Objectives Melatonin is commonly prescribed in paediatric population, but it is commercially only available in tablet form. Oral suspensions are useful pharmaceutical preparations faced with difficulties from the pharmaceutical industry to provide medicines to children. Ready-for-use suspending agents are used, although their compositions are unsuitable for children. A new liquid vehicle for oral suspensions, Inorpha, was commercialised with a formulation adapted in paediatrics. The stability of a melatonin oral suspension 2 mg/mL in Inorpha was studied. The physicochemical and microbiological stabilities were respectively evaluated during 6  and 4 months.

Methods A validated high-performance liquid chromatography method was used to assess the melatonin concentration under room temperature and refrigerated conditions weekly. A validated microbiological method was employed on 10 mL vials, on the one hand, and one 200 mL bottle opened every day, on the other hand, in order to evaluate respectively the microbiological stability before opening and under conditions of use.

Results After 18 weeks under room temperature and refrigerated conditions, samples held respectively a melatonin minimal concentration of 99.3% and 96.7% of initial concentration. After 6 months, melatonin concentration was 96.1% of initial concentration. Total aerobic microbial counts obtained during the 4-month microbiological study were <10 CFU/mL. None of the melatonin suspensions presented fungal growth.

Conclusions Variation of the active ingredient concentration did not exceed 5% after 6 months. Microbial enumeration assays demonstrated microbiological stability of suspensions over a period of 4 months. The microbiological quality was not altered by repeated openings of the bottle allowing production of multidose vials.