Study on the Optimal Conditions for Long-term Storage of Melatonin Powder as an In-hospital Preparation

Melatonin is often prescribed to children with epilepsy. It is supplied as an in-hospital preparation in Japan, but information on the storage conditions of such formulations is lacking. In this study, we used an analytical method to perform periodic measurements in order to establish expiration periods and storage conditions for compounded melatonin powder. A dispenser was used to envelop compounded melatonin powder in wrapping paper, after which the powders were preserved in 1) an opened transparent plastic bag, 2) a closed opaque plastic bag, 3) a closed transparent plastic bag with a desiccant, and 4) a closed opaque plastic bag with a desiccant. The bags were then stored at room temperature (15°C to 25°C), in a refrigerator (4°C), and in a freezer (-20°C). Chromatographic analysis was performed using an ODS-C18 column (40°C, ? = 210 nm) with a mobile phase consisting of methanol: 0.1 M sodium phosphate buffer (55:45, v/v) at a flow rate of 1 mL/min. The measurements were made at the following time points: 0 days, 7 days, 14 days, 28 days (1 month), 56 days (2 months), 84 days (3 months), 112 days (4 months), 140 days (5 months), and 168 days (6 months). The residual rates of melatonin for all time points, at all temperatures, under all conditions exceeded 95%. These experimental data suggested that the melatonin powder is stable for at least 6 months at room temperature of 15°C to 25°C.