Current Pharmaceutical Design - Volume 20, Issue 10, 2014

The blood-brain barrier (BBB) and blood-cerebrospinal fluid (BCSF) barriers are critical determinants of CNS homeostasis. Additionally, the BBB and BCSF barriers are formidable obstacles to effective CNS drug delivery. These brain barrier sites express putative influx and efflux transporters that precisely control permeation of circulating solutes including drugs. The study of transporters has enabled a shift away from “brute force” approaches to delivering drugs by physically circumventing brain barriers towards chemical approaches that can target specific compounds of the BBB and/or BCSF barrier. However, our understanding of transporters at the BBB and BCSF barriers has primarily focused on understanding efflux transporters that efficiently prevent drugs from attaining therapeutic concentrations in the CNS. Recently, through the characterization of multiple endogenously expressed uptake transporters, this paradigm has shifted to the study of brain transporter targets that can facilitate drug delivery (i.e., influx transporters). Additionally, signaling pathways and trafficking mechanisms have been identified for several endogenous BBB/BCSF transporters, thereby offering even more opportunities to understand how transporters can be exploited for optimization of CNS drug delivery. This review presents an overview of the BBB and BCSF barrier as well as the many families of transporters functionally expressed at these barrier sites. Furthermore, we present an overview of various strategies that have been designed and utilized to deliver therapeutic agents to the brain with a particular emphasis on those approaches that directly target endogenous BBB/BCSF barrier transporters.

Since the discovery of P-glycoprotein (P-gp) in brain microvessels composing the human blood-brain barrier (BBB), ATPbinding cassette (ABC) transporters have been recognized as bottlenecks in the development and delivery of neuropharmaceuticals. ABC transporters are expressed predominately at the plasma luminal membrane of brain capillary endothelial cells. These ABC transporters are responsible for the efflux of their substrates from the endothelial cells to the bloodstream against the concentration gradient and thus limit the entry of some drugs within the central nervous system (CNS). Advanced quantitative molecular biology tools allowed gene and protein quantification of the components of microvessels isolated from different species including human. Recently, positron emission tomography using radiolabelled probes that are substrates of ABC transporters allowed the determination of their functional activity at the human BBB. Here, we summarized new information regarding the relative expression, substrate recognition pattern for CNS drugs and functional activity of ABC transporters that are quantitatively expressed at the human BBB.

At the blood-brain barrier, ATP-binding cassette (ABC) transporters, such as, P-glycoprotein (MDR1, ABCB1) and breast cancer related protein (BCRP, ABCG2) limit CNS uptake of foreign chemicals. Thus, they are neuroprotective, but they also distinguish poorly between neurotoxicants and therapeutic drugs. So they are major obstacles to CNS pharmacotherapy. The present review is focused on new findings in animal models in vitro and in vivo showing that basal transport activity of P-glycoprotein and Bcrp can be rapidly and transiently reduced through targeting of specific signaling pathways within the brain capillary endothelium. Three pathways have been identified: estrogen signaling to Bcrp, vascular endothelial growth factor signaling to P-glycoprotein and TNFα/PKC/ sphingolipid signaling to P-glycoprotein. Translation of these results to the clinic could provide improved pharmacotherapy for a number of CNS diseases, including, brain cancer, neuroAIDS and epilepsy.

A major function of the blood brain barrier (BBB) and blood cerebrospinal fluid barrier (BCSFB) is to exert selective control over the flux of organic cations and anions into and out of the CNS compartment. These barriers are dynamic tissues that accomplish this task by expressing dozens of transporter proteins representing numerous transporter families. One such family, belonging to the Solute Carrier (SLC) superfamily, is the organic cation/anion/zwitterion (SLC22) family of transporters, which includes the organic cation transporters (OCTs/OCTNs) and organic anion transporters (OATs). SLC22 transporters interact with a broad range of compounds that include drugs of abuse, environmental toxins/toxicants, opioid analgesics, antidepressant and anxiolytic agents and neurotransmitters and their metabolites. Defining the transport mechanisms controlling the CNS penetration, disposition and clearance of such compounds is fundamental to advancing our understanding of the underlying mechanisms that regulate CNS homeostasis and impact neuronal health. Such information might help direct efforts to improve the efficacy and clinical outcomes of current and future therapeutic agents used in the treatment of CNS disorders. This review focuses on highlighting the identification of the SLC22 transporter family, current knowledge of OCT and OAT expression within the CNS (including brain capillaries, choroid plexus and brain regions relevant to monoaminergic neuronal signaling), and recent data regarding behavioral changes related to mood and anxiety disorders and altered responses to stimulants and antidepressants in SLC22 loss of functions models (knockout/knockdown). In vitro and in vivo evidence of SLC22 localization and transport characteristics within the CNS compartment are summarized.

