ADME

Question 1

The same dose of 2 different chemicals can give different concentrations in the body/at the target site due to differences in ADME. What does ADME stand for?

Answer 1

Absorption, Distribution, Metabolism, Elimination.

– Absorption: route and speed of uptake of the substance into the
body.

– Distribution: where the substance ends up in the body

– Metabolism/Biotransformation: how the substance is treated
chemically in the body

– Elimination/Excretion: by which route and how fast the substance
leaves the body

ADME can also change the toxicity of a compound.

Question 2

Which factors affect a toxicant’s ability to pass through cell membranes?

Answer 2

1. Size - the smaller, the easier/faster is the diffusion.
2. Lipophilicity - the more lipophilic the compound is, the faster it can diffuse through membranes. Small hydrophilic compounds can pass through aqueous pores.
3. Ionization - ionized/charged compounds are less lipophilic (more water soluble) and therefore diffuse slower through membranes than their non-ionized/neutral counterparts (which are more lipid soluble).

The rate/speed of the transport is proportional to the lipid solubility (octanol-water partition coefficient (KOW or LogP)).

Question 3

There are two ways for a compound to pass through a membrane, passive and active transport. Explain the two different ways of passive transport.

Answer 3

Passive transport means that no energy is invested in the transport of the compound. The most common way is through diffusion, where the compound is moved with it’s concentration gradient (from high to low concentration) in accordance with Fick’s law. The rate depends on size, ionization and lipophilicity of the compound.

Question 4

How can a toxicant pass membranes through active transport?

Answer 4

• There are several different transporters for toxicants, for example MDRs (multi-resistent drug proteins) which transport toxicant out from intestinal, brain endothelium, liver, kidney and placental cells to protect them.
• Organic-anion transporting proteins (Oatp) handles uptake of anions, bases and neutral molecules in hepatic cells (liver)
  Organic anion transporter (oat) – anion uptake in renal cells
  Organic cation transporter (oct) – cation uptake in renal and hepatic cells
• Several transporters in GI tract since the main point of GI tract is uptake of substances. Aid in uptake of nucleotides, metals, di- and tri-peptides. If a toxicant is similar to an endogenous compound that have specialized transporters, then the toxicant can hijack the transport system.

Pinocytsis is an alternative route to enter a cell.

Question 5

How is absorption defined in toxicology?

Answer 5

Absorbtion is the process when toxicants cross body membranes and enter the blood stream.

Question 6

There are several different routes of administration for uptake/absorption, and they are divided into to categories, which and what routes are included in them?

Answer 6

Enteral and parenteral administration.

Enteral = GI tract (sublingual, oral, rectal)
Parenteral = all the other ways.

Question 7

Which six routes are included in parenteral administration?

Answer 7

• Inhalation: lungs
• Intramuscular injection (i.m.): directly into muscle tissue
• Intraperitoneal injection (i.p.): injection into the peritoneum (body cavity).
• Subcutaneous injection (s.c.): injection is given in the fatty tissue, just under the skin.
• Intravenous injection (i.v.): directly into bloodstream (no absorption)
• Dermal or topical administration: on top of skin.

Question 8

Why is the GI tract one of the most common ways for a toxicant to enter the body?

Answer 8

The main function of the GI-tract is to absorb nutrients, so its adapted to maximize absorption with it’s large surface area and high blood flow which favors absorption of substances with its concentration gradient, and toxicants can be absorbed in the same way. The GI tract as absorption route is also highly relevant because it is the major site of unintentional exposure to a toxicant most often via food/drinks for adults but also due to hand to mouth movement in children. Also worth mentioning is that intentional overdoses most frequently occur via the oral route. A persons microflora can also alter toxicity of a xenobiotic.

Question 9

Discuss different factors important for the absorption of toxicants by the gastrointestinal tract. (Three compound properties and five for GI-tract itself)

Answer 9

Compound properties:
- Ionization: The GI tract also has a very differing pH, which enables acids and bases to exist in their non-ionized form at some point, which enables diffusion of them.
- Lipid solubility of compound: lipid-soluble compounds diffuse into the blood stream more than water soluble compounds.
- Size: The smaller the compound, the higher the rate of absorption: both by diffusion and pinocytosis of particulate matter. Uptake of nanoparticles is beginning to get traction.

