Oxidative degradation of melatonin in solution and solid-state pharmaceutical blends: HPLC-HRMS/MS degradant...

Melatonin (MLT) is used in pharmaceutical products, yet its oxidative degradation profile remains only partially defined, especially under formulation-relevant solid-state conditions. This study combined HPLC-PDA and HPLC-HRMS/MS to characterize MLT degradation in solution and in excipient-containing solid blends. Solution-phase forced degradation was performed with hydrogen peroxide, ACVA, and Cu(II), while solid-state stress studies were carried out in capsule-equivalent blends containing hydrogen peroxide and Cu(II), with EDTA used as a mechanistic mitigation probe. Five degradation products were detected overall and assigned at different confidence levels. Hydrogen peroxide generated DP-1, DP-2, and DP-3, whereas ACVA generated DP-4 and DP-5; Cu(II) selectively favored DP-4 in solution, indicating stress-dependent oxidative pathways. The chromatographic method resolved MLT from all observed degradation products and satisfactory mass balance was obtained under all tested solution stresses. In solid blends, MLT remained essentially stable under thermal control, degraded moderately in the presence of peroxide alone, and degraded substantially faster when peroxide and Cu(II) were combined. This acceleration was markedly attenuated by EDTA, supporting a metal-mediated catalytic contribution. The solid-state study should be interpreted as an accelerated worst-case mechanistic model rather than a predictive real-time stability study. A complementary computational toxicological screen did not indicate a strong overall mutagenic concern for the identified products, although AFMK (DP-3) may warrant targeted experimental clarification if required. The results expand the described oxidative degradation map of MLT and link degradant formation to microenvironmental factors relevant to impurity control and formulation-oriented stabilization strategies.