LEADING ARTICLE

Clin Pharmacokinet 2002; 41 (2): 81-92 0312-5963/02/0002-0081/$25.00/0 © Adis International Limited. All rights reserved.

The Impact of Efflux Transporters in the Brain on the Development of Drugs for CNS Disorders Eve M. Taylor NeoTherapeutics Inc., Irvine, California, USA

Abstract

The development of drugs to treat disorders of the CNS requires consideration of achievable brain concentrations. Factors that influence the brain concentrations of drugs include the rate of transport into the brain across the blood-brain barrier (BBB), metabolic stability of the drug, and active transport out of the brain by efflux mechanisms. To date, three classes of transporter have been implicated in the efflux of drugs from the brain: multidrug resistance transporters, monocarboxylic acid transporters, and organic ion transporters. Each of the three classes comprises multiple transporters, each of which has multiple substrates, and the combined substrate profile of these transporters includes a large number of commonly used drugs. This system of transporters may therefore provide a mechanism through which the penetration of CNS-targeted drugs into the brain is effectively minimised. The action of these efflux transporters at the BBB may be reflected in the clinic as the minimal effectiveness of drugs targeted at CNS disorders, including HIV dementia, epilepsy, CNS-based pain, meningitis and brain cancers. Therefore, modulation of these efflux transporters by design of inhibitors and/or design of compounds that have minimal affinity for these transporters may well enhance the treatment of intractable CNS disorders.

Evaluation of drug penetration into the brain should be an integral component of any drug development programme targeting disorders of the CNS. High pharmacological activity of a drug candidate in, for example, an in vitro screening assay will have little application to the treatment of a CNS disorder if the compound does not reach effective concentrations in the brain extracellular fluid (ECF). The blood-brain barrier (BBB), which creates and maintains a restricted extracellular environment in the CNS, comprises three ‘lines of defence’: (i) a physical barrier formed by tight junctions between endothelial cells of the brain capillaries and epithe-

lial cells of the choroid plexus and arachnoid membrane (figure 1); (ii) an enzymatic barrier conferred by an enrichment, in these endothelial and epithelial cells, of degradative enzymes; and (iii) transporters that mediate the efflux of compounds from brain to blood. The following review will focus on efflux transporters. 1. Efflux of Drugs from the Brain Once a drug enters the brain, whether by transport across the BBB after systemic administration or by direct administration into the CNS, it may return to the blood via three routes (figure 2). It

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b

Blood Fenestrated capillary endothelial cells Choroid plexus epithelial cells

a

Blood

CSF

Tight junction

Capillary endothelial cells Cells

ECF

Cells

Tight junction Superior sagittal sinus

Ependyma

ECF

Dura mater

Arachnoid villus

Choroid plexus

Lateral ventricle Third ventricle Fourth ventricle

c Central canal

Subdural space Blood Arachnoid cells CSF Cells

ECF

Tight junction Pial membrane

Fig. 1. Sites of the blood-brain barrier (BBB). The BBB comprises tight junctions between: (a) the endothelial cells of brain capillaries; (b) epithelial cells of the choroid plexus; and (c) epithelial cells of the arachnoid membrane (reproduced from Kandel et al.,[1] with permission). ECF = extracellular fluid; CSF = cerebrospinal fluid.

may cross the endothelial cells of the brain capillaries, cross the epithelial cells of the choroid plexus, or it may return to the circulation by bulk flow of cerebrospinal fluid (CSF) and reabsorption at the arachnoid villi. Although compounds may simply diffuse out of the brain across the endothelial and epithelial cells of the BBB, it has been known since the early 1960s that compounds may also be actively transported out of the brain into the blood. The first demonstration was by Pappenheimer et al.,[2] who showed that iodopyracet and phenolsulfonphthalein were transported from CSF to blood and that the transport of © Adis International Limited. All rights reserved.

iodopyracet was inhibited by phenolsulfonphthalein and p-aminohippurate. Subsequently, the saturable disappearance from the brain of organic cations,[3] iodide,[4-6] potassium[7,8] and penicillin[9] was demonstrated and it is now known that the active efflux of nutrients, metabolites, peptides, hormones and neurotransmitters serves to maintain brain homeostasis.[10] Of particular relevance to this discussion is the demonstration that efflux mechanisms play a role in limiting the penetration of drugs into the CNS; it has been shown that anti-HIV drugs, antispastic agents, immunosuppresants, analgesics, anticonvulsants, antibacterials, anticancer agents, imagClin Pharmacokinet 2002; 41 (2)

