CHAPTER 2 Membrane Transporters and Drug Response
27
(e.g., the thiazolidinedione troglitazone). Thus, uptake and efflux transporters determine the plasma and tissue concentrations of endogenous compounds and xenobiotics, thereby influencing either systemic or site-specific drug toxicity.
BASIC MECHANISMS OF MEMBRANE TRANSPORT TRANSPORTERS VERSUS CHANNELS
Both channels and transporters facilitate the membrane permeation of inorganic ions and organic compounds. Channels have two primary states, open and closed, that are stochastic phenomena.
Only in the open state do channels act as pores for their selected ions, allowing permeation across the plasma membrane. After opening, channels return to the closed state as a function of time.
FIGURE 2–1 Roles of membrane transporters in pharmacokinetic pathways. Membrane transporters (T) play roles in pharmacokinetic pathways (drug absorption, distribution, metabolism, and excretion), thereby setting systemic drug levels. Drug levels often drive therapeutic and adverse drug effects.
FIGURE 2–2 Hepatic drug transporters. Membrane transporters, shown as hexagons with arrows, work in concert with phase 1 and phase 2 drug-metabolizing enzymes in the hepatocyte to mediate the uptake and efflux of drugs and their metabolites.
In contrast, a transporter forms an intermediate complex with the substrate (solute); thereafter, a conformational change in the transporter induces substrate translocation to the other side of the membrane. Because of these different mechanisms, turnover rates differ markedly between chan-nels and transporters. Turnover rate constants of typical chanchan-nels are 106–108s–1, whereas those of transporters are, at most, 101–103s–1. Because transporters form intermediate complexes with specific compounds, transporter-mediated membrane transport is characterized by saturability and inhibition by substrate analogs.
The basic mechanisms involved in solute transport across the plasma membrane include pas-sive diffusion, facilitated diffusion, and active transport. Active transport can be further subdi-vided into primary and secondary active transport. These mechanisms are depicted in Figure 2–4.
FIGURE 2–3 Major mechanisms by which transporters mediate adverse drug responses. Three cases are given. The left panel of each case provides a cartoon representation of the mechanism; the right panel shows the resulting effect on drug levels. (Top panel) Increase in the plasma concentrations of drug due to a decrease in the uptake and/or secretion in clearance organs such as the liver and kidney. (Middle panel) Increase in the concentration of drug in toxicological target organs due either to the enhanced uptake or to reduced efflux of the drug. (Bottom panel) Increase in the plasma concentration of an endogenous compound (e.g., a bile acid) due to a drug’s inhibiting the influx of the endogenous com-pound in its eliminating or target organ. The diagram also may represent an increase in the concentration of the endoge-nous compound in the target organ owing to drug-inhibited efflux of the endogeendoge-nous compound.
CHAPTER 2 Membrane Transporters and Drug Response
29
PASSIVE DIFFUSION Simple diffusion of a solute across the plasma membrane involves three processes: partition from the aqueous to the lipid phase, diffusion across the lipid bilayer, and repartition into the aqueous phase on the opposite side. Diffusion of any solute (including drugs) occurs down an electrochemical potential gradient of the solute and is dependent on both its chem-ical and electrchem-ical potential.
FACILITATED DIFFUSION Membrane transporters may facilitate diffusion of ions and organic compounds across the plasma membrane; this facilitated diffusion does not require energy input. Just as in passive diffusion, the transport of ionized and nonionized compounds across the plasma membrane occurs down their electrochemical potential gradient. Therefore, steady state will be achieved when the electrochemical potentials of the compound on both sides of the membrane become equal.
ACTIVE TRANSPORT Active transport requires energy input and transports solutes against their electrochemical gradients, leading to the concentration of solutes on one side of the plasma membrane and the creation of potential energy in the electrochemical gradient formed. Active transport plays an important role in the uptake and efflux of drugs and other solutes. Depending on the driving force, active transport can be subdivided into primary and secondary active transport (Figure 2–4).
Primary Active Transport Membrane transport that directly couples with ATP hydrolysis is called primary active transport. ABC transporters are examples of primary active trans-porters. They contain one or two highly conserved ATP binding cassettes that exhibit ATPase activity. ABC transporters mediate the unidirectional efflux of many solutes across biological membranes.
