In vitro models of the blood-brain barrier to polar permeants: comparison of transmonolayer flux measurements and cell uptake kinetics using cultured cerebral capillary endothelial cells

Development of a physiologically-based pharmacokinetic model of the rat central nervous system

Central nervous system (CNS) drug disposition is dictated by a drug's physicochemical properties and its ability to permeate physiological barriers. The blood-brain barrier (BBB), blood-cerebrospinal fluid barrier and centrally located drug transporter proteins influence drug disposition within the central nervous system. Attainment of adequate brain-to-plasma and cerebrospinal fluid-to-plasma partitioning is important in determining the efficacy of centrally acting therapeutics. We have developed a physiologically-based pharmacokinetic model of the rat CNS which incorporates brain interstitial fluid (ISF), choroidal epithelial and total cerebrospinal fluid (CSF) compartments and accurately predicts CNS pharmacokinetics. The model yielded reasonable predictions of unbound brain-to-plasma partition ratio (Kpuu,brain) and CSF:plasma ratio (CSF:Plasmau) using a series of in vitro permeability and unbound fraction parameters. When using in vitro permeability data obtained from L-mdr1a cells to estimate rat in vivo permeability, the model successfully predicted, to within 4-fold, Kpuu,brain and CSF:Plasmau for 81.5% of compounds simulated. The model presented allows for simultaneous simulation and analysis of both brain biophase and CSF to accurately predict CNS pharmacokinetics from preclinical drug parameters routinely available during discovery and development pathways.

How well can in vitro brain microcapillary endothelial cell models predict rodent in vivo blood-brain barrier permeability?

The object of the study was to improve the blood-brain barrier (BBB) permeability in vitro-invivo correlations (IVIVC) between in vitro brain microcapillary endothelial cell (BMEC) models and the well-tested rodent in situ brain perfusion technique. Porcine, bovine, rat, mouse, and human in vitro BMEC apparent permeability values, P(e), (14 studies from several laboratories: 229 P(e), 60 compounds) were analyzed by a novel biophysical model encoded in a weighted nonlinear regression procedure to determine the aqueous boundary layer (ABL) thickness and the paracellular parameters: porosity-pathlength (dual-pore model), pore radius, and water channel electrostatic potential. The refined parameters were then used to transform the P(e) values into the transendothelial permeability (P(c)) values. Porcine BMEC mono-culture models showed tight junctions comparable to those reported in several Caco-2 studies. Bovine cultures were somewhat leakier. In the human primary cultured cell and the hCMEC/D3 cell line data, IVIVC based on P(e) values has r(2) = 0.14. With transformed permeability values, r(2) = 0.58. Comparable improvements were found in the other species data. By using the in vitro transendothelial P(c) values in place of the apparent P(e) values, IVIVC can be dramatically improved.

Biomedical Technologies for in vitro Screening and Controlled Delivery of Neuroactive Compounds

Cell culture models can provide information pertaining to the effective dose, toxiciology, and kinetics, for a variety of neuroactive compounds. However, many in vitro models fail to adequately predict how such compounds will perform in a living organism. At the systems level, interactions between organs can dramatically affect the properties of a compound by alteration of its biological activity or by elimination of it from the body. At the tissue level, interaction between cell types can alter the transport properties of a particular compound, or can buffer its effects on target cells by uptake, processing, or changes in chemical signaling between cells. In any given tissue, cells exist in a three-dimensional environment bounded on all sides by other cells and components of the extracellular matrix, providing kinetics that are dramatically different from the kinetics in traditional two-dimensional cell culture systems. Cell culture analogs are currently being developed to better model the complex transport and processing that occur prior to drug uptake in the CNS, and to predict blood-brain barrier permeability. These approaches utilize microfluidics, hydrogel matrices, and a variety of cell types (including lung epithelial cells, hepatocytes, adipocytes, glial cells, and neurons) to more accurately model drug transport and biological activity. Similar strategies are also being used to control both the spatial and temporal release of therapeutic compounds for targeted treatment of CNS disease.

