Pharmacokinetics of oxytetracycline administered intravenously to 4 to 5-day-old foals

Rhabdomyolysis and Acute Renal Failure Associated with Oxytetracycline Administration in Two Neonatal Foals Affected by Flexural Limb Deformity

Oxytetracycline (OTC) administration has become a frequent practice in equine neonatology for the treatment of flexural limb deformity. The cause of this condition remains unclear but clinical studies revealed that following IV administration of OTC a relaxation of the metacarpophalangeal joint occurs in foals affected by flexural deformity. Studies concluded that OTC administration in neonatal foals did not adversely affect the kidneys. Other adverse effects of OTC have never been reported. This report presents two cases with different outcomes of 3-day-old foals which presented acute collapse and progressive depression after OTC administration. The clinical aspects, the increased activity of serum enzymes indicative of muscular damage, the presence of myoglobin in urine were clear diagnostic indicators of severe rhabdomyolysis, and the gross and histological findings confirmed a myopathy associated with renal damage in one case. Adverse effects on the musculoskeletal and urinary systems in healthy foals were first reported and were probably associated with multiple doses administered to foals less than 24–48 h old and/or at dosing intervals less than 24–48 h. The risk of development of rhabdomyolysis and nephrotoxicity in neonatal foals treated with OTC for flexural deformity from now on should be considered.

In vitro effects of oxytetracycline on matrix metalloproteinase-1 mRNA expression and on collagen gel contraction by cultured myofibroblasts obtained from the accessory ligament of foals

OBJECTIVE

To determine the effects of oxytetracycline on matrix metalloproteinase-1 (MMP-1) mRNA expression and collagen gel contraction by equine myofibroblasts in an effort to explain the mechanistic basis for the pharmacologic treatment of flexural deformities in foals.

SAMPLE POPULATION

Cultured myofibroblasts from the accessory ligament (distal check ligament) of 6 foals.

PROCEDURE

Collagen gel scaffolds seeded with equine myofibroblasts were cultured in individual culture dishes containing complete media (Dulbecco's modified Eagle medium with 10% fetal bovine serum) and oxytetracycline (0, 12.5, 25, or 75 microg/mL) for 48 hours. After 24 hours, the gels were released from the bottom of the culture plate and allowed to contract. Photographs were taken at 0, 1, 2, 4, 6, 8, and 24 hours after release to assess the degree of collagen gel contraction. Additional gels were harvested at 2 hours after release for RNA isolation and reverse transcriptase-polymerase chain reaction assessment of the degree of MMP-1 mRNA expression.

RESULTS

Oxytetracycline induced a dose-dependent inhibition of collagen gel contraction by equine myofibroblasts. Oxytetracycline also induced a dose-dependent decrease in MMP-1 mRNA expression by equine myofibroblasts.

CONCLUSIONS AND CLINICAL RELEVANCE

Results of this study indicate that oxytetracycline inhibits tractional structuring of collagen fibrils by equine myofibroblasts through an MMP-1 mediated mechanism. In young foals, oxytetracycline administration may make the developing ligaments and tendons more susceptible to elongation during normal weight-bearing. Inhibition of normal collagen organization may provide the mechanistic explanation for the results seen following the pharmacologic treatment of flexural deformities in foals by oxytetracycline administration.

Biliary elimination kinetics and tissue concentrations of oxytetracycline after intravenous administration in hens

The pharmacokinetics of the biliary elimination of oxytetracycline (OTC) and tissue concentrations in certain organs were studied in 10 Leghorn hens. The animals were anaesthetized using xylazine/ketamine administered by the intramuscular (i.m.) route and were immobilized for right laparotomy. Both bile ducts were cannulated and a dose of 20 mg/kg of oxytetracycline hydrochloride was administered intravenously (i.v.). Samples of bile excreted were taken at predetermined intervals during 6 h. At 6 h animals were slaughtered and tissue samples of blood, liver, kidney, pancreas, spleen, heart, lung and pectoral muscle were taken. The values for OTC biliary elimination rate times were best fitted to a one-exponential equation. The maximum value for OTC biliary excretion rate (3.69+/-0.6 microg/min/kg) was reached at approximately 17.5 min (time to maximum concentration (tmax)). The first-order rate constant for the biliary excretion (k) and the half-life (t1/2) were 6.7x10(-3) min(-1) and 110.55 min, respectively. The mean value of area under the biliary excretion rate time curve (AUC) indicated that 839.77 microg/kg body weight (b.w.) were eliminated by the biliary route. The cumulative biliary excretion data indicated that approximately 4.20% of the dose was eliminated by this route, 3.28% being eliminated during the first 6 h and 0.92% thereafter. The highest mean concentrations were found in the kidney (35.82 microg/kg) and liver (16.77 microg/g). Significant differences were found between the concentrations of the various tissues studied. Plasma concentration was lower than that of the other tissues (except lung).