Biotransformation and Pharmacogenomics (Kruse)

Question 1

What is drug biotransformation?

Answer 1

it is the enzymatically driven process whereby a substance is changed from one chemical to another (to become more polar, possibly larger) as these are more easily excreted

can be anabolic or catabolic

Most metabolic products are less pharmacodynamically active than the parent drug

Question 2

What are the three main consequences of biotransformation?

Answer 2

inactivation

active compound to active compound

activation

Question 3

What is an example of drug inactivation?

What is an example of drug active compound to active compound?

What is an example of drug activation?

Answer 3

Acetylsalicylic acid to acetic acid and salicylate

Diazepam to oxazepam

L-dopa to dopamine (prodrug to active drug)

Question 4

Where does biotransformation occur?

Answer 4

Mostly occurs in the liver at some point between absorption into the general circulation and renal elimination

Question 5

What is the first pass effect?

Answer 5

the process by which oral drugs undergo extensive biotransformation after absorption prior to entering circulation

Note: Drugs administered parenterally do not undergo first-pass biotransformation

Question 6

How does the first pass effect change bioavailability?

Answer 6

the first pass effect greatly limits bioavailability of some drugs such that alternative routes of administration must be explored

ex: morphine, with oral bioavailability of 25% so that parenteral administration is preferred

Question 7

What are the results of Phase I reactions?

What are the results of Phase II reactions?

Answer 7

results in the biological inactivation of the drug

produce a metabolite with improved water solubility and increased molecular weight (enhances elimination)

Question 8

What sort of reactions occurs in Phase I?

Answer 8

enzymes that convert the parent drug to a more polar metabolite by unmasking or introducing a functional group

Most common: Oxidation, Reduction, Hydrolysis

These reactions are catabolic and their products can be more reactive/toxic than parent drug

Question 9

What sort of reactions occurs in Phase II reactions?

Answer 9

Conjugation occurs with endogenous substrates (glucoronic acid, sulfuric acid, acetic acid, amino acid) to improve water solubility and increase molecular weight

These reactions are anabolic

typically occur after Phase I (except in Isoniazid for TB tx)

Question 10

Phase I reactions are carried out by MFOs-what are some examples?

Answer 10

Cytochrome P450s

Flavin-containing monooxygenases

Epoxide hydrolases

These enzymes are located in lipophilic ER membranes of the liver and other tissues

Question 11

What is the main key enzyme in phase I reactions?

What are the main key enzymes in phase II reactions?

Answer 11

CYP3A4 (and other CYPs)

UGT=UDP-glucuronosyltransferase

GST-glutathione S transferase

NAT-N-acetyltransferase

TPMT-thiopurine methyltransferase

SULT-sulfotransferase

Question 12

What are some individual factors that effect biotransformation?

Answer 12

differences in the rate of metabolism and elimination

genetic factors like polymorphisms in CYPs and pharmacogenetic differences in enzyme expression levels

non-genetic factors like drug interactions, age/sex, circadian rhythm, body temp, liver size and function, nutritional and env. factors

Question 13

Genetic differences can effect the biotransformation of these two compounds

Answer 13

Succinylcholine

slow acetylator phenotype for N-acetyltranferase enzyme

Question 14

What are some CYP inducers?

Answer 14

phenobarbital

chronic ethanol use

aromatic hydrocarbons (benzo-a-pyrene) in tobacco smoke

rifamplin

St. John’s wort

Question 15

What are some CYP inhibitors?

Answer 15

grapefruit juice

inhibitors can be reversible and irreversible

Question 16

How can age affect biotransformation?

How do disease states affect biotransformation?

Answer 16

liver function and blood flow decrease with increasing age

liver disease and cardiac disease can disrupt drug biotransformation

Question 17

Discuss acetaminophen-induced hepatotoxicity

Answer 17

for an adult dose of 1.2g/d, 95% will undergo glucuronidation and sulfation and 5% will undergo P450-dependent GSH conjugation

when acetaminophen intake exceeds therapeutic doses glucuronidation and sulfation pathways are saturated and P450 pathway becomes more important. Hepatic GSH gets depleted faster than is regenerated

Toxic metabolites accumulate resulting in hepatotoxicity

Question 18

How does ETOH consumption effect acetaminophen biotransformation?

Answer 18

I *think* alcohol and tylenol together use up the glucuronidation and sulfation enzymes faster such that GSH has to be used and can cause hepatic damage more quickly.