Secondary Active Transport In secondary active transport, the transport across the plasma membrane of one solute S1against its concentration gradient is driven energetically by the trans-port of another solute S2in accordance with its concentration gradient. The driving force for this type of transport therefore is stored in the electrochemical potential created by the concentration difference of S2across the plasma membrane. Depending on the transport direction of the solute, secondary active transporters are classified as either symporters or antiporters. Symporters, also termed cotransporters, transport S2and S1in the same direction, whereas antiporters, also termed exchangers,move their substrates in opposite directions (Figure 2–4).
FIGURE 2–4 Classification of membrane transport mechanisms. Light blue circles depict the substrate. Size of the circles is proportional to the concentration of the substrate. Arrows show the direction of flux. Black squares represent the ion that supplies the driving force for transport (size is proportional to the concentration of the ion). Dark blue ovals depict transport proteins.
KINETICS OF TRANSPORT
The flux of a substrate (rate of transport) across the plasma membrane via transporter-mediated processes is characterized by saturability. The relationship between the flux v and substrate con-centration C in a transporter-mediated process is analogous to the rate of product formed by an enzyme and the concentration of substrate. The maximum transport rate (Vmax) is proportional to the density of transporters on the plasma membrane, and the Kmrepresents the substrate concen-tration at which the flux is half maximal. When C is small compared with the Km, the flux is increased in proportion to the substrate concentration (roughly linearly). If C is large compared with the Kmvalue, the flux approaches the maximal value (Vmax). The Kmand Vmaxvalues can be deter-mined by examining the flux at different substrate concentrations.
Transporter-mediated membrane transport of a substrate is also characterized by inhibition by other compounds. As with enzyme or receptor inhibition, this inhibition can be categorized as one of three types: competitive, noncompetitive, and uncompetitive.
Competitiveinhibition occurs when substrates and inhibitors share a common binding site on the transporter, resulting in an increase in the apparent Kmvalue. Noncompetitive inhibition occurs when the inhibitor allosterically affects the transporter in a manner that does not inhibit the forma-tion of an intermediate complex of substrate and transporter but does inhibit the subsequent translo-cation process. Uncompetitive inhibition assumes that inhibitors form a complex only with an intermediate substrate-transporter complex and inhibit subsequent translocation.
VECTORIAL TRANSPORT
The SLC transporters mediate either drug uptake or efflux, whereas ABC transporters mediate only unidirectional efflux. Asymmetrical transport across a monolayer of polarized cells, such as the epithelial and endothelial cells of brain capillaries, is called vectorial transport (Figure 2–5). Vec-torial transport is important in the efficient transfer of solutes across epithelial or endothelial barri-ers; it plays a major role in hepatobiliary and urinary excretion of drugs from the blood to the lumen and in the intestinal absorption of drugs and nutrients. In addition, efflux of drugs from the brain viabrain endothelial cells and brain choroid plexus epithelial cells involves vectorial transport.
For lipophilic compounds with sufficient membrane permeability, ABC transporters alone can achieve vectorial transport by extruding their substrates to the outside of cells without the help of influx transporters. For relatively hydrophilic organic anions and cations, coordinated uptake and efflux transporters in the polarized plasma membranes are necessary to achieve the vectorial move-ment of solutes across an epithelium. Common substrates of coordinated transporters are trans-ferred efficiently across the epithelial barrier. In the liver, a number of transporters with different substrate specificities are localized on the sinusoidal membrane (facing blood). These transporters are involved in the uptake of bile acids, amphipathic organic anions, and hydrophilic organic cations into hepatocytes. Similarly, ABC transporters on the canalicular membrane (facing bile) export such compounds into the bile. Overlapping substrate specificities between the uptake
FIGURE 2–5 Transepithelial or transendothelial flux. Transepithelial or transendothelial flux of drugs requires dis-tinct transporters at the two surfaces of the epithelial or endothelial barriers. These are depicted diagrammatically for transport across the small intestine (absorption), the kidney and liver (elimination), and the brain capillaries that com-prise the blood–brain barrier.
CHAPTER 2 Membrane Transporters and Drug Response
31
transporters (Na+/taurocholate cotransporting polypeptide [NTCP] and organic anion transporting polypeptide [OATP] family) and efflux transporters (BSEP, MRP2, P-glycoprotein, and BCRP) make the vectorial transport of organic anions highly efficient. Similar transport systems also are present in the intestine, renal tubules, and endothelial cells of the brain capillaries (Figure 2–5).
