absorption of xenobiotics - · pdf filetoxicants must cross one or several of these ......
TRANSCRIPT
MECHANISM OF TOXICITY
Absorption Of
Xenobiotics
� The skin, lungs, and alimentary canal are the
main barriers that separate higher organisms
from an environment containing a large number
of chemicals.
� Toxicants must cross one or several of these
incomplete barriers to exert deleterious effects.
� Only chemicals that are caustic and corrosive
(acids, bases, salts, oxidizers), which act directly
at the point of contact, are exceptions to this
generalization.
� A chemical absorbed into the bloodstream or
lymphatics through any of the major barriers is
distributed, at least to some extent, throughout
the body, including the site where it produces
damage (TARGET).
� A chemical may have one or several target organs
� several chemicals may have the same target organ or
organs.
� Toxicity is not concentration dependent
� Tissue having high deposition of toxicant may remain
unaffected (eg. DDT in adipose tissues)
ABSORPTION� Absorption is the transfer of a chemical from the site of exposure,
usually an external or internal body surface (e.g., skin, mucosa of the
alimentary and respiratory tracts), into the systemic circulation.
ABSORPTION� Absorption may be defined as the process by which a compound penetrates
one or more biological membranes to gain entry into the body.
� (stratified epithelium of the skin, alveolar membrane in lungs GI epithelium, capillary endothelium and finally cell membrane of the targets)
� Passive diffusion
� Pore transport
� Facilitated diffusion
� Active transport
� Ionic or Electrochemical diffusion
� Ion-Pair transport
� Endocytosis
ABSORPTIONCell membrane
ABSORPTIONCell membrane
� Toxicants cross membranes either by passive
processes in which the cell expends no energy or
by mechanisms in which the cell provides energy
to translocate the toxicant across its membrane.
� Passive processes
� Simple diffusion
� Filtration
� Special Transport
� Active Transport
� Xenobiotic Transporters
� Facilitated Diffusion
SIMPLE DIFFUSION
� Most toxicants cross
membranes by simple
diffusion.
� Chemicals traverse from
regions of higher
concentration to regions of
lower concentration without
any energy expenditure.
Small hydrophilic molecules (up to about 600 Da) permeate membranes through aqueous pores, in a process termed paracellular diffusion, whereas hydrophobic molecules diffuse across the lipid domain of membranes transcellular diffusion.The smaller a hydrophilic molecule is, the more readily it traverses membranes by simple diffusion through aqueous pores. Consequently, a small, water-soluble compound such as ethanol is rapidly absorbed into the blood from the GI tract and is distributed just as rapidly throughout the body by simple diffusion from blood into all tissues.
SIMPLE DIFFUSION
� Many chemicals are weak organic acids or bases which in solution are
ionized depending the pH of solution. The ionized form usually has
low lipid solubility and thus does not permeate readily through the
lipid domain of a membrane.
� Magnitude of absorption of ionizable chemicals depends on their
ionisation constant pKa.
FILTRATION
� When water flows in bulk across a porous
membrane, any solute small enough to pass
through the pores flows with it. Passage through
these channels is called filtration, as it involves
bulk flow of water caused by hydrostatic or
osmotic force.
� Depends on solubility of the chemical and size of
the pores.
ACTIVE TRANSPORT
� Active transport characterized by
� Movement of chemicals against electrochemical or concentration
gradients with expenditure of energy.
� saturability at high substrate concentrations
� Selectivity for certain
structural features of
chemicals
� competitive inhibition
by chemical or
compounds that are
carried by the same
transporter.
ACTIVE TRANSPORT
Significant advances in identifying and understanding the
carrier-mediated transport systems for xenobiotics have been
made in the recent years. In total, it is estimated that at least 5%
of all human genes are transporter related, indicative of the
importance of the transport function in normal biological and
toxicological outcomes. Transporters mediate the influx (uptake)
or efflux of xenobiotics and can be divided into two categories,
determined by whether they mediate active or facilitated transfer
of compounds.
Xenobiotic Transporter
ACTIVE TRANSPORT
� Many of the xenobiotic transporters belong to the large gene
superfamily of ATP binding cassete (ABC) transporters.
