D13 - Pharmacokinetics

Question 1

Pharmacogenetics definition

Answer 1

examines the effect of genetic differences in a single gene on drug response

Question 2

Pharmacogenomics definition

Answer 2

examines the effect of genetic differences in multiple genes across the genome on a drug response

Question 3

drug problems can be caused by variations in

Answer 3

• Pharmacokinetic - too much or too little drug at site of action
  
  • Pharmacodynamic - increased or reduced effect from a given concentration
  • Idiosyncratic - abnormal reaction in a few exposed individuals
  • Other - age, gender, body mass, diet, disease, other drugs etc.

Question 4

Pharmacogenetics

Answer 4

• Genes may influence pharmacokinetics by
  ○ Altering expression or function of proteins involved in absorption, distribution, metabolism or excretion (ADME) of drugs
  
  • Genes influence pharmacodynamics by
    ○ Altering expression or function of drug targets (receptors) or their downstream pathways

Question 5

Variability of drug response - key terms

Answer 5

• Beneficial or therapeutic response
  
  • Partial response
    ○ Need an increased dose for a sufficient response
  • Non-responsive
  • Full adverse response

Question 6

monogenic

Answer 6

allelic variation in a single gene

Question 7

Polygenic

Answer 7

allelic variation in greater than or equal to 2 genes

Question 8

Polymorphic

Answer 8

monogenic gene variant occurring at a population frequency of >1%

Question 9

Two types of genetic mutations underpinning variations

Answer 9

• Single base mutation - substitutes one nucleotide for another
  ○ Single nucleotide polymorphism (SNP)
  
  • Insertion or deletion of more than one nucleotide
    ○ Tandem repeat polymorphism = copy number variations (CNV)
    ○ Insertion/deletion polymorphism (indel)

Question 10

SNPs

Answer 10

single nucleotide polymorphism
- More than 3 million in the human genome
- Arise as a mutation and persist because non functional (>1%)
- Frequency can even increase because advantageous eg. G6PD & malaria resistance
- May lead to ethnic differences in SNP frequencies
- Site specific effects of SNPs
○ If there are mutations in upstream regulatory regions that can lead to either increase or reduction of expression in a protein
○ Mutations in the coding regions will lead to proteins that may have increased, similar, reduced or no activity of that particular enzyme
§ If there is a major functional disruption on the activity of the enzyme it leads to a monogenic disorder

Question 11

Favism

Answer 11

• First described by Pythagoras in 510BC
  
  • After eating fava beans (favism), some people became jaundiced and had red urine, sometimes fatal in children
  • Common in Mediterranean area - endemic malaria
  • 1956 - alving et al. showed it was due to a deficiency in glucose-6-phospahte dehydrogenase
  • G6PD gene is highly polymorphic - >400 variants
  • Most are silent but about 30 decrease activity
  • X-linked recessive - so symptomatic individuals almost always men
    ○ Unless the women has inherited defective alleles on both sides

Question 12

G6PD enzyme and pentose-phosphate pathways (hexose monophosphate shunt)

Answer 12

• First reaction in pentise phosphate pathway
  
  • Generates NADPH to provide reducing power in cells via reduced glutathione
  • (also generates 5-carbon sugars and ribose-5-phosphate)

Question 13

G6PD deficiency

Answer 13

• RBCs carry O2 - risk of free radical damage
  
  • Rely on G6PD/NADPH/glutathione for protection from oxidative damage
  • G6PD deficiency - haemolysis during O2 stress, eg. Infection and fava beans
    ○ Fava beans contain oxidative substances that cause problems with oxidation of haemoglobin
  • Drugs also create oxidative stress
    ○ Eg. Rasburicase - converts uric acid to allantoin and hydrogen peroxide

Question 14

• Primaquine

Answer 14

○ Modern day favism
○ Anti-malarial drug developed and used to treat all US troops in Korea (endemic malaria)
○ Mostly well tolerated but some (esp. African-Americans) developed jaundice and anaemia
○ Oxidised in the liver and oxidised metabolite converts Hb (Fe2+) to MetHb (Fe3+)
○ Inherited G6PD deficiency
§ Increases MetHb
§ RBCs susceptible to haemolysis
○ >2 dozen drug cause haemolytic anaemia in G6PD-deficient patients - sulphonamides, nitrofurans, NSAIDs, anti-malarials

Question 15

Succinylcholine/suxamethonium chloride

Answer 15

○ Neuromuscular blocking agent used to paralyse patients for surgery / intubation
○ Normally rapidly inactivated by plasma cholinesterase
○ 1/3500 people are homozygous for decreased cholinesterase activity
○ Prolonged post-operative respiratory paralysis
○ Toxicities associated with must commoner defects in drug metabolism subsequently explored

Question 16

Acetylation and drug metabolism

Answer 16

• Isoniazid - first line anti-TB drug, eliminated by acetylation
  
  • N-acetyltransferase (NAT2) is polymorphic
    ○ wild type (R
    ○ mutant variant which is deficient in acetylation activity (r
    ○ Slow (rr)
    ○ Intermediate (Rr)
    ○ And fast (RR)
  • Slow acetylators - hepatotoxicity and peripheral neuropathy
    ○ British/Indian 60%, Chinese 20%, Japanese 10%
  • Fast acetylators - lower drug bioavailability, poor response
  • NAT2 affects sulphonamides (antibiotics), procainamide (anti-arrhythmic), hydralazine (anti-hypertensive) and many other drugs