GI-tract itself:
- Surface area and high blood flow: The GI-tract has an enormous surface area with a high blood flow, which favors absorption by diffusion (from high to low concentration in surrounding tissues).
- Microflora: Can convert the toxicant to a more toxic compound or make it more easily absorbed.
- Enzyme biotransformation: There are many enzymes present in the GI tract that are responsible for making nutrients available for uptake that can convert toxicants into a form more readily absorbed.
- Residence time: the longer the compound or its metabolites stay in the GI tract, the higher the risk of it being absorbed.
- Intake of other compounds that increase or decrease absorption, eg grapefruit juice.

Question 10

Even particulate matter can be absorbed in the GI-tract, how? what determines absorption rate of particulate matter?

Answer 10

Small particulate matter is absorbed via diffusion in the GI-tract, and the size matters more than solubility or ionization. The absorption rate increases with decreasing particle diameter. Bigger particulate matter can be absorbed via pinocytosis, for example polystyrene latex can be taken up by pinocytosis, transported through the cell in vesicles.

Question 11

How is bioavailability (F) defined?

Answer 11

Bioavailability (F) is the unchanged fraction of the administered dose that enters the systemic circulation after oral administration.

An IV dose has 100% bioavailability since all of it reaches the systemic circulation, while an orally administered drug could have less if a lot is excreted before reaching the systemic regulation

Question 12

What four main factors can decrease bioavailability of an enterally administered compound? Provide an example for each.

Answer 12

1. Enzymatic degradation inside the digestive tract:
   – Proteases and peptidases split proteins into small peptides and
   amino acids.
   – Lipases split fat into three fatty acids and a glycerol molecule.
   – Amylases split carbohydrates such as starch and sugars into simple
   sugars such as glucose.
   – Nucleases split nucleic acids into nucleotides.
2. Ineffective intestinal absorption
   – Big particle size
   – Short residence time
3. Biotransformation
   - In epithelial cells of the intestine
   - In the liver
4. Elimination in the lungs

Question 13

There are many other factors that can decrease bioavailability of a compound, name three.

Answer 13

• Age: Newborns have higher GI tract pH and have other bacterial flora.
• Species: Anatomical differences (relative length of intestine segments) in ruminants vs omnivores vs carnivores, pH in different segments of the intestine affects the ratio of the nonionized vs the ionized form, Number and location of bacterial species differs a lot between species.

-

Question 14

What is “first pass effect” or “Presystemic elimination”?

Answer 14

First pass effect is the removal of chemicals in the epithelial cells of the intestines or lover before entrance into the systemic circulation. This reduces the orally administered compound’s concentration before reaching the systemic regulation –> lower absorption and lower bioavailability.

Question 15

The skin is a good protection, but it’s still an important route of exposure for xenobiotics/drugs. Why is it so important?

Answer 15

The skin is a huuuge organ (ca 2 m2 with about 1% pores) that is always in contact with ambient air and clothes, and is a common way of treatment with lotions and drugs. It’s also a target for some chemical warfare and pesticides such as organophosphates can penetrate the skin. Although it has seven layers that a toxicant need to pass through to reach the blood supply (systemic regulation) some toxicants can.

Question 16

Discuss different factors important for the absorption of toxicants by the skin. (Five for skin itself and two for compound properties)

Answer 16

Skin itself:
- Skin permeability: thickness and integrity of stratum corneum (rate-limiting). Permeability is a combination of both diffusivity and thickness of stratum corneum and this differs in different parts of the body (for example palms are thicker but higher diffusivity, while major part of skin is thin but lower diffusivity to aid in sensory input)
- Amount of dermal appendages: a higher amount of sweat glands, hair follicles, Sebaceous glands, the easier absorption can happen, since it’s more permeable than the stratum corneum and a more likely way of absorption for hydrophilic compounds.
- Stage of skin replenishing: since the skin replenish every 3-4 weeks, the skin can be varying states of permeability.
- Hydration: hydrated skin can increase absorption up to 30 fold!
- Ambient temperature: the higher the temp, the faster the absorption.

