Carrier-mediated transport of H1-antagonist at the blood-brain barrier: a common transport system of H1-antagonists and lipophilic basic drugs

Proteomics-Based Transporter Identification by the PICK Method: Involvement of TM7SF3 and LHFPL6 in Proton-Coupled Organic Cation Antiport at the Blood-Brain Barrier

A proton-coupled organic cation (H⁺/OC) antiporter working at the blood–brain barrier (BBB) in humans and rodents is thought to be a promising candidate for the efficient delivery of cationic drugs to the brain. Therefore, it is important to identify the molecular entity that exhibits this activity. Here, for this purpose, we established the Proteomics-based Identification of transporter by Crosslinking substrate in Keyhole (PICK) method, which combines photo-affinity labeling with comprehensive proteomics analysis using SWATH-MS. Using preselected criteria, the PICK method generated sixteen candidate proteins. From these, knockdown screening in hCMEC/D3 cells, an in vitro BBB model, identified two proteins, TM7SF3 and LHFPL6, as candidates for the H⁺/OC antiporter. We synthesized a novel H⁺/OC antiporter substrate for functional analysis of TM7SF3 and LHFPL6 in hCMEC/D3 cells and HEK293 cells. The results suggested that both TM7SF3 and LHFPL6 are components of the H⁺/OC antiporter.

Novel brain-targeting 3-n-butylphthalide prodrugs for ischemic stroke treatment

Currently, ischemic stroke is the leading cause of disability and death worldwide, and the performance of corresponding drugs is often unsatisfactory owing to the complex pathological processes and the impediment of the blood-brain barrier (BBB). Here, we employed various tertiary amino groups, including different linear, cyclic, and bimolecular drug structures, to modify 3-n-butylphthalide (NBP), a natural product used for ischemic stroke treatment, which has poor bioavailability, to generate a series of six prodrugs. These prodrugs showed significantly improved solubility and cellular uptake, which were primarily driven by putative pyrilamine cationic transporters. They also displayed more efficient brain delivery in vivo, reaching as high as 21.5-fold brain accumulation increase compared with NBP, leading to much higher bioavailability and stronger therapeutic effects. The toxicity of these molecules is also lower or similar to that of unmodified NBP. We showed that the tertiary amino group-modified NBP prodrugs are effective and safe for treating ischemic stroke with significantly enhanced druggability; hence, they have potential for further clinical development.

pH-Dependent Uptake and Sublethal Effects of Antihistamines in Zebrafish (Danio rerio) Embryos

Reported off-target effects of antihistamines in humans draw interest in ecotoxicity testing of first- and second-generation antihistamines, the latter of which have fewer reported side effects in humans. Because antihistamines are ionizable compounds, the pH influences uptake and toxicity and thus is highly relevant when conducting toxicity experiments. Zebrafish embryo toxicity tests were performed with the 3 first-generation antihistamines ketotifen, doxylamine, and dimethindene and the 2 second-generation antihistamines cetirizine and levocabastine at pH 5.5, 7.0, and 8.0. We detected effects on survival, phenotype, swimming activity, and heart rate for 4 antihistamines with the exception of levocabastine, which did not show any lethal or sublethal effects. When compared to lethal concentrations, effect concentrations neither of phenotype malformation nor of swimming activity or heart rate deviated by more than a factor of 10 from lethal concentrations, indicating that all sublethal effects were fairly nonspecific. First-generation antihistamines are weak bases and showed decreasing external effect concentrations with increasing neutral fraction, accompanied by increased uptake in the fish embryo. As a result, internal effect concentrations were independent from external pH. The pH-dependent toxicity originates from speciation-dependent uptake, with neutral species taken up in higher amounts than the corresponding ionic species. Cetirizine, which shifts from a zwitterionic to an anionic state in the measured pH range, did not show any pH-dependent uptake or toxicity. Environ Toxicol Chem 2019;00:1-11. © 2019 SETAC.

Recent advances in drug and nutrient transport across the blood-retinal barrier

INTRODUCTION

The blood-retinal barrier (BRB) is the barrier separating the blood and neural retina, and transport systems for low-weight molecules at the BRB are expected to be useful for developing drugs for the treatment of ocular neural disorders and maintaining a healthy retina. Areas covered: This review discusses blood-to-retina and retina-to-blood transport of drugs and nutrients at the BRB. In particular, P-gp (ABCB1/MDR1) has low impact on the transport of cationic drugs at the BRB, suggesting a significant role of novel organic cation transporters in influx and efflux transport of lipophilic cationic drugs between blood and the retina. The transport of pravastatin at the BRB involves transporters including organic anion transporting polypeptide 1a4 (Oatp1a4). Recent studies have shown the involvement of solute carrier transporters in the blood-to-retina transport of nutrients including riboflavin, L-ornithine, β-alanine, and L-histidine, implying that dipeptide transport at the BRB is minimal. Expert opinion: Novel organic cation transport systems and the elimination-dominant transport of pravastatin at the BRB are expected to be useful in systemic drug delivery to the neural retina without CNS side effects. The mechanism of nutrient transport at the BRB is expected to provide a new strategy for delivery of nutrient-mimetic drugs.

