In vitro potency and stability of fortified ophthalmic antibiotics

This study evaluates the in vitro potency, stability and contamination of the fortified ophthalmic antibiotic preparations of cefazolin, vancomycin, gentamicin and tobramycin stored for 4 weeks. The effects of the different solvents and storage temperatures on the antimicrobial potency and stability were also examined.

The fortified stock solution of cefazolin and vancomycin were prepared by reconstituting with 0.9% sodium chloride and with artificial tears. Gentamicin and tobramycin were prepared by adding parenteral forms into their commercial ophthalmic solutions. The antimicrobial potency was measured by the minimum bactericidal concentration for cefazolin (33.3 mg/mL) and vancomycin (31 mg/mL) against Staphylococcus aureus, and for tobramycin (13.5 mg/mL) and gentamicin (13.5 mg/mL) against Pseudomonas aeruginosa. The stability of solution was evaluated by measuring absorbance spectra and pH. During the study period the levels of contamination of the stock solutions were examined by culturing on blood and on Sabouraud-dextrose media.

There were no differences in the antimicrobial potency of the cefazolin, vancomycin and tobramycin within the 4-week period; however, the potency of gentamicin was decreased at both temperatures after 21 days. It was found that different solvents or storage temperatures had no effect on the potency. Throughout the 4-week period, no change was found in the absorbance spectra of gentamicin and tobramycin, whereas the absorbance spectra of cefazolin and vancomycin stored at 24 degrees C increased in both of the solvents used (P<0.05). In all of the antibiotics, pH changed at 24 degrees C after 7-10 days; however, at 4 degrees C, only the pH of tobramycin exhibited changes after 14 days (P<0.05). No contamination was detected in the stock solutions during the study period.

Topical fortified antibiotic solutions used for longer than 7 days should be stored at < or =4 degrees C, those stored at 24 degrees C should be discarded after 7 days.