Scientific and Regulatory Perspectives on Metabolizing Enzyme−Transporter Interplay and Its Role in Drug Interactions: Challenges in Predicting Drug Interactions

A Comprehensive Review of Antimicrobial Agents Against Clinically Important Bacterial Pathogens: Prospects for Phytochemicals

Antimicrobial resistance (AMR) hinders the effective treatment of a range of bacterial infections, posing a serious threat to public health globally, as it challenges the currently available antimicrobial drugs. Among the various modes of antimicrobial action, antimicrobial agents that act on membranes have the most promising efficacy. However, there are no consolidated reports on the shortcomings of these drugs, existing challenges, or the potential applications of phytochemicals that act on membranes. Therefore, in this review, we have addressed the challenges and focused on various phytochemicals as antimicrobial agents acting on the membranes of clinically important bacterial pathogens. Antibacterial phytochemicals comprise diverse group of agents found in a wide range of plants. These compounds have been found to disrupt cell membranes, inhibit enzymes, interfere with protein synthesis, generate reactive oxygen species, modulate quorum sensing, and inhibit bacterial adhesion, making them promising candidates for the development of novel antibacterial therapies. Recently, polyphenolic compounds have been reported to have proven efficacy against nosocomial multidrug-resistant pathogens. However, more high-quality studies, improved standards, and the adoption of rules and regulations are required to firmly confirm the clinical efficacy of phytochemicals derived from plants. Identifying potential challenges, thrust areas of research, and considering viable approaches is essential for the successful clinical translation of these compounds.

Drug-Food Interactions with a Focus on Mediterranean Diet

There is a growing interest among people in western countries for adoption of healthier lifestyle habits and diet behaviors with one of the most known ones to be Mediterranean diet (Med-D). Med-D is linked with daily consumption of food products such as vegetables, fruits, whole grains, seafood, beans, nuts, olive oil, low-fat food derivatives and limited consumption of meat or full fat food products. Med-D is well-known to promote well-being and lower the risk of chronic conditions such as cardiovascular diseases, diabetes, and metabolic syndrome. On the other hand bioactive constituents in foods may interfere with drugs’ pharmacological mechanisms, modulating the clinical outcome leading to drug-food interactions (DFIs). This review discusses current evidence for food products that are included within the Med-Dand available scientific data suggest a potential contribution in DFIs with impact on therapeutic outcome. Most cases refer to potential modulation of drugs’ absorption and metabolism such as foods’ impact on drugs’ carrier-mediated transport and enzymatic metabolism as well as potential synergistic or antagonistic effects that enhance or reduce the pharmacological effect for some drugs. Adherence to Med-D can improve disease management and overall well-being, but specific foods should be consumed with caution so as to not hinder therapy outcome. Proper patient education and consultation from healthcare providers is important to avoid any conflicts and side effects due to clinically significant DFIs.

Transporters in Regulatory Science: Notable Contributions from Dr. Giacomini in the Past Two Decades

Transporters govern the access of molecules to cells or their exit from cells, thereby controlling the overall distribution of drugs to their intracellular site of action. Clinically relevant drug-drug interactions mediated by transporters are of increasing interest in drug development. Drug transporters, acting alone or in concert with drug metabolizing enzymes, can play an important role in modulating drug absorption, distribution, metabolism, and excretion, thus affecting the pharmacokinetics and/or pharmacodynamics of a drug. Dr. Kathy Giacomini from the University of California, San Francisco is one of the world leaders in transporters and pharmacogenetics with key contributions to transporter science. Her contributions to transporter science are noteworthy. This review paper will summarize Dr. Giacomini's key contributions and influence on transporters in regulatory science in the past two decades. Regulatory science research highlighted in this review covers various aspects of transporter science including understanding the effect of renal impairment on transporters, transporter ontogeny, biomarkers for transporters, and interactions of excipients with transporters affecting drug absorption. Significance Statement This review paper highlights Dr. Giacomini's key contributions and influence on transporters in regulatory science in the past two decades. She has been at the cutting edge of science pertaining to drug transport, drug disposition, and regulatory science, leading to new era of translational sciences pertaining to drug disposition and transporter biology. Her research has and will continue to bring enormous impact on gaining new knowledge in guiding drug development and inspire scientists from all sectors in the field.

Cytochrome P450 and P-gp Mediated Herb-Drug Interactions and Molecular Docking Studies of Garcinol

Garcinol is an active constituent of Garcinia indica and Garcinia cambogia. Recent studies have proven that garcinol has anti-inflammatory, anti-cancer, and anti-oxidant activities. The objective of this study was to evaluate the inhibitory effects of garcinol on the activities of the drug metabolizing cytochrome P450 (CYP) isozymes to predict potential herb-drug interactions with co-administered drugs. Garcinol was incubated with a mixture of rat liver microsomes and eight CYP probe substrate cocktail under optimized incubation conditions and the samples were analyzed using a validated method on LC-MS/MS. Garcinol showed strong inhibition with IC50 values of CYP1A2 (7.6 µM), CYP2C9 (8.0 µM), CYP2B6 (2.1 µM), CYP2D6 (9.5 µM), and CYP3A4 (5.1 µM), respectively, and moderate inhibition towards CYP2C19 (16.4 µM) and CYP2E1 (19.0 µM). Molecular docking studies were performed on garcinol against the active sites of CYP2B6 and CYP3A4 proteins. These results further confirmed that the inhibitory activity of garcinol occurred by occupying the active sites of these human CYPs and by making favorable interactions with its key residues. In-vivo CYP inhibition studies were carried out in Sprague-Dawley rats. These results suggest garcinol may cause herb-drug interactions, mediated by inhibition of CYPs involved in drug metabolism in-vivo by altering the pharmacokinetic parameters like AUC and Cmax in a clinically significant manner. Garcinol was found to upregulate the expression and activity of P-gp in western blotting study and P-gp inhibition study in-vivo. These findings give a clear understanding to predict potential herb-drug/drug-drug interactions of garcinol for safe clinical use in future.

