GF120918, a P-glycoprotein modulator, increases the concentration of unbound amprenavir in the central nervous system in rats

ABCB1 and ABCG2 Regulation at the Blood-Brain Barrier: Potential New Targets to Improve Brain Drug Delivery

The drug efflux transporters ABCB1 and ABCG2 at the blood-brain barrier limit the delivery of drugs into the brain. Strategies to overcome ABCB1/ABCG2 have been largely unsuccessful, which poses a tremendous clinical problem to successfully treat central nervous system (CNS) diseases. Understanding basic transporter biology, including intracellular regulation mechanisms that control these transporters, is critical to solving this clinical problem.In this comprehensive review, we summarize current knowledge on signaling pathways that regulate ABCB1/ABCG2 at the blood-brain barrier. In Section I, we give a historical overview on blood-brain barrier research and introduce the role that ABCB1 and ABCG2 play in this context. In Section II, we summarize the most important strategies that have been tested to overcome the ABCB1/ABCG2 efflux system at the blood-brain barrier. In Section III, the main component of this review, we provide detailed information on the signaling pathways that have been identified to control ABCB1/ABCG2 at the blood-brain barrier and their potential clinical relevance. This is followed by Section IV, where we explain the clinical implications of ABCB1/ABCG2 regulation in the context of CNS disease. Lastly, in Section V, we conclude by highlighting examples of how transporter regulation could be targeted for therapeutic purposes in the clinic. SIGNIFICANCE STATEMENT: The ABCB1/ABCG2 drug efflux system at the blood-brain barrier poses a significant problem to successful drug delivery to the brain. The article reviews signaling pathways that regulate blood-brain barrier ABCB1/ABCG2 and could potentially be targeted for therapeutic purposes.

Influence of Massage and Skin Hydration on Dermal Penetration Efficacy of Nile Red from Petroleum Jelly-An Unexpected Outcome

The study aimed at comparing the influence of direct and indirect skin hydration as well as massage on the dermal penetration efficacy of active compounds. Nile red was used as a lipophilic drug surrogate and was incorporated into Vaseline (petroleum jelly). The formulation was applied with and without massage onto either dry skin or pre-hydrated, moist skin. It was expected that the occlusive properties of Vaseline in combination with massage and enhanced skin hydration would cause a superposition of penetration-enhancing effects, which should lead to a tremendous increase in the dermal penetration efficacy of the lipophilic drug surrogate. Results obtained were diametral to the expectations, and various reasons were identified for causing the effect observed. Firstly, it was found that Vaseline undergoes syneresis after topical application. The expulsed mineral oil forms a film on top of the skin, and parts of it penetrate into the skin. The lipophilic drug surrogate, which is dissolved in the mineral oil, enters the skin with the mineral oil, i.e., via a solvent drag mechanism. Secondly, it was found that massage squeezes the skin and causes the expulsion of water from deeper layers of the SC. The expulsed water can act as a water barrier that prevents the penetration of lipophilic compounds and promotes the penetration of hydrophilic compounds. Based on the data, it is concluded that dermal penetration is a complex process that cannot only be explained by Fick's law. It is composed of at least three different mechanisms. The first mechanism is the penetration of active ingredients with their solvents into the skin (convection, solvent drag), the second mechanism is the penetration of the active ingredient via passive diffusion, and the third mechanism can involve local penetration phenomena, e.g., the formation of liquid menisci and particle-associated penetration enhancement, which occur upon the evaporation of water and/or other ingredients from the formulation on top of the skin.

Exclusion of unsuitable CNS drug candidates based on their physicochemical properties and unbound fractions in biomatrices for brain microdialysis investigations

Microdialysis has been the only direct method of continuously measuring the unbound drug concentrations in extracellular fluid at a specific brain region with respect to time in the same animal. However, not every compound is suitable for microdialysis system as demonstrated by their inconsistent "by gain" and "by loss" in-vitro microdialysis probe recoveries leading to over- or under- estimated in-vivo concentrations. Therefore, our current study was proposed aiming to develop simple exclusion criteria for drug candidates that are not suitable for microdialysis system investigation. Through literature research, the properties ((LogP, pKa, water solubility and unbound fraction in plasma and brain) of drugs that have been reported for microdialysis studies were summarized. The exclusion criteria were developed by evaluating the impact of such properties on the consistency of in-vitro "by gain" and "by loss" recoveries of microdialysis probe. As a result, forty-five compounds were identified from literatures, among which doxorubicin, docetaxel, omeprazole, donepezil and phenytoin were found to have inconsistent in-vitro "by gain" and "by loss" microdialysis probe recoveries and subsequently selected for the exclusion criteria analysis. It was found that compounds with limited water solubility (less than 1 g/L) and unbound fraction in plasma (f_u,plasma less than 30%) and brain homogenate (f_u,brain less than 10%) were more likely to have inconsistent "by gain" and "by loss" microdialysis probe recoveries. Our proposed exclusion criteria were further validated using carbamazepine (limited water solubility only), DB213 (limited f_u,brain only) and piperine (both limited water solubility and limited f_u,plasma, f_u,brain). Our current proposed exclusion criteria will help excluding the CNS drug candidates that are highly unlikely suitable for brain microdialysis approach leading to a better success rate in brain microdialysis approach development.

The Brain Exposure Efficiency (BEE) Score

The blood-brain barrier (BBB), composed of microvascular tight junctions and glial cell sheathing, selectively controls drug permeation into the central nervous system (CNS) by either passive diffusion or active transport. Computational techniques capable of predicting molecular brain penetration are important to neurological drug design. A novel prediction algorithm, termed the Brain Exposure Efficiency Score (BEE), is presented. BEE addresses the need to incorporate the role of trans-BBB influx and efflux active transporters by considering key brain penetrance parameters, namely, steady state unbound brain to plasma ratio of drug (K_p,uu) and dose normalized unbound concentration of drug in brain (C_u,b). BEE was devised using quantitative structure-activity relationships (QSARs) and molecular modeling studies on known transporter proteins and their ligands. The developed algorithms are provided as a user-friendly open source calculator to assist in optimizing a brain penetrance strategy during the early phases of small molecule molecular therapeutic design.

HIV-1 Tat and opioids act independently to limit antiretroviral brain concentrations and reduce blood-brain barrier integrity

Poor antiretroviral penetration may contribute to human immunodeficiency virus (HIV) persistence within the brain and to neurocognitive deficits in opiate abusers. To investigate this problem, HIV-1 Tat protein and morphine effects on blood-brain barrier (BBB) permeability and drug brain penetration were explored using a conditional HIV-1 Tat transgenic mouse model. Tat and morphine effects on the leakage of fluorescently labeled dextrans (10-, 40-, and 70-kDa) into the brain were assessed. To evaluate effects on antiretroviral brain penetration, Tat+ and Tat- mice received three antiretroviral drugs (dolutegravir, abacavir, and lamivudine) with or without concurrent morphine exposure. Antiretroviral and morphine brain and plasma concentrations were determined by LC-MS/MS. Morphine exposure, and, to a lesser extent, Tat, significantly increased tracer leakage from the vasculature into the brain. Despite enhanced BBB breakdown evidenced by increased tracer leakiness, morphine exposure led to significantly lower abacavir concentrations within the striatum and significantly less dolutegravir within the hippocampus and striatum (normalized to plasma). P-glycoprotein, an efflux transporter for which these drugs are substrates, expression and function were significantly increased in the brains of morphine-exposed mice compared to mice not exposed to morphine. These findings were consistent with lower antiretroviral concentrations in brain tissues examined. Lamivudine concentrations were unaffected by Tat or morphine exposure. Collectively, our investigations indicate that Tat and morphine differentially alter BBB integrity. Morphine decreased brain concentrations of specific antiretroviral drugs, perhaps via increased expression of the drug efflux transporter, P-glycoprotein.

