Doxycycline concentration over time after storage in a compounded veterinary preparation.

OBJECTIVE: To determine the concentration of doxycycline compounded from doxycycline hyclate tablets into liquid formulations for oral administration in veterinary species and stored for 28 days.
DESIGN: Evaluation study. SAMPLE: Doxycycline hyclate tablets (100 mg) crushed and mixed with a 50:50 mixture of syrup and suspension vehicles for oral administration to produce 3 batches each of 2 doxycycline formulations: 33.3 and 166.7 mg/mL. PROCEDURES: Formulations were stored, protected from light, at room temperature (22° to 26°C [71.6° to 78.8°F]) and at a controlled cold temperature (refrigerated 2° to 8°C [35.6° to 46.4°F]). Doxycycline was extracted from the formulations, and concentration was measured by high-pressure liquid chromatography on days 0 (date of preparation), 1, 4, 7, 14, 21, and 28. Concentrations were compared with those of a US Pharmacopeial Convention reference standard. Formulation quality at each point was also assessed through color change, formulation consistency, and suspension uniformity.
RESULTS: On days 0, 1, 4, and 7, the concentration of each formulation was within 90% to 110% of the reference standard (range, 93% to 109%), which was deemed acceptable. However, doxycycline concentrations had decreased dramatically by day 14 and remained low for the duration of the study period. Doxycycline concentrations on days 14, 21, and 28 were all < 20% (range, 14% to 18%) of the reference standard, and the quality of the formulations decreased as well. No effect of storage temperatures on doxycycline concentration was identified. CONCLUSIONS AND CLINICAL RELEVANCE: The concentration of doxycycline, compounded from commercial tablets in the vehicles evaluated to yield doses of 33.3 and 166.7 mg/mL, cannot be assured beyond 7 days.