Role of efflux transport across the blood-brain barrier and blood-cerebrospinal fluid barrier on the disposition of xenobiotics in the central nervous system

Penetration of Organophosphate Triesters and Diesters across the Blood-Cerebrospinal Fluid Barrier: Efficiencies, Impact Factors, and Mechanisms

The penetration of organophosphate triesters (tri-OPEs) and diesters (di-OPEs) across the blood-brain barrier and their influencing factors remain unclear in humans. In this study, 21 tri-OPEs and 8 di-OPEs were measured in 288 paired serum and cerebrospinal fluid (CSF) samples collected in Jinan, China. Six tri-OPEs were frequently detected in both serum and CSF, with median concentrations ranging from 0.062 to 1.62 and 0.042-1.11 ng/mL, respectively. Their penetration efficiencies across the blood-CSF barrier (BCSFB) (R_CSF/serum, C_CSF/C_serum) were calculated at 0.667-2.80, and these efficiencies first increased and then decreased with their log K_ow values. The reduced penetration efficiencies of triphenyl phosphate (TPHP) and 2-ethylhexyl diphenyl phosphate (EHDPP) may be attributed to their strong binding affinities for human serum albumin and p-glycoprotein due to their high hydrophobicity and aryl structure, as indicated by molecular docking. This suggests that active efflux transport may be involved in the penetration of TPHP and EHDPP in addition to passive diffusion similar to the other four tri-OPEs. Di-OPEs were found in few serum samples and even fewer CSF samples, indicating their limited BCSFB permeability. This may be due to their high polarity, low hydrophobicity, and ionic state in blood. This study has important implications for understanding the neurotoxicity of tri-OPEs and di-OPEs and the underlying mechanisms.

Exosomal delivery of therapeutic modulators through the blood-brain barrier; promise and pitfalls

Nowadays, a large population around the world, especially the elderly, suffers from neurological inflammatory and degenerative disorders/diseases. Current drug delivery strategies are facing different challenges because of the presence of the BBB, which limits the transport of various substances and cells to brain parenchyma. Additionally, the low rate of successful cell transplantation to the brain injury sites leads to efforts to find alternative therapies. Stem cell byproducts such as exosomes are touted as natural nano-drug carriers with 50-100 nm in diameter. These nano-sized particles could harbor and transfer a plethora of therapeutic agents and biological cargos to the brain. These nanoparticles would offer a solution to maintain paracrine cell-to-cell communications under healthy and inflammatory conditions. The main question is that the existence of the intact BBB could limit exosomal trafficking. Does BBB possess some molecular mechanisms that facilitate the exosomal delivery compared to the circulating cell? Although preliminary studies have shown that exosomes could cross the BBB, the exact molecular mechanism(s) beyond this phenomenon remains unclear. In this review, we tried to compile some facts about exosome delivery through the BBB and propose some mechanisms that regulate exosomal cross in pathological and physiological conditions.

Development of a Region-Specific Physiologically Based Pharmacokinetic Brain Model to Assess Hippocampus and Frontal Cortex Pharmacokinetics

Central nervous system drug discovery and development is hindered by the impermeable nature of the blood-brain barrier. Pharmacokinetic modeling can provide a novel approach to estimate CNS drug exposure; however, existing models do not predict temporal drug concentrations in distinct brain regions. A rat CNS physiologically based pharmacokinetic (PBPK) model was developed, incorporating brain compartments for the frontal cortex (FC), hippocampus (HC), "rest-of-brain" (ROB), and cerebrospinal fluid (CSF). Model predictions of FC and HC C_max, t_max and AUC were within 2-fold of that reported for carbamazepine and phenytoin. The inclusion of a 30% coefficient of variation on regional brain tissue volumes, to assess the uncertainty of regional brain compartments volumes on predicted concentrations, resulted in a minimal level of sensitivity of model predictions. This model was subsequently extended to predict human brain morphine concentrations, and predicted a ROB C_max of 21.7 ± 6.41 ng/mL when compared to "better" (10.1 ng/mL) or "worse" (29.8 ng/mL) brain tissue regions with a FC C_max of 62.12 ± 17.32 ng/mL and a HC C_max of 182.2 ± 51.2 ng/mL. These results indicate that this simplified regional brain PBPK model is useful for forward prediction approaches in humans for estimating regional brain drug concentrations.

Transporter-mediated L-glutamate elimination from cerebrospinal fluid: possible involvement of excitatory amino acid transporters expressed in ependymal cells and choroid plexus epithelial cells

BACKGROUND

L-Glutamate (L-Glu) is the major excitatory neurotransmitter in the CNS, and its level in cerebrospinal fluid (CSF) is reported to be increased in neuroexcitatory diseases such as epilepsy. Since L-Glu concentration in the CSF is reported to be lower than that in plasma, it has been proposed that some mechanisms of L-Glu clearance from the CSF operate in the brain. The purpose of this study was to elucidate the major pathway of L-Glu elimination from rat CSF and the transporters responsible.

METHODS

Protein expression and localization of excitatory amino acid transporters were examined by immunohistochemical analysis using specific antibodies. In vivo elimination of L-Glu from rat CSF was evaluated by intracerebroventricular administration. An L-Glu uptake study by using primary-cultured rat ependymal cells and isolated rat choroid plexus was performed to characterize L-Glu transport mechanisms.

RESULTS

An immunohistochemical analysis has shown that excitatory amino acid transporter (EAAT) 1 and EAAT3, which are D-aspartate-sensitive and kainate-insensitive L-Glu transporters, are localized on the CSF-side of rat ependymal cells and choroid plexus epithelial cells, respectively. In contrast, the kainate-sensitive L-Glu transporter, EAAT2, is not expressed in these cells. In vivo L-Glu elimination clearance from the rat CSF (189 μL/(min · rat)) was 23-fold higher than the CSF bulk flow rate, indicating that facilitative process(es) are involved in L-Glu elimination from the CSF. The in vivo [(3)H]L-Glu elimination from the CSF was significantly inhibited by unlabeled L-Glu and D-aspartate, but not kainate. Moreover, unlabeled L-Glu and D-aspartate inhibited [(3)H]L-Glu uptake by rat ependymal cells and choroid plexus epithelial cells, whereas kainate had little effect.

CONCLUSION

It is suggested that EAAT1 in ependymal cells and EAAT3 in choroid plexus epithelial cells participate in L-Glu elimination from the CSF.