� The typical transporter consists of four domains, two
transmembrane domains (TMD) and two nucleotide binding
domains (NBD).
� TMD is highly hydrophobic and usually comprises six
transmembrane helices.
Xenobiotic Transporter
ACTIVE TRANSPORT
� ATP binding cassete (ABC)
Xenobiotic Transporter
ACTIVE TRANSPORT
� P-gp a member of ABC superfamily.
� Confer MDR (multi drug resistance) to cancer cells.
� Mdr1-type P-gps are ATP dependent drug transporters capable of
extruding avariety of hydrophobic organic chemicals from cells. And confer
MDR.
� Mdr2-type P-gps are phosphatidylcholine transporters and are not capable
to bind with most of the type1 substrates.
� MRP (multi resistant drug protein) family is another important
member of ABC transporter superfamily, excreting the chemicals
from cells.
� MRP1 has been isolated from multi drug resistant cancer cells.
� MRP2 and MRP3 are important in efflux of xenobiotics metabolites,
specially of glucoronated or reduced with glutathione.
Xenobiotic Transporter
ACTIVE TRANSPORT
� Breast cancer resistance protein (BCRP), originally isolated from
a breast cancer cell line, is expressed in normal and malignant
tissue.
� Play a role in the efflux transport of numerous endogenous and
xenobiotic sulfate conjugates
Xenobiotic Transporter
ACTIVE TRANSPORT
� Members of the ABC transport family are expressed
constitutively in many cells, and collectively they play important
roles in absorption from the GI tract and elimination into bile or
into urine for a diverse array of xenobiotics.
� They are also critical to maintaining the barrier function of
numerous tissues sites including the blood–brain barrier, the
blood–testis barrier, and the maternal–fetal barrier or placenta.
� They play a central role in the disposition and toxicity of
xenobiotics.
Xenobiotic Transporter
FACILITATED DIFFUSION
� Carrier mediated transport
� Similar to Active Transport with the exception
� Not occur against
concentration or electrical
gradient
� Dot not require ATP
� Movement of glucose at
basolateral membrane of GI
epithelium, in RBCs and central
nervous system.
FACILITATED DIFFUSION
� SLC (solute carriers)
� 300 genenes, 43 distinct families
� Glucose, neurotransmitter, nucleotides, metals and peptides.
� Mainly considered as influx pumps, solute can move
bidirectionally.
� SLCO, SLC21 and SLC22 play important role in xenobiotic
disposition.
� Important membrane transporting proteins mediating Na
independent transport, of xenbiotics, driving a variety of acid,
base and neutral endogenous compunds.
� Three of the SLCOs (OATP1B1,OATP1B3, andOATP2B1) have
been identified in human liver, play important role in xenobiotic
uptake by liver.
ROUTES OF ABSORPTION
� Xenobiotic have to cross membranes to get access
to the blood and tissues.
� There are no specific systems or pathways for the
sole purpose of absorbing toxicants.
� Xenobiotics penetrate membranes during
absorption by the same processes as do
biologically essential substances such as oxygen,
foodstuffs, and other nutrients.
ROUTES OF ABSORPTION
� The primary routes of exposure by which
xenobiotics can gain entry into the body are:
� Important for environmental exposure to food
and water contaminants. In addiation, the
main route for many pharmaceuticas.
� Important for environmental and occupational
exposure to air contaminants. Some
pharmaceuticals (such as nasal or oral aerosol
inhalers) utilize this route.
� An important environmental and
occupational exposure route. Many
consumer and pharmaceutical products are
applied directly to the skin.
ROUTES OF ABSORPTION
� Absorption may also occur from other sites, such
as the subcutis, peritoneum, or muscle, if a
chemical is administered by special routes.
� Enteral administration includes all routes
pertaining to the alimentary canal (sublingual,
oral, and rectal).
� Parenteral administration involves all other
routes (intravenous, intraperitoneal,
intramuscular, subcutaneous, etc.).
GI TRACT
� one of the most important sites where toxicants
are absorbed.
� Many environmental toxicants enter the food
chain and are absorbed together with food from
the GI tract.