Fetal Concentrations of Budesonide and Fluticasone Propionate: a Study in Mice

The study goal was to evaluate the transplacental transfer of two corticosteroids, budesonide (BUD) and fluticasone propionate (FP), in pregnant mice and investigate whether P-glycoprotein (P-gp) might be involved in reducing BUD transplacental transfer. Pregnant mice (N = 18) received intravenously either low (104.9 μg/kg) or high (1049 μg/kg) dose of [³H]-BUD or a high dose of [³H]-FP (1590 μg/kg). In a separate experiment, pregnant mice (N = 12) received subcutaneously either the P-gp inhibitor zosuquidar (20 mg/kg) or vehicle, followed by an intravenous infusion of [³H]-BUD (104.9 μg/kg). Total and free (protein unbound) corticosteroid concentrations were determined in plasma, brain, fetus, placenta, kidney, and liver. The ratios of free BUD concentrations in fetus versus plasma K_{(fetus, plasma, u, u)} 0.42 ± 0.17 (mean ± SD) for low-dose and 0.38 ± 0.18 for high-dose BUD were significantly different from K = 1 (P < 0.05), contrary to 0.87 ± 0.25 for FP, which was moreover significantly higher than that for matching high-dose BUD (P < 0.01). The BUD brain/plasma ratio was also significantly smaller than K = 1, while these ratios for other tissues were close to 1. In the presence of the P-gp inhibitor, K_{(fetus, plasma, u, u)} for BUD (0.59 ± 0.16) was significantly increased over vehicle treatment (0.31 ± 0.10; P < 0.01). This is the first in vivo study demonstrating that transplacental transfer of BUD is significantly lower than FP's transfer and that placental P-gp may be involved in reducing the fetal exposure to BUD. The study provides a mechanistic rationale for BUD's use in pregnancy.

Therapeutic Potential and Utility of Elacridar with Respect to P-glycoprotein Inhibition: An Insight from the Published In Vitro, Preclinical and Clinical Studies

The occurrence of efflux mechanisms via Permeability-glycoprotein (P-gp) recognized as an important physiological process impedes drug entry or transport across membranes into tissues. In some instances, either low oral bioavailability or lack of brain penetration has been attributed to P-gp mediated efflux activity. Therefore, the objective of development of P-gp inhibitors was to facilitate the attainment of higher drug exposures in tissues. Many third-generation P-gp inhibitors such as elacridar, tariquidar, zosuquidar, etc. have entered clinical development to fulfil the promise. The body of evidence from in vitro and in vivo preclinical and clinical data reviewed in this paper provides the basis for an effective blockade of P-gp efflux mechanism by elacridar. However, clinical translation of the promise has been elusive not just for elacridar but also for other P-gp inhibitors in this class. The review provides introspection and perspectives on the lack of clinical translation of this class of drugs and a broad framework of strategies and considerations in the potential application of elacridar and other P-gp inhibitors in oncology therapeutics.

Topical Treatment for Cutaneous Leishmaniasis: Dermato-Pharmacokinetic Lead Optimization of Benzoxaboroles

Cutaneous leishmaniasis (CL) is caused by several species of the protozoan parasite Leishmania, affecting an estimated 10 million people worldwide. Previously reported strategies for the development of topical CL treatments have focused primarily on drug permeation and formulation optimization as the means to increase treatment efficacy. Our approach aims to identify compounds with antileishmanial activity and properties consistent with topical administration. Of the test compounds, five benzoxaboroles showed potent activity (50% effective concentration [EC50] < 5 μM) against intracellular amastigotes of at least one Leishmania species and acceptable activity (20 μM < EC50 < 30 μM) against two more species. Benzoxaborole compounds were further prioritized on the basis of the in vitro evaluation of progression criteria related to skin permeation, such as the partition coefficient and solubility. An MDCKII-hMDR1 cell assay showed overall good permeability and no significant interaction with the P-glycoprotein transporter for all substrates except LSH002 and LSH031. The benzoxaboroles were degraded, to some extent, by skin enzymes but had stability superior to that of para-hydroxybenzoate compounds, which are known skin esterase substrates. Evaluation of permeation through reconstructed human epidermis showed LSH002 to be the most permeant, followed by LSH003 and LSH001. Skin disposition studies following finite drug formulation application to mouse skin demonstrated the highest permeation for LSH001, followed by LSH003 and LSH002, with a significantly larger amount of LSH001 than the other compounds being retained in skin. Finally, the efficacy of the leads (LSH001, LSH002, and LSH003) against Leishmania major was tested in vivo LSH001 suppressed lesion growth upon topical application, and LSH003 reduced the lesion size following oral administration.

Nanocarriers for brain specific delivery of anti-retro viral drugs: challenges and achievements

HIV/AIDS is a global pandemic and the deleterious effects of human immunodeficiency virus in the brain cannot be overlooked. Though the current anti-retro viral therapy is able to reduce the virus load in the peripheral tissues of the body, the inability of the anti-retro viral drugs to cross the blood brain barrier, as such, limits its therapeutic effect in the brain. The development of newer, successful nanoparticulate drug delivery systems to enhance the feasibility of the anti-retro viral drugs to the brain, offers a novel strategy to treat the AIDS-related neuronal degradation. This review summarised the neuropathogenesis of neuroAIDS, the challenges and achievements made in the delivery of therapeutics across the BBB and the use of nanocarriers as a safe and effective way for delivering anti-retro viral drugs to the brain.

ABCG2 and ABCB1 Limit the Efficacy of Dasatinib in a PDGF-B-Driven Brainstem Glioma Model

Dasatinib is a multikinase inhibitor in clinical trials for glioma, and thus far has failed to demonstrate significant efficacy. We investigated whether the ABC efflux transporters ABCG2 and ABCB1 expressed in the blood-brain barrier (BBB), are limiting the efficacy of dasatinib in the treatment of glioma using genetic and pharmacologic approaches. We utilized a genetic brainstem glioma mouse model driven by platelet-derived growth factor-B and p53 loss using abcg2/abcb1 wild-type (ABC WT) or abcg2/abcb1 knockout mice (ABC KO). First, we observed that brainstem glioma tumor latency is significantly prolonged in ABC KO versus ABC WT mice (median survival of 47 vs. 34 days). Dasatinib treatment nearly doubles the survival of brainstem glioma-bearing ABC KO mice (44 vs. 80 days). Elacridar, an ABCG2 and ABCB1 inhibitor, significantly increases the efficacy of dasatinib in brainstem glioma-bearing ABC WT mice (42 vs. 59 days). Pharmacokinetic analysis demonstrates that dasatinib delivery into the normal brain, but not into the tumor core, is significantly increased in ABC KO mice compared with ABC WT mice. Surprisingly, elacridar did not significantly increase dasatinib delivery into the normal brain or the tumor core of ABC WT mice. Next, we demonstrate that the tight junctions of the BBB of this model are compromised as assessed by tissue permeability to Texas Red dextran. Finally, elacridar increases the cytotoxicity of dasatinib independent of ABCG2 and ABCB1 expression in vitro In conclusion, elacridar improves the efficacy of dasatinib in a brainstem glioma model without significantly increasing its delivery to the tumor core. Mol Cancer Ther; 15(5); 819-29. ©2016 AACR.

