Midterm #1 (10/14 - 10/19)

Question 1

Antagonists and withdrawal

Answer 1

Antagonist increases total number of receptors: “overshoot” phenomena upon drug withdrawal. Response to physiological concentrations of agonist becomes exaggerated.

Question 2

Agonists and withdrawal

Answer 2

Agonist causes receptor down-regulation: upon agonist withdrawal too few receptors to produce effective stimulation by endogenous agonist. Example is clonidine an agonist at the alpha-adrenergic receptor that reduces blood pressure. Drug withdrawal can cause hypertensive crisis.

Question 3

Drug threshold

Answer 3

there is a minimum number of receptors that must be occupied before any drug effect is detectable.

On the receptor occupancy vs. maximal effect curve, you see effect when some % of the receptors become bound.

Example: Inhibition of the fast step in a biological process. No effect of inhibition will be seen until enough drug has been added so that the fast step becomes the rate limiting step.

EC50 > Kd and drug potency decreases.

Question 4

Receptor desensitization

Answer 4

Frequent or continuous exposure to agonists often results in a reduction in receptor response. This is called tachyphylaxis or desensitization. It is a mechanism to prevent over-stimulation that could lead to cellular damage or other adverse effects. But it can also lead to the development of drug tolerance, which is a more gradual process.

Question 5

PK tolerance

Answer 5

change at the drug level
1. The amount of drug that reaches the site of action is reduced due to either decreased absorption, decreased penetration to site of action, increased metabolism, or increased clearance.

2. Metabolic tolerance: continuous drug exposure induces synthesis of enzymes that degrades or inactivates the drug. Example is ethanol inducing alcohol dehydrogenase which is the rate limiting enzyme in the oxidative pathway for ethanol metabolism.

Question 6

PD tolerance

Answer 6

change at the receptor level. Either through receptor uncoupled from the signaling pathway or total number of receptors in the system is decreased.

Examples include nicotinic acetylchole, beta-adrenergic GPCR and dopamine. Receptor down-regulation, stress-induced are other mechanisms of PD tolerance

Question 7

Example PD tolerance: nicotinic acetylcholine

Answer 7

Desensitized state is caused by prolonged exposure to the agonist, ACh. The receptor undergoes a conformation change that blocks the responsiveness of the receptor to agonist binding.

Question 8

Example PD tolerance: beta-adrenergic receptor (GPCR)

Answer 8

tolerance to agonist (cAMP) over time.

upon addition of agonist you see a response but desensitization occurs quickly.

Once agonist is removed from system re-sensitization occurs. If not enough time is allocated to re-sensitization in the absence of the agonist, a decreased response will occur with agonist binding.

When beta-arrestin is phosphorylated, the GPCR can’t activate the AC, the receptor is internalized and either degraded in continued agonist signaling or moved to the membrane again after re-sensitization.

Question 9

PD tolerance: receptor down-regulation

Answer 9

occurs after continuous exposure to agonist and is a long-term reduction in receptor numbers. Mechanism is increased degradation of receptor or decreased synthesis of the receptor.

Question 10

PD tolerance: stress-induced

Answer 10

osmotic stress, UV light irradiation, acute psychological stress. Example is the med students and they measured levels of adrenergic receptor binding before and during finals week. During finals week, you produce so many catecholamines that you desensitize the receptor

Question 11

PD Example: Dopamine and Amphetamines

Answer 11

Dopaminergic neurotransmission. dopamine is cleared from the synapse by re-uptake transporters. These regulate the system by avoiding constant excitation.

Dopamine is required for downstream stimulatory effects of amphetamines. Amphetamines competitively inhibit the DA transporter and interferes with VMAT function and filling of synaptic vesicles with DA.

There is an increase in cytoplasmic DA which reverses the direction of DAT and increases extracellular DA.

With continuous use, the effects of amphetamines are decreased because DA loss and depletion of DA in cell.

Question 12

Cross-tolerance

Answer 12

state of decreased responsiveness to one drug resulting from repeated administration of another drug

Question 13

Quantal drug effects

Answer 13

Choose a desired magnitude of effect and treat it as an all-or-none effect. Then for each drug dose you either achieve the desired magnitude or you have no effect (all-or-none). Plotted on log(D) vs response where response is either zero then goes to 100% at a give dose.