Monocarboxylate transporters (MCTs) are known to mediate the transport of short chain monocarboxylates such as lactate, pyruvate and butyrate. Currently, fourteen members of this transporter family have been identified by sequence homology, of which only the first four members (MCT1- MCT4) have been shown to mediate the proton-linked transport of monocarboxylates. Another transporter family involved in the transport of endogenous monocarboxylates is the sodium coupled MCTs (SMCTs). These act as a symporter and are dependent on a sodium gradient for their functional activity. MCT1 is the predominant transporter among the MCT isoforms and is present in almost all tissues including kidney, intestine, liver, heart, skeletal muscle and brain. The various isoforms differ in terms of their substrate specificity and tissue localization. Due to the expression of these transporters in the kidney, intestine, and brain, they may play an important role in influencing drug disposition. Apart from endogenous short chain monocarboxylates, they also mediate the transport of exogenous drugs such as salicylic acid, valproic acid, and simvastatin acid. The influence of MCTs on drug pharmacokinetics has been extensively studied for γ-hydroxybutyrate (GHB) including distribution of this drug of abuse into the brain and the results will be summarized in this review. The physiological role of these transporters in the brain and their specific cellular localization within the brain will also be discussed. This review will also focus on utilization of MCTs as potential targets for drug delivery into the brain including their role in the treatment of malignant brain tumors.

The delivery of many drugs to the central nervous system (CNS) is limited due to the restrictive nature of the blood-brain barrier (BBB). The reduced paracellular diffusion and the presence of various drug efflux transporters in the brain microvessel endothelial cells forming the BBB make effective treatment of brain tumors with chemotherapeutic agents particularly problematic. While Pglycoprotein (P-gp) plays an important role in limiting BBB permeability of chemotherapeutic agents, other drug efflux transporters such as breast cancer resistance protein (BCRP) and multidrug resistance-associated proteins (MRPs) are likely to impact on chemotherapeutic levels within the brain and brain tumor. The current review examines the restrictive role that drug efflux transporters have in the delivery of chemotherapeutic agents to the brain. Consideration of different approaches taken to minimize the impact of drug efflux transporters in the BBB and improve chemotherapeutic response in treating brain tumors is also discussed.

Cerebral ischemia is one of the major causes of disability worldwide. In cerebral ischemic stroke, occlusion of a major cerebral artery by an embolus or local thrombosis can result in transient or permanent reduction of cerebral blood flow to a portion of the brain, resulting in deprivation of glucose and oxygen. Since the brain relies on a continuous supply of nutrients and ions via mostly carrier mediated processes across the blood-brain barrier (BBB), any irregularity in these transport mechanisms dramatically affects neuronal function and outcome after acute and chronic stroke. Despite numerous encouraging breakthroughs in preclinical stroke studies that evaluate monotherapies and a prevailing neurocentric approach that has yielded disappointing clinical translation, preclinical stroke studies investigating additional therapeutic targets might be used more effectively in combination with thrombolysis. In this context, this current review discusses the current understanding of dysfunctionional BBB nutrient and ion transport mechanisms involved in stroke pathophsyiology as novel therapeutic approaches. Recognition of the important role of the neurovascular unit in pathophysiology of stroke will provide new opportunities to treat ischemic brain injury, and maintenance of nutrient and ionic homeostasis should facilitate brain repair after stroke.

Molecular transporters that are expressed in brain, especially at the blood-brain barrier (BBB), are increasingly recognized as possible therapeutic targets in the treatment of neurodegenerative disorders such as Alzheimer’s disease and Parkinson’s disease. Some ATP-binding cassette (ABC) transporters, particularly P-glycoprotein (ABCB1), MRP1 (ABCC1) and BCRP (ABCG2), have been implicated in the clearance of neurotoxic polypeptides that characteristically accumulate in the brain, such as amyloid-β (Aβ) peptides in Alzheimer’s disease. Several lines of evidence also implicate lipid transporters of the A-branch of ABC transporters in pathogenesis. Induction of transporters via the activation of specific nuclear receptors may represent a novel approach to restoring diminished BBB function. On the other hand, transporters in the brain capillary endothelium regulate the permeation of therapeutic compounds into the brain. In addition to the export pumps that limit brain entry of exogenous substances, SLC-type uptake transporters, especially of the OCT (SLC22A) family, are of potential relevance in that they mediate not only the uptake of several drugs used for the treatment of neurodegenerative diseases, but also of certain neurotoxins. Here, we summarize recent findings and novel strategies targeting transporters to reduce brain pathology or to improve drug therapy.