REGULATION OF TRANSPORTER EXPRESSION
Transporter expression can be regulated transcriptionally in response to drug treatment and pathophysiological conditions, resulting in induction or down-regulation of transporter mRNAs.
A number of nuclear receptors form heterodimers with the 9-cis-retinoic acid receptor (RXR) in regulating drug-metabolizing enzymes and transporters. Such receptors include pregnane X receptor (PXR/NR1I2), constitutive androstane receptor (CAR/NR1I3), farnesoid X receptor (FXR/NR1H4), peroxisome proliferator-activated receptor a (PPARa), and retinoic acid receptor (RAR). Except for CAR, these are ligand-activated nuclear receptors that, as heterodimers with RXR, bind specific elements in the enhancer regions of target genes. CAR has constitutive tran-scriptional activity that is antagonized by inverse agonists such as androstenol and androstanol and induced by barbiturates. PXR (SXR in humans) is activated by synthetic and endogenous steroids, bile acids, and drugs such as clotrimazole, phenobarbital, rifampin, sulfinpyrazone, ritonavir, carbamazepine, phenytoin, sulfadimidine, taxol, and hyperforin (a constituent of St.
John’s wort). Table 2–1 summarizes the effects of drug activation of nuclear receptors on trans-porter expression. There is an overlap of substrates between CYP3A4 and P-glycoprotein, and PXR mediates coinduction of CYP3A4 and P-glycoprotein, supporting their cooperation in effi-cient detoxification.
TRANSPORTER SUPERFAMILIES IN THE HUMAN GENOME
SLC TRANSPORTERS The SLC superfamily includes 43 families and contains ∼300 human genes. Many of these genes are associated with genetic diseases (Table 2–2). SLC trans-porters transport diverse ionic and nonionic endogenous compounds and xenobiotics, acting either as facilitated transporters or as secondary active symporters or antiporters.
ABC SUPERFAMILY The ABC superfamily consists of 49 genes, each containing one or two conserved ABC regions. The ABC region—the core catalytic domain of ATP hydrolysis—contains Walker A and B sequences and an ABC transporter-specific signature C sequence. The ABC regions of these proteins bind and hydrolyze ATP, and the proteins use the energy for uphill transport of their substrates across the membrane. Although some ABC superfamily transporters contain only a single ABC motif, they form homodimers (BCRP/ABCG2) or heterodimers (ABCG5 and ABCG8) that exhibit a transport function. ABC transporters in prokaryotes are involved in the import of essential compounds that cannot be obtained by passive diffusion (e.g., sugars, vitamins, and metals). Most ABC genes in eukaryotes transport compounds from the cytoplasm to the outside or into an intra-cellular compartment (e.g., endoplasmic reticulum, mitochondria, and peroxisomes).
ABC transporters are divided into seven groups based on their sequence homology (Table 2–3).
They are essential for many cellular processes, and mutations in at least 13 of the genes cause or contribute to human genetic disorders.
In addition to conferring multidrug resistance, an important pharmacological aspect of these transporters is xenobiotic export from healthy tissues. In particular, MDR1/ABCB1, MRP2/
ABCC2, and BCRP/ABCG2 have been shown to be involved in overall drug disposition.
Properties of ABC Transporters Related to Drug Action
The tissue distribution of drug-related ABC transporters is summarized in Table 2–4, together with information about typical substrates.
