week 2

Question 1

body compartments

Answer 1

Drugs distribute into compartments in the body where they may be stored, metabolized, excreted or exert their pharmacological effect

Question 2

body’s compartments include

Answer 2

1) Interstitial Space – The extracellular fluid that surrounds cells. Low molecular weight, water soluble drugs distribute in the interstitial space.
2. Total body water – Includes the fluid in the interstitial space, intracellular fluid and the plasma.
3. Plasma – The non-cell containing component of blood. Drugs strongly bound to plasma protein and high molecular weight drugs typically distribute in plasma.
4. Adipose Tissue – The body’s fat. Lipid soluble (lipophilic) drugs distribute into adipose tissue.
5. Muscle – Some drugs bind tightly to muscle tissue.
6. Bone – Some drugs adsorb onto the crystal surface of bone with eventual incorporation into the crystal lattice. Bone can be a reservoir for the slow release of some drugs.
7. Other tissues

Question 3

Drug distribution is determined by:

Answer 3

1. Blood flow to tissues.
2. Ability of drug to move out of capillaries.
3. Ability of drug to move into cells.

 The more drug that distributes out of the blood, the lower the concentration of drug in the
blood.

Question 4

Blood flow to tissues

Answer 4

 Blood flow to tissues is a key determinant of drug distribution.

 In well perfused tissues such as the liver, kidney and brain, drug distribution is rapid.

 Distribution to tissues with lower blood flow such as skin, fat and bone is much slower.

Question 5

implcations for altered blood flow

Answer 5

Neonates have limited blow flow and therefore may have limited drug distribution.

 Poor blood flow rarely limits drug distribution in adult patients however some exceptions do exist.

Question 6

Poor blood flow rarely limits drug distribution in adult patients however some exceptions do
exist - examples:

Answer 6

o Patients with heart failure or shock may have reduced blood flow and therefore altered drug distribution.

o Solid tumours have low regional blood flow. The outer portion of tumours has a high blood flow but the blood flow progressively decreases towards the middle. Therefore it is difficult to attain high drug concentrations within solid tumours.

o Abscesses (infection filled with pus) have no blood supply and are therefore difficult to treat with antibiotics. They are often drained prior to drug therapy.

Question 7

Ability of drug to move out of capillaries

Answer 7

 With the exception of the brain, drug movement out of the capillaries into the interstitial space
occurs rapidly due to the permeable nature of the capillary wall.

 Drugs move out of the capillary through fenestrations.

Question 8

. Ability of drug to move into cells

Answer 8

 Once drugs leave the vasculature they must enter their target organ/cells to have an effect.

 The cell membrane is a significant barrier to drugs reaching their targets.

 In order for drugs to enter cells they must be sufficiently lipophilic to cross the cell membrane or
be carried by an uptake transporter into the cell.

 Some drugs are extruded (removed) from cells by efflux transporters.

Question 9

P-GLYCOPROTEIN (P-GP)

.

Answer 9

 P-glycoprotein is the most widely studied
efflux transporter.

 P-gp plays an important role in the
distribution of drugs

P-glycoprotein is an active transporter which
means that it requires energy (ATP) in order
to transport drugs against a concentration
gradient.

Question 10

“P” in P-gp

Answer 10

Although the “P” in P-gp stands for
permeability, it is helpful to remember the
word Protective when you think of P-gp.

 P-gp is protective because it facilitates drug
efflux from cells, promotes drug excretion
and protects the body from exposure to drugs
and other toxins.

Question 11

why p-gp is considered protective in

liver, intestine, kidney, brain

Answer 11

P-gp in the liver pumps drugs into the bile to facilitate excretion.

In the intestine, P-gp pumps drugs into the lumen preventing absorption into the blood.

In the kidney P-gp pumps drugs into the lumen facilitating excretion.

In the brain P-gp pumps drugs into the blood limiting
exposure in the brain.

Question 12

plasma protein binding

Answer 12

In plasma, drugs can be bound to plasma proteins or free (unbound).

 Only free drug is available to elicit a pharmacological response.

 Proteins are large and therefore drugs that are bound to plasma proteins are unable to pass
through capillary fenestrations.

Question 13

There are two major plasma proteins that bind drugs in plasma:

Answer 13

1. Albumin – Has a high affinity for lipophilic and anionic (i.e. weakly acidic) drugs. Albumin is responsible for the majority of protein binding.
2. Alpha 1 acid glycoprotein – Binds primarily cationic (i.e. weakly basic) and very hydrophilic drugs.

Question 14

Is Plasma Protein Binding Reversible?

Answer 14

 The binding of drugs to plasma proteins is reversible.

