Rate-limiting steps in hepatic drug clearance: comparison of hepatocellular uptake and metabolism with microsomal metabolism of saquinavir, nelfinavir, and ritonavir

Mechanisms of altered hepatic drug disposition during pregnancy: small molecules

INTRODUCTION

Pregnancy alters the systemic exposure and clearance of many hepatically cleared drugs that are commonly used by obstetric patients. Understanding the molecular mechanisms underlying the changes in factors that affect hepatic drug clearance (blood flow, protein binding, and intrinsic clearance) is essential to more precisely predict systemic drug exposure and dose requirements in obstetric patients.

AREAS COVERED

This review (1) summarizes the anatomic, physiologic, and biochemical changes in maternal hepatic, cardiovascular, endocrine, and renal systems relevant to hepatic drug clearance and (2) reviews the molecular mechanisms underlying the altered hepatic metabolism and intrinsic clearance of drugs during pregnancy via a comprehensive PubMed search. It also identifies knowledge gaps in the molecular mechanisms and factors that modulate hepatic drug clearance during pregnancy.

EXPERT OPINION

Pharmacokinetic studies have shown that pregnancy alters systemic exposure, protein binding, and clearance of many drugs during gestation in part due to pregnancy-associated decreases in plasma albumin, increases in organ blood flow, and changes in the activity of drug-metabolizing enzymes (DMEs) and transporters. The changes in the activity of certain DMEs and transporters during pregnancy are likely driven by hormonal-changes that inhibit their activity or alter the expression of these proteins through activation of transcription factors.

Proof of Concept of an All-in-One System for Measuring Hepatic Influx, Egress, and Metabolic Clearance Based on the Extended Clearance Concept

Hepatic clearance (CLH ) prediction is a critical parameter to estimate human dose. However, CLH underpredictions are common, especially for slowly metabolized drugs, and may be attributable to drug properties that pose challenges for conventional in vitro absorption, distribution, metabolism, and elimination (ADME) assays, resulting in nonvalid data, which prevents in vitro to in vivo extrapolation and CLH predictions. Other processes, including hepatocyte and biliary distribution via transporters, can also play significant roles in CLH Recent advances in understanding the interplay of metabolism and drug transport for clearance processes have aided in developing the extended clearance model. In this study, we demonstrate proof of concept of a novel two-step assay enabling the measurement of multiple kinetic parameters from a single experiment in plated human primary hepatocytes with and without transporter and cytochrome P450 inhibitors-the hepatocyte uptake and loss assay (HUpLA). HUpLA accurately predicted the CLH of eight of the nine drugs (within twofold of the observed CLH ). Distribution clearances were within threefold of observed literature values in standard uptake and efflux assays. In comparison, the conventional suspension hepatocyte stability assay poorly predicted the CLH The CLH of only two drugs was predicted within twofold of the observed CLH Therefore, HUpLA is advantageous by enabling the measurement of enzymatic and transport processes concurrently within the same system, alleviating the need for applying scaling factors independently. The use of primary human hepatocytes enables physiologically relevant exploration of transporter-enzyme interplay. Most importantly, HUpLA shows promise as a sensitive measure for low-turnover drugs. Further evaluation across different drug characteristics is needed to demonstrate method robustness. SIGNIFICANCE STATEMENT: The hepatocyte uptake and loss assay involves measuring four commonly derived in vitro hepatic clearance endpoints. Since endpoints are generated within a single test system, it blunts experimental error originating from assays otherwise conducted independently. A key advantage is the concept of removing drug-containing media following intracellular drug loading, enabling the measurement of drug reappearance rate in media as well as the measurement of loss of total drug in the test system unencumbered by background quantities of drug in media otherwise present in a conventional assay.

Involvement of multiple cytochrome P450 isoenzymes in drug interactions between ritonavir and direct oral anticoagulants

Herein, we aimed to determine the significance of drug interactions (DIs) between ritonavir and direct oral anticoagulants (DOACs) and identify the involved cytochrome P450 (CYP) isoenzymes. Using an in vitro cocktail method with human liver microsomes (HLM), we observed that ritonavir strongly inhibited CYPs in the following order: CYP3A, CYP2C8, CYP2D6, CYP2C9, CYP2C19, CYP2B6, and CYP2J2 (IC50: 0.023-6.79 μM). The degree of CYP2J2 inhibition was inconclusive, given the substantial discrepancy between the HLM and human expression system. Selective inhibition of CYP3A decreased the O-demethylation of apixaban by only 13.4%, and the involvement of multiple CYP isoenzymes was suggested, all of which were inhibited by ritonavir. Multiple CYP isoenzymes contributed also to the metabolism of rivaroxaban. Replacement of the incubation medium with phosphate buffer instead of HEPES enhanced apixaban hydroxylation. On surveying the FDA Adverse Event Reporting System, we detected that the signal of the proportional reporting ratio of "death" and found increase for "hemoglobin decreased" (12.5-fold) and "procedural hemorrhage" (201.9-fold) on administering apixaban with ritonavir; these were far less significant for other CYP3A inhibitors. Overall, these findings suggest that co-administration of ritonavir-boosted drugs with DOACs may induce serious DIs owing to the simultaneous inhibition of multiple CYP isoenzymes.

Kinetics of metabolism of deltamethrin and cis- and trans-permethrin in vitro. Studies using rat and human liver microsomes, isolated rat hepatocytes and rat liver cytosol

The kinetics of metabolism of deltamethrin (DLM) and cis- and trans-permethrin (CPM and TPM) was studied in male Sprague-Dawley rat and human liver microsomes. DLM metabolism kinetics was also studied in isolated rat hepatocytes, liver microsomes and cytosol. Apparent intrinsic clearance (CL_int) values for the metabolism of DLM, CPM and TPM by cytochrome P450 (CYP) and carboxylesterase (CES) enzymes in rat and human liver microsomes decreased with increasing microsomal protein concentration. However, when apparent CL_int values were corrected for nonspecific binding to allow calculation of unbound (i.e., corrected) CL_int values, the unbound values did not vary greatly with microsomal protein concentration. Unbound CL_int values for metabolism of 0.05-1 μM DLM in rat liver microsomes (CYP and CES enzymes) and cytosol (CES enzymes) were not significantly different from rates of DLM metabolism in isolated rat hepatocytes. This study demonstrates that the nonspecific binding of these highly lipophilic compounds needs to be taken into account in order to obtain accurate estimates of rates of in vitro metabolism of these pyrethroids. While DLM is rapidly metabolised in vitro, the hepatocyte membrane does not appear to represent a barrier to the absorption and hence subsequent hepatic metabolism of this pyrethroid.

