprotein will be important in defining, controlling, and improving oral bioavailability of CYP3A/P-glycoprotein substrates.

Gender effects in pharmacokinetics and pharmacodynamics

There are a number of examples of sex differences in drug pharmacokinetics and pharmacodynamics. Recent advances in the characterisation of specific isozymes involved in drug metabolism now allow for the preliminary identification of enzyme systems that are affected by sex. While current data are somewhat limited and not in complete agreement, the majority of studies show that apparent cytochrome P450 (CYP) 3A4 activity is higher in women than in men, whereas the activity of many other systems involved in drug metabolism may be higher in men than in women. Women and men also show different pharmacodynamic responses to a variety of drugs. While the clinical significance of these sex differences remains to be determined, we anticipate that they will be most important in the administration of drugs that have a narrow therapeutic range. In addition, sex differences in drug metabolism may be involved in the higher incidence of adverse reactions to drugs in women compared with men. Further research is needed to determine the scope and significance of these sex differences. Female-specific issues such as pregnancy, menopause, oral contraceptive use and menstruation may also have profound effects on drug metabolism. These effects can often be clinically important. Pregnancy may increase the elimination of antiepileptic agents, reducing their efficacy. Oral contraceptive use can interfere with the metabolism of many drugs and, conversely, certain drugs can impair contraceptive efficacy. More research is needed to determine the impact of menopause, hormone replacement and menstruation on drug therapy.

Role of P-glycoprotein and cytochrome P450 3A in limiting oral absorption of peptides and peptidomimetics

Cytochrome P450 3A4 (CYP3A4), the major phase I drug metabolizing enzyme in humans, and the MDR1 gene product P-glycoprotein (P-gp) are present at high concentrations in villus tip enterocytes of the small intestine and share a significant overlap in substrate specificity. A large body of research both in vitro and in vivo has established metabolism by intestinal CYP3A4 as a major determinant of the systemic bioavailability of orally administered drugs. More recently it has been recognized that drug extrusion by intestinal P-gp can both reduce drug absorption and modulate the effects of inhibitors and inducers of CYP3A-mediated metabolism. There is relatively little data regarding the effects of CYP3A and P-gp on peptide drugs; however, studies with the cyclic peptide immunosuppresant cyclosporine as well as peptidomimetics such as the HIV-protease inhibitor saquinavir (Invirase) and a new cysteine protease inhibitor K02 (Morpholine-Urea-Phe-Hphe-Vinyl sulfone; Axys Pharmaceuticals) provide some insight into the impact of these systems on the oral absorption of peptides.

The role of transporters in the pharmacokinetics of orally administered drugs

Drug transporters are recognized as key players in the processes of drug absorption, distribution, metabolism, and elimination. The localization of uptake and efflux transporters in organs responsible for drug biotransformation and excretion gives transporter proteins a unique gatekeeper function in controlling drug access to metabolizing enzymes and excretory pathways. This review seeks to discuss the influence intestinal and hepatic drug transporters have on pharmacokinetic parameters, including bioavailability, exposure, clearance, volume of distribution, and half-life, for orally dosed drugs. This review also describes in detail the Biopharmaceutics Drug Disposition Classification System (BDDCS) and explains how many of the effects drug transporters exert on oral drug pharmacokinetic parameters can be predicted by this classification scheme.

Predicting drug disposition, absorption/elimination/transporter interplay and the role of food on drug absorption

