Mechanisms by which 2',3'-dideoxyinosine (ddI) crosses the guinea-pig CNS barriers; relevance to HIV therapy

Important roles of transporters in the pharmacokinetics of anti-viral nucleoside/nucleotide analogs

INTRODUCTION

Nucleoside analogs are an important class of antiviral agents. Due to the high hydrophilicity and limited membrane permeability of antiviral nucleoside/nucleotide analogs (AVNAs), transporters play critical roles in AVNA pharmacokinetics. Understanding the properties of these transporters is important to accelerate translational research for AVNAs.

AREAS COVERED

The roles of key transporters in the pharmacokinetics of 25 approved AVNAs were reviewed. Clinically relevant information that can be explained by the modulation of transporter functions is also highlighted.

EXPERT OPINION

Although the roles of transporters in the intestinal absorption and renal excretion of AVNAs have been well identified, more research is warranted to understand their roles in the distribution of AVNAs, especially to immune privileged compartments where treatment of viral infection is challenging. P-gp, MRP4, BCRP, and nucleoside transporters have shown extensive impacts in the disposition of AVNAs. It is highly recommended that the role of transporters should be investigated during the development of novel AVNAs. Clinically, co-administered inhibitors and genetic polymorphism of transporters are the two most frequently reported factors altering AVNA pharmacokinetics. Physiopathology conditions also regulate transporter activities, while their effects on pharmacokinetics need further exploration. Pharmacokinetic models could be useful for elucidating these complicated factors in clinical settings.

Role and modulation of drug transporters in HIV-1 therapy

Current treatment of human immunodeficiency virus type-1 (HIV-1) infection involves a combination of antiretroviral drugs (ARVs) that target different stages of the HIV-1 life cycle. This strategy is commonly referred to as highly active antiretroviral therapy (HAART) or combined antiretroviral therapy (cART). Membrane-associated drug transporters expressed ubiquitously in mammalian systems play a crucial role in modulating ARV disposition during HIV-1 infection. Members of the ATP-binding cassette (ABC) and solute carrier (SLC) transporter superfamilies have been shown to interact with ARVs, including those that are used as part of first-line treatment regimens. As a result, the functional expression of drug transporters can influence the distribution of ARVs at specific sites of infection. In addition, pathological factors related to HIV-1 infection and/or ARV therapy itself can alter transporter expression and activity, thus further contributing to changes in ARV disposition and the effectiveness of HAART. This review summarizes current knowledge on the role of drug transporters in regulating ARV transport in the context of HIV-1 infection.

Interactions of tenofovir, Lamivudine, abacavir and Didanosine in primary human cells

Certain triple nucleoside/tide reverse transcriptase inhibitor (NRTI) regimens containing tenofovir (TDF) have been associated with rapid early treatment failure. The mechanism is unknown, but may be at the level of drug transport. We measured the lipophilicity of the drugs [³H]-lamivudine (3TC), -didanosine (ddI), -TDF and -ABC. Peripheral blood mononuclear cells (PBMCs) were used to evaluate drug–drug interactions at the level of drug transport. PBMCs were measured for the expression of P-glycoprotein (P-gp), multidrug resistance-associated protein-1 (MRP-1) and breast cancer resistance protein (BCRP) by flow cytometry. The rank order of the lipophilicity of the drugs were ABC⋙3TC≥ddI>TDF. The accumulation of [³H]-3TC, -ddI and -TDF were temperature sensitive (suggesting facilitated transport), in contrast to [³H]-ABC. ABC reduced the accumulation of [³H]-3TC, and cell fractionation experiments suggested this was mainly in membrane-bound [³H]-3TC. ABC/TDF and ABC/ddI increased the accumulation of [³H]-3TC and 3TC/TDF also increased the accumulation of [³H]-TDF. In contrast, none of the NRTI/NtRTI incubations (alone or in combination) altered the accumulation of [³H]-ABC and -ddI. PBMC expression of P-gp, MRP1 and BCRP were detected, but none correlated with the accumulation of the drugs. The high failure rates seen with TDF, ABC and 3TC are not fully explained by an interaction at transporter level.

