Current Drug Metabolism - Volume 5, Issue 1, 2004

P-glycoprotein / MDR1 was the first member of the ATP-binding cassette (ABC) transporter superfamily to be identified in a eukaryote. In eukaryotes, ABC proteins can be classified into three major groups based on function: transporters, regulators, and channels. MDR1 / P-glycoprotein is a prominent member of eukaryotic export-type ABC proteins. MDR1 / P-glycoprotein extrudes a very wide array of structurally dissimilar compounds, all lipophilic and ranging in mass from approximately 300 to 2000 Da, including cytotoxic drugs that act on different intracellular targets, steroid hormones, peptide antibiotics, immunosuppressive agents, calcium channel blockers, and others. Nucleotide binding and hydrolysis by MDR1 / P-glycorptrotein is tightly coupled with its function, substrate transport. ATP binding and hydrolysis were extensively analyzed with the purified MDR1 / P-glycoprotein. The vanadate-induced nucleotide trapping method was also applied to study the hydrolysis of ATP by MDR1 / P-glycoprotein. When MDR1 hydrolyzes ATP in the presence of excess orthovanadate, an analog of inorganic phosphate, it forms a metastable complex after hydrolysis. Using this method, MDR1 / P-glycoprotein can be specifically photoaffinity-labeled in the membrane, if 8-azido-[α32P]ATP is used as ATP. Visualization of the structure, as well as the biochemical data, is needed to fully understand how MDR1 / Pglycoprotein recognizes such a variety of compounds and how it carries its substrates across the membrane using the energy from ATP hydrolysis. To do so, large amounts of pure and stable proteins are required. Heterologous expression systems, which have been used to express P-glycoprotein, are also described.

The human multidrug resistance gene (MDR1), spanning greater than 200 kb, encodes for the ATP-dependent membrane efflux transporter, P-glycoprotein (Pgp). Significant progress has been made in the discovery of MDR1 polymorphisms and the assessment of allelic frequencies. The search for key genetic determinants that predispose individuals to drugs that are substrates or inhibitors of Pgp has just begun. Reports in the literature, particularly focusing on the C3435T polymorphism, have provided discordant results with respect to functional modification in vitro, and Pgp expression and disposition of probe drugs in vivo. Due to the large size of the MDR1 gene, genotyping based on individual single nucleotide polymorphism (SNPs) analysis is not sufficient to predict functional consequences. Strong linkage disequilibrium has been detected between several MDR1 polymorphisms, and discrepancies in the literature may be due to the focus on the influence of single nucleotide variations instead of on linked nucleotide variations. Multiple SNPs found on the same chromosome are assigned to a specific haplotype, and some attempts have been made to determine the role of MDR1 haplotypes in Pgp variability. Most of the data for MDR1 haplotype have been predicted based on computational or mathematical models. However, molecular haplotyping techniques, analysis of linkages on the same chromosome directly by biophysical and biochemical means, may be needed to characterize haplotypes in individuals with a highly polymorphic and large gene like MDR1. Haplotype identification may prove to be vital in identifying the functional significance of MDR1 polymorphisms on disease susceptibility and drug disposition.

Several members of different families of the ATP-binding cassette (ABC) superfamily of transport proteins are capable of transporting an extraordinarily structurally diverse array of endo- and xenobiotics and their metabolites across cell membranes. Together, these transporters play an important role in the absorption, disposition and elimination of these chemicals in the body. In tumor cells, increased expression of these drug transporters is associated with resistance to multiple chemotherapeutic agents. In this review, current knowledge of the biochemical, physiological and pharmacological properties of nine members of the multidrug resistance protein (MRP)-related ABCC family (MRP1, MRP2, MRP3, MRP4, MRP5, MRP6, MRP7, ABCC11 and ABCC12) as well as the G family member, ABCG2/BCRP, are summarized. A focus is placed on the structural similarities and differences of these drug transporters as well as the molecular determinants of their substrate specificities and transport activities. Factors that regulate expression of the MRP-related proteins and ABCG2/BCRP are also reviewed.

Transporters for organic anions and organic cations in kidney, liver, intestine, brain, and placenta play essential roles in drug disposition. The cloning and characterization of these transporters have significantly advanced our understanding of the molecular and cellular mechanisms of the drug disposition process. This review aims at updating the recent knowledge of general properties, structure-function relationships, and regulation mechanisms of the organic anion transporters (OATs) and the organic cation transporters (OCTs). Such information will be essential for the design and development of new drugs to maximize therapeutic efficacy and minimize drug-induced toxicity as well as unwanted drug-drug interactions.

