Effect of spectroscopic properties on photostability of tamoxifen citrate polymorphs

The photostability of tamoxifen citrate polymorphs, forms A and B, was investigated by chromatographic and spectroscopic analyses including high-pressure liquid chromatography (HPLC), colorimetry and UV/vis solid-state absorption spectroscopy. On the basis of the results of photostability studies under irradiation by visible light and both UVA (320–400 nm) and a fraction of UVB (290–320 nm) light, form A was chemically unstable, whereas form B was stable against light irradiation. The surface color of pellets prepared with any of these crystal forms turned from white to brown; however, the extent of color change in cross-sections of form A pellet was deeper than that of form B pellet. The maximum peak of UV/vis solid-state absorption spectra of form A was observed at 337 nm within the UVA range and was in longer wavelength regions than form B, which exhibited the strong UV absorption mainly in UVB and UVC region. The results obtained suggested that the photodegradation followed by surface color change of form A crystal was caused by the selective absorption of photoenergy of UVA light irradiated by a xenon lamp.