Compound physical properties:
- Lipophilicity: the more lipophilic, the easier the absorption. Mostly extremely lipophilic compounds that can cause problems. hydrophilic compounds are mostly not absorbed.
- Size: the smaller, the quicker the absorption. compounds above 400 Da exhibit poor dermal absorption

Question 17

There are different mechanisms for polar and nonpolar substances in
stratum corneum, which?

Answer 17

• Polar xenobiotics diffuse through the outer surface of protein filaments.
• Nonpolar substances dissolve in and diffuse through the lipid matrix between the protein filaments: Rate proportional to lipid solubility inversely proportional to molecular weight (exception, super lipophilic compounds that can diffuse through cell membranes).

Question 18

There are many species differences in skin absorption, name two.

Answer 18

• The number of pores from sweat and sebaceous glands vary
• The thickness of skin and amount of body hair vary
• No skin (eg insects) result in some toxins being lethal to insects but not to mammals because of our skin protection.

These are not just species differences but differs in different body parts of organisms, which creates differential toxicity.

Question 19

Why are the lungs such a major exposure route for xenobiotics?

Answer 19

The lungs are evolved to maximize diffusion in order to enable breathing and are therefor very susceptible to xenobiotics hijacking the route. The surface area of the alveolar membrane is around 60-80 m2 and is surrounded by a capillary network, meaning the route to enter the systemic circulation is very short. In addition, the blood flow is very high in the capillaries which further facilitates absorption.

Question 20

What categories of toxicants can be absorbed by the lungs?

Answer 20

• Gases: (such as carbon monoxide)
• Vapors (a substance in the gas phase at a temperature lower than
  its critical point)
• Aerosols (a colloid of fine solid particles or liquid droplets, in air or
  another gas)
• Particles (such as soot)

Question 21

What structure/mechanism do we have in place to minimize absorption of toxicants in the lungs?

Answer 21

The mucus layer of the nose: before gas is inhaled all the way down into the lungs it has to pass through the nose, where water soluble gases and highly reactive gases can be retained in the film of liquid that covers the mucus layer. This acts as a “scrubber” of the air which minimize their absorption in the lungs. These compounds can then be removed by coughing and sneezing. The downside to this is that this can increase toxicity in the nasal area (eg formaldehyde).

This means that the absorption of gases in the lungs are mostly gases with low water solubility and low reactivity.

Question 22

Why is ionization and lipid solubility NOT rate limiting in absorption of the lungs? (as it is for skin and GI-tract)

Answer 22

Dissociation of acids and bases is not rate-limiting for the absorption in the lungs since ionized molecules have very low volatility and therefore are at very low concentration in the air.

Lipid solubility is not rate limiting in the lungs since the membrane to diffuse through is super thin and the blood flow is so high, so the diffusion happens almost momentarily regardless of lipid solubility.

Question 23

Explain what the “blood-to-gas-partition coefficient (Pf)” is.

Answer 23

When gas molecules are inhaled, they will diffuse from the lung/alveolar space into the blood and dissolve until an equilibrium is reached, i.e., no net movement of inhaled gas between the alveolar space and blood. The ratio blood:air is the blood-to-gas-partition coefficient (Pf) and is a unitless ratio that is unique for each gas. When equilibrium is reached, the concentration can change, but not the ratio between air and blood.

The higher the Pf, the higher solubility the gas has in the blood.

Question 24

The absorption rate in the lungs can be limited by 2 factors, which? explain them.

Answer 24

• Perfusion limited: Compounds with low Pf (low solubility in blood) lingers in the alveoli which doesn’t give room for any more to be absorbed until the blood concentration is lowered (Blood gets saturated quickly) → limited by speed of bloodstream (perfusion). Increase in perfusion will increase the rate of absorption, since more of the gas is taken away at each time unit.
• Respiration (Ventilation) limited: Compounds with a high Pf (high solubility in blood) is readily absorbed into the blood with each respiratory cycle since very little if any gas remaining in the alveoli each breath → limited by depth and speed of breath (respiration).