Efflux of CNS Drugs

ing agents and uricosuric compounds are actively transported from the brain.[11-22] The molecular characterisation of the transporters responsible for efflux of drugs is an active area of research. Here we discuss the transporters that have been localised to the brain and may be involved in drug efflux across the BBB. These transporters include P-glycoprotein, members of the multidrug resistance-associated protein (MRP) family, monocarboxylic acid transporters, and organic ion transporters. 2. P-Glycoprotein P-glycoprotein is the prototypical multidrug resistance (MDR) transporter, the discovery of which was based on its ability to confer drug resistance on cancer cells.[23,24] P-glycoprotein is a 170kD transmembrane phosphoglycoprotein from the ATP-binding cassette (ABC) family that acts as a promiscuous energy-dependent outward transport pump. Three isoforms of P-glycoprotein have been identified in rodents (mdr1a, mdr1b, mdr2) and two isoforms in humans (MDR1, MDR2). MDR1, mdr1a, and mdr1b confer MDR on cancer cells, and mdr1a has been implicated in the efflux of drugs from the CNS. A partial list of mdr1a (referred to as P-glycoprotein here) substrates is presented in table I. Both P-glycoprotein mRNA and protein are expressed in the brain of humans,[25-37] monkeys,[31] rats,[27,32,35,36,38-45] mice,[33,36,46-49] cows,[32,38,50-52] pigs[47] and hamsters.[53] In 1989, Cordon-Cardo et al. [25] and Thiebaut et al.[27] showed that Pglycoprotein expression in the brain was localised to capillary endothelial cells in both rats and humans. This has been confirmed in tissue sections,[28,29,37,44] isolated brain capillaries,[30,32,33,35,38,43,45,49] isolated luminal membranes from brain capillaries,[41] primary cultures of brain capillary endothelial cells[35,39,42,45,52] and immortalised cultures of brain capillary endothelial cells.[45,48] Recently, P-glycoprotein has also been found in choroid plexus in rats[36] and monkeys.[54] Despite the numerous studies outlined above, some controversy remains regarding the localisation © Adis International Limited. All rights reserved.

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of P-glycoprotein on brain capillaries. In 1997, Pardridge et al.[31] presented data suggesting the colocalisation of glial fibrillary acidic protein (an astrocyte marker) and P-glycoprotein in human brain capillaries, and argued that the localisation of P-glycoprotein to brain capillaries is in fact due to expression in astrocytic endfeet and not endothelial cells. Detailed discussion of this controversy is available elsewhere.[55,56] Regardless of its exact site of expression, gene knockout studies have provided compelling evidence in support of a role for P-glycoprotein at the BBB. Although mdr1a knockout mice do not display overt physiological abnormalities or decreased life span, BBB penetration of a wide range of drugs is greatly increased in these animals. These include Blood H A Blood

C

CSF bulk flow

F

Blood

E

Cells G E D F

A

C ECF

CSF

Brain capillary endothelial cells

G

Choroid plexus epithelial cells B

Fenestrated capillary endothelial cells

Fig. 2. Pathways of drug distribution in brain. Once a drug enters

the brain by systemic administration (A) or by direct delivery into the CNS (B), it will become distributed in the extracellular fluid (ECF) of the brain parenchyma (C) and the cerebrospinal fluid (CSF; D) and may interact with the cells of the brain (E). It may then return to the blood by crossing the brain capillary endothelial cells (F) or the choroid plexus epithelial cells (G), or by bulk flow of CSF through arachnoid villi (H).