Question 17

N-acetyltransferase (NAT2) is polymorphic

Answer 17

○ wild type (R
		○ mutant variant which is deficient in acetylation activity (r 
		○ Slow (rr)
		○ Intermediate (Rr)
		○ And fast (RR)

Question 18

Drug metabolism

Answer 18

• Metabolism usually converts drugs to metabolites that are more water soluble and thus more easily excreted
  
  • Drugs can convert pro drugs (eg. Codeine, tamoxifen) to active drugs
  • Can produce toxic metabolites (eg. Acetaldehyde)
  • Patients with
    ○ Increased concentration and increased half life
    § Decreased drug metabolism and increased adverse effects
    ○ Decreased drug concentration and decreased half life
    § Increased drug metabolism and reduced therapeutic effects
  • Eg. Succinylcholine, isoniazid, codeine, SSRIs etc. - but not just prescription drugs

Question 19

Alcohol metabolism

Answer 19

• Metabolised by alcohol dehydrogenase (ADH) to a toxic and carcinogenic metabolite, acetaldehyde
  
  • Aldehyde dehydrogenase 2 (ALDH2) then converts acetaldehyde to acetate
  • If the ADH pathway is saturated acutely or chronically, CYP2E1 activity in increased to metabolise the excess alcohol
  • ADH, ALDH2 and CYP2E1 polymorphisms exist in varying ethnic frequencies
  • Functional ADH2 and ALDH2 polymorphisms in many east Asians
    ○
    ○ Rapid acetaldehyde causes increase flushing syndrome
    ○ Protective against alcoholism
    ○ Increased acetaldehyde over time carcinogenic (upper GIT)

Question 20

TPMT and Thiopurines

Answer 20

• Azathioprine and mercaptopurine
  ○ Leukaemia and immunosuppressant drugs (azathioprine is a pro-drug of 6MCP)
  ○ Both are metabolised by TPMT - thiopurine S-methyl transferase
  ○ TPMT is polymorphic = enzyme may be high activity (H) or low/no activity (L)
  ○ 1:300 are homozygous TPMT/TPMT no TPMT activity - drug levels increase - potentially fatal myelotoxicity
  ○ Heterozygotes
  § Decreased activity
  § Nausea
  § Myelosuppression

Question 21

Drug target (pharmacodynamic polymorphisms

Answer 21

• Genetic variation in drug targets (eg. Receptors) affects efficacy
  
  • PD effects best explained by comparison with PK effects

Question 22

B2 adrenoceptor, asthma, and B2 agonists

Answer 22

• ADRB2 - B2 adrenoceptor gene
  
  • B2 agonists - bronchodilation
  • Salbutamol - asthma
  • ADRB2 polymorphism affect
    ○ Receptor expression levels
    ○ Downstream signalling
    ○ Clinical responses to B2 agonists
  • Arg16/Arg16 polymorphism (increased in African Americans)
  • Increase in acute response with 1 dose
  • Decrease in peak expiratory flow with regular salbutamol dosing

Question 23

Warfarin

Answer 23

• Oral anticoagulant with highly variable dosing to achieve therapeutic anticoagulation - up to 20x differences in patient dose requirements
  
  • Inhibits Vit K epoxide reductase - VKORC1 is a subunit
  • Only 50% patients in correct INR range
  • Serious adverse effects - bleeding (INR > 4.5) or clotting (<2)
  • Pharmacogenetics
    ○ PD - VKORC1 - 1639G > A allele - 2x decrease in expression
    ○ PK - CYP2C9 variants - decrease in warfarin metabolism
    § CYP4F2 variants - vit K metabolism
    ○ Age, sex, BMI, diet and other drugs (20%)

Question 24

Hypersensitivity reactions - abacavir

Answer 24

• Nucleoside analogue reverse transcriptase inhibitor
  
  • Safe and effective treatment for HIV - first line drug
  • ~5% patients develop potentially life threatening hypersensitivity reactions (fever, rash, GI and respiratory symptoms) - genetic
  • Immune mediated -MHC-I Ag presentation and activation of HLA-B*5701 in CD8 T+ cells - secrete TNFa and IFNy
  • HLA- B*5701 allele frequency is low
  • Pre-prescription phenotyping eliminates hypersensitivity reactions
  • Cost effective healthcare

Question 25

Pharmacogenetics Summary

Answer 25

• Pharmacogenetics is the study of the role of inheritance in inter-individual variations in response to drugs
  
  • Variation in drug response only became a major problem with the advent of the pharmacological era (20th century)
  • Gene variants (SNPs) underpin these variations
  • They commonly occur in drug metabolism genes
  • Ethnic differences are often associated with SNP variant distribution
  • The ultimate goal of pharmacogenetics is to provide individualised therapy that maximises drug eficacy and minimises drug toxicity