The distribution of nifurtimox across the healthy and trypanosome-infected murine blood-brain and blood-cerebrospinal fluid barriers

Nifurtimox, an antiparasitic drug, is used to treat American trypanosomiasis (Chagas disease) and has shown promise in treating central nervous system (CNS)-stage human African trypanosomiasis (HAT; sleeping sickness). In combination with other antiparasitic drugs, the efficacy of nifurtimox against HAT improves, although why this happens is unclear. Studying how nifurtimox crosses the blood-brain barrier (BBB) and reaches the CNS may clarify this issue and is the focus of this study. To study the interaction of nifurtimox with the blood-CNS interfaces, we used the in situ brain/choroid plexus perfusion technique in healthy and trypanosome-infected mice and the isolated incubated choroid plexus. Results revealed that nifurtimox could cross the healthy and infected blood-brain and blood-cerebrospinal fluid (CSF) barriers (K(in) brain parenchyma was 50.8 ± 9.0 μl · min(-1) · g(-1)). In fact, the loss of barrier integrity associated with trypanosome infection failed to change the distribution of [(3)H]nifurtimox to any significant extent, suggesting there is not an effective paracellular barrier for [(3)H]nifurtimox entry into the CNS. Our studies also indicate that [(3)H]nifurtimox is not a substrate for P-glycoprotein, an efflux transporter expressed on the luminal membrane of the BBB. However, there was evidence of [(3)H]nifurtimox interaction with transporters at both the blood-brain and blood-CSF barriers as demonstrated by cross-competition studies with the other antitrypanosomal agents, eflornithine, suramin, melarsoprol, and pentamidine. Consequently, CNS efficacy may be improved with nifurtimox-pentamidine combinations, but over time may be reduced when nifurtimox is combined with eflornithine, suramin, or melarsoprol.

Peptidyl alpha-ketoamides with nucleobases, methylpiperazine, and dimethylaminoalkyl substituents as calpain inhibitors

A series of peptidyl alpha-ketoamides with the general structure Cbz-L-Leu-D,L-AA-CONH-R were synthesized and evaluated as inhibitors for the cysteine proteases calpain I, calpain II, and cathepsin B. Nucleobases, methylpiperazine, and dimethylaminoalkyl groups were incorporated into the primed region of the inhibitors to generate compounds that potentially cross the blood-brain barrier. Two of these compounds (Cbz-Leu-D,L-Abu-CONH-(CH(2))(3)-adenin-9-yl and Cbz-Leu-D,L-Abu-CONH-(CH(2))(3)-(4-methylpiperazin-1-yl) have been shown to have useful concentrations in the brain in animals. The best inhibitor for calpain I was Cbz-Leu-D,L-Abu-CONH-(CH(2))(3)-2-methoxyadenin-9-yl (K(i) = 23 nM), and the best inhibitor for calpain II was Cbz-Leu-D,L-Phe-CONH-(CH(2))(3)-adenin-9-yl (K(i) = 68 nM). On the basis of the crystal structure obtained with heterocyclic peptidyl alpha-ketoamides, we have improved inhibitor potency by introducing a small hydrophobic group on the adenine ring. These inhibitors have good potential to be used in the treatment of neurodegenerative diseases.

Expression and functional activity of nucleoside transporters in human choroid plexus

Background

Human equilibrative nucleoside transporters (hENTs) 1-3 and human concentrative nucleoside transporters (hCNTs) 1-3 in the human choroid plexus (hCP) play a role in the homeostasis of adenosine and other naturally occurring nucleosides in the brain; in addition, hENT1, hENT2 and hCNT3 mediate membrane transport of nucleoside reverse transcriptase inhibitors that could be used to treat HIV infection, 3'-azido-3'-deoxythymidine, 2'3'-dideoxycytidine and 2'3'-dideoxyinosine. This study aimed to explore the expression levels and functional activities of hENTs 1-3 and hCNTs 1-3 in human choroid plexus.

