HLTH 340 Lecture A2W2013 1 HLTH 340 Lecture A2 Toxicokinetic processes: absorption (part-1) NOTICE:...

HLTH 340 Lecture A2W2013 1

HLTH 340

Lecture A2

Toxicokinetic processes:absorption (part-1)

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Basic Steps in Toxicological Analysis


Toxicokinetic processes- also termed pharmacokinetics, ADME, disposition

• toxicokinetics describes the movement of xenobiotic substances into and within the organism subsequent to an environmental exposure

– descriptive (semi-quantitative) analysis– quantitative analysis (mathematical formulas and graphs)– computer-based simulations (PB-PK models = physiologically-based pharmacokinetic models)

• Absorption controls entry of xenobiotics through the external membrane barriers into the blood (or lymphatic) circulation

– local effect (tissues near site of absorption)– regional effect (tissues downstream from site of absorption -- “first-pass effects”– systemic effect (throughout the body)

• Distribution determines the movement of xenobiotic molecules with the circuatory fluids and specific organs and tissues

• Metabolism (biotransformation) describes the biochemical processes that convert the original (parent) xenobiotic to various metabolic products (metabolites)

• Excretion controls the removal of the xenobiotic or its metabolites from the body


Toxicokinetic (ADME) processes


Toxicokinetic and toxicodynamic pathways jointly affect toxicity


Route of exposure Route of exposure

• The ROUTE (site) of exposure is an important determinant of the ultimate DOSE – different routes may result in different rates of absorption.

– Dermal (skin)

– Inhalation (lung)

– Oral (GI)

– Injection

• The ROUTE of exposure may be important if there are tissue-specific toxic responses.

• Toxic effects may be local (in a specific tissue) or systemic (throughout the organism)


Routes of Absorption, Distribution and Excretion

absorption

distribution

excretion

first-pass

effect


First-pass extraction: the hepatic portal vein carries absorbed nutrients and xenobiotics to the liver


Absorption of molecules across external and internal membrane barriers

transcellular

paracellular

passive diffusion

(non-selective)receptor-mediated transport

(selective)


Types of membrane transport mechanisms:active transport and passive transport

internal dose

(blood)

external dose

(site of absorption)

external dose

(site of absorption)


Intestinal absorption via passive diffusion using paracellular and transcellular permeation pathways

intercellular tight junction(can be open, closed, or ‘leaky’


Paracellular permeation through a membrane barrier occurs between adjacent cell membranes

The characteristics of the paracellular pathway are defined by specific junctional complexes that span the intercellular space. There are four types of complexes:

(1) zona occludens, or tight junction;(2) zona adherens, or intermediate junction;(3) desmosomes; and(4) gap junctions.

Specific proteins localized to each complex link adjacent cells and the cytoskeleton. Original models of the paracellular pathway as a static barrier are being replaced by a more dynamic model in which the junctional complexes are involved in signaling and regulation, most likely through protein phosphorylation or dephosphorylation.

The tight junction is the most apical complex and is believed to control permeability across the paracellular pathway through a series of strands and grooves. Molecular definition of the specific components of the tight junction ( eg , Z0-1, Z0-2, occludin, cingulin) may permit a clearer understanding of how the tight junction functions as a barrier for ions and macromolecules.

apical (outside)

baso-lateral (inside)


The tight junction (TJ) barrier structure forms pore structures between adjacent cell membranes

The TJ barrier consists of two components —physiological pores and pathologicalbreaks.

All epithelial TJs have a system of small approximately 8-angstrom pores that varies among cell types in ionic charge selectivity and in porosity, i.e. the apparent number of pores.

The mechanism controlling overall porosityis unclear, but it is known that preferences for ionic

charges is controlled by claudins.

The claudins form the pore structureor influence their size and shape. Each claudin has a characteristicinfluence on the permeability for smallcations and anions.

