VPT 311: General and Systemic Veterinary Pharmacology (2+1)

FACTORS THAT AFFECT DRUG ACTION

Factors modifying drug action:

• Body size: influenzes the conc. achieved at the site of action. Doses are normally given for adults of medium build. In extreme cases – dose to be actually calculated based on per kg dose.
  • Body surface area is also an important parameter- used in calculation of dose for anticancer drugs
  • (BSA in sq.m. = K x BW in Grams^2/3 x 10 ^–4 (K= 10 for dogs, 10.1 cats)
• Age: drug behavior differs in infants and children. – low GFR,
  • Blood Brain Barrier more permeable, absorption is altered poor metabolizing capacity up to 4 weeks of age
  • In the old, poor metabolizing capacity, low PP binding, slower Absorption – dose alteration needed in both cases
• Sex: Females smaller in size and may need lower dose.
  • Pregnancy induced changes – delayed absorption, plasma albumin falls, acid glycoprotein increases, renal blood flow increases, induction of CYP 450 enzymes – all these affect the kinetics of drug
  • Besides – possibility of drug affecting fetus or the young ones thro’ milk
• Species – changes in sp. in the pattern of metabolism – rabbits resist
  • Atropine, dogs are poor acetylators, cats deficient in glucuronidation
• Race – Blacks need higher atropine to dilate pupil Mongols need lower dose
  • Fewer reports of aplastic anemia after chloramphenicol use in Asians compared to the West
• Diet and plane of nutrition – drug action predictable in good plane of nutrition
• Genetics – G-6-PD deficiency hemolysis with antimalarial agents Malignant hyperthermia after halothane in some human individuals and pigs