� Accidental ingestion
� Intentional overdoses
� Absorption of toxicants can take place along the
entire GI tract, even in the mouth and the
rectum.
GI TRACT
� Absorption influenced by a number of factors
� pH
� pH affect ionization
� Ionization influence diffusion
� Non ionized forms are lipid soluble can be readily
absorbed.
� Other factors like mass action law, surface area,
and blood flow rate also have to be considered.
� pH may also influence the xenobiotic absorption
by affecting their integrity.
GI TRACT
� There are no specific systems or pathways for the sole
purpose of absorbing toxicants.
� The mammalian GI tract has numerous specialized
transport systems (carrier-mediated) for the absorption of
nutrients and electrolytes.
GI TRACT
� Some xenobiotics are absorbed by the same specialized
transport systems, thereby leading to potential competition
or interaction.
� 5-fluorouracil absorbed by the pyrimidine transport
system
� thallium by iron transport
� cobalt and manganese compete for the iron transport
system
� Lead by calcium transporter
GI TRACT
� Several proteins in the SLC families are expressed in the
intestine where they are predominantly localized on the apical
brush border membranes of the enterocytes and increase
uptake from the lumen into the enterocytes.
� There are also peptide transporters
(PEPT1) in the GI tract that
mediate the transport of peptide-
like drugs such as antibiotics,
particularly those containing a β-
lactam structure
GI TRACT
� Numerous xenobiotic transporters are expressed in the GI
tract where they function to increase or decrease
absorption of xenobiotics.
� The primary active efflux transporters such
as P-gp, MRP2, and BCRP are expressed on
enterocyte brush border membranes where
they function to excrete their subsrates into
the lumen, thereby decreasing the net
absorption of xenobiotics. MRP3 is also found
in the intestine, but is localized to the
basolateral membrane.
� P-gp expression in the intestine increases
from the duodenum to colon, whereas MRP2
expression is highest in the duodenum and
decreases to undetectable levels in the
terminal ileum and colon, and BCRP is found
throughout the small intestine and colon.
GI TRACT
� There will be a net reduction in the absorption of chemicals
that are substrates for these transporters, and this is a
desirable outcome for toxic chemicals.
� However, whereas limiting absorption
of toxicants and carcinogens is
beneficial, these transporters can also
function to limit the oral absorption of
drugs. For example, the
immunosuppressive drug cyclosporine
and the chemotherapeutic anticancer
drugs paclitaxel (taxol), colchicine, and
vincristine are not readily absorbed
from the GI tract because they are good
substrates for P-gp.
GI TRACT
� Absorption of a toxicant from the GI tract depends on its
physical properties, including lipid solubility, its
dissolution rate, charge, particle size etc.
� It also affected by the local factors in GI tract
� Motality
� Presence of Food
� Digestive enzymes
� Microbial flora
GI TRACT
� Absorption of xenobiotic depends on their retention time in
the digestive tract however an increased motality may
leads to increase absorption by favoring the stomach
emptying.
� pH, digestive enzymes and microbial flora may transform
the xenobiotics in favorable or unfavorable manner e.g.
snake venom is relatively non toxic when administered
orally.
� By inhibiting cytochrome P450 3A Grapefruit juice
increases the GI absorption of numerous pharmaceutical
agents (such as calcium-channel blockers and cholesterol-
lowering agents) and in some cases, this effect leads to toxic
or adverse reactions resulting from increased exposure to
the drugs.
GI TRACT
� Weak acids and bases will be
absorbed by simple diffusion to a
greater extent in the part of the GI
tract in which they exist in the most
lipid-soluble (non-ionized) form
(hydrophilic substances) will be
transported to the liver by the portal
vein.
� Highly hydrophilic substances
may be absorbed through
transporters (xenobiotics with similar
structures to endogenous substrates).
� Highly hydrophobic compounds may be absorbed into the lymphatic system via chylomicrons and drained into venous circulation near the heart.
� The greatest level of absorption for most ingested substances occurs in the small intestine.
