Permeability김소연

2013.11.08

Permeability Overview

Permeability Fundamentals

Permeability Effect

Permeability Structure Modification Strate-gies

Problem

Contents

Permeability is the velocity of molecule passage through a mem-brane barrier.

Permeability is a determinant of intestinal absorption and oral bioavailability.

Optimizing passive diffusion is productive because it is the pre-dominant mechanism for absorption of most commercial drugs.

Permeability is increased by removing ionizable groups, increasing Log P, and decreasing size and polarity.

Peameability Overview

Different membranes can have different permeabilities for a com-pound.

Permeability Fundamen-tals

Passive diffusion is driven by a concentration gradient (High->Low Concentration)

It has been estimated that 95% of commercial drugs are predominantly ab-sorbed in the GI tract by passive diffusion

It is important to remember that permeability is much higher for more liphophilic molecules than for polar molecule

pH and pKa play important roles in passive diffusions.

Passive Diffusion

Luminal Cyto-plasm Abluminal

lipid bilayer Mem-brane

Passive Diffusion

The passive diffusion of acids is much higher at low pH.

The passive diffusion of bases is low at low pH.

Passive Diffusion

For an acid passive diffusion is enhanced in the direction of the higher pH because of the ionization equilibrium.

On the other hand, base passive diffusion is enhanced in the direction of the lower pH.

Passive Diffusion

Compounds may be engulfed by the membrane, pass through the cell within the vesicle, and be released on the other side.

Minor interest for small molecule drug discovery.

Endocytosis

Compound binds to a transmembrane protein and moves through the membrane.

Active transport requires the expenditure of energy, com-monly two ATPs for each molecule transported.

Active uptake

GI tract or other organs are sometimes termed leaky because of the somewhat loose junctions between the cells that allow molecules to slip between.

Paracellular permeability in the GI tract is available primarily to com-pounds that have a molecular weight less than 180 Da and are polar.

Palacellular

Efflux, the active transport of compounds from inside the cell or membrane back into the lumenal space.

Pgp(p-glycoprotein) BCRP (breast cancer resistance protein)

Efflux

GI track – Passive diffusion – Absorption BBB – Active transport, efflux

Combined Permeability

Concentration gradient

pH polarity

Expression of transporter

Transporter Km

Affinity of the compound for various transporters

The size of the pores between membrane barrier

Factors that affect perme-ability

Permeability Effect

Living Sys-tem

Bioavailabil-ity

In vitro dis-covery

Cell based Activity as-

say

Compounds with low permeability typically have low bioavailabil-ity

In this case, passive diffusion is limited for the highly charged acidic compound (pKa= 4.5).

Bioavailability

Both Good enzyme activity and permeability were required in order to produce good bioactivity in the cell based assay.

Cell based Activity assay

Cell based Activity assay

Modification StrategiesPermeability Structure Modification Strate-

giesIonizable group to non-ionizable group

Add lipophilicity

Isosteric replacement of polar groups

Esterify carboxylic acid

Reduce hydrogen bonding and polarity

Reduce size

Add nonpolar side chain

Prodrug

Ionizable group to non-ionizable group

Modification Strategies

Add lipophilicity

Modification Strategies

Isosteric replacement of polar groups

Modification Strategies

Esterify carboxylic acid

Modification Strategies

Reduce hydrogen bonding and polarity

Modification Strategies

Reduce size

Modification Strategies

Add nonpolar side chain

Modification Strategies

Prodrug

Modification Strategies

1. What is the predominant permeability mechanism for absorption of most com-mercial drugs?

2. What are the structural properties of compounds that undergo paracellular permeation?

3. How will passive diffusion permeability change as pH increases from 4.5 to 8 for: (a) basic compound, (b) acidic compound?

4. List important permeability barriers for drug discovery.

5. Which of the following structural modifications likely will improve permeability?: (a) change an amine to a methyl, (b) add a hydroxyl group, (c) remove a propyl group, (d) change a carboxylic acid to an ethyl ester, (e) change a carboxylic acid

to a tetrazole.

6. For the following lead compared, what structural modifications could you make that might improve permeability?

Problem

7. Permeability is important for which of the following?: (a) absorption in intestine, (b) CYP metabolism, (c) BBB penetration, (d) dissolution in the intestinal lumen, (e) in vitro cell-based assay, (f) to reach intracellular targets in vivo.

8. Following are groups that could be added to a lead compound that is MW 300 and has ClogP 2.0. Rank them from lowest to highest predicted permeability of the prod-uct: (a) –CH3, (b) –OH, (c) –OCH3, (d) –COOH.

9. Following are groups that could be added to a lead compound that is MW 450 and has ClogP 4.5. Rank them from lowest to highest predicted permeability of the prod-uct: (a) –C6H5, (b) –CH3, (c) –C3H7.

10. Following are groups that could be added to a lead compound that is MW 250 and has ClogP 0.0. Rank them from lowest to highest permeability of the product: (a) –CH3, (b) –C6H11, (c) –C3H7

Problem

Permeability Method Overview

In Silico Permeability Method

In Vitro Permeability Method

In Depth Peameability Method

Contents

In silico methods are available for calculating in vivo and in vitro permeability, such as intestinal absorption, Caco-2, and parallel artificial membrane permeability assay (PAMPA).

High-throughput permeability methods utilize high-performance liquid chromatography (immobilized artificial membrane), artificial membranes (PAMPA), and cell layers(Caco-2).

In-depth study of permeability uses portal vein cannulation and in situ perfusion.

Permeability Method Overview

In Silico Permeability Method

IAM HPLC

Test compounds partition between the aqueous mobile phase and the phospholipid phase.

Compounds are rank ordered by k, which indicates a higher lipophilicity or phospholipids affinity. The parameters of this affin-ity correlate with permeation.

In Vitro Permeability Method

Cell layer Permeability assay

In Vitro Permeability Method

Caco-2

In Vitro Permeability Method

Artificial Membrane Permeability assay

In Vitro Permeability Method

• PAMPA reduces the cost and increases the throughput of permeabil-ity assays.

• Instead of a barrier made of living cells, the PAMPA barrier is made of phospholipids solubilized in along-chain hydrocarbon.

Comparisn of Caco-2 and PAMPA Method

In Vitro Permeability Method

• Comparison of Caco-2 and PAMPA permeability values from the same compound can provide in-sight on permeability mechanisms.

In Depth Permeability Methods