Comparative pharmacokinetics of a single and double dose of a conventional oxytetracycline formulation in sheep, to allow for therapeutic optimisation

Abstract

In the veterinary industry, long acting oxytetracycline formulations are loosely referred to as those formulations that only require a single dose at 20 mg/kg to achieve clinical cure and to be repeated after three days only if required. Short acting oxytetracycline formulations are recommended for use once a day for four days, at a dose of 10 mg/kg IV and 10mg/kg IM on day one, 10 mg/kg IM on day two and 5 mg/kg IM on days three and four. The primary objective of this study is to demonstrate that, based on pharmacokinetics, a double dose of a conventional short acting, 135mg/ml formulation of oxytetracycline has a longer action than a single dose of the same formulation. As a secondary objective the efficacy and safety of a single, double dose of a conventional oxytetracycline formulation are compared to multiple, single doses of a conventional formulation as well against a single dose of a long-acting formulation. Factors that influence the duration of action of a parenteral oxytetracycline formulation are reviewed, as are the pharmacokinetic / pharmacodynamic relationship of oxytetracycline. A single dose, randomized, two treatment, two sequence cross-over experimental design as described by Grizzle (1965) was selected for this study. The washout period between the two sequences was determined using at least 5 half-lives (11.1 hours x 5) of conventional oxytetracycline formulation , based on a study by Davey et al (1985) Although a wash-out period of 55.5 hours for a dose rate 20 mg/kg of oxytetracycline would have sufficed to ensure the absence of any residual drug in the central compartment of the experimental animals, it was decided to extend the washout period between treatment periods to 7 days (168 hours) for mainly practical reasons. Sample size determination was based on the rejection of the null hypothesis as described by Anderson and Hauck (1983). 5 animals per treatment group were selected. The sheep were equally and randomly assigned to either the group that would receive the 10 mg/kg dose first (group 1), or the group that would receive the 20 mg/kg dose first (group 2). For the cross over treatment (phase 2), the animals remained in the groups they were allocated to for phase 1, but group 1 received the 20 mg/kg dose and group 2 received the 10 mg/kg dose. The volume of oxytetracycline was calculated based on a product oxytetracycline content of 135 mg/ml. The blood sample collection procedure was the same for phases (treatments) 1 and 2. Time 0 was the time of treatment. Samples were collected into 10ml lithium heparinized vacutainer glass tubes with 19G disposable needles at the following intervals (hrs): 0, 0.25, 0.5, 1, 2, 4, 6, 9, 12, 24, 36, 48, 72, 96. The oxytetracycline concentrations in plasma were determined using validated High Performance Liquid Chromatographic methodology. The difference between the 2 sets of results emanating from phase 1 and phase 2 of the study are used as basis for presenting the results. Three pharmacokinetic parameters were used to compare the 2 treatments: Cmax (maximum plasma concentration) , AUCinf (Total area under the concentration curve) and T>0.5 (Time that the drug concentration remains above 0.5 ìg/ml). The geometric means of the results show that: The 20mg/kg treatment maintains levels above 0.5 ìg/ml significantly longer than the 10 mg/kg treatment. (37.4 hours versus 24 hours; p value 0.0013) The 20 mg/kg dose reaches a significantly higher concentration than does the 10 mg/kg dose. (6.59ìg/ml versus 3.55ìg/ml; p value 0.000000) The 20mg/kg treatment has an AUCinf which is greater than the 10 mg/kg treatment by a highly significant margin. (120.63 ìg/ml*hr versus 71.63 ìg/ml*hr; p value 0.000001) In demonstrating that a conventional oxytetracycline formulation administered intramuscularly at double dose provides drug plasma concentrations above MIC for an average duration of 13 hours longer than a single dose, the primary objective of the study was achieved. The study demonstrated that a single dose at 20 mg/kg of a conventional oxytetracycline formulation offers an acceptable alternative to conventional treatment regimes in terms of efficacy, target animal safety, as well as convenience to the user.