HIV and viral protein effects on the blood brain barrier

The blood brain barrier (BBB) plays a critical role in the normal physiology of the central nervous system (CNS) by regulating what crosses from the periphery into the brain. Damage to the BBB or alterations in transport systems may mediate the pathogenesis of many CNS diseases, including HIV-associated CNS dysfunction. HIV-1 infection can result in neuropathologic changes in about one half of infected individuals and also can result in damage to the BBB. HIV-1 and the HIV-1 viral proteins, Tat and gp120, cause alterations in the integrity and function of the BBB through both paracellular and transcellular mechanisms. The current review discusses HIV and viral protein-mediated injury to the BBB with a focus on the effects on tight junction proteins, barrier permeability, and drug efflux proteins.

Antiretroviral bioanalysis methods of tissues and body biofluids

Research in the many areas of HIV treatment, eradication and prevention has necessitated measurement of antiretroviral (ARV) concentrations in nontraditional specimen types. To determine the knowledgebase of critical details for accurate bioanalysis, a review of the literature was performed and summarized. Bioanalytical assays for 31 ARVs, including metabolites, were identified in 205 publications measuring various tissues and biofluids. 18 and 30% of tissue or biofluid methods, respectively, analyzed more than one specimen type; 35-37% of the tissue or biofluid methods quantitated more than one ARV. 20 and 76% of tissue or biofluid methods, respectively, were used for the analysis of human specimens. HPLC methods with UV detection predominated, but chronologically MS detection began to surpass. 40% of the assays provided complete intra- and inter-assay validation data, but only 9% of publications provided any stability data with even less for the prevalent ARV in treatments.

Plasma protein binding: from discovery to development

The importance of plasma protein binding (PPB) in modulating the effective drug concentration at pharmacological target sites has been the topic of significant discussion and debate amongst drug development groups over the past few decades. Free drug theory, which states that in absence of energy-dependent processes, after steady state equilibrium has been attained, free drug concentration in plasma is equal to free drug concentration at the pharmacologic target receptor(s) in tissues, has been used to explain pharmacokinetics/pharmacodynamics relationships in a large number of cases. Any sudden increase in free concentration of a drug could potentially cause toxicity and may need dose adjustment. Free drug concentration is also helpful to estimate the effective concentration of drugs that potentially can precipitate metabolism (or transporter)-related drug-drug interactions. Disease models are extensively validated in animals to progress a compound into development. Unbound drug concentration, and therefore PPB information across species is very informative in establishing safety margins and guiding selection of First in Human (FIH) dose and human efficacious dose. The scope of this review is to give an overview of reported role of PPB in several therapeutic areas, highlight cases where PPB changes are clinically relevant, and provide drug metabolism and pharmacokinetics recommendations in discovery and development settings.

The importance of plasma protein binding in drug discovery

Plasma protein binding of drugs is a well-recognised phenomena, but it is only recently that the implications for drug action in vivo have been fully appreciated. Plasma proteins, by virtue of their high concentration, control the free drug concentration in plasma and in compartments in equilibrium with plasma, thereby, effectively attenuating drug potency in vivo. The historical background and thermodynamic basis for the 'Free Drug Principle' is presented, along with special considerations for intracellular targets, deep compartments and α1-acid glycoprotein binding. Real and apparent exceptions to the principle are discussed along with a survey of citations from the recent medicinal chemistry literature.

Development and evaluation of a novel microemulsion formulation of elacridar to improve its bioavailability

The study objective was to develop a formulation of elacridar to overcome its dissolution-rate-limited bioavailability. Elacridar is a P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) inhibitor that has been used to improve the brain distribution of drugs that are substrates of P-gp and BCRP. The chronic use of elacridar is restricted because of the poor solubility leading to poor oral bioavailability. A microemulsion formulation using Cremophor EL, Carbitol, and Captex 355 (6:3:1) was developed. The elacridar microemulsion was effective in the inhibition of P-gp and Bcrp in Madin-Darby canine kidney II-transfected cells. Friend Leukemia Virus Strain B (FVB) mice were used to determine the bioavailability of elacridar after a 10 mg/kg dose of elacridar in the microemulsion, intraperitoneally (i.p.) and orally (p.o.); and the absolute bioavailability was determined to be 1.3 and 0.47, respectively. Coadministration of elacridar microemulsion i.p. with p.o. erlotinib in FVB mice improved the erlotinib brain penetration threefold. The current study shows that a microemulsion formulation of elacridar is effective in improving the bioavailability of elacridar and is an effective inhibitor of P-gp and Bcrp, in vitro and in vivo. It offers an alternative to the suspension and allows a decrease in the dose required to achieve a significant inhibitory effect at the blood-brain barrier.

Protein Adsorption Patterns and Analysis on IV Nanoemulsions-The Key Factor Determining the Organ Distribution

Intravenous nanoemulsions have been on the market for parenteral nutrition since the 1950s; meanwhile, they have also been used successfully for IV drug delivery. To be well tolerable, the emulsions should avoid uptake by the MPS cells of the body; for drug delivery, they should be target-specific. The organ distribution is determined by the proteins adsorbing them after injection from the blood (protein adsorption pattern), typically analyzed by two-dimensional polyacrylamide gel electrophoresis, 2-D PAGE. The article reviews the 2-D PAGE method, the analytical problems to be faced and the knowledge available on how the composition of emulsions affects the protein adsorption patterns, e.g., the composition of the oil phase, stabilizer layer and drug incorporation into the interface or oil core. Data were re-evaluated and compared, and the implications for the in vivo distribution are discussed. Major results are that the interfacial composition of the stabilizer layer is the main determining factor and that this composition can be modulated by simple processes. Drug incorporation affects the pattern depending on the localization of the drug (oil core versus interface). The data situation regarding in vivo effects is very limited; mainly, it has to be referred to in the in vivo data of polymeric nanoparticles. As a conclusion, determination of the protein adsorption patterns can accelerate IV nanoemulsion formulation development regarding optimized organ distribution and related pharmacokinetics.

Brain distribution and bioavailability of elacridar after different routes of administration in the mouse

The objective of this study was to determine the bioavailability and disposition of elacridar (GF120918; N-(4-(2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl)phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide) in plasma and brain after various routes of administration in the mouse. Elacridar is a potent inhibitor of P-glycoprotein and breast cancer resistance protein and has been used to examine the influence of these efflux transporters on drug distribution to brain. Friend leukemia virus strain B mice were administered 100 mg/kg elacridar either orally or intraperitoneally. The absolute bioavailability of elacridar after oral or intraperitoneal dosing was determined with respect to an intravenous dose of 2.5 mg/kg. At these doses, the absolute bioavailability was 0.22 for oral administration and 0.01 for intraperitoneal administration. The terminal half-life of elacridar was approximately 4 h after intraperitoneal and intravenous administration and nearly 20 h after oral dosing. The brain-to-plasma partition coefficient (Kp,brain) of elacridar increased as plasma exposure increased, suggesting saturation of the efflux transporters at the blood-brain barrier. The Kp,brain after intravenous, intraperitoneal, and oral dosing was 0.82, 0.43, and 4.31, respectively. The low aqueous solubility and high lipophilicity of elacridar result in poor oral absorption, most likely dissolution-rate-limited. These results illustrate the importance of the route of administration and the resultant plasma exposure in achieving effective plasma and brain concentrations of elacridar and can be used as a guide for future studies involving elacridar administration and in developing formulation strategies to overcome the poor absorption.