Physiologically-based pharmacokinetic modeling to evaluate in vitro-to-in vivo extrapolation for intestinal P-glycoprotein inhibition

As one of the key components in model‐informed drug discovery and development, physiologically‐based pharmacokinetic (PBPK) modeling linked with in vitro‐to‐in vivo extrapolation (IVIVE) is widely applied to quantitatively predict drug–drug interactions (DDIs) on drug‐metabolizing enzymes and transporters. This study aimed to investigate an IVIVE for intestinal P‐glycoprotein (Pgp, ABCB1)‐mediated DDIs among three Pgp substrates, digoxin, dabigatran etexilate, and quinidine, and two Pgp inhibitors, itraconazole and verapamil, via PBPK modeling. For Pgp substrates, assuming unbound Michaelis‐Menten constant (K_m) to be intrinsic, in vitro‐to‐in vivo scaling factors for maximal Pgp‐mediated efflux rate (J_max) were optimized based on the clinically observed results without co‐administration of Pgp inhibitors. For Pgp inhibitors, PBPK models utilized the reported in vitro values of Pgp inhibition constants (K_i), 1.0 μM for itraconazole and 2.0 μM for verapamil. Overall, the PBPK modeling sufficiently described Pgp‐mediated DDIs between these substrates and inhibitors with the prediction errors of less than or equal to ±25% in most cases, suggesting a reasonable IVIVE for Pgp kinetics in the clinical DDI results. The modeling results also suggest that Pgp kinetic parameters of both the substrates (K_m and J_max) and the inhibitors (K_i) are sensitive to Pgp‐mediated DDIs, thus being key for successful DDI prediction. It would also be critical to incorporate appropriate unbound inhibitor concentrations at the site of action into PBPK models. The present results support a quantitative prediction of Pgp‐mediated DDIs using in vitro parameters, which will significantly increase the value of in vitro studies to design and run clinical DDI studies safely and effectively.

Intestinal Excretion, Intestinal Recirculation, and Renal Tubule Reabsorption Are Underappreciated Mechanisms That Drive the Distribution and Pharmacokinetic Behavior of Small Molecule Drugs

Drug reabsorption following biliary excretion is well-known as enterohepatic recirculation (EHR). Renal tubular reabsorption (RTR) following renal excretion is also common but not easily assessed. Intestinal excretion (IE) and enteroenteric recirculation (EER) have not been recognized as common disposition mechanisms for metabolically stable and permeable drugs. IE and intestinal reabsorption (IR:EHR/EER), as well as RTR, are governed by dug concentration gradients, passive diffusion, active transport, and metabolism, and together they markedly impact disposition and pharmacokinetics (PK) of small molecule drugs. Disruption of IE, IR, or RTR through applications of active charcoal (AC), transporter knockout (KO), and transporter inhibitors can lead to changes in PK parameters. The impacts of intestinal and renal reabsorption on PK are under-appreciated. Although IE and EER/RTR can be an intrinsic drug property, there is no apparent strategy to optimize compounds based on this property. This review seeks to improve understanding and applications of IE, IR, and RTR mechanisms.

Interactions of janus kinase inhibitors with drug transporters and consequences for pharmacokinetics and toxicity

Introduction: Janus kinase inhibitors (JAKinibs) constitute an emerging and promising pharmacological class of anti-inflammatory or anti-cancer drugs, used notably for the treatment of rheumatoid arthritis and some myeloproliferative neoplasms.Areas covered: This review provides an overview of the interactions between marketed JAKinibs and major uptake and efflux drug transporters. Consequences regarding pharmacokinetics, drug-drug interactions and toxicity are summarized.Expert opinion: JAKinibs interact in vitro with transporters in various ways, as inhibitors or as substrates of transporters or as regulators of transporter expression. This may theoretically result in drug-drug interactions (DDIs), with JAKinibs acting as perpetrators or as victims, or in toxicity, via impairment of thiamine transport. Clinical significance in terms of DDIs for JAKinib-transporter interactions remains however poorly documented. In this context, the in vivo unbound concentration of JAKinibs is likely a key parameter to consider for evaluating the clinical relevance of JAKinibs-mediated transporter inhibition. Additionally, the interplay with drug metabolism as well as possible interactions with transporters of emerging importance and time-dependent inhibition have to be taken into account. The role drug transporters may play in controlling cellular JAKinib concentrations and efficacy in target cells is also an issue of interest.

Liquid Biopsy Enables Quantification of the Abundance and Interindividual Variability of Hepatic Enzymes and Transporters

Variability in individual capacity for hepatic elimination of therapeutic drugs is well recognized and is associated with variable expression and activity of liver enzymes and transporters. Although genotyping offers some degree of stratification, there is often large variability within the same genotype. Direct measurement of protein expression is impractical due to limited access to tissue biopsies. Hence, determination of variability in hepatic drug metabolism and disposition using liquid biopsy (blood samples) is an attractive proposition during drug development and in clinical practice. This study used a multi-"omic" strategy to establish a liquid biopsy technology intended to assess hepatic capacity for metabolism and disposition in individual patients. Plasma exosomal analysis (n = 29) revealed expression of 533 pharmacologically relevant genes at the RNA level, with 147 genes showing evidence of expression at the protein level in matching liver tissue. Correction of exosomal RNA expression using a novel shedding factor improved correlation against liver protein expression for 97 liver-enriched genes. Strong correlation was demonstrated for 12 key drug-metabolizing enzymes and 4 drug transporters. The developed test allowed reliable patient stratification, and in silico trials demonstrated utility in adjusting drug dose to achieve similar drug exposure between patients with variable hepatic elimination. Accordingly, this approach can be applied in characterization of volunteers prior to enrollment in clinical trials and for patient stratification in clinical practice to achieve more precise individual dosing.

Safety perspectives on presently considered drugs for the treatment of COVID-19

Intense efforts are underway to evaluate potential therapeutic agents for the treatment of COVID-19. In order to respond quickly to the crisis, the repurposing of existing drugs is the primary pharmacological strategy. Despite the urgent clinical need for these therapies, it is imperative to consider potential safety issues. This is important due to the harm-benefit ratios that may be encountered when treating COVID-19, which can depend on the stage of the disease, when therapy is administered and underlying clinical factors in individual patients. Treatments are currently being trialled for a range of scenarios from prophylaxis (where benefit must greatly exceed risk) to severe life-threatening disease (where a degree of potential risk may be tolerated if it is exceeded by the potential benefit). In this perspective, we have reviewed some of the most widely researched repurposed agents in order to identify potential safety considerations using existing information in the context of COVID-19.

