Long term physicochemical stability study of novel ophthalmic formulations combining ceftazidime and vancomycin with and without cyclodextrins

Abstract Objectives Ceftazidime (CZ) and Vancomycin (VM) are used to treat bacterial keratitis; however, their physicochemical incompatibility does not allow their co-administration. This incompatibility can be managed by buffering the mixture at an alkaline pH or by using cage molecules such as cyclodextrins (CD). The objective of this work was to compare the stability during 168 days of frozen storage of two formulations combining VA and CZ at a final concentration of 25 mg/mL: a CD-free formulation, at a pH=8.5 and a formulation with CD. Methods Beforehand, a stability indicating method (SIM) was developed. Samples were analysed after 1, 3 and 6 months, and after 12, 24 and 72 h after defrosting. Analyses performed were the following: visual inspection, chromaticity, turbidity, osmolality and pH measurements, particles counting, CZ and VM quantification, breakdown product research, and sterility assay. Results The developed SIM allowed the simultaneous quantification and breakdown products research of both VM and CZ, without interference of the breakdown products. The analyses showed the presence of a visually detectable precipitate and increased turbidity as early as the first day after thawing for CD-free formulation and on the third day for the formulation with CD. CZ concentrations systematically decreased after thawing for both formulations whilst VM concentrations remained stable. Osmolality and pH remained unchanged, and no microbial growth was detected throughout the study. Conclusions CD delayed precipitation by 48 h compared to the CD-free formulation but did not permanently eliminate it. Both formulations showed very limited physicochemical stability after thawing.