Novel Cassette Assay To Quantify the Outer Membrane Permeability of Five β-Lactams Simultaneously in Carbapenem-Resistant Klebsiella pneumoniae and Enterobacter cloacae

Poor penetration through the outer membrane (OM) of Gram-negative bacteria is a major barrier of antibiotic development. While β-lactam antibiotics are commonly used against Klebsiella pneumoniae and Enterobacter cloacae, there are limited data on OM permeability especially in K. pneumoniae Here, we developed a novel cassette assay, which can simultaneously quantify the OM permeability to five β-lactams in carbapenem-resistant K. pneumoniae and E. cloacae Both clinical isolates harbored a blaKPC-2 and several other β-lactamases. The OM permeability of each antibiotic was studied separately ("discrete assay") and simultaneously ("cassette assay") by determining the degradation of extracellular β-lactam concentrations via multiplex liquid chromatography-tandem mass spectrometry analyses. Our K. pneumoniae isolate was polymyxin resistant, whereas the E. cloacae was polymyxin susceptible. Imipenem penetrated the OM at least 7-fold faster than meropenem for both isolates. Imipenem penetrated E. cloacae at least 258-fold faster and K. pneumoniae 150-fold faster compared to aztreonam, cefepime, and ceftazidime. For our β-lactams, OM permeability was substantially higher in the E. cloacae compared to the K. pneumoniae isolate (except for aztreonam). This correlated with a higher OmpC porin production in E. cloacae, as determined by proteomics. The cassette and discrete assays showed comparable results, suggesting limited or no competition during influx through OM porins. This cassette assay allowed us, for the first time, to efficiently quantify the OM permeability of multiple β-lactams in carbapenem-resistant K. pneumoniae and E. cloacae Characterizing the OM permeability presents a critical contribution to combating the antimicrobial resistance crisis and enables us to rationally optimize the use of β-lactam antibiotics.IMPORTANCE Antimicrobial resistance is causing a global human health crisis and is affecting all antibiotic classes. While β-lactams have been commonly used against susceptible isolates of Klebsiella pneumoniae and Enterobacter cloacae, carbapenem-resistant isolates are spreading worldwide and pose substantial clinical challenges. Rapid penetration of β-lactams leads to high drug concentrations at their periplasmic target sites, allowing β-lactams to more completely inactivate their target receptors. Despite this, there are limited tangible data on the permeability of β-lactams through the outer membranes of many Gram-negative pathogens. This study presents a novel, cassette assay, which can simultaneously characterize the permeability of five β-lactams in multidrug-resistant clinical isolates. We show that carbapenems, and especially imipenem, penetrate the outer membrane of K. pneumoniae and E. cloacae substantially faster than noncarbapenem β-lactams. The ability to efficiently characterize the outer membrane permeability is critical to optimize the use of β-lactams and combat carbapenem-resistant isolates.

Involvement of organic anion-transporting polypeptides and organic cation transporter in the hepatic uptake of jatrorrhizine

Jatrorrhizine possesses a wide spectrum of pharmacological activities. However, the mechanism underlying hepatic uptake of jatrorrhizine remains unclear.Rat liver slices, isolated rat hepatocytes and human embryonic kidney 293 (HEK293) cells stably expressing human organic anion-transporting polypeptide (OATP) and organic cation transporter (OCT) were used to evaluate the hepatic uptake of jatrorrhizine in this study.Uptake of jatrorrhizine in rat liver slices and isolated rat hepatocytes was significantly inhibited by glycyrrhizic acid (Oatp1b2 inhibitor) and prazosin (Oct1 inhibitor), but not by ibuprofen (Oatp1a1 inhibitor) or digoxin (Oatp1a4 inhibitor). Uptake of jatrorrhizine in OATP1B3 and OCT1-HEK293 cells indicated a saturable process with the K_m of 8.20 ± 1.28 and 4.94 ± 0.55 μM, respectively. However, the transcellular transport of jatrorrhizine in OATP1B1-HEK293 cells was not observed. Rifampicin (OATP inhibitor) for OATP1B3-HEK293 cells and prazosin for OCT1-HEK293 cells could inhibit the uptake of jatrorrhizine with the IC₅₀ of 5.49 ± 1.05 and 2.77 ± 0.72 μM, respectively.The above data indicate that hepatic uptake of jatrorrhizine is involved in both OATP and OCT, which may have important roles in jatrorrhizine liver disposition and potential drug-drug interactions.

Impact of Intracellular Concentrations on Metabolic Drug-Drug Interaction Studies

Accurate prediction of drug-drug interactions (DDI) is a challenging task in drug discovery and development. It requires determination of enzyme inhibition in vitro which is highly system-dependent for many compounds. The aim of this study was to investigate whether the determination of intracellular unbound concentrations in primary human hepatocytes can be used to bridge discrepancies between results obtained using human liver microsomes and hepatocytes. Specifically, we investigated if Kp_uu could reconcile differences in CYP enzyme inhibition values (K_i or IC₅₀). Firstly, our methodology for determination of Kp_uu was optimized for human hepatocytes, using four well-studied reference compounds. Secondly, the methodology was applied to a series of structurally related CYP2C9 inhibitors from a Roche discovery project. Lastly, the Kp_uu values of three commonly used CYP3A4 inhibitors—ketoconazole, itraconazole, and posaconazole—were determined and compared to compound-specific hepatic enrichment factors obtained from physiologically based modeling of clinical DDI studies with these three compounds. Kp_uu obtained in suspended human hepatocytes gave good predictions of system-dependent differences in vitro. The Kp_uu was also in fair agreement with the compound-specific hepatic enrichment factors in DDI models and can therefore be used to improve estimations of enrichment factors in physiologically based pharmacokinetic modeling.

Modeling Gadoxetate Liver Uptake and Efflux Using Dynamic Contrast-Enhanced Magnetic Resonance Imaging Enables Preclinical Quantification of Transporter Drug-Drug Interactions

OBJECTIVES

The aim of this study was to model the in vivo transporter-mediated uptake and efflux of the hepatobiliary contrast agent gadoxetate in the liver. The efficacy of the proposed technique was assessed for its ability to provide quantitative insights into drug-drug interactions (DDIs), using rifampicin as inhibitor.

MATERIALS AND METHODS

Three groups of C57 mice were scanned twice with a dynamic gadoxetate-enhanced magnetic resonance imaging protocol, using a 3-dimensional spoiled gradient-echo sequence for approximately 72 minutes. Before the second magnetic resonance imaging session, 2 of the groups received a rifampicin dose of 20 (n = 7) or 40 (n = 7) mg/kg, respectively. Data from regions of interest in the liver were analyzed using 2 simplifications of a 2-compartment uptake and efflux model to provide estimates for the gadoxetate uptake rate (ki) into the hepatocytes and its efflux rate (kef) into the bile. Both models were assessed for goodness-of-fit in the group without rifampicin (n = 9), and the appropriate model was selected for assessing the ability to monitor DDIs in vivo.

RESULTS

Seven of 9 mice from the group without rifampicin were assessed for model implementation and reproducibility. A simple 3 parameter model (ki, kef, and extracellular space, vecs) adequately described the observed liver concentration time series with mean ki = 0.47 ± 0.11 min and mean kef = 0.039 ± 0.016 min. Visually, the area under the liver concentration time profile was reduced for the groups receiving rifampicin. Furthermore, tracer kinetic modeling demonstrated a significant dose-dependent decrease in the uptake (5.9- and 17.3-fold decrease for 20 mg/kg and 40 mg/kg, respectively) and efflux rates (2.2- and 7.9-fold decrease) compared with the first scan for each group.