Physiologically based pharmacokinetic modelling of drug penetration across the blood-brain barrier--towards a mechanistic IVIVE-based approach

Predicting the penetration of drugs across the human blood-brain barrier (BBB) is a significant challenge during their development. A variety of in vitro systems representing the BBB have been described, but the optimal use of these data in terms of extrapolation to human unbound brain concentration profiles remains to be fully exploited. Physiologically based pharmacokinetic (PBPK) modelling of drug disposition in the central nervous system (CNS) currently consists of fitting preclinical in vivo data to compartmental models in order to estimate the permeability and efflux of drugs across the BBB. The increasingly popular approach of using in vitro-in vivo extrapolation (IVIVE) to generate PBPK model input parameters could provide a more mechanistic basis for the interspecies translation of preclinical models of the CNS. However, a major hurdle exists in verifying these predictions with observed data, since human brain concentrations can't be directly measured. Therefore a combination of IVIVE-based and empirical modelling approaches based on preclinical data are currently required. In this review, we summarise the existing PBPK models of the CNS in the literature, and we evaluate the current opportunities and limitations of potential IVIVE strategies for PBPK modelling of BBB penetration.

The mastermind approach to CNS drug therapy: translational prediction of human brain distribution, target site kinetics, and therapeutic effects

Despite enormous advances in CNS research, CNS disorders remain the world's leading cause of disability. This accounts for more hospitalizations and prolonged care than almost all other diseases combined, and indicates a high unmet need for good CNS drugs and drug therapies.Following dosing, not only the chemical properties of the drug and blood-brain barrier (BBB) transport, but also many other processes will ultimately determine brain target site kinetics and consequently the CNS effects. The rate and extent of all these processes are regulated dynamically, and thus condition dependent. Therefore, heterogenious conditions such as species, gender, genetic background, tissue, age, diet, disease, drug treatment etc., result in considerable inter-individual and intra-individual variation, often encountered in CNS drug therapy.For effective therapy, drugs should access the CNS "at the right place, at the right time, and at the right concentration". To improve CNS therapies and drug development, details of inter-species and inter-condition variations are needed to enable target site pharmacokinetics and associated CNS effects to be translated between species and between disease states. Specifically, such studies need to include information about unbound drug concentrations which drive the effects. To date the only technique that can obtain unbound drug concentrations in brain is microdialysis. This (minimally) invasive technique cannot be readily applied to humans, and we need to rely on translational approaches to predict human brain distribution, target site kinetics, and therapeutic effects of CNS drugs.In this review the term "Mastermind approach" is introduced, for strategic and systematic CNS drug research using advanced preclinical experimental designs and mathematical modeling. In this way, knowledge can be obtained about the contributions and variability of individual processes on the causal path between drug dosing and CNS effect in animals that can be translated to the human situation. On the basis of a few advanced preclinical microdialysis based investigations it will be shown that the "Mastermind approach" has a high potential for the prediction of human CNS drug effects.

Tissue distribution and depuration kinetics of waterborne 14C-labeled light PAHs in mummichog (Fundulus heteroclitus)

Light polycyclic aromatic hydrocarbons (PAHs) of petrogenic origin are commonly found in estuaries and coastal areas. Though they are known to be toxic to fish, little is known about their uptake and tissue distribution. This paper reports on the results of a study on uptake, elimination, and tissue distribution of three waterborne 14C-labeled PAHs in the mummichog, Fundulus heteroclitus, using whole-body autoradiography. After a 24 h exposure to 1 μCi·L(-1) of 14C-naphthalene, 14C-1-naphthol, and 14C-phenanthrene, fish were transferred to clean water and tissue distribution examined after 0, 1, 3, 7, 14, and 21 days of depuration. All compounds were readily accumulated by fish and were also rapidly eliminated (t0.5 range=1.1 to 3.0 days). Most of the radioactivity in naphthalene- and phenanthrene-treated fish was found in gall bladder≫liver>intestinal lumen. In naphthol-exposed fish, an important labeling of some brain areas was observed. Brain of naphthalene-exposed fish was also labeled after 24 h depuration, indicating that exposure to naphthalene may result in metabolite accumulation in the brain. This is the first study showing that naphthalene, naphthol, and/or unidentified metabolite(s) can accumulate in brain tissues, which may impair normal brain function.

The effect of plasma protein binding on in vivo efficacy: misconceptions in drug discovery

Data from in vitro plasma protein binding experiments that determine the fraction of protein-bound drug are frequently used in drug discovery to guide structure design and to prioritize compounds for in vivo studies. However, we consider that these practices are usually misleading, because in vivo efficacy is determined by the free (unbound) drug concentration surrounding the therapeutic target, not by the free drug fraction. These practices yield no enhancement of the in vivo free drug concentration. So, decisions based on free drug fraction could result in the wrong compounds being advanced through drug discovery programmes. This Perspective provides guidance on the application of plasma protein binding information in drug discovery.

Nature and consequences of mammalian brain and CSF efflux transporters: four decades of progress

In the last 40 years, especially with the application of new neurochemical and molecular biological techniques, there has been explosive progress in understanding how certain ligands and drugs are transported across the blood-brain barrier and choroid plexus out of brain and CSF. In the CNS, there are several separate efflux transporters with very broad specificity that are responsible for much of the efflux transport. This review focuses on three such transporters: organic acid transporter-3, peptide transporter-2 and P-glycoprotein for which there is substantial new information including 'knockout' models in mice and, in one case, dogs. Moreover, the structural biology and transport mechanism of P-glycoprotein at 3.8 angstroms is described. The overall objective is to show how this new knowledge provides a more thorough understanding (e.g., of molecular mechanisms) of efflux transport and in several cases leads to clinically relevant information that allows better treatment of certain CNS disorders (e.g., meningitis and brain cancer).

The transport of manganese across the blood-brain barrier

The mammalian central nervous system (CNS) possesses a unique and specialized capillary adaptation, referred to as the blood-brain barrier (BBB). The BBB maintains an optimal neuronal microenvironment, regulating blood-tissue exchange of macromolecules and nutrients. The BBB is characterized by individual endothelial cells that are continuously linked by tight junctions, inhibiting the diffusion of macromolecules and solutes between adjacent endothelial cells. This review will focus on pertinent issues to BBB maintenance, and survey recent dogmas on the transport mechanisms for the essential metal, manganese, across this barrier. Specifically, putative carriers for manganese into and out of the brain will be discussed.

