Terpenes

Abstract

Terpenes are the largest and most diverse group of naturally occurring compounds found in plants. They can be classified according to the number of isoprene units, the most common being monoterpenes (C₁₀), sesquiterpenes (C₁₅), diterpenes (C₂₀), and triterpenes (C₃₀). Besides being the principal constituents of essential oils and playing fundamental roles in plants, many terpenes are extensively used in pharmaceutical and industrial applications ranging from flavours to fragrances and medicines. Several studies have already demonstrated the diversity of terpenes’ biological properties, including cancer chemopreventive effects, antimicrobial, antiviral, analgesic, anti-inflammatory, antifungal, antiparasitic, and other activities. This chapter compiles the various terpenes isolated from plants, their sources, biological activities and beneficial health effects, mechanism of action, extraction and applications, and the future perspective for using the terpenes as lead molecules in several areas of the industry.

1 Introduction

From a structural and chemical perspective, terpenes are the largest and most diverse class of natural compounds, including more than 50,000 known molecules (Cox-Georgian et al. 2019; Pasquini et al. 2021; Torres-Fajardo and Higuera-Piedrahita 2021). They originate from mevalonic acid and are composed almost entirely of carbon, oxygen, and hydrogen. Their presence in different parts of the plant (leaves, flowers, stems, buds, fruits, pods, seeds, roots, and barks) contributes to its aroma, colour, and flavour (Cox-Georgian et al. 2019; Uwineza and Waśkiewicz 2020). Structural diversity in terpenoids from natural products arises from the coupling chemistry employed to link C₅ isoprenoid precursors. Moreover, different terpenes or derived compounds have been described to perform distinct functions ranging from primarily structural (such as cholesterol in maintaining the membrane structure) to more functional ones such as carotenoid pigments or gibberellin a plant-derived terpene responsible for the regulation of cell growth and defence (Mostofian et al. 2020; Wang et al. 2019a). In addition, due to their allelopathic effects, terpenes have a natural role in attracting pollinators, healing plants’ injured tissues from herbivore attacks and naturally repelling insects and parasites (War et al. 2012; Maffei 2010; Nejia et al. 2013). Despite terpenes being defined as hydrocarbons having the five-carbon isoprene unit as their building block (Fig. 5.1), many authors use the term ‘terpenes’ more broadly to include also the terpenoids. These terpenes have their carbon skeleton modified by oxidation and rearrangement since methyl groups are generally moved or removed when oxygen atoms are added to the hydrocarbon molecules. Therefore, terpenes are simple hydrocarbons while terpenoids contain oxygen in distinct functional groups, such as alcohols, aldehydes, ketones, acids, esters, and ethers, increasing terpene’s chemical and functional diversity (Pasquini et al. 2021; Mewalal et al. 2017; Mosquera et al. 2021).

Most of the natural terpenes have the general formula (C₅H₈)n, and their thermal decomposition gives isoprene as one of the products (Fig. 5.1) (Mewalal et al. 2017; Pichersky and Raguso 2018; Caputi and Aprea 2011; Hanuš and Hod 2020). They can be classified based on the value of n (number of isoprene units) or the number of carbon atoms present in the structure. Mono- and sesquiterpenes (C₁₀ and C₁₅, respectively) and their derivatives are highly volatile compounds and frequently the main constituents of essential oils (EOs). At the same time, di- and triterpenes (C₂₀ and C_30, respectively) and their derivatives are more complex compounds being less volatile or even assumed as non-volatile. They are found in EOs at low concentrations, mainly from plant gums and resins (Bicas et al. 2009; Haberstroh et al. 2018; Yáñez-Serrano et al. 2018). Terpenes can also be classified as acyclic with an open structure, cyclic with one ring structure, and bi-, tri-, and tetra-cyclic with two, three, and four rings.

Plants synthesise diverse compounds unique to this class of chemicals and contribute to the vast number of existing terpenes and terpenoids. Some plants store or emit these compounds and deposit them in specific organs, such as resin ducts, performing a defensive function or emitting immediately after their biosynthesis. Abiotic and biotic stress factors such as drought, extreme temperature, pollution, or pathogen attack can interfere and rearrange the biosynthesis and emission of terpenes. However, the response may depend on the stressor type and intensity (Pasquini et al. 2021; Haberstroh et al. 2018; Blanch et al. 2007).

Besides their fundamental role in plants, many terpenes are widely used in pharmaceutical and industrial applications ranging from flavours to fragrances (Cox-Georgian et al. 2019). Extensive application of chemical techniques and biological tests has led to the identification, characterisation, and extraction of significant components of broad interest, especially concerning the recovery of terpenes with particular interest for cosmetic and other industries (Salha et al. 2021). Several studies have already demonstrated the diversity of their biological properties, including cancer chemopreventive effects, antimicrobial, antiviral, analgesic, anti-inflammatory, antifungal, antiparasitic, and other activities (Yang et al. 2020). Recently, there has been an increased interest in these compounds obtained from industries' crops since some specific terpenes have been identified as specialties biofuels (Mewalal et al. 2017; Greay and Hammer 2011; Guimarães et al. 2014; Tetali 2019).