Development of a new Japanese guideline on drug interaction for drug development and appropriate provision of information

Drug interactions, in particular with concomitant drugs having a narrow therapeutic range, sometimes cause serious adverse drug reactions or attenuation of the therapeutic effect. Therefore, evaluation of the characteristics and severities of possible drug interactions in drug development is essential to understand such interactions to help prevent any potential risk for patients. In Japan, a regulatory document which was notified in 2001 to outline the basic principles of drug interaction studies during drug development was revised as a new guideline after 17 years to present general procedures that are currently considered scientifically valid. This article aims to present an overview of development process of the new Japanese guideline for investigating drug interactions and show the impact of implementating this guideline on drug interaction evaluations, thereby providing future perspectives of regulatory activities on drug interactions.

The Effects of Clarithromycin on the Pharmacokinetics of Apixaban in Healthy Volunteers: A Single-Sequence Crossover Study

BACKGROUND

This was an open-label, phase I, nonrandomized, single-sequence, crossover study to evaluate the effect of concomitant administration of multiple doses of clarithromycin on the single-dose exposure, safety, and tolerability of apixaban in healthy subjects.

METHODS

In total, 19 subjects received a single oral dose of apixaban 10 mg on day 1. On day 4, subjects began receiving oral clarithromycin immediate release (IR) 500 mg twice daily (bid) for 4 days. On day 8, subjects received oral apixaban 10 mg and oral clarithromycin IR 500 mg bid. Oral clarithromycin IR 500 mg bid was given alone on days 9 and 10.

RESULTS

Compared with apixaban alone, coadministration of apixaban with clarithromycin resulted in increased apixaban exposure. The adjusted geometric mean ratio (GMR) was 1.299 (90% confidence interval [CI] 1.220-1.384) for peak plasma concentration (Cmax), whereas the adjusted GMR for the area under the concentration curve (AUC(INF)) was 1.595 (90% CI 1.506-1.698). The mean half-life and median time to Cmax of apixaban were comparable with and without concomitant administration of clarithromycin. Administration of apixaban and clarithromycin concomitantly did not result in increased adverse events compared with administration of either agent alone. All adverse events were mild in intensity.

CONCLUSIONS

Apixaban Cmax and AUC(INF) increased 30% and 60%, respectively, when multiple doses of clarithromycin were coadministered with apixaban versus administration of apixaban alone. The increase in apixaban Cmax and AUC(INF) with concomitant clarithromycin was less than that which has been observed when apixaban was given with ketoconazole. Administration of apixaban alone and in combination with clarithromycin bid was safe and generally well-tolerated by the healthy adult subjects in this study.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov identifier number NCT02912234.

Physiologically-Based Pharmacokinetic Modeling Approach to Predict Rifampin-Mediated Intestinal P-Glycoprotein Induction

Physiologically‐based pharmacokinetic (PBPK) modeling is a powerful tool to quantitatively describe drug disposition profiles in vivo, thereby providing an alternative to predict drug–drug interactions (DDIs) that have not been tested clinically. This study aimed to predict effects of rifampin‐mediated intestinal P‐glycoprotein (Pgp) induction on pharmacokinetics of Pgp substrates via PBPK modeling. First, we selected four Pgp substrates (digoxin, talinolol, quinidine, and dabigatran etexilate) to derive in vitro to in vivo scaling factors for intestinal Pgp kinetics. Assuming unbound Michaelis‐Menten constant (K_m) to be intrinsic, we focused on the scaling factors for maximal efflux rate (J_max) to adequately recover clinically observed results. Next, we predicted rifampin‐mediated fold increases in intestinal Pgp abundances to reasonably recover clinically observed DDI results. The modeling results suggested that threefold to fourfold increases in intestinal Pgp abundances could sufficiently reproduce the DDI results of these Pgp substrates with rifampin. Hence, the obtained fold increases can potentially be applicable to DDI prediction with other Pgp substrates.

Nanocapsules embedded in microparticles for enhanced oral bioavailability and efficacy of Lopinavir as an anti-AIDS drug

Lopinavir (LPV), an efficient drug for HIV infection treatment, was incorporated into biodegradable PLGA nanocapsules (NCs) embedded in microparticles (MCPs) using the spray-drying technique in an attempt to bypass the P-gp efflux and protect the drug from CYP3A pre-systemic metabolism without ritonavir (RTV). SEM observations confirmed the formation of NCs and their entrapment in the MCPs. LPV-loaded NCs and free LPV were released from the MCPs at pH of 7.4 as evidenced by in vitro release studies. Results obtained from rat studies showed a two-fold higher bioavailability of LPV following oral administration of the optimal formulation than Kaletra^®, the marketed drug, showing that when properly entrapped, LPV can be effectively protected from CYP degradation in the gut as well as from the liver following systemic absorption. It was also shown that serum derived from rats following LPV oral administration in two formulations and Kaletra^® significantly decreased the multiplication of HIV-1 in cultured SupT1 cells. Furthermore, the LPV formulations markedly restricted the titre of infectious HIV-1 production compared with Kaletra^® confirming the improved antiviral activity of LPV delivered in the rat blood circulation by the nanocapsules embedded in microparticle formulations.

Evaluation of transporters in drug development: Current status and contemporary issues

Transporters govern the access of molecules to cells or their exit from cells, thereby controlling the overall distribution of drugs to their intracellular site of action. Clinically relevant drug-drug interactions mediated by transporters are of increasing interest in drug development. Drug transporters, acting alone or in concert with drug metabolizing enzymes, can play an important role in modulating drug absorption, distribution, metabolism and excretion, thus affecting the pharmacokinetics and/or pharmacodynamics of a drug. The drug interaction guidance documents from regulatory agencies include various decision criteria that may be used to predict the need for in vivo assessment of transporter-mediated drug-drug interactions. Regulatory science research continues to assess the prediction performances of various criteria as well as to examine the strength and limitations of each prediction criterion to foster discussions related to harmonized decision criteria that may be used to facilitate global drug development. This review discusses the role of transporters in drug development with a focus on methodologies in assessing transporter-mediated drug-drug interactions, challenges in both in vitro and in vivo assessments of transporters, and emerging transporter research areas including biomarkers, assessment of tissue concentrations, and effect of diseases on transporters.

