HLTH 340

Lecture A2

Toxicokinetic processes:

absorption (part-1)

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Basic Steps in Toxicological Analysis W2013 HLTH 340 Lecture A2 2

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 - systemic effect (throughout the body) “first-pass effects” 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 W2013 HLTH 340 Lecture A2 3

Toxicokinetic (ADME) processes W2013 HLTH 340 Lecture A2 4

Toxicokinetic and toxicodynamic pathways jointly affect toxicity W2013 HLTH 340 Lecture A2 5

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) W2013 HLTH 340 Lecture A2 6

Routes of Absorption, Distribution and Excretion

absorption

first-pass effect

distribution

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excretion

7

First-pass extraction:

the hepatic portal vein carries absorbed nutrients and xenobiotics to the liver W2013 HLTH 340 Lecture A2 8

Absorption of molecules across external and internal membrane barriers

passive diffusion receptor-mediated transport (non-selective) (selective) transcellular paracellular

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Types of membrane transport mechanisms: active transport and passive transport

external dose (site of absorption) internal dose (blood)

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external dose (site of absorption)

10

Intestinal absorption via passive diffusion using

paracellular

and

transcellular

permeation pathways W2013 HLTH 340 Lecture A2

intercellular tight junction (can be open, closed, or

‘

leaky

’ 11

Paracellular

permeation through a membrane barrier occurs between adjacent cell membranes

apical (outside)

The characteristics of the paracellular pathway are defined by specific junctional complexes that span the intercellular space. There are four types of complexes: (1) (2) (3) (4) zona occludens, or tight junction; zona adherens, or intermediate junction; desmosomes; and 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.

baso-lateral (inside)

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The tight junction (TJ) barrier structure forms pore structures between adjacent cell membranes The TJ barrier consists of two components — physiological pores and pathological breaks.

claudins

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 porosity is unclear, but it is known that preferences for ionic charges is controlled by claudins. The claudins form the pore structure or influence their size and shape. Each claudin has a characteristic influence on the permeability for small cations 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.

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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 permeability

[C external ] >> [C internal ] 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 –

hydrophobicity

– – tendency 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 W2013 HLTH 340 Lecture A2 14

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.

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Molecular dynamics computer simulation of membrane diffusion during xenobiotic absorption W2013 HLTH 340 Lecture A2 16

• • • •

Lipophilic

and

hydrophilic

solubility lipid solubility affects

transcellular passive diffusion

biomembranes through the phospholipid 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

negatively charged phosphate groups PO 4 - positively charged choline groups N-[CH 3 ] 4 + charge distribution 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 - [polarity H-bonding + ionic interactions] – calculated rate of absorption = 1/size (MW) x 1/lipophilicity (log K o/w ) W2013 HLTH 340 Lecture A2 17

• • • • Partition coefficient is a quantitative measure of the degree of

lipophilicity

of a given molecule partition coefficient

(K p , K o/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 (H 2 O) ‘oil and water don’t mix’

K o/w

– = conc (octanol) / conc (water) K o/w > 1 is

lipophilic

K o/w <1 is

hydrophilic

K o/w = 0 - 1 is

amphiphilic (mixed) log K o/w

– often expressed in log example: K o/w = 1000 --> log K o/w 10 units = 3 (strongly lipophilic) W2013 HLTH 340 Lecture A2 18

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

Poor transcellular absorption and membrane permeation is more likely when:

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 K o/w is over

5

there are more than

10

H-bond acceptors in the molecular structure

(mainly N and O containing polar groups) W2013 HLTH 340 Lecture A2 19

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

o/w

k

o/w

W2013 k

o/w

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Effect of partition coefficient on absorption rate

strongly lipophilic extremely lipophilic

4 - 5 3 2 1

moderately lipophilic

log K p < 0 substances are poorly absorbed due to ionic interactions or H-bonding

hydrophilic

< 0 0 - 0.9

mixed or amphiphilic

log K p > 5 substances are poorly absorbed due to membrane trapping or lack of water solubility

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Absorption of large or non-permeable xenobiotic molecules can occur via cellular endocytosis W2013 HLTH 340 Lecture A2 22

Absorption into brain of manganese (Mn 2+ ) ions via active transport channels and cellular endocytosis

TMI slide (illustrative purposes only)

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