Latanoprost tends to degrade in an aqueous solution, and also adsorbs to the plastic containers. Moreover, the latanoprost content of the formulation can easily degrade, depending on the storage temperature, because of the very low drug concentration in the eye drops. Therefore, Xalatan^® Eye Drops should be stored at 2-8°C (in the refrigerator)for the reasons described above.

In this study, we investigated ways to improve the stability of latanoprost in aqueous solution in order to develop a formulation of latanoprost eye drops that can be stored at room temperature (1-30°C). Excipients used for opthalmological preparations were examined. The stability at 60°C was evaluated after 4 weeks in model formulations that included the excipient, which enables sterile filtration. As a result, the latanoprost content was not decreased in formulations that included polyoxyl 40 stearate and polyethylene glycol monostearate 25 at suitable concentrations. Then, we established a formulation of latanoprost eye drops that could be stored at room temperature. It was assumed that both the degradation of latanoprost and adsorption to the plastic containers were depressed by these excipients.

The formulation developed in this study would not need to be stored in the refrigerator either during the medication period of the patient or during distribution and storage at medical sites. It was also predicted that instructions on the use of these drugs would be simplified. Therefore, the latanoprost eye drops developed in this study would be very useful for patients and health care professionals.

The physicochemical and rheological properties of O/W type emulsions were investigated and evaluated by multiple light scattering analysis to enable topical drug formulation under various accelerated storage conditions (25-50°C). The multiple light scattering analyzer (Turbiscan^®) could quickly determine the physicochemical properties of the emulsions and measure initial concentrated O/W emulsions. This multiple light scattering analyzer could accurately estimate the physicochemical differences in emulsions from changes in their initial backscattering. Physicochemical stability, such as agglomeration/coalescence/creaming of the emulsions, was especially strongly related to their viscosity and droplet size. It was concluded that multiple light scattering analysis of O/W emulsions could be a novel and effective means of predicting of physicochemical stability in early drug formulation development.