Methods

Freshly-isolated pieces of lateral ventricle hCP, removed for various clinical reasons during neurosurgery, were obtained under Local Ethics Committee approval. Quantification of mRNAs that encoded hENTs and hCNTs was performed by the hydrolysis probes-based reverse transcription real time-polymerase chain reaction (RT-qPCR); for each gene of interest and for 18 S ribosomal RNA, which was an endogenous control, the efficiency of PCR reaction (E) and the quantification cycle (Cq) were calculated. The uptake of [³H]inosine by the choroid plexus pieces was investigated to explore the functional activity of hENTs and hCNTs in the hCP.

Results

RT-qPCR revealed that the mRNA encoding the intracellularly located transporter hENT3 was the most abundant, with E^-Cq value being only about 40 fold less that the E^-Cq value for 18 S ribosomal RNA; mRNAs encoding hENT1, hENT2 and hCNT3 were much less abundant than mRNA for the hENT3, while mRNAs encoding hCNT1 and hCNT2 were of very low abundance and not detectable. Uptake of [³H]inosine by the CP samples was linear and consisted of an Na⁺-dependent component, which was probably mediated by hCNT3, and Na⁺-independent component, mediated by hENTs. The latter component was not sensitive to inhibition by S-(4-nitrobenzyl)-6-thioinosine (NBMPR), when used at a concentration of 0.5 μM, a finding that excluded the involvement of hENT1, but it was very substantially inhibited by 10 μM NBMPR, a finding that suggested the involvement of hENT2 in uptake.

Conclusion

Transcripts for hENT1-3 and hCNT3 were detected in human CP; mRNA for hENT3, an intracellularly located nucleoside transporter, was the most abundant. Human CP took up radiolabelled inosine by both concentrative and equilibrative processes. Concentrative uptake was probably mediated by hCNT3; the equilibrative uptake was mediated only by hENT2. The hENT1 transport activity was absent, which could suggest either that this protein was absent in the CP cells or that it was confined to the basolateral side of the CP epithelium.

Drug interactions at the blood-brain barrier: fact or fantasy?

There is considerable interest in the therapeutic and adverse outcomes of drug interactions at the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB). These include altered efficacy of drugs used in the treatment of CNS disorders, such as AIDS dementia and malignant tumors, and enhanced neurotoxicity of drugs that normally penetrate poorly into the brain. BBB- and BCSFB-mediated interactions are possible because these interfaces are not only passive anatomical barriers, but are also dynamic in that they express a variety of influx and efflux transporters and drug metabolizing enzymes. Based on studies in rodents, it has been widely postulated that efflux transporters play an important role at the human BBB in terms of drug delivery. Furthermore, it is assumed that chemical inhibition of transporters or their genetic ablation in rodents is predictive of the magnitude of interaction to be expected at the human BBB. However, studies in humans challenge this well-established paradigm and claim that such drug interactions will be lesser in magnitude but yet may be clinically significant. This review focuses on current known mechanisms of drug interactions at the blood-brain and blood-CSF barriers and the potential impact of such interactions in humans. We also explore whether such drug interactions can be predicted from preclinical studies. Defining the mechanisms and the impact of drug-drug interactions at the BBB is important for improving efficacy of drugs used in the treatment of CNS disorders while minimizing their toxicity as well as minimizing neurotoxicity of non-CNS drugs.

The transport of anti-HIV drugs across blood-CNS interfaces: summary of current knowledge and recommendations for further research

The advent of highly active antiretroviral therapy (HAART), which constitutes HIV protease inhibitors, nucleoside reverse transcriptase inhibitors, non-nucleoside reverse transcriptase inhibitors and nucleotide reverse transcriptase inhibitors, has dramatically reduced the morbidity and mortality associated with human immunodeficiency virus (HIV) infection in resource-rich countries. However, this disease still kills several million people each year. Though the reason for therapeutic failure is multi-factorial, an important concern is the treatment and control of HIV within the central nervous system (CNS). Due to the restricted entry of anti-HIV drugs, the brain is thought to form a viral sanctuary site. This not only results in virological resistance, but also is often associated with the development of complications such as HIV-associated dementia. The CNS delivery of anti-HIV drugs is limited by the blood–brain and blood–CSF interfaces due to a combination of restricted paracellular movement, powerful metabolic enzymes and numerous transporters including members of the ATP binding cassette (ABC) and solute carrier (SLC) superfamilies. A better appreciation of the transporters present at the brain barriers will prove a valuable milestone in understanding the limited brain penetration of anti-HIV drugs in HIV and also aid the development of new anti-HIV drugs and drug combinations, with enhanced efficacy in the CNS. This review aims to summarise current knowledge on the transport of anti-HIV drugs across the blood–brain barrier and the choroid plexus, as well as provide recommendations for future research.