Prediction of Tumour Tissue Diffusion Coefficients of Hypoxia-Activated Prodrugs from Physicochemical Parameters

The therapeutic activity of anticancer agents depends critically on their ability to penetrate through tumour tissue to reach their target cells, a requirement that is especially important for hypoxia-activated prodrugs. Here we use multicellular layers (MCL) grown in vitro from HT29 colon carcinoma cells to measure tissue diffusion coefficients (Dmcl) of 67 structurally diverse benzotriazine di-N-oxides (analogues of the hypoxia-activated prodrug tirapazamine) plus four miscellaneous compounds. An algorithm was developed to predict Dmcl from physicochemical parameters (molecular weight, octanol/water partition coefficient at pH 7.4, number of hydrogen bond donors and acceptors); the fitted multivariate relationship had an explained variance (R2) of 0.907 and predictive power (Q2) of 0.879. Using a subset of nine compounds tested as a single cassette, the algorithm was shown to apply, with some adjustment of coefficients, to MCLs from three other tumour cell lines with differing cell packing densities (SiHa, HCT8-Ea, and HCT8-Ra). The demonstrated relationships provide tools for optimizing extravascular transport of anticancer agents during lead optimization.

Use of Z310 cells as an in vitro blood-cerebrospinal fluid barrier model: tight junction proteins and transport properties

Immortalized rat choroidal epithelial Z310 cells have the potential to become an in vitro model for studying transport of materials at blood-cerebrospinal fluid barrier (BCB) (Shi and Zheng, 2005) [Shi, L.Z., Zheng, W., 2005. Establishment of an in vitro brain barrier epithelial transport system for pharmacological and toxicological study. Brain Research 1057, 37-48]. This study was designed to demonstrate the presence of tight junction properties in Z310 cells and the functionality of Z310 monolayer in transport of selected model compounds. Western blot analyses revealed the presence of claudin-1, ZO-1, and occludin in Z310 cells. Transmission electron microscopy showed a "tight junction" type of structure in the sub-apical lateral membranes between adjacent Z310 cells. Real-time RT-PCR revealed that Z310 cells expressed representative transporters such as DMT1, MTP1, TfR, p-glycoprotein, ATP7A, ZnT1, ABCC1, Oat3, OCT1 and OB-Ra. Moreover, Z310 cells cultured in a two-chamber Transwell device possessed the ability to transport zidovudine (anionic drug), thyroxine (hormone), thymidine (nucleoside), and leptin (large polypeptide) with kinetic properties similar to those obtained from the in vitro model based on primary culture of choroidal epithelial cells. Taken together, these data indicate that the Z310 BCB model expresses major tight junction proteins and forms a tight barrier in vitro. The model also exhibits the ability to transport substances of various categories across the barrier.

Establishment of an in vitro brain barrier epithelial transport system for pharmacological and toxicological study

An immortalized Z310 murine choroidal epithelial cell line was recently established in this laboratory. The purposes of this study were (1) to investigate the presence of tight junction (TJ) proteins in Z310 cells and (2) to develop a Z310 cell-based in vitro brain barrier transport model. Real-time RT-PCR studies revealed that Z310 cells possess mRNAs encoding ZO-1, -2, and -3, claudin-1, -2, -4, and -8, occludin, and connexin-32. Confocal microscopic analyses confirmed the presence of claudin-1 and ZO-1 in Z310 cells at cell-cell contact sites. When Z310 cells were grown on a two-chamber Transwell device, the [14C]sucrose permeability coefficient and transepithelial electrical resistance (TEER) across the cell monolayer were 6 x 10(-4) cm/min and 61 omega-cm2, respectively. To improve the tightness of Z310 barrier, the cells were cultured in astrocyte-conditioned medium (ACM), or in the presence of eicosapentaenoic acids (EPA, 10 microM), epidermal growth factor (EGF, 100 ng/mL), or dexamethasone (1 microM) in the growth medium. Treatment with ACM, EPA, EGF and dexamethasone significantly increased the TEER by 33%, 38%, 40%, and 50% above controls, respectively. However, only dexamethasone significantly reduced [14C]sucrose paracellular permeability (-231% of controls). These data suggest that Z310 cells possess the TJ proteins. The presence of dexamethasone in the growth medium improves the tightness of Z310 cell monolayer to the level better than that of the primary culture of choroidal epithelial cells. The Z310 cell-based in vitro model appears to be suitable for transepithelial transport study of drugs and toxicants.