 In the diagram to the right, the free drug (yellow dot) is in equilibrium with plasma protein. If some of the free
drug is removed, some of the protein bound drug will
dissociate from the protein and become free

Question 15

albumin

Answer 15

Malnutrition, trauma, aging, liver and kidney disease
decrease plasma albumin concentration. This results in
an increase in free drug concentration which may
result in toxicity.

 malnourished patient has less albumin in
their blood and therefore a higher free concentration of
drug.

Question 16

Alpha 1 Acidic Glycoprotein

Answer 16

 Aging, trauma and hepatic inflammation (i.e. in hepatitis) cause increased alpha 1 acidic
glycoprotein concentration. This results in decreased free drug concentration which may lead to ineffective therapy.

 the trauma patient has more alpha 1 acidic glycoprotein in their blood and therefore a lower free
drug concentration.

Question 17

volume of distribution VD

Answer 17

Represents the APPARENT volume that a drug distributes into.
 Vd is the ratio of the total amount of drug in the body (D) to the plasma concentration of the
drug (C), therefore:
Vd = D/C

 It is important to note that Vd is NOT a physical, anatomical space, rather it is a calculated
volume that helps determine the relative distribution of a drug within the body.

 Some drugs have a Vd much larger than the volume of the body due to extensive binding to
tissue.

Question 18

Fluid Compartments in the Body

Answer 18

Plasma – The liquid (non-cell) portion of blood.

Interstitial Fluid – The fluid that surround the cells of the body.

Intracellular Fluid – The fluid inside cells.

Question 19

total body water of human + fluuid breakdown

Answer 19

total body water - 42L for 70kg person

intracellular fluid - 28 L

extracellular fluid - 14 L

• interstitial fluid - 10L
• plasma - 4L

Question 20

Drugs with a Small VD

Answer 20

Drugs with a small Vd have the following characteristics:
o Highly protein bound (retained in plasma).
o Large molecular weight (unable to pass through capillary fenestrations).

 These drugs are unable to leave the vascular space (plasma).

Therefore these drugs tend to distribute into the plasma volume which is approximately 0.057 L/kg (or ~ 4 L
in a 70 kg person).

drugs w small VD almost always distributes only in plasma

Question 21

Drugs with an Intermediate VD

Answer 21

 Drugs with an intermediate Vd tend to have the following characteristics:
o Low molecular weight (able to pass through capillary fenestrations).
o Very hydrophilic (can’t cross cell membranes).
o Intermediate protein binding.

 These drugs are able to leave the vascular space and enter the interstitial space however they are unable to enter cells. Therefore these drugs tend to distribute into the extracellular fluid (plasma + interstitial space).
 The extracellular space is ~ 0.2 L/kg (~ 14 L in a 70 kg person).
intermediate Vd drug (purple dots) distributes into the plasma and interstitial fluid but not in the intracellular fluid.

Question 22

Drug with a Large VD

Answer 22

 Drugs with a large Vd typically have the following characteristics:
o Low molecular weight (able to pass through capillary fenestrations).
o Lipophilic (able to cross cell membranes).
o Minimal protein binding.

 These drugs are able to leave the vascular space and the interstitial space. Therefore these drugs tend to distribute into body compartments such as fat, bone,
muscle and other tissues.
 Drugs with a large Vd typically distribute into greater than 0.2 L/kg.
 Keep in mind that these drugs may have a Vd larger than total body water! How is this possible? Remember that Vd is mathematically derived and is NOT an actual
physical volume.
 large Vd drug (black dots) distributes predominantly into the intracellular fluid.

Question 23

drug displacement from protein

Answer 23

Drug binding to protein is reversible.

 If two drugs are present in the blood, one drug may displace the other drug from plasma protein.

 The fate of the displaced drug
depends on its volume of distribution.

Question 24

drug displacement from protein

If a small Vd:

Answer 24

 When the Vd of the displaced drug is small, displaced drug does NOT distribute into tissues, it
stays in the plasma.

This means the free drug concentration increases.

Question 25

drug displacement from protein

If a large Vd:

Answer 25

 When the Vd of the displaced drug is large, displaced drug leaves the plasma and distributes into
the tissues.

This causes the total plasma drug concentration to decrease, and the apparent Vd to increase even further.

Question 26

BODY COMPOSITION AND DRUG DISTRIBUTION

Answer 26

 As we age our body composition changes.

 Elderly people have an increased proportion of body mass as fat.

Similarly, obese people have a
larger proportion of body mass as fat. Drugs that distribute in fat will have a larger Vd in obese
or elderly people than young healthy adults.