Oxidative stress and toxicity induced by the nucleoside reverse transcriptase inhibitor (NRTI)--2',3'-dideoxycytidine (ddC): relevance to HIV-dementia

Human immunodeficiency virus dementia (HIVD) is the most common form of dementia occurring among young adults. In HIVD, neuronal cell loss occurs in the absence of neuronal infection. With the advent of highly active anti-retroviral therapy (HAART), the incidence of HIVD has drastically reduced, though prevalence of milder forms of HIVD continues to rise. Though these agents have been used successfully in suppressing viral production, they have also been associated with a number of side effects. Here we examine the possible role of NRTIs, in particular 2',3'-dideoxycytidine (ddC), in the neuropathology of HIVD. Synaptosomes and isolated mitochondria treated and incubated for 6 h with CSF-achievable concentrations of ddC, i.e., 6-11 ng/ml, were found to show a significant increase in oxidative stress with 40 nM ddC as measured by protein carbonyls and 3-nitrotyrosine (3NT), effects that were not observed in the more tolerable NRTI, 3TC. Protection against protein oxidation induced by ddC was observed when brain mitochondria were isolated from gerbils 1 h after injection i.p. with the brain accessible antioxidant and glutathione mimetic, tricyclodecan-9-yl-xanthogenate (D609). In addition, there is a significant reduction in the levels of anti-apoptotic protein Bcl-2 and a significant increase in cytochrome c release and also a significant increase in the expression of pro-apoptotic protein caspase-3 after mitochondria were treated with 40 nM ddC. The results reported here show that ddC at 40 nM can induce oxidative stress, cause the release of cytochrome c, and in addition, reduce the levels of anti-apoptotic proteins, increase the levels of pro-apoptotic proteins, thereby increasing the possibility for induction of apoptosis. These findings are consistent with the notion of a possible role of the NRTIs, and in particular, ddC, in the mechanisms involved in HIVD.

The distribution of the anti-HIV drug, tenofovir (PMPA), into the brain, CSF and choroid plexuses

Background

Tenofovir disoproxil fumarate, a prodrug of the nucleotide reverse transcriptase inhibitor, tenofovir (9-[9(R)-2-(phosphonomethoxy)propyl]adenine; PMPA), was recently approved for use in the combination therapy of human immunodeficiency virus (HIV)-1 infection. This study was undertaken to understand PMPA distribution to the virus sanctuary sites located in the brain, CSF and choroid plexuses and to clarify its possible role in reducing the neurological problems associated with HIV infection.

Methods

The methods used included an established bilateral carotid artery perfusion of [³H]PMPA and a vascular marker, D-[¹⁴C]mannitol, in anaesthetised guinea-pigs followed by scintillation counting, HPLC and capillary depletion analyses. Movement of [³H]PMPA into the brain, cisternal CSF and lateral ventricle choroid plexus was also examined in the absence and presence of additional anti-HIV drugs and a transport inhibitor. Control and test groups were compared by ANOVA or Student's t-test, as appropriate.

Results

The distribution of [³H]PMPA in the cerebrum, cerebellum, pituitary gland and cerebral capillary endothelial cells was not significantly different to that measured for D-[¹⁴C]mannitol. However, [³H]PMPA accumulation was significantly higher than that of D-[¹⁴C]mannitol in the choroid plexus and CSF. Further experiments revealed no cross-competition for transport of [³H]PMPA by probenecid, a non-specific inhibitor of organic anion transport, or the nucleoside reverse transcriptase inhibitors into any of the CNS regions studied. The octanol-saline partition coefficient measurement for [³H]PMPA was 0.0134 ± 0.00003, which is higher that the 0.002 ± 0.0004 measured for D-[¹⁴C]mannitol in an earlier study.