Can occur at smaller doses of tylenol/ETOH (doesn’t nec. need to be a tylenol overdose or a ton of booze)

Question 19

Basic Vocab (It’s an LO, so I’m including it)

Pharmacogenetics

Pharmacogenomics

Allele

Polymorphism

SNPs

Answer 19

Pharmacogenetics: study of different drug response to allelic variation in genes affecting drug metabolism, efficacy and toxicity at genomic level

Pharmacogenomics: study of entire genome to assess multigenetic determinants of drug response

Allele: one of two or more alternative forms of a gene that arise by mutation and are found at same locus (CYP2D6*3 is variant for CYP2D6)

Polymorphism: variation in DNA sequence that is present at an allele frequency of 1% or greater in pop.

SNP: base pair substitution that occur in genome

Question 20

Genetics can effect dose-response in what two ways?

Answer 20

by variations in pharmacokinetics and pharmacodynamics

Question 21

Polymorphisms in CYP450s can cause what?

Answer 21

absent, decreased or increased enzyme activity

poor/slow metabolizers are at risk for toxic accumulation

Ultrafast metabolizers are at risk for being undertreated

Question 22

What are some common polymorphisms in Phase II reaction enzymes?

Answer 22

UDP glycosyltransferase and campothetcin

N-acetyltransferase and isoniazid

butyryl cholinesterase and succhinylcholine

Question 23

How does glucose-6-phosphate dehydrogenase deficiency affect the pharmacodynamic response?

Answer 23

most common disease-producing enxyme defect in humans

normally, G6PD produces NADPH which regenerates glutathione to protect cells from oxidative damage

inG6PD def. oxidative damage leads to hemolytic anemia in the presence of oxidants

Question 24

How do ryanodine receptor mutations cause malignant hyperthermia?

Answer 24

inhalation anesthetics, succinylcholine given during surgery cause an elevation in CA in the sarcoplasm of muscle leading to muscle rigidity, elevation of body temp and rhabdomyolysis

Question 25

What are some examples of genetic variations affecting pharmacokinetics and pharmacodynamics?

Answer 25

polygenic effects (variations at multiple gene loci)

polymorphisms in biotransforming enzymes or polymorphisms in drug targets can affect Warfarin biotransformation

Question 26

What specific variations effect Warfarin metabolism?

Answer 26

CYP2C9 is a phase I drug metabolizing enzyme that primarly acts on acidic drugs including Warfarin, phenytoin and NSAIDs

Vit. K epoxide reductase complex subunit 1 is the target of anticoagulant warfarin and polymorphisms in it can lead to increased sensitivity to warfarin

this means there can be a >10x difference in dose due to polymorphisms

Question 27

What are the goals of animal testing in drug development?

Answer 27

ID potential human toxicites without human testing

design tests to further define the toxic mechanisms

predict the specific and most relevant toxicities to be monitored in clinical trials

Question 28

What are some limitations to animal testing?

Answer 28

some toxicity and therapeutic values in animals do not translate to humans

rare/adverse events that occur in humans unlikely detected in preclinical animal testing

Question 29

What is a lead compound?

Answer 29

a chemical compounds that has pharmacoligical or biological activity and whose chemical strucutre is used as a starting point for chemical modifications in order to improve potency, selectivity or pharmacokinetic parameters

Question 30

What is a No-effect dose?

What is a minimum lethal dose?

What is the median lethal dose?

Answer 30

the max dose at which a specified toxic effect is not seen

The smallest dose that is observed to kill any experimental animal under a defined set of conditions

the dose that kills approx. 50% of the animals

Question 31

Clinical trial terminology (is LO, so is included here)

Control:

Randomized:

double blind:

single blind:

open label:

parallel trial:

crossover trial:

endpoint:

surrogate endpoint:

Answer 31

Control: established therapy or placebo againsts which the efficacy of a new agent can be compared

Randomized: pt’s entering the trial have an equal prob. of receiving the test or control

Double blind: no one knows who is getting experiment or control

Single blind: health prof. knows who is getting treatment/control but pt does not

Open lab: everyone knows who is getting tx/control

Parallel trial: at least regimens are tested simultaneously but pt’s are assigned to only one therapy

Crossover trial: pt’s receive each therapy in sequence and therefore serve as their own controls

Endpoint: measured to assess a drug’s effect

Surrogate endpoint: an outcome of therapy that predicts the real goal of therapy without being that goal

Question 32

Define investigational new drug (IND)

Answer 32

application process before beginning clinical trials; submission contains preclinical data from animal pharm and tox studies, manufacturing information and clinical protocols

Question 33

Define institutional review board (IRB)

Answer 33

the purpose of IRB review is to assure that appropriate steps are taken to protect the rights, safety and welfare of humans participating as subjects in the research; has the authority to approve, require modifications and to disapprove reeasrch; can be internal to each university/college or independent/commercial

Question 34

Define new drug application (NDA)

Answer 34

an application submitted by manufacturer of a drug to the FDA after clinical trials have been completed for a liscense to market the drug for a specified indication