 As people age they have a decreased percentage of muscle per total body mass. Therefore drugs that distribute into muscle will have a lower Vd.

Question 27

total body water - lowest to highest

Answer 27

old, obese, healthy, baby

Question 28

muscle mass - lowest to highest

Answer 28

baby, old, obese, healthy

Question 29

fraction of lipophilic drug in fat tissue - lowest to highest

Answer 29

baby, healthy, old, obese

Question 30

drug metabolism

Answer 30

 Metabolism is the enzyme mediated alteration of a drug’s structure.

 Metabolism is also referred to as biotransformation.

Question 31

Sites of drug metabolism include:

Answer 31

liver - primary site of drug metabolism

intestine - enterocytes that line gut are able to metabolize drugs

stomach - site for metabolism of alcohol

kidney - underappreciated as a metabolic organ

intestinal bacteria - normal bacterial flora play an important role in drug metabolism

Question 32

why do we need drug metabolism

Answer 32

 Drug metabolism is important in humans to protect us from a number of environmental toxins as
well as synthesize essential endogenous molecules.

Note that even things like vegetables considered to be
healthy would by toxic if we didn’t have enzymes to process them

Question 33

eg of exogenous toxins

Answer 33

wine, smoking, steak, coffee,

Question 34

e.g. endogenous

Answer 34

vit D synthesis
bile acid synthesis
cholesterol metabolism
steroid hormones
bilirubin

Question 35

Therapeutic Consequences of Drug Metabolism

Answer 35

 Drug metabolism can have several different consequences.

1) Increase water solubility of drugs to promote their excretion
Lipophilic -> Hydrophilic
2) Inactivate drugs
Active -> Inactive
3) Increase drug effectiveness
Active -> More active
4) Activate prodrugs (prodrugs are inactive until metabolized)
Prodrug (inactive) -> Active drug
5) Increase drug toxicity
Non-toxic -> Toxic

Question 36

First Order

Answer 36

 In most clinical situations the concentration of
drug is much lower than the metabolic
capacity of the body. In these situations drug
metabolism displays 1st order kinetics.

 In 1st order kinetics drug metabolism is
directly proportional to the concentration of
free drug.

 This means a constant fraction of drug is
metabolized per unit time

Question 37

Zero Order

Answer 37

 In zero order kinetics, the plasma drug concentration is much higher than the metabolic capacity of the body.

 In zero order kinetics drug metabolism is constant over time.

 This means a constant amount of drug is metabolized per unit time.

 One of the best examples of zero order kinetics is ethanol.

Question 38

first order graph

Answer 38

decreasing curve

concentration decreases faster when there are higher drug concentrations than at the end when the drug
concentrations are low.

The bottom panel shows the amount of drug and the amount of enzyme.

Notice how there is much more enzyme than there is drug. This is typical in drugs that display first order metabolism.

Question 39

zero order graph

Answer 39

straight decrease

constant amount of drug is eliminated over time. This means that the metabolism is independent of drug
concentration.

The bottom panel shows the amount of drug and the amount of enzyme.
Notice this time how there is much more drug than there is enzyme. This is typical in drugs that display zero order metabolism.

Question 40

FIRST PASS METABOLISM

Answer 40

 PO drugs may undergo significant
metabolism prior to entering the systemic
circulation. This is called 1st pass metabolism.

The result of 1st pass metabolism is a decreased
amount of parent drug that enters systemic
circulation.

Question 41

First pass metabolism can occur via:

Answer 41

1. Hepatocytes in the liver
2. Intestinal enterocytes
3. Stomach
4. Intestinal bacteria

Question 42

first pass metabolism in diff organs

Answer 42

stomach
alcohol -alcohol dehydrogenase -> acetaldehyde

intestine
drug -CYP enzymes -> metabolite

intestinal bacteria
drug -bacterial enzymes -> metabolite

liver
drug -CYP enzymes -> metabolite

Question 43

Extraction Ratio

Answer 43

 The amount of metabolism on the first pass through the liver can greatly determine a drug’s
bioavailability.

 Drugs are characterized as having high or low extraction ratio (ER) depending on how much
metabolism occurs on the first pass through the liver.

Question 44

High ER Drugs

Answer 44

 Have low oral bioavailability ( 1- 20%)

 PO doses are usually much higher than IV doses (to compensate for high first pass metabolism).

 Small changes in hepatic enzyme activity produce large changes in bioavailability.

 Very susceptible to drug-drug interactions

Question 45

Low ER Drugs

Answer 45

 Have high oral bioavailability ( > 80%) PO doses are usually similar to IV doses.

 Small changes in hepatic enzyme activity have little effect on bioavailability.

 Not very susceptible to drug-drug interactions.

