Principles of Clinical Pharmacology January 9, 2003 Module 2: Drug Metabolism and Transport Unit 6: Concentrative and Equilibrative Drug Transport

1
Principles of Clinical PharmacologyJanuary 9,
2003Module 2 Drug Metabolism and
TransportUnit 6 Concentrative and
Equilibrative Drug Transport

• Peter C. Preusch, Ph.D.
• Pharmacology, Physiology, and
• Biological Chemistry Division
• National Institute of General Medical Sciences

2
Objectives

• Vision, reality, and the path between.
• Methods of measuring drug transport in
  vitro and in vivo.
• Mechanisms of drug transport.
• Recent advances in understanding the role of
  membrane transport proteins.
• Clinical significance.

3
Measurements of Drug Distribution Reflect
Membrane Transport In Vivo

• Blood/tissue Samples, Biopsies, and Assays
• Autoradiography
• Perfusion/Cannulation Methods
• Note ability to biopsy lumen wall or collect shed
  cells in the same intestinal perfusionexperiments
  Loc-I-Gut
• In vivo P(eff) amoxicillin /- amiloiride (Na/H
  exchange inhibitor) no effect on low P(eff)
  drug
• Radiology - X-ray, PET, SPECT
• Magnetic Resonance Imaging
• Microdialysis

4
Measurements of MembraneTransport In Vitro

• Ussing chamber - excised tissue samples
• Everted gut sac - uptake from medium
• Uptake/efflux by membrane vesicles, liposomes,
  BLM, PAMPA, cells in culture (CHO)
• - filtration, centrifugation, oil-stop
  separatory assays
• Fluorescent (confocal) microscopy of cultured
  cells
• fluorescent drugs (mitoxantrone, rhodamine)
• Electrophysiology in cells (e.g., oocytes)
• Monolayer cell cultures on permeable supports
• Caco-2 cells, MDCKII, brain MVECs

5
Measurement of Transport inExcised Tissue Samples
Modified Ussing-chamber allows perfusion of
solutions on both sides of membrane holder,
control of pressure differential, measurement of
potential, conductivity, pH.
Adapted from Ref. 7.
6
Monolayer Epithelial Cell Culture
I.J. Hidalgo, in ref. 6, Models for Assessing
Drug Absorption and Metabolism (Borchardt, et
al., Eds.) Plenum Press, NY, 1996, p. 38.
7
Thermodynamics of Transport

• Transport of neutral species
• Ions transmembrane potentials
• Ionizable species proton gradients
• Metals and other titrants
• Macromolecular and cellular binding sites
• Coupled transport and ATP driven pumps
• Chemical conversions

8
Equilibrative Transport Compartment Model
??

-
pHo
pHi
?Gtransp
So
Si
SHo
SHi
?Gpump
KBo
KBi
S'i
SoBo
SiBi
9
Equations for Membrane Transport Thermodynamics
?Gtransp 2.303RT logSi/So nF?? ?Gpump
R 8.314 joules/mol?K 1.987 cal/mol?K F
96.5 Joules/mol-mV 23.06 cal/mol-mV ?
Si/So 10x _at_ 296?K (23C) 310K
(37C) ?(?G) 5.67 kJ/mol 5.936
kJ/mol 1.35 kcal/mol 1.41
kcal/mol ?(??) 58.5 mV 61.5 mV
pH pKa logS/SH
10
Examples Driving Force/Drug/Compartment

• Diffusion caffeine total body water
• Ion trapping Tc-Sestamibi heart mitochondria
• pH trapping quinidine renal excretion
• Binding warfarin plasma/liver ratio
• Active captopril GI absorption

11
Proposed Mechanisms of Tetracycline Uptake and
Efflux
12
Mechanisms of Transmembrane Drug Transport -
Example Drugs

• Paracellular diffusion - ions, mannitol, polymers
• Passive diffusion across lipid bilayer
• fluoroquinolones, tetracycline (hydrophobic)
• Diffusion through OM channels and porins
• B-lactams, tetracyclins (hydrophilic, charged)
• Facilitated diffusion
• imipenem, catechols, albomycin, albicin
• Active Transport
• aminoglycosides, cycloserine, phosphomycin,
  alaphosphin
• Vesicle Trafficking Mediated Transport
• polymers, peptide hormones, targeted delivery