This chapter compiles some of the major roles of terpenes and terpenoids isolated from plants. For different compounds, their biological activities and beneficial health effects are emphasised. Moreover, general aspects regarding biosynthesis and extraction are also included.

2 Chemistry and Biosynthesis of Terpenes

2.1 Classification

These compounds are part of the secondary metabolism of plant species and consist of isoprene units: 2-methylbuta-1,3-diene, (C₅H₈) (Fig. 5.1) linked to each other in numerous ways, most commonly by head to the tail arrangement but other arrays can be found (Bicas et al. 2009; Rubulotta and Quadrelli 2019).

Fig. 5.1

The structural form of an isoprene unit

According to the number of isoprene units, terpenes can be classified as hemiterpenes when having a single isoprene unit (C₅), monoterpenes (C₁₀) when two isoprene units are linked, sesquiterpenes (C₁₅), diterpenes (C₂₀), sesterterpenes (C₂₅), triterpenes (C₃₀), and tetraterpenes (C₄₀) (Table 5.1 and Fig. 5.1). When more than eight isoprene units are connected, they are named polyterpenes (Torres-Fajardo and Higuera-Piedrahita 2021; Guimarães et al. 2014; Tetali 2019; Rubulotta and Quadrelli 2019; Harman-Ware 2020a). The isopropyl part of 2-methylbutane is defined as the head and the ethyl residue as the tail. In mono-, sesqui-, di-, and sesterterpenes, the isoprene units are linked from head-to-tail; tri- and tetraterpenes contain one tail-to-tail connection the centre (Pichersky and Raguso 2018). Examples of these compounds are shown in Fig. 5.2.

Table 5.1 Classification of terpenes based on the isoprene units

Fig. 5.2

Structural forms of volatile, semi, and non-volatile terpenes

2.2 Biosynthesis

Overall, all terpenes originate, in part, from the C₅ substrate dimethylallyl pyrophosphate (DMAPP) and isopentenyl diphosphate (IPP), typically by initially condensing DMAPP with one or more IPP molecules. The type of compound formed is associated with the synthesis pathway (Petrović et al. 2019). Tri- and sesqui-terpenes are most likely synthesised from the precursors produced in the mevalonate pathway (MVA) in the cytoplasm, while hemi-, mono-, di-, and tetra-terpenes are generally synthesised from the precursors produced in the methylerythritol 4-phosphate (MEP) pathway in the chloroplast (Fig. 5.3) (Mewalal et al. 2017; Tetali 2019). Both pathways use IPP and its respective isomer DMAPP to form the isoprene unit (C₅). The main difference is that, while in the MVA pathway, the activated building blocks are synthesised from acetyl-CoA, in the MEP pathway, they are obtained from pyruvate with glyceraldehyde 3-phosphate. Molecules of IPP and DMAPP condense to form geranyl pyrophosphate (GPP) with 10 carbons, and this reaction is catalysed by GPP synthase. Then, a molecule of IPP will react with the GPP to form a molecule of farnesyl pyrophosphate (FPP), presenting 15 carbons. The FPP can further react with a molecule of IPP and form a geranylgeranyl pyrophosphate (GGPP) with 20 carbons. Each combination will cause the release of pyrophosphate. Overall, these compounds are considered linear precursors of all terpenes (Mewalal et al. 2017; Rubulotta and Quadrelli 2019; Harman-Ware 2020a; Petrović et al. 2019).

Fig. 5.3

Principal pathways of the biosynthesis of terpenes

3 Extraction Technologies

In general, extraction methods can be classified as conventional and non-conventional. The first group encompasses maceration, Soxhlet, steam distillation, and hydrodistillation; they require long extraction times, high amounts of solvents, and high energy costs. The second group encompasses supercritical fluid extraction, ultrasound-assisted extraction, and microwave-assisted extraction. Non-conventional methods are considered eco-friendly as they are associated with lower energy costs and the avoidance of toxic solvents and therefore have a lower environmental impact.

3.1 Conventional Extraction Methods for Low Molecular Weight (LMW) Terpenes

LMW terpenes (mono- and sesquiterpenes) are the main constituents of essential oil (Tongnuanchan and Benjakul 2014). The techniques most frequently used for their extraction are steam distillation and hydrodistillation, as both are considered the main methods to obtain essential oil from different anatomical parts of plants (Salha et al. 2021).