Question 25

Aerosols can also be absorbed via the lungs, what is an aerosol?

Answer 25

An aerosol is a colloid of fine solid particles or liquid droplets, in air or
another gas. For example fog, smoke or dust.

Question 26

What are the main factors that affect aerosol absorption in the lungs?

Answer 26

Aerosol size and water solubility of any chemical present in the aerosol.

• Size: The smaller the aerosol, the further in the lung it can go, bigger particles (> 5 μm) get stuck in nose mucus, intermediate (2-5 μm) can end up in throat and removed by cilia up to mouth and swallowed, and small (< 1 μm) and super small (nanoparticles) can be absorbed in alveoli.
• Water solubility: Lower solubility of the chemical in the aerosol/particle means lower clearance rate. There are several clearance mechanisms in the lungs such as phagocytosis by macrophages, penetration into the lymphatic system or aspired into the bronchial tube -> transported to the mouth and swallowed. Even though there are many clearance routes, it’s not so effective.

Question 27

There are four most common special routes of administration used in toxicity studies, which? Compare the absorption rate between them and explain why there are differences.

Answer 27

• Intraperitoneal injection (i.p.): injection into the peritoneum (body cavity). Rapid absorption of xenobiotics because of the rich blood supply/extensive vascular system and the relatively large surface area of the peritoneal cavity. Intraperitoneally administered compounds are absorbed primarily through the portal circulation and therefore must pass through the liver before reaching other organs by way of systemic circulation which gives the possibility for first pass effect.
• Intramuscular injection (i.m.): directly into muscle tissue, slower uptake/absorption but still enter directly to systemic blood.
• Subcutaneous injection (s.c.): injection is given in the fatty tissue, just under the skin. Slower uptake/absorption but still enter directly to systemic blood.
• Intravenous injection (i.v.): directly into bloodstream (process of
  absorption eliminated)

Question 28

After absorption comes distribution, how is distribution defined in toxicology?

Answer 28

Distribution is the process in which an absorbed chemical moves away from the site of absorption to other areas of the body.

Question 29

Distribution is generally a rapid process, but what determines the rate of distribution and final distribution?

Answer 29

The rate of distribution is usually determined by blood flow and diffusion rate out of capillaries to cells (same factors as absorption determines diffusion rate, just the other way around).

The final distribution is usually determined by a chemical’s affinity to the various tissues.

Question 30

A key concept in understanding the disposition o a toxicant is
its volume o distribution (Vd), what is volume o distribution (Vd)?

Answer 30

The volume o distribution (Vd) is a theoretical volume in which the chemical should be uniformly dissolved in, in order to produce the observed blood plasma level.

Question 31

The total body volume can be divided into three compartments, which?

Answer 31

The total body water is divided into 3 compartments:
─ Intracellular water (in cells)
─ Interstitial water (outside of cells and outside of systemic circulation, like ECM)
─ Plasma water (systemic circulation)
(Both Interstitial water and plasma water are included in extracellular water)

All of these are places in which a chemical can be dissolved in.

Question 32

Chemicals can be distributed in 1, 2 or all 3 compartments, and in different ratios. If a compound is only distributed in the plasma vs all compartments, what does this mean for the Vd?

Answer 32

• If a chemical distributes only to the plasma compartment (no tissue distribution), it has a high plasma concentration and a low Vd. A low Vd indicates that most of the chemical is bound to plasma proteins.
• If a chemical distributes throughout the body (into all compartments), it has a low plasma concentration and a high Vd. A high Vd indicates that most of the chemical is distributed into tissues.

However, the distribution of toxicants is more complex than this and strongly influenced by factors such as binding to and/or dissolution in storage deposits like fat, liver and bone.

Question 33

How do we calculate the Vd?

Answer 33

We calculate the Vd based on known mass put in and concentration in plasma observed.

If we put in 100mg of chemical and measure 10mg in plasma, we calculate Vd=100/10=10L (high Vd bc high conc in other compartments compared to plasma)

If we put in 100mg of chemical and measure 50mg in plasma, we calculate Vd=100/50=2L (low Vd bc low conc in other compartments compared to plasma).