Clin Pharmacokinet 2002; 41 (2)

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Table I. Tissue expression and partial substrate profiles for efflux transporters. Compounds in bold are those that have been shown to be substrates/inhibitors for multiple transporters Transporter

Tissue expression

Substrates/inhibitors

mdr1a

Kidney, intestine, liver, brain, testis, adrenal gland, pregnant uterus, bone marrow, heart, lung, thymus, spleen

Paclitaxel, colchicine, emetine, dactinomycin, pepstatin A, gramicidin D, aldosterone, dexamethasone, progesterone, cyclosporin, dexniguldipine, azidopine, tamoxifen, cortisol, valspodar, valinomycin, amiodarone, tacrolimus, nifedipine, mitomycin, mithramycin, sirolimus, reserpine, tirilazad, phenothiazines, doxorubicin, daunorubicin, vinblastine, vincristine, etoposide, quinidine, quinine, methotrexate, teniposide, digoxin, verapamil

MRP1

Muscle, testis, thyroid, adrenal gland, bladder, colon, small intestine, liver, kidney, heart, ovary, brain, blood cells

Temocaprilat, pravastatin, cefodizime, leukotriene C4, leukotriene D4, ampicillin, MK-571, novobiocin, epirubicin, etoposide, vincristine, vinblastine, dactinomycin, doxorubicin, daunorubicin, probenecid, indomethacin, glutathione conjugates, methotrexate, grepafloxacin

MRP2

Liver, kidney, intestine, peripheral nerve, brain

Cisplatin, irinotecan, 7-ethyl-10-hydroxycamptothecin (SN-38), glutathione conjugates, vinblastine, vincristine, etoposide, methotrexate, doxorubicin, p-aminohippurate, taurocholate, glycocholate

MRP3

Liver, spleen, bladder, adrenals, pancreas, lung, kidney, intestine, brain

Etoposide, vincristine, methotrexate, glutathione conjugates, leukotriene C4, glycocholate, teniposide

MRP4

Prostate, lung, muscle, pancreas, testis, Zidovudine monophosphate, GS-438 (PMEG), lamivudine, adefovir, zidovudine ovary, bladder, kidney, gallbladder, brain

MRP5

Brain, skeletal muscle, lung, heart, liver, kidney, bladder, spleen, salivary gland, testis, thyroid, nerve, gallbladder, adrenals, placenta, ovary, pancreas

MRP6

Liver, kidney, brain, intestine, gall bladder, BQ-123 ovary, thyroid, salivary gland, lung

OAT1

Kidney, skeletal muscle, placenta, brain

OAT2

Kidney, liver, brain

Prostaglandin E2, dicarboxylates, p-aminohippurate, salicylate, aspirin

OAT3

Liver, kidney, brain, eye

Estrone sulfate, indocyanine green, bumetamide, piroxicam, losartan, p-aminohippurate, ochratoxin A, cimetidine, probenecid, zidovudine, furosemide, benzylpenicillin

OAT4

Placenta, kidney

Oestrone sulfate, dehydroepiandrosterone, ochratoxin A

oatp1

Liver, kidney, lung, brain, skeletal muscle, colon

Conjugated and unconjugated bile acids, enalapril, [D-Pen(2,5)]-enkephalin (DPDPE), deltorphin II, corticosterone sulfate, spironolactone, estradiol 17α-d-glucuronide, bromosulfophthalein

oatp2

Brain, retina, kidney

Taurocholate, thyroxine, tri-iodothyronine, cholate, 17α-estradiol glucuronide, bromosulfophthalein

oatp3

Kidney, retina, liver

Oubain, taurocholate, thyroxine, tri-iodothyronine, digoxin, bromosulfophthalein

OCT1

Liver, kidney, intestine, heart, skeletal muscle, brain

Vecuronium, 2-chloroadenosine, 2’-deoxytubercidin, decynium-22, dopamine, norepinephrine, amantadine, taurocholate, choline, tetraethylammonium, 1-methyl-4-phenyl pyridum ionN-methylnicotinamide, zidovudine, serotonin (5-hydroxytryptamine)

OCT2

Kidney, brain

Nicotine, procainamide, quinine, quinidine, dopamine, norepinephrine, amantadine, TEA, MPP, N-methylnicotinamide, cimetidine, serotonin

OCT3

Placenta, intestine, heart, lung, brain, kidney

Choline, dimethylamiloride, desipramine, clonidine, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, TEA, MPP, cimetidine, N-methylnicotinamide, guanidine, serotonin

OCTN1

Placenta, brain, heart, uterus, kidney, trachea, spleen, bone marrow, fetal liver, skeletal muscle, lung, pancreas, spinal cord

Mepyramine (pyrilamine), ofloxacin, levofloxacin, cefaloridine, TEA, L-carnitine, cimetidine, procainamide, quinidine, quinine, verapamil, nicotine, clonidine

© Adis International Limited. All rights reserved.