CONCLUSIONS

This study presents the first in vivo implementation of a 2-compartment uptake and efflux model to monitor DDIs at the transporter-protein level, using the clinically relevant organic anion transporting polypeptide inhibitor rifampicin. The technique has the potential to be a novel alternative to other methods, allowing real-time changes in transporter DDIs to be measured directly in vivo.

Membrane transporter data to support kinetically-informed chemical risk assessment using non-animal methods: Scientific and regulatory perspectives

Humans are continuously exposed to low levels of thousands of industrial chemicals, most of which are poorly characterised in terms of their potential toxicity. The new paradigm in chemical risk assessment (CRA) aims to rely on animal-free testing, with kinetics being a key determinant of toxicity when moving from traditional animal studies to integrated in vitro-in silico approaches. In a kinetically informed CRA, membrane transporters, which have been intensively studied during drug development, are an essential piece of information. However, how existing knowledge on transporters gained in the drug field can be applied to CRA is not yet fully understood. This review outlines the opportunities, challenges and existing tools for investigating chemical-transporter interactions in kinetically informed CRA without animal studies. Various environmental chemicals acting as substrates, inhibitors or modulators of transporter activity or expression have been shown to impact TK, just as drugs do. However, because pollutant concentrations are often lower in humans than drugs and because exposure levels and internal chemical doses are not usually known in contrast to drugs, new approaches are required to translate transporter data and reasoning from the drug sector to CRA. Here, the generation of in vitro chemical-transporter interaction data and the development of transporter databases and classification systems trained on chemical datasets (and not only drugs) are proposed. Furtheremore, improving the use of human biomonitoring data to evaluate the in vitro-in silico transporter-related predicted values and developing means to assess uncertainties could also lead to increase confidence of scientists and regulators in animal-free CRA. Finally, a systematic characterisation of the transportome (quantitative monitoring of transporter abundance, activity and maintenance over time) would reinforce confidence in the use of experimental transporter/barrier systems as well as in established cell-based toxicological assays currently used for CRA.

Sandwich-Cultured Hepatocytes for Mechanistic Understanding of Hepatic Disposition of Parent Drugs and Metabolites by Transporter-Enzyme Interplay

Functional interplay between transporters and drug-metabolizing enzymes is currently one of the hottest topics in the field of drug metabolism and pharmacokinetics. Uptake transporter-enzyme interplay is important to determine intrinsic hepatic clearance based on the extended clearance concept. Enzyme and efflux transporter interplay, which includes both sinusoidal (basolateral) and canalicular efflux transporters, determines the fate of metabolites formed in the liver. As sandwich-cultured hepatocytes (SCHs) maintain metabolic activities and form a canalicular network, the whole interplay between uptake and efflux transporters and drug-metabolizing enzymes can be investigated simultaneously. In this article, we review the utility and applicability of SCHs for mechanistic understanding of hepatic disposition of both parent drugs and metabolites. In addition, the utility of SCHs for mimicking species-specific disposition of parent drugs and metabolites in vivo is described. We also review application of SCHs for clinically relevant prediction of drug-drug interactions caused by drugs and metabolites. The usefulness of mathematical modeling of hepatic disposition of parent drugs and metabolites in SCHs is described to allow a quantitative understanding of an event in vitro and to develop a more advanced model to predict in vivo disposition.

Pharmacokinetic alterations in poloxamer 407-induced hyperlipidemic rats

1. Plasma lipid profile abnormalities in hyperlipidemia can potentially alter the pharmacokinetics of a drug in a complex manner. To evaluate these pharmacokinetic alterations in hyperlipidemia and to determine the underlying mechanism(s), poloxamer 407-induced hyperlipidemic rats (HL rats), a well-established animal model of hyperlipidemia have been used. 2. In this review, we summarize findings on the pathophysiological and gene expression changes in drug-metabolizing enzymes and transporters in HL rats. We discuss pharmacokinetic changes in drugs metabolized primarily via hepatic cytochrome P450 (CYPs) in terms of alterations in hepatic intrinsic clearance (CL^'_int), free fraction in plasma (f_u) and hepatic blood flow rate (Q_H), depending on the hepatic excretion ratio, as well as drugs eliminated primarily by mechanisms other than hepatic CYPs. 3. For lipoprotein-bound drugs, increased binding to lipoproteins resulted in lower f_u values and volumes of distribution, with some exceptions. Generally, slower non-renal clearance (or total body clearance) of drugs that are substrates of hepatic CYP3A and CYP2C is well explained by the following factors: alterations in CL^'_int (due to down-regulation of hepatic CYPs), decreased f_u and/or possible decreased Q_H. 4. These consistent findings across studies in HL rats suggest more studies are needed at the clinical level for optimal pharmacotherapies for hyperlipidemia.

Importance of the Unstirred Water Layer and Hepatocyte Membrane Integrity In Vitro for Quantification of Intrinsic Metabolic Clearance

Prediction of clearance-a vital component of drug discovery-remains in need of improvement and, in particular, requires more incisive assessment of mechanistic methodology in vitro, according to a number of recent reports. Although isolated hepatocytes have become an irreplaceable standard system for the measurement of intrinsic hepatic clearance mediated by active uptake transport and metabolism, the lack of prediction reliability appears to reflect a lack of methodological validation, especially for highly cleared drugs, as we have previously shown. Here, novel approaches were employed to explore fundamental experimental processes and associated potential limitations of in vitro predictions of clearance. Rat hepatocytes deemed nonviable by trypan blue staining showed undiminished metabolic activity for probe cytochrome P450 (P450) substrates midazolam and propranolol; supplementation with NADPH enhanced these activities. Extensive permeabilization of the plasma membrane using saponin showed either full or minimal P450 activity, depending on the presence or absence of 1 mM NADPH, respectively. The shaking of incubations facilitated P450 metabolic rates up to 5-fold greater than static incubation, depending on intrinsic clearance, indicating the critical influence of the unstirred water layer (UWL). Permeabilization allowed static incubation metabolic rates to approach those of shaking for intact cells, indicating an artificially induced breakdown of the UWL. Permeabilization combined with shaking allowed an increased metabolic rate for saquinavir, resolving the membrane permeability limitation for this drug. These findings advance the interpretation of the rate-limiting processes involved in intrinsic clearance measurements and could be critical for successful in vitro prediction.

High hepatic exposure of furanocoumarins in Radix Angelica dahuricae is associated with transporter mediated active uptake

ETHNOPHARMACOLOGICAL RELEVANCE

Radix Angelica dahuricae (RAD), the roots of Angelica dahurica (Hoffm.) Benth. & Hook.f. ex Franch. & Sav, is a well-known traditional Chinese medicine (TCM) and has been used for centuries to treat headaches, toothaches, nose congestion, abscesses, furunculoses, and acne. This herb is also one of frequently reported TCMs showing the herb-drug interaction potential. Furanocoumarins are main bioactive components of RAD.

AIM OF THE STUDY

This study is designed to characterize the tissue distribution profiles of furanocoumarins after oral administration of RAD extract in rats and to explore the mechanism underlying the high hepatic exposure of the major furanocoumarins.

