Pharmacogenetics (Nerissa Dando, MD) Flashcards
What is the goal for non-responders? Toxic responders?
Drug efficacy; drug safety
What are the three possible effects of drug administration?
Therapeutic effect
Adverse effect
Lethal effect
Describe: Succinylcholine
Anesthetic agent (muscle relaxant) ADR: overparalysis
What type of drug is primaquine?
Antimalarial
What type of drug is isoniazid (INH)?
Anti-tuberculosis
Intake of primaquine causes ADRs among African-Americans where they develop (1) which leads to (2).
(1) G6PD deficiency
(2) hemolytic anemia
Who developed the concept of chemical sensitivity?
Archibald Garrod
Define: Alcaptonuria
Excretion of homogenateisic acid in urine due to congenital absence of homogentisate 1,2 dioxygenase.
No breakdown of phenylalanine and tyrosine.
Urine is dark.
Enumerate Mendel’s Four Principles of Inheritance
- Genes come in pairs.
- Dominance and recessiveness
- Principle of segregation
- Principle of independent assortment
How many possible codons are there?
64
What is the sequence of the start codon? Stop codons?
Start codon - AUG
Stop codons - UAA, UAG, UGA
Enumerate the basic patterns of inheritance
Autosomal Dominant
Autosomal Recessive
X-linked
Mitochondrial Inheritance
Give examples of X-linked diseases
G6PD deficiency
Pyridoxine sensitive anemia
Vasopressin resistance
T/F: Mitochondrial inheritance is secondary to exposure to aminoglycosides like streptomycin, gentamycin and amikacin.
True
i.e. aminoglycoside-induced deafness
What are the factors affecting drug sensitivity?
Genetics
Disease
Age
Drug related factors
Which groups of patients are most sensitive to the effects of drugs?
Geriatric
Preterm
Chronically Ill
What drug related factors affect drug sensitivity?
Pharmaceutical formulation
Route of administration
Drug interaction
Environmental factors
Enumerate extrinsic factors affecting drug sensitivity
Drug-food interaction (grapefruit juice, alcohol & histamine cause sedation)
Environmental chemicals
Vices
Enumerate intrinsic factors affecting drug sensitivity
Physiological and pathophysiological attributes of a person (age, gender, weight, etc.)
Patient characteristics
Define: Gene
Unit of heredity on a chromosome
Define: Alleles
Alternative forms of genes that occur in pairs
It includes all alleles present in the cell whether dominant or recessive, expressed or not.
Genotype
Physical manifestation of a trait
Phenotype
What method can predict whether an individual is a candidate for polymerphic metabolism?
Molecular Genotyping
What methods are commonly employed in molecular genotyping?
Polymerase chain reaction (PCR)
Restriction fragment length polymorphism mapping (RFLM)
Method accomplished by administering a probe to an individual followed by analysis of metabolites in blood or urine
Phenotyping
What anti-hypertensive drug is used to identify the activity of CYP450?
Drebisoquine
Enumerate the three potential applications of molecular genotyping in drug development
- Determine safety and efficacy of investigational new drug prior to approval
- Protect patients against ADRs by identifying poor metabolisers
- Establish an individual’s genetic predisposition to disease
Define: Monogenic
Due to allelic variation at a single gene
Define: Polygenic
Due to variations at two or more genes
What describes frequently occurring monogenic variants occurring at a frequency > 1%?
Polymorphic
What are the most common polymorphisms?
Single nucleotide polymorphisms (SNPs)
Copy number variations (CNVs)
What are less common polymorphisms?
Insertions
Deletions
What are the two types of gross rearrangements?
Inversions
Translocations
These are implicated in human diseases (i.e. cancer and Alzheimer’s) but most have no known clinical effects.
Single Nucleotide Polymorphisms
What are the different types of SNPs?
Missense
Nonsense
Frameshift by addition
Frameshift by deletion
How many SNPs have been identified in the human genome?
2 million
T/F: A polymorphic distribution follows a normal distribution.
False
70 - 90% of them are found in extremes.
Enumerate the project goals of the Human Genome Project
- Identify all 30,000 genes in human DNA
- Determine sequences of 3 billion base pairs
- Store the information
- Improve tools for data analysis
- Transfer technologies to private sector
- Address ELSI (ethical, legal and social issues) that may arise
When was a working draft of the human genome completed?
June 2000
When were analysis of the working draft published?
February 2001
T/F: To be important, SNPs must affect either function or amount of a protein.
True
What are the possible effects of genetic polymorphisms?
- Pharmacokinetic (enzymes, transporters and binding proteins)
- Pharmacodynamic (target proteins, receptors and ion channels)
- Idiosyncratic Responses (no explanation)
Refers to genetic loci in which variants occur with a frequency of 1 - 2%
Genetic Polymorphism
Twin and family studies enable discrimination of?
