Transport mechanism of an H1-antagonist at the blood-brain barrier: transport mechanism of mepyramine using the carotid injection technique

The putative proton-coupled organic cation antiporter is involved in uptake of triptans into human brain capillary endothelial cells

BACKGROUND

Triptans are anti-migraine drugs with a potential central site of action. However, it is not known to what extent triptans cross the blood-brain barrier (BBB). The aim of this study was therefore to determine if triptans pass the brain capillary endothelium and investigate the possible underlying mechanisms with focus on the involvement of the putative proton-coupled organic cation (H+/OC) antiporter. Additionally, we evaluated whether triptans interacted with the efflux transporter, P-glycoprotein (P-gp).

METHODS

We investigated the cellular uptake characteristics of the prototypical H+/OC antiporter substrates, pyrilamine and oxycodone, and seven different triptans in the human brain microvascular endothelial cell line, hCMEC/D3. Triptan interactions with P-gp were studied using the IPEC-J2 MDR1 cell line. Lastly, in vivo neuropharmacokinetic assessment of the unbound brain-to-plasma disposition of eletriptan was conducted in wild type and mdr1a/1b knockout mice.

RESULTS

We demonstrated that most triptans were able to inhibit uptake of the H+/OC antiporter substrate, pyrilamine, with eletriptan emerging as the strongest inhibitor. Eletriptan, almotriptan, and sumatriptan exhibited a pH-dependent uptake into hCMEC/D3 cells. Eletriptan demonstrated saturable uptake kinetics with an apparent Km of 89 ± 38 µM and a Jmax of 2.2 ± 0.7 nmol·min-1·mg protein-1 (n = 3). Bidirectional transport experiments across IPEC-J2 MDR1 monolayers showed that eletriptan is transported by P-gp, thus indicating that eletriptan is both a substrate of the H+/OC antiporter and P-gp. This was further confirmed in vivo, where the unbound brain-to-unbound plasma concentration ratio (Kp,uu) was 0.04 in wild type mice while the ratio rose to 1.32 in mdr1a/1b knockout mice.

CONCLUSIONS

We have demonstrated that the triptan family of compounds possesses affinity for the H+/OC antiporter proposing that the putative H+/OC antiporter plays a role in the BBB transport of triptans, particularly eletriptan. Our in vivo studies indicate that eletriptan is subjected to simultaneous brain uptake and efflux, possibly facilitated by the putative H+/OC antiporter and P-gp, respectively. Our findings offer novel insights into the potential central site of action involved in migraine treatment with triptans and highlight the significance of potential transporter related drug-drug interactions.

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.

Substrates of the Human Brain Proton-Organic Cation Antiporter and Comparison with Organic Cation Transporter 1 Activities

Many organic cations (OCs) may be transported through membranes by a genetically still uncharacterized proton-organic cation (H + OC) antiporter. Here, we characterized an extended substrate spectrum of this antiporter. We studied the uptake of 72 drugs in hCMEC/D3 cells as a model of the human blood–brain barrier. All 72 drugs were tested with exchange transport assays and the transport of 26 of the drugs was studied in more detail concerning concentration-dependent uptake and susceptibility to specific inhibitors. According to exchange transport assays, 37 (51%) drugs were good substrates of the H + OC antiporter. From 26 drugs characterized in more detail, 23 were consistently identified as substrates of the H + OC antiporter in six different assays and transport kinetic constants could be identified with intrinsic clearances between 0.2 (ephedrine) and 201 (imipramine) mL × minute⁻¹ × g protein⁻¹. Excellent substrates of the H + OC antiporter were no substrates of organic cation transporter OCT1 and vice versa. Good substrates of the H + OC antiporter were more hydrophobic and had a lower topological polar surface area than non-substrates or OCT1 substrates. These data and further research on the H + OC antiporter may result in a better understanding of pharmacokinetics, drug–drug interactions and variations in pharmacokinetics.

Differences in Cerebral Distribution between Imipramine and Paroxetine via Membrane Transporters at the Rat Blood-Brain Barrier

PURPOSE

The present study aimed to elucidate the transport properties of imipramine and paroxetine, which are the antidepressants, across the blood-brain barrier (BBB) in rats.