Question 14

frequency distribution

Answer 14

gaussian distribution. Used to determine the minimum dose required to produce the specified effect for each member of the population.

Question 15

Quantal Dose-response curves

Answer 15

frequency distribution describing population. ED50 is the effective dose at which 50% of the subjects respond and is the average effective dose in the population. On the cumulative curve you can still measure ED50

Question 16

Variations of the quantal dose-response curve

Answer 16

shifted to the right means less potent.

Broader curve means more variable dose to get desired effect. On a cumulative curve the steeper the slope the less varied.

You will see a population insensitive to a drug if the curve does not reach cumulative 100% responding.

Question 17

Endogenous receptor ligand concentration example: propranolol

Answer 17

competitive antagonist at beta-adrenergic receptor to slow heart rate of patients with elevated catecholamines. However, it has no effect on the high heart rate of over-trained marathon runners because they have reduced circulating catecholamine levels

Question 18

Endogenous receptor ligand concentration example: Saralisin

Answer 18

weak partial agonist at the angiotensin II receptor to lower blood pressure in hypertensive patients cause by increased angiotensin II production.

However, it raises blood pressure in patients with normal angiotensin II levels because there is less angiotensin binding and the addition of the partial agonist causes increased binding.

Question 19

3 methods to have beneficial and toxic effects of a drug

Answer 19

1. Drug activates a receptor which activates an effector to activate toxicity and benefit. Example is warfarin, which is hard to avoid toxicity because it is a result of what you want the drug to do
2. Drug activates two separate receptors, each activating separate effectors. One effector causes toxicity the other benefit. Example is beta-blockers. Activation in the lungs causes toxicity and activation in the heart causes benefit.
3. Drug activates one receptor which activates two effectors. One effector causes toxicity and the other benefit. Example is MAOI which activates the monoamide oxidase receptor. Tyramine effector causes toxicity and serotonin effector causes benefit.

Question 20

Therapeutic Index

Answer 20

1. describes the relative safety of a drug in a population = dose undesired effect/dose desired effect
2. Commonly use TD50/ED50. Measure both on a cumulative quantal dose-response curve
3. The wider the TI the safer the drug, and the less steep the slope for adverse effects is the more desirable because if you increase the drug dose a smaller amount of people will experience the adverse effect.
4. The higher the TI, the safer the drug. If TI is

Question 21

Therapeutic window

Answer 21

range of drug dosages that can be used effectively for treatment while staying within the safety range. Range is from minimum effective dose to minimum toxic dose.

Question 22

Pharmacogenetics

Answer 22

Study of the genetic basis for the variation in drug responses. Usually involves single genes with large single variant effect.

Question 23

Pharmacogenomics

Answer 23

The use of genomic methods to assess how variates in the human genome affects the response to drugs. Involves many variants in many genes causing smaller effects.

Question 24

Monogenic

Answer 24

activity determined by a single gene

Question 25

Multigenic

Answer 25

activity influenced by many different genes

Question 26

Goal of pharmacogenetics and pharmacogenomics

Answer 26

genotype patients in a population with a given disease to predict which patients will respond to therapy based on their genotype and whether they carry a marker for the drug toxicity or no response.

Question 27

1000 Genomes Project

Answer 27

Announced in 2008 with the intent to have an international effort to produce a public catalog of human genetic variations. Goal was to sequence 2500 unidentified people from 25 populations around the world, it was published in October 2015 in Nature.

Question 28

concerns of pharmacogenetics and pharmacogenomics

Answer 28

1. Cost: Costs a lot to generate and store a patient’s genetic information. Is it worth the cost?
2. Ethics: in maintenance and use of the data. Who has access, who owns and controls the information (patient or researcher privacy issues), reproduction issues, and commercialization of data

Question 29

Pharmacogenetic Approach

Answer 29

1. Pre-genomics: use an approach called forward genetics that involves the phenotype-to-genotype. Identify a phenotype to a drug based on how patients respond to the drug. Have “normal” individuals and “outliers” for a given drug response. Then we take genetic comparison in these two groups.
2. Post-genomics: Reverse genetics and is a genotype-to-phenotype approach. Identify differences in genomes between individuals and assess contribution to variability in drug response.