Improving the prediction of the brain disposition for orally administered drugs using BDDCS

In modeling blood-brain barrier (BBB) passage, in silico models have yielded ~80% prediction accuracy, and are currently used in early drug discovery. Being derived from molecular structural information only, these models do not take into account the biological factors responsible for the in vivo outcome. Passive permeability and P-glycoprotein (Pgp, ABCB1) efflux have been successfully recognized to impact xenobiotic extrusion from the brain, as Pgp is known to play a role in limiting the BBB penetration of oral drugs in humans. However, these two properties alone fail to explain the BBB penetration for a significant number of marketed central nervous system (CNS) agents. The Biopharmaceutics Drug Disposition Classification System (BDDCS) has proved useful in predicting drug disposition in the human body, particularly in the liver and intestine. Here we discuss the value of using BDDCS to improve BBB predictions of oral drugs. BDDCS class membership was integrated with in vitro Pgp efflux and in silico permeability data to create a simple 3-step classification tree that accurately predicted CNS disposition for more than 90% of 153 drugs in our data set. About 98% of BDDCS class 1 drugs were found to markedly distribute throughout the brain; this includes a number of BDDCS class 1 drugs shown to be Pgp substrates. This new perspective provides a further interpretation of how Pgp influences the sedative effects of H1-histamine receptor antagonists.

Assessment and modulation of phillyrin absorption by P-gp using Caco-2 cells and MDR1-MDCKII cells

The study was to investigate the absorption mechanism and transport modulation of phillyrin by P-gp in Caco-2 cells and MDR1-MDCKII cells. Three concentrations of phillyrin were tested in transport studies. The absorptive transports of phillyrin in the two cell models were not concentration-dependent which indicated passive diffusion as the dominating process in the test concentrations. The absorptive P (app) were 7.15, 6.39 and 10.03 × 10(-6) cm s(-1), respectively, for different concentrations (2.2, 4.8 and 8.4 μg ml(-1)) in Caco-2 cells. And the low absorptive P (app) was consistent with the low oral bioavailability of phillyrin observed in pharmacokinetic experiments. In transport inhibition experiment, the efflux inhibitors, verapamil and GF120918 can increase the absorption of phillyrin in Caco-2 cells which suggested the involvement of efflux transporters. In the further inhibition experiment in MDR1-MDCKII cells, the absorption was greatly increased and the efflux of phillyrin was competitively inhibited by verapamil and GF120918, which confirmed the involvement of P-gp in the efflux of phillyrin.

Discovery of novel orally bioavailable oxaborole 6-carboxamides that demonstrate cure in a murine model of late-stage central nervous system african trypanosomiasis

We report the discovery of novel boron-containing molecules, exemplified by N-(1-hydroxy-1,3-dihydrobenzo[c][1,2]oxaborol-6-yl)-2-trifluoromethylbenzamide (AN3520) and 4-fluoro-N-(1-hydroxy-1,3-dihydrobenzo[c][1,2]oxaborol-6-yl)-2-trifluoromethylbenzamide (SCYX-6759), as potent compounds against Trypanosoma brucei in vitro, including the two subspecies responsible for human disease T. b. rhodesiense and T. b. gambiense. These oxaborole carboxamides cured stage 1 (hemolymphatic) trypanosomiasis infection in mice when administered orally at 2.5 to 10 mg/kg of body weight for 4 consecutive days. In stage 2 disease (central nervous system [CNS] involvement), mice infected with T. b. brucei were cured when AN3520 or SCYX-6759 were administered intraperitoneally or orally (50 mg/kg) twice daily for 7 days. Oxaborole-treated animals did not exhibit gross signs of compound-related acute or subchronic toxicity. Metabolism and pharmacokinetic studies in several species, including nonhuman primates, demonstrate that both SCYX-6759 and AN3520 are low-clearance compounds. Both compounds were well absorbed following oral dosing in multiple species and also demonstrated the ability to cross the blood-brain barrier with no evidence of interaction with the P-glycoprotein transporter. Overall, SCYX-6759 demonstrated superior pharmacokinetics, and this was reflected in better efficacy against stage 2 disease in the mouse model. On the whole, oxaboroles demonstrate potent activity against all T. brucei subspecies, excellent physicochemical profiles, in vitro metabolic stability, a low potential for CYP450 inhibition, a lack of active efflux by the P-glycoprotein transporter, and high permeability. These properties strongly suggest that these novel chemical entities are suitable leads for the development of new and effective orally administered treatments for human African trypanosomiasis.

Concentration-dependent effects and intracellular accumulation of HIV protease inhibitors in cultured CD4 T cells and primary human lymphocytes

BACKGROUND

The intracellular and plasma concentrations of HIV protease inhibitors (HPIs) vary widely in vivo. It is unclear whether there is a concentration-dependent effect of HPIs such that at increasing concentration they may either block their own efflux (leading to 'autoboosting') or influx (leading to saturability/decreased intracellular accumulation).

METHOD

The effects of various concentrations (0-30 microM) of lopinavir, saquinavir, ritonavir and atazanavir on the accumulation of [(14)C]lopinavir, [(3)H]saquinavir, [(3)H]ritonavir and [(3)H]atazanavir, respectively, were investigated in CEM(parental), CEM(VBL) [P-glycoprotein (ABCB1) overexpressing], CEM(E1000) (MRP1 overexpressing) and in peripheral blood mononuclear cells (PBMCs). We also investigated the effects of inhibitors of ABCB1/ABCG2 (tariquidar), ABCC (MK571) and ABCC1/2 (frusemide), singly and in combination with HPIs, on cellular accumulation.

RESULTS

In all the cell lines, with increasing concentration of lopinavir, saquinavir and ritonavir, there was a significant increase in the cellular accumulation of [(14)C]lopinavir, [(3)H]saquinavir and [(3)H]ritonavir. Tariquidar, MK571 and frusemide (alone and in combination with lopinavir, saquinavir and ritonavir) significantly increased the accumulation of [(14)C]lopinavir, [(3)H]saquinavir and [(3)H]ritonavir. Ritonavir (alone or in combination with tariquidar) decreased the intracellular accumulation of [(3)H]ritonavir in PBMCs. Atazanavir decreased the accumulation of [(3)H]atazanavir in a concentration-dependent manner in all of the cells tested.

CONCLUSIONS

There are complex and variable drug-specific rather than class-specific effects of the HPIs on their own accumulation.

Targeting the brain--surmounting or bypassing the blood-brain barrier

The constituents of the blood-brain barrier, including its efflux transporter system, can efficiently limit brain penetration of potential CNS therapeutics. Effective extrusion from the brain by transporters is a frequent reason for the pharmaceutical industry to exclude novel compounds from further development for CNS therapeutics. Moreover, high transporter expression levels that are present in individual patients or may be generally associated with the pathophysiology seem to be a major cause of therapeutic failure in a variety of CNS diseases including brain tumors, epilepsy, brain HIV infection, and psychiatric disorders. Increasing knowledge of the structure and function of the blood-brain barrier creates a basis for the development of strategies which aim to enhance brain uptake of beneficial pharmaceutical compounds. The different strategies discussed in this review aim to modulate blood-brain barrier function or to bypass constituents of the blood-brain barrier.

