absorption of xenobiotics - · pdf filetoxicants must cross one or several of these ......

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

� 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.


ACTIVE TRANSPORT

� ATP binding cassete (ABC)


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.


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


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.


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.


� 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.


� 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.

absorption of xenobiotics - · pdf filetoxicants must cross one or several of these ......

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