Mechanism of brain targeting by dexibuprofen prodrugs modified with ethanolamine-related structures

The first molecular insights into how prodrugs modified with ethanolamine-related structures target the brain were generated using an in vitro BBB model and in situ perfusion technique. Prodrugs were delivered safely and efficiently to the brain through tight interaction with the anionic membrane of brain capillary endothelial cells, observed as a shift in zeta potential, followed by uptake into the cells. Prodrugs III and IV carrying primary and secondary amine modifications appeared to enter the brain via energy-independent passive diffusion. In contrast, besides the passive diffusion, prodrugs I and II carrying tertiary amine modifications also appeared to enter via an active process that was energy and pH dependent but was independent of sodium or membrane potential. This active process involved, at least in part, the pyrilamine-sensitive H(+)/OC antiporter, for which the N,N-diethyl-based compound II showed a much lower affinity than the N,N-dimethyl-based compound I, likely due to steric hindrance. These new insights into brain-targeting mechanisms may help guide efforts to design new prodrugs.

Brain-specific delivery of dopamine mediated by n,n-dimethyl amino group for the treatment of Parkinson's disease

Parkinson's disease (PD) has become one of the most deadly diseases due to a lack of effective treatment. Herein, N-3,4-bis(pivaloyloxy)dopamine-3-(dimethylamino)propanamide (PDDP), a brain-specific derivative of dopamine, was designed and synthesized, which consists of a brain targeted ligand, N,N-dimethyl amino group, and two dipivaloyloxy groups for lipophilic modification. PDDP was investigated both in vitro and in vivo by comparing with L-DOPA and another derivative (BPD) without N,N-dimethyl amino group. PDDP showed a more pronounced accumulation in mouse brain microvascular endothelial cells (bEnd.3) than BPD via an active transport process. The increased cellular uptake of PDDP was proven to be mediated by putative pyrilamine cationic transporters. Following intravenous administration, the concentration of PDDP in the brain was 269.28-fold and 6.41-fold higher than that of L-DOPA and BPD at 5 min, respectively. Additionally, PDDP effectively attenuated the striatum lesion caused by 6-hydroxydopamine (6-OHDA) in rats. More importantly, PDDP presented antioxidant and antiapoptotic effects on 6-OHDA-induced toxicity in human neuroblastoma cells (SH-SY5Y). Thus, N,N-dimethyl amino group-based PDDP represents an effective and safe treatment for PD.

Diphenhydramine has similar interspecies net active influx at the blood-brain barrier

In rats, oxycodone, diphenhydramine, and [4-chloro-5-fluoro-2-(3-methoxy-2-methyl-phenoxy)-benzyl]-methylamine (CE-157119) undergo net active influx at the blood-brain barrier (BBB) based on significantly greater interstitial fluid compound concentrations (CISF ) than unbound plasma compound concentrations (Cp,u ). Oxycodone and diphenhydramine have CISF :Cp,u of 3.0 and 5.5, respectively, while CE-157119 has an unbound brain compound concentration (Cb,u ):Cp,u of 3.90; Cb,u is a high-confidence CISF surrogate. However, only CE-157119 has published dog and nonhuman primate (nhp) neuropharmacokinetics, which show similar Cb,u :Cp,u (4.61 and 2.04, respectively) as rats. Thus, diphenhydramine underwent identical interspecies neuropharmacokinetics studies to determine if its net active BBB influx in rats replicated in dogs and/or nhp. The single-dose-derived rat Cb,u :Cp,u (3.90) was consistent with prior steady-state-derived CISF :Cp,u and similar to those in dogs (4.88) and nhp (4.51-5.00). All large animal interneurocompartmental ratios were ≤1.8-fold different than their rat values, implying that diphenhydramine has constant and substantial Cb,u -favoring disequilibria in these mammals. Accordingly, the applied Cb,u -forecasting methodology accurately predicted [estimated mean (95% confidence interval) of 0.84 (0.68, 1.05)] Cb,u from each measured Cp,u in large animals. The collective datasets suggest these Cb,u -preferring asymmetries are mediated by a species-independent BBB active uptake system whose identification, full characterization, and structure-activity relationships should be prioritized for potential exploitation.