3.1.1 Steam Distillation

Steam distillation is one of the ancient and officially approved methods for isolating essential oils from plant materials (Li et al. 2014) being used in commercial-scale production due to the low cost associated with the process (Giacometti et al. 2018). In this conventional type of extraction, the plant material is exposed to a distillation process with boiling water without the water entering in contact with the plant material. Heat plays a fundamental role in this method since the steam passes through the sample, destroying its morphology and releasing the volatile aromatic compounds, which are then carried by vapour that is then condensed, resulting in a separation of phases of distilled water-essential oil (Tongnuanchan and Benjakul 2014; Pateiro et al. 2018). Steam distillation can also be carried out under pressure, depending on the characteristics of the sample (Li et al. 2014). This method is used to create fine perfumes, obtain distillates that refine the flavours and aroma of food and drinks, and produce medicines from plants (Salha et al. 2021). Different studies reveal that steam distillation can extract significant quantities of terpenes, leading to good responses on bioactive assays (Salha et al. 2021; El Kharraf et al. 2021; Ragab et al. 2019).

3.1.2 Hydrodistillation

Hydrodistillation is a variant of steam distillation. Instead of using steam that passes through the sample, it relies on a solid–liquid maceration of plant material in the water that is heated to ebullition. The heat allows the release of molecules of interest/volatile compounds present in the plant cells, which are carried by water vapour in an azeotropic mixture and collected at the end of the process (Tongnuanchan and Benjakul 2014). This process obtains two phases, namely water and an oil-rich phase. Hydrodistillation with Clevenger® apparatus is described in the European Pharmacopoeia as a method adequate for volatile oil extraction from different herbal products such as medicinal plants and dried spices (Li et al. 2014; Giacometti et al. 2018; Tavakolpour et al. 2017). This method is one of the most widely used at laboratory scale, being described in several studies to extract essential oil from different plant matrices (Fagbemi et al. 2021; Jafari-Sales and Pashazadeh 2020; Baker et al. 2021).

3.1.3 Enfleurage method

Enfleurage is the oldest perfumery process commonly employed in the south of France. It is a traditional method of extraction of flowers’ essential oil by their contact with odourless cold fat or vegetable oil. In this way, the essential oil and aromas are absorbed by the fat/oil (Ali et al. 2015), preserving the essential oil from hydrolysis during the extraction (Oktavianawati et al. 2019). Enfleurage extraction is generally a lengthy process since it requires three days to be complete. Moreover, the fat may be liquefied several times to enhance its organoleptic characteristics. The subsequent addition of alcohol to the fat/oil allows the absorbed essential oil to recover (Ali et al. 2015). Enfleurage has a greater yield of extraction when compared to other methods (Paibon et al. 2011). Formerly it was used for extracting essential oil of all flowers, but modern methods have shown that better results can be obtained more economically (Picot-Allain et al. 2021). However, some exceptions still remain such as the cases of jasmine (Jasminum L.) and tuberose (Agave amica L.), which do not tolerate other extraction methods (Barrales-Cureño et al. 2021; Roopashree and Naik 2019; Poucher 1993).

3.2 Conventional Extraction Methods for High Molecular Weight (HMW) Terpenes

Diterpenes, triterpenes, esters, and waxes are examples of high molecular weight molecules present in plant material. Di- and triterpenes are considered semi- or non-volatile terpenes compared with mono- and sesquiterpenes (LMW). Therefore, non-polar organic solvents are commonly used to extract HMW terpenes (Wang et al. 2014). However, for pharmaceutical, cosmetic, and food industries, the utilisation of organic solvents is limited and maximum residues are strictly defined for the allowed solvents (Choi and Verpoorte 2019). The maceration and Soxhlet extraction methods are used traditionally to obtain gums and resins containing terpenes of high molecular weight (Bensebia et al. 2016). However, cold press extraction is also described in the literature as an industrial method for extracting HMW terpenes (Putnik et al. 2017; Chemat et al. 2012).

3.2.1 Cold Press Extraction

Considered an old but evergreen extraction method, the cold press is commonly used in the industrial field to extract oil from different matrices (Chemat et al. 2012). The cold pressing method consists of the physical process of crushing citrus peels, in which the essential oil is present. With the mechanical action of crushing, the oil is released and transferred to a natural sponge, from which it is finally removed. For essential oil extraction, the citrus genus is the most suitable for this type of operation. The oil obtained by this method adds better fruit odour characteristics than other oil extraction methods (Ali et al. 2015).