MATERIALS AND METHODS

The tissue distribution of nine furanocoumarins was determined in rats after an oral dose of 0.46g/kg RAD extract using high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). Unbound fractions (ƒ_u) of major furanocoumarins, including imperatorin (IM), isoimperatorin (IIM), bergapten (BER) and oxypeucedanin hydrate (OXYH), were measured in rat plasma and selected tissue homogenates (liver, kidney, lung and brain) with Rapid Equilibrium Dialysis (RED) method. The temperature dependent hepatic uptake of IM, IIM, BER and OXYH were evaluated in suspended rat primary hepatocytes at 4°C or 37°C by the oil-spin method. The uptake kinetics was conducted in the cells over a wide concentration range. The furanocoumarins were co-incubated with a panel of transporter inhibitors to investigate the involvement of uptake transporters in the hepatic uptake. The transcellular transport characteristics of IM, IIM, BER and OXYH were further assessed using Caco-2 cell monolayer model.

RESULTS

IM, IIM, BER and OXYH were found to be the major bioactive furanocoumarins in rat plasma and tissues, representing more than 90% exposure for all the detected furanocoumarins. The most concentrative organ of major furanocoumarins was the liver, with liver-to-plasma exposure ratio (K_p,_AUC) of 5.1, 6.5 and 4.7 for IM, IIM and BER, and 2.3 for OXYH, respectively. IM, IIM and BER also showed higher concentrations in the kidney with K_p above 2.2. The higher protein binding of the furanocoumarins partially contributed to their higher tissue exposure. In suspended rat primary hepatocyte, the hepatic uptake of IM, IIM, BER and OXYH was temperature-dependent, with considerably higher uptake at 37°C than at 4°C. Uptake kinetics indicated that the hepatic uptake of IM, IIM, BER and OXYH involved both active transport and passive diffusion processes. For IM, IIM and BER, the contribution of the active transport was greater than the passive process, with the CL_active/CL_uptake > 72%. Ritonavir (RTN) and cyclosporine A (CsA), the known inhibitors of organic anion transporting polypeptide (Oatp) significantly inhibited the hepatic uptake of IM and BER, while the inhibitor of the organic anion transporters (Oat) probenecid (PBC) remarkably reduced IIM uptake. In the Caco-2 cell model, the furanocoumarins were highly permeable in the apical to basolateral direction without notable active efflux.

CONCLUSION

The furanocoumarins rapidly and widely distributed into various tissues after oral dose of the RAD extract. IM, IIM, BER and OXYH were the major components detected in both plasma and tissues. Liver was the most distributed tissue of the total and free furanocoumarins. Non-specific protein binding contributed partially to the higher tissue exposures of these bioactive components. The Oatp and Oat mediated active uptake played the primal role in the high hepatic exposure of the furanocoumarins.

Renal organic anion transporters in drug-drug interactions and diseases

The kidney plays a vital role in maintaining systemic homeostasis. Active tubular secretion and reabsorption, which are mainly mediated by transporters, is an efficient mechanism for retaining glucose, amino acids, and other nutrients and for the clearance of endogenous waste products and xenobiotics. These substances are recognized by uptake transporters located in the basolateral and apical membranes of renal proximal tubule cells and are extracted from plasma and urine. Organic anion transporters (OATs) belong to the solute carrier (SLC) 22 superfamily and facilitate organic anions across the plasma membranes of renal proximal tubule cells. OATs are responsible for the transmembrane transport of anionic and zwitterionic organic molecules, including endogenous substances and many drugs. The alteration in OAT expression and function caused by diseases, drug-drug interactions (DDIs) or other issues can thus change the renal disposition of substrates, induce the accumulation of toxic metabolites, and lead to unexpected clinically outcome. This review summarizes the recent information regarding the expression, regulation, and substrate spectrum of OATs and discusses the roles of OATs in diseases and DDIs. These findings will enables us to have a better understanding of the related disease therapy and the potential risk of DDIs mediated by OATs.

Understanding the Interplay Between Uptake and Efflux Transporters Within In Vitro Systems in Defining Hepatocellular Drug Concentrations

One of the most holistic in vitro systems for prediction of intracellular drug concentrations is sandwich-cultured hepatocytes (SCH); however, a comprehensive evaluation of the utility of SCH to estimate uptake and biliary clearances and the need for additional kinetic parameters has yet to be carried out. Toward this end, we have selected 9 compounds (rosuvastatin, valsartan, fexofenadine, pravastatin, repaglinide, telmisartan, atorvastatin, saquinavir, and quinidine) to provide a range of physicochemical and hepatic disposition properties. Uptake clearances were determined in SCH and compared with conventional monolayer and suspension hepatocyte systems, previously reported by our laboratory. CL_uptake ranged from 1 to 41 μL/min/10⁶ cells in SCH which were significantly lower (1%-10%) compared with the other hepatocyte models. The hepatocyte-to-media unbound concentration ratio (Kp_u) has been assessed and ranged 0.7-59, lower compared with other hepatocyte systems (8-280). Estimates of in vitro biliary clearance (CL_bile) for 4 drugs were determined and were scaled to predict in vivo values using both intracellular concentration and media drug concentrations. These studies demonstrate that reduced uptake in rat SCH may limit drug access to canalicular efflux transport proteins and highlight the importance of elucidating the interplay between these proteins for accurate prediction of hepatic clearance.

Rate-Determining and Rate-Limiting Steps in the Clearance and Excretion of a Potent and Selective p21-Activated Kinase Inhibitor: A Case Study of Rapid Hepatic Uptake and Slow Elimination in Rat

BACKGROUND

Significant under-prediction of in vivo clearance in rat was observed for a potent p21-activated kinase (PAK1) inhibitor, GNE1.

OBJECTIVE

Rate-determining (rapid uptake) and rate-limiting (slow excretion) steps in systemic clearance and elimination of GNE1, respectively, were evaluated to better understand the cause of the in vitro-in vivo (IVIV) disconnect.

METHODS

A series of in vivo, ex vivo, and in vitro experiments were carried out: 1) the role of organic cation transporters (Oct or Slc22a) was investigated in transporter knock-out and wild-type animals with or without 1-aminobenzotriazole (ABT) pretreatment; 2) the concentration-dependent hepatic extraction ratio was determined in isolated perfused rat liver; and 3) excreta were collected from both bile duct cannulated and non-cannulated rats after intravenous injection.

RESULTS

After intravenous dosing, the rate-determining step in clearance was found to be mediated by the active uptake transporter, Oct1. In cannulated rats, biliary and renal clearance of GNE1 accounted for only approximately 14 and 16% of the total clearance, respectively. N-acetylation, an important metabolic pathway, accounted for only about 10% of the total dose. In non-cannulated rats, the majority of the dose was recovered in feces as unchanged parent (up to 91%) overnight following intravenous administration.

CONCLUSION

Because the clearance of GNE1 is mediated through uptake transporters rather than metabolism, the extrahepatic expression of Oct1 in kidney and intestine in rat likely plays an important role in the IVIV disconnect in hepatic clearance prediction. The slow process of intestinal secretion is the rate-limiting step for in vivo clearance of GNE1.