Exploiting nutrient transporters at the blood-brain barrier to improve brain distribution of small molecules

The blood-brain barrier (BBB) is a major physiological barrier for drugs that target CNS receptors or enzymes. Several methods exist by which permeability to the CNS can be increased, one of which is using native nutrient transporters to carry these drugs through the endothelial cells of the BBB. In this review, we focus on work that characterizes the use of nutrient transporters of the BBB in delivering drugs to the CNS.

Strategies for therapy of retinal diseases using systemic drug delivery: relevance of transporters at the blood-retinal barrier

INTRODUCTION

There is an increasing need for managing rapidly progressing retinal diseases because of the potential loss of vision. Although systemic drug administration is one possible route for treating retinal diseases, retinal transfer of therapeutic drugs from the circulating blood is strictly regulated by the blood-retinal barrier (BRB).

AREAS COVERED

This review discusses the constraints and challenges of drug delivery to the retina. In addition, this article discusses the properties of drugs and the conditions of the BRB that affect drug permeability. The reader will gain insights into the strategies for developing therapeutic drugs that are able to cross the BRB for treating retinal diseases. Further, the reader will gain insights into the role of BRB physiology including barrier functions, and the effect of influx and efflux transporters on retinal drug delivery.

EXPERT OPINION

When designing and selecting optimal drug candidates, it's important to consider the fact that they should be recognized by influx transporters and that efflux transporters at the BRB should be avoided. Although lipophilic cationic drugs are known to be transported to the brain across the blood-brain barrier, verapamil transport to the retina is substantially higher than to the brain. Therefore, lipophilic cationic drugs do have a great ability to increase influx transport across the BRB.

Involvement of an influx transporter in the blood-brain barrier transport of naloxone

Naloxone, a potent and specific opioid antagonist, has been shown in previous studies to have an influx clearance across the rat blood-brain barrier (BBB) two times greater than the efflux clearance. The purpose of the present study was to characterize the influx transport of naloxone across the rat BBB using the brain uptake index (BUI) method. The initial uptake rate of [(3)H]naloxone exhibited saturability in a concentration-dependent manner (concentration range 0.5 microM to 15 mM) in the presence of unlabeled naloxone. These results indicate that both passive diffusion and a carrier-mediated transport mechanism are operating. The in vivo kinetic parameters were estimated as follows: the Michaelis constant, K(t), was 2.99+/-0.71 mM; the maximum uptake rate, J(max), was 0.477+/-0.083 micromol/min/g brain; and the nonsaturable first-order rate constant, K(d), was 0.160+/-0.044 ml/min/g brain. The uptake of [(3)H]naloxone by the rat brain increased as the pH of the injected solution was increased from 5.5 to 8.5 and was strongly inhibited by cationic H(1)-antagonists such as pyrilamine and diphenhydramine and cationic drugs such as lidocaine and propranolol. In contrast, the BBB transport of [(3)H]naloxone was not affected by any typical substrates for organic cation transport systems such as tetraethylammonium, ergothioneine or L-carnitine or substrates for organic anion transport systems such as p-aminohippuric acid, benzylpenicillin or pravastatin. The present results suggest that a pH-dependent and saturable influx transport system that is a selective transporter for cationic H(1)-antagonists is involved in the BBB transport of naloxone in the rat.

Genetic deficiency of carnitine/organic cation transporter 2 (slc22a5) is associated with altered tissue distribution of its substrate pyrilamine in mice

Carnitine/organic cation transporter 2 (OCTN2) recognizes various cationic compounds as substrates in vitro, but information on its pharmacokinetic role in vivo is quite limited. This paper demonstrates altered tissue distribution of the OCTN2 substrate pyrilamine in juvenile visceral steatosis (jvs) mice, which have a hereditary defect of the octn2 gene. At 30 min after intravenous injection of pyrilamine, the tissue-to-plasma concentration ratio (K(p)) in the heart and pancreas was higher, whereas the K(p) in kidney and testis was lower in jvs mice compared with wild-type mice. Pyrilamine transport studies in isolated heart slices confirmed higher accumulation, together with lower efflux, of pyrilamine in the heart of jvs mice. The higher accumulation in heart slices of jvs mice was abolished by lowering the temperature, by increasing the substrate concentration, and in the presence of other H(1) antagonists or another OCTN2 substrate, carnitine, suggesting that OCTN2 extrudes pyrilamine from heart tissue. On the other hand, the lower distribution to the kidney of jvs mice was probably due to down-regulation of a basolateral transporter coupled with OCTN2, because, in jvs mice, (i) the K(p) of pyrilamine in kidney assessed immediately after intravenous injection (approximately 1 min) was also lower, (ii) the urinary excretion of pyrilamine was lower, and (iii) the uptake of pyrilamine in kidney slices was lower. The renal uptake of pyrilamine was saturable (K(m) approximately 236 microM) and was strongly inhibited by cyproheptadine, astemizole, ebastine and terfenadine. The present study thus indicates that genetic deficiency of octn2 alters pyrilamine disposition tissue-dependently.