Question 34

The site of accumulation can be the site of major toxicity, but more often it is not. What is important to remember when it comes to storage of a chemical and it’s target organ(s)/tissue(s)?

Answer 34

It is always the free molecules of a chemical that are dangerous! If bound to another organ or tissue than the target (often higher accumulation in kidneys/liver), the chemical is “inactive” which can be seen as a protective measure but even if bound/stored, there is always an exchange into the plasma with elimination/biotransformation of the unbound fraction of the compound due to the laws of equilibrium.

The half life of stored compounds can be very long due to this, and drastic changes such as dieting or pregnancy can increase the toxicity of stored compounds.

Question 35

There are several plasma proteins that can bind xenobiotics and endogenous compounds. Which is the most common xenobiotic binding protein?

Answer 35

Albumin is the most common xenobiotic-binding protein and is also the
most abundant plasma protein. It has six binding regions, is capable of binding more different endogenous compounds (and exogenous) than any other human plasma protein.

Side note, evans blue is a dye used in many different aspect and it binds almost 100% to albumin, very useful to calculate Vd and look at integrity of barriers such as BBB.

Question 36

The binding of toxicants to plasma proteins is reversible, which is an important concept in toxicology for two reasons, which and why?

Answer 36

1. Since toxicity is dependent on the amount of UNBOUND toxicant, so a toxicant with high affinity to plasma proteins will manifest as less toxic compared to one with low affinity. Toxicants with high affinity can have delayed toxicity: severe toxic reactions can occur due to another agent causing them to be displaced, which increase the equilibrium in the target organ and cause toxicity.
2. Xenobiotics with high affinity can also compete with endogenous compounds, which can lead to such high levels of the unbound endogenous compound that it leads to toxicity.

Large species and intraspecies differences in levels of plasma protein can cause the same dose toxic/non-toxic for different individuals.

Question 37

Why are the liver and kidney such central depositing organs?

Answer 37

Both the liver and kidney play a big role in metabolism/excretion of compounds in the body and because of this they have evolved to filter out different compounds from the blood through active transport, and therefore have a high capacity of binding to different compounds. Examples of proteins in these organs are
- Ligandin (binding protein for steroids and bilirubin) has high affinity for organic acids.
- Metallothionein binds heavy metals (Cd, Hg, Zn)

These two organs probably concentrate more toxicants than do all the other organs combined.

Question 38

Which type of compounds are likely to be stored in fat? What consequences does this have for an obese vs a skinny person?

Answer 38

Highly lipophilic compounds are readily stored get highly concentrated in fat, as they rapidly can diffuse through cell membranes and have a high solubility in fat. Organic environmental pollutants like DDT, Chlordane, PCBs (polychlorinated biphenyls) and PBDEs (polybrominated diphenyl ethers) are some examples of highly lipophilic toxicants that gets stored in fat.

The toxicity of highly lipophilic compounds is expected to be less severe in an obese person since more can be bound, but with rapid fat mobilization such as big weight loss or starvation severe toxicity can occur as the toxicant concentration in systemic circulation increases.

Question 39

Bone is also a potential storage depot for toxicants, what is the main mechanism behind this and how severe is it?

Answer 39

The main mechanism behind bone storage is that toxicants such as heavy metals in the fluid surrounding bone can diffuse and exchange with bone constituents. Similarity in size and charge facilitates transfer, for example:
- Fluoride (F-) displaces OH -
- Strontium2+ and lead 2+ displace Ca 2+

Deposition and reversible storage of toxicants in bone is dynamic and may or may not be detrimental. For instance, lead is not toxic to bone,
but the chronic effects of fluoride deposition (skeletal fluorosis) and radioactive strontium (osteosarcoma and other neoplasms) are well documented. Although deposition of some xenobiotics are not dangerous, their release might be, as this is a reversible process.

Question 40

The central nervous system is very vulnerable to toxicity, but there are four major anatomical and physiologic reasons why some toxicants do not readily enter the CNS, which?