Unfortunately, antiepileptic drug therapy fails to control seizure activity in a relevant percentage of epilepsy patients. Epidemiological data as well as findings in human epileptic tissue and in rodent models indicate that drug resistance is a multi-factorial phenomenon with various factors contributing to therapeutic failure. Enhanced efflux transport of antiepileptic drugs as a consequence of seizure-associated up-regulation of transporters such as P-glycoprotein constitutes one factor discussed in this context. Evidence exists that expression rates of P-glycoprotein correlate with drug response in rodent models and in patients. Moreover, add-on of a Pglycoprotein modulator proved to be efficacious in a rat model of drug-resistant epilepsy. Further proof is obviously needed regarding the relative functional relevance of blood-brain barrier efflux for antiepileptic drug efficacy in epilepsy patients. Ongoing studies with positron emission tomography using transporter substrate radiotracers might provide further information. However, these studies also face major challenges considering the complexity of various factors affecting the kinetics of radiotracers in central nervous system pathologies.

Membrane-associated drug transporters are important determinants of antiretroviral drug disposition in the central nervous system during HIV-1 infection. A number of influx and efflux transport proteins expressed at the blood-brain barrier, blood-cerebrospinal fluid barrier and in brain parenchyma cellular compartments (i.e., astrocytes, microglia) have been implicated in the traffic of many antiretroviral drugs into and out of the brain. In particular, members of the ATP-binding cassette membrane associated transporter superfamily and Solute Carrier family are known to be involved in the efflux and/or influx of drugs, respectively. As a result, changes in the functional expression of these transporters can alter the disposition and distribution of drugs in the brain. Moreover, antiretroviral therapy itself and/or pathological events (i.e., inflammation, oxidative stress) associated with viral infection may affect the functional expression of these transporters. This review summarizes recent knowledge on the role of drug transporters in regulating brain antiretroviral drug transport in the context of HIV-1 infection.

The blood-brain barrier (BBB) represents a large obstacle for the treatment of central nervous system diseases. Targeting endogenous nutrient transporters that transcytose the BBB is one promising approach to selectively and noninvasively deliver a drug payload to the brain. The main limitations of the currently employed transcytosing receptors are their ubiquitous expression in the peripheral vasculature and the inherent low levels of transcytosis mediated by such systems. In this review, approaches designed to increase the repertoire of transcytosing receptors which can be targeted for the purpose of drug delivery are discussed. In particular, combinatorial protein libraries can be screened on BBB cells in vitro or in vivo to isolate targeting peptides or antibodies that can trigger transcytosis. Once these targeting reagents are discovered, the cognate BBB transcytosis system can be identified using techniques such as expression cloning or immunoprecipitation coupled with mass spectrometry. Continued technological advances in BBB genomics and proteomics, membrane protein manipulation, and in vitro BBB technology promise to further advance the capability to identify and optimize peptides and antibodies capable of mediating drug transport across the BBB.

Over the recent years there has been a greater appreciation in the important roles drug transporters play in drug-drug interactions (DDI), safety and effectiveness of drugs. Notable consequence of this recognition includes the white paper published by the International Transporter Consortium (ITC) and the guidance documents drafted by regulatory agencies for investigating transporter-mediated DDIs during drug development. While DDIs as a result of transporter-mediated alterations in drug absorption, disposition, or excretion are typically undesirable, there are exceptions. When specific transporters selectively regulate the exposure of a drug at the site of action and/or toxicity, the use of these transporters as molecular targets has been proposed as a promising strategy for tissue-selective drug delivery to enhance efficacy or mitigate toxicity. Furthermore, membrane transporters play a pivotal role in the transport of nutrients and endogenous compounds into or out of cells to sustain cell survival. Genetic polymorphism of drug transporters as well as transporterinhibiting drugs can alter the transporter functional activity and/or protein expression, causing transporter-specific diseases. Therefore, investigating drug-transporter interactions is a critical aspect in candidate drug selection, in order to enhance the pharmacological effects and/or prevent the unintended off-target toxicity. The goal of this review is to provide the drug discovery scientists with a cadre of concepts beyond the ITC White Paper that facilitate rational drug design for optimal safety and efficacy. To that end, this review focuses on the following aspects: 1) regulatory landscape on drug transporter-mediated DDIs, 2) transporter related organ toxicity, 3) utility of drug transporters for target organ delivery, and 4) to highlight the diseases known thus far that are associated with variants of transporter genes.