Involvement of the pyrilamine transporter, a putative organic cation transporter, in blood-brain barrier transport of oxycodone

The purpose of this study was to characterize blood-brain barrier (BBB) transport of oxycodone, a cationic opioid agonist, via the pyrilamine transporter, a putative organic cation transporter, using conditionally immortalized rat brain capillary endothelial cells (TR-BBB13). Oxycodone and [3H]pyrilamine were both transported into TR-BBB13 cells in a temperature- and concentration-dependent manner with Km values of 89 and 28 microM, respectively. The initial uptake of oxycodone was significantly enhanced by preloading with pyrilamine and vice versa. Furthermore, mutual uptake inhibition by oxycodone and pyrilamine suggests that a common mechanism is involved in their transport. Transport of both substrates was inhibited by type II cations (quinidine, verapamil, and amantadine), but not by classic organic cation transporter (OCT) substrates and/or inhibitors (tetraethylammonium, 1-methyl-4-phenylpyridinium, and corticosterone), substrates of OCTN1 (ergothioneine) and OCTN2 (L-carnitine), or organic anions. The transport was inhibited by metabolic inhibitors (rotenone and sodium azide) but was insensitive to extracellular sodium and membrane potential for both substrates. Furthermore, the transport of both substrates was increased at alkaline extracellular pH and decreased in the presence of a protonophore (carbonyl cyanide-p-trifluoromethoxyphenylhydrazone). Intracellular acidification induced with ammonium chloride enhanced the uptakes, suggesting that the transport is driven by an oppositely directed proton gradient. The brain uptake of oxycodone measured by in situ rat brain perfusion was increased in alkaline perfusate and was significantly inhibited by pyrilamine. These results suggest that blood-brain barrier transport of oxycodone is at least partly mediated by a common transporter with pyrilamine, and this transporter is an energy-dependent, proton-coupled antiporter.

Contribution of Carrier-Mediated Transport Systems to the Blood–Brain Barrier as a Supporting and Protecting Interface for the Brain; Importance for CNS Drug Discovery and Development

The blood-brain barrier (BBB) forms an interface between the circulating blood and the brain and possesses various carrier-mediated transport systems for small molecules to support and protect CNS function. For example, the blood-to-brain influx transport systems supply nutrients, such as glucose and amino acids. Consequently, xenobiotic drugs recognized by influx transporters are expected to have high permeability across the BBB. On the other hand, efflux transporters, including ATP-binding cassette transporters such as P-glycoprotein located at the luminal membrane of endothelial cells, function as clearance systems for metabolites and neurotoxic compounds produced in the brain. Drugs recognized by these transporters are expected to show low BBB permeability and low distribution to the brain. Despite recent progress, the transport mechanisms at the BBB have not been fully clarified yet, especially in humans. However, an understanding of the human BBB transport system is critical, because species differences mean that it can be difficult to extrapolate data obtained in experimental animals during drug development to humans. Recent progress in methodologies is allowing us to address this issue. Positron emission tomography can be used to evaluate the activity of human BBB transport systems in vivo. Proteomic studies may also provide important insights into human BBB function. Construction of a human BBB transporter atlas would be a most important advance from the viewpoint of CNS drug discovery and drug delivery to the brain.

The Use of Microdialysis for the Study of Drug Kinetics: Central Nervous System Pharmacokinetics of Diphenhydramine in Fetal, Newborn, and Adult Sheep

The central nervous system (CNS) pharmacokinetics of the H(1) receptor antagonist diphenhydramine (DPHM) were studied in 100- and 120-day-old fetuses, 10- and 30-day-old newborn lambs, and adult sheep using in vivo microdialysis. DPHM was administered i.v. at five infusion rates, with each step lasting 7 h. In all ages, cerebrospinal fluid (CSF) and extracellular fluid (ECF) concentrations were very similar to each other, which suggests that DPHM between these two compartments is transferred by passive diffusion. In addition, the brain-to-plasma concentration ratios were >or=3 in all age groups, suggesting the existence of a transport process for DPHM into the brain. Both brain and plasma DPHM concentrations increased in a linear fashion over the dose range studied. However, the ECF/unbound plasma and CSF/unbound plasma DPHM concentration ratios were significantly higher in the fetus and lambs (approximately 5 to 6) than in the adult (approximately 3). The factors f(CSF) and f(ECF), the ratios of DPHM areas under the curves (AUCs) in CSF and ECF to the plasma DPHM AUC, respectively, decreased with age, indicating that DPHM is more efficiently removed from the brain with increasing age. The extent of plasma protein binding of the drug increased with age. This study provides evidence for a transporter-mediated mechanism for the influx of DPHM into the brain and also for an efflux transporter for the drug, whose activity increases with age. Moreover, the higher brain DPHM levels in the fetus and lamb compared with the adult may explain the greater CNS effects of the drug at these ages.