13
Transcellular vs Paracellular Pathways
Transepithelial Resistance (O cm2) renal tubule
6-7 gallbladder 20-30 intestine
30-100 chroid plexus 80 colon
290-500 Caco-2
230-1000 Gastric mucosa gt1700 urinary bladder
gt2000
N-trimethyl chitosan chloride coadmin increased
permeation of nonopeptide buserelin in Caco-2
cells and enhanced bioavailability in rats from
0.8 to 6-13
14
Details of Tight Junction
EM of Caco-2
zona occluden zona adherens desmosome Ca, IP3,
PKC, CamK, MLCK
15
Apparatus for On-Line Fluorescence Measurement of
Transport in Epithelial Cell Cultures
MDCKII ? MDR1 ? SDZ PSC 833 Daunorubicin ?ex
480, ?em 590 FITC-dextran ?ex 480, ?em
525 Trans Epithelial Resistance (TER) 300 -
600 ??mm2
Ref. 8 Wielinga, et al., J. Pharm. Sci, 88(12),
1340, 1999.
16
Paracellular versus Transcellular Transport
Ref. 8 Wielinga, et al., J. Pharm. Sci, 88(12),
1340, 1999.
17
Paracellular Permeability Enhancers

• Examples Cachelators, bile salts, anionic
  surfactants, medium chain FAs, alkyl glycerols,
  cationic polymers, cytochalsin D, hormones,
  TNF-a, enterotoxins, zonula occludens toxin (V.
  cholerae)
• Substrates Ions, mannitol, ceftoxin, dextrans,
  proteins
• Advantages
• hydrophilic macromolecular substrates
• avoids intracellular degradation
• Disadvantages
• toxicity due high mM concentrations needed
• non-selectivity of substrate transport
• Concern systemic toxicity of lumenal contents,
  blood brain barrier effects (intended and/or not)

18
Pinocytosis, Endocytosis, and Receptor Mediated
Transcytosis

• Pinocytosis (cell sipping - non-mediated)
• non-specific, non-saturable, bulk fluid phase
  uptake, large particles, polymer-conjugates,
  obsolete term?
• Endocytosis (receptor-mediated uptake)
• specific, relevant to macromolecules, used to
  deliver small molecules as prodrugs, mediates
  clearance
• insulin, growth hormone, erythropoetin, G-CSR,
  ILs
• Transcytosis (receptor-mediated uptake and
  secretion on the translateral surface)
• useful for macromolecules and small molecule
  prodrugs, GI, BBB, and pulmonary epithelia.
• Protein translocation domain fusions (mechanism?)
• Antennapedia homeodomain, HIV TAT protein, R7

19
Transcytosis Delivery of Prodrug
vesicular transport
Brain
Blood
From Bickel Pardridge in Ref. 22, p. 30.
Transferrin receptor-mediated transcytosis of an
mAB-avidin-biotin-disulfide cross-linked
vasoactive intestinal peptide.
Endothelial Cell
TfR, VitB12R, FcRn, PigR are under commercial
development.
20
Passive Diffusion

• Characteristics of passive diffusion
• kin kout, net rate k(So - Si),
  non-selective
• Model Membranes (experimental systems)
• monolayers, bilayers, liposomes, BLMs, IAMs
• Membrane Models (functional/mathematical)
• structural, electrical, single/multiple barrier,
  partition adsorption/diffusive, unstirred layers
• Simulation of bilayers and transport
• molecular dynamics - diffusion within bilayer
• QSAR - structure/transport correlations

21
Molecular Dynamics Simulation of Membrane
Diffusion
From Bassolino-Klimas, Alper and Stouch, ref.
16. See also ref. 17. Snapshot from 10 nsec MD
simulation in 100 fs steps. Showed hopping
motions of 8 Å over ca 5 psec vs RMS motions of
1.5 Å. Motions differ in center and near
surface, both differ from bulk organic.
Rotational isomerizations (gauche/trans) gate
channels between voids. Differing motions
available to adamantane, nifedipine.
22
QSAR of Transport

• Hansch Equation
• log (1/C) -k(logP)2 k'(logP) ?? k"
• C dose or S for effect (ED50, IC50, rate)
• logP partition coef or ? lipophilicity factor
• ? Hammett electronic substituent effects
• k, k', k", ? regression coefficients
• Free-Wilson Model
• BA ?ajXj ?
• BA biological activity (e.g., log(1/c))
• aj substituent constant, Xj substituent
  presence, ? overall average activity