Answer 40

1. Tightly jointed cells: The capillary endothelial cells of the CNS are tightly joined, leaving few or no pores between the cells, which prevents diffusion of small and medium sized polar compounds through paracellular pathways.
2. numerous glial cells: The capillaries in the CNS are to a large extent surrounded by glial cell processes (astrocytes), which secrete chemical actors that modulate endothelial permeability, maintaining low protein conc in interstitial fluid etc. The numerous amount also result in more membranes for compounds to diffuse through, which lowers the rate of diffusion of lipid soluble compounds.
3. Low protein conc in interstitial fluid: The protein concentration in the interstitial fluid o the CNS is much lower than that in other body fluids, limiting the movement of water-insoluble compounds by paracellular transport, which is possible in a largely aqueous medium only when such compounds are bound to proteins.
4. Endothelial cells contain multi-drug- resistant proteins (mdr) that exude chemicals back into the blood.

Remember: The BBB is different in different areas of the brain, ventricles, parts of hypothalamus, and pineal body are more permeable due to the nature of their functions (sensory input).

Question 41

What factors affect the rate of uptake into the brain?

Answer 41

• Lipid-solubility and degree of ionization is important
  High lipid-solubility -> increases the rate of uptake
  High degree of ionization -> decreased rate of uptake
• Very lipophilic compounds have limited access to the brain because of strong binding to plasma lipoproteins that can’t get trough.
• Some toxicants mimic compounds that are actively taken up by the brain, eg methylmercury which binds to cysteine, mimicking methionine, which is actively absorbed by neutral amino acid carrier.

Remember: Some lipophilic compounds may enter the brain, but are so efficiently removed by these transporters that they never reach appreciable concentrations. THe BBB is not fully developed at birth, and this is one reason why some chemicals are more toxic to newborns than to adults

Question 42

What factors affect the rate of uptake in the placenta?

Answer 42

The placental barrier is constructed for keeping xenobiotics out but also to take a lot of compounds in, such as nutrients, gasses etc and dispose of excretions, which is a very delicate process that’s mediated through hormonal control. The same factors that affect membrane permeability elsewhere in the body are applied here; degree of ionization, lipophilicity, protein binding, molecular weight, blood flow, and the concentration gradient across the placenta.

Question 43

Compare the placenta to the BBB in terms of barrier integrity, and differences.

Answer 43

Not many xenobiotics are actively transported over the placenta, as is also the case for the BBB. Most toxic agents pass over the placenta by simple diffusion and most important feature is lipid/water solubility

The placenta also have biotransformation systems, preventing some chemicals from reaching the fetus. This is not the case for the BBB.

The placenta have several transport proteins that actively transport compounds from the fetus to the mother, similar to the BBB.

The placenta is not as good/precise of a barrier as BBB, as viruses (e.g., rubella virus), cellular pathogens (e.g., syphilis spirochetes), and globulin antibodies can traverse the placenta. But both have extensive transport systems to keep xenobiotics out.

Question 44

Toxicants in mother and fetus, usually have the same concentration under steady state conditions, but the concentration of toxicants in fetal tissues depends on the ability of the tissue to concentrate the toxicant, which differs between mother and fetus. Give some examples of differences in tissue affinity between mother and fetus.

Answer 44

• Differences in plasma protein concentration – diphenylhydantoin higher in mothers, since fetuses have lower plasma protein concentration.
• Liver not fully developed in fetuses – lower levels of xenobiotics in fetal liver compared to maternal.
• Blood-brain barrier not fully developed – higher concentrations of lead and dimethylmercury in fetal brain.
• Lower body fat content of the fetus – lower levels of highly lipophilic chemicals in fetuses (TCDD, DDT).

Question 45

What is the definition of excretion in toxicology?

Answer 45

Excretion is the removal of xenobiotics from the blood and their return to the external environment via body secretions: urine, feces, exhalation, sweat, saliva, tears and milk.

All body secretions have the ability to excrete toxins

Question 46

Many organs are involved in the excretion of toxins, but which are the three most important ones? Start with the most important, then second then third.

Answer 46

• The most important organ is the kidney excreting chemicals in urine
• The second important route is faeces, through biliary excretion
• The third route is through the lungs, mainly gases

Question 47

The kidney is the most important organ in excretion, why?

