Lect 5

• Pharmacogenetics involves the study of genetic differences in an individual's response to drugs, whether that individual is a human or veterinary patient.
• 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.
• Drug therapy effectiveness and safety is different from one individual to another due to genetic differences.
• Genetic differences can affect genes coding for drug metabolizing enzymes, drug transporters, plasma protein (drug binding), and drug receptors.
• Individual variations to drug therapy can cause lack of therapeutic efficacy and unexpected harmful effects (toxicity).
• Pharmacogenetics combines the areas of pharmacokinetics, how genetic differences alter ADME, and pharmacodynamics, how genes alter receptors and signal molecules.
• Polymorphism in CYP1A2 in dogs is caused by a mutation (c1117C>T) in the coding region of CYP1A2 that generates a premature stop codon, leading to a complete lack of hepatic CYP1A2 protein expression and enzymatic function.
• Affected breeds for CYP2D15 include Beagles.
• The mutation in CYP1A2 was identified in approximately 15% of the beagle colony in a pharmaceutical company.
• Single nucleotide polymorphisms cause changes in enzymatic activity.
• Substrate drugs for CYP2D15 include celecoxib, propranolol, dextromethorphan, and imipramine.
• Animals with a defective CYP2D15 show a reduced metabolization rate, classifying them as poor metabolizers.
• Affected breeds include Irish Wolfhound, Beagles, and Berger Blanc Suisse.
• Individualization of drug therapy can help predict those patients at high risk for developing drug toxicity and can help identify those patients that are most likely to benefit from a particular drug.
• A mutation in a DNA molecule can result in a base change that creates a codon for the same amino acid, resulting in no change in protein structure or function.
• A mutation in a DNA molecule can result in a different amino acid, or the creation of a stop codon, causing changes in protein structure and function.
• Genotypes: +/+ = wild type, +/ - = heterozygous, -/ - = homozygous mutated.
• MDR1 Defect in Dogs: In intact dogs, an intact P-gp limits drug entry into the organism after oral administration, promoting drug elimination into the bile and urine, and restricting drug penetration across the BBB.
• In MDR1 mutant dog, there is an absence of activity in the location of the gallbladder.
• MDR1 drug substrates used in veterinary medicine.
• MDR1 +/+ dog (normal) MDR1 -/+ dog.
• Dogs homozygous for the normal MDR1 allele, can receive 2000 µg/Kg of ivermectin as a single dose without signs of toxicity.
• Nuclear scintigraphy was used to image the head of two Collies after IV injection of P-gp substrate 99m Tc-MIBI.
• In MDR1 normal dog, there is intense uptake of 99m TC-MIBI in the gallbladder.
• No clinical studies in dogs have been conducted on gastric secretory inhibitors, antiemetics, antibiotics, antifungals, glucocorticoids, anticonvulsants, antiparasitic agents, antineoplastic drugs, antiemetics, antidiarrheal agents, cardiovascular drugs, and safe at the therapeutic dose of antiparasitic agents.
• With defective P-gp function, P-gp substrate accumulated within brain tissue.
• Dogs, homozygous for this mutation (MDR1 - / - ), are highly sensitive to many MDR1-transported drugs.
• MDR1(−/−) dogs, a defect protein increases drug absorption (bioavailability?), reduces biliary and urinary elimination, and increases permeation of blood-tissue barriers (e.g., BBB, blood testis barrier, and blood placenta barrier).
• Dogs homozygous for the deletion experience adverse neurological effects after a single dose of 120 µg/Kg ivermectin.
• P-glycoprotein (ABCB1, MDR1) polymorphism in dogs (MDR1 defect) was originally described in the Collie (ivermectin sensitive) and related breeds (Shetland Sheepdog, Australian Shepherd, Border Collie, etc.).
• Depression, ataxia, somnolence, salivation, tremor, and coma are clinical signs of ivermectin toxicity in MDR1 defective Collies.
• ABC transporters harness energy from ATP to transport substrates, including many drugs used in veterinary and human medicine against large concentration gradients.
• Dogs and cats with defective P-glycoprotein (MDR1) have exquisite sensitivity to adverse effects.
• The defective MDR1 protein is shorter than the wild type protein and nonfunctional.
• P-glycoprotein (ABCB1, MDR1) is expressed in dogs and relevance in drug disposition.
• Variations in a given gene may be present rarely in a population, or in relatively large numbers in a population.
• 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.
• The P-glycoprotein (ABCB1, MDR1) polymorphism in dogs (MDR1 defect) is caused by a deletion (nt230 del - 4 bp) in the ABCB1-gene, which results in a frameshift such that several premature stop codons are generated very early in the translation process (formation of shortened protein).
• Polymorphisms are defined as genetic variations occurring at a frequency of 1% or greater in the population (species of interest).