 Take many passes through the liver via the systemic circulation before they are completely metabolized.

Question 46

TYPES OF DRUG METABOLISM

Answer 46

 Drug metabolism is broadly divided into 2 phases, phase I metabolism and phase II metabolism

Question 47

phase I metabolism

Answer 47

Convert lipophilic drugs to more polar molecules by introducing or unmasking polar functional groups such as hydroxyl (-OH) or amine (-NH2).

 Involves oxidation, reduction and hydrolysis reactions.

 Mediated by cytochrome P450 enzymes, esterases and dehydrogenases.

 Metabolites formed can be more active, less active or equally active as the parent drug.

Question 48

phase II metabolism

Answer 48

Increase the polarity of lipophilic drugs by conjugation reactions (addition of large water soluble
molecule to drug).

 Conjugates include glucuronic acid (a sugar), sulfate (-SO4), acetate or amino acids (i.e. glycine).

 Metabolites are less active than the parent drug.

Exception: Morphine 6-glucuronide is a more potent analgesic (pain reliever) than morphine.

Question 49

Intracellular Site of Drug Metabolizing Enzymes

Answer 49

Phase I – Phase I drug metabolizing enzymes are
localized to the smooth endoplasmic reticulum (ER).
- smooth endoplasmic reticulum

Phase II – Phase II drug metabolizing enzymes are
localized predominantly in the cytosol of the cell with
the exception of glucuronidation which is localized to
the smooth ER
- cytosol; exception - glucuronidation occurs in smooth ER

Question 50

CYTOCHROME P-450 DRUG METABOLIZING ENZYMES

Answer 50

CYPs are a large family of drug metabolizing enzymes.
 CYPs are the predominant phase I drug metabolizing enzyme system.
 The majority of drug metabolism in the body is performed by hepatic CYP enzymes.

There are 12 families of CYPs with 3 accounting for the majority of drug metabolism.
 Malnutrition can decrease CYP activity as these enzymes require dietary protein, iron, folic acid
and zinc for full activity.
 CYP3A4 metabolizes the largest fraction of currently marketed drugs

Question 51

CYTOCHROME P-450 DRUG METABOLIZING ENZYMES

- nomenclature + chemical formula

Answer 51

 CYPs oxidize drugs by inserting one atom of oxygen into the drug molecule producing water as a
by product.

Drug + O2 + NADPH + H+ → Drugoxidized + H20 + NADP+

CYP3A4

3-family
A- sub-family
4 - isozyme

Question 52

Phase II drug metabolizing enzymes

Answer 52

1) UDP-glucuronosyltransferases (UGTs)
2. Sulfotransferases (SULTs)
3. Glutathione S Transferases (GSTs)
4. N-acetyltransferases (NATs)
5. Thiopurine Methyltransferase (TPMT)

 Unlike CYPs, the fraction of drugs metabolized by phase II enzymes is relatively equally split between UGTs, SULTs, GSTs and NATs.

Question 53

UDP-glucuronosyltransferases (UGTs)

Answer 53

 Are localized in the smooth endoplasmic reticulum and are part of phase II drug metabolism.
 Catalyze the transfer of a glucuronic acid (sugar) to a drug.
 Glucuronidated drugs are more polar and therefore more easily excreted.
 There are 19 human UGT enzymes.

                             UGT Drug + UDP-glucuronic acid → drug-glucuronide + UDP

Question 54

Sulfotransferases (SULTs)

Answer 54

 Are cytosolic phase II drug metabolizing enzymes.
 Catalyze the transfer of a sulfate group to a hydroxyl group of drugs.
 Sulfated drugs are more polar and therefore more easily excreted.
 There are 11 human SULT enzymes.

             SULT Drug + sulfate → drug-sulfate

Question 55

Glutathione S Transferases (GSTs)

Answer 55

 Are phase II drug metabolizing enzymes that may be cytosolic or microsomal.
 Catalyze the transfer of a glutathione molecule to a drug.
 Glutathione (GSH) is an intracellular anti-oxidant.
 Transfer of a glutathione onto a reactive (i.e. toxic) drug renders the metabolite less toxic.
 There are over 20 human GST enzymes.

                GST Reactive Drug + GSH → drug-GSH

Question 56

N-acetyltransferases (NATs)

Answer 56

 Are cytosolic phase II drug metabolizing enzymes.
 Catalyze the transfer of an acetyl group from acetyl CoA to a drug.
 Subject to genetic polymorphisms which is a major cause in variability to drug response.
 There are 2 human NAT enzymes, NAT 1 and NAT2.