Blood-brain barrier transport of pramipexole, a dopamine D2 agonist

The blood-brain barrier (BBB) transport of pramipexole, a potent dopamine receptor agonist with high efficacy for Parkinson's disease, was mainly characterized using immortalized rat brain capillary endothelial cells (RBEC)1 as an in vitro BBB model. [(14)C]Pramipexole uptake by RBEC1 was dependent on temperature and pH, but not sodium ion concentration or membrane potential. The uptake was inhibited by several organic cations including pyrilamine. Mutual inhibition was observed between pramipexole and pyrilamine. In addition, [(14)C]pramipexole uptake was stimulated by preloading unlabeled pramipexole. RT-PCR analysis for organic cation transporters (rOCT1-3, rOCTN1-2) in RBEC1 was performed. The mRNA level of rOCTN2 was the highest, followed by rOCTN1, while expression of rOCT1, rOCT2 and rOCT3 was negligible. The brain uptake of [(14)C]pramipexole, which was measured by the in situ rat brain perfusion technique, was significantly inhibited by unlabeled pramipexole. These results suggest that pramipexole is, at least in part, transported across the BBB by an organic cation-sensitive transporter. The pramipexole transport in RBEC1 was pH-dependent, but sodium- and membrane potential-independent.

A pharmacokinetic study of diphenhydramine transport across the blood-brain barrier in adult sheep: potential involvement of a carrier-mediated mechanism

The purpose of this study was to examine the disposition of diphenhydramine (DPHM) across the ovine blood-brain barrier (BBB). In six adult sheep, we characterized the central nervous system (CNS) pharmacokinetics of DPHM in brain extracellular fluid (ECF) and cerebrospinal fluid (CSF) using microdialysis in two experiments. In the first experiment, DPHM was administered via a five-step i.v. infusion (1.5, 5.5, 9.5, 13.5, and 17.5 microg/kg/min; 7 h per step). Average steady-state CNS/total plasma concentration ratios (i.e., [CNS]/[total plasma]) for steps 1 to 5 ranged from 0.4 to 0.5. However, average steady-state [CNS]/[free plasma] ratios ranged from 2 to 3, suggesting active transport of DPHM into the CNS. Plasma protein binding averaged 86.1 +/- 2.3% (mean +/- S.D.) and was not altered with increasing drug dose. Plasma, CSF, and ECF demonstrated biexponential pharmacokinetics with terminal elimination half-lives (t1/2beta) of 10.8 +/- 5.4, 3.6 +/- 1.0, and 5.3 +/- 4.2 h, respectively. The bulk flow of CSF and transport-mediated efflux of DPHM may explain the observed higher CNS clearances. In the second experiment, DPHM was coadministered with propranolol (PRN) to examine its effect on blood-brain CSF and blood-brain ECF DPHM relationships. Plasma total DPHM concentration decreased by 12.8 +/- 6.3% during PRN, whereas ECF and CSF concentrations increased (88.1 +/- 45.4 and 91.6 +/- 34.3%, respectively). This increase may be due to the inhibitory effect of PRN on a transporter-mediated efflux mechanism for DPHM brain elimination.

EFFECT OF P-GLYCOPROTEIN ON INTESTINAL ABSORPTION AND BRAIN PENETRATION OF ANTIALLERGIC AGENT BEPOTASTINE BESILATE

The antiallergic agent bepotastine besilate is a nonsedating, second-generation H1-antagonist with high oral absorption and negligible distribution into brain. To clarify the role of P-glycoprotein (P-gp) in the pharmacokinetics of bepotastine, intestinal absorption and brain penetration studies were performed. [(14)C]Bepotastine transport in P-gp-overexpressed LLC-PK1 cells indicated that bepotastine was a substrate of P-gp. The affinity of bepotastine to P-gp estimated by ATPase activity assay was low, with a K(m) value of 1.25 mM. After i.v. administration, the brain/plasma free concentration ratio in mdr1-knockout mice was 3 times higher than that in wild-type mice. The in situ intestinal absorption studies of [(14)C]bepotastine in rats showed a clear regional difference, showing highest permeability at the upper part of small intestine with a decreasing permeability in the descending part of small intestine. The apparent absorption rate constant (ka) of [(14)C]bepotastine in the small intestine was greatly increased by cyclosporin A and verapamil, especially in the distal portion, and the site-specific absorption of [(14)C]bepotastine disappeared. The concentration dependence of ka of [(14)C]bepotastine was observed with a higher ka at higher concentration (20 mM) compared with that at lower concentration (1 microM). In conclusion, bepotastine is a substrate for P-gp, and P-gp clearly limited the brain distribution of bepotastine, whereas the effect of P-gp on intestinal absorption of bepotastine was minimal, presumably because of high membrane permeability at the upper region of small intestine where P-gp is less expressed. Such intestinal absorption property of bepotastine is distinctly different from the low membrane-permeable P-gp substrate fexofenadine.

