Zinc finger nuclease-mediated gene knockout results in loss of transport activity for P-glycoprotein, BCRP, and MRP2 in Caco-2 cells.

Membrane transporters P-glycoprotein [P-gp; multidrug resistance 1 (MDR1)], multidrug resistance-associated protein (MRP) 2, and breast cancer resistance protein (BCRP) affect drug absorption and disposition and can also mediate drug-drug interactions leading to safety/toxicity concerns in the clinic. Challenges arise with interpreting cell-based transporter assays when substrates or inhibitors affect more than one actively expressed transporter and when endogenous or residual transporter activity remains following overexpression or knockdown of a given transporter. The objective of this study was to selectively knock out three drug efflux transporter genes (MDR1, MRP2, and BCRP), both individually as well as in combination, in a subclone of Caco-2 cells (C2BBe1) using zinc finger nuclease technology. The wild-type parent and knockout cell lines were tested for transporter function in Transwell bidirectional assays using probe substrates at 5 or 10 µM for 2 hours at 37°C. P-gp substrates digoxin and erythromycin, BCRP substrates estrone 3-sulfate and nitrofurantoin, and MRP2 substrate 5-(and-6)-carboxy-2',7'-dichlorofluorescein each showed a loss of asymmetric transport in the MDR1, BCRP, and MRP2 knockout cell lines, respectively. Furthermore, transporter interactions were deduced for cimetidine, ranitidine, fexofenadine, and colchicine. Compared with the knockout cell lines, standard transporter inhibitors showed substrate-specific variation in reducing the efflux ratios of the test compounds. These data confirm the generation of a panel of stable Caco-2 cell lines with single or double knockout of human efflux transporter genes and a complete loss of specific transport activity. These cell lines may prove useful in clarifying complex drug-transporter interactions without some of the limitations of current chemical or genetic knockdown approaches.

Pharmacokinetic interaction of the antiparasitic agents ivermectin and spinosad in dogs.

Neurological side effects consistent with ivermectin toxicity have been observed in dogs when high doses of the common heartworm prevention agent ivermectin are coadministered with spinosad, an oral flea prevention agent. Based on numerous reports implicating the role of the ATP-binding cassette drug transporter P-glycoprotein (P-gp) in ivermectin efflux in dogs, an in vivo study was conducted to determine whether ivermectin toxicity results from a pharmacokinetic interaction with spinosad. Beagle dogs were randomized to three groups treated orally in parallel: Treatment group 1 (T01) received ivermectin (60 µg/kg), treatment group 2 (T02) received spinosad (30 mg/kg), and treatment group 3 (T03) received both ivermectin and spinosad. Whereas spinosad pharmacokinetics were unchanged in the presence of ivermectin, ivermectin plasma pharmacokinetics revealed a statistically significant increase in the area under the curve (3.6-fold over the control) when ivermectin was coadministered with spinosad. The majority of the interaction is proposed to result from inhibition of intestinal and/or hepatic P-gp-mediated secretory pathways of ivermectin. Furthermore, in vitro Transwell experiments with a human multidrug resistance 1-transfected Madin-Darby canine kidney II cell line showed polarized efflux at concentrations ≤ 2 µM, indicating that spinosad is a high-affinity substrate of P-gp. In addition, spinosad was a strong inhibitor of the P-gp transport of digoxin, calcein acetoxymethyl ester (IC(50) = 3.2 µM), and ivermectin (IC(50) = 2.3 µM). The findings suggest that spinosad, acting as a P-gp inhibitor, increases the risk of ivermectin neurotoxicity by inhibiting secretion of ivermectin to increase systemic drug levels and by inhibiting P-gp at the blood-brain barrier.

Two branched polar groups and polar linker moieties of thiophene amide derivatives are essential for MRP2/ABCC2 recognition.

Previously we demonstrated that the torsion angle between two biphenyl rings forming a three-dimensional conformation is the determinant factor for multi-drug resistance protein 2 (Mrp2/Abcc2) interaction [1]. More recently, we reported a heterocyclic compound, 1-(1-(4-bromophenyl)-3-carbamoyl-1H-pyrazol-4-yl) urea that shares the polar head groups with the biphenyl-substituted heterocycles, is highly secreted from bile by Mrp2/Abcc2, [2]. Collectively we hypothesized that the two branched polar groups and linkers might be essential with proposed Mrp2/Abcc2 recognition fitting in two primarily positive regions deep in the binding site. To test the hypothesis, a discovery lead compound (Compound 1) was examined to confirm the Mrp2/Abcc2 involvement resulting in hepatobiliary secretion in rats. The structural requirement of Mrp2/Abcc2 recognition was further explored in a series of thiophene amides derivatives divided into eight structural classes, with structural changes focused on the amide linker orientation or substitution from amide and sulfonamide to alkene, alkane, or alkyne linkers. In Caco-2 cell bidirectional transport assays and Mrp2/Abcc2 membrane vesicle uptake assays, the involvement of Mrp2/Abcc2 mediated transport was confirmed in structural classes 1 - 5, which contains polar amide or sulfonamide linker, but not in classes 6 - 8 with non-polar aliphatic linker. The Mrp2/Abcc2 recognition showed strong correlation with structural descriptors in predictive Bayesian model, as well as with polar surface area and lipophilicity (LogP). The result provided valuable information for predicting transporter recognition in silico, for improved predictions of transporter involved ADME in early drug discovery.