Thioguanine, mercaptopurine, S-(2,4 dinitrophenyl)glutathione, fluorescein diacetate, 8-bromo-cyclic GMP, zaprinast, N2,2′-O-dibutyryl-cyclic GMP, trequinsin, sildenafil, sulfinpyrazone, cadmium chloride, potassium antimonyl tartrate, adefovir, probenecid, indomethacin, glutathione conjugates, cyclic GMP, cyclic AMP, zidovudine

Prostaglandins, uric acid, β-lactam antibacterials, aciclovir, zalcitabine, salicylurate, naproxen, phenacetin, glutarate, α-ketoglutarate, levofloxacin, probenecid, methotrexate, folate, p-aminohippurate, indomethacin, grepafloxacin, salicylate, zidovudine, aspirin (acetylsalicylate), cefaloridine, ochratoxin A, cyclic AMP, cyclic GMP, furosemide, benzylpenicillin

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Table I. Contd Transporter

Tissue expression

Substrates/inhibitors

OCTN2

Kidney, trachea, spleen, bone marrow, skeletal muscle, heart, placenta, brain

Glycine betaine, butyrobetaine, acetylcarnitine, L-carnitine, TEA, guanidine

OCTN3

Testes, kidney

L-Carnitine

MCT1

Intestine, ovary, testis, prostate, thymus, spleen, kidney, muscle, liver, placenta, brain, heart

Benzoic acid, mersalyl acid, formate, bicarbonate, acetate, glyoxylate, oxamate, glycolate, proprionate, oxobutyrate, α-cyanocinnamate and derivatives, quercitin, phloretin, niflumic acid, 3-isobutyl-1-methylxanthine, 4,4′-substituted stilbene-2,2′-disulfonates (e.g. DIDS, SITS), pyruvate, lactate, salicylate

MCT2

Testis, liver, brain, heart, muscle, kidney, skin

Lactate, pyruvate

MCT3

Retinal pigment epithelium

Lactate

MCT4

Leucocyte, intestine, ovary, testis, prostate, thymus, spleen, skeletal muscle, lung, placenta, heart

Lactate

MCT5

Intestine, ovary, testis, prostate, kidney, Not known; assumed to be monocarboxylic acids muscle, liver, placenta, heart

MCT6

Leucocyte, intestine, testis, prostate, Not known; assumed to be monocarboxylic acids kidney, placenta, heart, kidney, muscle, lung

MCT7

Leucocyte, intestine, ovary, testis, placenta, pancreas, muscle, lung, heart, brain

MCT8

Intestine, ovary, testis, prostate, thymus, Not known; assumed to be monocarboxylic acids pancreas, liver, kidney, muscle, placenta, brain, heart

Not known; assumed to be monocarboxylic acids

MCT = monocarboxylic acid transporter; mdr = multidrug resistance; MRP = multidrug resistance-associated protein; OAT = organic anion transporter; oatp = organic anion transporter protein; OCT = organic cation transporter; OCTN = organic cation/carnitine transporter.

ivermectin,[57,58] dexamethasone,[58] cyclosporin,[58] ondansetron,[59] loperamide,[59] morphine,[58] vinblastine,[57] digoxin,[58,60] quinolone antibacterial agents,[61] indinavir, saquinavir and nelfinavir.[62] The functional implication of such observations is well illustrated by the fact that ivermectin, commonly used in tropical medicine, agriculture and veterinary science to treat infections and known for its high safety margin in vertebrates, is lethally neurotoxic in mdr1a knockout animals.[57] P-glycoprotein has been shown to transport hydrophilic acids such as methotrexate;[63] however, the classic P-glycoprotein substrate is a hydrophobic, amphipathic molecule with a planar ring system, molecular weight greater than 400, and a positive charge at pH 7.4.[64] The fact that P-glycoprotein is capable of transporting hydrophobic molecules greatly complicates the design of CNS drugs. Research efforts to date have largely focused on the development of drugs that are hydrophobic and me© Adis International Limited. All rights reserved.

