9 – Pharmacogenomics

Question 1

Why isn’t the response of a drug therapy always uniform?

Answer 1

• Differences in PK or PD
  o Between patients
  o Within same patient
• NOT variability in drug products

Question 2

Example: interindividual variability with warfarin

Answer 2

• Anti-coagulation=prolong prothrombin times
• Don’t want to be TOO LONG or TOO short
• *different people require different doses to get a desired effect

Question 3

Pharmacogenomics

Answer 3

• Influence of GENETIC VARIATION within a population on drug therapy
  o May alter PK (ADME) and PD
• *we do NOT know what ‘type’ of animal we will be treating
• Ex. time course or receptors

Question 4

PD effects: adrenergic receptor mutations

Answer 4

• Won’t respond the same way when the drug tries to bind to it
• Down regulation or up regulation

Question 5

Multidrug resistance (mdr) gene: ABCB1 gene

Answer 5

• Codes for P-glycoprotein (P-gp)=ABCB1 protein
  o Membrane transporter pump (apical membrane)
• ‘knockout’ mice (gene deleted): normal until got mites
  o Drug: ivermectin
  o MICE dropped dead
• *collie bred dogs

Question 6

White feet, don’t treat

Answer 6

• Collie breed dogs (white feet)
• Do NOT treat with Ivermectin=susceptible to ivermectin toxicity

Question 7

Why are collies ivermectin sensitive?

Answer 7

• Deletion mutation in mdr (ABCB1) gene = produced a frame shift (premature stop codon) = get a non-functional P-gp

Question 8

Homozygous gene mutation of mdr gene

Answer 8

• Adverse effects to normally safe doses of ivermectin

Question 9

Heterozygote gene mutation of mdr gene

Answer 9

• May show toxicity at increased ivermectin doses

Question 10

P-glycoprotein expressed in many cell types

Answer 10

• Intestinal epithelial cells
• Brain capillary (BBB) endothelial cells
• Biliary canaliculus cells
• Renal proximal tubule cells
• Placenta and testes

Question 11

What is the function of P-gp?

Answer 11

• Actively transports chemicals from INSIDE the cell to OUTSIDE the cell
  o *active=can function against EXTREME concentration gradients

Question 12

P-gp evolved as a

Answer 12

• Protective mechanism to decrease body’s exposure to toxic xenobiotics
• “the bouncer”
• *works on many drug classes (why first called ‘mdr gene’)

Question 13

P-gp and oral drug absorption

Answer 13

• Expressed on intestinal epithelial apical membranes=can REDUCE oral bioavailability of substrate drugs
• *if have ABCB1 gene deletion=INCREASED oral bioavailability to many drugs

Question 14

Drugs that are P-gp substrates may also be substrates for

Answer 14

• CYP 3A enzymes

Question 15

Absorption ‘steps’

Answer 15

• Need to get absorbed (avoid P-gp)
• Then make it through the liver (avoid CYP3A enzymes)
• Enter capillary portal vein

Question 16

What can be used to increase oral bioavailability?

Answer 16

• Cyclosporine
• Ketoconazole: another substrate for P-gp and CYP enzymes
  *Inhibits P-gp efflux activity and CYP3A metabolic activity
• *need to be very careful as toxicity can occur

Question 17

P-gp and drug excretion

Answer 17

• Transports drugs into urine/bile=enhancing excretion
• Decreases renal or biliary excretion (ex. reduced P-gp) can INCREASE drug exposure (=AUC) and elimination half life

Question 18

Vincristine toxicity and ABCB1 genetics

Answer 18

• Thrombocytopenia=more in those with a mutated gene
• Neutropenia grade=high in mutant dogs
• *3 WT/WT with neutropenia were border collies
  o ABCB1 was normal, possible a CYP gene mutation?

Question 19

P-gp and brain capillary endothelial cells

Answer 19

• Function as part of BBB by pumping drugs out of CNS

Question 20

P-gp and drug distribution: ABCB1 gene deletion

Answer 20

• *increased brain concentrations of many drugs
• Ex. ivermectin, moxidectin, corticosteroids, Ioperamide (AVERMECTIC TOXICITY)

Question 21

Avermectin toxicity

Answer 21

• Ivermectin and related drugs
• Not a problem when part of heartworm preventative dose (concentration is so little)

Question 22

Breed incidence

Answer 22

• Can try and use to know how it may be to use the drug

Question 23

Get a dog and want to know if it has P-gp mutation

Answer 23

• TEST before treating
• Check epithelial OR blood sample
• 1-2 week turn around

Question 24

What about CYP polymorphisms?

Answer 24

• Variances in enzymes expression or function=major impact on drug metabolism and clearance
• May account for prolonged anesthesia in Greyhounds
• *no pharmacogenomic test available yet for individual dogs

Question 25

Cat pharmacogenomics?

Answer 25

• Only 1/8 of cats had similar ABCB1 gene deletion as in dogs
• *but affected cats, had multiple other point mutations throughout the ABCB1 gene

Question 26

ABCG2 transporter genes

Answer 26

• Encode for another transporter
• Expressed in similar tissues as P-gp (ex. breast cancer RP (BCRP))

Question 27

Humans with ABCG2 polymorphisms have

Answer 27

• Decreased transporter activity=increase exposure to substrate drugs

Question 28

Feline ABCG2 transporter genes

Answer 28

• Several AA differences at conserved sites
• *lead to decreased transporter function
• **CONSISTENT in all cats (NOT just a sub-population)
• Ex. fluroquinolone-induced retinal toxicity OR acetaminophen-induced toxicity?