Evaluation of the immortalized human brain capillary endothelial cell line BB19 as a human cell culture model for the blood–brain barrier

In vitro models of the blood-brain barrier (BBB) play a major role in the study of BBB permeability of drug candidates. However, most established in vitro models use cells of non-human origin, which is not optimal for the prediction of brain permeability in humans. The aim of this study was to assess the human brain capillary endothelial cell line BB19 for its usefulness as an in vitro model of the human BBB. Restrictive tight junctions are a prerequisite for drug transport studies. Sucrose permeability of BB19 cells on different filters was compared to porcine brain capillary endothelial cells (BCEC). Tightness of BB19 cell monolayers still needs further optimization. Hardly any discrimination between Sucrose and Propranolol (P(app) = 1.30 x 10(-5) vs. 2.18 x 10(-5) cm/s) was seen. Cells showed an improvement towards a more primary BCEC morphology with C6 conditioned medium, dexamethasone, and 1,25-dihydroxyvitamin D3. The presence of P-glycoprotein (P-gp), MRP4 and BCRP, ABC-transporters located in the BBB, has been shown on mRNA level, by immunostaining, and western blot. MRP1, MRP2, MRP5, OAT3, and OAT4 were also detected by RT-PCR. Functional properties of the BBB were shown with uptake of propranolol, morphine, and sucrose. Uptake studies with daunomycin and the P-gp inhibitor verapamil showed functional activity of P-gp. We conclude that BB19 cells might be feasible as a human in vitro model of the BBB for drug uptake studies. However, for the assessment of transport studies, further improvements of this model are necessary.

Alteration at translational but not transcriptional level of transferrin receptor expression following manganese exposure at the blood-CSF barrier in vitro

Manganese exposure alters iron homeostasis in blood and cerebrospinal fluid (CSF), possibly by acting on iron transport mechanisms localized at the blood-brain barrier and/or blood-CSF barrier. This study was designed to test the hypothesis that manganese exposure may change the binding affinity of iron regulatory proteins (IRPs) to mRNAs encoding transferrin receptor (TfR), thereby influencing iron transport at the blood-CSF barrier. A primary culture of choroidal epithelial cells was adapted to grow on a permeable membrane sandwiched between two culture chambers to mimic blood-CSF barrier. Trace (59)Fe was used to determine the transepithelial transport of iron. Following manganese treatment (100 microM for 24 h), the initial flux rate constant (K(i)) of iron was increased by 34%, whereas the storage of iron in cells was reduced by 58%, as compared to controls. A gel shift assay demonstrated that manganese exposure increased the binding of IRP1 and IRP2 to the stem loop-containing mRNAs. Consequently, the cellular concentrations of TfR proteins were increased by 84% in comparison to controls. Assays utilizing RT-PCR, quantitative real-time reverse transcriptase-PCR, and nuclear run off techniques showed that manganese treatment did not affect the level of heterogeneous nuclear RNA (hnRNA) encoding TfR, nor did it affect the level of nascent TfR mRNA. However, manganese exposure resulted in a significantly increased level of TfR mRNA and reduced levels of ferritin mRNA. Taken together, these results suggest that manganese exposure increases iron transport at the blood-CSF barrier; the effect is likely due to manganese action on translational events relevant to the production of TfR, but not due to its action on transcriptional, gene expression of TfR. The disrupted protein-TfR mRNA interaction in the choroidal epithelial cells may explain the toxicity of manganese at the blood-CSF barrier.

