Leishmaniases are parasitic diseases caused by more than 20 different Leishmania species, with approximately 0.7–1.0 million new cases each year occurring in 98 countries around the world (World Health Organisation). This disease complex presents two main clinical manifestations: visceral leishmaniasis (VL), which is caused by the Leishmania donovani and L. infantum species and can cause fever, weight loss, hepatosplenomegaly, anemia, among others. This disease can be fatal if acute and untreated; and tegumentary leishmaniasis (TL), which is the most common clinical form of leishmaniases and is caused by distinct parasite species (Burza et al., 2018). TL present three main clinic forms: cutaneous leishmaniasis (CL), mucosal leishmaniasis (ML), and diffuse-cutaneous leishmaniasis (DCL). These diseases cause since self-limiting lesions until scars in mucous membranes, leading to the patient morbidity (Aronson and Joya, 2019; Kaye et al., 2020). TL can be evoked by infection with several Leishmania species, such as Leishmania braziliensis, L. guyanensis, L. amazonensis, among others; and parasitological and molecular evaluations have shown that L. braziliensis is the most important etiological agent of the disease in the Americas, and responsible by severe clinic manifestations in the patients (Volpedo et al., 2021).

Parasitological diagnosis is considered to be the gold standard for TL, in which the visualization of the parasite in skin or mucosal lesion samples, its isolation in cultures, and/or its inoculation in animals are all considered to be valid diagnostic techniques (Skraba et al., 2014). Although these methods present high specificity, the sensitivity is variable, between 60% and 95% in cultures and smears (Duthie et al., 2018; Oyama et al., 2018). The molecular parasitological diagnosis has also been performed and presents specific advantages, such as higher sensitivity than conventional parasitological techniques; however, it is expensive and requires a specialized laboratory structure (Boni et al., 2017; Galluzzi et al., 2018).

Treatment against TL is hampered due to the toxicity of the drugs, high cost, and/or the emergence of resistant strains (Amato et al., 2008; Roatt et al., 2020). Pentavalent antimonials are considered the first-choice treatment; however, they can cause severe adverse effects in patients (Ponte-Sucre et al., 2017). Other compounds, such as deoxycholate amphotericin B and its liposomal formulations, pentamidine, paramomycin, among others, may also be used, but them all cause adverse effects, which limit their use and adherence to treatment (Pradhan et al., 2022). In addition, in endemic regions of TL, the treatment failure rate can reach 50% of all cases (Prates et al., 2017), which indicate the requirement for the development of new, safer, and more effective antileishmanial drugs.

Therefore, the development of new strategies to treat leishmaniasis has become a priority. In this light, natural products and synthetic molecules have traditionally played an important role in drug discovery and were the basis of most early medicines (Cheuka et al., 2016; Singh et al., 2022). Vanillin is an organoleptic compound of natural vanilla acquired from the pods of an orchid named Vanilla planifolia. It presents a chemical structure that allows for its the use in distinct biological applications, since vanillin and its derivatives have shown anticancer (Rakoczy et al., 2021), antibacterial (Mok et al., 2020), antidiabetic (Salau et al., 2021), and antileishmanial (Freitas et al., 2023) activities.

In a recent study developed by our research group, two novel synthetic derivatives from vanillin, called 3s [4-(2-hydroxy-3-(4-octyl-1H-1,2,3-triazol-1-yl)propoxy)-3-methoxybenzaldehyde] and 3t [4-(3-(4-decyl-1H-1,2,3-triazol-1-yl)-2-hydroxypropoxy)-3-methoxybenzaldehyde], were evaluated regarding their in vitro antileishmanial activity against L. infantum, L. amazonensis, and L. braziliensis promastigotes and intra-amastigotes (Freitas et al., 2023). Results showed that both compounds were effective and selective against parasites, and they reduced the parasite load in treated and infected murine macrophages. The mechanism of action evaluated in L. infantum revealed that 3s and 3t altered the parasite mitochondrial membrane potential, leading to reactive oxygen species production, an increase in lipid bodies, and changes in the cell cycle, causing the parasite's death.

In this context, in the present study, we evaluated the in vivo antileishmanial activity of 3s and 3t against L. amazonensis infection. For this, BALB/c mice were infected with parasite promastigotes and later received 3s and 3t, which were administered pure or incorporated into Poloxamer P407-based micelles, which were used with delivery and adjuvant purposes (Tavares et al., 2019a, 2020; Mendonça et al., 2022). Other animals were treated with AmpB or its liposomal formulation (Ambisome®), and the lesion development, parasite burden, cellular and humoral responses, and biochemical markers were evaluated one and 30 days after treatment.