D13 - Pharmacokinetics Flashcards
Pharmacogenetics definition
examines the effect of genetic differences in a single gene on drug response
Pharmacogenomics definition
examines the effect of genetic differences in multiple genes across the genome on a drug response
drug problems can be caused by variations in
- 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.
Pharmacogenetics
- 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
- Genes influence pharmacodynamics by
Variability of drug response - key terms
- Beneficial or therapeutic response
- Partial response
○ Need an increased dose for a sufficient response - Non-responsive
- Full adverse response
- Partial response
monogenic
allelic variation in a single gene
Polygenic
allelic variation in greater than or equal to 2 genes
Polymorphic
monogenic gene variant occurring at a population frequency of >1%
Two types of genetic mutations underpinning variations
- 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)
- Insertion or deletion of more than one nucleotide
SNPs
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
Favism
- 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
G6PD enzyme and pentose-phosphate pathways (hexose monophosphate shunt)
- 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)
G6PD deficiency
- 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
- Primaquine
○ 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
Succinylcholine/suxamethonium chloride
○ 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
Acetylation and drug metabolism
- 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
- N-acetyltransferase (NAT2) is polymorphic
N-acetyltransferase (NAT2) is polymorphic
○ wild type (R ○ mutant variant which is deficient in acetylation activity (r ○ Slow (rr) ○ Intermediate (Rr) ○ And fast (RR)
Drug metabolism
- 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
Alcohol metabolism
- 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)
TPMT and Thiopurines
- 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
Drug target (pharmacodynamic polymorphisms
- Genetic variation in drug targets (eg. Receptors) affects efficacy
- PD effects best explained by comparison with PK effects
B2 adrenoceptor, asthma, and B2 agonists
- 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
Warfarin
- 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%)
Hypersensitivity reactions - abacavir
- 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
Pharmacogenetics Summary
- 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
