Introduction to Veterinary Pharmacokinetics:

Administration (routes)

General Principles

Oral Administration

Advantages

Disadvantages

Location of processes involved in the absorption of oral drugs.

Process Primary Location Secondary Location(s)
Tablets disintegrate (a suspension forms) Stomach Duodenum for enteric coated forms
Drug dissolves from suspension Stomach Duodenum
Drug in lipid suspension may be picked up by lacteals (absorption) Duodenum, jejunum, ileum  
Drug in solution crosses mucosa (absorption) Duodenum, jejunum Stomach, Ileum, colon

Patient and Pharmaceutical Factors

Regional differences

Stomach - most species (Abomasum - ruminants, C3 - camelids)

Rumenoreticulum (C1 & C2 - camelids)

Small Intestine (all species)

Colon/Rectum (all species)

Intramuscular Administration

Advantages

Disadvantages

Injectable dose forms

DrugVehicleDose Form
Water SolWater SolAqueous Solution
Water SolLipid SolSuspension
Lipid SolWater SolSuspension
Lipid SolLipid SolLipid Solution

Processes

  1. Drug in suspension or lipid solution dissolves in tissue fluid (takes time, may slow absorption)
  2. Drug in aqueous solution only has to MIX with the tissue fluid (immediately available for absorption).
  3. Drug in tissue fluid solution diffuses into capillaries
  4. Drug (in solution) in capillaries is carried to circulatory system.

Any of these processes can be "rate limiting" for absorption. If multiple steps must occur, the SLOWEST process controls the overall rate of absorption.

Example:

Assume that a bolus injection roughly spherical

  • an aqueous solution mixes with tissue fluid for rapid absorption. The drug is already dissolved in "water"
    • dissolution in tissue fluid is not rate-limiting
    • Entry of drug into circulatory system limited (only) by rate of blood flow to the tissue (how fast it can be carried away).
    • blood flow varies by body region/muscle group, so exercise may affect absorption rate
    • Occasionally the vehicle may mix with tissue fluid and the drug may fall out of solution. The "falls" out of solution in the tissue and dissolves very slowly.
      • Produces tissue residues
      • Reduces effect
  • a bolus of a lipid soluble vehicle remains relatively intact (spherical?). Lipid vehicle must "degrade" or dissolve very slowly.
    • Mixing and dissolution in tissue fluid occurs from surface of bolus
    • entry of drug into circulatory system limited by rate of drug "dissolution". (Movement from the "bolus" to the tissue fluid).

Patient and Pharmaceutical Factors

Subcutaneous Administration

Advantages

Disadvantages

Process

Patient and Pharmaceutical Factors

Topical

Advantages

Disadvantages

Process

Patient and Pharmaceutical Factors

Vehicle effects

Special routes of administration - seek guidance before attempting:

Intraperitoneal
Intrathecal
Intra-articular
"Regional" (Intra-arterial, Interosseous, Intravenous with tournequet)
Per rectum
  • Reserved for emergency
  • Absorption like enteral