Toward the prediction of CNS drug-effect profiles in physiological and pathological conditions using microdialysis and mechanism-based pharmacokinetic-pharmacodynamic modeling

Our ultimate goal is to develop mechanism-based pharmacokinetic (PK)-pharmacodynamic (PD) models to characterize and to predict CNS drug responses in both physiologic and pathologic conditions. To this end, it is essential to have information on the biophase pharmacokinetics, because these may significantly differ from plasma pharmacokinetics. It is anticipated that biophase kinetics of CNS drugs are strongly influenced by transport across the blood-brain barrier (BBB). The special role of microdialysis in PK/PD modeling of CNS drugs lies in the fact that it enables the determination of free-drug concentrations as a function of time in plasma and in extracellular fluid of the brain, thereby providing important data to determine BBB transport characteristics of drugs. Also, the concentrations of (potential) extracellular biomarkers of drug effects or disease can be monitored with this technique. Here we describe our studies including microdialysis on the following: (1) the evaluation of the free drug hypothesis; (2) the role of BBB transport on the central effects of opioids; (3) changes in BBB transport and biophase equilibration of anti-epileptic drugs; and (4) the relation among neurodegeneration, BBB transport, and drug effects in Parkinson's disease progression.

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

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

Rapid elimination of cefaclor from the cerebrospinal fluid is mediated by a benzylpenicillin-sensitive mechanism distinct from organic anion transporter 3

The purpose of this study was to investigate the carrier-mediated elimination of cephalosporins from the cerebrospinal fluid (CSF) via the choroid plexus. Cefaclor and cefalexin are structural analogs with similar lipophilicity, differing by only one functional group (cefaclor, -Cl; cephalexin, -CH(3)), and they are substrates of rat peptide transporter PEPT2 with similar transport activities. However, cefaclor was cleared from the CSF more rapidly than cefalexin after intracerebroventricular administration (the elimination rate constants were 0.11 and 0.050 min(-1), respectively). The elimination of cefaclor from the CSF was inhibited by benzylpenicillin, but not by glycylsarcosine (GlySar), whereas GlySar, but not benzylpenicillin, had an inhibitory effect on the elimination of cefalexin from the CSF. The uptake of cefaclor by the freshly isolated rat choroid plexus was saturable, with a K(m) value of 250 muM, and the uptake clearance corresponding to saturable components accounts for the major part of the in vivo clearance from the CSF (17 versus 26 mul/min, respectively). The uptake of cefaclor by the choroid plexus was inhibited by benzylpenicillin, but not by GlySar. However, the inhibitory effect of benzylpenicillin was weaker than expected from its own K(m) value, and furthermore, organic anion transporter (Oat)3 substrates (cimetidine or p-aminohippurate) had no effect. These results suggest that cefaclor and cefalexin are eliminated from the CSF by different transporters, and rapid elimination of cefaclor from the CSF is accounted for by a benzylpenicillin-sensitive mechanism distinct from Oat3. A slight modification of a single chemical group of cephalosporins can greatly affect the contribution of the transporters involved, and their duration in the CSF.

Drug transporters: their role and importance in the selection and development of new drugs

Drug transporters expressed in various tissues play a significant role in drug disposition. By regulating the function of such transporters, it may be possible to eventually develop drugs with ideal pharmacokinetic profiles. In this article, we summarize the significant role played by drug transporters in drug disposition, focusing particularly on their potential use during the drug development process. The ability to manipulate transporter function offers the opportunity of being able to deliver a drug to the target organ, avoiding distribution to other organs (thereby reducing the chance of toxic side-effects), controlling the elimination process, and/or improving oral bioavailability. During drug development, it would be very useful to be able to select a lead compound that may or may not interact with transporters, depending on whether such an interaction is desirable. The use of specific inhibitors of transporters is also an attractive approach to controlling drug disposition, leading to improved efficacy. Currently, optimizing the pharmacokinetic properties of a drug during the early stages of its development is widely accepted as being of great importance. High-throughput screening systems using transporter gene transfected cells or computational (in silico) approaches are efficient tools for assessing transport activity during the early stage of drug development. In addition, drug-drug interactions involving drug transporters and functional genetic polymorphisms of drug transporters are also described. It would also be extremely valuable to be able to quantitatively predict inter-individual pharmacokinetic differences caused by transporter polymorphisms or pharmacokinetic changes caused by drug-drug interactions involving transporters during drug development.

Identification of P-glycoprotein substrates and inhibitors among psychoactive compounds--implications for pharmacokinetics of selected substrates

The pharmacokinetics of antipsychotic drugs has become an integral part in understanding their pharmacodynamic activity and clinical effects. In addition to metabolism aspects, carrier-mediated transport, particularly secretion by ABC transporters, has been discussed as potentially relevant for this group of therapeutics. In this study, the psychoactive compounds perphenazine, flupentixol, domperidone, desmethyl clozapine, haloperidol, fluphenazine, fluvoxamine, olanzapine, levomepromazine, perazine, desmethyl perazine, clozapine, quetiapine and amisulpride were characterized in terms of P-glycoprotein (P-gp) affinity and transport. Experimental methods involved a radioligand displacement assay with [3H]talinolol as radioligand and transport--as well as transport inhibition--studies of the P-gp substrate [3H]talinolol across Caco-2 cell monolayers. In addition, the physicochemical descriptors log P and deltalog P were determined to test potential correlations between transporter affinity and lipophilicity parameters. All of the tested antipsychotics showed affinity to P-gp albeit their IC50 values (concentration of competitor that displaced 50% of the bound radioligand) differed by a factor exceeding 1000, when compared using the transport inhibition assay. From the group of P-gp substrates, amisulpride and fluphenazine were selected for in-vivo drug-drug interaction studies in rats to demonstrate the in-vivo relevance of the in-vitro findings. Compounds were administered by intraperitoneal injection either alone or in combination with 50 mg kg(-1) ciclosporin. The concentration versus time profiles for both drugs were followed in serum as well as in brain tissues. Significant differences between the treatments with the antipsychotic alone versus the combination of antipsychotic with ciclosporin were found for amisulpride. The distribution of amisulpride to the brain was increased and systemic serum levels were likewise increased indicating decreased systemic clearance for the combination regimen. For fluphenazine, systemic levels with and without co-administration of ciclosporin were comparable while higher brain-to-serum concentration ratios were found after co-administration of ciclosporin. The findings are explained on the basis of the limited contribution of P-gp-mediated transport to the elimination of fluphenazine and to a direct effect with respect to its distribution into the brain.

