HLTH 340
Lecture A2
Toxicokinetic processes:
absorption (part-1)
NOTICE: These materials are subject to Canadian copyright and are presented here as images published in journals and books for which the University of Waterloo
holds a licensed electronic subscription. These materials are provided to HLTH 340 students for their exclusive use though a non-public courseware system (UWLEARN) and the images are restricted to the use of HLTH 340 students. Reproduction, transmittal, copying, or posting of these images by students in any form,
electronic or physical, is strictly prohibited.
W2013
HLTH 340 Lecture A2
1
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 -- “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
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
excretion
W2013
HLTH 340 Lecture A2
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
(selective)
(non-selective)
transcellular
paracellular
W2013
HLTH 340 Lecture A2
9
Types of membrane transport mechanisms:
active transport and passive transport
external dose
(site of
absorption)
W2013
internal
dose
(blood)
HLTH 340 Lecture A2
external dose
(site of
absorption)
10
Intestinal absorption via passive diffusion using
paracellular and transcellular permeation pathways
intercellular tight junction
(can be open, closed, or ‘leaky’
W2013
HLTH 340 Lecture A2
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.
W2013
HLTH 340 Lecture A2
baso-lateral
(inside)
12
The tight junction (TJ) barrier structure forms pore
structures between adjacent cell membranes
claudins
The TJ barrier consists of two components —
physiological pores and pathological
breaks.
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 factoralpha.
W2013
HLTH 340 Lecture A2
13
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)
–
–
•
absorption rate for passive diffusion is determined by 3 major factors
–
–
–
•
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)
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
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.
W2013
HLTH 340 Lecture A2
15
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 through the phospholipid
biomembranes
•
hydrophilic (water soluble)
–
–
–
–
–
–
•
lipophilic (fat and oil 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
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
–
–
W2013
lipophilicity = hydrophobicity - [polarityH-bonding + ionic interactions]
calculated rate of absorption = 1/size (MW) x 1/lipophilicity (log Ko/w)
HLTH 340 Lecture A2
17
Partition coefficient is a quantitative measure of the
degree of lipophilicity of a given molecule
•
partition coefficient (Kp, Ko/w)
–
•
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)
–
•
measures relative degree of solubility in lipid (lipophilicity) and water (hydrophilicity)
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
–
W2013
example:
Ko/w = 1000 --> log Ko/w = 3 (strongly lipophilic)
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 Ko/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)
ko/w
ko/w
ko/w
W2013
HLTH 340 Lecture A2
20
Effect of partition coefficient on absorption rate
extremely
lipophilic
strongly
lipophilic
4-5
3
2
moderately
lipophilic
1
log Kp < 0 substances are
poorly absorbed due to
ionic interactions or H-bonding
hydrophilic
W2013
<0
log Kp > 5 substances are poorly absorbed
due to membrane trapping
or lack of water solubility
0 - 0.9 mixed or
amphiphilic
HLTH 340 Lecture A2
21
Absorption of large or non-permeable xenobiotic
molecules can occur via cellular endocytosis
W2013
HLTH 340 Lecture A2
22
Absorption into brain of manganese (Mn2+) ions via
active transport channels and cellular endocytosis
TMI slide (illustrative purposes only)
W2013
HLTH 340 Lecture A2
23