DRUG TRANSPORT IN THE BRAIN - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 417

the bile, 45 overexpression of human MDR1 or rodent mdr1a/mdr1b confers the MDR
phenotype. 40 , 46 In humans, the MDR1 gene product is 1280 amino acids in length
and consists of two homologous halves, each made up of six transmembrane do-
mains and one ATP-binding site. 40 Mammalian Pgp may possess between two and
four oligosaccharide side chains (i.e., glycosylations) on the first extracellular loop. 40
In addition, phosphorylation is a hallmark of mature Pgp, and these posttransla-
tional modifications are often observed in the linker region between transmembrane
domains six and seven. 40
Pgp was identified initially in Chinese hamster ovary cells selected for resistance
to colchicine, 42 where it functions as an energy-dependent efflux pump that can ex-
trude several amphipathic pharmacological agents. Since its discovery, many drugs
have been shown to be Pgp substrates, including naturally occurring chemotherapeu-
tic drugs such as anthracyclines (doxorubicin, daunorubicin, mitoxantrone), Vinca
alkaloids (vincristine, vinblastine), epipodophyllotoxins (etoposide, teniposide), and
taxanes (taxol, taxotere) as well as immunosuppressive agents (cyclosporin A and its
analog, PSC833), cardiac glycosides (digoxin), antibiotics (rifampin, erythromycin,
ciprofloxacin, grepafloxacin), antiallergenics (bepotastine, fexofenadine), beta block-
ers (talinolol, celiprolol), antiepileptics (phenytoin), steroid hormones (dexametha-
sone, cortisol), antimycotic agents (ketoconazole, itraconazole), histamine receptor
antagonists (cimetidine, ranitidine), and HIV-1 protease inhibitors (saquinavir, in-
dinavir, ritonavir, nelfinavir, amprenavir). Other therapeutic compounds that have
also been shown to be Pgp substrates include morphine and atorvastatin. 47 Currently
identified Pgp transport inhibitors include calcium channel blockers (verapamil),
calmodulin antagonists (trifluoperazine), quinolines (quinidine), cyclosporin A, and
the HIV-1 protease inhibitors. Many of these compounds act as both Pgp substrates
and inhibitors and may interact with Pgp at more than one binding site. 47 Several
studies have shown that Pgp is directly involved in regulating the brain accumulation
of these therapeutic agents. 48 - 53 For example, in vivo studies in mdr1a/1b knockout
mice have shown that HIV-1 protease inhibitors (e.g., saquinavir, indinavir, nelfinavir,
ritonavir) exhibit four- to 36-fold increases in brain drug accumulation. 54 , 55
In the CNS, Pgp expression has been investigated primarily at the brain vascu-
lar barriers (i.e., BBB and BCSF barrier). Although Pgp has been localized on the
apical side of the choroid plexus epithelia, 56 the location of the protein in the brain
microvessel endothelial cells that constitute the blood-brain barrier has been contro-
versial. While several luminal membrane isolation studies, immunohistochemistry,
and immunofluorescence laser scanning confocal microscopy studies have localized
Pgp to the luminal surface of the mammalian brain endothelium, 57 - 59 others have
identified the localization of Pgp on neighboring astrocyte foot processes. 60 , 61 Re-
cently, our laboratory has reported Pgp localization on both the luminal and abluminal
sides of the brain microvascular endothelium. 62 Furthermore, we have characterized
the functional expression of Pgp in a rat microglia cell line (MLS-9), 63 cultured
rat astrocytes, 64 and in a rat brain microvessel endothelial cell line and in situ in
rat brain capillaries. 65 Recently, Pgp expression has also been reported in human
glioma cell lines. 66 In addition to the plasma membrane, the subcellular localization
of Pgp in brain cellular compartments has also been investigated. Previous studies