Involvement of Rat Organic Anion Transporter 3 in the Uptake of an Organic Herbicide, 2,4-Dichlorophenoxyacetate, by the isOlated Rat Choroid Plexus

2,4-Dichlorophenoxyacetic acid (2,4-D) is an anionic herbicide. The purpose of the present study is to examine whether organic anion transporter 3 (Oat3; Slc22a8) is solely responsible for the uptake of 2,4-D by the isolated rat choroid plexus (CP). When expressed in LLC-PK1 cells, rOat3 was mainly localized to the basolateral membrane. Although there was no vectorial transport of 2,4-D in the control LLC-PK1 cells, expression of rOat3 increased the basal-to-apical transport of 2,4-D fourfold without affecting the transcellular transport in the opposite direction. The basal-to-apical transport of 2,4-D in rOat3-LLC was saturable with a K(m) value of 20 microM. The uptake of 2,4-D by the isolated rat CP was determined using the centrifugal filtration method. Saturable uptake of 2,4-D was observed in the isolated rat CP with a K(m) value of 22 microM. Probenecid and substrates of rOat3, such as p-aminohippurate, benzylpenicillin, and cimetidine, inhibited the uptake of 2,4-D by the isolated rat CP. Their K(i) values were comparable with those for the uptake of benzylpenicillin by the isolated rat CP, which is mainly mediated by rOat3. Furthermore, benzylpenicillin was a competitive inhibitor for the uptake of 2,4-D by the isolated rat CP. These results suggest that 2,4-D and benzylpenicillin share the same transporter for their uptake by the isolated rat CP, and rOat3 is the most likely candidate transporter.

Development of an in vitro blood–brain barrier model based on immortalized porcine brain microvascular endothelial cells

Immortalized porcine brain microvessel endothelial cells (PBMEC/C1-2) were used to develop a model for measurement of blood-brain barrier permeation of central nervous system active drugs. Previous studies showed that a system using C6 astrocyte glioma conditioned medium leads to cell layers with transendothelial electrical resistance values up to 300 Omega cm(2) and a permeability coefficient P(e) of 3.24 +/- 0.14 x 10(-4) cm/min for U-[(14)C]sucrose, which is in good agreement to published values and thus indicates the formation of tight junctions in vitro. However, commercially available inserts for the Transwell system were not permeable for highly lipophilic compounds, such as diazepam. Systematic studies with different insert showed, that inserts with a pore width of 1 microm proved to be optimal for permeation studies of lipophilic compounds. Permeability studies with a set of three benzodiazepines further supported this finding.

Do Multidrug Resistance‐Associated Protein‐1 and ‐2 Play Any Role in the Elimination of Estradiol‐17β‐Glucuronide and 2,4‐Dinitrophenyl‐S‐Glutathione Across the Blood–Cerebrospinal Fluid Barrier?

The purpose of this study was to examine the role of multidrug resistance-associated protein-1 and -2 (Mrp1 and Mrp2) in the efflux transport of organic anions across the blood-cerebrospinal fluid (CSF) barrier. The CSF concentration of estradiol-17beta-glucuronide (E(2)17betaG) and 2,4-dinitrophenyl-S-glutathione (DNP-SG) in the CSF after intracerebroventricular and intravenous injection were compared between wild-type and Mrp1 gene knockout mice. There was no significant difference in the apparent CSF elimination rate constants of E(2)17betaG (0.158 and 0.145 min(-1)) and DNP-SG (0.116 and 0.0779 min(-1)) between wild-type and Mrp1 knockout mice, respectively. After intravenous administration of E(2)17betaG, its brain-to-serum and CSF-to-serum concentration ratios in Mrp1 knockout mice were not significantly different from those in the wild-type. Results from in vivo and in vitro studies using Eisai hyperbilirubinemic rats, in which Mrp2 is hereditarily deficient, were similar to those using normal rats. Quantitative polymerase chain reaction (PCR) showed that the expression level of Mrp4 and Mrp5 was several times higher than that of Mrp1, whereas the expression levels of Mrp2, Mrp3, and Mrp6 were negligible or low. Therefore, Mrp4 and Mrp5 may contribute to the efflux transport of E(2)17betaG and DNP-SG from the CSF.

Brain barrier systems: a new frontier in metal neurotoxicological research

The concept of brain barriers or a brain barrier system embraces the blood-brain interface, referred to as the blood-brain barrier, and the blood-cerebrospinal fluid (CSF) interface, referred to as the blood-CSF barrier. These brain barriers protect the CNS against chemical insults, by different complementary mechanisms. Toxic metal molecules can either bypass these mechanisms or be sequestered in and therefore potentially deleterious to brain barriers. Supportive evidence suggests that damage to blood-brain interfaces can lead to chemical-induced neurotoxicities. This review article examines the unique structure, specialization, and function of the brain barrier system, with particular emphasis on its toxicological implications. Typical examples of metal transport and toxicity at the barriers, such as lead (Pb), mercury (Hg), iron (Fe), and manganese (Mn), are discussed in detail with a special focus on the relevance to their toxic neurological consequences. Based on these discussions, the emerging research needs, such as construction of the new concept of blood-brain regional barriers, understanding of chemical effect on aged or immature barriers, and elucidation of the susceptibility of tight junctions to toxicants, are identified and addressed in this newly evolving field of neurotoxicology. They represent both clear challenges and fruitful research domains not only in neurotoxicology, but also in neurophysiology and pharmacology.