TISSUE DISTRIBUTION OF DRUG-RELATED ABC TRANSPORTERS
MDR1 (ABCB1), MRP2 (ABCC2), and BCRP (ABCG2) are all expressed in the apical side of the intestinal epithelia, where they extrude xenobiotics, including many clinically relevant drugs. Key to the vectorial excretion of drugs into urine or bile, ABC transporters are expressed in polarized tissues, such as kidney and liver: MDR1, MRP2, and MRP4 (ABCC4) on the brush-border mem-brane of renal epithelia, and MDR1, MRP2, and BCRP on the bile canalicular memmem-brane of hepa-tocytes. Some ABC transporters are expressed specifically on the blood side of the endothelial or epithelial cells that form barriers to the free entrance of toxic compounds into tissues: the BBB (MDR1 and MRP4 on the luminal side of brain capillary endothelial cells), the blood–
cerebrospinal fluid (CSF) barrier (MRP1 and MRP4 on the basolateral blood side of choroid plexus epithelia), the blood–testis barrier (MRP1 on the basolateral membrane of mouse Sertoli
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Regulation of Transporter Expression by Nuclear Receptors Transcription
Transporter Species Factor Ligand (Modulated by Drugs) Effect of Ligand
MDR1 (P-gp) Human PXR Rifampin ↑ Transcription activity (promoter assay)
(600 mg/day, 10 days) ↑ Expression in duodenum in healthy subjects Rifampin ↓ Oral bioavailability of digoxin in healthy subjects
(600 mg/day, 10 days)
Rifampin ↓ AUC of talinolol after IV and oral administration in healthy (600 mg/day, 9 days) subjects
MRP2 Human PXR Rifampin ↑ Expression in duodenum in healthy subjects
(600 mg/day, 9 days)
Rifampin/hyperforin ↑ Expression in human hepatocytes FXR GW4064/chenodeoxycholate ↑ Expression in HepG2 cells
Mouse PXR PCN/dexamethasone ↑ Expression in mouse hepatocyte
CAR Phenobarbital ↑ Expression in hepatocyte of PXR KO mice (promoter assay) Rat PXR/FXR/CAR PCN/GW4064/phenobarbital ↑ Expression in rat hepatocytes
PXR/FXR/CAR ↑ Transcription activity (promoter assay)
BSEP Human FXR Chenodeoxycholate, GW4064 ↑ Transcription activity (promoter assay)
Ntcp Rat SHP1 ↓ RAR mediated transcription
OATP1B1 Human SHP1 Indirect effect on HNF1α expression
OATP1B3 Human FXR Chenodeoxycholate ↑ Expression in hepatoma cells
MDR2 Mouse PPARα Ciprofibrate (0.05% w/w in diet) ↑ Expression in the liver
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Table 2–2
Families in the Human Solute Carrier Superfamily
Number Selected
Gene of Family Drug
Name Family Name Members Substrates Examples of Linked Human Diseases
SLC1 High-affinity glutamate and neutral amino acid 7 Amyotrophic lateral sclerosis
transporter
SLC2 Facilitative GLUT transporter 14
SLC3 Heavy subunits of the heteromeric amino acid 2 Melphalin Classic cystinuria type I transporters
SLC4 Bicarbonate transporter 10 Hemolytic anemia, blindness–auditory impairment
SLC5 Na+glucose cotransporter 8 Glucosfamide Glucose–galactose malabsorption syndrome
SLC6 Na+- and Cl–-dependent neurotransmitter transporter 16 Paraoxetine, fluoxetine X-linked creatine deficiency syndrome
SLC7 Cationic amino acid transporter 14 Melphalan Lysinuric protein intolerance
SLC8 Na+/Ca2+exchanger 3 Asymmetrical dimethylarginine
SLC9 Na+/H+exchanger 8 Thiazide diuretics Congenital secretory diarrhea
SLC10 Na+bile salt cotransporter 6 Benzothiazepine Primary bile salt malabsorption SLC11 H+coupled metal ion transporter 2 Hereditary hemochromatosis SLC12 Electroneutral cation–Cl–cotransporter family 9 Gitelman’s syndrome SLC13 Na+–sulfate/carboxylate cotransporter 5 Sulfate, cysteine conjugates
SLC14 Urea transporter 2 Kidd antigen blood group
SLC15 H+–oligopeptide cotransporter 4 Valacyclovir
SLC16 Monocarboxylate transporter 14 Salicylate, atorvastatin Muscle weakness SLC17 Vesicular glutamate transporter 8 Sialic acid storage disease SLC18 Vesicular amine transporter 3 Reserpine Myasthenic syndromes
SLC19 Folate/thiamine transporter 3 Methotrexate Thiamine-responsive megaloblastic anemia SLC20 Type III Na+–phosphate cotransporter 2
