Pharmacogenetics Flashcards
What is pharmacogenetics? What is important to remember?
The branch of pharmacology that involves identifying genetics variations leading to
interindividual differences in drug response
REMEMBER: Each patient is unique!!
What is the difference between pharmacogenetics and pharmacogenomics?
Pharmacogenetics refers to monogenetic (single gene) variants that affect a patient‘s
response to a particular drug
Pharmacogenomics refers to the entire spectrum of genes that are involved in
determining a patient‘s response to a particular drug
What is the goal of drug therapy?
The goal of the drug therapy is to produce a specific pharmacologic effect in a patient
without producing adverse effects
What are the main causes of drug response variability?
____ and ____ of drugs are different from one individual to another.
Effectivity and safety of drugs is different from one individual to another
What can individual variations in drug therapy cause?
• Lack of therapeutic efficacy
• Unexpected harmful effects (Toxicity)
What can pharmacogenetics alter both? Where do polymorphisms occur?
Pharmacogenetics can alter both drug efficacy
and the likelihood of toxicity
Polymorphisms in drug receptors, drug
transporters and in drug metabolizing enzymes
What makes up pharmacogenetics?
Pharmacogenetics combines the areas of pharmacokinetics, how genetic differences
alter ADME, and pharmacodynamics, how genes alter receptors and signal molecules
Why is pharmacogenetics important?
Why is this important?
Individualization of drug therapy has two important clinical implications:
• It can help predict those patients at high risk for developing drug toxicity. In those cases, a lower
drug dose or an alternative drug should be use
• It can help identify those patients that are most likely to benefit from a particular drug
What does the human genome contain in terms of nucleotide bases, and genes?
The human genome contains approximately 3 billion nucleotide bases, representing
roughly 30,000 genes
What is a gene?
A gene is the DNA sequence containing a series of codons (three consecutive
nucleotide bases form a specific codon) that specify a particular protein
What is a mutation?
A mutation alters the sequence of nucleotide bases in a DNA molecule. This in turn
alters transcribed RNA, creating a different codon
What is a silent mutation?
The mutation results in a base change that creates a codon for the same amino acid. There is no change in protein structure or function.
What is a regular mutation?
The mutation results in a different amino acid, or the creation of a stop codon, the change in protein structure and function can be deleterious
What are homozygous genotypes?
If an individual has two identical alleles, that individual is said to have a homozygous genotype
What are heterozygous genotypes?
If an individual has two different alleles, that individual is said to have a heterozygous genotype
What is important to remember about phenotypes in individuals?
The phenotype of each individual with regard to a specific gene is the outward, physical manifestation of a given genotype. That outward physical manifestation might be something immediately obvious, or
it may not be apparent until a particular drug is administered to that individual
What is found at each gene locus?
At each gene locus, an individual carriers two alleles, one from each parent
What are polymorphisms? What is their usual frequency?
- Variations in a given gene may be present rarely in a population, or in relatively large numbers in a population
• Polymorphisms are defined as genetic variations occurring at a frequency of 1% or greater in the population (species of interest
What genes in humans are polymorphic? What diseases occur from mutations in humans?
In humans, many of the genes encoding cytochrome P450 enzymes are polymorphic, whereas some inherited human diseases such as cystic fibrosis are caused by rare mutations occurring in less than 1% of the population
How can identification of a mutation be useful?
or some diseases, identification of a specific mutation may be used to provide specific treatment approaches for the patient and in the case of veterinary patients, guide breeding decisions, too
What are the most relevant genetic defects in veterinary medicine that affects drug therapy?
- MDR1 defect
• Cytochrome P450 enzymes
• Glucuronyltransferase
• N-Acetyltransferase
What is the MDR1 mutation in dogs? What breeds are sensitive?