Transport-Metabolism Interplay of Atazanavir in Rat Hepatocytes

The aim of this study was to explore the mechanisms governing the intra- to extracellular unbound concentration ratio (Kpu,u) for the HIV protease inhibitor atazanavir (ATV) in rat hepatocytes. We had previously proposed a new method to determine Kpu,u by using the unbound Km values from metabolism studies with suspended rat hepatocytes and rat liver microsomes. Following that method, we determined that the value of ATV Kpu,u was 0.32, indicating that ATV hepatocellular clearance is uptake rate-limited. This hypothesis was supported by the linear correlation between Kpu,u and active uptake clearance (P = 0.04; R(2)=0.82) in the presence of increasing concentrations of the uptake transport inhibitor losartan. Moreover, in contrast to an expected increase of Kpu,u upon inhibition of ATV metabolism, a decrease of Kpu,u was observed, suggesting an increased impact of sinusoidal efflux. In summary, involvement of active uptake transport does not guarantee high intracellular accumulation; however, it has a key role in regulating intracellular drug concentrations and drug metabolism. These findings will help improve future in vitro-to-in vivo extrapolations and likewise physiologically based pharmacokinetic models.

Hepatic Clearance Prediction of Nine Human Immunodeficiency Virus Protease Inhibitors in Rat

This study aimed to determine the rate-limiting step in the overall hepatic clearance of the marketed human immunodeficiency virus (HIV) protease inhibitors (PI) in rats by predicting the experimentally determined hepatic in vivo clearance of these drugs based on in vitro clearance values for uptake and/or metabolism. In vitro uptake and metabolic clearance values were determined in suspended rat hepatocytes and rat liver microsomes, respectively. In vivo hepatic clearance was determined after intravenous bolus administration in rats. Excellent in vitro-in vivo correlation (IVIVC; R(2) = 0.80) was observed when metabolic intrinsic Cl values were used, which were determined in vitro at a single concentration corresponding to the blood concentration observed in rats in vivo at the mean residence time. On the contrary, poor IVIVC was observed when in vitro metabolic Cl values based on full Michaelis-Menten profiles were used. In addition, the use of uptake Cl values or a combination of both uptake and metabolic clearance data led to poor predictions of in vivo clearance. Although our findings indicate a key role for metabolism in the hepatic clearance of several HIV PI in rats, subsequent simulations revealed that inhibition of hepatic uptake can lead to altered hepatic clearance for several of these drugs.

Verapamil hepatic clearance in four preclinical rat models: towards activity-based scaling

The current study was designed to cross-validate rat liver microsomes (RLM), suspended rat hepatocytes (SRH) and the isolated perfused rat liver (IPRL) model against in vivo pharmacokinetic data, using verapamil as a model drug. Michaelis-Menten constants (Km), for the metabolic disappearance kinetics of verapamil in RLM and SRH (freshly isolated and cryopreserved), were determined and corrected for non-specific binding. The 'unbound' Km determined with RLM (2.8 µM) was divided by the 'unbound' Km determined with fresh and cryopreserved SRH (3.9 µM and 2.1 µM, respectively) to calculate the ratio of intracellular to extracellular unbound concentration (Kpu,u). Kpu,u was significantly different between freshly isolated (0.71) and cryopreserved (1.31) SRH, but intracellular capacity for verapamil metabolism was maintained after cryopreservation (200 vs. 191 µl/min/million cells). Direct comparison of intrinsic clearance values (Clint) in RLM versus SRH, yielded an activity-based scaling factor (SF) of 0.28-0.30 mg microsomal protein/million cells (MPPMC). Merging the IPRL-derived Clint with the MPPMC and SRH data, resulted in scaling factors for MPPGL (80 and 43 mg microsomal protein/g liver) and HPGL (269 and 153 million cells/g liver), respectively. Likewise, the hepatic blood flow (61 ml/min/kg b.wt) was calculated using IPRL Clint and the in vivo Cl. The scaling factors determined here are consistent with previously reported CYP450-content based scaling factors. Overall, the results show that integrated interpretation of data obtained with multiple preclinical tools (i.e. RLM, SRH, IPRL) can contribute to more reliable estimates for scaling factors and ultimately to improved in vivo clearance predictions based on in vitro experimentation.

Rationalizing underprediction of drug clearance from enzyme and transporter kinetic data: from in vitro tools to mechanistic modeling

Over the years, there has been an increase in the number and quality of available in vitro tools for the assessment of clearance. Complexity of data analysis and modelling of corresponding in vitro data has increased in an analogous manner, in particular for the simultaneous characterization of transporter and metabolism kinetics, together with intracellular binding and passive diffusion. In the current chapter, the impact of different factors on the in vitro-in vivo extrapolation of clearance will be addressed in a stepwise manner, from the selection of the most adequate in vitro system and experimental design/condition to the corresponding modelling of data generated. The application of static or physiologically based pharmacokinetic models in the prediction of clearance will be discussed, highlighting limitations and current challenges of some of the approaches. Particular focus will be on the ability of in vitro and in silico predictive tools to overcome the trend of clearance underprediction. Improvements made as a result of inclusion of extrahepatic metabolism and consideration of transporter-metabolism interplay across different organs will be discussed.

A perspective on the prediction of drug pharmacokinetics and disposition in drug research and development

Prediction of human pharmacokinetics of new drugs, as well as other disposition attributes, has become a routine practice in drug research and development. Prior to the 1990s, drug disposition science was used in a mostly descriptive manner in the drug development phase. With the advent of in vitro methods and availability of human-derived reagents for in vitro studies, drug-disposition scientists became engaged in the compound design phase of drug discovery to optimize and predict human disposition properties prior to nomination of candidate compounds into the drug development phase. This has reaped benefits in that the attrition rate of new drug candidates in drug development for reasons of unacceptable pharmacokinetics has greatly decreased. Attributes that are predicted include clearance, volume of distribution, half-life, absorption, and drug-drug interactions. In this article, we offer our experience-based perspectives on the tools and methods of predicting human drug disposition using in vitro and animal data.

Prediction of human pharmacokinetics in 2013 and beyond

The utility of in vitro generated kinetic data to provide quantitative prediction of in vivo pharmacokinetic behavior is well established and forms a cornerstone of many research projects in drug metabolism and disposition, particularly within the pharmaceutical industry. This issue provides several excellent examples of the use of in vitro techniques for prediction of human pharmacokinetics (PK). The general area of in vitro-in vivo extrapolation (IVIVE) is broad and hence the spectrum of topics covers various aspects drug clearance and distribution and drug-drug interactions. Some articles were commissioned whereas others were identified during the reviewing process. Overall they provide a snapshot of activity at the end of 2013. They document that whereas the translation of some in vitro approaches is now established, other areas are in their infancy and need more development.

