Incorporation of fixed oils into spearmint oil-loaded nanoemulsions and their influence on characteristic and cytotoxic properties against human oral cancer cells

Spearmint oil (SMO) has been revealed to inhibit oral cancer cells. Its activity increases when it is incorporated into nanoemulsions. However, SMO alone cannot be prepared into stable nanoemulsions without an Ostwald ripening inhibitor. This study aimed to evaluate the effect of an Ostwald ripening inhibitor, i.e., fixed oil, on the formation and characteristic of anticancer nanoemulsions. Various fixed oils, namely, virgin coconut oil (VCO; C12), palm oil (PMO; C16), olive oil (OLO; C18:1), soybean oil (SBO; C18:2), and perilla oil (PRO; C18:3), were selected, and their abilities to stabilize nanoemulsions were compared. Results indicated that the formation and characteristic of nanoemulsions were dependent on the type of fixed oils and the ratio of SMO to fixed oils. Nevertheless, the stable nanoemulsions with nearly the same droplet size could be prepared at a ratio of 80:20 regardless of the type of fixed oils. The stable nanoemulsions might be a result of specific molecular interactions among surfactants, SMO, and fixed oils. Among the nanoemulsions with fixed oils, 80:20 SMO:PRO nanoemulsion showed the most potent cytotoxic effect against oral cancer cells and therefore was selected for further study. The influence of excipients on the properties, including droplet size, carvone content, and cytotoxic activity, of the formulated nanoemulsions were comparatively evaluated after storage at 40 °C for 6 months. Both nanoemulsions had excellent stability. An apoptotic property assay was employed to probe the cell death mechanism. 80:20 SMO:PRO nanoemulsion could induce cancer cell death via the intrinsic apoptosis pathway, which was the desirable pathway for candidate anticancer agents. In conclusion, nanoemulsions containing SMO with cytotoxic effects against oral cancer cells were successfully prepared by selecting specific amounts and types of surfactants and fixed oils. This research might serve as a basis for fabricating stable nanoemulsions containing other volatile oils.