Conclusion

There is negligible transport of [³H]PMPA across the blood-brain barrier, but it can cross the blood-CSF barrier. This is a reflection of the differing physiological and functional characteristics of the blood-CNS interfaces. Self- and cross-inhibition studies did not suggest the involvement of a transport system in the CNS distribution of this drug. However, the ability of PMPA to accumulate in the choroid plexus tissue, but not the cerebral capillary endothelial cells, and the hydrophilic nature of PMPA, does point to the possibility of a transporter at the level of the choroid plexus. PMPA that has crossed the choroid plexus and is in the CSF could treat HIV-infected perivascular and meningeal macrophages, but it is unlikely to reach the infected microglia of deep brain sites.

Enhanced Prospects for Drug Delivery and Brain Targeting by the Choroid Plexus–CSF Route

The choroid plexus (CP), i.e., the blood-cerebrospinal fluid barrier (BCSFB) interface, is an epithelial boundary exploitable for drug delivery to brain. Agents transported from blood to lateral ventricles are convected by CSF volume transmission (bulk flow) to many periventricular targets. These include the caudate, hippocampus, specialized circumventricular organs, hypothalamus, and the downstream pia-glia and arachnoid membranes. The CSF circulatory system normally provides micronutrients, neurotrophins, hormones, neuropeptides, and growth factors extensively to neuronal networks. Therefore, drugs directed to CSF can modulate a variety of endocrine, immunologic, and behavioral phenomema; and can help to restore brain interstitial and cellular homeostasis disrupted by disease and trauma. This review integrates information from animal models that demonstrates marked physiologic effects of substances introduced into the ventricular system. It also recapitulates how pharmacologic agents administered into the CSF system prevent disease or enhance the brain's ability to recover from chemical and physical insults. In regard to drug distribution in the CNS, the BCSFB interaction with the blood-brain barrier is discussed. With a view toward translational CSF pharmacotherapy, there are several promising innovations in progress: bone marrow cell infusions, CP encapsulation and transplants, neural stem cell augmentation, phage display of peptide ligands for CP epithelium, CSF gene transfer, regulation of leukocyte and cytokine trafficking at the BCSFB, and the purification of neurotoxic CSF in degenerative states. The progressively increasing pharmacological significance of the CP-CSF nexus is analyzed in light of treating AIDS, multiple sclerosis, stroke, hydrocephalus, and Alzheimer's disease.

Factors affecting delivery of antiviral drugs to the brain

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

Interaction of nucleoside analogues with nucleoside transporters in rat brain endothelial cells

A number of nucleoside analogues, consisting of antiviral compounds and agents designed as adenosine A1 receptor agonists, were examined for nucleoside transporter affinity using an in vitro model of the blood-brain barrier (BBB), the rat brain endothelial cell line, RBE4. Structure-activity relationships (SAR) were also performed to identify the key structural requirements for transporter recognition and the suitability of these systems for carrier-mediated strategies to deliver therapeutics across the BBB. Adenosine receptor agonists did not show transport affinity for concentrative nucleoside carriers, but exhibited affinity for equilibrative systems (Ki=10.8-97.9 microM) within the range of Kms for natural substrates. However, none of the antiviral compounds tested in this study showed affinity for either class of nucleoside transporter. SAR studies suggest that the hydroxyl group located at the 3'-position of the ribose moiety is an essential requirement for transporter recognition. This may explain the inability of nucleoside derived anti-viral compounds to use these systems despite the significant structural homology with naturally occurring nucleosides. Sites have also been identified which accommodate structural additions with retention of carrier affinity, suggesting that compounds which fail to penetrate the BBB could be attached to these sites for carrier-mediated delivery using a prodrug strategy.