tabolically stable, thus enabling the drug to bypass the physical and enzymatic BBB. It is clear that such compounds may still face the barrier provided by efflux transporters and, in fact, success in the pursuit of hydrophobicity to enable transport across the physical BBB may enhance the P-glycoprotein substrate characteristics of the compound, resulting instead in decreased brain penetration. This highlights the importance of considering all aspects of BBB physiology in the development of CNS drugs. 3. Multidrug Resistance-Associated Protein Family Multidrug resistance-associated protein (MRP) 1, a 190kD ABC transporter protein, was the first non-P-glycoprotein MDR-conferring transporter identified.[65] It transports a wide range of compounds (table I) many of which are actively transported out of the brain. The expression and role of Clin Pharmacokinet 2002; 41 (2)

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MRP1 in the brain is still being clarified. The initial examination of MRP1 expression in human tissues demonstrated that MRP is expressed in the brain[66] and this has been confirmed in mice and rats using isolated microvessels and tissue sections.[42,45,51,67,68] However, others showed little or no staining for MRP1 in the brain.[34,35,69-71] MRP1 is consistently expressed in cultured brain capillary endothelial cells,[35,42,45,48,51,70,72,73] although this may be a culture-dependent phenomenon.[35,70] Recently, Rao et al.[36] and Nishino et al.[74] demonstrated MRP in the choroid plexus from rats. There is, therefore, a suggestion that MRP1 may play a role in the blood-brain or blood-CSF barriers. Both Wijnholds et al.[75] and Lorico et al.[76] have generated MRP1 deficient mice in their studies. Although these mice are generally healthy, they do demonstrate a lack of high-affinity glutathione conjugate pump activity in erythrocytes, elevated glutathione levels in most tissues (not brain), reduced inflammatory response to a topical irritant, and increased sensitivity to anticancer drugs.[75-77] Neither of these groups reported any BBB-related deficits and, specifically, Wijnholds et al.[75] did not see any increased uptake of etoposide into the brain in mrp1-/- mice. They proposed that etoposide efflux by P-glycoprotein in these animals may have masked any deficit in MRP1-mediated transport of the drug, and so they compared BBB penetration of etoposide in mdr1a/mdr1b double knock outs (DKOs) and mrp1/mdr1a/mdr1b triple knockouts (TKOs).[71] DKO and TKO animals had similar total brain concentrations of [3H]etoposide after intravenous administration, but TKOs had increased CSF etoposide concentrations compared with DKOs, indicating that MRP1 may play a role specifically at the blood-CSF barrier. Recently, MRP1 homologues MRP2 to MRP6 have been identified,[78,79] and all have been shown to mediate drug efflux in vitro. Analysis of human brain tissue by Northern blot and ribonuclease protection assay demonstrated that MRP5 was highly expressed in the brain.[78-80] More recently, Miller et al.[68] detected MRP2 in the capillary endothelial cells of isolated rats and porcine brain capillaries, © Adis International Limited. All rights reserved.

Taylor

and Zhang et al.[51] demonstrated expression of MRP4, 5 and 6 in cultured bovine brain capillary endothelial cells and isolated bovine brain capillaries. MRP2, 4, and 5 substrates (table I) include etoposide, zidovudine, probenecid and mercaptopurine, all of which are actively transported out of the brain. Further investigation is required to confirm a role for these homologues in the efflux of drugs from the brain. 4. Monocarboxylic Acid Transporters Monocarboxylic acid transporters (MCTs) are ubiquitously expressed and serve to transport pyruvate, lactate and other metabolites bidirectionally across membranes.[81,82] MCTs have been implicated in both the influx[83] and efflux of compounds across the BBB. Probenecid,[14] mercaptopurine,[15] aluminium citrate[84,85] and AIT-082[86] are transported out of the brain by a mechanism inhibited by monocarboxylic acids, and MCTs have been implicated in each case. Furthermore, a role in the transport of compounds across the BBB is implied by the expression of both MCT1 and MCT2.[87-89] MCT1 is expressed in brain capillary endothelial cells, ependymocytes, astrocytic endfeet, pericytes and choroid plexus.[88,90-92] MCT2 is abundant in astrocytic endfeet, ependymocytes and neuropil.[93] Six new MCTs (MCT3 to MCT8, using the most recent nomenclature[81]) have been cloned from human tissue, and both MCT7 and 8 are expressed in the brain.[89,94] Their role in the BBB is yet to be determined. 5. Organic Ion Transporters Four major families of organic ion transporters have been identified: organic anion transporters (OAT), organic cation transporters (OCT), organic anion transporter proteins (oatp) and organic cation/carnitine transporters (OCTN).[95-97] Multiple members of each family have been identified: OAT1 to OAT4, OCT1 to OCT3, oatp1 to oatp4 and OCTN1 to OCTN3. Each family was originally described on the basis of its function in the kidney and liver, major sites for organic ion elimination, Clin Pharmacokinet 2002; 41 (2)

