title: In vitro potency and stability of fortified ophthalmic antibiotics.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Ophthalmology, Cumhuriyet University, School of Medicine, Sivas, Turkey.
authors.forename: M K
authors.initials: MK
authors.lastname: Arici
authors.forename: Z
authors.initials: Z
authors.lastname: Sümer
authors.forename: C
authors.initials: C
authors.lastname: Güler
authors.forename: O
authors.initials: O
authors.lastname: Elibol
authors.forename: G
authors.initials: G
authors.lastname: Saygi
authors.forename: S
authors.initials: S
authors.lastname: Cetinkaya
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002437:Cefazolin / Q000494:pharmacology; D004355:Drug Stability; D004356:Drug Storage; D005839:Gentamicins / Q000494:pharmacology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008826:Microbial Sensitivity Tests; D009883:Ophthalmic Solutions / Q000494:pharmacology; D011550:Pseudomonas aeruginosa / Q000187:drug effects* / Q000254:growth & development; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000254:growth & development; D013696:Temperature; D014031:Tobramycin / Q000494:pharmacology; D014640:Vancomycin / Q000494:pharmacology
chemical_list: D000900:Anti-Bacterial Agents; D005839:Gentamicins; D009883:Ophthalmic Solutions; D014640:Vancomycin; D002437:Cefazolin; D014031:Tobramycin