Homeostatic capabilities of the choroid plexus epithelium in Alzheimer's disease

As the secretory source of vitamins, peptides and hormones for neurons, the choroid plexus (CP) epithelium critically provides substances for brain homeostasis. This distributive process of cerebrospinal fluid (CSF) volume transmission reaches many cellular targets in the CNS. In ageing and ageing-related dementias, the CP-CSF system is less able to regulate brain interstitial fluid. CP primarily generates CSF bulk flow, and so its malfunctioning exacerbates Alzheimers disease (AD). Considerable attention has been devoted to the blood-brain barrier in AD, but more insight is needed on regulatory systems at the human blood-CSF barrier in order to improve epithelial function in severe disease. Using autopsied CP specimens from AD patients, we immunocytochemically examined expression of heat shock proteins (HSP90 and GRP94), fibroblast growth factor receptors (FGFr) and a fluid-regulatory protein (NaK2Cl cotransporter isoform 1 or NKCC1). CP upregulated HSP90, FGFr and NKCC1, even in end-stage AD. These CP adjustments involve growth factors and neuropeptides that help to buffer perturbations in CNS water balance and metabolism. They shed light on CP-CSF system responses to ventriculomegaly and the altered intracranial pressure that occurs in AD and normal pressure hydrocephalus. The ability of injured CP to express key regulatory proteins even at Braak stage V/VI, points to plasticity and function that may be boosted by drug treatment to expedite CSF dynamics. The enhanced expression of human CP 'homeostatic proteins' in AD dementia is discussed in relation to brain deficits and pharmacology.

Do antidepressants regulate how cortisol affects the brain?

Although the effects of antidepressants on glucocorticoid hormones and their receptors are relevant for the therapeutic action of these drugs, the molecular mechanisms underlying these effects are unclear. Studies in depressed patients, animals and cellular models have demonstrated that antidepressants increase glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) expression and function; this, in turn, is associated with enhanced negative feedback by endogenous glucocorticoids, and thus with reduced resting and stimulated hypothalamic-pituitary-adrenal (HPA) axis activity. In a series of studies conducted over the last few years, we have shown that antidepressants modulate GR function in vitro by inhibiting membrane steroid transporters that regulate the intracellular concentration of glucocorticoids. In this paper, we will review the effects of membrane steroid transporters and antidepressants on corticosteroid receptors. We will then present our unpublished data on GR live microscopy in vitro, showing that ligand-induced translocation of the GR starts within 30 seconds and is completed within minutes. Furthermore, we will present our new data using an in situ brain perfusion model in anaesthetised guinea-pigs, showing that entry of cortisol to the brain of these animals is limited at the blood-brain barrier (BBB). Finally, we will present a comprehensive discussion of our published findings on the effects of chemically unrelated antidepressants on membrane steroid transporters, in mouse fibroblasts and rat cortical neurones. We propose that antidepressants in humans could inhibit steroid transporters localised on the BBB and in neurones, like the multidrug resistance p-glycoprotein, and thus increase the access of cortisol to the brain and the glucocorticoid-mediated negative feedback on the HPA axis. Enhanced cortisol action in the brain might prove to be a successful approach to maximise therapeutic antidepressant effects.

The Distribution of the HIV Protease Inhibitor, Ritonavir, to the Brain, Cerebrospinal Fluid, and Choroid Plexuses of the Guinea Pig

Anti-human immunodeficiency virus (HIV) drug penetration into the brain and cerebrospinal fluid (CSF) is necessary to tackle HIV within the CNS. This study examines movement of [(3)H]ritonavir across the guinea pig blood-brain and blood-CSF barriers and accumulation within the brain, CSF, and choroid plexus. Ritonavir is a protease inhibitor, used in combination therapy (often as a pharmacoenhancer) to treat HIV. Drug interactions at brain barrier efflux systems may influence the CNS penetration of anti-viral drugs, thus the influence of additional protease inhibitors, nucleoside reverse transcriptase inhibitors, and non-nucleoside reverse transcriptase inhibitors on [(3)H]ritonavir CNS distribution was explored. Additionally, the involvement of transporters on [(3)H]ritonavir passage across the brain barriers was assessed. Results from in situ brain perfusions and capillary depletion analysis demonstrated that [(3)H]ritonavir uptake into the guinea pig brain was considerable (6.6 +/- 0.7 ml/100 g at 30 min, vascular space corrected), although a proportion of drug remained trapped in the cerebral capillaries and did not reach the brain parenchyma. CSF uptake was more limited (2.2 +/- 0.4 ml/100 g at 30 min), but choroid plexus uptake was abundant (176.7 +/- 46.3 ml/100 g at 30 min). [(3)H]Ritonavir brain and CSF uptake was unaffected by neither inhibitors of organic anion transport (probenecid and digoxin) or P-glycoprotein (progesterone), nor by any additional anti-HIV drugs, indicating that brain barrier efflux systems do not significantly limit brain or CSF [(3)H]ritonavir accumulation in this model. [(3)H]Ritonavir uptake into the perfused choroid plexus was significantly reduced by nevirapine and abacavir, additional perfusion studies, and isolated incubated choroid plexus experiments were carried out in an attempt to further characterize the transporter involved.