In vitro trans-monolayer permeability calculations: often forgotten assumptions

In designing effective therapeutic strategies, novel drugs must exhibit favorable pharmacokinetic properties. The physicochemical characteristics of a drug, such as pK(a), molecular weight, solubility and lipophilicity, will influence the way the drug partitions from the aqueous phase into membranes, and thus, will influence its ability to cross cellular barriers, such as the lining of the gastrointestinal tract and the blood-brain barrier. Physicochemical characteristics also influence the degree to which a drug is able to cross a barrier layer, and the route by which it does this; whether transcellular (across the cells)-by diffusion, carrier-mediated transport or transcytosis-or paracellular-by diffusing through the tight junctions between the cells. The in vitro model systems that are currently employed to screen the permeation characteristics of a drug often represent a compromise between high throughput with low predictive potential and low throughput with high predictive potential. Here, we will examine the way in which in vitro cellular permeability assays are often performed and the assumptions that are implied but sometimes forgotten, and we will make simple suggestions for improving the methodological techniques and mathematical equations used to determine drug permeability.

Valproic acid uptake by bovine brain microvessel endothelial cells: role of active efflux transport

The basis for low brain permeability of valproic acid (VPA) appears to be the result of efflux transport at the blood-brain barrier (BBB); however, the identity of the putative efflux transporter has not been investigated. The objective of our studies was to determine whether the multidrug resistance-associated protein (MRP) might be involved in efflux transport of VPA. Brain microvessel endothelial cells (BMEC) were isolated from cow brains and grown to confluence. MRP messenger RNA (mRNA) in BMEC were verified by reverse transcriptase-polymerase chain reaction (RT-PCR). Functional activity was demonstrated using the steady-state retention of calcein and MRP inhibitors, indomethacin (IND) and probenecid (PRB). Probenecid (0.50 mM) and indomethacin (10 microM) produced a 26 and 13% ( P<0.05 ) elevation in steady-state cellular VPA uptake following a 30-min-incubation with tracer 3H-VPA and 30 microM cold VPA. In contrast, at higher concentrations of probenecid (2 mM) and indomethacin (500 microM), an 11 and 31% reduction in VPA uptake was observed. The biphasic pattern of VPA uptake suggested concurrent inhibition of uptake and efflux transporters by the inhibitor with differing sensitivities, i.e. the efflux transporter being more susceptible to inhibition than the influx transporter. Similar results were obtained in the MRP overexpressing cell line A549. Overall, the results suggest that MRP(s) is(are) involved in the efflux transport of VPA, but do not preclude the possible contribution(s) of other organic anion transporters. The findings also adds to the growing evidence that up-regulation of active drug efflux transporters at the BBB may contribute to the development of drug resistance to antiepileptic drug therapy.

Blood-brain barrier efflux transport

Efflux transport at the blood-brain barrier (BBB) limits the brain tissue exposure to a variety of potential therapeutic agents, including compounds that are relatively lipophilic and would be predicted to permeate the endothelial lining of the brain microvasculature. Recent advances in molecular and cell biology have led to identification of several specific transport systems at the blood-brain interface. Refinement of classical pharmacokinetic experimentation has allowed assessment of the structural specificity of transporters, the impact of efflux transport on brain tissue exposure, and the potential for drug-drug interactions at the level of BBB efflux transport. The objective of this minireview is to summarize efflux transporter characteristics (location, specificity, and potential inhibition) for transport systems identified in the BBB. A variety of experimental approaches available to ascertain or predict the impact of efflux transport on net brain tissue uptake of substrates also are presented. The potential impact of efflux transport on the pharmacodynamics of agents acting in the central nervous system are illustrated. Finally, general issues regarding the role of identifying efflux transport as part of the drug development process are discussed.

