Pharmacogenetics

Question 1

What is the definition of Pharmacogenetics?

Answer 1

• Refers to the study of inherited differences in drug metabolism and response
• Single gene/phenotype

Question 2

What does Pharmacogenetics aim to do?

Answer 2

Links differences in gene structure (polymorphisms) to drug metabolism and response. Genetic Variation (genotype) has interaction with drug metabolism and response (phenotype)

Question 3

What are pharmacogenetic consequences on pharmacokinetics?

Answer 3

Decreased first-pass effect

• Greater bioavailability
• Higher peak concentration

Reduced parent drug elimination

• Longer half-life
• More/fewer active metabolites

Altered concentration-effect relationship between poor and good metabolisers

Question 4

How can Pharmacogenetics be applied?

Answer 4

• Drug development: More targeted, more powerful drugs
• Individualisation of therapy: The right drug and the right dose for every patient
• Predicting ADRs (& reducing mortality)
• Defining susceptibility
• Identifying potential addiction
• Reducing cost of health care
• Don’t treat non-responders (stratification)
• Don’t treat those most susceptible to toxicity (stratification)
• Adjust dose to maximise efficacy while avoiding toxicity

Question 5

What are clinical applications of Pharmacogenetics?

Answer 5

• Anti-coagulation: Warfarin
• Psychiatry: Tricyclic anti-depressants, Atomoxetine
• Oncology: Thiopurines (TPMT), 5-fluorouracil, Herceptin (Her-2/neu), Tamoxifen (CYP2D6/ABCC2), Cetuximab (KRAS)
• Cardiovascular: Statins, Clopidogrel
• Pain control: Codeine, methadone
• Epilepsy: Phenytoin, Carbamazepine
• Infection: Abacavir

Question 6

What s the effect of Pharmacognetics on Cholinesterase?

Answer 6

Enzyme required to breakdown short acting muscle relaxant suxamethonium or mivacurium

Genetic variants result in prolonged paralysis requiring ventilation support

Determine patient’s phenotype, Family studies

• UU phenotype is usual/normal activity
• UA and UF may have increased risk
• FF, FS and AF intermediate sensitivity

Question 7

What are feature of Thiopurine Drugs?

Answer 7

• Used in the routine treatment of inflammatory and autoimmune diseases
• Highly efficacious but ADRs are common
• TPMT testing used routinely to tailor individual drug dose
• Steroid sparing reagent

Question 8

What are the TPMT genetics?

Answer 8

• TPMT coded by 27 kb gene, 10 exons, located on 6p22.3
• Wild-type is TPMT*1
• Very low activity associated with ~41 variants, most common = TPMT*3 (80-90%)

Question 9

How is TPMT tested?

Answer 9

Phenotype testing is first-line test

Genotype testing appropriate in some patient groups

• Children with acute lymphocytic leukaemia

Recommended in NICE guidelines for Crohn’s disease, Recommended in BNF

TPMT phenotype/genotype can prospectively identify

• Deficient patients at risk of life-threatening toxicity
• Heterozygotes who will respond well to low-dose aza therapy
• Patients with high TPMT activity who will need high doses from the start

Question 10

What is the epidemiolgy of Thiopurine drugs?

Answer 10

In caucasians:

• 1 in 300 - Very low TPMT activity
• 6 – 11% - Intermediate TPMT activity
• 2% - Ultra high TPMT activity

Very low TPMT activity can lead to potentially fatal myelosuppression when treated with standard drug doses

Question 11

Describe how dosing of Azathioprine and 6-MP is affected by Pharmacognenetics?

Answer 11

Phenotype

AZA Dosing

6-MP Dosing

Normal/High Activity

Start with normal starting dose (2-3 mg/kg/d) then disease specific guidelines

Start with normal starting dose (1.5 mg/kg/d)

Intermediate Activity

Start at 30-80% of target dose (e.g. 1-1.5 mg/kg/d) and titrate to tolerance

Start at 30 – 80% of target dose (e.g. 0.75 mg/kg/d)

Deficient Activity

Consider Alternative agents. If using AZA drastically reduce dose to 10% of target and only give thrice weekly

For non-malignant conditions consider alternative non-thiopurine immunosuppressant therapy

Question 12

What are enzymes of Drug metabolism?

