Pharmacotherapeutic group: Antibacterials for systemic use, Tetracyclines
ATC Vet code: QJ01AA02
Pharmacodynamic properties
Doxycycline is a second generation, broad spectrum tetracycline.
It is active against a large number of Gram positive and Gram negative pathogens including strains resistant to first generation tetracyclines.
It is essentially bacteriostatic; it inhibits the bacterial protein synthesis by blocking binding of transfer RNA to the messenger RNA-ribosome complex.
There are several known mechanisms of resistance to tetracyclines such as doxycycline but the two most common are energy dependant efflux systems and ribosomal protection proteins.
Resistance is mainly mediated by efflux pumps or ribosomal protection proteins. Cross-resistance among tetracyclines is common but depends on resistance mechanisms: i.e. mutation in efflux pumps that renders resistance to tetracycline may still be sensitive to doxycycline.
Pharmacokinetic particulars
After oral administration in dogs and cats at the recommended dose of 10 mg/kg, doxycycline reaches the maximal plasma concentration (Tmax) within 24 hours. The peak concentration (Cmax) is 1.4 μg/ml and 4.3 μg/ml in dogs and cats respectively. The oral bioavailablilty of doxycycline after repeated administration is approximately 45% in both species and is not affected by the presence of food.
In spite of a high protein binding rate, the volume of distribution of doxycycline is high demonstrating that doxycycline is broadly distributed in organs and tissues. This is due to high liposolubility of doxycycline.
Doxycycline is mainly excreted as unchanged drug and eliminated in faeces and urine. Mean elimination half-life is 8.37 hours in cats.