Photostability of indomethacin in model gelatin capsules: Effects of film thickness and concentration of titanium dioxide on the coloration and photolytic degradation.

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The dependence of the photostability of indomethacin on the capsule shell thickness and on the concentration of titanium dioxide used as an opacificient was investigated using gelatin films prepared by casting as models of hard gelatin capsules. The film thickness and concentration of opacificient were varied in the ranges of 50-150μm and 0-1.5% of the amount of dried films, respectively. Films and indomethacin tablets were exposed to a 400 W mercury vapor lamp for 120 min, then coloration of the enclosed indomethacin was measured by tristimulus colorimetry, and evaluated as Hunter's color difference. The chemical stability of the drug was determined by following the ultraviolet spectrophotometric change of a solid sample. There was a good linear relationship between color difference values and the square root of exposure time at all concentrations and thicknesses. The coloration rate constants calculated from the slopes of these lines were affected by both parameters, and decreased significantly as the film thickness was increased at every concentration of opacificient. However, the effect of concentration seemed to disappear above 1% addition. The coloration rate was directly dependent on the average transmittance of films over the wavelength range relating to the photostability of indomethacin. Degradation of indomethacin followed apparently sequential first-order kinetics. Apparent conversions after 120-min exposure as determined from the UV spectra were also affected by both parameters and were below 3% at thicknesses above 80μm at every opacificient concentration tested ; this represents excellent protection of indomethacin.

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