METHODS

In vivo influx and efflux transport of imipramine and paroxetine across the BBB were tested using integration plot analysis and a combination of brain efflux index and brain slice uptake studies, respectively. Conditionally immortalized rat brain capillary endothelial cells, TR-BBB13 cells, were utilized to characterize imipramine and paroxetine transport at the BBB in vitro.

RESULTS

The in vivo influx clearance of [3H]imipramine and [3H]paroxetine in rats was determined to be 0.322 mL/(min·g brain) and 0.313 mL/(min·g brain), respectively. The efflux clearance of [3H]imipramine and [3H]paroxetine was 0.380 mL/(min·g brain) and 0.126 mL/(min·g brain), respectively. These results suggest that the net flux of paroxetine, but not imipramine, at the BBB in vivo was dominated by transport to the brain from the circulating blood. The uptake of imipramine and paroxetine by TR-BBB13 cells exhibited time- and temperature-dependence and one-saturable kinetics with a Km of 37.6 μM and 89.2 μM, respectively. In vitro uptake analyses of extracellular ion dependency and the effect of substrates/inhibitors for organic cation transporters and transport systems revealed minor contributions to known transporters and transport systems and the difference in transport properties in the BBB between imipramine and paroxetine.

CONCLUSIONS

Our study showed the comprehensive outcomes of imipramine and paroxetine transport at the BBB, implying that molecular mechanism(s) distinct from previously reported transporters and transport systems are involved in the transport.

[Development of Novel Methodology and Its Application for Clarifying the Transport Function of the Blood-brain Barrier]

The blood-brain barrier (BBB) consists of brain capillary endothelial cells linked by tight junctions and serves to regulate the transfer of endogenous compounds and xenobiotics between the circulating blood and brain interstitial fluid. We have developed a methodology to characterize brain-to-blood efflux transport in vivo, using the Brain Efflux Index and an in vitro culture model of the BBB, i.e., a conditionally immortalized cell line of the neurovascular unit. Employing these methods, we showed that the BBB plays an important role in protecting the brain by transporting neurotransmitters, neuromodulators, metabolites, uremic toxins, and xenobiotics together with atrial natriuretic peptide from the brain interstitial fluid to the circulating blood. We also developed a highly selective, sensitive LC-MS/MS method for simultaneous protein quantification. We found significant species differences in the expression amounts of various BBB transporter proteins among mice, rats, marmosets, cynomolgus monkeys, and humans. Among transporter proteins at the BBB, multidrug resistance protein 1 (Mdr1/Abcb1) is known to generate a concentration gradient of unbound substrate drugs between the blood and brain. Based on measurements of the intrinsic efflux transport rate of Mdr1 and the protein expression amounts of Mdr1 in mouse brain capillaries and Mdr1-expressing cell lines, we predicted the unbound drug concentration gradients of 7 drugs in the mouse brain in vivo. This was the first successful prediction of in vivo drug transport activity from in vitro experimental data and transporter protein concentration in tissues. This methodology and findings should greatly advance central nervous system barrier research.

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.

The histaminergic system is involved in psychological stress-induced hyperthermia in rats

The histaminergic system modulates numerous physiological functions such as wakefulness, circadian rhythm, feeding, and thermoregulation. However, it is not yet known if this system is also involved in psychological stress-induced hyperthermia (PSH) and, if so, which histamine (H) receptor subtype mediates the effect. Therefore, we investigated the effects of pretreatments with intraperitoneal injections of mepyramine (an H1 receptor inverse agonist), cimetidine (an H2 receptor antagonist), and ciproxifan (an H3 receptor inverse agonist) on cage-exchange stress-induced hyperthermia (a model of PSH) by monitoring core body temperature (T_c) during both light (10:00 am-12:00 pm) and dark (10:00 pm-12:00 am) phases in conscious, freely moving rats. We also investigated the effects of these drugs on stress-induced changes in locomotor activity (L_a) to rule out the possibility that effects on T_c are achieved secondary to altered L_a Cage-exchange stress increased T_c within 20 min followed by a gradual decrease back to baseline T_c during both phases. In the light phase, mepyramine and cimetidine markedly attenuated PSH, whereas ciproxifan did not affect it. In contrast, in the dark phase, mepyramine dropped T_c by 1°C without affecting cage-exchange stress-induced hyperthermia, whereas cimetidine and ciproxifan did not affect both postinjection T_c and PSH Cage-exchange stress induced an increase in L_a, especially in the light phase, but none of these drugs altered cage-exchange stress-induced L_a in either circadian rhythm phase. These results suggest that the histaminergic system is involved in the physiological mechanisms underlying PSH, particularly through H1 and H2 receptors, without influencing locomotor activity.