3.2.2 Maceration, Heat-Assisted Extraction, and Soxhlet Extraction

Maceration is a common solid–liquid extraction used in the nutraceutical field (Chemat et al. 2012). In this extraction method, the sample is soaked and agitated to increase cell permeability, releasing the compounds of interest, such as terpenes (Chia et al. 2020). This process is performed at ambient temperature or in the cold, and compounds are extracted based on diffusion and mass-transfer phenomena. However, to increase the extraction efficiency, the temperature can be applied. In that case, the process is called heat-assisted extraction (HAE) (Rocchetti et al. 2019). The constant agitation in dynamic maceration or heat-assisted extraction increases the solubilisation of the compounds of interest, generating higher extraction yields. Using different solvents or their mixtures can facilitate the mass transfer of the compounds of interest (Garcia-Vaquero et al. 2020). Soxhlet extraction occurs by adding the sample into a cellulose container inserted into the extraction chamber, situated on top of a collecting flask and under a reflux condenser, making this approach a hybrid continuous-discontinuous technique (Fernandez-Pastor et al. 2017; López-Bascón and Luque de Castro 2020).

The use of volatile non-polar solvents and high extraction times are generally perceived as disadvantages of these techniques (de Melo et al. 2020). In addition, these extraction methods have demonstrated poor selectivity. Another aspect being considered is that HAE and Soxhlet frequently use high temperatures, which can degrade thermo-labile bioactive compounds (Palmieri et al. 2020). However, there is the possibility of coupling the Soxhlet apparatus to other equipment to improve the extraction efficiencies such as ultrasound extraction and (Subramanian et al. 2016) microwave-assisted Soxhlet (Prados-Rosales et al. 2002) though the most used is still extraction by convection heat.

Because of the mentioned drawbacks of these methodologies, the search for more environmentally friendly extractions has driven the development of new extraction technologies aiming to reduce the use of toxic solvents and energy costs while improving the extracts' quality and yield.

3.3 Novel Non-Conventional Extraction Technologies

In order to reduce energy costs, waste generation, and the use of environmentally friendly solvents, new extraction technologies are being considered viable alternatives for the future (Fig. 5.4), being less aggressive than conventional technologies and many times more profitable in terms of extraction yield. Green extraction methods such as supercritical fluid extraction, ultrasound-assisted extraction (UAE), and microwave-assisted extraction (MAE) have increased the efficiency of the extraction processes of terpenes in general (Bensebia et al. 2016).

Fig. 5.4

Extraction methods considered as being eco-friendly

3.3.1 Supercritical Fluid Extraction (SFE)

Supercritical Fluid Extraction (SFE) is a suitable and efficient technology considered an environmentally friendly process. SFE allows the processing of plant material at low temperatures, limiting thermal degradation and avoiding toxic solvents (Tongnuanchan and Benjakul 2014). Supercritical fluids such as CO₂, propane, butane, or ethylene are used as solvents for various applications. CO₂ is the supercritical solvent of choice in the extraction of flavour and fragrance compounds since it is colourless, odourless, safe, pure, cost-effective, non-flammable, and recyclable at relatively low pressures near room temperature. Higher CO₂ density (above 500 kg/m³) favours the yield of extraction of diterpenes, triterpenes, esters, and waxes (Jasna et al. 2009; Reverchon et al. 1995). In short, the solvent is forced to pass through a metallic filter submitted to cooling through heat exchangers. The cooled solvent is then pressurised using a high-pressure pump. In this stage, a co-solvent like ethanol may be mixed to enhance the solubilisation of compounds with greater polarity (Capuzzo et al. 2013). The solvent flows through the feedstock to extract its soluble compounds in supercritical conditions. Subsequently, the extract is precipitated in the separation vessel due to depressurisation (Priyanka 2018).

3.3.2 Microwave-Assisted Extraction (MAE)

Microwave is a non-contact heat source that can achieve more effective and selective heating, resulting in a distillation process that is completed in minutes instead of hours as in conventional extractions (Li et al. 2014). Microwaves are electromagnetic spectrum radiation ranging in frequency from 300 MHz (radio radiation) to 300 GHz. The principal mechanism of microwave equipment is based on two principles. The first is related to the interaction of electric and magnetic fields, resulting in dielectric and magnetic losses leading to heating. The second principle refers to the dipole rotation that occurs when dipole molecules try to align themselves with the alternating electric field in a microwave-produced medium, with heat being produced due to this rotation (Vinatoru et al. 2017).

Microwave extraction is a technique that aims to achieve higher efficiency, which is given by its selective heating. The microwave extraction technique may or may not be accompanied by solvent addition to the extraction. In addition to the frequency used in the extraction and the type of solvent (or the absence of solvent), parameters such as temperature, time, and sample mass are essential in determining the extraction yield. Moreover, besides being used in microwave-assisted hydrodistillation for the extraction of essential oils, it also allows the combination with conventional organic solvents for the extraction of less volatile terpenes (Liu et al. 2018; Sarfarazi et al. 2020). The short extraction time and the possibility of low MAE temperatures make this an excellent technique for extracting terpenes from plants (Isidore et al. 2021).