The passage of material larger than approximately 8-angstroms shows no charge selectivity. This small pathway may represent a pathological break between cells. Such disruptions can arise in response to proinflammatory factors like interferon-gamma and tumor necrosis factor-alpha.

claudins


Transcellular passive diffusion is the commonest type of absorption across membrane barriers

• passive diffusion - a process that requires no molecular transport system or energy source (random migration by individual solute molecules)

– passive diffusion cannot concentrate substances across membrane barrier (no pumping action)– bidirectional -- flow of molecules will follow the concentration gradient in either direction (in or out of tissue)

• absorption rate for passive diffusion is determined by 3 major factors– surface area through which diffusion is occurring (membrane lining of gut, lung, and skin)– concentration gradient [Cexternal] >> [Cinternal]– permeability of the substance through the membrane barrier

• permeability is typically determined by each substance’s physicochemical properties– molecular weight

• smaller molecules (MW < 500 daltons) are often able to migrate through biomembranes by passive diffusion• over 80% of effective drugs have a MW < 450 daltons

– hydrophobicitytendency of a substance to dissolve preferentially in fatty or oily biological media, but not in water

– ionization• molecules that carry positively or negatively charged functional groups have ionic properties• charged ionic groups experience electrostatic interactions with ionic phospholipid membrane groups

– polarity (hydrogen bonding)molecules with uneven electrical charge distribution (polar compounds) form H-bonds with water


Lipid sieve model of cell membrane

The ‘lipid sieve’ model helps to explain how small molecules that are lipophilic can permeate through the cellular phospholipid membrane by passive diffusion

hydrophilic molecules cannot permeate the membrane unless there is a specific paracellular transport channel or membrane-associated active transport pump.


Molecular dynamics computer simulation of membrane diffusion during xenobiotic absorption


Lipophilic and hydrophilic solubility• lipid solubility affects transcellular passive diffusion through the phospholipid

biomembranes

• hydrophilic (water soluble)– ionic molecules carry one or more positive or negative charges– polar molecules carry partial positive or negative charges– phospholipid molecules on the membrane surface contain a zwitterionic charge distribution

negatively charged phosphate groups PO4--

positively charged choline groups N-[CH3]4+

– charged phospholipid groups will repel or bind ionic hydrophiles via electrostatic interactions– charged phospholipid groups will form hydrogen bonds (H-bonds) with uncharged hydrophiles that have

polar functional groups (esters, amides, etc.)– most hydrophiles cannot pass across membranes by transcelluar passive diffusion

• lipophilic (fat and oil soluble)– electrically neutral molecules with no positive or negative charges– no electrostatic repulsion or H-bond attraction at the membrane surface– readily penetrate into and through the the non-polar interior of biomembranes– many small lipophiles can pass through biomembranes by transcellular passive diffusion– usually small lipophiles can be more readily absorbed than most small hydrophiles

• lipophilicity factors are used to predict passive absorption of drugs and xenobiotics– lipophilicity = hydrophobicity - [polarityH-bonding + ionic interactions]– calculated rate of absorption = 1/size (MW) x 1/lipophilicity (log Ko/w)


Partition coefficient is a quantitative measure of the degree of lipophilicity of a given molecule

• partition coefficient (Kp, Ko/w)– measures relative degree of solubility in lipid (lipophilicity) and water (hydrophilicity)

• measure concentration of xenobiotic in 2-phase solvent mixture– oily non-aqueous phase solvent (octanol) and watery aqueous phase (H2O)

– ‘oil and water don’t mix’

• Ko/w = conc (octanol) / conc (water)– Ko/w > 1 is lipophilic Ko/w<1 is hydrophilic Ko/w = 0 - 1 is amphiphilic (mixed)

• log Ko/w often expressed in log10 units– example: Ko/w = 1000 --> log Ko/w = 3 (strongly lipophilic)


Lipinski’s ‘rule of five’ for predicting xenobiotic absorption by transcellular passive diffusion

• there are more than 5 H-bond donors in the molecular structure (mainly OH and NH groups)

• the molecular weight is over 500

• the molecule’s log Ko/w is over 5

• there are more than 10 H-bond acceptors in the molecular structure (mainly N and O containing polar groups)

Poor transcellular absorption and membrane permeation is more likely when:


Effect of lipophilicity on the absorption rate of 3 related xenobiotic substances (barbiturate drugs)

ko/w

ko/w

ko/w


Effect of partition coefficient on absorption rate

1

2

3

0 - 0.9

< 0

4 - 5

hydrophilic

mixed oramphiphilic

moderately lipophilic

extremely lipophilic

strongly lipophilic

log Kp > 5 substances are poorly absorbed due to membrane trapping

or lack of water solubilitylog Kp < 0 substances are poorly absorbed due to

ionic interactions or H-bonding


Absorption of large or non-permeable xenobiotic molecules can occur via cellular endocytosis


Absorption into brain of manganese (Mn2+) ions via active transport channels and cellular endocytosis

TMI slide (illustrative purposes only)

HLTH 340 Lecture A2W2013 1 HLTH 340 Lecture A2 Toxicokinetic processes: absorption (part-1) NOTICE:...

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