The ability to predict drug disposition involves concurrent consideration of many chemical and physiological variables and the effect of food on the rate and extent of availability adds further complexity due to postprandial changes in the gastrointestinal (GI) tract. A system that allows for the assessment of the multivariate interplay occurring following administration of an oral dose, in the presence or absence of meal, would greatly benefit the early stages of drug development. This is particularly true in an era when the majority of new molecular entities are highly permeable, poorly soluble, extensively metabolized compounds (BDDCS Class 2), which present the most complicated relationship in defining the impact of transporters due to the marked effects of transporter-enzyme interplay. This review evaluates the GI luminal environment by taking into account the absorption/transport/elimination interplay and evaluates the physiochemical property issues by taking into account the importance of solubility, permeability and metabolism. We concentrate on the BDDCS and its utility in predicting drug disposition. Furthermore, we focus on the effect of food on the extent of drug availability (F), which appears to follow closely what might be expected if a significant effect of high fat meals is inhibition of transporters. That is, high fat meals and lipidic excipients would be expected to have little effect on F for Class 1 drugs; they would increase F of Class 2 drugs, while decreasing F for Class 3 drugs.

Differentiation of absorption and first-pass gut and hepatic metabolism in humans: studies with cyclosporine

The low and variable bioavailability of cyclosporine has been attributed to poor absorption. However, recent studies have suggested that intestinal first-pass metabolism exerts a significant effect on bioavailability. We describe theory and methods to differentiate the contribution from oral absorption and intestinal and hepatic metabolism to overall cyclosporine bioavailability. Analysis of data from previous studies in our laboratories shows that in the absence of intestinal metabolism, cyclosporine absorption from its presently available dosage form averages at least 65% +/- 12% in healthy volunteers and 77% +/- 19% in kidney transplant patients. Analysis also suggests that the extraction ratio for cyclosporine in the gut is approximately twice the hepatic extraction and that cyclosporine absorption does not present a problem, with an average of 86% of the drug absorbed intact from its commercially available product in healthy volunteers. The boundary condition analysis described should have broad application in the differentiation of factors responsible for poor bioavailability.

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.

Drug absorption interactions between oral targeted anticancer agents and PPIs: is pH-dependent solubility the Achilles heel of targeted therapy?

A majority of the novel orally administered, molecularly targeted anticancer therapies are weak bases that exhibit pH-dependent solubility, and suppression of gastric acidity with acid-reducing agents could impair their absorption. In addition, a majority of cancer patients frequently take acid-reducing agents to alleviate symptoms of gastroesophageal reflux disease, thereby raising the potential for a common but underappreciated drug-drug interaction (DDI) that could decrease the exposure of anticancer medication and result in subsequent failure of therapy. This article is a review of the available clinical literature describing the extent of the interaction between 15 orally administered, small-molecule targeted anticancer therapies and acid-reducing agents. The currently available clinical data suggest that the magnitude of this DDI is largest for compounds whose in vitro solubility varies over the pH range 1-4. This range represents the normal physiological gastric acidity (pH ~1) and gastric acidity while on an acid-reducing agent (pH ~4).

Mechanisms of clinically relevant drug interactions associated with tacrolimus

The clinical management of tacrolimus, a macrolide used as immunosuppressant after transplantation, is complicated by its narrow therapeutic index in combination with inter- and intraindividually variable pharmacokinetics. As a substrate of cytochrome P450 (CYP) 3A enzymes and P-glycoprotein, tacrolimus interacts with several other drugs used in transplantation medicine, which also are known CYP3A and/or P-glycoprotein inhibitors and/or inducers. In clinical studies, CYP3A/P-glycoprotein inhibitors and inducers primarily affect oral bioavailability of tacrolimus rather than its clearance, indicating a key role of intestinal P-glycoprotein and CYP3A. There is an almost complete overlap between the reported clinical drug interactions of tacrolimus and those of cyclosporin. However, in comparison with cyclosporin, only few controlled drug interaction studies have been carried out, but tacrolimus drug interactions have been extensively studied in vitro. These results are inconsistent and are of poor predictive value for clinical drug interactions because of false negative results. P-glycoprotein regulates distribution of tacrolimus through the blood-brain barrier into the brain as well as distribution into lymphocytes. Interaction of other drugs with P-glycoprotein may change tacrolimus tissue distribution and modify its toxicity and immunosuppressive activity. There is evidence that ethnic and gender differences exist for tacrolimus drug interactions. Therapeutic drug monitoring to guide dosage adjustments of tacrolimus is an efficient tool to manage drug interactions. In the near future, progress can be expected from studies evaluating potential pharmacokinetic interactions caused by herbal preparations and food components, the exact biochemical mechanism underlying tacrolimus toxicity, and the potential of inhibition of CYP3A and P-glycoprotein to improve oral bioavailability and to decrease intraindividual variability of tacrolimus pharmacokinetics.