Targeting regulation of ABC efflux transporters in brain diseases: a novel therapeutic approach

Blood-brain barrier efflux transporters limit the brain penetration and efficacy of various central nervous system drugs. In several CNS diseases, therapy- or pathophysiology-associated transcriptional activation of efflux transporters further strengthens the barrier function. Targeting the regulatory pathways that drive efflux transporter expression in different diseases represents an intriguing approach for prevention of these events thereby promoting delivery to the brain and enhancing or restoring drug efficacy. In particular, the approach holds the promise to preserve basal transporter expression and activity, which is of specific relevance in view of the protective function of efflux transport. The elucidation of the signaling cascades involved in transporter regulation is a major presupposition for the development of preventive strategies. Orphan nuclear receptors as well as the Wnt/beta-catenin signaling pathway have been implicated in drug-induced changes in transporter expression. Targeting these xenobiotic sensors is therefore discussed as a means to optimize brain delivery and therapeutic outcome. Relevant progress has also been made with the identification of key signaling events that drive P-glycoprotein expression in response to pathophysiological mechanisms. In the epileptic brain, complex signaling events involving cyclooxygenase-2 activity trigger P-glycoprotein expression in response to glutamate release and activation of endothelial NMDA receptors. Moreover, reactive oxygen species and inflammatory cytokines have been identified as regulatory factors which might affect P-glycoprotein in several CNS diseases. Recent data substantiated several interesting targets in the respective signaling cascades thereby rendering a basis for the ongoing development of innovative approaches to optimize central nervous system drug brain penetration and efficacy.

Lung microdialysis study of levofloxacin in rats following intravenous infusion at steady state

Lung microdialysis has been used with rats to investigate antibiotic distribution after single-dose administration. However, conducting such experiments after intravenous infusion at steady state would constitute a more convenient alternative, which was evaluated here, using levofloxacin (LVX) as a test compound. Microdialysis probes were inserted in blood and muscle, used as a comparator, between 9:00 a.m. and 11:00 a.m. Intravenous LVX infusion was started 6 h later and maintained until the end of the experiment at a rate of 1.0 mg.h(-1). Lung microdialysis probes were inserted on the morning of the next day. Rats were kept anesthetized during dialysate collection. In vivo probe recoveries were estimated by retrodialysis using a calibrator method, with ciprofloxacin (CIP) as the calibrator. LVX and CIP were analyzed in dialysates by high-performance liquid chromatography. The steady-state tissue-to-blood unbound-drug concentration ratios were 1.00 +/- 0.15 in muscle tissues and 1.06 +/- 0.40 in lungs, suggesting passive distribution of LVX in tissue. Although providing no information on rate of distribution, microdialysis investigations following drug infusion at steady state appear to be an interesting approach for characterization of antibiotic distribution in rat lungs.

Central nervous system pharmacokinetics of the Mdr1 P-glycoprotein substrate CP-615,003: intersite differences and implications for human receptor occupancy projections from cerebrospinal fluid exposures

The central nervous system (CNS) distribution and transport mechanisms of the investigational drug candidate CP-615,003 (N-[3-fluoro-4-[2-(propylamino)ethoxy]phenyl]-4,5,6,7-tetrahydro-4-oxo-1H-indole-3-carboxamide) and its active metabolite CP-900,725 have been characterized. Brain distribution of CP-615,003 and CP-900,725 was low in rats and mice (brain-to-serum ratio < 0.2). Cerebrospinal fluid (CSF)-to-serum ratios of CP-615,003 were 6- to 8-fold lower than the plasma unbound fraction in rats and dogs. In vitro, CP-615,003 displayed quinidine-like efflux in MDR1-expressing Madin-Darby canine kidney II cells. The brain-to-serum ratio of CP-615,003 in mdr1a/1b (-/-) mice was approximately 7 times that in their wild-type counterparts, confirming that impaired CNS distribution was explained by P-gp efflux transport. In contrast, P-gp efflux did not explain the impaired CNS penetration of CP-900,725. Intracerebral microdialysis was used to characterize rat brain extracellular fluid (ECF) distribution. Interestingly, the ECF-to-serum ratio of the P-gp substrate CP-615,003 was 7-fold below the CSF-to-serum ratio, whereas this disequilibrium was not observed for CP-900,725. In a clinical study, steady-state CSF exposures were measured after administration of 100 mg of CP-615,003 b.i.d. The human CSF-to-plasma ratios of CP-615,003 and CP-900,725 were both approximately 10-fold below their ex vivo plasma unbound fractions, confirming impaired human CNS penetration. Preliminary estimates of CNS receptor occupancy from human CSF concentrations were sensitive to assumptions regarding the magnitude of the CSF-ECF gradient for CP-615,003 in humans. In summary, this case provides an example of intersite differences in CNS pharmacokinetics of a P-gp substrate and potential implications for projection of human CNS receptor occupancy of transporter substrates from CSF pharmacokinetic data when direct imaging-based approaches are not feasible.

Tariquidar (XR9576): a P-glycoprotein drug efflux pump inhibitor

P-glycoprotein actively transports structurally unrelated compounds out of cells, conferring the multidrug resistance phenotype in cancer. Tariquidar is a potent, specific, noncompetitive inhibitor of P-glycoprotein. Tariquidar inhibits the ATPase activity of P-glycoprotein, suggesting that the modulating effect is derived from the inhibition of substrate binding, inhibition of ATP hydrolysis or both. In clinical trials, tariquidar is tolerable and does not have significant pharmacokinetic interaction with chemotherapy. In patients, inhibition of P-glycoprotein has been demonstrated for 48 h after a single dose of tariquidar. Studies to assess a possible increase in toxicity of chemotherapy and the impact of P-glycoprotein inhibition on tumor response and patient outcome are ongoing. Tariquidar can be considered an ideal agent for testing the role of P-glycoprotein inhibition in cancer.

Pharmacokinetic considerations in the treatment of CNS tumours

Despite aggressive therapy, the majority of primary and metastatic brain tumour patients have a poor prognosis with brief survival periods. This is because of the different pharmacokinetic parameters of systemically administered chemotherapeutic agents between the brain and the rest of the body. Specifically, before systemically administered drugs can distribute into the CNS, they must cross two membrane barriers, the blood-brain barrier (BBB) and blood-cerebrospinal fluid (CSF) barrier (BCB). To some extent, these structures function to exclude xenobiotics, such as anticancer drugs, from the brain. An understanding of these unique barriers is essential to predict when and how systemically administered drugs will be transported to the brain. Specifically, factors such as physiological variables (e.g. blood flow), physicochemical properties of the drug (e.g. molecular weight), as well as influx and efflux transporter expression at the BBB and BCB (e.g. adenosine triphosphate-binding cassette transporters) determine what compounds reach the CNS. A large body of preclinical and clinical research exists regarding brain penetration of anticancer agents. In most cases, a surrogate endpoint (i.e. CSF to plasma area under the concentration-time curve [AUC] ratio) is used to describe how effectively agents can be transported into the CNS. Some agents, such as the topoisomerase I inhibitor, topotecan, have high CSF to plasma AUC ratios, making them valid therapeutic options for primary and metastatic brain tumours. In contrast, other agents like the oral tyrosine kinase inhibitor, imatinib, have a low CSF to plasma AUC ratio. Knowledge of these data can have important clinical implications. For example, it is now known that chronic myelogenous leukaemia patients treated with imatinib might need additional CNS prophylaxis. Since most anticancer agents have limited brain penetration, new pharmacological approaches are needed to enhance delivery into the brain. BBB disruption, regional administration of chemotherapy and transporter modulation are all currently being evaluated in an effort to improve therapeutic outcomes. Additionally, since many chemotherapeutic agents are metabolised by the cytochrome P450 3A enzyme system, minimising drug interactions by avoiding concomitant drug therapies that are also metabolised through this system may potentially enhance outcomes. Specifically, the use of non-enzyme-inducing antiepileptic drugs and curtailing nonessential corticosteroid use may have an impact.