Computation of log BB values for compounds transported through carrier-mediated mechanisms using in vitro permeability data from brain microvessel endothelial cell (BMEC) monolayers

PURPOSE

To explore the possibility of determining in vivo log BB values (the logarithm value of brain to plasma concentration ratio) from in vitro permeability data measured in brain microvessel endothelial cell (BMEC) monolayers.

METHODS

An equilibrium mathematical model was developed: log BB = log(Ca/Cb) + log Kbr:pl, where Cb and Ca are the drug concentrations at equilibrium in the basolateral (B) and apical (A) sides of BMECs in an A-to-B directional diffusion system and Kbr:pl is the brain-plasma partition coefficient. With this model, murine log BB values were calculated for 24 pharmaceutical compounds, mostly Pgp substrates.

RESULTS

Calculated log BB values correlated well to experimental values (r2 = 0.854, slope = 0.907 +/- 0.080), demonstrating that the model could reasonably predict brain penetration for compounds that are involved in carrier-mediated transport mechanisms. For a second data set that included volatile organic compounds (log BB = log Kbr:pl), log Kbr:pl values were also shown to correlate well with their respective experimental log BB values (r2 = 0.876, slope = 0.973 +/- 0.082), demonstrating that log Kbr:pl is an excellent descriptor for log BB when a compound penetrates the blood-brain barrier by passive diffusion only.

CONCLUSION

The equilibrium model demonstrated a reasonable ability to compute in vivo log BB values, regardless of the involvement or mechanisms of carrier-mediated transport.

Drug efflux transporters in the CNS

The central nervous system (CNS) contains important cellular barriers that maintain homeostasis by protecting the brain from circulating toxins and through the elimination of toxic metabolites generated in the brain. The barriers that limit the concentration of toxins and xenobiotics in the interstitial fluids of the CNS are the capillary endothelial cells of the blood-brain barrier (BBB) and the epithelial cells of the blood-cerebrospinal fluid barrier (BCSFB). Both of these barriers have cellular tight junctions and express transport systems which serve to actively transport nutrients into the brain, and actively efflux toxic metabolites and xenobiotics out of the brain. This review will focus on the expression and function of selected drug efflux transporters in these two barriers, specifically the multidrug resistance transporter, p-glycoprotein, and various organic anion transporters, such as multidrug resistance-associated proteins, organic anion transporter polypeptides, and organic anion transporters. These transport systems are increasingly recognized as important determinants of drug distribution to, and elimination from, different compartments of the CNS. Consequences of drug efflux transporters in barriers of the CNS include limiting the distribution of substrates that are beneficial to treat CNS diseases, and increasing the possibility of drug-drug interactions that may lead to untoward toxicities. Therefore, the study of these transporters is important in examining the various determinants of drug delivery to the CNS.

Establishment and characterization of an immortalized Z310 choroidal epithelial cell line from murine choroid plexus

The choroid plexus plays a wide range of roles in brain development, maturation, aging process, endocrine regulation, and pathogenesis of certain neurodegenerative diseases. To facilitate in vitro study, we have used a gene transfection technique to immortalize murine choroidal epithelial cells. A viral plasmid (pSV3neo) was inserted into the host genome of primary choroidal epithelia by calcium phosphate precipitation. The transfected epithelial cells, i.e., Z310 cells, that survived from cytotoxic selection expressed SV40 large-T antigen throughout the life span, suggesting a successful gene transfection. The cells displayed the same polygonal epithelial morphology as the starting cells by light microscopy. Immunocytochemical studies demonstrate the presence of transthyretin (TTR), a thyroxine transport protein known to be exclusively produced by the choroidal epithelia in the CNS, in both transfected and starting cells. Western blot analyses further confirm the production and secretion of TTR by these cells. The mRNAs encoding transferrin receptor (TfR) were identified by Northern blot analyses. The cells grow at a steady rate, currently in the 110th passage with a population doubling time of 20-22 h in the established culture. When Z310 cells were cultured onto a Trans-well apparatus, the cells formed an epithelial monolayer similar to primary choroidal cells, possessing features such as an uneven fluid level between inner and outer chambers and an electrical resistance approximately 150-200 omega-cm(2). These results indicate that immortalized Z310 cells possess the characteristics of choroidal epithelia and may have the potential for application in blood-CSF barrier (BCB) research.