Role of cationic drug-sensitive transport systems at the blood-cerebrospinal fluid barrier in para-tyramine elimination from rat brain

Background

para-Tyramine (p-TA) is a biogenic amine which is involved in multiple neuronal signal transductions. Since the concentration of p-TA in dog cerebrospinal fluid (CSF) has been reported to be greater than that in plasma, it is proposed that clearance of cerebral p-TA is important for normal function. The purpose of this study was to examine the role of the blood–brain barrier and blood-cerebrospinal fluid barrier (BCSFB) in p-TA clearance from the brain.

Methods

In vivo [³H]p-TA elimination from rat cerebral cortex and from CSF was examined after intracerebral and intracerebroventricular administration, respectively. To evaluate BCSFB-mediated p-TA transport, [³H]p-TA uptake by isolated rat choroid plexus and conditionally immortalized rat choroid plexus epithelial cells, TR-CSFB3 cells, was performed.

Results

The half-life of [³H]p-TA elimination from rat CSF was found to be 2.9 min, which is 62-fold faster than that from rat cerebral cortex. In addition, this [³H]p-TA elimination from the CSF was significantly inhibited by co-injection of excess unlabeled p-TA. Thus, carrier-mediated p-TA transport process(es) are assumed to take part in p-TA elimination from the CSF. Since it is known that transporters at the BCSFB participate in compound elimination from the CSF, [³H]p-TA transport in ex vivo and in vitro models of rat BCSFB was examined. The [³H]p-TA uptake by isolated rat choroid plexus and TR-CSFB3 cells was time-dependent and was inhibited by unlabeled p-TA, indicating carrier-mediated p-TA transport at the BCSFB. The p-TA uptake by isolated choroid plexus and TR-CSFB3 cells was not reduced in the absence of extracellular Na⁺ and Cl⁻, and in the presence of substrates of typical organic cation transporters. However, this p-TA uptake was significantly inhibited by cationic drugs such as propranolol, imipramine, amantadine, verapamil, and pyrilamine. Moreover, p-TA uptake by TR-CSFB3 cells took place in an oppositely-directed H⁺ gradient manner. Therefore, this suggested that p-TA transport at the BCSFB involves cationic drug-sensitive transport systems which are distinct from typical plasma membrane organic cation transporters.

Conclusion

Our study indicates that p-TA elimination from the CSF is greater than that from the cerebral cortex. Moreover, it is suggested that cationic drug-sensitive transport systems in the BCSFB participate in this p-TA elimination from the CSF.

Involvement of Proton-Coupled Organic Cation Antiporter in Varenicline Transport at Blood-Brain Barrier of Rats and in Human Brain Capillary Endothelial Cells

Varenicline is a selective partial α₄β₂ nicotinic acetylcholine receptor agonist, which is used to help achieve smoking cessation. Here, we investigated varenicline transport at the blood-brain barrier by means of in vivo microdialysis, in situ brain perfusion, and brain efflux index measurements in rats, and in vitro uptake studies in human brain capillary endothelial cells. Microdialysis demonstrated that varenicline is actively transported from blood to brain in rats. Blood-to-brain uptake transport of varenicline, as measured by the in situ brain perfusion technique, was strongly inhibited by diphenhydramine, a potent inhibitor of proton-coupled organic cation (H⁺/OC) antiporter. However, brain efflux index study showed that brain-to-blood efflux transport of varenicline was not inhibited by diphenhydramine. In human brain capillary endothelial cells, varenicline was taken up time- and concentration-dependently. The uptake was dependent on an oppositely directed proton gradient, but was independent of extracellular sodium and membrane potential. The uptake was inhibited by a metabolic inhibitor, and by substrates of H⁺/OC antiporter, but not by substrates or inhibitors of OCTs, OCTNs, PMAT, and MATE1, which are known organic cation transporters. The present results suggest that the H⁺/OC antiporter contributes predominantly to varenicline uptake at the blood-brain barrier.

