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PASSAGE OF PASSAGE OF XENOBIOTICS ACROSS XENOBIOTICS ACROSS
BIOLOGICAL BIOLOGICAL MEMBRANESMEMBRANES
I.I. PHYSICOCHEMICAL PHYSICOCHEMICAL DETERMINANTS OF THE DETERMINANTS OF THE PASSAGE OF XENOBIOTICS PASSAGE OF XENOBIOTICS ACROSS BIOLOGICAL ACROSS BIOLOGICAL MEMBRANESMEMBRANES
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A. Membrane CharacteristicsA. Membrane Characteristics
1. Membrane Composition
Membrane Type Phospholipid ProteinGeneral 40% 60%
Inner mitochondrial 20-25% 75-80%
Myelin 75% 25%
phospholipid{
polar headnon-polar tail
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A. Membrane CharacteristicsA. Membrane Characteristics 1. Membrane 1. Membrane CompositionComposition2. Membrane Structure2. Membrane Structure
Fluid-mosaic model of Singer and Nicholson (Science 175:720-731, 1972
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A. Membrane CharacteristicsA. Membrane CharacteristicsB. Drug CharacteristicsB. Drug Characteristics
•molecular weight, shape, sizemolecular weight, shape, sizeTissue Estimated Pore Radius
jejunem 7.5 A
ileum 3.5A
•lipid solubilitylipid solubility•ionizationionization
•solubility in unstirred layer around cellsolubility in unstirred layer around cell
cell
unstirredlayer
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II. II. MECHANISMS OF BIOTRANSPORTMECHANISMS OF BIOTRANSPORTBiotransportBiotransport
The translocation of a solute from one The translocation of a solute from one side of a biological barrier to the other side of a biological barrier to the other side in the intact form.side in the intact form.
A. Passive DiffusionA. Passive Diffusion
semi-permeable membrane
External Internal
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Fick’s Law of DiffusionFick’s Law of Diffusion
21 CCh
DAK
dt
dQ p
dQ/dt - rate of diffusion D - diffusion coefficient
A - surface area of membrane Kp - partition coefficienth - membrane thickness
C1 - C2 = concentration difference for solute
Generally, C1>>C2
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ABSORPTION FROM RAT ABSORPTION FROM RAT STOMACH STOMACH AND SMALL AND SMALL
INTESTINEINTESTINE % absorbed in 1 hr % absorbed in 10 min
Drug from stomach from small intestine
phenobarbital 17 52pentobarbital 24 55promethazine 0 38ehtanol 38 64
Data from: Magnussen MP. Acta Pharmacol Toxicol 26:130, 1968.
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Increase in Surface Surface AreaStructure (relative to cylinder) sq cm
simplecylinder
1 3,300
Folds ofKerckring
3 10,000
Villi 30 100,000
Microvilli 600 2,000,000
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COMPARISON OF BARBITURATE COMPARISON OF BARBITURATE ABSORPTION FROM RAT COLONABSORPTION FROM RAT COLON
Barbiturate Kp % Absorbed
barbitalphenobarbitalpentobarbitalsecobarbital
0.74.82851
12203040
Data from: Schanker LS. J Pharmacol Exp Ther 123:81, 1958.
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EFFECT OF pH ON INTESTINAL ABSORPTION IN EFFECT OF pH ON INTESTINAL ABSORPTION IN THE ISOLATED RAT SMALL INTESTINETHE ISOLATED RAT SMALL INTESTINE
Acids 5-nitrosalicylic salicylic acetylsalicylic benzoic
pKa
2.33.03.54.2
% absorbed at
pH 4
40 64 41 62
pH 5
27 35 27 36
pH 7
0 30 --- 35
pH 8
0 10 --- 5
Bases aniline 4.6 40 48 58 61 aminopyrine 5.0 21 35 48 52 quinine 8.4 9 11 41 54
Data from: Schanker LS, J Pharmacol Exp Ther 123:81, 1958.