Various modes of genetic transmission
Dominance-recessivity relationships
T/F: Pharmacogenomics is a subset of pharmacogenetics.
False
It’s the other way around.
Define: Pharmacogenomics
Study of variations of DNA AND RNA characteristics related to drug response
Define: Pharmacogenetics
Study of genetically controlled variations in DNA sequence related to drug response
According to pharmacogenomics, what is the key to creating personalised drugs with greater efficacy and safety?
Understanding an individual’s genetic make-up
Define: Poor Metabolizers
Slower-than-usual metabolism leading to toxicity
May be due to liver or kidney failure
Define: Ultra-rapid Metabolizers
Observed therapeutic failure
Variability in drug target genes
T/F: There is poor efficacy of prodrugs in slow metabolizers and ADR risk in ultra-rapid metabolizers.
True
Give examples of prodrugs.
Codeine
Tamoxifen
Clopidogrel
What is seen in CYP2D6 poor metabolises of debrisoquine?
Marked hypotension
Note: This is not an ADR.
What group has the largest incidence of G6PD deficiency?
Indians-Parsees
T/F: Newborn screening cannot detect G6PD deficiency.
False
Enumerate the drugs demonstrated to precipitate haemolytic anemia in G6PD deficients
Acetanilide Methylene blue Nalidixic acid Naphthalene Sulfapyridine Sulfanilamide Sulfamethixazole Primaquine Sulfacetamide Nitrofurantoin Nitrates
Example of an unusual drug response is malignant hyperthermia following the use of what?
Halothane (anaesthetic agent)
What is the classic presentation of malignant hyperthermia?
Rapid rise in body temperature Muscle rigidity Tachycardia Rhabdomyolysis Severe acidosis Hyperkalemia
What is the antidote for malignant hyperthermia?
L.V. dantrolene
What causes malignant hyperthermia?
Mutations in the ryanodine receptor
In cases of Warfarin resistance, by how much does the dose need to be increased?
7 - 20 times recommended therapeutic dose
In cases of folate resistance, by how much does the dose need to be increased?
40 - 1000 times daily recommended dose
T/F: Fast acetylators are susceptible to phenytoin toxicity when phenytoin and isoniazid are given together.
False
Slow acetylators!
T/F: Individualizing therapy is important for drugs with a wide therapeutic range.
False
Narrow!
Increased risk of cancer is associated with polymorphisms in what genes?
CYP (2A1, 1A2 and 2E1)
Glutathione transferases (GSMT1, GSTT1)
Epoxide hydrolase
NAT2 - N-acetyl transferase 2
T/F: Drug clearance typically removes drugs via enzymatic transformation into more hydrophobic metabolites.
False
Hydrophilic!
T/F: Cytochrome P450 enzymes are responsible for Phase II oxidative metabolism of steroid hormones, xenobiotics and carcinogens.
False
Phase I oxidative metabolism!
What CYPs are important for Phase I?
CYP3A4, CYP2C19 and CYP2D6
What enzymes are important for Phase II?
TPMT, NAT1 & NAT2
What are the mechanisms of variation in drug response?
- Modification of absorption of xenobiotic
- Modified metabolism resulting in modification of rate, pattern of elimination, detoxification or activation of a given xenobiotic
- Modification in the abundance, affinity or function of a target enzyme or receptor
What factors influence absorption of the xenobiotic?
- Rapid metabolizer
- Decreased abundance of transport protein
- Enhanced excretion by multi-drug resistance protein
T/F: When the main metabolic pathway leading to a harmless metabolite is blocked, parent drug could accumulate and cause toxicity.
True
T/F: Deficiency of methemoglobin reductase is inherited as an autosomal recessive trait.
True
T/F: Methemoglobin reductase homozygotes are predisposed to develop methemoglobinemia when exposed to nitrates and other stress.
False
Heterozygotes!
Give an example of a modification in the abundance, affinity or function of a target enzyme or receptor.
- Silent variants of serum cholinesterase associated with succinylcholine
- Poor metaboliser variants of CYP2D6 and CYP2C19
Describe the metabolism of ethanol by the body
Ethanol -> acetaldehyde (by ADH) -> acetate (by ALDH)
T/F: The rate of ethanol metabolism of genetically controlled.
True
T/F: Atypical ADH exhibits higher catalytic activity and typically contains a variant alpha 2 subunit.
False
Variant beta 2 subunit!
What ethnic group has the largest distribution of atypical ADH?
Swiss (20%)
How does the atypical B2 subunit of alcohol dehydrogenase differ from the typical?
CGC to CAC transition resulting in an Arg47His substutition
What does the atypical ADH subunit mutation cause?
