Opportunities and challenges for the nasal administration of nanoemulsions.

Nasal delivery has become a growing area of interest for drug administration as a consequence of several practical advantages, such as ease of administration and non-invasiveness. Moreover, the avoidance of hepatic first-pass metabolism and rapid and efficient absorption across the permeable nasal mucosa offer a promising alternative to other traditional administration routes, such as oral or parenteral delivery. In fact, nasal delivery has been proposed for a number of applications, including local, systemic, direct nose-to-brain and mucosal vaccine delivery. Nanoemulsions, due to their stability, small droplet size and optimal solubilization properties, represent a versatile formulation approach suitable for several administration routes. Nanoemulsions demonstrated great potential in nasal drug delivery, increasing the absorption and the bioavailability of many drugs for systemic and nose-to-brain delivery. Furthermore, they act as an active component, i.e. an adjuvant, in nasal mucosal vaccinations, displaying the ability to induce robust mucosal immunity, high serum antibodies titres and a cellular immune response avoiding inflammatory response. Interestingly, nanoemulsions have not been proposed for the treatment of local ailments of the nose. Despite the promising results in vitro and in vitro, the application of nanoemulsions for nasal delivery in humans appears mainly hindered by the lack of detailed toxicology studies to determine the effect of these formulations on the nasal mucosa and cilia and the lack of extensive clinical trials.