Answer 47

Toxic compounds are excreted with the urine in the same way by-
products from the body are excreted, so the kidneys have evolved to filter out anything that isn’t needed in the body anymore (including toxicants). The kidney receives about 25 % of the cardiac output, which is A LOT, and 20% of that is filtered through the glomeruli.

Question 48

How does excretion by the kidney work in broad terms?

Answer 48

Blood enter the nephron (functional unit of the kidney) in the afferent arteriole and goes into the glomerulus where smaller molecules (<60kDa) are pressed out into the proximal tubule and the remaining blood goes out through the efferent arteriole. The filtrate then goes through the proximal tubule down through the loop of Henle, where a lot of reabsorption occur to keep the acid-base-, water- and electrolyte level in balance. The filtrate remaining goes through the distal tube and out into the collecting duct leading the bladder.

Question 49

What four factors affect the rate of excretion by the kidney?

Answer 49

Factors that affect excretion in the kidney are:
- size/plasma protein binding: too big molecules can’t be filtered out, so toxins bound to plasma proteins can’t be filtered out.
- compound specific properties: polar compounds and ions are favored for excretion while lipid soluble are readily reabsorbed.
- pH: Since urine is slightly acidic, bases are ionized (less lipid soluble) and therefore can’t diffuse through surrounding cell membranes, and are readily excreted instead.
- active secretion: The kidney and renal cells contain many active transporters made for excretion of xenobiotics.

Question 50

When are acids and bases ionized/not ionized?

Answer 50

At low pH bases are ionized, meaning less lipid soluble
– harder to cross membranes
– therefore low reabsorption, but instead excretion.
At low pH acids are nonionized, meaning lipid soluble,
– easier to cross membranes
– therefore reabsorbed and not excreted in urine

At higher pH the other way around: acids are ionized and bases are non-ionized.

Question 51

How can knowledge about acid/base ionization be used to treat xenobiotic toxicity from an acid (weak)?

Answer 51

Different treatments can increase excretion, which is what you want:

By highering the pH we increase the ionized form and get increased excretion.

– Phenobarbital poisoning treated with sodium bicarbonate as Phenobarbital is a weak organic acid (pKa 7.2) and by highering the pH
we get increases in the ionized form of phenobarbital which leads to increased excretion.

– Salicylate (an important active metabolite of aspirin (acetylsalicylic
acid) excretion can also be increased with sodium bicarbonate utilizing the same mechanism.

Question 52

In which other ways than filtration can toxicants be excreted in urine?

Answer 52

• Passive diffusion from plasma to distal tube, important for organic acids and bases.
• Active secretion from extra cellular fluid or blood via transporters into renal proximal tubule cells to be excreted into urine.

Examples of transporters:
* OCT – organic-cation transporter - small organic cations and many drugs and environmental toxins
* OCTN – organic cation transporter novel – low efficiency for cations, wide range of molecules
* OAT – organic-anion transporter - small and hydrophilic organic anions
* OATP – organic-anion transporting polypeptide - anions, neutral and cationic compounds; wide range of drugs, toxins and poisons

Question 53

What are some differences between filtration and renal secretion?

Answer 53

Filtration is a much more effective process than renal secretion, but renal secretion can excrete protein bound toxicants and larger compounds which filtration can’t.

Active transport/renal secretion can be corrupted by competition from non toxic compounds, which slows down secretion. But this can also e useful to increase the time before a therapeutic drug is excreted.

Question 54

Since the kidneys handle a large portion of chemical homeostatic processes, reabsorption happens readily. What consequences can this have for toxicity?

Answer 54

Toxicants that bind to small proteins that are filtered through the glomeruli can be absorbed in the proximal tubule and reabsorbed back in blood which in turn can lead to accumulation and toxicity in the body or in the kidney itself. Examples:

» Cadmium binds to metallothionein -> complex reabsorbed -> toxic effects in tubular cells.
» Limonene and trimethyl pentane binds to α 2 -globulin, taken up into
proximal tubule cells and produce hyaline droplet nephropathy and
tumors.

Question 55

Many of the regular factors like age, disease and genetics can affect excretion in the kidney, but age and species are the most prevalent ones, explain.