23
QSAR of TransportSelected from V.Austel E.
Kutter in Ref. 18.

• ABSORPTION - log (abs), log Perm, log k
• Barbiturates Gastric log PCHCL3/w
• Sulfonamides Gastric log Pisoam-OAC/w
• Anilines Gastric pKa
• Xanthines Intestine Distribution Coef
• C-glycosides Intestine log Po/w, Rm
• Excretion - log (excreted), log ClR, log k
• Penicillins Biliary logP, Rm
• Suflathiazoles Biliary logPo/w, pKa
• Sufapyridines Renal Rm, pKa
• Sulfonamides Renal ?, pKa
• Amphetamines Renal logPh/w

24
QSAR Conclusions

• Passive Diffusion is a function of
• Lipophilicity (logPo/w or CLOGP)
• GI (0.5-2.0), buccal (4-4.5), topical (gt2.0)
• Hydrogen bond donors/acceptors, polarity/charge
• Water solubility (measured or calculated)
• melting point, solvation energy, pH/buffers
• pKa - fraction of neutral species available
• mw - D ? 1/?mw mw lt 500 Da
• Confounding factors - inaccurate data,
  paracellular transport, mediated transport

25
http//www.simulations-plus.com/pdf_files/aaps_200
0_report.pdf Neural Net models trained on up to
1337 compounds.
26
Mediated Transport Facilitated Diffusion and
Active Transport

• Rates gt passive, solute specific, high Q10
• Non-symmetrical (kin ? kout at Si So)
• Saturable transport - Michaelis-Menten
• Inhibitable - competitive, non-competitive
• Regulated - inducibility repression
• Tissue specific- differential expression
• Energy dependent - active transport
• primary pumps - respiration, photosyn, ATPase
• secondary transporters (coupled to H, Na etc.)

27
Membrane Transporter Models Circa 1991
Transporter
Channel
Pore
28
Membrane Transporter Models Circa 2001
KscA
OmpA
GlpF
FepA
From http//blanco.biomol.uci.edu/mptopo/
29
Cell Culture and Molecular Biology Methods (I)

• Isolation of MXR genes (Ref. 25).
• Cells cultured from patients w/ resistant tumors.
• mitoxantrone uptake measured microscopically
• Cells grown under progressively selective
  conditions mitoxantrone, adriamycin, verapamil
• Isolation of differentially expressed mRNA as
  cDNA clones and cDNA sequencing.
• Northern analysis of mRNA expression levels.
• Southern analysis of gene copy amplification.
• Quantitative PCR analysis of expression levels in
  non-selected resistant cells.

30
Cell Culture and Molecular Biology Methods (II)

• Isolation of BCRP genes (Ref. 26-27)
• cells same as from Ref. 25
• cultured under selective conditions w/
  doxorubicin and verapamil
• RNA fingerprinting used to isolate cDNAs.
• transfection of non-selected cells confers
  resistance to mitoxantrone, doxorubicin,
  daunorubicin
• reduced uptake (dauno), enhance efflux (rhodamine
  123)
• Northern/Southern analysis of various cell lines

31
Cell Culture and Molecular Biology Methods (III)

• Isolation of MOAT-B,C,D (Ref. 28)
• cMOAT (cannicular multispecific organic anion
  transporter) MRP previously isolated
• MOAT-B isolated by PCR
• Homology search against EST datase suggests
  MOAT-C MOAT-D
• EST probe isolation of cDNA from human
• RNA blot analysis of tissue expression library
• chromosomal location by FISH

32
Cell Culture and Molecular Biology Methods (IV)

• Other Cloning Methods
• Expression cloning in oocytes
• Homologous hybridization
• Cloning by RT-PCR with degenerate primers
• Cloning by functional complementation

33
Cell Culture and Molecular Biology Methods (V)

• What have you got? MXR, BCRP, MDR, MRP, ABC
• Homology search against database - BLAST
• Sequence alignments and phylogenetic trees
• Hydropathy analysis and transmembrane topology
  predictions - Kyte-Dolittle
• ATP binding and other consensus motifs
• Homology modeling from known transporters
• Inferences about possible substrates/functions

34
Biochemistry and Biophysics

• Functional characterization
• Expression of Transport Activity in Vitro
• Substrate structure/activity profiles and
  co-substrates (GSH, ATP, H, Na), uncouplers
• Tissue distribution - EST database, RNA
  expression levels, antibodies, in situ methods
• Phenotypes in Knock Out Rodents
• Subcellular localization microscopy
• Isolation, purification, reconstitution
• Structural biology - EM, X-ray, NMR
• Mechanism of substrate transport and energy
  coupling - enzymology, inhibition, drug design