Mechanisms of Cefadroxil Uptake in the Choroid Plexus: Studies in Wild-Type and PEPT2 Knockout Mice

The choroid plexus uptake of [(3)H]cefadroxil was studied in peptide transporter 2 (PEPT2) wild-type and null mice as a function of temperature, transport inhibitors, pH, and saturability. At normal pH (7.4) and temperature (37 degrees C), the uptake of 1 microM cefadroxil was reduced by 83% in PEPT2(-/-) mice as compared with PEPT2(+/+) mice (p < 0.001). A further reduction was achieved in null animals by reducing the temperature to 4 degrees C, or by adding saturating concentrations of unlabeled cefadroxil or p-aminohippurate (p < 0.05). Glycylsarcosine coadministration could inhibit the uptake of cefadroxil in PEPT2(+/+) mice (p < 0.01) but not PEPT2(-/-) mice. Although a proton-stimulated uptake of cefadroxil was demonstrated in PEPT2(+/+) mice (pH 6.5 versus pH 7.4; p < 0.01), no pH dependence was observed in PEPT2(-/-) mice. Kinetic parameters for cefadroxil (without p-aminohippurate) in wild-type mice were: V(max) = 5.4 pmol/mg/min, K(m) = 34 microM, and K(d) = 0.0069 microl/mg/min; in the presence of p-aminohippurate, the parameters were: V(max) = 4.1 pmol/mg/min, K(m) = 27 microM, and K(d) = 0.0064 microl/mg/min. In null animals, the kinetic parameters of cefadroxil (without p-aminohippurate) were: V(max) = 2.7 pmol/mg/min, K(m) = 110 microM, and K(d) = 0.0084 microl/mg/min; in the presence of p-aminohippurate, only a K(d) = 0.010 microl/mg/min was observed. Based on kinetic and inhibitor analyses, it was determined that (under linear conditions), 80 to 85% of cefadroxil's uptake in choroid plexus is mediated by PEPT2, 10 to 15% by organic anion transporter(s), and 5% by nonspecific mechanisms. These findings demonstrate that PEPT2 is the primary transporter responsible for cefadroxil uptake in the choroid plexus. Moreover, the data suggest a role for PEPT2 in the clearance of peptidomimetics from cerebrospinal fluid.

Functional characterization of rat brain-specific organic anion transporter (Oatp14) at the blood-brain barrier: high affinity transporter for thyroxine

Oatp14/blood-brain barrier-specific anion transporter 1 (Slc21a14) is a novel member of the organic anion transporting polypeptide (Oatp/OATP) family. Northern blot analysis revealed predominant expression of Oatp14 in the brain, and Western blot analysis revealed its expression in the brain capillary and choroid plexus. Immunohistochemical staining indicated that Oatp14 is expressed in the border of the brain capillary endothelial cells. When expressed in human embryonic kidney 293 cells, Oatp14 transports thyroxine (T4; prothyroid hormone) (Km = 0.18 mum), as well as amphipathic organic anions such as 17beta estradiol-d-17beta-glucuronide (Km = 10 mum), cerivastatin (Km = 1.3 mum), and troglitazone sulfate (Km = 0.76 mum). The uptake of triiodothyronine (T3), an active form produced from T4, was significantly greater in Oatp14-expressed cells than in vector-transfected cells, but the transport activity for T3 was approximately 6-fold lower that for T4. The efflux of T4, preloaded into the cells, from Oatp14-expressed cells was more rapid than that from vector-transfected cells (0.032 versus 0.006 min-1). Therefore, Oatp14 can mediate a bidirectional transport of T4. Sulfobromophthalein, taurocholate, and estrone sulfate were potent inhibitors for Oatp14, whereas digoxin, p-aminohippurate, or leukotriene C4, or organic cations such as tetraetheylammonium or cimetidine had no effect. The expression levels of Oatp14 mRNA and protein were up- and down-regulated under hypo- and hyperthyroid conditions, respectively. Therefore, it may be speculated that Oatp14 plays a role in maintaining the concentration of T4 and, ultimately, T3 in the brain by transporting T4 from the circulating blood to the brain.

Impact of drug transporter studies on drug discovery and development

Drug transporters are expressed in many tissues such as the intestine, liver, kidney, and brain, and play key roles in drug absorption, distribution, and excretion. The information on the functional characteristics of drug transporters provides important information to allow improvements in drug delivery or drug design by targeting specific transporter proteins. In this article we summarize the significant role played by drug transporters in drug disposition, focusing particularly on their potential use during the drug discovery and development process. The use of transporter function offers the possibility of delivering a drug to the target organ, avoiding distribution to other organs (thereby reducing the chance of toxic side effects), controlling the elimination process, and/or improving oral bioavailability. It is useful to select a lead compound that may or may not interact with transporters, depending on whether such an interaction is desirable. The expression system of transporters is an efficient tool for screening the activity of individual transport processes. The changes in pharmacokinetics due to genetic polymorphisms and drug-drug interactions involving transporters can often have a direct and adverse effect on the therapeutic safety and efficacy of many important drugs. To obtain detailed information about these interindividual differences, the contribution made by transporters to drug absorption, distribution, and excretion needs to be taken into account throughout the drug discovery and development process.

Choroid plexus controls brain availability of anti-HIV nucleoside analogs via pharmacologically inhibitable organic anion transporters

OBJECTIVE

In AIDS, early suppression of the viral load in the central nervous system is critical for the efficacy of antiretroviral therapy, in order to prevent the emergence of a reservoir of resistant strains of virus, and brain impairment in late stages of the infection. The blood-cerebrospinal fluid (CSF) interface (i.e. the choroidal epithelium) constitutes the most direct route to reach the ventricular meningeal and perivascular infected macrophages, and may modulate the cerebral biodisposition of antiretroviral drugs through various transport systems. Our aim was to address nucleoside drug transfer specifically across the blood-CSF interface, and identify the possible mechanisms involved in their transport.

METHODS

Drug influx and efflux were measured using an in vitro cellular model that reproduces the barrier and transport properties of the blood-CSF interface in vivo. Transport mechanisms were investigated by competition studies.

RESULTS

The CSF influx rate of zidovudine was the highest, although moderate, followed by that of stavudine. The permeability coefficients of the other drugs tested were low. Zidovudine influx into the CSF is independent of thymidine transport systems, and more importantly is limited by an efflux mechanism. This efflux involves an apical (CSF-facing) carrier belonging to the solute carrier (Slc) 22 family of organic anion transporters, and can be inhibited by a therapeutic concentration of benzbromarone.

CONCLUSIONS

The demonstration and characterization of this efflux mechanism is the basis for the development of specific inhibitory agents in view to increase the delivery of antiretroviral nucleoside analogs to the brain.