Determination of intracellular unbound concentrations and subcellular localization of drugs in rat sandwich-cultured hepatocytes compared with liver tissue

Prediction of clinical efficacy, toxicity, and drug-drug interactions may be improved by accounting for the intracellular unbound drug concentration (C(unbound)) in vitro and in vivo. Furthermore, subcellular drug distribution may aid in predicting efficacy, toxicity, and risk assessment. The present study was designed to quantify the intracellular C(unbound) and subcellular localization of drugs in rat sandwich-cultured hepatocytes (SCH) compared with rat isolated perfused liver (IPL) tissue. Probe drugs with distinct mechanisms of hepatocellular uptake and accumulation were selected for investigation. Following drug treatment, SCH and IPL tissues were homogenized and fractionated by differential centrifugation to enrich for subcellular compartments. Binding in crude lysate and cytosol was determined by equilibrium dialysis; the C(unbound) and intracellular-to-extracellular C(unbound) ratio (K(pu,u)) were used to describe accumulation of unbound drug. Total accumulation (K(pobserved)) in whole tissue was well predicted by the SCH model (within 2- to 3-fold) for the selected drugs. Ritonavir (K(pu,u) ∼1) was evenly distributed among cellular compartments, but highly bound, which explained the observed accumulation within liver tissue. Rosuvastatin was recovered primarily in the cytosolic fraction, but did not exhibit extensive binding, resulting in a K(pu,u) >1 in liver tissue and SCH, consistent with efficient hepatic uptake. Despite extensive binding and sequestration of furamidine within liver tissue, a significant portion of cellular accumulation was attributed to unbound drug (K(pu,u) >16), as expected for a charged, hepatically derived metabolite. Data demonstrate the utility of SCH to predict quantitatively total tissue accumulation and elucidate mechanisms of hepatocellular drug accumulation such as active uptake versus binding/sequestration.

Hepatic basolateral efflux contributes significantly to rosuvastatin disposition II: characterization of hepatic elimination by basolateral, biliary, and metabolic clearance pathways in rat isolated perfused liver

Basolateral efflux clearance (CLBL) contributes significantly to rosuvastatin (RSV) elimination in sandwich-cultured hepatocytes (SCH). The contribution of CLBL to RSV hepatic elimination was determined in single-pass isolated perfused livers (IPLs) from wild-type (WT) and multidrug resistance-associated protein 2 (Mrp2)-deficient (TR(-)) rats in the absence and presence of the P-glycoprotein and breast cancer resistance protein (Bcrp) inhibitor, elacridar (GF120918); clearance values were compared with SCH. RSV biliary clearance (CLBile) was ablated almost completely by GF120918 in TR(-) IPLs, confirming that Mrp2 and Bcrp primarily are responsible for RSV CLBile. RSV appearance in outflow perfusate was attributed primarily to CLBL, which was impaired in TR(-) IPLs. CLBL was ≈ 6-fold greater than CLBile in the linear range in WT IPLs in the absence of GF120918. Recovery of unchanged RSV in liver tissue increased in TR(-) compared with WT (≈ 25 versus 6% of the administered dose) due to impaired CLBL and CLBile. RSV pentanoic acid, identified by high-resolution liquid chromatography-tandem mass spectroscopy, comprised ≈ 40% of total liver content and ≈ 16% of the administered dose in TR(-) livers at the end of perfusion, compared with ≈ 30 and 3% in WT livers, consistent with impaired RSV excretion and "shunting" to the metabolic pathway. In vitro-ex vivo extrapolation between WT SCH and IPLs (without GF120918) revealed that uptake clearance and CLBL were 4.2- and 6.4-fold lower, respectively, in rat SCH compared with IPLs; CLBile translated almost directly (1.1-fold). The present IPL data confirmed the significant role of CLBL in RSV hepatic elimination, and demonstrated that both CLBL and CLBile influence RSV hepatic and systemic exposure.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Clinical significance of organic anion transporting polypeptides (OATPs) in drug disposition: their roles in hepatic clearance and intestinal absorption

Organic anion transporting polypeptide (OATP) family transporters accept a number of drugs and are increasingly being recognized as important factors in governing drug and metabolite pharmacokinetics. OATP1B1 and OATP1B3 play an important role in hepatic drug uptake while OATP2B1 and OATP1A2 might be key players in intestinal absorption and transport across blood-brain barrier of drugs, respectively. To understand the importance of OATPs in the hepatic clearance of drugs, the rate-determining process for elimination should be considered; for some drugs, hepatic uptake clearance rather than metabolic intrinsic clearance is the more important determinant of hepatic clearances. The importance of the unbound concentration ratio (liver/blood), K(p,uu) , of drugs, which is partly governed by OATPs, is exemplified in interpreting the difference in the IC(50) of statins between the hepatocyte and microsome systems for the inhibition of HMG-CoA reductase activity. The intrinsic activity and/or expression level of OATPs are affected by genetic polymorphisms and drug-drug interactions. Their effects on the elimination rate or intestinal absorption rate of drugs may sometimes depend on the substrate drug. This is partly because of the different contribution of OATP isoforms to clearance or intestinal absorption. When the contribution of the OATP-mediated pathway is substantial, the pharmacokinetics of substrate drugs should be greatly affected. This review describes the estimation of the contribution of OATP1B1 to the total hepatic uptake of drugs from the data of fold-increases in the plasma concentration of substrate drugs by the genetic polymorphism of this transporter. To understand the importance of the OATP family transporters, modeling and simulation with a physiologically based pharmacokinetic model are helpful.

Interaction between HIV protease inhibitors (PIs) and hepatic transporters in sandwich cultured human hepatocytes: implication for PI-based DDIs

Although HIV protease inhibitors (PIs) produce profound metabolic interactions through inactivation/inhibition of CYP3A enzymes, their role as victims of transporter-based drug-drug interactions (DDIs) is less well understood. Therefore, this study investigated if the PIs, nelfinavir (NFV), ritonavir (RTV), lopinavir (LPV) or amprenavir (APV) were transported into sandwich-cultured human hepatocytes (SCHH), and whether OATPs contributed to this transport. The findings showed that, except for (3) H-APV, no significant decrease in the total hepatocyte accumulation of the (3) H-PIs was detected in the presence of the corresponding unlabeled PI, indicating that the uptake of the other PIs was not mediated. Further, hepatocyte biliary efflux studies using (3) H-APV and unlabeled APV confirmed this decrease to be due to inhibition of sinusoidal influx transporter(s) and not the canalicular efflux transporters. Moreover, this sinusoidal transport of APV was not OATP-mediated. The results indicate that the hepatic uptake of NFV, RTV or LPV was primarily mediated by passive diffusion. The hepatic uptake of APV was mediated by an unidentified sinusoidal transporter(s). Therefore, NFV, RTV or LPV will not be victims of DDIs involving inhibition of hepatic influx transporters; however, the disposition of APV may be affected if its sinusoidal transport is inhibited.

