Investigating ABCB1-Mediated Drug-Drug Interactions: Considerations for In vitro and In vivo Assay Design.

BACKGROUND: ABCB1 is a key ABC efflux transporter modulating the pharmacokinetics of a large percentage of drugs. ABCB1 is also a site of transporter mediated drug-drug interactions (tDDI). It is the transporter most frequently tested for tDDIs both in vitro and in the clinic. OBJECTIVE: Understanding the limitations of various in vitro and in vivo models, therefore, is crucial. In this review we cover regulatory aspects of ABCB1 mediated drug transport as well as inhibition and the available models and methods. We also discuss protein structure and mechanistic aspects of transport as ABCB1 displays complex kinetics that involves multiple binding sites, potentiation of transport and probe-dependent IC50 values. RESULTS: Permeability of drugs both passive and mediated by transporters is also a covariate that modulates apparent kinetic values. Levels of expression as well as lipid composition of the expression system used in in vitro studies have also been acknowledged as determinates of transporter activity. ABCB1-mediated clinical tDDIs are often complex as multiple transporters as well as metabolic enzymes may play a role. This complexity often masks the role of ABCB1 in tDDIs. CONCLUSION: It is expected that utilization of in vitro data will further increase with the refinement of simulations. It is also anticipated that transporter humanized preclinical models have a significant impact and utility.

Membrane Assays to Characterize Interaction of Drugs with ABCB1.

ATP-binding cassette sub-family B member 1 (ABCB1) [P-glycoprotein (P-gp), multidrug resistance protein 1 (MDR1)] can affect the pharmacokinetics, safety, and efficacy of drugs making it important to identify compounds that interact with ABCB1. The ATPase assay and vesicular transport (VT) assay are membrane based assays that can be used to measure the interaction of compounds with ABCB1 at a lower cost and higher throughput compared to cellular-based assays and therefore can be used earlier in the drug development process. To that end, we tested compounds previously identified as ABCB1 substrates and inhibitors for interaction with ABCB1 using the ATPase and VT assays. All compounds tested interacted with ABCB1 in both the ATPase and VT assays. All compounds previously identified as ABCB1 substrates activated ABCB1-mediated ATPase activity in the ATPase assay. All compounds previously identified as ABCB1 inhibitors inhibited the ABCB1-mediated transport in the VT assay. Interestingly, six of the ten compounds previously identified as ABCB1 inhibitors activated the basal ATPase activity in activation assays suggesting that the compounds are substrates of ABCB1 but can inhibit ABCB1 in inhibition assays. Importantly, for ATPase activators the EC50 of activation correlated with the IC50 values from the VT assay showing that interactions of compounds with ABCB1 can be measured with similar levels of potency in either assay. For ATPase nonactivators the IC50 values from the ATPase inhibition and VT inhibition assay showed correlation. These results demonstrate the utility of membrane assays as tools to detect and rank order drug-transporter interactions.

Synthesis and ABCG2 inhibitory activity of novel fumitremorgin C analogs--specificity and structure activity correlations.

The Ko family of fumitremorgin C analogs are potent and selective ABCG2 inhibitors. However, the most potent Ko compounds carry an ester linkage in their side-chain that makes them chemically and metabolically less stable. We have synthesized 16 tricyclic and 28 tetracyclic novel analogs devoid of ester linkages and tested them for ABCG2 inhibition potency and specificity. Unlike in the tricyclic analog group, potent ABCG2 inhibitory compounds were found among the tetracyclic analogs. The most potent compounds carried the 3S,6S,12aS configuration. We observed a marked stereospecificity as compounds with the 3S,6S,12aS configuration were at least 18-fold more potent inhibitors than their diastereoisomeric pairs with a 3S,6R,12aS configuration. This stereospecificity was not observed in ABCB1 and ABCC1 inhibition. Therefore, a single chiral center confers specificity for ABCG2 over ABCB1 and ABCC1. This is quite unexpected considering the large multivalent drug binding site these transporters harbor.

The use of microdialysis techniques in mice to study P-gp function at the blood-brain barrier.