Predicting drug-drug interactions through drug structural similarities and interaction networks incorporating pharmacokinetics and pharmacodynamics knowledge

Drug–drug interactions (DDIs) may lead to adverse effects and potentially result in drug withdrawal from the market. Predicting DDIs during drug development would help reduce development costs and time by rigorous evaluation of drug candidates. The primary mechanisms of DDIs are based on pharmacokinetics (PK) and pharmacodynamics (PD). This study examines the effects of 2D structural similarities of drugs on DDI prediction through interaction networks including both PD and PK knowledge. Our assumption was that a query drug (Dq) and a drug to be examined (De) likely have DDI if the drugs in the interaction network of De are structurally similar to Dq. A network of De describes the associations between the drugs and the proteins relating to PK and PD for De. These include target proteins, proteins interacting with target proteins, enzymes, and transporters for De. We constructed logistic regression models for DDI prediction using only 2D structural similarities between each Dq and the drugs in the network of De. The results indicated that our models could effectively predict DDIs. It was found that integrating structural similarity scores of the drugs relating to both PK and PD of De was crucial for model performance. In particular, the combination of the target- and enzyme-related scores provided the largest increase of the predictive power.Graphical abstract

Assessment of Amino Acid/Drug Transporters for Renal Transport of [18F]Fluciclovine (anti-[18F]FACBC) in Vitro

[¹⁸F]Fluciclovine (trans-1-amino-3-[¹⁸F]fluorocyclobutanecarboxylic acid; anti-[¹⁸F]FACBC), a positron emission tomography tracer used for the diagnosis of recurrent prostate cancer, is transported via amino acid transporters (AATs) with high affinity (K_m: 97-230 μM). However, the mechanism underlying urinary excretion is unknown. In this study, we investigated the involvement of AATs and drug transporters in renal [¹⁸F]fluciclovine reuptake. [¹⁴C]Fluciclovine (trans-1-amino-3-fluoro[1-¹⁴C]cyclobutanecarboxylic acid) was used because of its long half-life. The involvement of AATs in [¹⁴C]fluciclovine transport was measured by apical-to-basal transport using an LLC-PK1 monolayer as model for renal proximal tubules. The contribution of drug transporters herein was assessed using vesicles/cells expressing the drug transporters P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), multidrug resistance-associated protein 4 (MRP4), organic anion transporter 1 (OAT1), organic anion transporter 3 (OAT3) , organic cation transporter 2 (OCT2), organic anion transporting polypeptide 1B1 (OATP1B1), and organic anion transporting polypeptide 1B3 (OATP1B3). The apical-to-basal transport of [¹⁴C]fluciclovine was attenuated by l-threonine, the substrate for system alanine-serine-cysteine (ASC) AATs. [¹⁴C]Fluciclovine uptake by drug transporter-expressing vesicles/cells was not significantly different from that of control vesicles/cells. Fluciclovine inhibited P-gp, MRP4, OAT1, OCT2, and OATP1B1 (IC₅₀ > 2.95 mM). Therefore, system ASC AATs may be partly involved in the renal reuptake of [¹⁸F]fluciclovine. Further, given that [¹⁸F]fluciclovine is recognized as an inhibitor with millimolar affinity for the tested drug transporters, slow urinary excretion of [¹⁸F]fluciclovine may be mediated by system ASC AATs, but not by drug transporters.

Transporters as Drug Targets in Neurological Diseases

Membrane transport proteins have central physiological function in maintaining cerebral homeostasis. These transporters are expressed in almost all cerebral cells in which they regulate the movement of a wide range of solutes, including endogenous substrates, xenobiotic, and therapeutic drugs. Altered activity/expression of central nervous system (CNS) transporters has been implicated in the onset and progression of multiple neurological diseases. Neurological diseases are heterogeneous diseases that involve complex pathological alterations with only a few treatment options; therefore, there is a great need for the development of novel therapeutic treatments. To that end, transporters have emerged recently to be promising therapeutic targets to halt or slow the course of neurological diseases. The objective of this review is to discuss implications of transporters in neurological diseases and summarize available evidence for targeting transporters as decent therapeutic approach in the treatment of neurological diseases.

Fluticasone furoate and vilanterol trifenatate combination therapy for the treatment of asthma

INTRODUCTION

Combination inhaled corticosteroids (ICS) and long acting β2-adrenergic agonists (LABA) are used in a stepwise fashion for patients whose asthma is not controlled by low dose ICS alone. Adherence is one of the main issues facing clinicians in the control of asthma symptoms with currently available combination inhalers requiring twice-daily (BD) inhalation. Fluticasone furoate (FF) and vilanterol trifenatate (VI) both have prolonged retention in the lung with effects on lung function over 24-hours and as such the combination has been proposed for once-daily (OD) dosing.

AREAS COVERED

The stepwise pharmacologic approach to asthma management is addressed, followed by a detailed assessment of the literature pertaining to the efficacy, tolerability and safety of FF/VI combination therapy for the treatment of asthma. Expert commentary: Doses of 100/25µg and 200/25µg inhaled OD, have similar improvements in lung function, asthma control as well as rates of side effects to one of the currently available BD ICS/LABA combinations, fluticasone propionate and salmeterol (FP/SAL) but have not been compared with other commonly used combinations. It is hoped that OD dosage of FF/VI can improve adherence and hence asthma control in these patients, however evidence to support this has yet to become available.

Key Findings from Preclinical and Clinical Drug Interaction Studies Presented in New Drug and Biological License Applications Approved by the Food and Drug Administration in 2014

Regulatory approval documents contain valuable information, often not published, to assess the drug-drug interaction (DDI) profile of newly marketed drugs. This analysis aimed to systematically review all drug metabolism, transport, pharmacokinetics, and DDI data available in the new drug applications and biologic license applications approved by the U.S. Food and Drug Administration in 2014, using the University of Washington Drug Interaction Database, and to highlight the significant findings. Among the 30 new drug applications and 11 biologic license applications reviewed, 35 new molecular entities (NMEs) were well characterized with regard to drug metabolism, transport, and/or organ impairment and were fully analyzed in this review. In vitro, a majority of the NMEs were found to be substrates or inhibitors/inducers of at least one drug metabolizing enzyme or transporter. In vivo, when NMEs were considered as victim drugs, 16 NMEs had at least one in vivo DDI study with a clinically significant change in exposure (area under the time-plasma concentration curve or Cmax ratio ≥2 or ≤0.5), with 6 NMEs shown to be sensitive substrates of cytochrome P450 enzymes (area under the time-plasma concentration curve ratio ≥5 when coadministered with potent inhibitors): paritaprevir and naloxegol (CYP3A), eliglustat (CYP2D6), dasabuvir (CYP2C8), and tasimelteon and pirfenidone (CYP1A2). As perpetrators, seven NMEs showed clinically significant inhibition involving both enzymes and transporters, although no clinically significant induction was observed. Physiologically based pharmacokinetic modeling and pharmacogenetics studies were used for six and four NMEs, respectively, to optimize dosing recommendations in special populations and/or multiple impairment situations. In addition, the pharmacokinetic evaluations in patients with hepatic or renal impairment provided useful quantitative information to support drug administration in these fragile populations.

