7 Pharmacogenetics and Pharmacokinetics

Question 1

Why do patients show different responses ?

Answer 1

Pharmacodynamic response - IC50 or EC50 might differ between patients so dose of drug may need to be changed for an individual patient

Pharmacokinetics - Patients may need to be treated differently due to the difference in pharmacokinetic properties of drug in individual patients

Question 2

Genetic code is redundant ..

Answer 2

• several ways of coding for a particular amino acid, may be 4 codons coding for a specific amino acid

Question 3

SNPs

Answer 3

Single nucleotide polymorphisms

Question 4

•Synonymous SNP

Answer 4

both alleles of a gene encode the same polypeptide

Question 5

•Non-synonymous SNP

Answer 5

the alleles encode a different polypeptide

Question 6

Pharmacogenetics

Answer 6

The study of how individual patients’ genome affects their response to a drug

Question 7

Monogenic vs Polygenic

Answer 7

– Monogenic – a single gene affects the response

– Polygenic - multiple genes affect the response

Question 8

Succinylcholine (suxamethonium)
& butyrylcholinesterase G209A
polymorphism

Answer 8

Suxamethonium is a drug used to cause relaxation of muscle, facilitate surgery or introduce a tube into patients airway.
Acts as a depolarising blocker. Not degraded by acetylcholinerase. Drug has a prolonged activity at the nicotinic acetylcholine receptor, stimulates it to cause continuous release of calcium from SR until the SR is devoid of calcium and as a consequence there is no further muscle contraction.

-More rapid degradation Encoded by G209
-Slow degradation
Encoded by A209 (SNP)
Patients at risk of prolonged paralysis

Question 9

Isoniazid & N-acetyl transferase

Answer 9

Isoniazid used to treat TB
Major ADR – peripheral neuropathy

Some people acetylate faster than others

• Several different SNPs associated with the NAT2 gene •27 different alleles known
• “Fast acetylators”
• patients efficiently metabolize isoniazid
• “Slow acetylators”
• patients who slowly metabolize isoniazid, at greater risk of peripheral nerve damage
• NB other drugs are also metabolized by N-acetyltransferase 2 • eg procainamide (used to treat arrhythmia)

Question 10

Cytochrome P450 2D6

Answer 10

• Causes of patients with reduced 2D6 P450 activity
  • allele encodes enzyme with compromised catalytic activity
  • deletion of gene

– Patients with reduced 2D6 function:
• increased response to drugs that are metabolically inactivated by 2D6 (eg nortryptyline)
• decrease response to pro-drugs that are activated by 2D6 (eg codeine, tamoxifen) codeine gets converted to morphine and only then it has its analgesic properties, tamoxifen is used for breast cancer - partial agonist at oestrogen receptor, also activated by 2D6 genotype

• Patients with increased 2D6 P450 activity
  • multiple copies of 2D6
  • “ultrarapid metabolizers”
  • nortryptyline isn’t activated by 2DP but codeine is

Question 11

6 mercaptopurine & thiopurine S- methyl transferase (TPMT)

Answer 11

6 mercaptopurine
•Cytotoxic chemotherapy for leukaemia
•antimetabolite – inhibits synthesis of purine nucleotides so not enough purine available for cells to synthesise DNA

• Narrow therapeutic window
• Major ADR – myelosuppression •Metabolized by TPMT

Question 12

Thiopurine S-methyl transferase

TPMT

Answer 12

Exon - coding
Inton - non-coding

G460A Ala154Thr and A719G Tyr240Cys reduce the catalytic activity of TPMT, so will increase chance of patients suffering from myelosuppression

Question 13

TPMT & trimodal distribution

Answer 13

Trimodal distribution - inherit 2 copies of most genes and have 2 alleles of each gene which can be functional or non functional. If both alleles are functional the patient will have high TPMT activity. If only one allele is non functional then we see intermediate TPMT activity. Some patients may have 2 non functional alleles and so will have low TPMT activity

3 groups - classical mendelian genetics (almost) single gene

Question 14

Polygenic pharmacogenetics: Warfarin

Answer 14

Polygenic because multiple genes show variations that determine the patients response to warfarin

Warfarin used to prevent blood clots/ excessive bleeding

• Metabolized by Cyp 2C9
– Two variants substantially reduce Cyp enzyme activity
• Arg144Cys
• Ile358Leu

• VKORC
– vitamin k epoxide reductase
– target of warfarin
– different alleles encode variants of enzyme with different sensitivities to warfarin

Question 15

Polygenic pharmacogenetics:

cisplatin, carboplatin, oxaliplatin

Answer 15

•Platinum based compounds used as chemotherapy for cancer which platinate DNA and prevent replication
consequence - cells can no longer go through the cell cycle as DNA cant be replicated and therefore the cells undergo apoptosis

• Cleared by conjugation to glutathione.
- glutathione is a tripeptide as it contains 3 AA’s (glutamate, cysteine and glycine)

• There are several enzymes (genes) involved in clearance of platinum compounds
• Many of these have several polymorphisms
– Eg Glutathione S transferase
• There are >8 isoforms of GST
• The genes encoding these may have no, one or both allele deleted
• Each allele may have several SNPs

Key point of last slide - Significant complexity!

Question 16

Genome wide association studies

Answer 16

Clinical trial subjects receive drug

• Subjects who respond to drug
• Subjects who don’t respond to drug

Genome wide comparision of DNA of responders and non-responders

Identify key genetic differences that are associated with the response

Confirm that these factors predict response in 2nd cohort

Clinically useful biomarker to predict response

Question 17

Example of genome wide association study: Statins and

OATP1B1

Answer 17

•Statins used to reduce
cholesterol –inhibit synthesis in liver
•Major ADR is myopathy (muscle pain)
• Patients with T521C SNP in mRNA sequence more likely to experience myopathy

OATP1B1 removes statins from the blood and takes them into the liver

Question 18

OATP1B1 T521C SNP: model

Answer 18

• C at 521 impairs uptake into hepatocytes (site of action of statin)
• Greater AUC
• Greater accumulation of drug in muscle tissue
• Both alleles need to be affected (i.e. C)for significant myopathy

T521C prevents drug from being pumped into the liver (doesn’t get metabolised either) and goes straight into the systemic circulation and gets into the muscle and causes myopathy (muscle pain)