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.

Oxymorphone active uptake at the blood-brain barrier and population modeling of its pharmacokinetic-pharmacodynamic relationship

The aim of this study was to characterize the blood-brain barrier (BBB) transport and pharmacokinetics-pharmacodynamics (PKPD) relationship of oxymorphone and to further elucidate its possible contribution to oxycodone analgesia. The BBB transport of oxymorphone was studied using microdialysis in male Sprague-Dawley rats. Samples from microdialysis blood and brain probes, brain tissue, and plasma were analyzed by liquid chromatography with tandem mass spectrometry. The effect was measured as tail-flick latency. The study consisted of a PKPD experiment with combined microdialysis and antinociceptive measurements (n = 8), and another antinociceptive effect experiment (n = 9) using a 10 times lower dose. The combined data were analyzed with an integrated PKPD model in nonlinear mixed effect modeling utilizing a specific method (M3) for handling missing PD observations. The concentration of unbound oxymorphone was higher in brain than in blood, with a ratio of 1.9 (RSE, 9.7%), indicating active uptake at the BBB. The integrated PKPD model described the oxymorphone BBB transport and PKPD relationship successfully, with an EC50 in the brain of 63 ng/mL, and the M3 method was able to address the issue of censored observations. Oxymorphone has active uptake transport at the BBB in rats, with moderate uptake clearance to the brain. Its contribution to analgesia after oxycodone administration is not significant.

Functional expression of a proton-coupled organic cation (H+/OC) antiporter in human brain capillary endothelial cell line hCMEC/D3, a human blood-brain barrier model

Background

Knowledge of the molecular basis and transport function of the human blood–brain barrier (BBB) is important for not only understanding human cerebral physiology, but also development of new central nervous system (CNS)-acting drugs. However, few studies have been done using human brain capillary endothelial cells, because human brain materials are difficult to obtain. The purpose of this study is to clarify the functional expression of a proton-coupled organic cation (H⁺/OC) antiporter in human brain capillary endothelial cell line hCMEC/D3, which has been recently developed as an in vitro human BBB model.

Methods

Diphenhydramine, [³H]pyrilamine and oxycodone were used as cationic drugs that proved to be H⁺/OC antiporter substrates. The in vitro uptake experiments by hCMEC/D3 cells were carried out under several conditions.

Results

Diphenhydramine and [³H]pyrilamine were both transported into hCMEC/D3 cells in a time- and concentration-dependent manner with K_m values of 59 μM and 19 μM, respectively. Each inhibited uptake of the other in a competitive manner, suggesting that a common mechanism is involved in their transport. The diphenhydramine uptake was significantly inhibited by amantadine and quinidine, but not tetraethylammonium and 1-methyl-4-phenylpyridinium (substrates for well-known organic cation transporters). The uptake was inhibited by metabolic inhibitors, but was insensitive to extracellular sodium and membrane potential. Further, the uptake was increased by extracellular alkalization and intracellular acidification. These transport properties are completely consistent with those of previously characterized H⁺/OC antiporter in rat BBB.

Conclusions

The present results suggest that H⁺/OC antiporter is functionally expressed in hCMEC/D3 cells.

Physicochemical selectivity of the BBB microenvironment governing passive diffusion--matching with a porcine brain lipid extract artificial membrane permeability model

PURPOSE

To mimic the physicochemical selectivity of the blood-brain barrier (BBB) and to predict its passive permeability using a PAMPA model based on porcine brain lipid extract (PBLE 10%w/v in alkane).

METHODS

Three PAMPA (BD pre-coated and PBLE with 2 different lipid volumes) models were tested with 108 drugs. Abraham solvation descriptors were used to interpret the in vitro-in vivo correlation with 282 in situ brain perfusion measurements, spanning over 5 orders of magnitude. An in combo PAMPA model was developed from combining measured PAMPA permeability with one H-bond descriptor.

RESULTS

The in combo PAMPA predicted 93% of the variance of 197 largely efflux-inhibited in situ permeability training set. The model was cross-validated by the "leave-many-out" procedure, with q(2) = 0.92 ± 0.03. The PAMPA models indicated the presence of paramembrane water channels. Only the PBLE-based PAMPA-BBB model with sufficient lipid to fill all the internal pore space of the filter showed a wide dynamic range window, selectivity coefficient near 1, and was suitable for predicting BBB permeability.