Contribution of organic anion transporter 3 (Slc22a8) to the elimination of p-aminohippuric acid and benzylpenicillin across the blood-brain barrier

The role of rat organic anion transporter 3 (rOat3; Slc22a8) in the efflux transport at the blood-brain barrier (BBB) was characterized. The expression of rOat1, rOat2, and rOat3 in the brain capillary endothelial cells (BCEC) was examined using reverse transcription-polymerase chain reaction analysis, which showed that there was no expression of rOat1 or rOat2, but moderate expression of rOat3. The expression of rOat3 in the BCEC was further confirmed by Western blotting. Immunohistochemical staining showed that rOat3 is located on the abluminal and, possibly, luminal membrane of the BCEC. The contribution of rOat3 to the efflux of para-aminohippuric acid (PAH) and benzylpenicillin (PCG), substrates of rOat3, from the cerebrum into the blood circulation across the BBB was evaluated using the Brain Efflux Index method. PAH and PCG were eliminated from the cerebrum with rate constants of 0.039 and 0.043 min-1, respectively, and the elimination was saturated at high substrate concentrations. Taking account of the dilution in the brain, the Km values for the elimination of PAH and PCG were estimated to be 168 and 29 micro M, respectively. The efflux of PAH and PCG across the BBB was inhibited in a dose-dependent manner by unlabeled PCG and PAH, respectively. The Ki value of PAH for the efflux of PCG was 106 micro M and that of PCG for the efflux of PAH was 58 micro M. These values were comparable with their Km values, suggesting that they share the same efflux mechanism at the BBB. Furthermore, cimetidine and pravastatin, which are also substrates and inhibitors of rOat3, significantly inhibited the efflux of PAH and PCG from the cerebrum. These results suggest that rOat3 is responsible for the elimination of PAH and PCG from the brain across the BBB.

In vitro study of the functional expression of organic anion transporting polypeptide 3 at rat choroid plexus epithelial cells and its involvement in the cerebrospinal fluid-to-blood transport of estrone-3-sulfate

The cerebrospinal fluid-to-blood efflux transport of estrone-3-sulfate (E(1)S) via the blood-cerebrospinal fluid barrier (BCSFB) may play an important role in regulating E(1)S levels in the brain. Here, we investigated the efflux transport of E(1)S at the BCSFB using conditionally immortalized rat choroid plexus epithelial cells (TR-CSFB) and identified the responsible transporter. The [(3)H]E(1)S uptake by TR-CSFB cells was composed of saturable and nonsaturable components, and the K(m) and V(max) values of the saturable component were determined to be 16.8 +/- 5.1 microM and 12.3 +/- 2.3 pmol/min/mg of protein, respectively. [(3)H]E(1)S uptake was inhibited by probenecid, cholate, taurocholate, sulfobromophthalein, dehydroepiandrosterone sulfate, triiodothyronine, thyroxin, and digoxin but not by p-aminohippuric acid, gamma-aminobutyric acid, or methotrexate, suggesting the involvement of organic anion transporting polypeptide (oatp) in the uptake. Reverse transcription-polymerase chain reaction analysis revealed that oatp3 was expressed in TR-CSFB cells and isolated rat choroid plexus, although oatp1 was not detected in either. Xenopus laevis oocytes expressing oatp3 exhibited [(3)H]E(1)S uptake activity with a K(m) of 8.09 +/- 2.83 microM and V(max) of 8.02 +/- 0.87 pmol/h/oocyte. Moreover, oatp3 is localized at the brush-border membrane of choroid plexus epithelial cells. These results suggest that oatp3 is involved in the E(1)S efflux transport at the BCSFB.

Expression of Human Organic Anion Transporters in the Choroid Plexus and Their Interactions With Neurotransmitter Metabolites

The purpose of the present study was to elucidate the expression of human organic anion transporter 1 (hOAT1) and hOAT3 in the choroid plexus of the human brain and their interactions with neurotransmitter metabolites using stable cell lines. Immunohistochemical analysis revealed that hOAT1 and hOAT3 are expressed in the cytoplasmic membrane and cytoplasm of human choroid plexus. Neurotransmitter metabolites, namely, 5-methoxyindole-3-acetic acid (5-MI-3-AA), homovanillic acid (HVA), vanilmandelic acid (VMA), 3,4-dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindole-3-acetic acid (5-HI-3-AA), N-acetyl-5-hydroxytryptamine (NA-5-HTT), melatonin, 5-methoxytryptamine (5-MTT), 3,4-dihidroxymandelic acid (DHMA), 5-hydroxytryptophol, and 5-methoxytryptophol (5-MTP), but not methanephrine (MN), normethanephrine (NMN), and 3-methyltyramine (3-MT), at 2 mM, inhibited para-aminohippuric acid uptake mediated by hOAT1. On the other hand, melatonin, 5-MI-3-AA, NA-5-HTT, 5-MTT, 5-MTP, HVA, 5-HI-3-AA, VMA, DOPAC, 5-hydroxytryptophol, and MN, but not 3-MT, DHMA, and NMN, at 2 mM, inhibited estrone sulfate uptake mediated by hOAT3. Differences in the IC(50) values between hOAT1 and hOAT3 were observed for DHMA, DOPAC, HVA, 5-HI-3-AA, melatonin, 5-MI-3-AA, 5-MTP, 5-MTT, and VMA. HOAT1 and hOAT3 mediated the transport of VMA but not HVA and melatonin. These results suggest that hOAT1 and hOAT3 are involved in the efflux of various neurotransmitter metabolites from the cerebrospinal fluid to the blood across the choroid plexus.

Considerations in the use of cerebrospinal fluid pharmacokinetics to predict brain target concentrations in the clinical setting: implications of the barriers between blood and brain

In the clinical setting, drug concentrations in cerebrospinal fluid (CSF) are sometimes used as a surrogate for drug concentrations at the target site within the brain. However, the brain consists of multiple compartments and many factors are involved in the transport of drugs from plasma into the brain and the distribution within the brain. In particular, active transport processes at the level of the blood-brain barrier and blood-CSF barrier, such as those mediated by P-glycoprotein, may lead to complex relationships between concentrations in plasma, ventricular and lumbar CSF, and other brain compartments. Therefore, CSF concentrations may be difficult to interpret and may have limited value. Pharmacokinetic data obtained by intracerebral microdialysis monitoring may be used instead, providing more valuable information. As non-invasive alternative techniques, positron emission tomography or magnetic resonance spectroscopy may be of added value.

