Bioavailability and bioequivalence of veterinary drug dosage forms, with particular reference to horses: an overview.

The research paper discusses how the method of administration and the formulation of veterinary medication affect how drugs are absorbed, which then impacts the intensity and duration of the drug’s effect. This study also discusses how the location of the administration site can affect the concentration of the drug in the animal’s plasma and how absorption directly impacts the rate of drug elimination. The article further studies the impact of feeding times relative to oral drug dosing in horses and the convenience of oral paste formulations. It concluded by examining the assessment of bioequivalence which relies on the concept that preparations of a drug which provide essentially similar plasma concentration profiles should produce the same therapeutic effect.

Factors Affecting Bioavailability

• The research discusses how the route of administration and the formulation of the drug dosage form can affect its bioavailability, which includes the rate and extent of absorption of a drug. This, in turn, can impact the duration and intensity of the drug’s pharmacological effect on the animal.
• It highlights that the site of injection can affect the plasma concentration profile of drugs administered as aqueous suspensions or sustained-release parenteral preparations.
• The phenomenon called ‘flip-flop’ is discussed, where the rate of drug elimination is influenced by the rate of absorption, thereby increasing the apparent half-life of the drug.

Absorption and Impact of Feeding Time

• Absorption is shown to approximate a first-order process, and the absorption half-life or the mean absorption time can provide a close estimate of the rate of absorption.
• The research notes that the feeding time relative to oral dosing can affect systemic availability and also the pattern of absorption in horses, with mention of specific drugs like rifampin, trimethoprim, and phenylbutazone.

Oral Pastes and Bioequivalence

• Oral paste formulations are discussed as a convenient method to administer drugs, providing precision in dosage compared with powders or granules added to feed. They could also potentially provide sustained release of medication.
• The study further delves into the assessment of bioequivalence, which leans on the idea that preparations of a drug which provide similar plasma concentration profiles should yield the same therapeutic effect. The concept of relative bioavailability is used, incorporating a reference dosage form and a measure of the uncertainty of the estimate.