Cocrystallization with nicotinamide promotes naproxen photodegradation in the solid-state

In this study, we evaluated the photostability of a cocrystal of naproxen (NPX) and nicotinamide (NA) for the development of the photostabilization strategy. NPX photodegradation during ultraviolet-light (UV) irradiation was estimated by high-performance liquid chromatography (HPLC). The photostability of a NPX-NA cocrystal was less than NPX in the solid-state. Furthermore, NPX was photodegraded faster in the presence of NA at a higher concentration when the NPX-NA mixtures were UV-irradiated. The values of residual amounts of NPX in UV-irradiated NPX-NA cocrystal and mixture were 76.60 ± 3.85% and 70.06 ± 4.09%, respectively, which were significantly lower compared with that of UV-irradiated NPX powder (83.57 ± 2.15%). However, in the case that the effect of NA in the suspension of NPX was investigated, NA had no effect on NPX photodegradation. Residual amounts of NPX in UV-irradiated suspensions of NPX and NPX-NA cocrystal were comparable (33.13 ± 6.02% and 30.22 ± 2.09%, respectively). These results suggested that NA might promote NPX photodegradation only in the solid-state on account of that NA molecule could deliver the excitation energy to NPX molecule. This is the first study focused on the photochemical behavior of NPX-NA cocrystal and mixture and suggests that the presence of NA might induce the change of photostability of NPX in the powder form.