Expression and functional characterization of rat organic anion transporter 3 (rOat3) in the choroid plexus

We reported previously that an efficient efflux system for benzylpenicillin (PCG) is located on the choroid plexus (CP). In this study, we investigated the involvement of rat organic anion transporter 1 (rOat1; Slc22a6) and rOat3 (Slc22a8) in the uptake of PCG and p-aminohippurate (PAH) by the CP. Western blot analysis indicates the expression of rOat3, but not rOat1, on the CP, and immunohistochemical staining shows that rOat3 is localized on the brush border membrane of the choroid epithelial cells. PCG and PAH were found to be taken up by isolated rat CP, with K(m) values of 111 and 354 microM, respectively. A mutual inhibition study suggests that the same transporter is responsible for the uptake of PCG and PAH by isolated rat CP. This was confirmed by examining the effect of organic anions and cimetidine on their uptake. Estradiol-17beta-glucuronide and cimetidine were found to be selective inhibitors of rOat3. The inhibition constants of the inhibitors including estradiol-17beta-glucuronide and cimetidine were comparable for the uptake of PAH and PCG by isolated rat CP. In addition, these values were also comparable with those for rOat3, but not with those for rOat1. These results suggest that rOat3 is mainly responsible for the uptake of PCG and PAH by isolated rat CP, and it functions as one of the detoxification systems on the CP by removing its substrates from the cerebrospinal fluid.

Role of transporters in the tissue-selective distribution and elimination of drugs: transporters in the liver, small intestine, brain and kidney

Cumulative studies have revealed the importance of transporters in drug disposition in the body. Recently, organic anion transporters such as organic anion transporting polypeptides (OATPs), organic anion transporters (OATs) and multidrug resistance associated proteins (MRPs) have been identified. Their broad substrate specificity as well as the multiplicity of transporter gene products make these transporters suitable detoxification systems in the body. OATPs and OATs are responsible for the hepatic and renal uptake of organic anions, respectively, while MRP2 is a major transporter involved in the biliary excretion of organic anions. OATPs and MRP2 are involved in the hepatobiliary transport of pravastatin and temocaprilat. These are good examples of hepatobiliary transport maximizing their pharmacological effects, but minimizing their side-effects. Taking into consideration tissue-selective expression and substrate specificity, transporters are useful for delivering small molecules to target tissues. MRPs are also suggested to be involved in the barrier function in the small intestine, blood-brain barrier and blood-cerebrospinal fluid barriers by extruding their ligands into the luminal side. In this manuscript, we have summarized recent studies by others and ourselves on the role of these transporters in the tissue selective distribution and elimination of drugs.

Efflux transport systems for drugs at the blood-brain barrier and blood-cerebrospinal fluid barrier (Part 1)

Penetration through the blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB) is necessary if a drug is to achieve the required concentration for a desired pharmacological effect. Efflux transport systems at the BBB and BCSFB provide a protective barrier function by removing drugs from the brain or cerebrospinal fluid and transferring them to the systemic circulation, respectively; several transporters at the BBB and BCSFB have been identified. Efflux transport should be taken into consideration during drug development to improve brain penetration and to avoid drug-drug interactions involving these transporters and subsequent side effects.

Efflux transport systems for drugs at the blood-brain barrier and blood-cerebrospinal fluid barrier (Part 2)

Penetration of the blood-brain barrier or blood-cerebrospinal fluid barrier is necessary if a drug is to achieve the required concentration for a desired pharmacological effect. Efflux transport systems at such barriers provide protection for the CNS by removing drugs from the brain or cerebrospinal fluid, and transferring them to the systemic circulation. In Part 2 of this review, in vivo and in vitro studies of efflux transport via these barriers are discussed, with reference to the transporters previously described in Part 1(1).

Organic anion transport across the choroid plexus

Several organic anion transport systems have recently been identified and localized at the apical and basolateral plasma membrane domains of choroid plexus epithelial cells. These organic anion transporters include (1) indirectly coupled Na(+)/dicarboxylate cotransport and dicarboxylate/organic anion exchange, which is represented on the molecular level by a member of the "kidney"-type organic anion transporter (OAT) family at the apical plasma membrane domain; (2) the organic anion transporting polypeptide 1 (Oatp1) and Oatp2, which both mediate typical "liver"-like organic anion transport activities at the apical and basolateral plasma membrane domains, respectively; and (3) the multidrug resistance protein Mrp1/MRP1 at the basolateral plasma membrane domain, and the P-glycoprotein Mdr1/MDR1 at an apical and subapical membrane vesicle compartment. This cellular transport polarity can account, at least in part, for the previously suggested physiologic transport properties of the choroid plexus epithelium and provides a framework for the identification and localization of additional organic anion transporters involved in the absorption and/or excretion of drugs and drug metabolites at the choroid plexus.

Oatp2 mediates bidirectional organic solute transport: a role for intracellular glutathione

One member of the OATP family of transporters, rat Oatp1, functions as an anion exchanger that is driven in part by the glutathione (GSH) electrochemical gradient, indicating that other OATP-related transporters may also be energized by this mechanism. The present study examined whether rat Oatp2 is also an anion exchanger, and, if so, whether it is energized by the GSH electrochemical gradient. As with Oatp1, uptake of 10 microM [(3)H]taurocholate in Oatp2-expressing Xenopus laevis oocytes was trans-stimulated by intracellular 0.2 mM unlabeled taurocholate, indicating bidirectional transport. Interestingly, [(3)H]taurocholate uptake in Oatp2-expressing oocytes was also trans-stimulated when oocytes were preloaded with GSH, S-methylglutathione, S-sulfobromophthalein-glutathione, S-dinitrophenyl glutathione, or ophthalmic acid (a GSH analog) but not by glutarate or N-acetylcysteine, suggesting that GSH derivatives and conjugates may function as intracellular substrates for Oatp2. Support for this hypothesis was provided by the demonstration of enhanced [(3)H]GSH and [(3)H]S-(2,4-dinitrophenyl)-glutathione efflux in Oatp2-expressing oocytes. However, in contrast to Oatp1, extracellular GSH failed to cis-inhibit uptake of [(3)H]taurocholate or [(3)H]digoxin in Oatp2-expressing oocytes, indicating that the stimulatory effect of high intracellular GSH concentrations is not due to a coupled exchange mechanism. Taken together, the results indicate that Oatp2 mediates bidirectional transport of organic anions by a GSH-sensitive facilitative diffusion mechanism and suggest that this transporter may play a role in cellular export of specific organic molecules.