Intestinal absorption of novel-dipeptide prodrugs of saquinavir in rats

Saquinavir (SQV) was the first human immuno-virus-1 (HIV-1) protease inhibitor approved by FDA. However, P-glycoprotein (P-gp), an efflux pump limits its oral and brain bioavailabilities. The objective of this study is to investigate whether prodrug modification of SQV to dipeptide prodrugs Valine-Valine-Saquinavir (Val-Val-SQV) and Glycine-Valine-Saquinavir (Gly-Val-SQV) targeting intestinal peptide transporter can enhance intestinal permeability of SQV by circumventing P-gp mediated efflux. Single pass intestinal perfusion experiments in rat jejunum were performed to calculate the absorption rate constant and intestinal permeability of SQV, Val-Val-SQV and Gly-Val-SQV. Equimolar concentration (25 microM) of SQV, Val-Val-SQV and Gly-Val-SQV were employed in the perfusion studies. Perfusion experiments were also carried out in the presence of cyclosporine (10 microM) and glycyl-sarcosine (20 mM). Absorption rate constants in rat jejunum (ka) for SQV, Val-Val-SQV and Gly-Val-SQV were found to be 14.1+/-3.4x10(-3), 65.8+/-4.3x10(-3), and 25.6+/-5.7x10(-3) min(-1), respectively. Enhanced absorption of Val-Val-SQV and Gly-Val-SQV relative to SQV can be attributed to their translocation by the peptide transporter in the jejunum. Significant permeability enhancement of SQV across rat jejunum was observed in the presence of cyclosporine 10 microM (P-gp inhibitor). However, permeability of Val-Val-SQV was unchanged in the presence of cyclosporine suggesting lack of any interaction of the prodrug with efflux pump. Intestinal absorption of Val-Val-SQV was significantly inhibited in the presence of gly-sar indicating the involvement of peptide transporter in intestinal absorption. In conclusion, peptide transporter targeted prodrug modification of P-gp substrates could lead to shielding of these drug molecules from efflux pumps.

Role of drug efflux carriers in the healthy and diseased brain

The blood-brain barrier is a natural diffusion barrier, which expresses active carriers extruding drugs on their way to the brain back into the blood against concentration gradients. Whereas these so-called adenosine triphosphate-binding cassette (ABC) transporters prevent the brain entry of toxic compounds under physiological conditions, they complicate pharmacotherapies in neurological disease. Recent observations in animal models of ischemic stroke, drug-resistant epilepsy, and brain cancer showed that the prototype of ABC transporters, ABCB1, is upregulated on brain injury, deactivation of this carrier considerably enhancing the accumulation of neuroprotective, antiepileptic, and chemotherapeutic compounds. These studies provide the proof of concept that the efficacy of brain-targeting drugs may significantly be improved when drug efflux is blocked. Under clinical conditions, efforts currently are made to enhance drug accumulation by selecting new compounds that do not bind to efflux carriers or deactivating ABC transporters by targeted downregulation or pharmacological inhibition. We predict that strategies aiming at circumventing drug efflux may greatly facilitate progress in neurological therapies.

Role of P-glycoprotein in tissue uptake of indinavir in rat

The effect of p-glycoprotein inhibition on tissue distribution of indinavir, an anti-HIV (human immunodeficiency virus) protease inhibitor drug, has been evaluated. Indinavir was co-administered intravenously in rats along with a p-glycoprotein inhibitor, PSC833, and the drug concentrations in plasma and various tissues were determined using a HPLC method. Additionally, initial uptake clearance of indinavir was evaluated in the brain and testes. The highest increasing effect of p-glycoprotein inhibition on the tissue uptake ratios of indinavir was found in central nervous system (CNS). The estimated tissue extraction the drug was indicative of (i) limited drug entry to brain parenchyma, which was increased significantly by p-glycoprotein inhibition, (ii) non-restricted drug entry to testes, heart and spleen, which was increased significantly in the case of heart and decreased in the case of testes and spleen as a result of p-glycoprotein inhibition, and (iii) drug accumulation in liver and small intestine and, to a lesser extent, kidney, which was not affected by p-glycoprotein inhibition. The uptake clearances of indinavir by brain parenchyma in PSC833-treated and control rats were 68.80+/-8.65 and 21.63+/-4.28 micro/min/g and the corresponding values for the testes were 39.84+/-4.90 and 36.65+/-2.54 microl/min/g. The difference was significant only in the case of brain parenchyma (P<0.001). These data showed that p-glycoprotein inhibition increases the CNS uptake of indinavir markedly and has some transient minor effects on drug uptake by some other tissues.

Applicability of reverse microdialysis in pharmacological and toxicological studies

A recent application of microdialysis is the introduction of a substance into the extracellular space via the microdialysis probe. The inclusion of a higher amount of a drug in the perfusate allows the drug to diffuse through the microdialysis membrane to the tissue. This technique, actually called as reverse microdialysis, not only allows the local administration of a substance but also permits the simultaneous sampling of the extracellular levels of endogenous compounds. Local effects of exogenous compounds have been studied in the central nervous system, hepatic tissue, dermis, heart and corpora luteae of experimental animals by means of reverse microdialysis. In central nervous studies, reverse microdialysis has been extensively used for the study of the effects on neurotransmission at different central nuclei of diverse pharmacological and toxicological agents, such as antidepressants, antipsychotics, antiparkinsonians, hallucinogens, drugs of abuse and experimental drugs. In the clinical setting, reverse microdialysis has been used for the study of local effects of drugs in the adipose tissue, skeletal muscle and dermis. The aim of this review is to describe the principles of the reverse microdialysis, to compare the technique with other available methods and finally to describe the applicability of reverse microdialysis in the study of drugs properties both in basic and clinical research.