Higher ethanol Vmax
Higher Km for the homozygous B2B2
What are the three genotypes for aldehyde dehydrogenase?
- Homozygote normal
- Heterozygote
- Deficient homozygote
What causes the “flush” and feelings of discomfort associated with ethanol intake?
Acetaldehyde accumulation
Describe: cyclophosphamide polymorphic biotransformation
Antineoplastic agents that cause cystitis
Familial pyrimidinemia and pyridinuria are associated with what?
5-fluorouracil induced toxicity
Major toxic manifestation of 5FU in DPD occurs in what tissue?
Bone marrow
What are the two phenotypes for mephenytoin hydroxylation polymorphism?
- Poor metaboliser
2. Extensive metabolizer
Mephenytoin hydroxylation is mediated by what enzyme?
CYP2C19
What are the types of parathion poisoning and paraoxon polymorphism?
PON1 & PON2
What causes the hydrolysis of succinylcholine?
Pseudocholinesterase
How do you document pseudocholinesterase deficiency?
History taking
Unexpected persistent respiratory paralysis for a prolonged period of time
Enumerate a few examples of choline ester compounds
Succinylcholine
Mivacarium
Cocaine
Atypical plasma cholinesterase causes (1) that lasts (2)
(1) paralysis
(2) 2 - 6 mins
Note: Patients deficient in the enzyme experience 1-hour long paralysis episodes.
How many % of succinylcholine is inactivated by normal functioning pseudocholinesterase?
90 - 95%
T/F: Flaccid paralysis of skeletal muscle develops within 1 minute of succinylcholine binding
True
What is the cause of pseudocholinesterase deficiency?
Abnormal alleles that code for the enzyme
Where is the gene coding for pseudocholinesterase located?
E1 locus on the long arm of chromosome 3
How many % of the population carries one or more of the atypical gene alleles for pseudocholinesterase gene?
4%
How is the dibucaine-resistant genetic variant form of pseudocholinesterase detected?
Percent inhibition of hydrolysis of benzyl choline caused by addition of dibucaine (dibucaine number)
What is the DN value for normal homozygotes? for the atypical dibucaine resistant genotype?
80%; 20%
How is the fluoride-resistant pseudocholinesterase enzyme variant detected?
Percent inhibition of benzyl choline hydrolysis when fluoride is added to assay
What is the FN value for normal homozygotes? for the atypical homozygotes?
60%; 36%
What is the most severe form of atypical pseudocholinesterase?
Silent form
T/F: Only 1 in 100,000 individuals are homozygous for the silent Es genotype with no detectable pseudocholinesterase enzyme activity.
True
What causes the silent Es genotypes?
Gene mutations by frameshift or stop codon
What is the cause of fish odor syndrome?
Excessive excretion of trimethylaminuria (TMA) in urine, sweat and breath
What is the prodrug used to check the activity of CYP2D6?
Debrisoquine
T/F: If you give a patient debrisoquine and there is marked hypotension, the phenotypic presentation is ultra rapid metabolism.
False
Poor metabolism!
What are drugs that can cause increased toxicity in ultra rapid metabolizers?
Codeine-narcotic and opioid
What are drugs with an extremely narrow therapeutic index?
Antiarrythmitics
Antidepressants
Antipsychotics
What are the two N-acetyltransferase isozymes?
NAT1 and NAT2
T/F: Filipinos are fast acetylators.
True
NAT2 allele polymorphism produces what phenotypes?
Rapid acetylator
Intermediate acetylator
Slow acetylator
T/F: Due to NAT2 polymorphism, there is a higher incidence and/or severity of adverse drug reactions to isoniazid, hydrazine, procainamide and sulfamethaxazole.
True
NAT2 activates and deactivates many of the (1) and (2) amine carcinogens.
(1) Aromatic
(2) Heterocyclic
T/F: There is no association between acetylation polymorphism and urinary bladder & colorectal cancers.
False
There is!
Slow acetylators have an 8-fold higher risk of what compared to fast acetylators?
Cancer
Rapid acetylators are at increased risk of what from acetylation of heterocyclic amines found in cooked meats?
Colon cancer
Where has human NAT2 been localised?
Short arm of chromosome 8
What allele is associated with rapid acetylators?
NAT2*4
What alleles are associated with poor acetylators?
NAT25B, NAT26A and NAT2*14A
What complications does thiopurinemethyltransferase (TPMT) polymorphism cause?
Severe coma or depression after giving certain anti-cancer drugs
In AFL, patients may develop myosuppression
Patients with TPMT polymorphism will experience toxicity after administration of what?
6-mercaptopurine
T/F: TPMT is found in patients with acute lymphoblastic leukaemia.
True
With pharmacogenetics, we aim for (1) and (2).
(1) efficacy
(2) safety