"Target-Site" Drug Metabolism and Transport

The recent symposium on "Target-Site" Drug Metabolism and Transport that was sponsored by the American Society for Pharmacology and Experimental Therapeutics at the 2014 Experimental Biology meeting in San Diego is summarized in this report. Emerging evidence has demonstrated that drug-metabolizing enzyme and transporter activity at the site of therapeutic action can affect the efficacy, safety, and metabolic properties of a given drug, with potential outcomes including altered dosing regimens, stricter exclusion criteria, or even the failure of a new chemical entity in clinical trials. Drug metabolism within the brain, for example, can contribute to metabolic activation of therapeutic drugs such as codeine as well as the elimination of potential neurotoxins in the brain. Similarly, the activity of oxidative and conjugative drug-metabolizing enzymes in the lung can have an effect on the efficacy of compounds such as resveratrol. In addition to metabolism, the active transport of compounds into or away from the site of action can also influence the outcome of a given therapeutic regimen or disease progression. For example, organic anion transporter 3 is involved in the initiation of pancreatic β-cell dysfunction and may have a role in how uremic toxins enter pancreatic β-cells and ultimately contribute to the pathogenesis of gestational diabetes. Finally, it is likely that a combination of target-specific metabolism and cellular internalization may have a significant role in determining the pharmacokinetics and efficacy of antibody-drug conjugates, a finding which has resulted in the development of a host of new analytical methods that are now used for characterizing the metabolism and disposition of antibody-drug conjugates. Taken together, the research summarized herein can provide for an increased understanding of potential barriers to drug efficacy and allow for a more rational approach for developing safe and effective therapeutics.

Evaluation of Adverse Drug Properties with Cryopreserved Human Hepatocytes and the Integrated Discrete Multiple Organ Co-culture (IdMOC(TM)) System

Human hepatocytes, with complete hepatic metabolizing enzymes, transporters and cofactors, represent the gold standard for in vitro evaluation of drug metabolism, drug-drug interactions, and hepatotoxicity. Successful cryopreservation of human hepatocytes enables this experimental system to be used routinely. The use of human hepatocytes to evaluate two major adverse drug properties: drug-drug interactions and hepatotoxicity, are summarized in this review. The application of human hepatocytes in metabolism-based drug-drug interaction includes metabolite profiling, pathway identification, P450 inhibition, P450 induction, and uptake and efflux transporter inhibition. The application of human hepatocytes in toxicity evaluation includes in vitro hepatotoxicity and metabolism-based drug toxicity determination. A novel system, the Integrated Discrete Multiple Organ Co-culture (IdMOC) which allows the evaluation of nonhepatic toxicity in the presence of hepatic metabolism, is described.

Inhibition of P-glycoprotein Gene Expression and Function Enhances Triptolide-induced Hepatotoxicity in Mice

Triptolide (TP) is the major active principle of Tripterygium wilfordii Hook f. and very effective in treatment of autoimmune diseases. However, TP induced hepatotoxicity limited its clinical applications. Our previous study found that TP was a substrate of P-glycoprotein and its hepatobiliary clearance was markedly affected by P-gp modulation in sandwich-cultured rat hepatocytes. In this study, small interfering RNA (siRNA) and specific inhibitor tariquidar were used to investigate the impact of P-gp down regulation on TP-induced hepatotoxicity. The results showed that when the function of P-gp was inhibited by mdr1a-1 siRNA or tariquidar, the systemic and hepatic exposures of TP were significantly increased. The aggravated hepatotoxicity was evidenced with the remarkably lifted levels of serum biomarkers (ALT and AST) and pathological changes in liver. The other toxicological indicators (MDA, SOD and Bcl-2/Bax) were also significantly changed by P-gp inhibition. The data analysis showed that the increase of TP exposure in mice was quantitatively correlated to the enhanced hepatotoxicity, and the hepatic exposure was more relevant to the toxicity. P-gp mediated clearance played a significant role in TP detoxification. The risk of herb-drug interaction likely occurs when TP is concomitant with P-gp inhibitors or substrates in clinic.

Tacrine sinusoidal uptake and biliary excretion in sandwich-cultured primary rat hepatocytes

PURPOSE. The knowledge of hepatic disposition kinetics of tacrine, a first cholinesterase inhibitor was approved by FDA for the treatment of Alzheimer's disease (AD), would help to understand its hepatotoxicity, its therapeutic effect, and improve the management of patients with AD. The current study aims to characterize tacrine hepatic transport kinetics and study the role of organic cation transporters (OCTs), P-glycoprotein (P-gp) and multidrug resistance-associated protein (MRP2) in tacrine sinusoidal uptake and biliary excretion. METHODS. Modulation of tacrine hepatic uptake and efflux, biliary excretion index (BEI%), were performed in sandwich-cultured primary rat hepatocytes (SCHs) using transporters inhibitors. Conformation of the integrity of SCHs model was established by capturing images with light-contrast and fluorescence microscopy. RESULTS. Tacrine uptake in SCHs was carrier-mediated process and saturable with apparent Km of 31.5±9.6 µM and Vmax of 908±72 pmol/min/mg protein. Tetraethyl ammonium (TEA), cimetidine and verapamil significantly reduced tacrine uptake with more pronounced effect observed with verapamil which caused 3-fold reduction in tacrine uptake, indicating role for OCTs. Tacrine has a biliary excretion in SCHs with maximum BEI% value of 22.9±1.9% at 10 min of incubation. Addition of MK571 and valspodar decreased the BEI% of tacrine by 40 and 60% suggesting roles for canalicular MRP2 and P-gp, respectively. CONCLUSIONS. Our results show that in addition to metabolism, tacrine hepatic disposition is carrier-mediated process mediated by sinusoidal OCTs, and canalicular MRP2 and P-gp.

