Membrane Transporters in Drug Development

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Abstract

Membrane transporters have wide, but specific tissue distributions. They can impact on multiple endogenous and xenobiotic processes. Knowledge and awareness within the pharmaceutical industry of their impact on drug absorption, distribution, metabolism and elimination (ADME) and drug safety is growing rapidly. Clinically important transporter-mediated drug–drug interactions (DDIs) have been observed. Up to nine diverse transporters are implicated in the DDIs of a number of widely prescribed drugs, posing a significant challenge to the pharmaceutical industry. There is a complex interplay between multiple transporters and/or enzymes in the ADME and pharmacogenomics of drugs. Integrating these different mechanisms to understand their relative contributions to ADME is a key challenge.

Many different factors complicate the study of membrane transporters in drug development. These include a lack of specific substrates and inhibitors, non-standard in vitro tools, and competing/complementary mechanisms (e.g. passive permeability and metabolism).

Discovering and contextualizing the contribution of membrane transporters to drug toxicity is a significant new challenge.

Drug interactions with key membrane transporters are routinely assessed for central nervous system (CNS) drug discovery therapies, but are not generally considered across the wider drug discovery. But, there is interest in utilizing membrane transporters as drug delivery agents.

Computational modeling approaches, notably physiology-based/pharmacokinetic (PB/PK) modeling are increasingly applied to transporter interactions, and permit integration of multiple ADME mechanisms. Because of the range of tissues and transporters of interest, robust transporter, in vitro to in vivo, scaling factors are required. Empirical factors have been applied, but absolute protein quantitation will probably be required.

Introduction

Compartmentalization at intracellular, cellular and tissue levels is fundamental to the function and well being of living organisms. Higher organisms contain a complex system of physical barriers, which help to control systemic, tissue and cellular exposure to both xenobiotics and endogenous molecules. Specific mechanisms have evolved which selectively absorb nutrients and excrete waste products across these barriers, and to ensure that endogenous substances are maintained at normal levels in the body. These same systems can also modulate the absorption, distribution, metabolism and elimination (ADME) of xenobiotic substances. Molecules with a net charge (positive or negative) will have restricted access to cells and tissues, unless facilitated by uptake transporters, because of their inability to cross the plasma membrane. Conversely highly permeable molecules should generally have good penetration, unless restricted by the activity of efflux transporters. Most endogenous molecules exist as charged species. Small changes in pH, differences in protein binding and/or relative solubility between extra- and intracellular milieu may also influence the net concentrative effect of a transporter. Both uptake and efflux transporters are present in most, if not all, cell types. Whilst the overall transporter complement will influence intracellular drug concentrations, their effects may also be quite subtle. The fundamental action of a membrane transporter protein is either to maintain a substrate in equilibrium, or to establish a concentration gradient across a membrane, or tissue barrier, which could not otherwise be achieved.

There are approximately 400 transporter-like genes expressed in humans. These are categorized into two major superfamilies: the solute carrier (SLC) (Heddiger, 2010) and ATP-binding cassette (ABC) transporters (Muller, 2006). The ABC superfamily is significantly smaller (48 members) than the SLC (over 300 members). Both superfamilies are further categorized, based on similarity of function and/or gene sequence. Both transporter superfamilies appear to have broad substrate specificities, ranging from metal ion transport, bile salts, sugars, hormones, amino and nucleic acids, small peptides and nucleosides, and of course, xenobiotics. All the ABC and many of the SLC transporters behave as active transporters. It is convenient to think of ABC transporters as maintainers of low intracellular concentration of their substrates (i.e. efflux transporters), and SLC transporters as establishers of high intracellular concentrations (i.e. uptake transporters), but there are exceptions. ABC transporters (e.g. P-glycoprotein, also referred to as Pgp or MDR1) directly hydrolyze ATP, SLC transporters create concentration gradients by co-transport and/or exchange of ions, or act as facilitative transporters. Although it is generally accepted that Pgp (and perhaps other ABC transporters) takes its substrates from the plasma membrane, and solute transporters from the free fraction in the blood or cytosol, crystal structures are not available and so the precise mechanisms involved are still a matter of investigation.

Section snippets

Location, Orientation, and Function

Membrane transporters are membrane bound proteins with multiple trans-membrane spanning domains and specific cellular locations and membrane orientations, which define their tissue/cellular function. Knowledge of transporter tissue distribution and cellular localization is essential if one is to understand the role of a particular transporter in a given organ or cell. For example, Pgp is a luminally expressed efflux transporter. It is widely expressed in tissues, most notably in the

Historical Perspective

Active drug transport in vivo was first observed many decades ago. It was of particular interest for renally cleared molecules where altered renal function was associated with disease states (e.g. uric acid clearance and gout; Gutman and Yu, 1957, Gutman and Yu, 1958). Although the precise mechanisms remained unknown, in the 1950s scientists and clinicians successfully extended the elimination half-life of, the then rare drug, penicillin to extend its therapeutic window by co-administration of

Studying and Contextualizing Transporter Interactions: Application and Interpretation of Data

Currently, nine membrane transporters (Pgp, BCRP, OATP1B1, OATP1B3, OCT1, OCT2, OAT1, OAT3, and BSEP) are considered by transporter DDI specialists and/or regulators as important DDI investigation targets for new drugs. The International Transporter Consortium (ITC) have endeavored to contextualize the risks posed through DDI (Giacomini et al., 2010), and to provide both commercial organizations and regulatory bodies with guidelines on when and how to investigate transporter DDI liabilities.

The Transporter Toolkit, its Opportunities and Challenges

Although the transporter toolkit is extensive, the availability, utility, and throughput of the tools is mixed, and translation of output for human outcomes is variable (Table II). The experimental study of transporters is probably the most challenging of the DMPK sciences for a host of reasons:

  • The transporter must be expressed in a membrane, in the correct orientation, and if in a polarized system, at the correct location on the plasma membrane.

  • Drug substance must be measured either

Conclusion

Knowledge and awareness of drug transporters and their impact on drug ADME and safety has grown rapidly across the pharmaceutical industry, the scientific community, and in the clinic. Transporter drug interactions have been demonstrated clinically, and are now accepted as significant risk factors for drug development and therapeutic use. Regulatory expectations are high, and pharmaceutical companies are currently expected to provide interaction information for at least nine drug transporters

Acknowledgments

The author would like to acknowledge former colleagues at GlaxoSmithKline R&D Ltd. for their many stimulating and knowledgeable discussions on membrane transporter mechanisms, their investigation and understanding, particularly Joseph Polli, Harma Ellens, Kathryn Kenworthy, Joan Humphreys, Grant Generaux, James Clarke, Catherine Cartwright, Michael Hobbs and Catherine Booth-Genthe.

Conflict of Interest: The author is an independent DMPK consultant, and is a former employee of GlaxoSmithKline R&D

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