Effect of Application Device Design on Scalp Delivery of Topical Minoxidil Formulations

Equivalence of OTC formulations is generally assessed based on pharmaceutical attributes such as active ingredient content, dissolution behavior, and stability. However, for topical liquid formulations that are directly applied to the skin or scalp, the container design and usability may also influence the applied dose and the amount that reaches the target site. Appropriate design of containers or applicator devices is therefore essential for ensuring the effectiveness and safety of OTC topical products. In this study, we investigated the effects of container-tip geometry and hair density on scalp delivery of minoxidil (MXD) using four marketed MXD topical formulations. A scalp model was prepared by covering filter paper with human hair, and we evaluated: (1) the amount of MXD dispensed per actuation from each container, (2) the amount of MXD that reached beneath the hair layer, and (3) the in vitro skin permeation using porcine ear skin. Increased hair density reduced MXD delivery to the scalp for all formulations, especially for containers with larger tip ends. These findings suggest that ensuring the efficacy and reproducibility of MXD-containing OTC topical formulations requires not only optimizing drug permeability and formulation characteristics, but also considering hair density and container-tip geometry in designing appropriate usage conditions.