Pharmacokinetic data on doxycycline and its distribution in different biological fluids in female goats

A pharmacokinetic study of doxycycline after intravenous administration at 5 mg/kg body weight in goats revealed that a concentration of greater than or equal to 0.5 microgram/ml was maintained for 5 min-2 h, 4-12 h, 2-12 h and 5 min-greater than 48 h in plasma, interstitial fluid, milk and urine respectively. The low t1/2 alpha of 0.73 +/- 0.11 h and high t1/2 beta of 16.63 +/- 1.58 h show that the drug is rapidly distributed but slowly eliminated from the body. The tissue:plasma concentration of 4.86 +/- 1.06 during the elimination phase [K12/(K21-beta)] indicates a high expected tissue concentration, which is supported by similarly increased drug concentration in interstitial fluid and milk. The high Vdarea of 9.78 +/- 0.86 L/kg observed denotes that, apart from its wide distribution, the drug may be stored in fat depots as it is known to be highly lipophilic. As the drug maintained a therapeutic concentration for a shorter time in plasma, and the calculated dose rate for maintaining a minimal plasma concentration of 0.5-1.5 micrograms/ml is relatively high, it may not be of much use in treating septicaemia in this species. Since the observed tissue:plasma concentration was higher and a therapeutic concentration was maintained in interstitial fluid and milk for longer, the drug can be used for other systemic infections at a lower dose rate than that required for treating septicaemia. As the drug maintained a very high concentration in urine, it may be of particular value in treating urinary tract infections caused by sensitive micro-organisms.