Pharmacokinetics of flunixin meglumine in healthy foals less than twenty-four hours old.

OBJECTIVE

To determine pharmacokinetic variables that describe the disposition of flunixin after i.v. administration of flunixin meglumine to foals < 24 hours old.

ANIMALS: 6 healthy foals, 2 males and 4 females (mean age, 11.6 hours; range, 6 to 22.5 hours).

PROCEDURE: Flunixin (as flunixin meglumine) was administered to foals at a dosage of 1.1 mg/kg of body weight. Flunixin concentration in plasma samples was analyzed, using gas chromatography/mass spectroscopy. Concentration versus time profiles were analyzed according to standard pharmacokinetic techniques. Blood samples were obtained from foals by jugular venipuncture at defined intervals over a 48-hour period. Samples were centrifuged, and plasma was frozen at -70 C until analyzed. One-, two-, and three-compartment analyses were conducted. The most appropriate model was determined by Akaike's information criterion analysis.

RESULTS

Plasma concentration versus time profiles were best described, using a two-compartment open model. Clearance was significantly lower than that determined for older foals and adult horses. Volume of distribution was larger than that determined for adults. Mean plasma half-life for healthy foals < 24 hours old was 8.5 hours.

CONCLUSIONS AND CLINICAL RELEVANCE: Although additional factors (eg, dehydration or sepsis) must be considered on a case-by-case basis, flunixin meglumine should be administered differently to foals < 24 hours old, compared with adults. Under similar clinical circumstances, doses in foals should be increased by as much as 1.5 times to induce comparable therapeutic concentrations; longer dose intervals, on the basis of clinical response, would be necessary to avoid drug toxicity.