Transporter-mediated drug delivery: recent progress and experimental approaches

A comprehensive list of drug transporters has recently become available as a result of extensive genome analysis, as well as membrane physiology and molecular biology studies. This review covers recent progress in identification and characterization of drug transporters, illustrative cases of successful drug delivery to, or exclusion from, target organs via transporters, and novel experimental approaches to therapeutics using heterologously transduced transporters in tissues. We aim to provide clues that could lead to efficient strategies for the use of transporters to deliver drugs and/or to optimize lead compounds.

Small molecular drug transfer across the blood-brain barrier via carrier-mediated transport systems

Because of the physiological nature of the blood-brain barrier (BBB), transport of chemical compounds between blood and brain has been widely believed to occur by means of passive diffusion, depending upon the lipophilicity of the compounds. However, discrepancies exist between the lipophilicity and apparent BBB permeation properties in many cases, and these discrepancies can be ascribed to the existence of multiple mechanisms of drug transport through the BBB. Molecular identification and functional analysis of influx transport proteins (from blood to brain) and efflux transport proteins (from brain to blood) have progressed rapidly. Therefore, the BBB is now considered to be a dynamic interface that controls the influx and efflux of a wide variety of substances, including endogenous nutrients and exogenous compounds such as drugs, to maintain a favorable environment for the CNS. This review focuses on the role of transport systems in the uptake of xenobiotics, including organic anionic/cationic and neutral drugs, across the BBB into the brain, as well as on strategies to increase drug delivery into the brain by blocking efflux transport protein function, or to reduce CNS side effects by modulating BBB transport processes.

INFLUX AND EFFLUX TRANSPORT OF H1-ANTAGONIST EPINASTINE ACROSS THE BLOOD-BRAIN BARRIER

We investigated influx and efflux transporters involved in blood-brain barrier transport of the nonsedative H1-antagonist epinastine. The basal-to-apical transport of [14C]epinastine was markedly higher than that in the opposite direction in LLC-GA5-COL150 cells stably transfected with human multidrug resistance (MDR)1 gene. The brain-to-plasma concentration ratio of [14C]epinastine in mdr1a/b(-/-) mice was 3.2 times higher than that in wild-type mice. The uptake of both [3H]mepyramine and [14C]epinastine into immortalized rat brain capillary endothelial cells (RBEC)1 showed temperature and concentration dependence. The kinetic parameters, K(m), V(max), and uptake clearance (V(max)/K(m)), of the initial uptake of [3H]mepyramine and [14C]epinastine by RBEC1 were 150 microM, 41.8 nmol/min/mg protein, and 279 microl/min/mg protein for mepyramine and 10.0 mM, 339 nmol/min/mg protein, and 33.9 microl/min/mg protein for epinastine, respectively. The uptake of [3H]mepyramine and [14C]epinastine by RBEC1 was inhibited by organic cations such as quinidine, amantadine, and verapamil, but not by other organic cations, tetraethyl ammonium, guanidine, and carnitine. Organic anions such as benzoic acid, estrone-3-sulfate, taurocholate, and neutral digoxin were not inhibitory. Furthermore, some cationic H1 antagonists (chlorpheniramine, cyproheptadine, ketotifen, and desloratadine) inhibited the [3H]mepyramine and [14C]epinastine uptake into RBEC1. In conclusion, the present study demonstrated that the combination of efficient efflux transport by P-glycoprotein and poor uptake by the influx transporter, which is identical with that responsible for the uptake of mepyramine, account for the low brain distribution of epinastine.