Stable knock-down of efflux transporters leads to reduced glucuronidation in UGT1A1-overexpressing HeLa cells: the evidence for glucuronidation-transport interplay

Efflux of glucuronide is facilitated by the membrane transporters including BCRP and MRPs. In this study, we aimed to determine the effects of transporter expression on glucuronide efflux and cellular glucuronidation. Single efflux transporter (i.e., BCRP, MRP1, MRP3, or MRP4) was stably knocked-down in UGT1A1-overexpressing HeLa cells. Knock-down of transporters was performed by stable transfection of short-hairpin RNA (shRNA) using lentiviral vectors. Glucuronidation and glucuronide transport in the cells were characterized using three different aglycones (i.e., genistein, apigenin, and emodin) with distinct metabolic activities. BCRP knock-down resulted in significant reductions in excretion of glucuronides (42.9% for genistein glucuronide (GG), 21.1% for apigenin glucuronide (AG) , and 33.7% for emodin glucuronide (EG); p < 0.01) and in cellular glucuronidation (38.3% for genistein, 38.6% for apigenin, and 34.7% for emodin; p < 0.01). Knock-down of a MRP transporter led to substantial decreases in excretion of GG (32.3% for MRP1, 36.7% for MRP3, and 36.6% for MRP4; p < 0.01) and AG (59.3% for MRP1, 24.7% for MRP3, and 34.1% for MRP4; p < 0.01). Also, cellular glucuronidation of genistein (38.3% for MRP1, 32.3% for MRP3, and 31.1% for MRP4; p < 0.01) and apigenin (40.6% for MRP1, 32.4% for MRP3, and 34.6% for MRP4; p < 0.001) was markedly suppressed. By contrast, silencing of MRPs did not cause any changes in either excretion of EG or cellular glucuronidation of emodin. In conclusion, cellular glucuronidation was significantly altered by decreasing expression of efflux transporters, revealing a strong interplay of glucuronidation with efflux transport.

Expression of drug transporters in human kidney: impact of sex, age, and ethnicity

Background

Differences in expression of drug transporters in human kidney contribute to changes in pharmacokinetics and toxicokinetics of a variety of drug compounds. The basal expression levels of genes involved in drug transport processes in the kidney introduces differences in bioavailability, distribution, and clearance of drugs, possibly influencing drug efficacy and adverse reactions. Sex differences in gene expression of transporters are a key cause of differences in sex-dependent pharmacokinetics, which may characterize many drugs and contribute to individual differences in drug efficacy and toxicity. Therefore, evaluating the expression of drug transporters in normal human kidneys is important to better understand differences in drug bioavailability, distribution, and clearance of drugs in humans. Other factors such as age and ethnicity may also contribute to individual differences in gene expression of drug transporters in the human kidney.

Methods

Quantitative real-time PCR (QRT-PCR) was performed to determine the gene expression of 30 drug transporters in 95 age-matched normal human kidney tissues. Multiple Student’s t-tests (Sidak-Bonferroni correction) and two-way ANOVA (Bonferroni correction) analyses were used to determine statistically significant differences.

Results

In the 30 transporter genes examined, sex, ethnicity, and age differences in gene expression were exhibited in normal human kidney tissue. These changes in expression were not found to be differentially significant. However, sex-age and sex-ethnicity interactions were found to be statistically significant. For sex-age interactions, SCL22A12 was found to be significantly higher expressed in females <50 years compared to males <50 years. Expression levels of SLC22A2, SLC22A12, SLC6A16, and ABCB6 were significantly higher in females <50 years compared to females ≥50 years. In sex-ethnicity interactions, expression levels of ATP7B and KCNJ8 were found to be significantly higher in African American females compared to European American females. Also, the expression of SLC31A2 was significantly higher in European American males compared to European American females.

Conclusions

Sex, age, and ethnic differences impacted the expression of drug transporters in normal human kidneys, which suggests that the analysis of gene expression of drug transporters will aid in improving the usage/dosage of drug therapies influencing personalized medicine and susceptibility to adverse drug reactions.

Is there more to learn about cytochrome P450 enzymes?

The cytochrome P450 system is a group of enzymes, found mainly in the liver and gut mucosa, that plays a crucial role in controlling the concentrations of many endogenous substances and drugs.1 The activity of the individual enzymes can vary over time and from person to person in response to diet, medicines or exposure to environmental pollutants. It has been almost 15 years since DTB reviewed the significance of the cytochrome P450 system and its relevance to prescribing (Why bother about cytochrome p450 enzymes?).1 A lot has changed in healthcare over that time, but has our understanding of this important aspect of drug metabolism altered significantly? Here we provide a brief overview of the function of cytochrome P450 enzymes and look at some of the concepts that have become established, or have begun to emerge, since the publication of our previous article.

Biomarkers and human hepatocytes

The accuracy of preclinical safety evaluation to predict human toxicity is hindered by species difference in drug metabolism and toxic mechanism between human and nonhuman animals. In vitro human-based experimental systems allowing the assessment of human-specific drug properties represent a logical and practical approach to provide human-specific information. An advantage of in vitro approaches is that they require only limited amounts of time and resources, and, most importantly, do not invoke harm to human patients. Human hepatocytes, with complete hepatic metabolizing enzymes, transporters and cofactors, represent a practical and useful experimental system to assess drug metabolism. The use of human hepatocytes to evaluate two major adverse drug properties, drug–drug interactions and hepatotoxicity, are reviewed. The application of human hepatocytes in metabolism-based drug–drug interactions includes metabolite profiling, pathway identification, CYP450 inhibition, CYP450 induction, and uptake and efflux transporter inhibition. The application of human hepatocytes in toxicity evaluation includes in vitro hepatotoxicity and metabolism-based drug toxicity determination. Correlation of drug toxicity with proteomics and genomics data may allow the discovery of clinical biomarkers for early detection of liver toxicity.

The effect of ketoconazole on the pharmacokinetics and pharmacodynamics of inhaled fluticasone furoate and vilanterol trifenatate in healthy subjects

AIM

To investigate the effects of the cytochrome P450 3A4 (CYP3A4) inhibitor ketoconazole on the pharmacokinetics (PK) and pharmacodynamics of fluticasone furoate (FF) and vilanterol trifenatate (VI).

METHODS

Two double-blind, randomized, placebo-controlled, two-way crossover studies in healthy subjects. In study 1, subjects received single doses of ketoconazole (400 mg) or placebo on days 1-6, with a single dose of inhaled VI (25 μg) on day 5. Pharmacodynamic and PK data were obtained up to 48 h following the VI dose. In study 2, subjects received once daily ketoconazole (400 mg) or placebo for 11 days, with FF/VI (200/25 μg) for the final 7 days. Pharmacodynamic and PK data were obtained up to 48 h following the day 11 dose.

