Unraveling the Impact of the Oil Phase on the Physicochemical Stability and Skin Permeability of Melatonin Gel Formulations

Melatonin's antioxidant properties make it a valuable component in anti-aging semisolid topical products. This study explores the role of Pemulen^®, an acrylic-based viscosifying agent, in stabilizing cream-gel formulations. Remarkably, even at low concentrations (0.4%), Pemulen^® successfully produced physicochemical stable topical formulations. In this work, the impact of the ratio of the oily phase-comprising olive oil and isopropyl myristate from 0 to 20%-was investigated to understand the internal microstructure effect on skin permeability, rheological properties, and stability. The formulations exhibited pseudoplastic behavior, with a significant positive correlation (<i>p</i>-value < 0.1) between the oily phase ratio, viscosity, spreadability, skin adhesiveness, and permeability. Formulations without the oil phase exhibited greater skin permeability. However, higher oily phase content enhanced viscosity, spreadability, and skin adhesion. Given that melatonin primarily degrades through oxidation, incorporating antioxidant excipients in semisolid formulations is crucial for maintaining its chemical stability. A quality by design (QbD) approach was used to assess the impact of four excipients-(a) DL-α-tocopheryl acetate (0.05%), (b) ascorbic acid (0.1%), (c) ethylene diamine tetraacetic acid (0.1%), and (d) sodium metabisulphite (0.5%)-on melatonin's stability. Our findings indicate that maintaining the physical stability of the formulation with a 20% oil phase is more critical for protecting melatonin from oxidation than merely adding antioxidant excipients.