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21 CCh
DAK
dt
dQ p
Since D, Kp, and h are constant for a given drug/membrane; and given that C1>>C2:
1PCdt
dQ
Where P - permeability constant
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II. II. MECHANISMS OF BIOTRANSPORTMECHANISMS OF BIOTRANSPORT
A. Passive DiffusionA. Passive Diffusion
B. Carrier-Mediated BiotransportB. Carrier-Mediated Biotransport
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CHARACTERISTICS OF CARRIER-CHARACTERISTICS OF CARRIER-MEDIATED TRANSPORTMEDIATED TRANSPORT
Facilitated Diffusion Active Transport
Utilization of energy no yes
Movement against a concentration gradient no yes
Exhibits saturation yes yes
Example substances riboflavin, Vit B12 5-flurouracil
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xenobiotic
transport
protein
out
in
out
in
Proposed Model for Carrier-Mediated TransportProposed Model for Carrier-Mediated Transport
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0
10
20
30
40
50
60
70
% Dose excreted
5 10 30
Dose (mg)
Effect of Food on the Absorption of Riboflavin
Control
+ Food
Data from: Levy G, Jusko WJ. J Pharm Sci 55:285-289, 1966.
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Membrane Transporters and Their Membrane Transporters and Their SubstratesSubstrates
TransporterTransporter SubstratesSubstratesAmino acid transportersAmino acid transporters baclofen, cyclosporin, L-dopa, baclofen, cyclosporin, L-dopa,
gabapentin, methyldopagabapentin, methyldopa
Peptide transportersPeptide transporters -lactam antibiotics, ACE -lactam antibiotics, ACE inhibitors, inhibitors, (hPEPT1, HPT1)(hPEPT1, HPT1) cephalexin, cyclosporin, cephalexin, cyclosporin, methyldopamethyldopa
Nucleoside transportersNucleoside transporters zidovudine, zalcitabine, zidovudine, zalcitabine, dipyridamoledipyridamole (CNT1, CNT2)(CNT1, CNT2)
Organic anion transportersOrganic anion transporters ceftriaxone, benzoic acid, ceftriaxone, benzoic acid, methotrexatemethotrexate (OATP1, OATP3, OATP8)(OATP1, OATP3, OATP8) pravastatin pravastatin
Organic cation transportersOrganic cation transporters thiamine, desipramine, thiamine, desipramine, quinidine, quinidine, (OCT1,OCT2)(OCT1,OCT2) midazolam, verapamil midazolam, verapamil
Bile acid transportersBile acid transporters chlorambucil, thyroxinechlorambucil, thyroxine (IBAT/ISBT)(IBAT/ISBT)
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II. II. MECHANISMS OF BIOTRANSPORTMECHANISMS OF BIOTRANSPORT
A. Passive DiffusionA. Passive DiffusionB. Carrier-Mediated BiotransportB. Carrier-Mediated Biotransport
C. Cellular EffluxC. Cellular Efflux
Key ABC Efflux TransportersKey ABC Efflux Transporters
P-glycoprotein: P-glycoprotein: MDR1 (ABCB1)MDR1 (ABCB1)Multidrug Resistance Protein: Multidrug Resistance Protein: MRP1 MRP1 (ABCC1)(ABCC1)Breast Cancer Resistance Protein: Breast Cancer Resistance Protein: BCRP BCRP (ABCG2)(ABCG2)
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Plasma levels of saquinavir versus time after oral administration in Plasma levels of saquinavir versus time after oral administration in wild type (open circles) and Mdr1awild type (open circles) and Mdr1a-/--/-/1b/1b-/--/- mice. mice. From: Huisman MT, et al. P-From: Huisman MT, et al. P-glycoprotein limits oral availability, brain and fetal penetration of saquinavir even with glycoprotein limits oral availability, brain and fetal penetration of saquinavir even with high doses of ritonavir. high doses of ritonavir. Mol PharmacolMol Pharmacol 59:806-813, 2001 59:806-813, 2001
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Consequence of the Efflux Transporter P-glycoproteinConsequence of the Efflux Transporter P-glycoprotein
1) Limited drug absorption
enterocytepgp
Gut lumen
2) Enhanced drug elimination2) Enhanced drug eliminationProximal tubule cells
Tubule lumen
hepatocytes
bile3) Limited distribution
Endothelial cells
capillary
Brain or testessyncytiotrophoblast
Maternal blood
lymphocyte
Adapted from: Fromm MF. Trends in Pharmacol Sci 25:423, 2004
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From: http://bigfoot.med.unc.edu/watkinsLab/website/hEnt.htm
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From: Hunter J, Hirst BH. Intestinal secretion of drugs. The role of P-glycoprotein and related drug efflux systems in limiting oral drug absorption. Advanced Drug Delivery Reviews 25:129-157, 1997.