Efflux of CNS Drugs

and there is accumulating evidence for a role of each in the brain. Members of each family transport compounds that are actively transported out of the brain (table I). For example, OAT1 transports zidovudine, aciclovir and methotrexate, oatp3 transports digoxin, OCT2 transports quinidine and N-methylnicotinamide, and OCTN1 transports both quinidine and verapamil. There is evidence that OAT1 and OAT3,[98,99] OCT1 to OCT3,[100-104] oatp1 to oatp3[105-110] and OCTN1 to OCTN3[111-115] are expressed in the brain, and their role in the brain is under investigation. To date, the saturable efflux of dehydroepiandrosterone sulphate, an oatp2 substrate, has been demonstrated in immortalised mouse brain capillary endothelial cells,[109] and 17β-estradiol 17β-D-glucuronide, an oatp substrate, was transported out of the brain by a probenecidsensitive mechanism.[74] 6. Clinical Modulation of Efflux Pumps in the Brain The consequences of efflux transporters at the BBB may be readily apparent in clinical treatment. The minimal effectiveness in some patients of HIV drugs on AIDS dementia, antibacterials on CNS infections, anticonvulsants on epilepsy, opioids on CNS-based pain and chemotherapy on brain tumours may be explained, at least in part, by the efficiency with which these transporters function. Furthermore, the expression of these transporters may well be induced by age,[116] by the disorders that clinicians seek to treat[28] or by the drugs used in a therapeutic regimen. Certainly the last is a major problem in oncology, where drug-induced MDR is a factor in the intractable nature of some cancers. Modulation of MDR is a priority in oncology and is becoming a major focus in the development of CNS-active drugs. The concept of modulating efflux mechanisms to increase brain penetration of drugs was introduced in the 1961 study by Pappenheimer et al.[2] They showed that the efflux of iodopyracet from CSF was inhibited by phenolsulfonphthalein and by p-aminohippurate, and they suggested that the inhibition of efflux mechanisms © Adis International Limited. All rights reserved.

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might enhance brain penetration of drugs such as penicillin that had been shown to penetrate the brain poorly. Indeed, Fishman,[9] Dixon et al.[117] and Spector and Lorenzo[20] went on to show that probenecid, a drug commonly used to inhibit renal clearance of antibacterials, increases the CSF concentrations of penicillin. In more recent studies the efflux of didanosine,[17,118] zidovudine,[18,119-121] γ-aminobutyric acid,[122] mercaptopurine,[15] leukotriene C4,[123] valproic acid[11] and etoposide[124] has been inhibited by coadministration of a wide range of substances used as putative specific inhibitors of efflux transporters. However, it is very apparent that there is extensive overlap of the substrate profiles of all the transporters discussed here (table I). Most notably probenecid, a commonly used efflux inhibitor used in BBB studies to elucidate the mechanism of transport, is a substrate/inhibitor of MRP1, MRP5, OAT1, OAT3 and MCT1. Similarly, quinidine and verapamil, repeatedly described in the literature as P-glycoprotein-specific substrates/inhibitors, interact with P-glycoprotein, OCT2 and OCTN1. Therefore, for clinical modulation, enhanced pharmaceutical research, and detailed elucidation of transport mechanisms, specific transport inhibitors are required. Many companies are developing novel and specific efflux inhibitors primarily for the inhibition of MDR in cancer patients (table II). The most well characterised efflux inhibitor currently in development is valspodar (SDZ PSC 833), a non-immunosuppressive cyclosporin analogue that specifically inhibits P-glycoprotein. Its utility in the inhibition of drug efflux from the brain, and thereby its enhancement of drug penetration into the brain, has been investigated and it has been shown to increase the uptake of colchicine, vinblastine, digoxin, quinidine, vincristine and cyclosporin into the brain[125-129] and increase neurosensitivity to cyclosporin, quinidine, and ivermectin.[130] In addition to modulating efflux transporters with small molecule pharmacological agents, it may be possible to use anti-MDR antibodies, protein kinase C inhibitors, antisense oligonucleotides or ribozymes Clin Pharmacokinet 2002; 41 (2)

