This paper addresses two critical issues concerning the guidelines adopted by the ICH on the photostability testing: the quinine actinometry method and the light/radiation exposure map distribution of the photostability chamber. Using a qualified non-commercial photostability chamber tests were performed using quinine and physical actinometry and compared the results to those which are used as the basis of the ICH guidelines. The statistical analysis on the results showed that: (i) the calibration curve of the quinine solution depends on its concentration and on its location in the chamber; (ii) the quinine actinometry method currently recommended by the ICH guidelines should not be generalized to any photostability chamber.

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