Intestinal MDR transport proteins and P-450 enzymes as barriers to oral drug delivery

Cytochrome P-450 3A4 (CYP3A4), the major phase I drug metabolizing enzyme in humans, and the multidrug efflux pump, MDR or P-glycoprotein (P-gp), are present at high levels in the villus tip enterocytes of the small intestine, the primary site of absorption for orally administered drugs. These proteins are induced or inhibited 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. A series of studies from our laboratory of cyclosporine and tacrolimus in humans and a novel cysteine protease inhibitor in rats, dosed concomitantly with inhibitors and inducers of CYP3A4 and P-gp, suggest that gut extraction can be modeled using measures of intestinal metabolism and absorption rate, the latter reflecting changes in P-gp. Results evaluating a preliminary model applied to the CYP3A substrate drugs midazolam, indinavir, saquinavir, and rifabutin suggest that the model may be useful for predicting in vivo intestinal metabolism from in vitro data.

Identification of the hepatic protein targets of reactive metabolites of acetaminophen in vivo in mice using two-dimensional gel electrophoresis and mass spectrometry

Liver toxicity following an overdose of acetaminophen is frequently considered a model for drug-induced hepatotoxicity. Extensive studies over many years have established that such toxicity is well correlated with liver protein arylation by acetaminophen metabolites. Identification of protein targets for covalent modifications is a challenging but necessary step in understanding how covalent binding could lead to liver toxicity. Previous approaches suffered from technical limitations, and thus over the last 10 years heroic efforts were required to determine the identity of only a few target proteins. We present a new mass spectrometry-based strategy for identification of all target proteins that now provides a comprehensive survey of the suite of liver proteins modified. After administration of radiolabeled acetaminophen to mice, the proteins in the liver tissue lysate were separated by two-dimensional polyacrylamide gel electrophoresis. In-gel digestion of the radiolabeled gel spots gave a set of tryptic peptides, which were analyzed by matrix-assisted laser desorption ionization mass spectrometry. Interrogation of data bases based on experimentally determined molecular weights of peptides and product ion tags from postsource decay mass spectra was employed for the determination of the identities of modified liver proteins. Using this method, more than 20 new drug-labeled proteins have been identified.

Effects of renal failure on drug transport and metabolism

Renal failure not only alters the renal elimination, but also the non-renal disposition of drugs that are extensively metabolized by the liver. Although reduced metabolic enzyme activity in some cases can be responsible for the reduced drug clearance, alterations in the transporter systems may also be involved in the process. With the development of renal failure, the renal secretion of organic ions mediated by organic anion transporters (OATs) and organic cation transporters (OCTs) is decreased. 3-Carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF) and other organic anionic uremic toxins may directly inhibit the renal excretion of various drugs and endogenous organic acids by competitively inhibiting OATs. In addition, the expression of OAT1 and OCT2 was reduced in chronic renal failure (CRF) rats. Renal failure also impairs the liver uptake of drugs and organic anions, such as bromosulphophthalein (BSP), indocyanine green (ICG), and thyroxine, where organic anion transport polypeptides (OATPs) are the major transporters. Most previous studies have been done in animals or cell culture, very often in rat models, but these are presumed to reflect the presentation of advanced renal disease in humans as well. Recent studies demonstrate that the uremic toxins CMPF and indoxyl sulfate (IS) can directly inhibit rOatp2 and hOATP-C in hepatocytes. The protein content of the liver uptake transporters Oatp1, 2, and 4 were significantly decreased in CRF rats. Decreased activity of the intestinal efflux transporter, P-glycoprotein (P-gp), was also observed in CRF rats, with no significant change of protein content, suggesting that uremic toxins may suppress P-gp function. However, increased protein levels of multidrug resistance-associated protein (MRP) 2 in the kidney and MRP3 in the liver were found in CRF rats, suggesting an adaptive response that may serve as a protective mechanism. Increases in drug areas under the curve (AUCs) in subjects with advanced renal disease for drugs that are not renally excreted are consistent with uremic toxin effects on either intestinal or hepatic cell transporters, metabolizing enzymes, or both. In conclusion, alterations of drug transporters, as well as metabolic enzymes, in patients with renal failure can be responsible for reduced drug clearance.

