Pharmacogenetics

Question 1

Differentiate between pharmacogenetics and pharmacogenomics.

Answer 1

Pharmacogenetics is the study of how variations in a single gene affect an individual’s response to a specific drug
while
Pharmacogenomics is the study of how the entire genome (the complete set of genes) influences an individual’s response to drugs.

Pharmacogenetics: Single gene
Pharmacogenomics: Whole genome and complex gene interactions.

Pharmacogenetics: Individual genetic differences in drug response.
Pharmacogenomics: Broader genetic influences, considering entire populations and their genetic diversity.

Question 2

Mention 4 discoveries that spurred interest in pharmacogenetics.

Answer 2

1. Abnormal (low) butyrylcholinesterase in psychiatric patients who showed prolonged muscular paralysis after administration of succinylcholine prior to electrotherapy
2. Haemolysis in African American males with G6DP deficiency when treated with primaquine
3. The identification of the proportion of slow acetylators in certain ethnic groups
4. Attribution of peripheral neuropathy to slow acetylation of isoniazid due to genetic diversity in the enzyme N-acetyltransferase.

Question 3

Mention 3 genetic differences in drug metabolism.

Answer 3

1. Genetic polymorphism
2. Phase I and II metabolism
3. P-glycoprotein (P-gp)

Question 4

Genetic polymorphism categorises individuals into four (4) different metaboliser phenotypes which are:

Answer 4

1. Poor metabolisers (PM)
2. Intermediate metabolisers (IM)
3. Extensive metabolisers (EM)
4. Ultrarapid metabolisers (UM)

Question 5

What is the clinical implication of the metaboliser phenotypes?

Answer 5

PM-IM: Prodrugs are metabolised slowly into active metabolite; may lead to accumulation of prodrug.
Active drug is slowly metabolised to inactive metabolite, with potential for drug accumulation. Lower dosage of medication is required.

UM: Prodrug is metabolised quickly to active drug.
Active drug is quickly metabolised to inactive metabolites, leading to potential therapeutic failure.
Higher dosage of active drug may be required.

Question 6

Briefly discuss Phase I & II metabolism as a genetic difference in drug metabolism.

Answer 6

Drug metabolism takes place mostly in the liver and is divided into two major categories, phase I (oxidation, reduction, and hydrolysis reactions), which is largely mediated by CYP450 enzymes, and phase II metabolism (conjugation reactions).

Phase I metabolism enzymes are responsible for approximately 59% of the adverse drug reactions.

With a total of 57 genes encoding for CYP450 enzymes, specific CYP450 Enzymes such as CYP2D6, CYP2C9, and CYP2C19 are highly polymorphic and account for upward of about 40% of hepatic phase I metabolism.

Question 7

What is P-glycoprotein (P-gp) ?

Answer 7

It is a membrane-bound, ATP-dependent transport system responsible for the efflux of a variety of xenobiotics from cells to the extracellular fluid, usually against their concentration gradient.
The more the expression of P-gp, the greater the efflux potential of xenobiotics such as anticancer drugs.

Question 8

Mention 4 substrates of P-gp.

Answer 8

i. Carvedilol
ii Diltiazem
iii. Digoxin
iv. Phenobarbital

Question 9

Mention 3 inhibitors of P-gp.

Answer 9

i. Carvedilol
ii Diltiazem
iii. Phenytoin

Question 10

Highlight some of the CYP450 isoenzyme polymorphisms as well as their drug substrate and clinical implication.

Answer 10

1. CYP2D6| Tamoxifen (used to treat breast CA)| CYP2D6 catalyses the metabolic conversion to its active form endoxifen. This results in variable therapeutic outcomes to treatment of estrogen-sensitive cancers.
2. CYP2D6| Codeine |Codeine is converted to the active form morphine.

Infants who are breastfed milk by ultrarapid metaboliser (UM) mothers on codeine results in excessive codeine activation to morphine (lipophilic) which may result in fatal respiratory depression.

3. CYP2C9/ CYP2C19 | S-Warfarin | The presence of CYP2C9 mutations is associated with a reduction in the metabolism of S-warfarin.