[Biopharmaceutical studies on molecular mechanisms of membrane transport]

By incorporating the transporter-mediated or receptor-mediated transport process in physiologically based pharmacokinetic models, we succeeded in the quantitative prediction of plasma and tissue concentrations of beta-lactam antibiotics, insulin, pentazocine, quinolone antibacterial agents, and inaperizone and digoxin. The author's research on transporter-mediated pharmacokinetics focuses on the molecular and functional characteristics of drug transporters such as oligopeptide transporter, monocarboxylic acid transporter, anion antiporter, organic anion transporters, organic cation/carnitine transporters (OCTNs), and the ATP-binding cassette transporters P-glycoprotein and MRP2. We have successfully demonstrated that these transporters play important roles in the influxes and/or effluxes of drugs in intestinal and renal epithelial cells, hepatocytes, and brain capillary endothelial cells that form the blood-brain barrier. In the systemic carnitine deficiency (SCD) phenotype mouse model, juvenile visceral steatosis (jvs) mouse, a mutation in the OCTN2 gene was found. Furthermore, several types of mutation in human SCD patients were found, demonstrating that OCTN2 is a physiologically important carnitine transporter. Interestingly, OCTNs transport carnitine in a sodium-dependent manner and various cationic drugs transport it in a sodium-independent manner. OCTNs are thought to be multifunctional transporters for the uptake of carnitine into tissue cells and for the elimination of intracellular organic cationic drugs.

Investigation on the influx transport mechanism of pentazocine at the blood-brain barrier in rats using the carotid injection technique

The influx transport mechanism of pentazocine (PTZ) at the blood-brain barrier (BBB) was investigated in rats using the carotid injection technique. The uptake kinetics of PTZ into the rat brain exhibited saturability, which occurred by both nonsaturable and carrier-mediated transport processes. The in vivo kinetic parameters were estimated as follows: the maximal uptake rate (Jmax), 3.6 +/- 1.2 micromol/min/g brain and the apparent Michaelis constant (K1), 3.7 +/- 1.7 mM for the saturable component of PTZ into the brain, and the nonsaturable uptake rate constant (Kd), 0.06 +/- 0.04 ml/min/g brain. The uptake of PTZ by the brain was strongly inhibited by lidocaine, imipramine and propranolol, and also by H1-antagonists such as mepyramine, diphenhydramine. In addition, narcotic-antagonist analgesic (buprenorphine, butorphanol or eptazocine) and an opioid antagonist (naloxone) significantly inhibited PTZ transport. These results suggest that PTZ permeates into the brain via a carrier-mediated transport system, which may widely recognize the cationic drugs.

Blood-brain barrier transport of H1-antagonist ebastine and its metabolite carebastine

The transport mechanism of the non-sedative H1-antagonist ebastine and its first-pass carboxylic acid metabolite carebastine at the blood-brain barrier (BBB) was studied. In rats, the brain uptake index (BUI) value of [14 C]carebastine was significantly lower than that of [14 C]ebastine. The BUI value of [14 C]carebastine was greatly increased by the addition of non-labeled carebastine. The steady-state uptake of [14 C]carebastine by P-glycoprotein-overexpressing K562/ADM cells was significantly lower than that by their parental drug-sensitive cell line K562. The decreased steady-state uptake of [14 C]carebastine by K562/ADM cells was reversed by verapamil. Steady-state uptake of [14 C]carebastine by primary cultured bovine brain capillary endothelial cells (bovine BCECs) was increased in the presence of metabolic inhibitors and verapamil. Non-labeled carebastine increased the steady-state uptake of a P-glycoprotein substrate, [3 H]vincristine, by bovine BCECs. The initial uptake of [3 H]mepyramine by bovine BCECs and RBEC1 (an immortalized cell line from rat brain capillary endothelial cells) was strongly inhibited by ebastine, while zwitterionic carebastine was slightly inhibitory. The values of brain-to-plasma unbound concentration ratio (Kp,f) in mdr1a(-/-) mice were increased 5.3-fold and 4.2-fold for [14 C ebastine and for [14 C]carebastine, respectively, compared with those in mdr1a(+/+) mice. Non-radiolabeled carebastine increased the Kp,f values of [14 C]carebastine in both types of mice. In conclusion, carebastine was shown to be a substrate for P-glycoprotein-mediated efflux from the brain at the BBB. A second efflux system may also be involved. The relatively low affinity of the uptake transport system for carebastine also limits the brain distribution of ebastine/carebastine.

[Studies on the mechanism of subcellular distribution of basic drugs based on their lipophilicity]

This paper described the studies on the mechanism of subcellular distribution of lipophilic weak bases. Although the tissue distribution of basic drugs appeared to decrease with time simply in parallel with their plasma concentration, their subcellular distribution in various tissues exhibited a variety of patterns. Basic drugs were distributed widely in various tissues, but were concentrated in lung granule fraction, where their accumulation was dependent on their lipophilicity and lysosomal uptake. As the plasma concentration of drugs decreased after maximum level, the contribution of lysosomes to their subcellular distribution increased. The uptake of the basic drugs into lysosomes depended both on their intralysosomal pH and on the drug lipophilicity. As the lipophilicity of the basic drugs increased, they accumulated more than the values predicted from the pH-partition theory and raised the intralysosomal pH more potently, probably owing to their binding with lysosomal membranes with or without additional intralysosomal aggregation. These phenomena should be considered as a basis of drug interaction in clinical treatments.