• Caused by a deletion (4 bp) in the multidrug resistance gene (MDR1-gene, ABCB1-
gene) which leads to the formation of shortened protein (P-glycoprotein). This causes defect in MDR1 which leads to inactivation of the protein and decreased protection from drug accumulation in critical tissues ( brain, kidneys, spleen, ect)
• Originally described in the Collie (ivermectin sensitive) and related breeds (Shetland
Sheepdog, Australian Shepherd, Border Collie, etc) Can also be seen in mix-breeds.
What is P- glycoprotein?
Is a transmembran protein that was first described in highly resistant tumor cell lines. This tumor cells were cross resistant to various anticancer agents
Is an ATP-dependent carrier which works as an efflux transporter which pumps drugs out of the cell and is a functional component of several body barriers
What body barriers is P- glycoprotein a functional component of?
• Intestine
• Kidney
• Brain (blood-brain barrier)
• Testicles (blood-testis barrier)
• Placenta
• Liver (bile canaliculi)
Where can you see MDR1 expression? What is the difference between dogs that are homozygous for the defect vs dogs that are homozygous for normal allele?
What is the difference in drugs that can be given to a dog with a homozygous MDR1 defect and a normal dog?
Dogs homozygous for the normal MDR1 allele, can receive 2000 µg/Kg of ivermectin as a single dose without signs of toxicity
Dogs homozygous for the deletion experience adverse neurological effects after a single dose of 120 µg/Kg ivermectin
- Essentially, alot more drug makes it past important barrier into these critical tissues.
What is occurring in this image?
What is occurring in this image?
How does P-gp work within dogs with an intact P-gp?
• limits drug entry into the organism after oral administration
• promotes drug elimination into the bile and urine
• restricts drug penetration across the blood-brain barrier
How does P-gp work within dogs with a defective P-gp?
What category of drugs can cause a adverse reaction in dogs who have the defective allele for MDR1?
P-gp transported drugs can cause an increase in adverse effects in MDR1(-/-) dogs, especially when treated with macrocyclic lactones like ivermecti
What can you see in terms of clinical signs, for patients who have neurotoxicity to ivermectin with an MDR1 defect?
What can occur with MDR1 substrates, and what is the determinant of this?
Depending on the dosis and the mechanism of action, drugs that are MDR1 substrates can cause severe depression leading to death
If a dog recovers from MDR1 neurotoxicity, how long does it take for them to seem fully recovered?
Takes almost 2 weeks to recover, if they survive.
What signs can you see of MDR1 issues in dogs with Heterozygous alleles?
- Dogs heterozygous for this allele typically wont show signs, unless they are given very high doses.
What dogs are sensitive ( genes not breeds) to MDR1 transported drugs?
Dogs, homozygous for this mutation (MDR1-/-) are highly sensitive to many MDR1-transported drugs
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What three breeds in order from most to least, have the highest allele frequency for the defect?
Collie, Australian Shepherd, Shetland Sheepdog.
FYI: What drugs have increased toxicity in MDR1 defective individuals?
FYI: What drugs are safe at therapeutic doses in MDR1 defective individuals?
FYI: What drugs have no studies in their level of toxicities for MDR1 defective individuals? Can they cause toxicities?
yes
What breeds of cats can you see MDR1 defects in? What is the difference in the issue within the gene?
• The MDR1 defect has been detected in cats of the breeds Maine Coon which showed neurological toxicity after ivermectin application. Defect has also been detected in Siamese, Ragdolls, Turkish Angora
• Affected cats show a homozygous 2-bp deletion in the MDR1 gene (MDR1 nt1930(del2))
Where is Cytochrome p450 enzymes found? What are they important for?
• Cytochrome are heme-containing proteins located in the membrane of the endoplasmic reticulum
• CYPs catalyze phase I biotransformation reactions (oxidation and reduction)
What is a CYP1A2 deficiency? What is the cause?
Single nucleotide polymorphisms (SNPs) → nonsense mutation (C1117T) that produces a change in the reading frame, a premature stop codon, and as a result, a shortened protein (no heme-binding domain)
What happens to dogs that have a homozygous combination for the mutant allele? What are they considered?