Nucleoside transporters mediate cellular uptake of physiologic nucleosides for nucleic acid synthesis in the salvage pathways in many cell types. These transporters also play an important role in in vivo disposition and intracellular targeting of many nucleoside analogs used in anticancer and antiviral drug therapy. In mammalian cells, there are two major nucleoside transporter gene families: the equilibrative nucleoside transporters (ENTs) and the concentrative nucleoside transporters (CNTs). The ENTs are facilitated carrier proteins and the CNTs are Na+-dependent secondary active transporters. Recent molecular cloning of a number of ENT and CNT transporters has greatly advanced our understanding of the molecular and cellular mechanisms by which nucleosides and nucleoside analogs are transported across biological membranes. In this manuscript, we review the structure, function, tissue distribution, and cellular localization of various cloned mammalian nucleoside transporters. Information on transporter interaction with various nucleoside drugs and analogs is presented. Current knowledge on the regulation of nucleoside transporters in various cell types and tissues is reviewed. The therapeutic significance of nucleoside transporters is discussed along with emerging data from recent clinical studies.

Proton-coupled peptide transporters, localized at brush-border membranes of intestinal and renal epithelial cells, play important roles in protein absorption and the conservation of peptide-bound amino nitrogen. These transporters also have significant pharmacological and pharmacokinetic relevance to the transport of various peptide-like drugs such as β-lactam antibiotics. The identification and molecular characterization of H+ / peptide cotransporters (PEPT1 and PEPT2) have facilitated the clarification of many aspects of these transporters such as the structure / function relationship and regulation. Recent findings that intestinal PEPT1 can transport L-valine ester prodrugs such as valacyclovir provided a major step forward toward the development of novel drug delivery systems. It has been demonstrated that peptide transporters, which have a similar substrate specificity to PEPT1 and PEPT2, but possess other distinct functional properties, are localized at basolateral membranes of intestinal and renal epithelial cells. This review highlights the recent advances in our knowledge of the cellular and molecular nature of PEPT1, PEPT2 and the basolateral peptide transporters.

The blood-brain barrier (BBB) and blood-CSF barrier (BCSFB) represent the main interfaces between the central nervous system (CNS) and the peripheral circulation. Drug exposure to the CNS is dependent on a variety of factors, including the physical barrier presented by the BBB and the BCSFB and the affinity of the substrate for specific transport systems located at both of these interfaces. It is the aggregate effect of these factors that ultimately determines the total CNS exposure, and thus pharmacological efficacy, of a drug or drug candidate. This review discusses the anatomical and biochemical barriers presented to solute access to the CNS. In particular, the important role played by various efflux transporters in the overall barrier function is considered in detail, as current literature suggests that efflux transport likely represents a key determinant of overall CNS exposure for many substrates. Finally, it is important to consider not only the net delivery of the agent to the CNS, but also the ability of the agent to access the relevant target site within the CNS. Potential approaches to increasing both net CNS and target-site exposure, when such exposure is dictated by efflux transport, are considered.

The oral route of drug administration remains the most popular and convenient route of administration, despite its many shortcomings and challenges. Although the advantages associated with this route far outweigh any limitations, a prominent limitation relates to the interactions of drugs with intestinal membrane transporters. The complexities of these interactions and their impact on drug absorption and absorption variability are only now becoming recognized. The rapidly growing awareness of transporter-mediated secretion, saturable absorption, and even the concerted actions of transporters in intestinal drug absorption and secretion has attracted the attention of pharmaceutical scientists in academia, the pharmaceutical industry and the regulatory agencies. This is evidenced by the recent rapid accumulation of data in the literature, the routine conducting of transport studies in the discovery and development of drugs, and finally by the recognition of the importance of transporter (e.g. P-glycoprotein and OATP) mediated secretion of drugs by regulatory authorities such as the U.S. Food and Drug Administration. In this mini-review, we focus on the handful of absorptive and secretory transporters that have been relatively well studied and illustrate the impact of these intestinal transporters on oral drug absorption using published reports from preclinical and clinical studies.

Any treatment of a pregnant woman with medication (drugs) de facto results in the treatment of her unborn child, even when her unborn child is not the target of drug therapy. This is because, in most instances, the placenta is not a complete barrier to the passage of drugs from the maternal to the fetal compartment. This barrier is in part due to the presence of various efflux transporters in the placenta. The placenta is also richly endowed with influx transporters. In this article, we will review the physiological characteristics of the placenta and how it functions as a barrier to passage of drugs into the fetal compartment. In addition, we will review placental transporters that are important in modulating the exposure of the fetus to drugs and, therefore, the efficacy and toxicity of such drugs towards the fetus.

The article by von Montfoort et al. was inadvertently published in an earlier issue of this journal. The readers are referred to volume 4, issue 3, p. 185-211.