The solid-state stability of doxycycline, trimethoprim and sulfamethoxypyridazine alone or in combination was evaluated for 135 days (20 weeks). A validated HPLC assay was developed for their simultaneous determination and applied to assess their solid-state stability alone and in binary combinations upon extemporaneous preparation. The results showed that the drugs are stable based on ICH Q1A (R2) protocol for at least 135 days at 25±2ºC and at 60±5% of relative humidity and protected from light. Similarly, the solid-state stability of the combination mixtures of doxycycline and sulfamethoxypyridazine, doxycycline and trimethoprim, and trimethoprim and sulfamethoxypyridazine were also shown stable in the above conditions. These findings provide evidence for the suitability of preparation of extemporaneous formulations which might help for safe and efficacious treatments in veterinary medicine.

Key words: Doxycycline, Solid-state stability, Sulfamethoxypyridazine, Trimethoprim, Veterinary medicine