• Dogs homozygote of the mutant allele (m/m) are considered “poor metabolizers” • As a result, drugs become high concentrated in blood → toxicity
What breeds are affected by the CYP1A2 deficiency?
Affected breeds: Beagle, Irish Wolfhound, Whippet, Dalmatian, Australian Shepherd
FYI: What drugs are CYP1A2 substrates?
CYP1A2 substrate drugs: clomipramine, lidocaine, naproxen, ondansetron,
propranolol, verapamil
What can be seen in terms of metabolite concentrations of radequinil in dogs normal for CYP1A2 ad those with deficiencies?
Increased concentrations of the drug AC-3933 (radequinil, dogs (poor metabolizing, PM-Beagles) a cognitive enhancer) and its metabolites in normal dogs (A, enhanced metabolizers) and CYP1A (-/-) dogs (B, poor metabolizers)
What is a CYP2B11 deficiency? What is the mechanism? What is seen in animals with this defect? What breeds are usually affected?
• Single nucleotide polymorphisms (SNPs) identified (mechanism not fully understood)
• Animals with a defective CYP2B11 show an increased bioavailability and toxicity
• Affected breeds: Greyhound
FYI: What drugs are CYP2B11 substrates?
• Substrate drugs: Propofol, ketoconazole, thiopental (extended recovery from anesthesia in affected
dogs)
What is a CYP2D15 deficiency? What is seen in animals with this deficiency? What are the affected breeds?
• Single nucleotide polymorphisms (SNPs) cause changes in enzymatic activity
• Animals with a defective CYP2D15 show a reduced metabolization rate
• Affected breeds: Beagle
FYI: What drugs are CYP2D15 substrates?
• Substrate drugs: celecoxib, propranolol, dextromethorphan, imipramine
What are the N- Acetyltransferase? What animal is it absent in? What is present in the other companion animal?
Both N-acetyltransferase genes are absent in dogs
• Cats lack NAT2 but express NAT1
What is seen in animals with a lack of N-acetyltransferase?
Increase the risk for hypersensitivity reactions and adverse effects from drugs that are metabolized by N-acetyltransferase enzyme: sulfonamides, procainamide, hydralazine and other drugs
What is a Glucuronyl transferase deficiency? What is it caused by? What can you see in patients with this deficiency?
Deletion on the uridinephosphate glucuronydil transferase gene (UGT1A6 gene) leads to an extremely reduced glucuronydation capacity → accumulation of the drug and toxic adverse effects
How is the drug processed if the patient has a glycuronyl transferase deficiency?
The accumulating drug will be then via the alternative metabolic pathway using cytochrome P450 detoxified
What is glycuronyl transferase important for?
conjugation/ making the drug water soluble.
What animals are affected by glycuronyl transferase deficiency?
Affected species/breeds: Cats/all breeds, lions, civet cats
When the alternative pathway is used with detoxified Cytochrome p450, what is produced. What is this product detoxified with? Why do the patients show deficienies?
This produces the metabolite N-acetyl-p-benzo-quinon-imin (NAPQI) which is a harmful metabolite. NAPQI is detoxified using glutathione (GSH). Due to the low GSH reserves in cats
What is caused by NAPQI accumulation? What can be seen?
NAPQI accumulates causing liver necrosis and methemoglobin formation (Heinz bodies) in the red blood cells
What is seen in this image?
Heinz bodies
FYI: What drugs are p-aminophenol derivatives?
phenacetine, paracetamol, phenylbutazone
What is a drug, that is common in homes, that a curious cat may find? What may be seen in these cats if ingested? Why?
Drug: paracetamol
Complications: Liver Necrosis/ Heinz Body Anemia
Why? : Glucuronyl transferase deficiency/ N-acetyl-p-benzo-quinon-imin (NAPQI) production