An integrated assay system involving dual/triple-probe microdialysis techniques in rats was developed earlier for testing interactions with P-glycoprotein (P-gp) at the blood-brain barrier using quinidine/PSC-833 as a P-gp substrate/inhibitor combination. The aim of the present study was to expand our assay system to mice using microdialysis with simultaneous sampling of blood and brain and to compare the result with a primary mouse brain endothelial cell monolayer (pMBMEC) assay. Brain penetration of quinidine was dose dependent in both anesthetized and awake mice after intraperitoneal drug administration. PSC-833 pretreatment caused a 2.5- to 3.4-fold increase in quinidine levels of brain dialysate samples in anesthetized or awake animals, after single or repeated administration of PSC-833. In pMBMEC, a 2.0- to 2.5-fold efflux ratio was observed in the transcellular transport of quinidine. The P-gp-mediated vectorial transport of quinidine was eliminated by PSC-833. These results indicate that quinidine with PSC-833 is a good probe substrate-reference inhibitor combination for testing drug-drug interactions with P-gp in the in vivo and in vitro mouse systems. With increasing number of humanized transgenic mice, a test system with mouse microdialysis experimentation becomes more important to predict drug-drug interactions in humans.

ABCG2 modulates chlorothiazide permeability--in vitro-characterization of its interactions.

We are showing that chlorothiazide, a diuretic, is an ABCG2 substrate. It is a Biopharmaceutics Classification System/Biopharmaceutics Drug Distribution and Classification System (BCS/BDDCS) Class IV drug with low bioavailability. Therefore, we tested if chlorothiazide interacts with major apically located intestinal efflux transporters. Our data show that chlorothiazide is transported by ABCG2 with a Km value of 334.6 µM and does not interact with ABCB1 or ABCC2. The chlorothiazide-ABCG2 interaction results in a vectorial transport in MDCKII-BCRP and Caco-2 cells with efflux ratios of 36 and 8.1 respectively. Inhibition of ABCG2 in Caco-2 cells reduced the efflux ratio to 1.4, suggesting that ABCG2 plays a role in limiting chlorothiazide bioavailability in humans.

ATP-binding cassette B1 transports seliciclib (R-roscovitine), a cyclin-dependent kinase inhibitor.

Seliciclib, a cyclin-dependent kinase inhibitor, is a promising candidate to treat a variety of cancers. Pharmacokinetic studies have shown high oral bioavailability but limited brain exposure to the drug. This study shows that seliciclib is a high-affinity substrate of ATP-binding cassette B1 (ABCB1) because it activates the ATPase activity of the transporter with an EC(50) of 4.2 µM and shows vectorial transport in MDCKII-MDR1 cells, yielding an efflux ratio of 8. This interaction may be behind the drug's limited penetration of the blood-brain barrier. ABCB1 overexpression, on the other hand, does not confer resistance to the drug in the models tested. These findings should be considered when treatment strategies using seliciclib are designed.

Mouse Bsep ATPase assay: a nonradioactive tool for assessment of the cholestatic potential of drugs.

The mouse ortholog of the human bile salt export pump (BSEP) transporter was expressed in a baculovirus-infected insect cell (Sf9) system to study the effect of membrane cholesterol content on the transporter function. The transport activity of cholesterol-loaded mouse Bsep-HAM-Sf9 vesicles was determined in a vesicular transport assay with taurochenodeoxycholate (TCDC), a known BSEP substrate. Mouse Bsep transports TCDC at a high rate that can be sensitively detected in the ATPase assay. Cholesterol upload of the Sf9 membrane potentiates both TCDC transport and TCDC-stimulated ATPase activities. Inhibitory effect of BSEP interactors on probe substrate transport was tested in both vesicular transport and ATPase assays using cholesterol-loaded membrane vesicles. A good rank order correlation was found between IC(50) values measured in TCDC-stimulated mBsep ATPase assay and in the human BSEP vesicular transport assay utilizing taurocholate (TC) as probe substrate. This upgraded form of the mouse Bsep-HAM ATPase assay is a user friendly, sensitive, nonradioactive method for early high-throughput screening of drugs with BSEP-related cholestatic potential. It may complement the human BSEP-mediated taurocholate vesicular transport inhibition assay.