Investigation of the mechanism of enhancement of central nervous system delivery of 2'-beta-fluoro-2',3'-dideoxyinosine via a blood-brain barrier adenosine deaminase-activated prodrug

Enhanced central nervous system (CNS) delivery of certain poorly penetrating 2',3'-dideoxynucleosides has been achieved by designing prodrugs that are substrates for enzymes, such as adenosine deaminase (ADA), that are present at high activities in brain tissue. In this study, the potential role of adenosine deaminase localized within the endothelial cells of the blood-brain barrier (BBB) in providing enhanced intracellular and CNS delivery of an ADA-activated prodrug is assessed in vitro using cell culture models of the BBB. The kinetics of uptake and bioconversion of 2'-beta-fluoro-2',3'-dideoxyadenosine (F-ddA), a model ADA-activated prodrug of 2'-beta-fluoro-2',3'-dideoxyinosine, were determined in primary cultured bovine brain microvascular endothelial cells. Model-based simulations of CNS availability derived from in vitro estimates of parameters for simple passive diffusion and ADA-catalyzed deamination suggest that ADA that is localized within the BBB plays an important role in the conversion of F-ddA to 2'-beta-fluoro-2',3'-dideoxyinosine during its passage across the BBB. Consistent with in vivo observations, these simulations demonstrate that elevated levels of certain enzymes, such as ADA, in the brain microvascular endothelial cells of the BBB may be exploited in the design of brain-targeted prodrugs or drug-carrier conjugates, which brain tissue selectively converts.

Progress and limitations in the use of in vitro cell cultures to serve as a permeability screen for the blood-brain barrier

A relatively simple, widely applicable, and robust in vitro method of predicting blood-brain barrier (BBB) permeability to central nervous system-acting drugs is an increasing need. A cell-based model offers the potential to account for transcellular and paracellular drug diffusional processes, metabolism, and active transport processes, as well as nondefined interactions between a drug and cellular material that may impact upon a membrane's overall permeability profile. Any in vitro BBB cell model to be utilized for the transendothelial BBB permeability screening of potential central nervous system drugs must display reproducible solute permeability, and a number of other general criteria including: a restrictive paracellular barrier; a physiologically realistic cell architecture; the functional expression of key transporter mechanisms; and allow ease of culture to meet the technical and time constraints of a screening program. This article reviews the range of in vitro cell-based BBB models available, including the primary/low passage bovine and porcine brain endothelial cultures as well as the spectrum of immortalized brain endothelial cell lines that have been established. The article further discusses the benefits and limitations of exploiting such systems as in vitro BBB permeability screens.

Integration of mass spectrometry into early-phase discovery and development of central nervous system agents