35
Structure of MsbA from E. coli A Homolog of the
Multidrug Resistance ATP Binding Cassette (ABC)
TransportersGeoffrey Chang and Christopher B.
Roth, Science Sep 7 2001 1793-1800.
36
Structure of bacterial oxalate transporter a
paradigm for the multifacilitator superfamily.T.
Hirai, et al. (Subramaniam lab, NIH), Nature
Structural Biology 9(8) 597-600. Low (6.5 ?)
resolution based on EM of 2D crystals.
37
Membrane Transporter Families

• ABC Superfamily
• ABC peptide transporter family
• P-glycoprotein (MDR) family
• MDR1a,1b,2,3 - organic cations, lipids (PC)
• MRP1,2,3 - organic anions, GSX conjugates
• cMOAT - canalicular multispecific organic anion
  transporter MRP2
• cBAT - canalicular bile acid transporter
• Porins Channels

• Major Facilitator Superfamily
• POT - proton coupled oligopeptide transporter
• NT - Na coupled nucleotide transporter
• NTCP - N coupled taurocholate protein
• OATP - polyspecific organic anion transport
  protein
• OAT-K1 - renal methotrexate transporter
• OCT - organic cation transporter - electrogenic
• RFC - reduced folate carrier
• sGSHT - glutathione conjugate transporter

38
(No Transcript)
39
Organic Cation Substrates (MDR OCT)
(From Zhang, Brett, Giacomini, Ref. 32)
40
Substrates of cMOAT(canalicular multispecific
organic anion transporter)Selected from Table IV
in Chap. 14 in ref 42a.

• glutathione disulfide
• leukotrienes (C4, D4, E4, N-acetyl-E4)
• glutathione conjugates (e.g., DNP,
  bromosulfophthalein, metals Sb, As, Bi, Cd, Cu,
  Ag, Zn)
• glucuronide conjugates (bilirubin, T3,
  p-nitrophenol, grepafloxacin)
• bile acid conjugates (glucuronides and sulfates)
• organic anions (folates, methotrexate,
  ampicillin, ceftiaxone, cefadozime,
  grepafloxacin, prevastatin, temocaprilate)

41
(No Transcript)
42
Drug Uptake by Endogenous Transporters in the
Small Intestine Lee, et al., Adv.Drug Delivery
Reviews, 2001. Table 1.

• Transporter
• Amino Acid
• Organic Anion
• Nucleoside
• Oligopeptide
• Monocarboxylic Acid
• Organic Cation

• Substrates
• L-DOPA, gabapentin
• Captopril, acyclovir
• Didanosine, idoxuridine
• ?-Lactam antibiotics
• Valproic acid, pravastatin
• Cimetidine, verapamil

43
Nucleotide Transporters of Mammalian Cells
(CNT1)
Cloned from kidney. Apically expressed. N1 SPNT
or CNT2 N2 CNT1
From C.E. Cass, in Ref. 31, Fig. 3, p.
413. es,ei sensitivity versus
nitrobenzylthioinosine
44
Nucleoside Drug TransportersAdapted fromC.E.
Cass (Tables 1-4) in Ref 31.

• Cladribine (Cl-dAdo) Leukemia es, ei, N1, N5
• Cytarabine (araC) Leukemia es, ei
• 2-Fludarabine (F-araA) Leukemia es, N1, N5
• Pentostatin (dCF) Leukemia es
• Floxidine (F-dUrd) Colon Cancer es, ei
• Didanosine (ddI) HIV es, NB
• Zalcitabine (ddC) HIV es, N2
• Zidovudine (AZT) HIV N2
• Acyclovir (ACV) HSV NB
• Gancyclovir (GCV) HSV es, NB
• Vidarabine (araA) HSV es, ei, N1
• Idoxuridine (IdUrd) HSV es
• Trifluridine (F3-dThd) HSV ND
• Ribavirin (RBV) RNA/DNA ND

45
Tissue Uptake and Intracellular Drug Transport
(subcellular PK)
mito doxo
AZT
NT
MXR
Place Holder - Figure TBN
MDR
RFC
OC
MTP
MTX
AT
MRP
VATP
PEPT
GSX
H
valcyclo
46
Exploiting Nutrient Transporters to Enhance Drug
Bioavailability

• Valacyclovir is an amino acid ester prodrug of
  the antiviral drug acyclovir.
• Oral biovailability (AUC) is increased in humans
  3-5x.
• Intestinal permeability in a rat perfusion model
  is increased 3-10x. Effect is specific (SAR),
  stereospecific (L), saturable, and inhibitable by
  PEPT1 subsrates (cephalexin, dipeptides), and by
  gly-acyclovir, val-AZT.
• Competitive with 3H-gly-sarc in CHO/hPEPT1 cells.
  
