Spilanthol-rich essential oil obtained by microwave-assisted extraction from Acmella oleracea (L.) R.K. Jansen and its nanoemulsion: Insecticidal, cytotoxic and anti-inflammatory activities

Acmella oleracea (L.) R.K. Jansen, also named jambù, is an edible and medicinal plant native to Brazil but extensively cultivated all over the world due to its diverse utilizations in food, cosmetics, pharmaceutics, and pest management science. Most of applications are related to the presence of active ingredients, namely alkylamides, among which spilanthol is the most important one. On the other hand, the plant biomass resulting from the large-scale cultivation of A. oleracea may yield an essential oil (EO), which can be used for some industrial purposes, for instance as larvicide against vector mosquitoes. This EO is mainly characterized by mono- and sesquiterpenes, however also spilanthol can be detected in the mixture. To maximize the content of spilanthol in the A. oleracea EO, A. oleracea aerial parts obtained from a cultivation in central Italy were subjected to microwave-assisted extraction (MAE). This procedure allowed to obtain a higher yield and spilanthol content when compared with traditional hydrodistillation (0.47 v 0.22 %, and 13.31 vs 2.24 %, respectively). The EO terpene fraction was mainly represented by β-pinene (10.8 %), myrcene (12.3 %), (E)-caryophyllene (19.4 %) and α-humulene (1.1 %). The A. oleracea EO was encapsulated into a nanoemulsion (NE) containing 6 % EO using a high-energy method. The formulation physical stability was assessed by optical microscope and DLS analyses at different timepoints showing stability up to one year of storage. The A. oleracea EO and its NE, along with the isolated spilanthol, were evaluated for acute toxicity against the 3rd instar larvae of Culex quinquefasciatus, a filariasis and arbovirus vector of public health importance, achieving LC₅₀ values of 16.1, 407.5, and 3.1 μL/L, respectively. The larvicidal activity seemed to be not dependent on the interaction with the cholinergic system, as shown by the high IC₅₀ values of A. oleracea EO and spilanthol exhibited in the anti-acetylcholinesterase (AChE) assay. Testing LC₃₀ values, the A. oleracea-borne products showed a significant impact on larval development, fecundity, fertility and natality of C. quinquefasciatus. Testing LC₃₀ values, the A. oleracea EO-NE provided a higher efficacy than spilanthol, reducing the adult fertility, in terms of egg hatchability (%) and the overall abundance of F₁ larvae. These results can be considered as promising for the development of new mosquito larvicides. The safety use of A. oleracea EO, NE and spilanthol was demonstrated in assays on mammalian fibroblasts and microglia cells, showing low level of cytotoxicity coupled with protective effects against inflammation.