Distribution of oxytetracycline in the healthy and diseased postpartum genital tract of cows

SUMMARY Previously determined pharmacokinetic parameters of the disposition of oxytetracycline (otc) in cows were used to predict an iv priming dose and a rate of constant iv infusion of otc sufficient to approach steady-state equilibrium of the drug between plasma and genital tissue concentrations in healthy postpartum cows and postpartum cows with endometritis. After 8 hours’ constant iv infusion at rates calculated to maintain a plasma concentration of approximately 5 μg/ml, the mean concentrations of otc in plasma (μg/ml) and tissues (μg/g) of healthy and infected postpartum cows, respectively, were: plasma, 4.95 and 5.23; uterine tissue, 3.65 and 4.18; and ovarian tissue, 4.57 and 4.53. The mean plasma to genital tissue ratios for healthy and infected postpartum cows, respectively, were 1.38 and 1.31 for uterine tissue and 1.09 and 1.16 for ovarian tissue. There were no significant differences (P > 0.28) between healthy and infected postpartum cows in any of the parameters investigated. However, plasma-to-genital tissue ratios of concentrations of otc in postpartum cows as a group were significantly higher (P ≤ 0.03) than those previously determined in cycling cows. Computer-simulated uterine tissue concentrations of otc, after twice daily iv doses of 11 mg of otc/kg of body weight, indicated that this dosage regimen would provide postpartum uterine tissue concentrations > 5 μg/g during most of the dosage interval.