Question 30

Polymorphisms

Answer 30

defined as a variation in DNA sequence that occurs at a frequency greater than 1% in a population.

Question 31

Single Nucleotide Polymorphisms (SNPs):

Answer 31

1. Single base pair change that occurs every 300-1000 nucleotides. About 10 * 10^6 SNPs/person. Have to occur in coding region of genes.Two types:
2. Nonsynonymous: different amino acid (missense) or stop codon (nonsense)
3. Synonymous: produces same amino acid. Example is ABCB1 which is a Pg-p, still produces same codon but affects transport of protein to membrane and may result in less protein made. So, still may see a different phenotype.
4. Insertions/deletions (Indels): change amino acids, introduce extra amino acids or remove amino acid or can shorten a protein. Causes a frameshift, unless you insert/delete a multiple of 3.

Question 32

Phenotype-driven Terminology

Answer 32

1. Autosomal: not located on a sex chromosome
2. Recessive: phenotype only evident when gene is altered on both chromosomes
3. Dominant: phenotype evident when gene is altered on only one chromosome
4. Co-dominant: phenotype is intermediate with one altered and one normal gene on the chromosomes

Question 33

G6PD deficiency

Answer 33

First examples of pharmacogenetics.

X-linked monogenetic trait

Development of severe hemolytic anemia after treatment with many anti-malarial drugs.

Question 34

G6PD and African American Troops

Answer 34

In 1950’s there was large-scale use of primaquine on US troops, some developed jaundice and anemia and it was more common among African American troops.

Theory is that African Americans have lower levels of G6PD because the organism responisble for Malaria cannot replicate as quickly in an environment with low G6PD, so the mutation is advantageous to people who come from regions with high malaria rates.

Question 35

Drug Induced Acute Hemolytic Anemia

Answer 35

Linked to markedly reduced G6PD activity in red blood cells. There are 187 known mutations in G6PD but none cause complete loss in enzyme activity

RBC depend on G6PD to produce the reducing agent NADPH

Low G6PD activity makes RBCs very sensitive to oxidizing drugs, like Primaquine, which leads to cell lysis.

Question 36

G6PD experiment in prisoners

Answer 36

Later in 1962, human volunteers in a penitentiary where given primaquine daily. They found that the % of hematocrit decreased after about 1 week, % reticulocytes (immature RBC) increased and the half life of RBC was markedly decreased compared to normal.

Question 37

Pharmacogenetics in Clinical Practice.

Answer 37

Requires 3 primary types of evidence

1. Screens of multiple human tissues linking the polymorphism to a trait
2. Complementary preclinical functional studies indicating the polymorphism is possibly linked to a phenotype.
3. Multiple supportive clinical phenotype/genotype association studies

Question 38

Oncology TPMT Example

Answer 38

TPMT = thiopurine methyltransferase. TPMT is the main metabolizer of chemotherapeutic drugs such as 6-MP (6-mercaptopurine) and azathipurine which are used to treat blood cancers, mainly childhood leukemias.

Thiopurine drugs are converted into thioguanine nucleotides which are cytotoxic to cells.

Deficiency in TPMT activity leads to severe hematopoetic toxicity associated with treatment and potential mortality.

In pharmacogenetic tests they found that patients homozygous for the variant required larger dose reductions compared to the wild-type but overall incidence of relapse was similar to the wild-type.

Question 39

FDA approved pharmacogenetic tests

Answer 39

1. TPMT gene for 6-MP due to toxicity
2. CYP2D6 gene for tamoxifen due to decreased efficacy
3. UGT1A1 gene for irinotecan due to toxicity
4. CYP2D6 gene for codeine due to ineffective analgesia

Question 40

Oncology tamoxifen example

Answer 40

tamoxifen needs to be converted to endoxifen to be active. Conversion is catalyzed by the polymorphic enzyme CYP2D6.

There are EM, IM and PM when considering CYP2D6.

Efficacy of tamoxifen is likely low in 6-105 of European population due to deficiency in enzyme activity.