GI TRACT
� The liver and first-pass metabolism serve as a defense against most xenobiotics. The liver is the organ with the highest metabolic capacity for xenobiotics
• Polar substances that are absorbed:
• go to the liver via the portal vein.
• may undergo first-pass metabolism
or presystemic elimination in
gastric and/or liver cells where
xenobiotics may be biotransformed.
• can be excreted into the bile without
entrance into the systemic circulation
or enter the systemic circulation.
GI TRACT
Lipophilic, non-polar substances (e.g.
polycyclic aromatic hydrocarbons)
� Ride on the “coat-tails” of lipids via
micelles and follow lipid absorption to the
lymphatic system (via chylomicrons) to the
lungs.
� Non-polar substances may by-pass first-pass metabolism. e.g.
PAH have selective toxicity in the lung, where they may be
metabolically activated.
INHALATION (LUNG)
� Toxicants absorbed by the
lung are:
� Gases (e.g. carbon monoxide,
nitrogen dioxide, sulfur
dioxide, phosgene)
� Vapors or volatile liquids (e.g.
benzene and carbon
tetrachloride)
� Aerosols
INHALATION (LUNG)
INHALATION (LUNG)
� The absorption of inhaled gases and vapors starts in the nasal cavity which has:
� Turbinates, which increase the surface area for increased absorption (bony projections in the breathing passage of the nose improving smell).
� Mucosa covered by a film of fluid.
� The nose can act as a “scrubber” for water-soluble gases and
highly reactive gases, partially protecting the lungs from
potentially injurious insults (e.g. formaldehyde, SO2).
-Rats develop tumors in the nasal turbinates when exposed to
formaldehyde.
INHALATION (LUNG)
Absorption of gases differs from
intestinal and percutaneous
absorption of compounds because:
� Ionized molecules are of very low
volatility, so their ambient air
concentration is insignificant.
� Epithelial cells lining the alveoli
(type I pneumocytes) are very thin
and the capillaries are in close
contact with the pneumocytes, so
the diffusion distance is very short.
� Chemicals absorbed by the lungs are rapidly removed by the
blood (3-4 seconds for blood to go through lung capillary
network).
INHALATION (LUNG)
� When a gas is inhaled into the lungs, gas molecules diffuse from the alveolar space into the blood and then dissolve.
� The gas molecules partition between the air and blood during the absorptive phase, and between blood and other tissues during the distributive phase.
inhalation bypasses first-pass metabolism.
EXAMPLES OF TOXICANT GASES OR VOLATILE LIQUIDS
� Carbon monoxide—binds hemoglobin (with >200x affinity compared to O2) and displaces oxygen leading to impaired oxygenation of tissues, energy impairment, and death.
� Chloroform—anesthetic that depresses the nervous system, but can also be metabolized to phosgene, a reactive metabolite that modifies proteins and causes toxicity in lung, kidney, and liver.
� Sarin gas—chemical warfare agent (recently used in Syria) that causes excessive neuronal excitation, convulsions, seizures, tearing, salivation, suffocation, and death through inhibition of acetylcholinesterase.
� Carbon tetrachloride—volatile liquid used widely as a cleaning agent and refrigerant, currently banned—greenhouse gas and carbon tetrachloride can be bioactivatedin the liver to produce a potent hepatotoxin.
� Benzene—largely found in crude oil, but also found in tobacco smoke and used to be found in glues, paints, and detergents—benzene metabolism leads to bioactivatedcarcinogens that cause leukemia.
AEROSOLS AND PARTICLES
Deposited in nasopharyngeal region (or mouth).
� Removed by nose wiping, blowing or sneezing.
� The mucous blanket of the ciliated nasal surface can propel
insoluble particles by movement of cilia and be swallowed.
� Soluble particles can dissolve in mucus and be carried to the
pharynx or nasal epithelia and into blood. (asbestos-lung cancer)
Size Site of Absorption
Deposited in tracheobronchiolar regions of the lungs.
� Cleared by retrograde movement of mucus layer in ciliated portion of
respiratory tract.
� Coughing can increase expulsion rate.
� Particles can be swallowed and absorbed from the GI tract.
