**1. Introduction**

Traditional or folk medicine relies heavily on the use of compounds-rich plants, like those of the Asteraceae family, of which many such species are commercially available in the form of herbal preparations. These are particularly rich in a wide range of natural terpenoids named sesquiterpene lactones [1] that, in some cases, are considered the active principles of such therapeutic plants [2]. Structurally speaking, sesquiterpene lactones are terpenes that have in common a basic structure of 15 carbons (thus the prefix sesqui-) resulting from biosynthesis involving three isoprene units with a cyclical structure along with a fused α-methylene-γ-lactone ring [3]. Sesquiterpene synthases catalyze a common biosynthesis route for sesquiterpene lactones, based on the cyclization of farnesyl

phosphate resulting from the 2-C-methyl-D-erythritol-4-phosphate (MEP) and mevalonate (MVA) pathways of dimethylalyl diphosphate and isopentenyl diphosphate precursors in chloroplasts and cytosol, respectively [4]. However, sesquiterpene lactones have very different chemical structures regarding the type and position of the substituents, as well as the size of the non-lactone ring [5,6]. For this reason, in structural terms, sesquiterpene lactones are organized into several subclasses: eudesmanolide (a 6/6 bicyclic structure), guaianolide and pseudoguaianolide (both 5/7 bicyclic compounds), germacranolide (with a 10-membered ring) and xanthanolide (containing a non-cyclic carbon chain and a seven-membered ring) [7,8] (Figure 1).

**Figure 1.** Basic chemical structure of each of the sesquiterpene lactone subclasses: eudesmanolide, guaianolide, pseudoguaianolide, germacranolide, and xanthanolide.

Some of these subclasses have compounds that exhibit a wide range of biological activities. These range from antitumor [9] to anti-inflammatory, including antimalarial, antimicrobial, antioxidant [10], neuroprotective [11], hepatoprotective, and immune-stimulant properties [12,13]. Regarding the structure/activity relationship of these compounds, it appears that the α-methylene-γ-lactone nucleus has a crucial role in almost all their observed biological effects, such as cytotoxic, antitrypanosomal, and anti-inflammatory actions [12,14]. Other specific structural moieties of sesquiterpene lactone seem to influence their activity. For example, the presence of electrophilic sites associated with medium/high lipophilicity increase antimycobacterial activity, while a double bond exo to the cyclopentenone ring seems to favor anti-inflammatory activity [7,8,14]. The interaction of the <sup>α</sup>,β-unsaturated cyclopentenone nucleus with the target depends largely on the geometry of the molecule, which is also a factor that influences the level of activity exhibited by sesquiterpene lactones [7,15]. Moreover, the number of alkylating groups in the structure of sesquiterpene lactones contributes to the level of activity they display, two groups being the optimal number [7,15]. The structure/activity relationship specific to each sesquiterpene lactone presented in this review will be discussed in detail throughout Section2. Scientific evidence shows an actionmechanism common to sesquiterpenelactones. The structural elements α-methylene-γ-lactone and <sup>α</sup>,β-unsaturated cyclopentenone act as alkylating groups on proteins found in cells through Michael addition, especially their thiol groups. They thereby affect cell functionality, i.e., gene regulation, protein synthesis, and cell metabolism [15–17].

In recent years, the scientific community has already shown an oustanding growth of interest in the sesquiterpene lactones, largely due to the success of artemisinin—one of the best known—and the broad spectrum of activities exhibited by this compound's chemical family [18–20]. Thus, the high number of studies published concerning their isolation from new natural sources, total and semi-synthetic syntheses, and evaluation of pharmacological potential, is not surprising. Natural sesquiterpene lactones exhibit poor pharmacokinetic properties due mainly to their low bioavailability, deriving from low solubility in water. As a result, in order to overcome these limitations, research interest in the synthesis of their derivatives has increased [21,22]. In addition, structural modification and synthesis has also allowed for an in-depth knowledge of their chemical properties, as well as the establishment of structure-activity relationships.

In terms of health promotion, the applications of sesquiterpene lactones and their derivatives are a key research area. From this point of view, in vivo, preclinical and clinical studies are those that allow a more realistic assessment of their medicinal potential [23].

The literature on this topic is extensive, especially for the most successful compounds; however, studies on less known sesquiterpenes and their derivatives show that these compounds deserve more attention, since they could play an important part in future human health maintenance. Therefore, this review aims to point out the results of the most impactful and recent in vivo and (pre)clinical studies of these insufficiently explored compounds and derivatives, which could be a valuable alternative in the development of new therapeutic drugs.

#### **2. Sesquiterpene Lactones with Significant In Vivo Activity**