Dependence of nelfinavir brain uptake on dose and tissue concentrations of the selective P-glycoprotein inhibitor zosuquidar in rats

Most reverse transcriptase and protease inhibitors used in highly active antiretroviral therapy for treating human immunodeficiency virus (HIV) infections exhibit poor penetration into the brain, raising the concern that the brain may be a sanctuary site for the development of resistant HIV variants. This study explores the relationship between the dose and plasma and brain concentrations of zosuquidar and the effect of this selective P-glycoprotein inhibitor on central nervous system penetration of the HIV protease inhibitor nelfinavir maintained at steady state by intravenous infusions in rats. Nelfinavir was infused (10 mg/kg/h) for up to 10 h with or without concurrent administration of an intravenous bolus dose of 2, 6, or 20 mg/kg zosuquidar given at 4 h. Brain tissue and plasma were analyzed for both drug concentrations. Brain tissue/plasma nelfinavir concentration ratios (uncorrected for the vascular contribution) increased nonlinearly with zosuquidar dose from 0.06 +/- 0.03 in the absence of zosuquidar and 0.09 +/- 0.02 between 2 and 6 h after 2 mg/kg zosuquidar to 0.85 +/- 0.19 after 6 mg/kg and 1.58 +/- 0.67 after 20 mg/kg zosuquidar. Zosuquidar brain tissue/plasma concentration ratios exhibited a similar abrupt increase from 2.8 +/- 0.3 after a 2 mg/kg dose to approximately 15 after the 6 and 20 mg/kg doses. The apparent threshold in the plasma concentration of zosuquidar necessary to produce significant enhancement in brain uptake of nelfinavir appears to be close to the plasma concentrations associated with the maximum tolerated dose reported in the literature after repeated dosing of zosuquidar in patients.

A combined cell based approach to identify P-glycoprotein substrates and inhibitors in a single assay

The objective of this project was to develop a cell based in vitro experimental procedure that can differentiate P-glycoprotein (P-gp) substrates from inhibitors in a single assay. Caco-2 cells grown to confluency on 12-well Transwell were used for this study. The efflux permeability (B to A) of P-gp specific probe (viz., digoxin) in the presence of test compounds (e.g. substrates, inhibitors and non-substrates of P-gp) was monitored, and the influx permeability (A to B) of test compounds was evaluated after complete P-gp blockade. Radiolabelled digoxin was added on the basolateral side with buffer on the apical side. The digoxin concentration appearing on the apical side represents digoxin efflux permeability during the control phase (0-1 h period). After 1 h, a test compound (10 microM) was added on the apical side. The reduced efflux permeability of digoxin suggests that the added test compound is an inhibitor. The influx permeability of test compound is also determined during the 1-2 h study period by measuring the concentration of the test compound in the basolateral side. At the end of 2 h, a potent P-gp inhibitor (GF120918) was added. The increased influx permeability of test compound during the 2-3 h incubation period indicates that the added test compound is a substrate. Samples were taken from both sides at the end of 1-3 h and the concentrations of the test compounds and digoxin were quantitated. Digoxin efflux permeability remained unchanged when incubated with P-gp substrates (e.g., etoposide, rhodamine123, taxol). However, when a P-gp inhibitor was added to the apical side, the digoxin efflux (B to A permeability) was significantly reduced (ketoconazole=51% reduction) as expected. The influx permeability of substrates increased significantly (rhodamine123=70%, taxol=220%, digoxin=290%) after the P-gp inhibitor (GF120918) was introduced, whereas the influx permeability of P-gp inhibitor and non-substrates was not affected by GF120918. Thus, this combined assay provides an efficient cell based in vitro screening tool to simultaneously distinguish compounds that are P-gp substrates from P-gp inhibitors.

Transport of HIV protease inhibitors through the blood-brain barrier and interactions with the efflux proteins, P-glycoprotein and multidrug resistance proteins

HIV protease inhibitors (HPIs) have limited penetration into the brain. This poor transport through the blood-brain barrier is mainly due to active efflux by proteins such as P-glycoprotein (P-gp) preventing drugs from clearing the brain of the virus. The present paper focuses on cerebral uptake of HPIs and interactions between HPIs and efflux proteins, either as substrates or modulators. Most of the studies described HPIs as P-gp substrates. Studies are more controversial when investigating HPIs as inhibitors of P-gp. HPIs seem to be able to inhibit efflux proteins of in vitro cell models but with limited consequences in vivo. Moreover, after repeated administrations of HPIs, most of them are also able to induce the expression and functionality of P-gp. For these reasons, certain combinations of HPIs may not efficiently increase brain uptake of HPIs as would combinations of more potent efflux inhibitors.

Drug resistance in brain diseases and the role of drug efflux transporters

Resistance to drug treatment is an important hurdle in the therapy of many brain disorders, including brain cancer, epilepsy, schizophrenia, depression and infection of the brain with HIV. Consequently, there is a pressing need to develop new and more effective treatment strategies. Mechanisms of resistance that operate in cancer and infectious diseases might also be relevant in drug-resistant brain disorders. In particular, drug efflux transporters that are expressed at the blood-brain barrier limit the ability of many drugs to access the brain. There is increasing evidence that drug efflux transporters have an important role in drug-resistant brain disorders, and this information should allow more efficacious treatment strategies to be developed.

Quantitative assessment of HIV-1 protease inhibitor interactions with drug efflux transporters in the blood-brain barrier

PURPOSE

To quantitatively characterize the drug efflux interactions of various HIV-1 protease inhibitors in an in vitro model of the blood-brain barrier (BBB) and to compare that with HIV-1 protease inhibitor stimulated P-glycoprotein (P-gp)-ATPase activity.

METHODS

Cellular accumulation of the P-gp sensitive probe, rhodamine 123 (R123), and the mixed P-gp/multidrug resistance-associated protein (MRP) probe, 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF), were evaluated in primary cultured bovine brain microvessel endothelial cells (BBMEC) in the presence of various concentrations of HIV-1 protease inhibitors. The potency (IC50) and efficacy (Imax) of the drugs in the cell accumulation assays for P-gp and/or MRP was determined and compared to activity in a P-gp ATPase assay.

RESULTS

For R123 (P-gp probe), the rank order potency for inhibiting R123 accumulation in the BBMEC was saquinavir=nelfinavir>ritonavir=amprenavir>indinavir. This correlated well with the rank order affinity in the P-gp ATPase assay. The rank order potency for MRP-related drug efflux transporters, was nelfinavir>ritonavir>saquinavir>amprenavir>indinavir.

CONCLUSIONS

HIV-1 protease inhibitors potently interact with both P-gp and MRP-related transporters in BBMEC. Characterization of the interactions between the HIV-1 protease inhibitors and drug efflux transporters in brain microvessel endothelial cells will provide insight into potential drug-drug interactions and permeability issues in the BBB.

Role of drug efflux transporters in the brain for drug disposition and treatment of brain diseases

The blood-brain barrier (BBB) serves as a protective mechanism for the brain by preventing entry of potentially harmful substances from free access to the central nervous system (CNS). Tight junctions present between the brain microvessel endothelial cells form a diffusion barrier, which selectively excludes most blood-borne substances from entering the brain. Astrocytic end-feet tightly ensheath the vessel wall and appear to be critical for the induction and maintenance of the barrier properties of the brain capillary endothelial cells. Because of these properties, the BBB only allows entry of lipophilic compounds with low molecular weights by passive diffusion. However, many lipophilic drugs show negligible brain uptake. They are substrates for drug efflux transporters such as P-glycoprotein (Pgp), multidrug resistance proteins (MRPs) or organic anion transporting polypeptides (OATPs) that are expressed at brain capillary endothelial cells and/or astrocytic end-feet and are key elements of the molecular machinery that confers the special permeability properties to the BBB. The combined action of these carrier systems results in rapid efflux of xenobiotics from the CNS. The objective of this review is to summarize transporter characteristics (cellular localization, specificity, regulation, and potential inhibition) for drug efflux transport systems identified in the BBB and blood-cerebrospinal fluid (CSF) barrier. A variety of experimental approaches available to ascertain or predict the impact of efflux transport on brain access of therapeutic drugs also are described and critically discussed. The potential impact of efflux transport on the pharmacodynamics of agents acting in the CNS is illustrated. Furthermore, the current knowledge about drug efflux transporters as a major determinant of multidrug resistance of brain diseases such as epilepsy is reviewed. Finally, we summarize strategies for modulating or by-passing drug efflux transporters at the BBB as novel therapeutic approaches to drug-resistant brain diseases.