CONCLUSION

BBB permeability can be predicted by in combo PAMPA. Its speed and substantially lower cost, compared to in vivo measurements, make it an attractive first-pass screening method for BBB passive permeability.

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.

Uptake of codeine into intestinal epithelial (Caco-2) and brain endothelial (RBE4) cells

Orally administered codeine has to permeate both the intestinal and the blood-brain barrier in order to act as analgesic and cough suppressant. In this study we characterized the uptake of codeine at intestinal epithelial (Caco-2) and brain endothelial (RBE4) cells. At both cell types, uptake of [(3)H]codeine was independent of an inwardly directed Na(+) gradient. Uptake was, however, strongly stimulated by an outwardly directed H(+) gradient and inhibited by the protonophore FCCP. [(3)H]Codeine uptake into Caco-2 cells was strongly temperature dependent. In the presence of excess amounts of unlabeled codeine, the uptake was inhibited by up to 87% (Caco-2) or 94% (RBE4), respectively. Synthetic opioids and some non-opioid organic cations like propranolol, pyrilamine and quinidine potently inhibited [(3)H]codeine uptake. Several prototype substrates of known transporters for amino acids, neurotransmitters and organic cations were ineffective. Our data are consistent with a hypothetic saturable, H(+)-dependent (antiport) mechanism not yet identified on a molecular level. The pH dependence of codeine uptake and its intracellular accumulation can partially also be explained by a model comprising diffusional membrane permeation of unionized species of codeine followed by codeine sequestration into acidic vesicles and distribution into cellular lipids.

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.

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.

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 of transport mechanism of pentazocine across the blood-brain barrier using the in situ rat brain perfusion technique

To characterize pentazocine (PTZ) transport across the blood-brain barrier (BBB), the cerebrovascular permeability-surface area product (PS(inf)) of PTZ was determined by a well-established in situ rat brain perfusion technique. The uptake kinetics of PTZ by the rat brain exhibited saturability, which indicates the simultaneous mechanisms of carrier-mediated transport and passive diffusion. The kinetic parameters were estimated as follows: maximal influx rate (V(max)), 27.2 +/- 5.2 nmol/s/g brain; apparent Michaelis constant (K(m)) for the saturable component of PTZ uptake, 2.9 +/- 0.5 mM; nonsaturable uptake rate constant (K(d)), 1.5 +/- 0.3 microL/s/g brain. BBB transport of PTZ was significantly inhibited by cationic drugs such as diphenhydramine, propranolol, and eptazocine (a narcotic-antagonist analgesic), but not by choline, suggesting that the PTZ transport system is shared by cationic drugs. Furthermore, co-perfusion of verapamil caused a significant (two-fold) increase in the BBB permeability to PTZ. This finding indicates that PTZ may be a substrate of the endogenous BBB efflux transport system, P-glycoprotein. These findings demonstrate that the primary mechanism governing the uptake of PTZ by the brain is carrier-mediated transport, not passive diffusion.

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.

ReN 1869, a novel tricyclic antihistamine, is active against neurogenic pain and inflammation

The tricyclic compound (R)-1-(3-(10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-ylidene)-1-propyl)-3-piperidine carboxylic acid (ReN 1869) is a novel, selective histamine H(1) receptor antagonist. It is orally available, well tolerated, easily enters the central nervous system (CNS) but no adverse effects are seen in mice at 300 mg/kg. ReN 1869 at 0.01-10 mg/kg is antinociceptive in tests of chemical nociception in rodents (formalin, capsaicin, phenyl quinone writhing) but not in thermal tests (hot plate and tail flick). ReN 1869 amplifies the analgesic action of morphine but does not show tolerance after chronic dosing. Moreover, the compound is effective against inflammation of neurogenic origin (antidromic nerve stimulation, histamine-evoked edema) but not in carrageenan-induced inflammation. We suggest that ReN 1869, via H(1) blockade, counteracts the effect of histamine liberated from activated mast cells and inhibits pain transmission in the dorsal spinal cord. ReN 1869 represents a new class of antihistamines with pain-relieving properties that probably is mediated centrally through histamine H(1) receptors but alternative mechanisms of action cannot be excluded.