3.3.3 Ultrasound-Assisted Extraction (UAE)

Ultrasound-assisted extraction is a solid–liquid extraction that uses waves with ultrasonic frequencies between 20 and 100 kHz, which enhances the extraction of bioactive compounds from plants (Wen et al. 2018). Waves at these frequencies generate cavitation and vibration, which are responsible for creating cycles of expansion and compression, causing the formation of bubbles that promote and facilitate cellulose destruction and the release of compounds of interest into the extraction solvent (Isidore et al. 2021; Villalva et al. 2021). In addition to frequency, the propagation of ultrasonic waves is related to power (W). Despite UAE not requiring the use of high temperatures commonly associated with other extraction methodologies, temperature and extraction time are also considered relevant parameters in this method (Garcia-Perez et al. 2021; Turrini et al. 2021; Dzah et al. 2020). To date, UAE has proven to be a valuable extraction technique for obtaining terpenes present in the essential oils of botanical species (Chemat et al. 2012; Santos et al. 2019; Munekata et al. 2020).

3.3.4 Enzymatic-Assisted Extraction

Enzyme-assisted extraction has been intensively studied in the last decade. Since plants have a cell wall composed of resistant polysaccharides that can interfere in extracting bioactive compounds and essential oils (Tirgarian et al. 2019). Enzymes like cellulase, xylanase, and pectinase can degrade cell wall components, enabling more efficient extraction of bioactive compounds and improving the bioactive content of essential oils and extracts. Influential factors that determine the extraction efficiency of enzymatic-assisted extraction include solvent pH, reaction temperature, enzyme concentration, and enzyme type (Miljanović et al. 2020). Some authors have considered that enzymatic treatment also promotes changes in the molecular structure of compounds by converting phytochemicals into final products with higher bioavailability and bioactivities (Kitrytė et al. 2018; Martins et al. 2016a).

3.3.5 Micelle-Mediated Extraction

Also known as cloud-point extraction, it is a surfactant-based extraction technique. The micelle-mediated extraction procedure involves forming hydrophobic species by complexation with an organic ligand in a surfactant medium (Paleologos et al. 2005). The solution becomes cloudy and separates in two phases, i.e. aqueous phase and surfactant rich coacervate phase (Chatterjee et al. 2017). The distribution of solute is more in the surfactant rich phase due to solubilisation in the micelles, thereby leading to its extraction. Therefore, several studies have focused on the choice of the surfactant and chelating agent. Nonionic surfactants are the most common surfactants in cloud-point extraction (Hinze and Pramauro 1993).

3.4 Separation of Terpenes

Essential oils are complex mixtures as they are frequently composed of several terpenes and terpenoids. Because some terpenes are prone to oxidation phenomena, thus contributing to off-flavours and limiting essential oil’s application in food, cosmetic, and pharmaceutical industries, they are frequently submitted to deterpenation (i.e. separation of terpenes from terpenoids) to maintain the quality of the final product. The separation of terpenes significantly improves the oil's stability, solubility, and storage requirements, thus allowing for their industrial applications (Ozturk et al. 2018).

3.5 Identification and Quantification Techniques

Various methods to identify and quantify terpenes have been widely studied. Most often, those require the use of techniques such as gas chromatography (GC), high-performance liquid chromatography (HPLC), capillary electrophoresis, thin-layer chromatography, and SFE chromatography to separate the compounds of interest (Kumar et al. 2016; Xu et al. 2018a). Jiang et al. (2016) proposed different protocols for the extraction/isolation and analysis of terpenoids according to their polarity and size. The analysis was mainly based on chromatographic techniques, such as HPLC and GC. Among those, GC coupled to mass spectrometry detection (GC-MS) is considered the gold-standard technique being the most frequently used to identify volatile terpenes (Louw 2021). On the other hand, for less volatile compounds, such as triterpenes, LC-MS-MS is preferred as an alternative to GC-MS due to its remarkable capacity in the identification of compounds, with the advantages of being also faster and requiring minimum sample preparation (Garg et al. 2020).