                NAT Drug + Acetyl CoA → Acetylated Drug + CoA

Question 57

Thiopurine Methyltransferase (TPMT)

Answer 57

 Are cytosolic phase II drug metabolizing enzymes.
 Catalyze the transfer of a methyl group from S-adenosylmethionine to a drug.
 Subject to genetic polymorphisms. Although rare, these polymorphisms have dramatic effect on
drug safety (more later).

                  TPMT Drug + S-adenosylmethionine → Drug-CH3 + Methionine

Question 58

factors affecting drug metabolism

Answer 58

1. Age
2. Drug interactions (enzyme inducers and enzyme inhibitors).
3. Disease state
4. Genetic Polymorphisms

Question 59

age

Answer 59

The expression and activity of drug metabolizing enzymes changes as we age.

 For example, infants have almost no CYP activity. It takes babies approximately 1 year after birth
until they have a reasonable level of drug metabolizing enzymes.

 By age 2, babies have the same amount of drug metabolizing enzymes as adults do.

Question 60

drug interactions

Answer 60

enzyme inducers

enzyme inhibitors

Question 61

enzyme induction

Answer 61

Induction is a process where a cell synthesizes an enzyme in response to a drug or other chemical.

 Certain CYP isozymes are susceptible to induction by drugs.

 The consequence of CYP induction is increased drug metabolism.

 Enzyme induction plays an important role in drug interactions (more later).

Question 62

Consequences of increased drug metabolism may include:

Answer 62

1. Decreased plasma drug concentration.
2. Decreased drug activity (if metabolite is inactive).
3. Increased drug activity (if metabolite is active).

Question 63

enzyme inhibition

Answer 63

Some drugs and natural compounds can inhibit CYPs.

 The consequence of CYP inhibition is decreased drug metabolism.

 Decreased drug metabolism may result in:

1) Higher plasma drug concentration.
2) Increased therapeutic effect of drugs.
3) Increased drug toxicity.

Question 64

disease state

Answer 64

Disease can play a critical role in determining CYP activity.

 Diseases that decrease CYP activity include:

1) Liver disease
2) Kidney Disease
3) Inflammatory diseases
4) Infection

Question 65

Genetic Polymorphisms

Answer 65

 Genes for some drug metabolizing enzymes have genetic polymorphisms also known as single
nucleotide polymorphisms (SNPs).

 A SNP is a change of a single nucleotide (A, T, G or C) in our DNA.

 SNPs often affect the protein (i.e. drug metabolizing enzyme) that is produced.

 There are a number of SNPs in drug metabolizing enzymes that cause pronounced differences to
the response of drugs.

Question 66

Phase I SNPs

Answer 66

CYP2C9

CYP2D6

Question 67

CYP2C9

Answer 67

Metabolizes the anticoagulant drug warfarin.

 Polymorphism of CYP2C9 results in an enzyme with decreased activity.

 Patients with a polymorphism in CYP2C9 require a lower dose of warfarin.

 If the dose is not lowered, patients may experience extensive bleeding, a side effect of warfarin.

Question 68

CYP2D6

Answer 68

Metabolizes codeine to morphine, morphine is a more potent analgesic than codeine.

 Extensive metabolizers are considered have normal enzymatic activity.

 Intermediate metabolizers have reduced metabolic activity whereas poor metabolizers have almost no metabolic activity.

 Ultra-rapid metabolizers have significantly increased CYP2D6 activity.

 Ultra-rapid metabolizers possess multiple copies of the CYP2D6 gene.

Question 69

CYP2D6 has many genetic polymorphisms that can result in 4 distinct phenotypes:

Answer 69

Ultra-rapid metabolizer (UM)
Extensive metabolizer (EM)
Intermediate metabolizer (IM)
poor metabolizer (PM).

Question 70

Phase II SNPs

Answer 70

UGT1A1

NAT2

Question 71

UGT1A1

Answer 71

 Is part of the UGT family of enzymes.

 Glucuronidates the anti-cancer compound SN-38 (the active metabolite of irinotecan).

 Polymorphisms in UGT1A1 decrease its activity.

 Patients with UGT1A1 polymorphisms are at increased risk of diarrhea and dose limiting bone marrow suppression (potentially fatal).

Question 72

NAT2

Answer 72

 Acetylates the drug isoniazid (used to treat tuberculosis), caffeine and various cancer causing
chemicals.

 There are over 23 different SNPs in the NAT2 gene.

 Patients are classified as either rapid or slow acetylators based on their genotype.

 Slow acetylators are more susceptible to isoniazid toxicity (neuropathy, hepatotoxicity) than rapid acetylators.

 Slow acetylators have a higher risk for developing certain types of cancer.