Primary hepatocyte cultures as prominent in vitro tools to study hepatic drug transporters

Before any drug can be placed on the market, drug efficacy and safety must be ensured through rigorous testing. Animal models are used for this purpose, though currently increasing attention goes to the use of alternative in vitro systems. In particular, liver-based testing platforms that allow the prediction of pharmacokinetic (PK) and pharmacotoxicological properties during the early phase of drug development are of interest. They also enable the screening of potential effects on hepatic drug transporters. The latter are known to affect drug metabolism and disposition, thereby possibly underlying drug-drug interactions, which, in turn, may result in liver toxicity. Clearly, stable in vivo-like functional expression of drug transporters in hepatic in vitro settings is a prerequisite to be applicable in routine PK and pharmacotoxicological testing. In the first part of the article, an updated overview of hepatic drug transporters is provided, followed by a state-of-the-art review of drug-transporter production and activity in primary hepatocyte cultures (PHCs), being the gold-standard in vitro system. Specific focus is hereby put on strategies to maintain long-term functional expression, in casu of drug transporters, in these systems. In the second part, the use of PHCs to assess hepatobiliary transport and transporter-mediated interactions is outlined.

Use of mechanistic modeling to assess interindividual variability and interspecies differences in active uptake in human and rat hepatocytes

Interindividual variability in activity of uptake transporters is evident in vivo, yet limited data exist in vitro, confounding in vitro-in vivo extrapolation. The uptake kinetics of seven organic anion-transporting polypeptide substrates was investigated over a concentration range in plated cryopreserved human hepatocytes. Active uptake clearance (CL(active, u)), bidirectional passive diffusion (P(diff)), intracellular binding, and metabolism were estimated for bosentan, pitavastatin, pravastatin, repaglinide, rosuvastatin, telmisartan, and valsartan in HU4122 donor using a mechanistic two-compartment model in Matlab. Full uptake kinetics of rosuvastatin and repaglinide were also characterized in two additional donors, whereas for the remaining drugs CL(active, u) was estimated at a single concentration. The unbound affinity constant (K(m, u)) and P(diff) values were consistent across donors, whereas V(max) was on average up to 2.8-fold greater in donor HU4122. Consistency in K(m, u) values allowed extrapolation of single concentration uptake activity data and assessment of interindividual variability in CL(active) across donors. The maximal contribution of active transport to total uptake differed among donors, for example, 85 to 96% and 68 to 87% for rosuvastatin and repaglinide, respectively; however, in all cases the active process was the major contributor. In vitro-in vivo extrapolation indicated a general underprediction of hepatic intrinsic clearance, an average empirical scaling factor of 17.1 was estimated on the basis of seven drugs investigated in three hepatocyte donors, and donor-specific differences in empirical factors are discussed. Uptake K(m, u) and CL(active, u) were on average 4.3- and 7.1-fold lower in human hepatocytes compared with our previously published rat data. A strategy for the use of rat uptake data to facilitate the experimental design in human hepatocytes is discussed.

Utility of drug depletion-time profiles in isolated hepatocytes for accessing hepatic uptake clearance: identifying rate-limiting steps and role of passive processes

Drug depletion-time profiles in isolated hepatocytes, as well as microsomes, have become a standard method of assessing hepatic metabolic clearance in vitro. There is a previously described adaptation of the depletion approach to allow determination of hepatic uptake by transporters in addition to metabolism (Drug Metab Dispos 35:859-865, 2007). Dual incubations are performed where one set of incubations undergo conventional methodology, whereas for the second set, cells and media are separated for determination of drug loss from the media. The utility of this dual incubation approach has been assessed using eight drugs (atorvastatin, clarithromycin, erythromycin, fexofenadine, pitavastatin, repaglinide, rosuvastatin, and saquinavir) with a range of active uptake, passive permeability, cell binding, and metabolic characteristics. Four of these compounds (fexofenadine, rosuvastatin, pitavastatin, and atorvastatin) show a biphasic time profile when assessing drug loss from media indicative of hepatic uptake before elimination within the hepatocyte, which is distinct from the time profile in a conventional incubation, and show higher clearances. The four other compounds (clarithromycin, saquinavir, erythromycin, and repaglinide) show identical depletion-time profiles (and clearances) in both sets of incubations. Whether or not the biphasic nature (and higher clearance) is evident, indicating transporter activity for a particular drug, appears to be dependent on its passive permeability. Using the parameter K(pu) to reflect the relative importance of hepatic transporters versus passive diffusion, a value of 10 was identified as a cutoff for whether the biphasic nature was evident; those compounds in excess of 10 show this characteristic clearly. There appears to be no relationship between the presence of the biphasic nature and any other parameter, including cellular binding, extent of metabolism, or the magnitude of active uptake.

Simultaneous assessment of uptake and metabolism in rat hepatocytes: a comprehensive mechanistic model

Kinetic parameters describing hepatic uptake in hepatocytes are frequently estimated without appropriate incorporation of bidirectional passive diffusion, intracellular binding, and metabolism. A mechanistic two-compartment model was developed to describe all of the processes occurring during the in vitro uptake experiments performed in freshly isolated rat hepatocytes plated for 2 h. Uptake of rosuvastatin, pravastatin, pitavastatin, valsartan, bosentan, telmisartan, and repaglinide was investigated over a 0.1 to 300 μM concentration range at 37°C for 2 or 45-90 min; nonspecific binding was taken into account. All concentration-time points were analyzed simultaneously by using a mechanistic two-compartment model describing uptake kinetics [unbound affinity constant (K(m,u)), maximum uptake rate (V(max)), unbound active uptake clearance (CL(active,u))], passive diffusion [unbound passive diffusion clearance (P(diff,u))], and intracellular binding [intracellular unbound fraction (fu(cell))]. When required (telmisartan and repaglinide), the model was extended to account for the metabolism [unbound metabolic clearance (CL(met,u))]. The CL(active,u) ranged 8-fold, reflecting a 11-fold range in uptake K(m,u), with telmisartan and valsartan showing the highest affinity for uptake transporters (K(m,u) <10 μM). Both P(diff,u) and fu(cell) span over two orders of magnitude and reflected the lipophilicity of the drugs in the dataset. An extended incubation time allowed steady state to be reached between media and intracellular compartment concentrations and reduced the error in certain parameter estimates observed with shorter incubation times. Active transport accounted for >70% of total uptake for all drugs investigated and was 4- and 112-fold greater than CL(met,u) for telmisartan and repaglinide, respectively. Modeling of uptake kinetics in conjunction with metabolism improved the precision of the uptake parameter estimates for repaglinide and telmisartan. Recommendations are made for uptake experimental design and modeling strategies.