4 Pharmacological Activities

The presence of diverse functional groups such as phenols, esters, aldehydes, ketones, and alcohols in secondary metabolites produced plants are related to their variety of biological effects due to different action sites available (Masyita et al. 2022; Boukhatem 2020; Singh and Sharma 2015). Many of these bioactive compounds are volatiles obtained in the form of essential oils. Individually or as a mixture of compounds, they have been used in folk medicine for their disinfectant and preservative effects since ancient times. Nowadays, different terpenes and terpenoids are also used as active ingredients in modern pharmaceuticals (Pasquini et al. 2021; Yang et al. 2020; Petrović et al. 2019; Paduch et al. 2007). Many of these novel drugs are based on the knowledge of folk medicine. Two of the most successful terpenes in the market today are used against malaria (artemisinin and its derivatives) (Krishna et al. 2008) and different cancers (taxol and its derivatives) (Cox-Georgian et al. 2019; Yared and Tkaczuk 2012). Several studies have been demonstrating the bioactivity of different terpenes, including those related to antitumour, anti-inflammatory, antiviral, antimicrobial, antiparasitic, antioxidant, antidepressant, antidiabetic, and against cardiovascular diseases, among other pharmacological activities (Cox-Georgian et al. 2019; Yang et al. 2020; Paduch et al. 2007). Additionally, they can be employed as a natural alternative to ward off insects for their anti-insect properties (Salha et al. 2021; Petrović et al. 2019; Singh and Sharma 2015). It has been estimated that the market of terpene-based pharmaceuticals grows every year, with sales of several million dollars (Wang et al. 2005; Kim et al. 2019). Accordingly, the research on the biological potential of terpene-core compounds is of increasing interest to develop new drugs in contemporary medicine. Several terpenes and/or their derivatives presenting biological activity, namely mono-, sesquiterpenes, and their derivatives, are frequently found as complex mixtures in different essential oils. Therefore, many studies have focused on separating and isolating these compounds towards a deep knowledge of their specific activities. These studies also showed that the combination of specific compounds could lead to improved results concerning biological potential, probably due to a synergic effect (Boukhatem 2020; Singh and Sharma 2015). Moreover, it is essential to highlight that those different enantiomers will have different biological potentials, and the configuration should be considered (Zhu et al. 2017). Some of the most common terpenes and their described biological potential are compiled in Tables 5.1, 5.2, 5.3, and 5.4.

4.1 Monoterpenes with Relevant Pharmacological Activities

Fig. 5.5

Example of monoterpenes with relevant biological activity described in literature

Figure 5.5 shows the chemical structures of different monoterpenes that have been associated with relevant biological activities. Several studies so far have evidenced a correlation between the antitumour activity of some essential oils and their monoterpene composition, showing that these compounds can play an important role in cancer prevention and treatment. The antitumoural activity can occur through multiple mechanisms in different steps, such as preventing the interaction of carcinogens in the initiation stage, inhibiting the development and migration of cancer cells, and inducing cancer cell apoptosis and, therefore, tumour regression. According to Paduch et al. (2007), the post-translational isoprenylation of proteins that regulate the growth of cells is the most important mechanism influenced by monoterpenes. Up until now, various works have reported that different monoterpenes exhibit antitumour activity. Limonene and perillyl alcohol, as well as their derivatives and stereoisomers, have been described to have a potent antitumour activity (da Silva et al. 2021) that allows the apoptosis of different tumours, including prostate, pancreas, breast, colon, and liver, both in vitro (Rabi and Bishayee 2009; Shi and Gould 2002; Bardon et al. 2002; Crowell 1999) and in vivo tests, specifically in nude mice (Lu et al. 2004; Gould et al. 1994) and human patients with early and advanced cancer (Boukhatem 2020; Vigushin et al. 1998; Miller et al. 2013; Zielińska-Błajet and Feder-Kubis 2020; Chebet et al. 2021). Also, by inducing apoptosis and cell cycle arrest, thymoquinone, naturally found in Nigella sativa (black cumin), exhibits activity against breast, skin, brain, bile duct, and non-small cell lung cancers both in studies in vitro (cancer cell lines) and in vivo (rats) (Majdalawieh et al. 2017; Khader and Eckl 2014). The main compound from Cannabis sativa essential oil, β-myrcene, presents a significant antiproliferative action against some cancer cell lines, such as breast and human lung carcinoma and leukaemia (Bai and Tang 2020; Tomko et al. 2020; Surendran et al. 2021). In a study with camphene isolated from Piper cernuum (Piperaceae), the growth of a subcutaneous tumour was inhibited in both human tumour cell lines and B16F10-Nex2 murine melanoma, suggesting its promising role in tumour therapy (Girola et al. 2015). Another study demonstrated that carvone could reduce pulmonary adenoma and fore-stomach cancer formation against different cell lines (Patel and Thakkar 2014; Pina et al. 2022). Geraniol has been demonstrated to have both preventive and therapeutic potential against different types of tumours (lung, colon, prostate, pancreatic, and liver) when tested in different cancer cell lines but also in vivo using rats (Cho et al. 2016; Galle et al. 2014; Carnesecchi et al. 2001; Kim et al. 2011; Burke et al. 1997). As demonstrated by different studies, this can be due to the inhibition of the expression of the HMG-CoA (3-hydroxy-3-methylglutaryl coenzyme A) reductase gene in tumour cells. Therefore, geraniol has been suggested as a potential multitarget drug in chemotherapy since it can intervene in different processes such as cell proliferation, apoptosis, autophagy, and metabolism (Yang et al. 2020). By inhibiting the NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) activity, terpineol and its isomers have shown antitumoural activity when tested in cell lines (breast, lung, prostate, ovarian, and leukaemia) (Khaleel et al. 2018; Hassan et al. 2010). Similarly, the isomers thymol and carvacrol have also evidenced interesting antitumour properties. Several studies in vitro and in vivo have shown that thymol and carvacrol induced apoptosis, cytotoxicity, cell cycle arrest, and antimetastatic activity, and also displayed different antiproliferative effects and inhibition of signalling pathways (Sampaio et al. 2021; Islam et al. 2019; Fan et al. 2015). Linalool, a monoterpene found in many aromatic plants, has also demonstrated a high potential as an antitumoural compound since it promotes autophagy and apoptosis of tumour cells (prostate, breast, colorectal, and liver) either in mice or in vitro using different cell lines (Yang et al. 2020; Iwasaki et al. 2016; Pan and Zhang 2019; Chang and Shen 2014; Cerchiara et al. 2015; Xiu-Bin et al. 2015). In addition to its antitumoural effects, linalool and its derivatives also have anti-inflammatory activity since they can relieve the symptoms of inflammation by affecting the nuclear translocation of NF-κB and related pathways (Petrović et al. 2019; Ma et al. 2015). According to Ma et al. (2015), linalool inhibits acute lung inflammation in mice by producing interleukin-6 (IL-6), IL-1β, IL-8, tumour necrosis factor-alpha (TNF-α), and monocyte chemoattractant protein-1 (MCP-1), which are involved in neuroinflammatory processes and several diseases. Other monoterpenes such as 1,8-Cineole, the main compound of Eucalyptus sp., and terpinen-4-ol, the main compound of Melaleuca alternifolia, have also demonstrated anti-inflammatory activity as they can suppress the production of several pro-inflammatory substances such as prostaglandins, tumour necrosis factor-alpha (TNF-α), and interleukin 1 beta (IL-1β) (Salha et al. 2021; Paduch et al. 2007). Paeoniflorin and its derivatives can inhibit the production of inflammatory factor nitric oxide (NO), interleukin 6 (IL-6), and TNF-α induced by lipopolysaccharides (Yang et al. 2020; Singh and Sharma 2015). The reduction of pro-inflammatory interleukins production is also the mechanism of action of α-pinene, explaining the anti-inflammatory activity of plants whose essential oil is rich in this compound, such as pine trees (Kim et al. 2015a). Besides the mentioned monoterpenes, borneol, β-myrcene, citronellol are also known for their anti-inflammatory activities (Salha et al. 2021; Paduch et al. 2007).

