## 1 Introduction  
Cancer is among the leading causes of death in most countries worldwide and is estimated to be responsible for 19.3 million new cases and 10 million deaths in 2020 [1]. The burden of cancer incidence and mortality is expanding rapidly, with a 47% increase in new cancer cases (28.4 million) projected for 2040 [1]. By far, lung cancer is the leading cause of cancer death (1.8 million), followed by liver (830,000), stomach (780,000), breast (685,000), and colon (575,000) cancer. The expansion of precision medicine and innovative therapeutic approaches, including targeted therapy and immunotherapy, in the last few years has provided excellent results in some instances. However, a wide range of constraints, such as ineffectiveness, resistance, unfavorable side effects, and extremely expensive costs, have lowered expectations for these treatments [2]. This scenario highlights the necessity to investigate new therapeutic options and/or improve the established ones. Nature is an excellent source of molecules for innovation in drug discovery. More than 50% of all anticancer drugs approved in the world between 1940 and 2010 were natural products or their derivatives [3; 4]. Among natural products in general and phytochemicals in particular, essential oils (EOs) have gained special interest because of their broad diversity of bioactivities [4]. Essential oils are complex, lipid-soluble, and volatile chemicals produced by aromatic plants that act as secondary metabolites. They are natural mixtures (20-60 compounds) characterized by high concentrations (20-70%) of two or three main components, particularly terpenes and terpenoids [5; 6; 7]. Essential oils are obtained from the leaves, roots, and stems of plants by hydrodistillation, steam distillation, or dry distillation, and from fruit peels by cold pressing [7]. In nature, they play a critical function in plant defense against bacteria, fungi, insects, and herbivores, as well as pollinating insect attractants [5; 6; 7]. Since ancient times, EOs have been used for the prevention and treatment of a variety of diseases. Their chemopre and chemotherapeutic effects have been widely documented. In addition, in vitro studies have demonstrated that EO extracts employed as single agents selectively target cancer cells, with no or significantly reduced cytotoxicity against healthy cells [6; 7; 8]. The complex mixtures found in EO compositions should be considered an advantage because the potential multi-target and synergistic effects of the individual components are in accordance with the goal of cancer treatments to evade resistance mechanisms [9; 10; 11]. Notwithstanding the anticancer benefits described, there are several drawbacks to using EO, including degradation by oxidation and evaporation, decomposition by ultraviolet light and heat, and low water solubility [12]. The nanoencapsulation of EO into lipid nanoparticles appears to be a strategy to solve these issues [13; 14; 15]. Solid lipid nanoparticles (SLN) emerged in the 1990s for the encapsulation of lipophilic drugs as an alternative to conventional lipid colloidal carriers such as emulsions and liposomes [15; 16]. The primary benefits of SLN include their high physical stability, high drug loading, controlled drug release profile, surface modification to enhance passive and active cell targeting, and protection of labile drugs from environmental degradation [17; 18; 19]. SLNs are biocompatible, non-toxic, and easily produced on a large scale [19]. Moreover, the inclusion of liquid lipids (such as EO) into the solid lipid matrix of SLN promotes their reorganization into nanostructured lipid carriers (NLC) [20; 21], improving their stability, encapsulation efficiency, and drug loading capacity, and limiting the expulsion of cargo molecules during storage [17; 18; 19].