The early-phase discovery and development of useful central nervous system (CNS) agents present ample opportunities to exploit mass spectrometry and provide detailed compound/mixture characterization, or to make the process faster and/or more economic. Neuropeptide FF antagonists and centrally active thyrotropin-releasing hormone analogues were used as specific examples in this work. We evaluated the characterization of focused libraries of peptide derivatives by electrospray ionization, tandem mass spectrometry and liquid chromatography/tandem mass spectrometry on a quadrupole ion trap and nanoelectrospray on a Fourier transform ion cyclotron resonance mass spectrometer. Immobilized artificial-membrane chromatography was employed as a model to predict/rank new agents against lead compounds for their potential to reach the central nervous system in pharmacologically significant amounts. Measuring brain concentrations in rodents after the intravenous administration of test compounds was used as an in vivo approach, and we took advantage of microdialysis sampling that furnished samples without interfering tissue matrix and afforded the estimation of extracellular concentrations in a localized part of the brain. Overall, making atmospheric-pressure ionization mass spectrometry an integral part of the process has played a major role in increasing throughput, selectivity, specificity and detection sensitivity and thereby providing useful information about the extent or mechanism of transport and metabolic activation/inactivation in early-phase discovery and development of CNS agents.

The blood-brain barrier and oncology: new insights into function and modulation

The efficacy of chemotherapy for malignant primary or metastatic brain tumours is still poor. This is at least partly due to the presence of the blood-brain barrier (BBB). The functionality of the BBB can be explained by physicochemical features and efflux pump mechanisms. An overview of the literature is presented with emphasis on oncology. The BBB consists of capillary endothelial cells that lack fenestrations and are connected together with continuous tight junctions, with a high electrical resistance. Permeability of tight junctions can be increased in vitro by contraction of the cytoskeleton, caused by bradykinin agonists. Different efflux pumps are present in the BBB. Examples are P-glycoprotein (P-gp), organic anion transporters, (OAT) and multidrug-resistance-associated proteins (MRP)(1 and 3). These pumps act as a multi-specific efflux pump for various chemotherapeutic drugs. Experiments have shown that P-gp can be inhibited by different non-chemotherapeutic substrates such as cyclosporin A. The functionality in vivo of P-gp can be measured with positron emission tomography and [(11)C]-verapamil or with single photon emission computer tomography and(99m)Tc-sestamibi. MRP(1)and MRP(3)act as organic anion transporters that in vitro act as efflux pumps for substances that are conjugated or co-transported with glutathione and glucuronide, respectively. Methotrexate has been recently demonstrated to be transported by MRP(1)and MRP(3). Results of studies which demonstrate the clinical relevance and applicability of BBB modulators are eagerly awaited.

Use of cultured cerebral capillary endothelial cells in modeling the central nervous system availability of 2',3'-dideoxyinosine

The biochemical and physiological mechanisms responsible for the limited central nervous system (CNS) uptake of dideoxynucleoside reverse transcriptase inhibitors currently used to treat HIV-1 infection in humans are poorly understood. In vitro models of the blood-brain barrier (BBB) offer an attractive alternative to in vivo or in situ animal studies for understanding the role of the blood-brain barrier in regulating brain tissue concentrations of these agents. In the present study, the kinetics of 2', 3'-dideoxyinosine (ddI) uptake and purine nucleoside phosphorylase (PNP) mediated catabolism in primary cultures of bovine brain microvessel endothelial cells (BBMECs) were determined in order to ascertain the importance of both transport and metabolism governing the CNS availability of this purine dideoxynucleoside. Initial rates of ddI uptake as a function of ddI donor concentration suggest the involvement of both passive diffusion and carrier-mediated processes. These studies confirm earlier in vivo findings that transporters may play a role in regulating the CNS concentration of ddI. Analysis of ddI uptake and metabolite accumulation in BBMECs over longer time intervals (beyond the intial rate region) provide substantial in vitro evidence for an enzymatic BBB for ddI. Simulations of the CNS availability of ddI derived from in vitro estimates of parameters for passive diffusion, carrier-mediation, and metabolism indicate that the fraction of ddI entering the BBB cells which actually reaches the brain parenchyma may be quite low (< 2%) due to metabolism by PNP localized within the BBB, consistent with the low CNS delivery of ddI observed in vivo. Transporters and metabolic enzymes within the BBB may function in coordinated fashion to reduce the CNS concentrations of both rapidly metabolized and poorly metabolized dideoxynucleosides.