• Enhanced, saturable, inhibitable mucosal to
  serosal transport demonstrated in CACO-2 cells
  and accompanied by hydrolysis. Serosal to
  mucosal transport is passive.
• Rationale applied by Roche to design of
  valgancylcovir.
• XenoPort, Inc. working on gabapeptin-XP

47
Drug Interactions Drug Transport

• Digoxin - non-metabolized substrate for PgP
• Verapamil, amiodarone, and quinidine increase
  plasma levels, reduce renal and non-renal
  clearance, increase blood/brain barrier
  transport.
• Dose adjustment may be needed in 50 of cases.
• St. John's wort (Hypericum perforatum) decreased
  digoxin AUC by 25 after 10 days treatment
  through induction of PgP.
• HIV Protease Inhibitors
• Amprenavir clearance reduced by nelfinavir (-41)
  and by indinavir (-54), but not saquinavir.
• FDA warning against Hypericum supplements

48
Drug Resistance Reversal

• MDR1 (P-glycoprotein) drug efflux pump
• Multiple trials of multiple agents recent
  efforts at inhibiting transcription
• Steady state digoxin therapy was established in
  normal healthy volunteers (1 mg then 0.125
  mg/day). Initiation of valspodar (400 mg
  followed by 200 mg twice per day) caused
  immediate and progressive increases in digoxin
  AUC (211) and decreases in total body, renal,
  and non-renal clearance (-67, -73, -58) after
  5 days.
• BCRP (breast cancer resistance protein or ABCG2)
• Inhibited by fungal toxin fumitremorgin C, but
  neurotoxic side effects
• Kol143 and other derived analogs developed
  inhibit BCRP, but not PgP or MRP
• Non-toxic in mice, increased oral availability of
  topotecan in mice
• RFC (reduced folate carrier) - antifolate drugs
  (methotrexate)
• Resistant leukemia cell lines were selected by
  stepwise doses
• Cross resistance (gt2000x) to five novel
  hydrophilic antifolates shown
• Intracellular folate levels reduced, increased
  requirement 42x
• Hypersensitive to hydrophobic antifolates
• Mutations clustered in exons 2 and 3, TMD1

49
Microbial Drug Transport and Resistance
Mechanisms

• Mechanisms of Drug Uptake in Bacteria
• OM porins, periplasmic binders, and IM pumps
• B-lactam channels - imipenem resistance
• nutrient uptake transporters - amino-glycosides
• siderophore uptake is a drug delivery target
• Mechanisms of Drug Efflux in Bacteria
• Major facilitatory (MF) family
• RND family (AcrAB, EmrAB, TolC)
• SMR (small multidrugresistance pumps)
• ABC (ATP binding cassette) family

50
Structures of MDR substrates
(From K. Lewis, Ref. 47).
51
Topology Models of Microbial Multidrug
Resistance Pumps
From K. Lewis, Ref. 47, Fig. 2.
52
Pharmacogenetics of Transport (I)

• P-glycoprotein
• Polymorphism in cell cultures
• Gly185Val - selected for colchicine sensitivity
• Ser893Ala - naturally occuring polymorphism
• RFLP predicts ivermectin neurotoxicity sensitive
  P-gp-deficient mice.
• Polymorphisms in 24 normal volunteers
• Function effects on digoxin uptake measured
• 7 intron, 3 wobble, 3' 5' non-coding, 3 a.a.
  changes C/T 3435 lowers 2-fold
• Polymorphisms now known in Pgp, MRP1 and MRP2

53
Pharmacogenetics of Transport (II)

• OATP-C (organic anion transporting polypeptide-C)
• liver specific uptake transporter
• multiple SNPs detected, including 14
  non-synonymous, gene frequency depends on race
• 16 assessed in vitro, 8 result in reduced
  transport, esp. T521C (val174ala) occurs in 14
  European- and G146C (gly488ala) in 9 of
  African-Americans
• OCT1 (organic cation transporter 1)
  electrogenic uptake, drugs neurotransmitters
• 25 variations identified in 57 Caucasian samples
• 3 (arg61cys, cys88arg, gly401ser) reduce
  transport and occur frequently (9/1, 0.6, 22)

