Membrane transporters in drug development

Abstract

Membrane transporters can be major determinants of the pharmacokinetic, safety and efficacy profiles of drugs. This presents several key questions for drug development, including which transporters are clinically important in drug absorption and disposition, and which in vitro methods are suitable for studying drug interactions with these transporters. In addition, what criteria should trigger follow-up clinical studies, and which clinical studies should be conducted if needed. In this article, we provide the recommendations of the International Transporter Consortium on these issues, and present decision trees that are intended to help guide clinical studies on the currently recognized most important drug transporter interactions. The recommendations are generally intended to support clinical development and filing of a new drug application. Overall, it is advised that the timing of transporter investigations should be driven by efficacy, safety and clinical trial enrolment questions (for example, exclusion and inclusion criteria), as well as a need for further understanding of the absorption, distribution, metabolism and excretion properties of the drug molecule, and information required for drug labelling.

Figure 1. selected human transport proteins for drugs and endogenous substances

Transporters in plasma membrane domains of intestinal epithelia, hepatocytes, kidney proximal tubules and brain capillary endothelial cells are presented. Those coloured in red indicate that the selected transporters are described in detail in this manuscript. Those coloured in blue indicate that the transport proteins are of importance but are not described in this manuscript. a | Intestinal epithelia contain in their apical (luminal) membrane several uptake transporters including one or more members of the organic anion transporting polypeptide (OATP) family; peptide transporter 1 (PEPT1; SLC15A1); ileal apical sodium/bile acid co-transporter (ASBT; SLC10A2); and monocarboxylic acid transporter 1 (MCT1; SLC16A1). The apical ATP-dependent efflux pumps include multidrug resistance protein 2 (MRP2; ABCC2); breast cancer resistance protein (BCRP; ABCG2); and P-glycoprotein (P-gp; MDR1, ABCB1). The basolateral membrane of intestinal epithelia contains organic cation transporter 1 (OCT1; SLC22A1); heteromeric organic solute transporter (OSTα–OSTβ); and MRP3 (ABCC3). b | Human hepatocyte uptake transporters in the basolateral (sinusoidal) membrane include the sodium/taurocholate co-transporting peptide (NTCP; SLC10A1); three members of the OATP family (OATP1B1 (SLCO1B1), OATP1B3 (SLCO1B3) and OATP2B1 (SLCO2B1)); organic anion transporter 2 (OAT2; SLC22A7) and OAT7 (SLC22A9); and OCT1. Efflux pumps in the hepatocyte basolateral membrane include MRP3, MRP4 (ABCC4) and MRP6 (ABCC6). Apical (canalicular) efflux pumps of the hepatocyte comprise P-gp; bile-salt export pump (BSEP or SPGP; ABCB11); BCRP (ABCG2); and MRP2. In addition, multidrug and toxin extrusion protein 1 (MATE1; SLC47A1) is located in the apical hepatocyte membrane. c | Kidney proximal tubules contain in the apical (luminal) membrane OAT4 (SLC22A11); urate transporter 1 (URAT1; SCL22A12); PEPT1 and PEPT2 (SLC15A2); MRP2 and MRP4; MATE1 and MATE2-K (SLC47A2); P-gp; organic cation/ergothioneine transporter (OCTN1; SLC22A4); and organic cation/carnitine transporter (OCTN2; SLC22A5). Basolateral uptake transporters in proximal tubule epithelia include OATP4C1 (SLCO4C1); OCT2; and OAT1, OAT2 and OAT3 (SLC22A8). d | Apical (luminal) transport proteins of brain capillary endothelial cells contributing to the function of the blood–brain barrier include the uptake transporters OATP1A2 and OATP2B1; and the efflux pumps P-gp, BCRP, MRP4 and MRP5 (ABCC5). Note that localization of transporters to particular membranes and tissues is sometimes controversial; therefore, the International Transporter Consortium erred on the conservative side in only showing the localization of transporters for which good evidence exists.

Figure 2. Decision tree for computer modelling of transporter proteins

Ligand-based methods such as pharmacophore and 3D-quantitative structure–activity relationship modelling (3D-QSAR) will be required when crystal structure templates are not available or can be used to complement existing homology models. Generally, a high-throughput in vitro assay is used to generate a data set of transporter ligands (substrates or inhibitors) and their corresponding activity values (Km/Jmax, Ki, or percentage inhibition). a | Data sets are split into a training set used to construct a model and a test set to validate the model. b | Criteria for model acceptance depend on its ability to successfully predict biological activity of test set molecules. c | Acceptable models may be combined to generate synergistic consensus models. d | To generate homology models with high confidence, the membrane topology of both the target and scaffold proteins must match. Modelling of transporters with divergent topology should be attempted with great caution as it is currently unclear how the arbitrary deletion or insertion of a membrane helix may distort the overall helical packing of membrane proteins. e | To further increase fidelity, models should be optimized using molecular dynamics simulations (MDS), preferably embedded in a lipid bilayer surrounded by an aqueous environment. f | When all criteria are satisfied, homology models may be used to determine regions of substrate binding or interaction and mapping of possible permeation pathways. g | Biochemical validation — for example, substituted cysteine accessibility method (SCAM) — will therefore be needed. TMD, transmembrane domain.