SLC21/ Organic anion transporter 11 Pravastatin
SLC22SLC0 Organic cation/anion/zwitterion transporter 18 Pravastatin, metformin Systemic carnitine deficiency syndrome SLC23 Na+-dependent ascorbate transporter 4 Vitamin C
SLC24 Na+/(Ca2+-K+) exchanger 5
SLC25 Mitochondrial carrier 27 Senger’s syndrome
SLC26 Multifunctional anion exchanger 10 Salicylate, ciprofloxacin Congenital Cl–-losing diarrhea
(Continued)
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Families in the Human Solute Carrier Superfamily (Continued)
Number Selected
Gene of Family Drug
Name Family Name Members Substrates Examples of Linked Human Diseases
SLC27 Fatty acid transporter protein 6
SLC28 Na+-coupled nucleoside transport 3 Gemcitabine, cladribine SLC29 Facilitative nucleoside transporter 4 Dipyridamole, gemcitabine
SLC30 Zinc efflux 9
SLC31 Copper transporter 2 Cisplatin
SLC32 Vesicular inhibitory amino acid transporter 1 Vigabatrin
SLC33 Acetyl-CoA transporter 1
SLC34 Type II Na+–phosphate cotransporter 3 Autosomal-dominant hypophosphatemic rickets SLC35 Nucleoside-sugar transporter 17 Leukocyte adhesion deficiency type II SLC36 H+-coupled amino acid transporter 4 D-Serine,D-cycloserine
SLC37 Sugar-phosphate/phosphate exchanger 4 Glycogen storage disease non-1a SLC38 System A and N, Na+-coupled neutral amino 6
acid transporter
SLC39 Metal ion transporter 14 Acrodermatitis enteropathica
SLC40 Basolateral iron transporter 1 Type IV hemochromatosis
SLC41 MgtE-like magnesium transporter 3
SLC42 Rh ammonium transporter (pending) 3 Rh-null regulator SLC43 Na+-independent system-L-like amino acid transporter 2
CHAPTER 2 Membrane Transporters and Drug Response
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Table 2–3
The ATP Binding Cassette (ABC) Superfamily in the Human Genome and Linked Genetic Diseases
Number of
Gene Name Family Name Family Members Examples of Linked Human Diseases
ABCA ABC A 12 Tangier disease (defect in cholesterol transport;
ABCA1), Stargardt syndrome (defect in retinal metabolism; ABCA4)
ABCB ABC B 11 Bare lymphocyte syndrome type I (defect in
antigen-presenting; ABCB3 and ABCB4), progressive familial intrahepatic cholestasis type 3 (defect in biliary lipid secretion; MDR3/ABCB4), X-linked sideroblastic anemia with ataxia (a possible defect in iron homeostasis in mitochondria; ABCB7), progressive familial intrahepatic cholestasis type 2 (defect in biliary bile acid excretion; BSEP/ABCB11) ABCC ABC C 13 Dubin–Johnson syndrome (defect in biliary bilirubin
glururonide excretion; MRP2/ABCC2),
pseudoxanthoma (unknown mechanism; ABCC6), cystic fibrosis (defect in chloride channel regulation; ABCC7), persistent hyperinsulinemic hypoglycemia of infancy (defect in inwardly rectifying potassium conductance regulation in pancreatic B cells; SUR1)
ABCD ABC D 4 Adrenoleukodystrophy (a possible defect in
peroxisomal transport or catabolism of very long-chain fatty acids; ABCD1)
ABCE ABC E 1
ABCF ABC F 3
ABCG ABC G 5 Sitosterolemia (defect in biliary and intestinal excretion of plant sterols; ABCG5 and ABCG8)
cells and MDR1 in several types of human testicular cells), and the blood–placenta barrier (MDR1, MRP2, and BCRP on the luminal maternal side and MRP1 on the antiluminal fetal side of pla-cental trophoblasts).
MRP/ABCC Family
The substrates of transporters in the MRP/ABCC family are mostly organic anions. Both MRP1 and MRP2 accept glutathione and glucuronide conjugates, sulfated conjugates of bile salts, and nonconjugated organic anions of an amphipathic nature (at least one negative charge and some degree of hydrophobicity). They also transport neutral or cationic anticancer drugs, such as vinca alkaloids and anthracyclines, possibly via a cotransport or symport mechanism with reduced glutathione. MRP3 also has a substrate specificity that is similar to that of MRP2 but with a lower transport affinity for glutathione conjugates compared with MRP1 and MRP2.