Answer 12

Phase 1 (oxidative) – SER

• CYP 1A2, CYP2B, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4,NADPH-quinone oxidoreductase

Phase 2 (conjugative) - cytosol

• Glutathione S-transferase, N-acetyltransferase, UDP-glucuronosyltransferase, Sulphotransferases

Question 13

What are cytochrome P450 enzymes?

Answer 13

• Superfamily of enzymes that catalyze the metabolism of drugs and other substances
• Endogenous substrates of CYPs include eicosanoids, estradiol, arachidonic acids, cholesterol, vitamin D and neurotransmitters
• Genetic polymorphisms of CYPs may affect the enzyme catalytic activity
• Association with both disease states and ADRs
• 57 CYP genes and 58 pseudogenes arranged into 18 families and 43 subfamilies in man

Question 14

What are types of Cytochrome P450 enzymes?

Answer 14

CYP2D6

• Debrisoquine, Tricyclics, Antipsychotics, SSRI’s, Anti-arrhythmics, Anti-hypertensives, Morphine derivs

CYP2C19

• Sedatives, Barbiturates, Tricyclics

CYP3A4

• Tricyclics, Antipsychotics, SSRI’s, Sedatives

CYP1A2

• Tricyclics, Antipsychotics, SSRI’s, Sedatives

CYP2C9

• Antiepileptics, Anticoagulants

Question 15

What are features ofWarfarin?

Answer 15

• Narrow therapeutic window requires careful monitoring and strict compliance
• Multiple clinically important drug interactions and erratic safety profile
• 10-fold variability around dose, target INR and side-effects
• Polymorphisms account for 30-50% of the variability in dosing
• Very commonly prescribed for prophylaxis and treatment of thromboembolic disorders, atrial fibrillation and systemic embolism after MI

Question 17

What are is involved in Warfarin paharmacogenetic testing?

Answer 17

• CYP2C9
• Target of warfarin inhibition
• Converts vit K epoxide to quinone (vit K recycling)
• Deficiency (1639 G>A) means less warfarin target and smaller dose requirement
• Reduction in hospitalisations when starting doses are informed by the patient genotypes VKORC1/CYP2CP (demonstrated in 2010)

Question 18

What are features of CYP2D6?

Answer 18

​Most extensively characterised polymorphic drug-metabolising enzyme (20-25% of all clinically used drugs)

• more than 75 allelic variants described
• more than 15 encode an inactive enzyme or no enzyme at all
• other alleles encode enzyme with reduced, normal or increased enzyme activity

Question 19

What are categories for phenotyping of metabolism of psychoactive drugs?

Answer 19

• Poor metabolizers (PM)
• Intermediate metabolizers (IM)
• Extensive metabolizers (EM)
• Ultrarapid metabolizers (UM)

Question 20

What is the KRAS Gene?

Answer 20

• Important role in cell growth and tumour development
• Gene can be mutated or normal in colorectal ca cells
• If KRAS is mutated, then anti-EGFR therapies such as cetuximab are not effective and should not be used
• KRAS gene mutations occur in about 40% of colorectal ca. patients
• Patients diagnosed with metastatic colon ca. should be tested for KRAS mutation status to determine eligibility for anti-EGFR Rx

Question 21

What is Hercptin?

Answer 21

• HER2 gene associated with 25-30% of early-stage breast cancers
• Overexpression of HER2 is associated with enhanced tumour aggressiveness
• Herceptin (Trastuzumab) is a monoclonal antibody against HER2
• Only effective in patients with tumours that overexpress HER2

Question 22

What is the reason for slow uptake of Pharmacogenetics?

Answer 22

• Lack of good phenotypic data or access to appropriate patient samples
• Lack of participation by pharma
• Lack of technological and methodological tools for PGx discovery
• Lack of good evidence base for clinical use
• Lack of interest/competence on part of clinicians to use information
• Lack of timeliness of results

Question 23

What is Genome wide association studies?

Answer 23

• Possible due to technology advances
• GWAS are hypothesis-free
• Many genetic variants associated with various drug responses and adverse effects e.g. Genetic risk variant for myopathy in patients treated with simvastatin