Clinically, warfarin maintenance dosing requirements are lower in patients with CYP2C92 polymorphisms and further reduced in patients with CYP2C93 variants.

Question 11

What are the clinical implications or benefits of pharmacogenetics?

Answer 11

1. Adverse Drug Reactions
   One benefit of understanding pharmacogenomics is the possibility of a decrease in the number of ADRs. The polymorphic forms of CYPP450s are responsible for the development of idiosyncratic ADRs. Also 56% of drugs cited in ADR studies are metabolized by polymorphic phase I enzymes, of which 86% are P450s.
2. Pharmacogenetic Testing prior to prescribing and advanced screening for disease: The pharmacogenetic tests mentioned on drug labels can be classified as “test required,” “test recommended,” and “information only.”
3. Decrease in the overall cost to the health-care system caused by ineffective drug therapy.
4. Selective and potent drugs for therapy.
5. Accurate methods of determining appropriate drug dosages

Question 12

Currently, four drugs are required to have pharmacogenetic testing performed before they are prescribed. Name them.

Answer 12

1. Cetuximab
2. Trastuzumab
3. Maraviroc
4. Dasatinib

Question 13

What are the pharmacogenetic biomarkers for the drug labelled for Required Tests?

Answer 13

1. KRAS gene mutation test is perfomed for Cetuximab, Adagrasib and Sotorasib.
   [Cetuximab, a monoclonal antibody targeting epidermal growth factor receptor (EGFR), is ineffective in patients with KRAS mutations due to continuous activation of KRAS protein independent of EGFR. Thus, KRAS mutation status is used to exclude such patients from Cetuximab therapy. Conversely, Sotorasib and Adagrasib inhibit the mutated KRAS protein, making them effective in treating cancers with KRAS mutations, such as non-small cell lung cancer.
2. HER2 Receptor positive test is performed for Trastuzumab.
   [ In HER2-positive individuals, gene amplification leads to high HER2 expression, promoting uncontrolled cell proliferation and aggressive breast cancer. Trastuzumab, a monoclonal antibody targeting HER2, inhibits its function and triggers immune-mediated cancer cell destruction. HER2-positive status is a biomarker for Trastuzumab therapy eligibility.]
3. CCR5-tropic HIV-1 test for Maraviroc
   [ Maraviroc is effective against strains of HIV-1 that use the CCR5 co-receptor to enter and infect host cells. Before initiating maraviroc therapy, a tropism test is performed to determine if the HIV-1 strain in the patient is CCR5-tropic. This ensures that maraviroc will be effective]
4. Philadelphia chromosome positive for Dasatinib.
   [A genetic biomarker that qualifies a patient for the use of Dasatinib in acute lymphoblastic leukemia]

Question 14

What are the pharmacogenetic biomarkers for the drugs labelled for Recommended Tests?

Answer 14

1. HLAB*1502 (Human leukocyte antigen B) for Carbamazepine
   [use of carbamazepine in patients with this genetic biomarker will produce skin rashes (ADR) associated with Steven-Johnson Syndrome.]
2. . CYP2C9 variants/VKORC1 variants for Warfarin
   [variants of CYP2C9 reduce the enzyme’s activity, slowing warfarin metabolism, leading to higher blood levels and increased bleeding risk. These patients need lower warfarin doses. VKORC1, the target of warfarin, is crucial for blood clotting. VKORC1 variants result in lower enzyme expression, increasing sensitivity to warfarin.]
3. TPMT (Thiopurine methyl transferase) variants for Azathioprine
   [Azathioprine is a prodrug converted into 6-mercaptopurine (6-MP) in the body. TPMT catalyzes the methylation of 6-MP into inactive metabolites. Genetic polymorphisms in the TPMT gene can result in decreased or absent enzyme activity, leading to a high risk of severe toxicity.]
4. G6PD deficiency for Primaquine and Rasburicase
5. CYP2D6 for Tamoxifen and Codeine

Question 15

List the tests for drugs labelled “For Information Only”.

Answer 15

1. CYP2C9 variants for Celecoxib
2. N-Acetyl transferase (NAT) variants for Isoniazid and Rifampin
3. CYP2D6 variants for Tamoxifen, Fluoxetine and Atomoxetine