Hydroxyurea transport across the blood-brain and blood-cerebrospinal fluid barriers of the guinea-pig

Hydroxyurea is used in the treatment of HIV infection in combination with nucleoside analogues, 2'3'-didehydro-3'deoxythymidine (D4T), 2'3'-dideoxyinosine or abacavir. It is distributed into human CSF and is transported from the CSF to sub-ependymal brain sites, but its movement into the brain directly from the blood has not been studied. This study addressed this by a brain perfusion technique in anaesthetized guinea-pigs. The carotid arteries were perfused with an artificial plasma containing [14C]hydroxyurea (1.6 microm) and a vascular marker, [3H]mannitol (4.6 nm). Brain uptake of [14C]hydroxyurea (8.0 +/- 0.9%) was greater than [3H]mannitol (2.4 +/- 0.2%; 20-min perfusion, n = 8). CSF uptake of [14C]hydroxyurea (5.6 +/- 1.5%) was also greater than [3H]mannitol (0.9 +/- 0.3%; n = 4). Brain uptake of [14C]hydroxyurea was increased by 200 microm hydroxyurea, 90 microm D4T, 350 microm probenecid, 25 microm digoxin, but not by 120 microm hydroxyurea, 16.5-50 microm D4T, 90 microm 2'3'-dideoxyinosine or 90 microm abacavir. [14C]Hydroxyurea distribution to the CSF, choroid plexus and pituitary gland remained unaffected by all these drugs. The metabolic half-life of hydroxyurea was > 15 h in brain and plasma. Results indicate that intact hydroxyurea can cross the brain barriers, but is removed from the brain by probenecid- and digoxin-sensitive transport mechanisms at the blood-brain barrier, which are also affected by D4T. These sensitivities implicate an organic anion transporter (probably organic anion transporting polypeptide 2) and possibly p-glycoprotein in the brain distribution of hydroxyurea and D4T.

Effect of transport inhibitors and additional anti-HIV drugs on the movement of lamivudine (3TC) across the guinea pig brain barriers

To treat human immunodeficiency virus (HIV) within the central nervous system (CNS), levels of anti-HIV drugs in the brain must reach therapeutic concentrations. The ability of (-)-2'-deoxy-3'-thiacytidine (3TC; lamivudine) to cross the blood-brain and blood-cerebrospinal fluid (CSF) barriers, alone and in combination with additional nucleoside analogs, was investigated. The bilateral in situ guinea pig brain perfusion method, linked to high-performance liquid chromatography analyses, was used to examine 3TC uptake into brain and CSF simultaneously. The influence of transport inhibitors and additional nucleoside analogs on this uptake was investigated. 3TC movement across the blood-CSF barrier was examined in more detail by the isolated choroid plexus model. 3TC movement across the brain barriers and subsequent accumulation in the brain and CSF was low. However, 3TC uptake from blood into choroid plexus (a potential CNS target for HIV treatment) was significant, and was facilitated by a digoxin-sensitive transporter. Another transporter was identified, which removed 3TC from the choroid plexus. Abacavir, 2'3'-didehydro-3'deoxythymidine, and 3'-azido 3'-deoxythymidine did not interact with 3TC at either of the brain barriers to affect CNS concentrations of 3TC. However, a significant interaction between 3TC and 2'3'-dideoxyinosine was observed at the choroid plexus, and it may prove beneficial to select drug combinations where no such interaction is indicated.