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.

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.

A novel chemical delivery system for brain targeting

Two different chemical approaches for brain drug delivery and targeting are proposed in the present review. One is a chemical drug delivery using a ring-closure reaction to the hydrophilic quaternary thiazolium compound in the brain. The other is a chemical drug targeting utilizing the nutrient receptor (transporter) system on the blood-brain barrier. The brain delivery system has been optimized and it was demonstrated that the brain delivery of three drugs, a drug for Parkinson's disease, an excitatory amino acid antagonist and a free radical scavenger, were improved by the conjugation with cis-2-formylaminoethenylthio derivatives in vivo. As for the brain targeting system, it was demonstrated that the conjugation with L-Glu improved the drug's brain distribution via the L-Glu excitatory and/or transport receptors in vitro and in vivo. These findings suggest that the concepts of two chemical approaches will contribute to the development of new central nervous system drugs.

Drug transfer across the blood-brain barrier and improvement of brain delivery

The blood-brain barrier is formed by the endothelial cells of the brain capillaries. Its primary characteristic is the impermeability of the capillary wall due to the presence of complex tight junctions and a low endocytic activity. Essential nutrients are delivered to the brain by selective transport mechanisms, such as the glucose transporter and a variety of amino acid transporters. Although most drugs enter the brain by passive diffusion through the endothelial cells depending on their lipophilicity, degree of ionization, molecular weight, relative brain tissue and plasma bindings, some others can use specific endogenous transporters. In such cases, binding competition on the transporter with endogenous products or nutrients can occur and limits drug transfer. The blood-brain barrier can be a major impediment for the treatment of diseases of the central nervous system, since many drugs are unable to reach this organ at therapeutic concentrations. Various attempts have been made to overcome the limiting access of drugs to the brain, e.g. chemical modification, development of more hydrophobic analogs or linking an active compound to a specific carrier. Transient opening of the blood-brain barrier in humans has been achieved by intracarotid infusion of hypertonic mannitol solutions or of bradykinin analogs. Another way to increase or decrease brain delivery of drugs is to modulate the P-glycoprotein (P-gp) whose substrates are actively pumped out the cell into the capillary lumen. Many P-gp inhibitors or inducers are available to enhance the therapeutic effects of centrally acting drugs or to decrease central adverse effects of peripherally active drugs.

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

The blood-brain barrier (BBB) transport system for H1-antagonists was studied using primary cultured bovine brain capillary endothelial cells (BCEC). The uptake of [3H]mepyramine was inhibited by various H1-antagonists. Ketotifen competitively inhibited [3H]mepyramine uptake with an inhibition constant (Ki) of 46.8 microM. Lipophilic basic drugs such as propranolol, lidocaine and imipramine significantly inhibited [3H]mepyramine uptake. In particular, propranolol inhibited [3H]mepyramine uptake competitively at an inhibition constant (Ki) of 51.1 microM. Moreover, in ATP-depleted BCEC, [3H]mepyramine uptake was stimulated by preloading with H1-antagonists and lipophilic basic drugs. These results indicated that H1-antagonists are transported across the BBB via a carrier-mediated transport system common to lipophilic basic drugs.

Carrier-mediated transport of H1-antagonist at the blood-brain barrier: mepyramine uptake into bovine brain capillary endothelial cells in primary monolayer cultures

The transport mechanism of the H1-antagonist mepyramine at the blood-brain barrier (BBB) was studied by using primary cultured monolayers of bovine brain capillary endothelial cells (BCEC). The initial uptake of [3H]mepyramine into the BCEC showed strong temperature and concentration dependency, indicating that it involves both saturable and nonsaturable processes. Transport at the luminal membrane may be the rate-limiting process in the transcellular transport, since the values of the uptake coefficient of [3H]mepyramine at the luminal membrane (609 microliters/mg protein/min) and the transcellular permeability coefficient (488 microliters/mg protein/min) are very similar. The initial uptake of [3H]mepyramine was not affected by metabolic inhibitors, but was stimulated by preloading with the drug. Mepyramine appears to be transported into the BCEC by a carrier-mediated transport system which does not require metabolic energy, probably via a facilitated diffusion mechanism.