Terpenes have shown several other interesting biological properties, such as neuroprotection, and some are being studied to treat depression. According to Cox-Georgian et al. (2019), antidepressant properties are evidenced for linalool and pinene (active principles of Litsea glaucescens, Tagetes lucida and flowers of lavender) by interacting with the serotonin-1A receptors (5HT_1A) of the serotonergic pathway (Bonilla-Jaime et al. 2015). When inhaled, the monoterpenes car-3-ene, borneol, verbenol, and pinocarveol have also evidenced antidepressant properties (Pasquini et al. 2021). Some studies showed that myrcene, linalool, citronellol, 1,8-cineole, α- and γ-terpinene have neuroprotective functions owing to their antioxidant effect (Salha et al. 2021; Petrović et al. 2019; Singh and Sharma 2015). Furthermore, borneol can act as an essential neuroprotective agent against Alzheimer’s disease due to its free radical scavenging activity (Yang et al. 2020). Several studies indicate that β-myrcene, menthol, camphor, citronellol, terpineol and its isomers, linalool and linalyl acetate, have analgesic or sedative properties. Therefore, menthol is frequently used topically as an analgesic agent, with the advantage of presenting antipruritic, counterirritant, and antiseptic activity. Similarly, camphor is traditionally used in different preparations, including liniments and cream formulations (Cox-Georgian et al. 2019; Salha et al. 2021; Singh and Sharma 2015; Paduch et al. 2007). Several monoterpenes, including terpineol and its isomers, 1,8-cineole, d-limonene, pinene, nerol, and geranial, as well as the essential oils containing them, have shown antimicrobial activity, which is most probably related to the lipophilic structure of these compounds (Pasquini et al. 2021). These compounds can easily pass through lipid bilayers, causing morphological changes in the cell membrane and cytoplasm, resulting in the leakage of intracellular material (Carson and Riley 1995). Especially terpinen-4-ol, 1,8-cineole and linalool possess antibacterial activity against bacteria isolated from the oral cavity, skin, and respiratory tract (Pasquini et al. 2021; Zengin and Baysal 2014; Wang et al. 2019b). The antimicrobial potential of menthol, particularly against Escherichia coli and Candida albicans has also been demonstrated (Gupta et al. 2021). Carvone and perillaldehyde were also able to inhibit the growth of C. albicans, with the former being effective against Listeria monocytogenes, Enterococcus faecium, and Escherichia coli (Yang et al. 2020; Singh and Sharma 2015).