54
Pharmacogenetics of Transport (III)
Pharmacogenetics Network - UCSF Project

• http//pharmacogenetics.ucsf.edu
• OCT2 Transporter - renal tubule basolateral
• Adverse Effects - procainamide, clonidine
• Chromosome locus 6q26
• Aliases
• - Organic Cation Transporter 2
• - Solute Carrier Family 22, Member 2
• - SLC22A2
• Links to NCBI Data
• OMIM On-line Mendelian Inheritance in Man
• LocusLink Data
• Reference Sequence
• Homo Sapiens mRNA for OCT2 from kidney.
• Gene Structure Introns/Exons
• Transmembrane Topology Prediction

• Variants occur with frequency of 15
• Coding regions and Exon/Intron boundaries
• For 247 DNA samples from Coriell Institute
• SNPs found at
• Synonomous 130, 223, 297, 401, 466, 502, 529
• Non-Syn 54, 161, 165 (2), 270 (2), 400, 432
• Cellular phenotyping Data to be gathered.
• Clinical studies Data to be gathered

                                             
                     
  1   2   3   4   5   6   7   8   9   10   11  
55
Pharmacogenomics of Transport (I)

• Classification by mechanism, origin, topology,
  domain structure, energetics, energy source,
  substrate specificity, sequences, 3D structures
• BLAST (Basic Local Alignment Search Tool)
• INCA (Integrative Neighborhood Cluster Analysis)
• T.C. W.X.Y.Z (Saier et al)., e.g., MDR1
  3.A.1.201
• W type and energy source (3 primary
  transporter)
• Z transporter family or superfamily(3.A P-P
  cleavage)
• Y transporter subfamily (3.A.1 ABC family)
• Z substrate transported (3.A.1.201 multiple
  drugs)
• http//www.biology.ucsd.edu/msaier/transport/titl
  epage.html

56
Pharmacogenomics of Transport (II)
http//www.biology.ucsd.edu/ipaulsen/transport/fi
ndings.html
57
Pharmacogenomics of Transport (III)

• Database of Bacterial Transporters (Saier
  Paulsen)
• http//www.biology.ucsd.edu/ipaulsen/transport/in
  dex2.html
• TIGR (The Institute for Genome Research
• EGAD (Expressed Gene Anatomy Database)
• http//www.tigr.org/docs/tigr-scripts/egad
• Human Membrane Transporter Database
• Sadee et al (AAPS Pharm Sci)
• search by transporter, family, tissues/organ,
  substrates/drugs
• http//lab.digibench.net/transporter/
• Human SNPs Consortium - search transporter gt 58
  hits 12/28/01
• http//snp.cshl.org/
  175 hits 12/31/02
• Other databases http//wwwncbi.nlm.nih.gov/

58
Membrane Transporter Families

• H/Dipeptide Symporters
• Proton-dept oligopeptide transporters (POT)
• 70 cloned from nature 2 from humans (PEPT1 and
  PEPT2)
• Comparisons between worm, rat, mouse sequences
  identified two additional human POTs (hPHT1 and
  hPHT2) related to cloned rodent histidine/peptide
  transporters.
• Facilitative Glucose Transporter Family
• Sodium/Glucose and Sodium/Nucleoside
• Amino Acid Transporters
• Sodium Neurotransmitter Symporters
• ABC Transporters

59
Pharmacogenomics of Transport (IV)Expression
Patterns using MicroArray Chips
In vivo permeabilities measured in human duodenum
using perfusion methods. In vitro permeabilities
measured using Caco-2 cells. Expression patterns
of 12,599 gene sequences analyzed using GeneChip
(including 443 expected ADME genes). Sun, et
al., 2002.

• Results Functional Genomics
• 37-47 of genes (26-44 of ADME genes) expressed
  in both cell types, but gt1,000 sequences showed
  gt5x variation between cell types. Variation gt3x
  for gt70 transporters detected.
• In vivo/in vitro permeability correlated well (R2
  85) for passively absorbed drugs.
• Variations (3-35x) above expected passive values
  were observed for mediated absorption and
  correlated with differences (2-595x) in gene
  expression.
• Interhuman variability (3-294 of mean) for 31
  of genes.