Kinetic characterization of rat hepatic uptake of 16 actively transported drugs

To explore the determinants of hepatic uptake, 16 compounds were investigated with different physicochemical and disposition characteristics, including five statins, three sartans, saquinavir, ritonavir, erythromycin, clarithromycin, nateglinide, repaglinide, fexofenadine, and bosentan. Freshly isolated rat hepatocytes in suspension were used with the oil-spin method to generate kinetic parameters. Clearances, via passive diffusion (P(diff)) and active uptake (CL(active), characterized by maximal uptake rate and K(m)), were estimated from the initial uptake rate data over a 0.01 to 100 μM concentration range. The K(m) values had a range of 15-fold, with 10 of the 16 drugs with K(m) < 10 μM (median 6 μM). Both CL(active) and P(diff) ranged over 100-fold (median 188 and 14 μl/min/10⁶ cells). Assessment of the relative contribution of P(diff) and CL(active) indicated that, at low concentrations (approximately 0.1 μM), the active process contributes >80% to the overall uptake for 13 drugs. Although high P(diff) values were obtained for ritonavir and repaglinide, active process contributed predominantly to uptake; in contrast, high passive permeability dominates over transporter-mediated uptake for saquinavir over the full concentration range. For bosentan and erythromycin, active and passive processes were equally important. Hepatocyte-to-medium unbound concentration ratio was >10 for 9 of the 16 drugs, ranging from 2 to 494 for bosentan and atorvastatin, respectively. Some drugs showed extensive intracellular binding (fraction unbound range 0.01-0.6), which was not correlated with active uptake. LogD₇.₄ correlated significantly with P(diff) and the extent of intracellular binding but not with active uptake. This study provides systematic assessment of the role of active uptake relative to the passive process; implications of the findings are discussed.

Influence of drug transport proteins on the pharmacokinetics and drug interactions of HIV protease inhibitors

Protease inhibitors, a class of antiretroviral agents frequently used in the treatment of HIV infection, interact with numerous transport proteins resulting in clinically significant drug-drug interactions (DDIs). This review focuses on the proteins that transport protease inhibitors and directly influence the pharmacokinetics of these drugs, as well as the transport proteins that are inhibited or induced by protease inhibitors. Clinically relevant DDIs involving drug transporters and protease inhibitors, either as "victim" drugs or as "perpetrator" drugs, and the pharmacokinetic consequences of such interactions are highlighted. A summary of transporter-mediated processes underlying the toxicity of protease inhibitors is provided. Finally, the effect of HIV infection or co-infection on drug transport proteins, and the implications for protease inhibitor pharmacokinetics is discussed. Transport proteins significantly influence the pharmacokinetics, efficacy and toxicity profiles of this important class of drugs.

Comparative use of isolated hepatocytes and hepatic microsomes for cytochrome P450 inhibition studies: transporter-enzyme interplay

Accurate assignment of the concentration of victim drug/inhibitor available at the enzyme active site, both in vivo and within an in vitro incubation, is an essential requirement in rationalizing and predicting drug-drug interactions. Inhibitor accumulation within the liver, whether as a result of active transport processes or intracellular binding, may best be accounted for using hepatocytes rather than hepatic microsomes to estimate in vitro inhibitory potency. The aims of this study were to compare K(i) values determined in rat liver microsomes and freshly isolated rat hepatocytes of four cytochrome P450 (P450) inhibitors (clarithromycin, enoxacin, nelfinavir, and saquinavir) with known hepatic transporter involvement and a range of uptake (cell/medium concentration ratios 20-3000) and clearance (10-1200 μl/min/10(6) cells) properties. Inhibition studies were performed using two well established P450 probe substrates (theophylline and midazolam). Comparison of unbound K(i) values showed marked differences between the two in vitro systems for inhibition of metabolism. In two cases (clarithromycin and enoxacin, both low-clearance drugs), inhibitory potency in hepatocytes markedly exceeded that in microsomes (10- to 20-fold), and this result was consistent with their high cell/medium concentration ratios. For nelfinavir and saquinavir (high-clearance, extensively metabolized drugs), the opposite trend was seen in the K(i) values: despite very high cell/medium concentration ratios, stronger inhibition was evident within microsomal preparations. Hence, the consequences of hepatic accumulation resulting from uptake transporters vary according to the clearance of the inhibitor. This study demonstrates that transporter-enzyme interplay can result in differences in inhibitory potency between microsomes and hepatocytes and hence drug-drug interaction predictions that are not always intuitive.

Mechanistic insight from in silico pharmacokinetic experiments: roles of P-glycoprotein, Cyp3A4 enzymes, and microenvironments

Saquinavir exhibits paradoxical transport across modified Caco-2 cell monolayers (doi: 10.1124/jpet.103.056390) expressing P-glycoprotein and Cyp3A4. The data implicate complicated intracellular transport mechanisms. Drawing on recent discrete event modeling and simulation advances, we built an in silico analog of the confluent, asymmetric cell monolayer used in the cited work. We call it in silico experimental Caco-2 (cell monolayer) culture (ISECC). Concrete, working, hypothesized spatial mechanisms were implemented. Validation was achieved when in silico experimental results met similarity measure (SM) expectations that targeted 16 wet-lab experimental conditions. Initial mechanistic hypotheses turned out to be necessary parts of a more complicated explanation. We progressed through four stages of an iterative refinement and validation protocol that enabled and facilitated discovery of plausible, new mechanistic details. The process exercised abductive reasoning, a primary means of scientific knowledge creation and creative cognition. The ISECC that survived the most stringent SM challenge produced transport data that was statistically indistinguishable from referent wet-lab observations. It required a 7:1 ratio of apical transporters to metabolizing enzymes, a 97% reduction of efflux activity by an inhibitor, a biased distribution of metabolizing enzymes, heterogeneous intracellular spaces, and restrictions on intracellular drug movement. Experimenting on synthetic analogs such as ISECC provides a former unavailable means of discovering new mechanistic details and testing their plausibility. The approach thus provides a powerful new expansion of the scientific method: an independent, scientific means to challenge, explore, better understand, and improve any inductive mechanism and, importantly, the assumptions on which it rests.

Design, data analysis, and simulation of in vitro drug transport kinetic experiments using a mechanistic in vitro model

The use of in vitro data for quantitative predictions of transporter-mediated elimination in vivo requires an accurate estimation of the transporter Michaelis-Menten parameters, V(max) and K(m), as a first step. Therefore, the experimental conditions of in vitro studies used to assess hepatic uptake transport were optimized regarding active transport processes, nonspecific binding, and passive diffusion (P(dif)). A mechanistic model was developed to analyze and accurately describe these active and passive processes. This two-compartmental model was parameterized to account for nonspecific binding, bidirectional passive diffusion, and active uptake processes based on the physiology of the cells. The model was used to estimate kinetic parameters of in vitro transport data from organic anion-transporting peptide model substrates (e.g., cholecystokinin octapeptide deltorphin II, fexofenadine, and pitavastatin). Data analysis by this mechanistic model significantly improved the accuracy and precision in all derived parameters [mean coefficient of variations (CVs) for V(max) and K(m) were 19 and 23%, respectively] compared with the conventional kinetic method of transport data analysis (mean CVs were 58 and 115%, respectively, using this method). Furthermore, permeability was found to be highly temperature-dependent in Chinese hamster ovary (CHO) control cells and artificial membranes (parallel artificial membrane permeability assay). Whereas for some compounds (taurocholate, estrone-3-sulfate, and propranolol) the effect was moderate (1.5-6-fold higher permeability at 37 degrees C compared with that at 4 degrees C), for fexofenadine a 16-fold higher passive permeability was seen at 37 degrees C. Therefore, P(dif) was better predicted if it was evaluated under the same experimental conditions as V(max) and K(m), i.e., in a single incubation of CHO overexpressed cells or rat hepatocytes at 37 degrees C, instead of a parallel control evaluation at 4 degrees C.