Preclinical and clinical evidence for the collaborative transport and renal secretion of an oxazolidinone antibiotic by organic anion transporter 3 (OAT3/SLC22A8) and multidrug and toxin extrusion protein 1 (MATE1/SLC47A1).

N-({(5S)-3-[4-(1,1-dioxidothiomorpholin-4-yl)-3,5-difluorophenyl]-2-oxo-1,3-oxazolidin-5-yl}methyl)acetamide (PNU-288034), an oxazolidinone antibiotic, was terminated in phase I clinical development because of insufficient exposure. Analysis of the drug pharmacokinetic and elimination profiles suggested that PNU-288034 undergoes extensive renal secretion in humans. The compound was well absorbed and exhibited approximately linear pharmacokinetics in the oral dose range of 100 to 1000 mg in human. PNU-288034 was metabolically stable in liver microsomes across species, and unchanged drug was cleared in the urine by an apparent active renal secretion process in rat and monkey (two to four times glomerular filtration rate) but not dog. In vitro studies conducted to characterize the transporters involved demonstrated PNU-288034 uptake by human organic anion transporter 3 (OAT3; K(m) = 44 +/- 5 microM) and human multidrug and toxin extrusion protein 1 (hMATE1; K(m) = 340 +/- 55 microM). The compound was also transported by multidrug resistance P-glycoprotein and breast cancer resistance protein. In contrast, human organic cation transporter 2, human OAT1, and hMATE2-K did not transport PNU-288034. Coadministration of PNU-288034 and the OAT3 inhibitor probenecid significantly increased PNU-288034 plasma area under the curve (170%) and reduced both plasma and renal clearance in monkey. Coadministration of PNU-288034 and cimetidine, a MATE1 inhibitor, also reduced plasma clearance in rat to a rate comparable with probenecid coadministration. Collectively, our results demonstrated a strong in vitro-in vivo correlation for active renal secretion coordinated through the vectorial transport process of OAT3 and MATE1, which ultimately resulted in limiting the systemic exposure of PNU-288034.

Evaluation of drug transporter interactions in drug discovery and development.

Drug transporters play an important role in the absorption, distribution, excretion and toxicity of both endogenous and exogenous compounds. Transporters may act as physiological 'gatekeepers' in the regulation of the pharmacological and/or toxicological effects of drugs by limiting distribution to tissues responsible for their effect and/or toxicity. This review will first provide a brief outline of the characteristics of membrane bound drug transporter families and their respective roles in regulating drug pharmacokinetics. This background then provides the context for a discussion of the characterization of a drug candidate as a substrate, inhibitor and/or inducer of drug transporter(s), followed by an assessment of the in vitro and in vivo preclinical methods used in drug discovery and development for screening molecules to identify potential transporter interactions. Finally, specific examples of the translation of in vitro findings to the in vivo effects are discussed to link the current understanding of the impact of drug transporters to clinical pharmacology. Thus, the goal is to provide the drug discovery scientist with a cadre of concepts, strategies, and tools for ultimately making rational decisions in drug design and delivery resulting in the optimization of drug concentrations at the target of pharmacology.

Saturation of multidrug-resistant protein 2 (mrp2/abcc2)-mediated hepatobiliary secretion: nonlinear pharmacokinetics of a heterocyclic compound in rats after intravenous bolus administration.

Multidrug-resistant protein 2 (MRP2/ABCC2), expressed on the canalicular membrane of hepatocytes, mediates the secretion of conjugated or nonconjugated compounds into bile and plays an important role in physiology and drug elimination. A heterocyclic compound, BPCPU [1-(1-(4-bromophenyl)-3-carbamoyl-1H-pyrazol-4-yl) urea], which was metabolically stable in vitro in rat liver microsomes and freshly isolated rat hepatocytes, demonstrated a saturable nonlinear pharmacokinetic profile in the rat. Polarized efflux was observed for this compound in Caco-2 cells, with a low K(m) = 1.06 +/- 0.06 microM. The Caco-2 efflux was dose-dependent and saturable. Coadministration of 25 microM MK571 ([3-[[3-[2-(7-chloroquinolin-2-yl)vinyl]phenyl]-(2-dimethylcarbamoylethylsulfanyl)methylsulfanyl] propionic acid]), an MRP inhibitor, blocked the polarized efflux in Caco-2 cells. In contrast, this compound did not inhibit calcein efflux in MRP2 gene-transfected Madin-Darby canine kidney cells, suggesting that it is a substrate, not an inhibitor, of the MRP2/ABCC2 transporter. To investigate the mechanism for the nonlinear pharmacokinetics, bile duct-cannulated rats were used to obtain time profiles of plasma concentration, biliary, and urinary excretion after intravenous administration at various doses. The plasma clearance increased remarkably with decreased dose, from 1.5 ml/min/kg at 5 mg/kg to 14.9 ml/min/kg at 0.05 mg/kg. A dose-dependent biliary excretion also was observed. The results revealed that saturation of hepatobiliary secretion played a role in the dose-dependent changes in total body clearance and biliary clearance. Saturating concentrations of the Mrp2/Abcc2 substrate, BPCPU, causing decreased hepatobiliary clearance could be the major cause for the nonlinear pharmacokinetics observed in rats.