Bioavailability of cyclosporine with concomitant rifampin administration is markedly less than predicted by hepatic enzyme induction

The pharmacokinetics of cyclosporine was studied in six healthy volunteers after administration of the drug orally (10 mg/kg) and intravenously (3 mg/kg) with and without concomitant rifampin administration. Both blood and plasma (separated at 37 degrees C) samples were analyzed for cyclosporine concentration. For blood and plasma, respectively, clearances of cyclosporine were calculated to be 0.30 and 0.55 L/hr/kg, values for volume of distribution at steady state were 1.31 and 1.68 L/kg, and bioavailabilities were 27% and 33% during the pre-rifampin phase. Post-rifampin phase clearances of cyclosporine were 0.42 and 0.79 L/hr/kg, values for volume of distribution at steady state were 1.36 and 1.35 L/kg, and bioavailabilities were 10% and 9% for blood and plasma, respectively. Rifampin not only induces the hepatic metabolism of cyclosporine but also decreases its bioavailability to a greater extent than would be predicted by the increased metabolism. The decreased bioavailability most probably can be explained by an induction of intestinal cytochrome P450 enzymes, which appears to be markedly greater than the induction of hepatic metabolism.

The effects of ketoconazole on the intestinal metabolism and bioavailability of cyclosporine

The pharmacokinetics of cyclosporine were studied in the blood of five normal healthy volunteers (two men and three women) after each received oral and intravenous cyclosporine alone and with concomitant oral ketoconazole. Administration of ketoconazole caused a significant decrease in intravenous cyclosporine clearance (0.18 +/- 0.05 L/kg/hr versus 0.32 +/- 0.09 L/hr/kg) and a significant increase in cyclosporine oral bioavailability (56.4% +/- 11.7% versus 22.4% +/- 4.8%) compared with values before ketoconazole administration. Steady-state volume of distribution for intravenously administered cyclosporine was unchanged (1.26 +/- 0.44 L/kg versus 1.10 +/- 0.27 L/kg). Hepatic bioavailability (1 - hepatic extraction ratio) calculated for intravenous cyclosporine increased by 11% in the presence of ketoconazole (86.3% +/- 3.7% versus 75.2% +/- 6.6% without ketoconazole), which accounts for only one third of the observed increase in cyclosporine oral bioavailability. Because it is unlikely that ketoconazole had a significant effect on either cyclosporine absorption or hepatic blood flow, the increase in cyclosporine bioavailability observed in this study is most likely explained by inhibition of gastrointestinal cytochrome P450 enzymes.

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.

The role of BCS (biopharmaceutics classification system) and BDDCS (biopharmaceutics drug disposition classification system) in drug development

Biopharmaceutics Classification System and Biopharmaceutics Drug Distribution Classification System are complimentary, not competing, classification systems that aim to improve, simplify, and speed drug development. Although both systems are based on classifying drugs and new molecular entities into four categories using the same solubility criteria, they differ in the criterion for permeability and have different purposes. Here, the details and applications of both systems are reviewed with particular emphasis of their role in drug development.