In-vitro red blood cell partitioning of doxycycline

Objective: In-vitro red blood cell (RBC) partitioning of doxycycline was studied to determine whether doxycycline penetrates RBC and its concentration was assayed keeping in view its high lipophilicity. Materials and Methods: Standardization of doxycycline was performed in whole blood and plasma of cattle by microbiological assay using Bacillus subtillis ATCC 6633 as indicator organizm. Actual concentration of the drug was obtained by comparing zone inhibition with standard graph and the extent of partitioning was mathematically calculated. Results: The R(2) value of standard graph for doxycycline was 0.9934 and 0.9727 for plasma and whole blood, respectively. Overall, RBC partitioning of doxycycline was found to be 18.40 +/- 1.70%. Conclusions: Overall RBC partitioning of doxycycline indicated low penetration into RBC. Plasma is the fluid suggested for pharmacokinetic evaluation of doxycycline.