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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.