Affinity for the P-glycoprotein efflux pump at the blood-brain barrier may explain the lack of CNS side-effects of modern antihistamines

First generation H1 receptor antagonists are often associated with adverse CNS effects such as sedation, whereas modern, second generation antihistamines are generally non-sedating. The difference in therapeutic profile is mainly due to the poor CNS penetration of the modern derivatives. Current explanations for the differential ability of classical and modern antihistamines to cross the blood-brain barrier (BBB), based on differences in lipophilicity or protein binding, are inadequate. We have tested the hypothesis that non-sedating antihistamines fail to enter the CNS due to recognition by the P-glycoprotein (Pgp) drug efflux pump expressed on the luminal surface of cerebral endothelial cells forming the BBB in vivo. The ability of several sedating and non-sedating antihistamines to affect the uptake of the Pgp model substrate [3H]-colchicine was examined using the immortalised rat brain endothelial cell line, RBE4, an established in vitro model of the BBB expressing Pgp. All second generation antihistamines tested, significantly increased net accumulation of [3H]-colchicine to a level similar to that caused by the Pgp inhibitor verapamil. By contrast, the first generation antihistamines showed no affinity for Pgp. The results indicate that differences in the ability of classical and modern antihistamines to interact with Pgp at the BBB may determine their CNS penetration and as a consequence the presence or absence of central side-effects.

Transporter-mediated permeation of drugs across the blood-brain barrier

Drug distribution into the brain is strictly regulated by the presence of the blood-brain barrier (BBB) that is formed by brain capillary endothelial cells. Since the endothelial cells are connected to each other by tight junctions and lack pores and/or fenestrations, compounds must cross the membranes of the cells to enter the brain from the bloodstream. Therefore, hydrophilic compounds cannot cross the barrier in the absence of specific mechanisms such as membrane transporters or endocytosis. So, for efficient supply of hydrophilic nutrients, the BBB is equipped with membrane transport systems and some of those transporter proteins have been shown to accept drug molecules and transport them into brain. In the present review, we describe mainly the transporters that are involved in drug transfer across the BBB and have been molecularly identified. The transport systems described include transporters for amino acids, monocarboxylic acids, organic cations, hexoses, nucleosides, and peptides. Most of these transporters function in the direction of influx from blood to brain; the presence of efflux transporters from brain to blood has also been demonstrated, including P-glycoprotein, MRPs, and other unknown transporters. These efflux transporters seem to be functional for detoxication and/or prevention of nonessential compounds from entering the brain. Various drugs are transported out of the brain via such efflux transporters, resulting in the decrease of CNS side effects for drugs that have pharmacological targets in peripheral tissues or in the reduction of efficacy in CNS because of the lower delivery by efflux transport. To identify the transporters functional at the BBB and to examine the possible involvement of them in drug transports by molecular and physiological approaches will provide a rational basis for controlling drug distribution to the brain.

Non-cardiac adverse effects of antihistamines (H1-receptor antagonists)

Antihistamines, available without prescription in many countries, are generally considered to be safe medications; however, the old first-generation H1 antagonists commonly cause adverse central nervous system (CNS) effects, even when administered in usual doses. Patients may not be aware of these effects and do not necessarily develop tolerance to them. In contrast, the new, second-generation H1 antagonists are relatively free from adverse effects in the CNS, primarily because they do not cross the blood-brain barrier and block the important neurotransmitter function of histamine. Most of the H1 antagonists in current use are unlikely to cause cardiac toxicity. There is no evidence that H1 antagonists, which have been approved by regulatory agencies, have carcinogenic, tumour-promoting, or teratogenic effects in humans.

Carrier-mediated or specialized transport of drugs across the blood-brain barrier

In the drug development process, it remains a difficult task to regulate the entry of the drugs. However, recent progress in studies of the transporter-mediated influx and efflux of endogenous and exogenous compounds, including synthetic drugs, across the blood-brain barrier (BBB) is beginning to provide a rational basis for controlling drug distribution to the brain. This paper describes mechanisms established in the last decade for carrier-mediated influx and efflux of drugs and endocytosis of biologically active peptides across the BBB. The transport systems at the BBB described here are the uptake transporters for nutrients, such as amino acids and hexoses, monocarboxylates, amines, carnitine and glutathione and efflux transporters, such as P-glycoprotein and multiple organic anion transporters. Delivery of cationized peptides across the BBB via adsorptive-mediated endocytosis is also described. By utilizing such highly specific transport mechanisms, it should be possible to establish strategies to regulate the entry of candidate drugs, including peptides, into the brain.