Stability of cefazolin sodium eye drops

Objective: Assessing the stability of cefazolin sodium in preservative-free and preservative-containing eye drops. Method: Extemporaneous formulations of eye drops were prepared from a commercially-available parenteral product of cefazolin sodium: eye drops ‘A’ contained 50 mg/ml of cefazolin sodium in 0·45% w/v sodium chloride solution, and eye drops ‘B’ contained 50 mg/ml, 0·005% w/v thiomersal and 1% w/v glycerol in water-for-injection. Cefazolin sodium concentrations in these eye drops were monitored by a stability-indicating HPLC assay method. Measurements of pH and osmolality, as well as tests for microbial contamination, were conducted. Results: The eye drops stored at 4 °C were stable for 42 days with minimal changes in pH and osmol- ality, but eye drops stored at room temperature were only stable for a few days with greater incre- ments in pH and osmolality. None of the samples cultured had bacterial or fungal growth after 7 days of incubation. Conclusion: Extemporaneously prepared formula- tions of cefazolin are unstable at room temperature and should be stored in a refrigerator.