Microdialysis—theoretical background and recent implementation in applied life-sciences

In the past decade microdialysis has become a method of choice in the study of unbound tissue concentrations of both endogenous and exogenous substances. Microdialysis has been shown to offer information about substances directly at the site of action while being well tolerable and safe. The large variety of its field of application has been demonstrated. However, a few challenges have to be met to make this method generally applicable in routine applications. This review will provide an overview over theoretical aspects that have to be considered during the implementation of microdialysis. Moreover, a comparison between microdialysis and other tissue sampling techniques will demonstrate advantages and limitations of the methods mentioned. Subsequently, it will present a critical synopsis of a variety of scientific/biomedical applications of this method with emphasis on the most recent literature, focussing on target tissues while giving examples of substances examined. It is concluded that microdialysis will be of great value in future investigations of pharmacokinetics, pharmacodynamics and in monitoring of disease status and progression.

Factors affecting delivery of antiviral drugs to the brain

Although the CNS is in part protected from peripheral insults by the blood-brain barrier and the blood-cerebrospinal fluid barrier, a number of human viruses gain access to the brain, replicate within this organ, or sustain latent infection. The efficacy of antiviral drugs towards the cerebral viral load is often limited as both blood-brain interfaces impede their cerebral distribution. For polar compounds, the major factor restricting their entry lies in the tight junctions that occlude the paracellular pathway across these barriers. For compounds with more favourable lipid solubility properties, CNS penetration will be function of a number of physicochemical factors that include the degree of lipophilicity, size and ability to bind to protein or red blood cells, as well as other factors inherent to the vascular and choroidal systems, such as the local cerebral blood flow and the surface area available for exchange. In addition, influx and efflux transport systems, or metabolic processes active in both capillary endothelial cells and choroid plexus epithelial cells, can greatly change the bioavailability of a drug in one or several compartments of the CNS. The relative importance of these various factors with respect to the CNS delivery of the different classes of antiviral drugs is illustrated and discussed.

Blood-brain barrier active efflux transporters: ATP-binding cassette gene family

The blood-brain barrier (BBB) contributes to brain homeostasis by protecting the brain from potentially harmful endogenous and exogenous substances. BBB active drug efflux transporters of the ATP-binding cassette (ABC) gene family are increasingly recognized as important determinants of drug distribution to, and elimination from, the CNS. The ABC efflux transporter P-glycoprotein (Pgp) has been demonstrated as a key element of the BBB that can actively transport a huge variety of lipophilic drugs out of the brain capillary endothelial cells that form the BBB. In addition to Pgp, other ABC efflux transporters such as members of the multidrug resistance protein (MRP) family and breast cancer resistance protein (BCRP) seem to contribute to BBB function. Consequences of ABC efflux transporters in the BBB include minimizing or avoiding neurotoxic adverse effects of drugs that otherwise would penetrate into the brain. However, ABC efflux transporters may also limit the central distribution of drugs that are beneficial to treat CNS diseases. Furthermore, neurological disorders such as epilepsy may be associated with overexpression of ABC efflux transporters at the BBB, resulting in pharmacoresistance to therapeutic medication. Therefore, modulation of ABC efflux transporters at the BBB forms a novel strategy to enhance the penetration of drugs into the brain and may yield new therapeutic options for drug-resistant CNS diseases.

The effects of protease inhibitors and nonnucleoside reverse transcriptase inhibitors on p-glycoprotein expression in peripheral blood mononuclear cells in vitro

Several antiretroviral compounds have been shown to be substrates for the efflux protein P-glycoprotein (P-gp) although few studies have investigated the effects of drug on expression of this protein. Here, an in vitro system has been adopted to investigate the effects of protease inhibitors (PIs) and nonnucleoside reverse transcriptase inhibitors (NNRTIs) on P-gp expression in peripheral blood mononuclear cells (PBMCs). PBMCs isolated from healthy volunteers were incubated with 10 or 100 microM PI (saquinavir, ritonavir, lopinavir, indinavir, nelfinavir, amprenavir) or 10 microM NNRTI (efavirenz, nevirapine) for 72 hours. Surface P-gp expression was measured by flow cytometry and compared with vehicle-incubated controls. Toxicity was assessed by MTT assay and the effects of each compound were compared between individuals with differing genotypes at position 3435 of exon 26 of MDR1, which was assigned by restriction fragment length polymorphism. Significant increases in median P-gp expression were observed following incubation with 10 microM nelfinavir (10.2 versus 6.7% P-gp-positive cells) and efavirenz (10.0 versus 6.7% P-gp-positive cells). No significant differences in induction were observed between genotypes (CC, CT, TT). Following incubation with 100 microM PI, significant upregulation of P-gp occurred except with amprenavir. However, nelfinavir, ritonavir, and lopinavir caused marked toxicity, indicating that at higher concentrations, the increase in P-gp may be at least partially related to a stress response. These results indicate the potential of some PIs and NNRTIs to induce P-gp expression in PBMCs in vitro.

Structure-activity relationship: analyses of p-glycoprotein substrates and inhibitors

OBJECTIVE

A large number of structurally and functionally diverse compounds act as substrates or modulators of p-glycoprotein (p-gp). Some of them possess multiple drug resistance (MDR)-reversing activity, but only a small number of them have entered clinical study. In order to uncover the factors which exert a significant impact on the interaction between substrates/modulators and p-gp, we have performed structure-activity relationship (SAR) analyses, including molecular modelling, two-dimensional (2D) and three-dimensional (3D) parameter-frame-setting analysis, quantitative structure activity relationship (QSAR) analysis among substrates/modulators, as well as clinically promising MDR-reversing agents.

METHODS

The physicochemical parameters C log P, CMR and all regression equations were derived by using C log P version 4.0 and the latest CQSAR software, respectively. Molecular modelling and all other parameter calculations were performed by using HyperChem version 5.0 program, after geometry optimization and energy minimization using the AM1 semiempirical method.

RESULTS

SAR analyses indicate that MDR reversal activity is correlated with the lipophilicity (C log P), molecular weight (log Mw), longest chain (Nlc) of the molecule and the energy of the highest occupied orbital (Ehomo). In addition, the presence of a basic tertiary nitrogen atom in the structure is also an important contributor to p-gp inhibitory activity. Some separation in space is achieved for different subsets of p-gp substrates and inhibitors using Nlc, C log P and Ehomo as three independent parameters in the 3D-parameter-frame setting.

CONCLUSION

A highly effective p-gp modulator candidate should possess a log P value of 2.92 or higher, 18-atom-long or longer molecular axis, and a high Ehomo value, as well as at least one tertiary basic nitrogen atom. The results obtained may be useful in explaining drug-p-gp interactions for different compounds, including drug interactions and the development of new MDR chemosensitizers.