RESULTS

In study 1, there was no effect of co-administration of ketoconazole and VI on pharmacodynamic or PK parameters. In study 2, co-administration of ketoconazole and FF/VI had no effect on 0-4 h maximal heart rate or minimal blood potassium {treatment difference [90% confidence interval (CI)] -0.6 beats min(-1) (-5.8, 4.5) and 0.04 mmol l(-1) (-0.03, 0.11), respectively}, whilst there was a 27% decrease in 24 h weighted mean serum cortisol [treatment ratio (90% CI) 0.73 (0.62, 0.86)]. Co-administration of ketoconazole increased [percentage change (90% CI)] FF area under the curve (0-24) and maximal plasma concentration by 36% (16, 59) and 33% (12, 58), respectively, and VI area under the curve (0-t') and maximal plasma concentration by 65% (38, 97) and 22% (8, 38), respectively.

CONCLUSION

Co-administration of FF/VI or VI with ketoconazole resulted in a less than twofold increase in systemic exposure to FF and VI. There was no increase in β-agonist systemic pharmacodynamic effects, while serum cortisol was decreased by 27%. Co-administration of FF/VI with strong CYP3A4 inhibitors has the potential to increase systemic exposure to both fluticasone furoate and vilanterol, which could lead to an increase in the potential for adverse reactions.

Proteoliposomes as tool for assaying membrane transporter functions and interactions with xenobiotics

Proteoliposomes represent a suitable and up to date tool for studying membrane transporters which physiologically mediate absorption, excretion, trafficking and reabsorption of nutrients and metabolites. Using recently developed reconstitution strategies, transporters can be inserted in artificial bilayers with the same orientation as in the cell membranes and in the absence of other interfering molecular systems. These methodologies are very suitable for studying kinetic parameters and molecular mechanisms. After the first applications on mitochondrial transporters, in the last decade, proteoliposomes obtained with optimized methodologies have been used for studying plasma membrane transporters and defining their functional and kinetic properties and structure/function relationships. A lot of information has been obtained which has clarified and completed the knowledge on several transporters among which the OCTN sub-family members, transporters for neutral amino acid, B0AT1 and ASCT2, and others. Transporters can mediate absorption of substrate-like derivatives or drugs, improving their bioavailability or can interact with these compounds or other xenobiotics, leading to side/toxic effects. Therefore, proteoliposomes have recently been used for studying the interaction of some plasma membrane and mitochondrial transporters with toxic compounds, such as mercurials, H₂O₂ and some drugs. Several mechanisms have been defined and in some cases the amino acid residues responsible for the interaction have been identified. The data obtained indicate proteoliposomes as a novel and potentially important tool in drug discovery.

Evidence of drug-drug interactions through uptake and efflux transport systems in rat hepatocytes: implications for cellular concentrations of competing drugs

For drugs with hepatobiliary transport across hepatocytes, the interplay between uptake and efflux transporters determines hepatic concentrations of drugs, but the evolution over time of these concentrations is difficult to measure in humans other than with magnetic resonance imaging contrast agents in the liver. Gadobenate dimeglumine (BOPTA) is a contrast agent used in liver magnetic resonance imaging that enters into human hepatocytes through organic anion transporting polypeptides (OATP) and exits unchanged into bile through the multiple resistance-associated protein 2 (MRP2). Rifampicin (RIF) is transported by the same membrane proteins and may compete with BOPTA for hepatic uptake. Simultaneous drug-drug interactions through uptake and efflux transport systems in hepatocytes according to the cellular concentrations of competing drugs were never investigated. In perfused rat liver preparations, we demonstrate how the drug-drug interactions through transporters determine cellular concentrations of the competing drugs BOPTA and RIF, and we show that the cellular concentrations by modulating transport through membranes regulate the rat Oatp-Mrp2 interplay. Moreover, drug interactions through transporters change greatly over time.

Intracellular drug concentrations and transporters: measurement, modeling, and implications for the liver

Intracellular concentrations of drugs and metabolites are often important determinants of efficacy, toxicity, and drug interactions. Hepatic drug distribution can be affected by many factors, including physicochemical properties, uptake/efflux transporters, protein binding, organelle sequestration, and metabolism. This white paper highlights determinants of hepatocyte drug/metabolite concentrations and provides an update on model systems, methods, and modeling/simulation approaches used to quantitatively assess hepatocellular concentrations of molecules. The critical scientific gaps and future research directions in this field are discussed.

Investigating the enteroenteric recirculation of apixaban, a factor Xa inhibitor: administration of activated charcoal to bile duct-cannulated rats and dogs receiving an intravenous dose and use of drug transporter knockout rats

The study described here investigated the impact of intestinal excretion (IE; excretion of drug directly from circulation to intestinal lumen), enteroenteric recirculation (EER), and renal tubule recirculation (RTR) on apixaban pharmacokinetics and disposition. The experimental approaches involve integrating apixaban elimination pathways with pharmacokinetic profiles obtained from bile duct-cannulated (BDC) rats and dogs receiving i.v. doses together with oral administration of activated charcoal (AC). Additionally, the role of P-gp (P-glycoprotein; abcb1) and BCRP (breast cancer resistance protein; abcg2) in apixaban disposition was evaluated in experiments using transporter inhibitors and transporter knockout (KO) rats. Approximately 20-50% of an apixaban i.v. dose was found in feces of BDC rats and dogs, suggesting IE leading to fecal elimination and intestinal clearance (IC). The fecal elimination, IC, and systemic clearance of apixaban were increased upon AC administration in both BDC rats and dogs and were decreased in BDC rats dosed with GF-120918, a dual BCRP and P-gp inhibitor). BCRP appeared to play a more important role for absorption and intestinal and renal elimination of apixaban than P-gp in transporter-KO rats after oral and i.v. dosing, which led to a higher level of active renal excretion in rat than other species. These data demonstrate that apixaban undergoes IE, EER, and RTR that are facilitated by efflux transporters. Intestinal reabsorption of apixaban could be interrupted by AC even at 3 hours post-drug dose in dogs (late charcoal effect). This study demonstrates that the intestine is an organ for direct clearance and redistribution of apixaban. The IE, EER, and RTR contribute to overall pharmacokinetic profiles of apixaban. IE as a clearance pathway, balanced with metabolism and renal excretion, helps decrease the impacts of intrinsic (renal or hepatic impairment) and extrinsic (drug-drug interactions) factors on apixaban disposition.