(asbestosis—lung fibrosis, wheezing)
Penetrates to alveolar sacs of lungs and is absorbed into blood or cleared
through lymphatic system after being scavenged by alveolar macrophages.
(asbestos and silica dust can cause silicosis—cough, shortness of
breath,inflammation, immunodeficiency through damaging pulmonary
macrophages)
>5µm
2 - 5µm
<1µm
ROUTES OF EXPOSURES: DERMAL (SKIN)
Human skin comes into
contact with many toxic
agents.
Fortunately, the skin is not
very permeable and is a good
barrier
for separating organisms
from their environment.
FACTORS FOR DERMAL ABSORPTION
� To be absorbed through the skin, a toxicant must
pass through the epidermis or the appendages
(sweat and sebaceous glands and hair follicles).
� Once absorbed through the skin, toxicants must
pass through several tissue layers before
entering the small blood and lymph capillaries in
the dermis.
� The rate-determining barrier in the dermal absorption of
chemicals is the epidermis—especially the stratum corneum
(horny layer), the upper most layer of the epidermis.
� The cell walls are chemically resistant, two-times thicker than
for other cells and dry, and in a keratinous semisolid state
with much lower permeability for toxicants by diffusion—the
stratum corneum cells have lost their nuclei and are
biologically inactive (dead).
� Once a toxicant is absorbed through the stratum corneum,
absorption through the other epidermal layers is rapid.
ALL TOXICANTS MOVE ACROSS THE STRATUM
CORNEUM BY PASSIVE DIFFUSION
� Polar substances diffuse through the outer
surface of protein filaments of the hydrated
stratum corneum.
� Non-polar molecules dissolve and diffuse through
the lipid matrix between protein filaments.
� The rate of diffusion is proportional to lipid
solubility and inversely proportional to molecular
weight.
� Once absorbed, the toxicant enters the
systemic circulation by-passing first-pass
metabolism.
FACTORS THAT AFFECT STRATUM CORNEUM
ABSORPTION OF TOXICANTS
1. Hydration of the stratum corneum
� The stratum corneum is normally 7% hydrated
which greatly increases permeability of toxicants.
(10-fold better than completely dry skin)
� On additional contact with water, toxicant
absorption can increase by 2- to 3-fold.
FACTORS THAT AFFECT STRATUM CORNEUM
ABSORPTION OF TOXICANTS
2. Damage to the stratum corneum
� Acids, alkalis and mustard gases injure the
epidermis and increase absorption of toxicants.
� Burns and skin diseases can increase permeability
to toxicants.
3. Solvent Administration
� Carrier solvents and creams can aid in increased
absorption of toxicants and drugs (e.g.
dimethylsulfoxide (DMSO)).
SPECIAL ROUTES OF EXPOSURE
TOXICANTS USUALLY ENTER THE BLOODSTREAM AFTER
ABSORPTION THROUGH THE SKIN, LUNGS OR GI TRACT.
SPECIAL ROUTES INCLUDE:
1. Subcutaneous injection (SC) (under the skin)-by-passesthe epidermal barrier, slow absorption but directly intosystemic circulation; affected by blood flow
2. Intramuscular injection (IM) (into muscle)-slowerabsorption than IP but steady and directly into systemiccirculation; affected by blood flow
3. Intraperitoneal injection (IP) (into the peritoneal cavity)-quick absorption due to high vascularization and largesurface area absorbed primarily into the portal circulation (toliver—first-pass metabolism) as well as directly into thesystemic circulation.
4. Intravenous injection (IV) (into blood stream) -directlyinto systemic circulation
TOXICITY IS DEPENDENT ON ROUTE OF
EXPOSURE
SUMMARY ON ABSORPTION
� Route of exposure and physicochemical properties
of xenobiotic determine how a chemical is absorbed
and whether it goes through first-pass metabolism
or is subjected to systemic circulation.
� The degree of ionization and the lipid solubility of
chemicals are very important for oral and
percutaneous exposures.
� For exposure to aerosols and particles, the size and
water solubility are important.
� For dermal absorption, polarity, molecular weight
and carrier solvent of the toxicant and hydration of
the epidermis are important.