MRP3 is expressed on the sinusoidal side of hepatocytes and is induced under cholestatic con-ditions. MRP3 functions to return toxic bile salts and bilirubin glucuronides into the blood cir-culation. MRP4 and MRP5 pump nucleotide analogs and clinically important anti–human immunodeficiency virus (HIV) drugs. No substrates for MRP6 have been identified that explain MRP6-associated pseudoxanthoma.
ABC TRANSPORTERS IN DRUG ABSORPTION AND ELIMINATION
Systemic exposure to orally administered digoxin is increased by coadministration of MDR1 inducers and negatively correlated with MDR1 protein expression in the intestine. MDR1 is also expressed on the brush-border membrane of renal epithelia, and its function can be monitored using digoxin (>70% excreted in the urine). MDR1 inhibitors (e.g., quinidine, verapamil, vaspo-dar, spironolactone, clarithromycin, and ritonavir) all markedly reduce renal digoxin excretion. In view of this, drugs with narrow therapeutic windows (e.g., digoxin) should be used with great care if MDR1-based drug–drug interactions are likely.
36
ABC Transporters Involved in Drug Absorption, Distribution, and Excretion Transporter Name Tissue Distribution Physiological Function Substrates
MDR1 Liver Detoxification of xenobiotics? Characteristics:Neutral or cationic compounds with bulky structure
(ABCB1) Kidney
Intestine Anticancer drugs: etoposide, doxorubicin, vincristine
BBB Ca2+channel blockers: diltiazem, verapamil
BTB HIV protease inhibitors: indinavir, ritonavir
BPB Antibiotics/antifungals: erythromycin, ketoconazole
Hormones: testosterone, progesterone
Immunosuppressants: cyclosporine, FK506 (tacrolimus) Others: digoxin, quinidine
MRP1 Ubiquitous (kidney, Leukotriene (LTC4) secretion Characteristics:Amphiphilic with at least one negative net charge (ABCC1) BCSFB, BTB) from leukocyte Anticancer drugs: vincristine (with GSH), methotrexate
Glutathione conjugates: LTC4, glutathione conjugate of ethacrynic acid Glucuronide conjugates: estradiol-17-D-glucuronide, bilirubin mono(or bis)
glucuronide
Sulfated conjugates: estrone-3-sulfate (with GSH) HIV protease inhibitors: saquinavir
Antifungals: grepafloxacin
Others: folate, GSH, oxidized glutathione
MRP2 Liver Excretion of bilirubin glucuronide Characteristics:Amphiphilic with at least one negative net charge (similar
(ABCC2) Kidney and GSH into bile to MRP1)
Intestine Anticancer drugs: methotrexate, vincristine
BPB Glutathione conjugates: LTC4, GSH conjugate of ethacrynic acid
Glucuronide conjugates: estradiol-17-D-glucuronide, bilirubin mono(or bis) glucuronide
Sulfate conjugate of bile salts: taurolithocholate sulfate HIV protease inhibitors: indinavir, ritonavir
Others: pravastatin, GSH, oxidized glutathione
MRP3 Liver ? Characteristics:Amphiphilic with at least one negative net charge
(ABCC3) Kidney (Glucuronide conjugates are better substrates than glutathione conjugates.)