In vitro approach to predict post-translational phosphorylation response to mixtures

While exposure to chemical mixtures is an everyday reality, an understanding of their combined effects, and any potential prediction thereof, is extremely limited. Realistic exposures potentially consist of hundreds to thousands of chemicals per day, but even relatively simple binary mixture interactions can be inherently difficult to predict based upon the lack of temporal and spatial mechanisms for the individual constituents. To this end, we explore the concept of capitalizing on downstream convergence of intracellular signal transduction to experimentally simplify the means of determining xenobiotics that, when combined, could result in increased or unexpected toxicity. In a proof of principle study, we exposed HepG2 cells to deguelin, a natural isoflavonoid, alone and in combination with KCN, and determined the relative post-translational phosphorylation responses to several key proteins related to mitochondrial outer membrane permeabilization. Dose-dependent phosphorylation activity provides a clear identification of threshold response to low-level exposures, and crosstalk amongst selected proteins correctly forecasts mixtures interactions that may lead to increased toxicity. We then used Bliss Independence to determine if the experimentally measured mixture phosphorylation responses could be predicted with individual responses. Independence accurately predicted mixture interactions for deguelin and KCN (87.5%). To more fully exhaust independence as a model for determining potential pharmacodynamic interactions, we exposed HepG2 cells to deguelin and staurosporine, a broad kinase inhibitor; independence accurately predicted these mixture responses (77.5%). In this study, we demonstrate the potential of a new in vitro approach for the prediction of toxic mixtures interactions that is fundamentally driven by the interdependence of signal transduction and apoptosis.

Role of transporters in drug interactions

Over the past few decades, a tremendous amount of work has been done on the molecular characterization of transport proteins in animals and humans, leading to a better understanding of the physiological roles of a number of transport proteins. Furthermore, there is increasing preclinical and clinical evidence to support the importance of transport proteins in the pharmacokinetics and toxicokinetics of a wide variety of structurally diverse drugs. As a consequence, the degree of expression and functionality of transport proteins may directly affect the therapeutic effectiveness, safety and target specificity of drugs. Recently, there has also been increased awareness about potential drug-drug, drug-herb and drug-food interactions involving transporters. Traditionally, a change in metabolic clearance of a drug, particularly via cytochrome P450-mediated metabolism, has been considered the cause of many clinically important drug interactions. However, increasing evidence suggests that some drug interactions result from changes in the activity and/or expression of drug transporters. Accordingly, assessment of the clinical relevance of transporter-mediated drug interactions has become a regulatory issue during the drug approval process and also the evaluation of drug interaction potential has become an integral part of risk assessment during drug development processes. Therefore, this review will highlight the role of some selected drug transporters in drug interactions, as well as their clinical implication.

The evolution of the OATP hepatic uptake transport protein family in DMPK sciences: from obscure liver transporters to key determinants of hepatobiliary clearance

Over the last two decades the impact on drug pharmacokinetics of the organic anion transporting polypeptides (OATPs: OATP-1B1, 1B3 and 2B1), expressed on the sinusoidal membrane of the hepatocyte, has been increasingly recognized. OATP-mediated uptake into the hepatocyte coupled with subsequent excretion into bile via efflux proteins, such as MRP2, is often referred to as hepatobiliary excretion. OATP transporter proteins can impact some drugs in several ways including pharmacokinetic variability, pharmacodynamic response and drug-drug interactions (DDIs). The impact of transporter mediated hepatic clearance is illustrated with case examples, from the literature and also from the Pfizer portfolio. The currently available in vitro techniques to study the hepatic transporter proteins involved in the hepatobiliary clearance of drugs are reviewed herein along with recent advances in using these in vitro data to predict the human clearance of compounds recognized by hepatic uptake transporters.

Sensitivity of intravenous and oral alfentanil and pupillary miosis as minimal and noninvasive probes for hepatic and first-pass CYP3A induction

Systemic and oral clearances of alfentanil (ALF) are in vivo probes for hepatic and first-pass cytochrome P450 (CYP) 3A. Both ALF single-point plasma concentrations and miosis are surrogates for area under the concentration-time curve (AUC) and clearance and are minimal and noninvasive CYP3A probes. This investigation determined ALF sensitivity for detecting graded CYP3A induction and compared it with that of midazolam (MDZ). Twelve volunteers (sequential crossover) received 0, 5, 10, 25, or 75 mg oral rifampin for 5 days. MDZ and ALF were given intravenously and orally on sequential days. Dark-adapted pupil diameter was measured with blood sampling. Graded rifampin decreased plasma MDZ AUCs to 83, 76, 62, and 59% (intravenous (i.v.)) and 78, 66, 39, and 24% (oral) of control. Hepatic and first-pass CYP3A induction were detected comparably by plasma MDZ and ALF AUCs. Single ALF concentrations detected all CYP3A induction, whereas MDZ was less sensitive. ALF miosis detected induction of first-pass but not hepatic CYP3A.

Physiologically-based pharmacokinetics in drug development and regulatory science

The application of physiologically-based pharmacokinetic (PBPK) modeling is coming of age in drug development and regulation, reflecting significant advances over the past 10 years in the predictability of key pharmacokinetic (PK) parameters from human in vitro data and in the availability of dedicated software platforms and associated databases. Specific advances and contemporary challenges with respect to predicting the processes of drug clearance, distribution, and absorption are reviewed, together with the ability to anticipate the quantitative extent of PK-based drug-drug interactions and the impact of age, genetics, disease, and formulation. The value of this capability in selecting and designing appropriate clinical studies, its implications for resource-sparing techniques, and a more holistic view of the application of PK across the preclinical/clinical divide are considered. Finally, some attention is given to the positioning of PBPK within the drug development and approval paradigm and its future application in truly personalized medicine.

Use of in vivo animal models to assess pharmacokinetic drug-drug interactions

Animal models are used commonly in various stages of drug discovery and development to aid in the prospective assessment of drug-drug interaction (DDI) potential and the understanding of the underlying mechanism for DDI of a drug candidate. In vivo assessments in an appropriate animal model can be very valuable, when used in combination with in vitro systems, to help verify in vivo relevance of the in vitro animal-based results, and thus substantiate the extrapolation of in vitro human data to clinical outcomes. From a pharmacokinetic standpoint, a key consideration for rational selection of an animal model is based on broad similarities to humans in important physiological and biochemical parameters governing drug absorption, distribution, metabolism or excretion (ADME) processes in question for both the perpetrator and victim drugs. Equally critical are specific in vitro and/or in vivo experiments to demonstrate those similarities, usually both qualitative and quantitative, in the ADME properties/processes under investigation. In this review, theoretical basis and specific examples are presented to illustrate the utility of the animal models in assessing the potential and understanding the mechanisms of DDIs.