Concentrations of doxycycline in muscle tissue and muscle tissue fluid.

Doxycycline is known to have a marked tissue affinity, but there seems to be no previous studies where doxycycline levels have been compared in different compartments of a tissue. The purpose of the present investigation was to measure concentrations of doxycycline in serum, muscle and muscle tissue fluid using an experimental model in rabbits. After an intravenous infusion of doxycycline 10 mg/kg, samples were obtained at intervals for 4 h and concentrations were measured using microbiological methods. The levels of doxycycline in muscle tissue fluid followed closely the concentrations in serum and showed a similar rate of elimination. Doxycycline concentrations in muscle peaked later than the tissue fluid concentrations and were much higher than the corresponding serum level (mean ratio tissue/serum = 3.5).