37
Intestine Anticancer drugs: etoposide, methotrexate
Glutathione conjugates: LTC4, glutathione conjugate of 15-deoxy-delta prostaglandin J2
Glucuronide conjugates: estradiol-17-D-glucuronide, etoposide glucuronide Sulfate conjugates of bile salts: taurolithocholate sulfate
Bile salts: glycocholate, taurocholate Others: folate, leucovorin
MRP4 Ubiquitous (kidney, ? Characteristics:Nucleotide analogues
(ABCC4) prostate, lung, Anticancer drugs: 6-mercaptopurine, methotrexate
muscle, pancreas, Glucuronide conjugates: estradiol-17-D-glucuronide
testis, ovary, Cyclic nucleotides: cyclic AMP, cyclic GMP
bladder, gallbladder, HIV protease inhibitors: adefovir
BBB, BCSFB) Others: folate, leucovorin, taurocholate (with GSH)
MRP5 Ubiquitous ? Characteristics:Nucleotide analogues
(ABCC5) Anticancer drugs: 6-mercaptopurine
Cyclic nucleotides: cyclic AMP, cyclic GMP HIV protease inhibitors: adefovir
MRP6 Liver ? Anticancer drugs: doxorubicin*, etoposide*
(ABCC6) Kidney Glutathione conjugate of: LTC4
Other: BQ-123 (cyclic peptide ET-1 antagonist) BCRP Liver Normal heme transport during Anticancer drugs: methotrexate, mitoxantrone, (MXR) Intestine maturation of erythrocytes camptothecin analogs (SN-38, etc.), topotecan
(ABCG2) BBB Glucuronide conjugates: 4-methylumbelliferone glucuronide,
estradiol-17-D-glucuronide
Sulfate conjugates: dehydroepiandrosterone sulfate, estrone-3-sulfate Others: cholesterol, estradiol
MDR3 Liver Excretion of phospholipids Characteristics:Phospholipids
(ABCB4) into bile
BSEP Liver Excretion of bile salts into bile Characteristics:Bile salts (ABCB11)
ABCG5 and Liver Excretion of plant sterols into bile Characteristics:Plant sterols
ABCG8 Intestine and intestinal lumen
NOTE: Representative substrates and cytotoxic drugs with increased resistance (*) are included in this table (cytotoxicity with increased resistance is usually caused by the decreased accumulation of the drugs). Although MDR3 (ABCB4), BSEP (ABCB11), ABCG5, and ABCG8 are not directly involved in drug disposition, inhibition of these physiologically important ABC transporters will lead to unfavorable side effects.
Little clinically applicable information regarding MRP2 and BCRP drug-handling is avail-able. Most MRP2 or BCRP substrates also can be transported by the OATP family transporters on the sinusoidal membrane.
GENETIC VARIATION IN MEMBRANE TRANSPORTERS: IMPLICATIONS FOR CLINICAL DRUG RESPONSE
Inherited disorders of membrane transport have been identified (Tables 2–2 and 2–3), and poly-morphisms in membrane transporters that play a role in drug response are yielding new insights in pharmacogenetics (see Chapter 4). The most widely studied drug transporter is P-glycoprotein (MDR1, ABCB1); the ABCB1 genotype is associated with responses to anticancer drugs, antiviral agents, immunosuppressants, antihistamines, cardiac glycosides, and anticonvulsants. ABCB1 SNPs also have been associated with tacrolimus and nortriptyline neurotoxicity and susceptibility for developing ulcerative colitis, renal cell carcinoma, and Parkinson’s disease.
TRANSPORTERS INVOLVED IN PHARMACOKINETICS
Hepatic Transporters
HMG-CoA REDUCTASE INHIBITORS
Statins are cholesterol-lowering agents that reversibly inhibit HMG-CoA reductase, which cat-alyzes a rate-limiting step in cholesterol biosynthesis (see Chapter 35). Most of the statins in the acid form are substrates of uptake transporters that mediate hepatic uptake and enterohepatic cir-culation (Figures 2–5 and 2–6). In this process, hepatic uptake transporters such as OATP1B1 and efflux transporters such as MRP2 cooperate to produce vectorial transcellular transport of bisubstrates in the liver. The efficient first-pass hepatic uptake of statins by OATP1B1 helps them to exert their pharmacological effect and also minimizes the systemic drug distribution, thereby minimizing adverse effects in smooth muscle. Recently, two common SNPs in SLCO1B1 (OATP1B1) have been associated with elevated plasma levels of pravastatin.
DRUG–DRUG INTERACTIONS INVOLVING TRANSPORTER-MEDIATED HEPATIC UPTAKE Transporter-mediated hepatic uptake can cause drug–drug interactions among drugs that are actively taken up into the liver and metabolized and/or excreted in the bile. When an inhibitor of drug-metabolizing enzymes is highly concentrated in hepatocytes by active transport,
FIGURE 2–6 Transporters in the hepatocyte that function in the uptake and efflux of drugs across the sinusoidal membrane and efflux of drugs into the bile across the canalicular membrane. See text for details of the transporters pictured.
CHAPTER 2 Membrane Transporters and Drug Response
39
extensive inhibition of the drug-metabolizing enzymes may be observed because of the high concentration of the inhibitor in the vicinity of the drug-metabolizing enzymes.