Several factors, including age, can affect pharmacokinetics, which describes the fate of a drug in the body-absorption, distribution, metabolism, and excretion. Guidelines for use of drugs in neonates are not usually available. Neonates have a higher percentage of body water and decreased protein binding; thus, water-soluble drugs have a higher volume of distribution, necessitating higher doses, which may need to be accompanied by wider dosing intervals. Since neonates have less body fat than adults, drugs distributed to fat may need to be given at lower doses.
Pharmacokinetics is also altered in geriatric patients, most often related to organ dysfunction. For animals with renal disease, doses of drugs that are cleared by the kidneys should be altered. If the patient has lost liver function, it is best to select drugs that rely on the kidney for elimination rather than hepatic biotransformation. If a drug metabolized by the liver must be administered, use the lowest daily dose possible and split the daily dose into three or four divided doses.
Drug distribution can be altered in pregnant dams as well as in fetuses. Many drug labels carry the claim, "Not intended for use in pregnant animals" or "Sufficient studies have not been performed to establish the safety of this drug in pregnancy." If a drug needs to be given to a pregnant animal, understanding pharmacologic effects and pharmacokinetics can help predict its safety.
COMMENTARY: It is important to keep in mind the physiologic changes that occur in animals when both the drug and the dosing regimen are being chosen. Most information is extrapolated from laboratory animals or human reports. This paper gives a few specific examples of pharmacokinetic changes in veterinary patients.
Problems in drug therapy for animals: How diseases, physiologic condition, and body composition can alter drug therapy. Papich MG. ACVIM PROCEEDINGS, 2003, pp 45-47.