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Table II. Specific inhibitors of efflux transporters in development Stage of development

Agent

Company

Target

Phase 2-3

Valspodar (PSC-833)

Novartis

P-glycoprotein

Phase 2

Biricodar (VX-710)

Vertex

P-glycoprotein and MRP1

VX-853

Vertex

P-glycoprotein and MRP1 P-glycoprotein

Phase 1-2 Phase 1 Preclinical

XR-9576

Xenova

LY-335979

Eli Lilly

P-glycoprotein

R-101933

Janssen

P-glycoprotein

OC144-093

Ontogen

P-glycoprotein

MS-209

Mitsui Pharmaceuticals

MDR

TER-199

Telik

MRP1

SNF-4435C

Snow Brand Milk Products

MDR

CGP-41251

Novartis

P-glycoprotein

LY-329146

Eli Lilly

P-glycoprotein and MRP1

LY-117018

Eli Lilly

P-glycoprotein and MRP1

KR-30035

Korea Research Institute of P-glycoprotein Chemical Technology

Pyrrolidinemethyl diamide and carbamate derivatives, α, α -difluoroamides

Bristol-Myers-Squibb

MDR

1, 4-Dihydropyridine derivatives

Nikken Chemicals

MDR

MDR = multidrug resistance; MRP = multidrug resistance-associated protein.

to down-regulate the activity or expression of MDR transporters.[131,132] However, the very nature of MDR mechanisms, being of high capacity and low specificity, ensures that inhibition of such mechanisms to enhance BBB penetration will not be specific to a single drug and will enhance transport of many compounds into the brain. This may have dire consequences for patients, particularly those on multiple therapies. Design of drugs that have reduced affinity for brain efflux transporters or that can bypass the transporters may provide a more efficient solution to the problem of MDR in the BBB. Spector et al.[20] have demonstrated that some antibacterials, including the cephalosporin ceftriaxone and the carbapenem imipenem, have minimal affinity for the efflux system that is responsible for transporting penicillins out of the brain, and increased brain penetration of these drugs is associated with improved treatment of patients with bacterial meningitis.[21] More recently, Rousselle et al.[133] described increased penetration of the Pglycoprotein substrate doxorubicin into the brain when conjugated to peptides that readily cross the BBB and are not MDR substrates. © Adis International Limited. All rights reserved.

7. Conclusion With the unequivocal demonstration in P-glycoprotein knockout mice of a role for drug efflux in the preservation of the CNS environment, an area of research that dates back to the early 1960s is gaining momentum. Many efflux transporters, in addition to P-glycoprotein, have now been identified and their role in the CNS is under investigation. With their broad substrate profiles, which include many drugs actively transported from brain to blood, and their demonstrated expression in the brain, it is clear that these efflux transporters have an important role in the action or, more importantly, the inaction of drugs in the CNS. Therefore, the design of compounds that can modulate the efflux of drugs from the CNS and the design of new generation compounds with reduced affinity for efflux transporters may improve the treatment of CNS disorders. Acknowledgements The author would like to thank Drs Scott Wieland, Michelle Glasky and Mark Foreman for helpful comments on this manuscript, and Ben Aguillon for excellent assistance with graphic art.

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Correspondence and offprints: Eve M. Taylor, NeoTherapeutics Inc., 157 Technology Drive, Irvine, CA 92618, USA. E-mail: [email protected]

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