Veterinary Pharmacokinetics (VM8314):

"For nearly all veterinary patients and in nearly all cases, dosage is based on recommendations from references, trusted experts and/or drug labels. Pharmacokinetics are ONLY important to veterinarians if dosage modification improves clinical outcome (enhances efficacy, avoids intoxication)...

...and then pharmacokinetics are VERY important."-JRW

General Rules

  1. Drug action is proportional to the drug concentration at the site of action.
  2. Concentration predicts drug effects better than dose.
  3. Drugs only act when they are dissolved body water
    • The body water is divided into spaces by semipermiable membranes.
    • Drugs must be soluble in both water and lipid to move around in the body.
      • "Lipid soluble" and "Water soluble" are relative not absolute.
    • Equillibria between the spaces determine where a drug "is."
  4. Pharmacokinetics "happen" because physiologic processes happen.
    • The processes may happen differently in different species. Species differences in pharmacokinetics distinguish Veterinary pharmacology from Human pharmacology.
    • The processes may or may not be altered by disease. Disease-induced pharmacokinetic differences distinguish between basic and clinical pharmacology.
  5. We can only sample from the blood stream, not from all the places where the drug goes.
  6. We will consider drugs in the GI tract and remaining in injections sites to be outside the body. A drug molecule is not "in the body" until it is in the blood stream.
  7. We generally talk about a "fraction" of the drug that does "X" (eliminated from the body, absorbed from an injection site, etc.) Individual molecules are "on their own."
  8. Not everything that happens is "important".
    • If protein binding of a drug is <90%, the drug will not be involved in a protein binding interaction
    • If organ function is (at least) 80% of "normal", it's "normal"
    • If bioavailability is >80% it is not likely to be a factor in dosing.

Your understanding of pharmacokinetics may improve if you visualize what's going on at a microscopic level. Shown here is a section of skeletal muscle. Visualize a blood vessel in this field (remember to make sure the tip of your needle really IS in among muscle cells and not in a vessel). Visualize what happens to those neatly arrayed fibers when you push a ball of pharmaceutical product in between them.

No matter how they are administered, drugs (must) end up in the bloodstream. Nearly all pharmaceutical actions occur either in the interstitium (e.g. cell surface receptors) or inside of cells (e.g., intracytoplasmic receptors). Within each of these spaces the drug may be free in solution, bound to proteins or trapped as ions.

Definitions

TermDefinition
Dose
  1. noun - Amount of drug given at one time.
  2. verb - The act of administering a single dose.
Dosage
  1. noun - How large is each dose? How often is each dose given? by what route? and for how long?
  2. noun - The process by which you decided how to administer a drug.
    1. Looked it up
    2. Looked it up and adjusted it based on characteristics of THIS patient.
Equillibrium
  1. A state during which relative concentrations on opposite sides of a semi-permeable membrane stop changing. (Concentrations may rise or fall but the concentration ratio between the two sides of the membrane is constant.)