Answer 55

Many functions are not yet developed at birth, which may result in slower elimination of xenobiotics:
– Organic acid transport system that secretes penicillin is under-
developed in premature neonates (20% excretion)

There’s also big species differences:
– Excretion of weak organic acids vary a lot due to differences
in urine pH
– Filtration in glomeruli can differ due to protein-binding and
differences in plasma proteins between species
– Types of active transporters and affinity of the transporters
varies
– Species differences in biotransformation

Question 56

Fecal excretion is the second most important route of excretion for toxicants, name the four most important sources that contribute to fecal excretion of toxicants.

Answer 56

– Non-absorbed ingesta: Indigestible material, excessive nutrients, first pass effect.

– Biliary excretion: compounds absorbed in intestine, get to liver, is biotransformed and excreted in bile, and final excretion occur for compounds that have been biotransformed into excretable form (that are not reabsorbed).

– Intestinal excretion: Toxicants can diffuse from blood into intestinal lumen via diffusion (slow process), this can happen with some toxicants with low rates of biotransformation and/or low renal or biliary clearance.

– Intestinal wall and flora: Mucosal biotransformation and re-excretion into the intestinal lumen is common, chemicals not absorbed, in bile or excreted by the intestinal wall can be taken up by microorganisms (most common in the large intestine) and biotransformed (usually favoring reabsorption)

Question 57

What is reabsorption of xenobiotics in the intestine called?

Answer 57

Enterohepatic circulation.

Question 58

How does the enterohepatic circulation work?

Answer 58

1. Xenobiotics are ingested and absorbed in the small intestine
2. The liver excretes directly or biotransforms and excrete the compound in bile
3. The bile is secreted into the duodenum and the xenobiotics may be
   reabsorbed as it is, or
4. Intestinal enzymes or microbial flora may biotransform the complex,
   often favoring reabsorption

(arsenic is often reabsorbed due to microorganism biotransformation)

Question 59

Compounds are divided into three classes depending on their concentration ratio between bile vs plasma in the enterohepatic circulation, which and what do they contain?

Answer 59

– Class A: Ratio nearly 1. Ex. Na, K, glucose, Hg, Th, Cs, Co.

– Class B: Ratio greater than 1 (10-1000). Generally compounds rapidly
excreted in bile. Ex. bile acids, bilirubin, lead, arsenic, manganese.

– Class C: Ratio below 1. Ex. inulin, albumin, Zn, Fe, Au, Cr.

(The process of transport from plasma into bile of class A and C
compounds is not known)

Question 60

Which characteristics of the compound matter for excretion in bile?

Answer 60

– Usually low-molecular-weight compounds are poorly excreted in bile

– Compounds or conjugates with molecular weights over 325 are excreted in bile

Large species differences but excretion is more compound-specific than species-specific. Compounds excreted in bile enter the intestine and can either be excreted with feces or be reabsorbed.

Question 61

The third most important route of excretion of toxicants is through the lungs (exhalation). Which type of toxicants are eliminated this way and how does it work?

Answer 61

The lungs mainly eliminates substances that are in gas phase at body
temperature.

The mechanisms for elimination by the lungs have the exact same
characteristics as for uptake: equilibrium between alveolar air and blood and vapor pressure (Breathalizer, ethanol works on this basis).

Can be perfusion of ventilation limited depending on blood-to-gas-partition coefficient (Pf) (solubility in blood).

Low solubility ratio -> dependent on blood flow, perfusion limited -> Fast excretion.
High solubility ratio -> dependent on rate and depth of respiration, ventilation limited -> slow excretion

Question 62

Why is excretion via milk important?

Answer 62

1. Toxicants can be transferred from mothers to nursing offspring: redistribution and emptying of storage depots.
2. Compounds can be transferred from cow to human: intake of dairy products is higher in younger people, still not fully developed.
3. A lot of pollutants are lipid soluble and are readily excreted in milk.

Toxicants are excreted into milk via simple diffusion. basic compounds diffuse easier into milk (bc it’s acidic and basic compounds are ionized in acidic pH).

Big species differences in fat %.