Stability of trimethoprim in newly formulated liquid dosage form

Background: Extemporaneous preparation of trimethoprim oral suspension using a commercially available trimethoprim tablet, methylcellulose mucilage and syrup, is cumbersome and time-consuming and there is also an increased risk of dental caries when it is used for the prophylaxis of urinary tract infection.

Aim: To investigate the stability of trimethoprim suspension formulated in Ora-Plus/Ora-Sweet SF over a period of 63 days under two different storage conditions.

Methods: Commercially available trimethoprim tablets, Ora-Plus and Ora-Sweet SF were used to prepare trimethoprim suspension. The suspension (10 mg/mL) was then packaged in glass bottles and stored at 4 or 25°C. An aliquot was withdrawn immediately before (Day 0) and after 2, 4, 7, 14, 21, 30, 45 and 63 days. Samples were analysed by high-performance liquid chromatography to measure the changes in trimethoprim concentration before and after storage. Samples were assessed for changes in physical appearance (e.g. odour, caking and ease of resuspension) and pH. Microbial contamination was assessed at the end of 63 days of storage.

Results: Trimethoprim retained more than 95% of its initial concentration for at least 63 days when stored at 4 or 25°C. There were no apparent changes in physical appearance of the suspension and the pH of the suspension remained unchanged throughout the study period. No microbial growth was observed in the tested samples.

Conclusion: The newly formulated trimethoprim suspension is chemically, physically and microbiologically stable for at least 63 days, and therefore such formulation can now be extemporaneously prepared and given an acceptable shelf life.