Choroid plexus in the central nervous system: biology and physiopathology

Choroid plexuses (CPs) are localized in the ventricular system of the brain and form one of the interfaces between the blood and the central nervous system (CNS). They are composed of a tight epithelium responsible for cerebrospinal fluid secretion, which encloses a loose connective core containing permeable capillaries and cells of the lymphoid lineage. In accordance with its peculiar localization between 2 circulating fluid compartments, the CP epithelium is involved in numerous exchange processes that either supply the brain with nutrients and hormones, or clear deleterious compounds and metabolites from the brain. Choroid plexuses also participate in neurohumoral brain modulation and neuroimmune interactions, thereby contributing greatly in maintaining brain homeostasis. Besides these physiological functions, the implication of choroid plexuses in pathological processes is increasingly documented. In this review, we focus on some of the novel aspects of CP functions in relation to brain development, transfer of neuro-humoral information, brain/immune system interactions, brain aging, and cerebral pharmaco-toxicology.

Kinetic and biochemical analysis of carrier-mediated efflux of drugs through the blood-brain and blood-cerebrospinal fluid barriers: importance in the drug delivery to the brain

In this manuscript, our recent studies on the transporters on the blood-brain barrier and blood-cerebrospinal fluid (CSF) barrier responsible for the excretion of ligands from the central nervous system (CNS) to the blood are summarized. By comparing the brain entry of quinidine in normal and mdr 1a knock out mice, the predominant role of P-glycoprotein in the brain distribution of this compound was demonstrated. In addition to P-glycoprotein, the presence of transporters responsible for the efflux of organic anions from the brain has been suggested by a pharmacokinetic analysis of the CNS distribution of cefodizime, a third generation cephalosporin antibiotic. This suggestion was confirmed by demonstrating the presence of a specific mechanism for the elimination of p-aminohippuric acid from the brain after microinjection into the cerebral hemisphere. In vitro, the energy-dependent luminal preferential efflux of glutathione-bimane was demonstrated in a monolayer of MBEC4 cells which were derived from mouse brain endothelial cells. Studies with isolated membrane vesicles from MBEC4 cells suggested the presence of a primary active transporter(s) for organic anions, and Western blot analysis indicated the presence of multidrug resistance associated protein (MRP1) and/or its related transporters on MBEC4 cells and freshly isolated rat cerebral endothelial cells. The transcellular transport of 17beta estradiol 17beta-D-glucuronide (E(2)17betaG) across the choroid plexus was also demonstrated by examining the efflux of this compound from CSF after intracerebroventricular administration. The functional significance of organic anion transporting polypeptide (oatp-1) on the brush border membrane of the choroid plexus was demonstrated by comparing the uptake of E(2)17betaG into the isolated choroid plexus and oatp-1 transfected COS-7 cells; in addition, reverse transcription-polymerase chain reaction and Western blot analysis indicated the presence of MRP in the choroid plexus. Together with the direction of transcellular transport, the basolateral localization of MRP on the choroid plexus was suggested. By regulating the activity of these efflux transporters, it is possible to improve the brain entry of certain substrates.

Recent advances in the brain targeting of neuropharmaceuticals by chemical delivery systems

Brain-targeted chemical delivery systems represent a general and systematic method that can provide localized and sustained release for a variety of therapeutic agents including neuropeptides. By using a sequential metabolism approach, they exploit the specific trafficking properties of the blood-brain barrier and provide site-specific or site-enhanced delivery. After a brief description of the design principles, the present article reviews a number of specific delivery examples (zidovudine, ganciclovir, lomustine benzylpenicillin, estradiol, enkephalin, TRH, kyotorphin), together with representative synthetic routes, physicochemical properties, metabolic pathways, and pharmacological data. A reevaluated correlation for more than 60 drugs between previously published in vivo cerebrovascular permeability data and octanol/water partition coefficients is also included since it may be useful in characterizing the properties of the blood-brain barrier, including active transport by P-glycoprotein.

Organic cation transport in rat choroid plexus cells studied by fluorescence microscopy

Quinacrine uptake and distribution were studied in a primary culture of rat choroid plexus epithelial cells using conventional and confocal fluorescence microscopy and image analysis. Quinacrine rapidly accumulated in cells, with steady-state levels being achieved after 10-20 min. Uptake was reduced by other organic cations, e.g., tetraethylammonium (TEA), and by KCN. Quinacrine fluorescence was distributed in two cytoplasmic compartments, one diffuse and the other punctate. TEA efflux experiments indicated that more than one-half of intracellular organic cation was in a slowly emptying compartment. The protonophore monensin both emptied that TEA compartment and abolished punctate quinacrine fluorescence, suggesting that a large fraction of total intracellular organic cation was sequestered in acidic vesicles, e.g., endosomes. Finally, quinacrine-loaded vesicles were seen to move within the cytoplasm and to abruptly release their contents at the blood side of the cell; the rate of release was greatly reduced by the microtubule disrupter nocodazole.

Strategies to target kyotorphin analogues to the brain

The design, synthesis, and pharmacological evaluation of brain-targeted chemical delivery systems (CDS) for a kyotorphin analogue (Tyr-Lys) are described. The brain-targeted compound contains the active peptide in a packaged, disguised form, flanked between the lipophilic cholesteryl ester on the C-terminus and the 1, 4-dihydrotrigonellyl redox targetor, attached to the N-terminus through strategically selected L-amino acid(s) spacer. It was found that for successful brain targeting, the epsilon-amine of Lys needs to be also converted to a lipophilic function. Through sequential enzymatic bioactivation, the Tyr-Lys dipeptide is released in a sustained manner, producing significant and prolonged analgesic activity as demonstrated by the rat tail latency test. An alternate strategy was also employed. Lys was replaced by a redox amino acid pair, Nys+ left and right arrow Nys, the nicotinamide left and right arrow 1,4-dihydronicotinamide analogues of Lys (Nys+ is 2-amino-6-(3-carbamoyl-1-pyridiniumyl)hexanoic acid). The Nys form is lipophilic and facilitates delivery in addition to the C- and N-terminal lipophilic functions. Enzymatic oxidation to Nys+ provides the lock-in, followed by removal of the lipophilic groups, releasing Tyr-Nys+ from the brain-targeted analogue (BTRA). Nys+ was shown to be an effective substitution for Arg or Lys. The activities of the CDS and BTRA, respectively, were antagonized by naloxone, supporting the designed brain-targeted processes. The most potent compound is the two-proline spacer containing CDS (CDS-PP), followed by the BTRA.