Antifungal activity was also evidenced by α-terpinene, which exhibited activity similar to commercial drugs, and by some terpenoids such as citronellal, citronellol, nerol, geraniol, borneol, and geranial (Salha et al. 2021; Paduch et al. 2007; Abbaszadeh et al. 2014). Moreover, monoterpene mixtures, such as the one including terpinen-4-ol, α-pinene, β-pinene, 1,8-cineole linalool, and α-terpineol, have also the capacity of limiting fungi development (Marei et al. 2012; Wang et al. 2019c).

In addition, paeoniflorin, a monoterpene glycoside isolated from the root bark of Paeonia suffruticosa, showed in vitro antifungal activity against Candida albicans and was effective against carbapenem-resistant Klebsiella pneumoniae, which are emerging and highly resistant pathogens that can have a significant impact in clinical practice but also in food production (Qian et al. 2020). Besides their interest as antibacterial and antifungal compounds, some volatile terpenes are known for their antiviral potential. Among them are 1,8 cineole, thymol, α-terpineol, α-and γ-terpinene, carvone, carveol, α- and β-pinene, camphor, β-ocimene, p-cymene, citral, and isoborneol. This last compound has great antiviral activity against Herpes simplex Virus-1 (HSV-1) by inhibiting the virus replication and the glycosylation of viral proteins (Cox-Georgian et al. 2019; Pasquini et al. 2021). Another relevant activity associated with monoterpenes is their antiparasitic activity. For example, d-limonene, α-pinene, and β-pinene have antiplasmodial properties, especially against Plasmodium falciparum (Petrović et al. 2019; Boukhatem 2020), while thymol and menthol have anti-leishmanial and trypanocidal activity (Luna et al. 2019). The antiparasitic functions of terpenes are related to their interaction with Fe (II) groups of the infected erythrocytes, releasing free radicals that can kill parasites (Rodrigues Goulart et al. 2004). Monoterpenes are also considered very interesting as natural anti-insect compounds. Besides its anti-inflammatory, antioxidant, and analgesic activities, citronellol, neral, and geranial, as well as thymol and menthol and their derivatives, have anti-insect properties and thus can be potential alternatives to synthetic insecticides (Reis et al. 2016; Al Dawsari and Alam 2022).

Table 5.2 Relevant monoterpenes and their bioactivities

4.2 Sesquiterpenes with Relevant Pharmacological Activities

Fig. 5.6

Example of sesquiterpenes with relevant biological activity described in literature

The chemical structures of some relevant sesquiterpenes are shown in Fig. 5.6. Several sesquiterpenes, such as caryophyllene, caryophyllene oxide, germacrene, and patchoulene, have been attracting attention due to their interesting biological activity, including anti-inflammatory, analgesic, antiparasitic, and antitumour properties. A pharmacological study revealed that β-caryophyllene isolated from the leaves of M. koenigii exhibited promising antimalarial activity against the chloroquine-sensitive strain of Plasmodium sp. (Kamaraj et al. 2017). Additionally, several studies highlighted its anti-inflammatory and tissue-protective properties by modulating numerous signalling pathways and inhibiting inflammatory mediators, including cytokines (Jha et al. 2021). Its isomer humulene, or α-caryophyllene, given orally or by aerosol, exhibited anti-inflammatory properties in a murine model by reducing the inflammatory mediators (Rogerio et al. 2009). Caryophyllene oxide is also known for its similar properties since it evidenced anti-inflammatory activity in vivo and central and peripheral analgesia (Chavan et al. 2010). β-patchoulene also showed relevant anti-inflammatory activity in mice models with acute inflammation (Zhang et al. 2016a) being able to mediate a gastroprotective effect against the oxidative stress associated with a gastric ulcer, both in vitro and in vivo (Cox-Georgian et al. 2019). Other terpenoids with anti-inflammatory therapeutic potential are parthenolides (from Tanacetum parthenium L.), which suppress inflammation by inhibiting the activity of NF-κB (Mathema et al. 2012).

Besides having anti-inflammatory activity, β-caryophyllene can reduce alcohol-induced liver injury and has demonstrated neuroprotective properties against Parkinson's disease by acting on specific G-protein-coupled receptors, namely the cannabinoid CB2R, which is a receptor that reduces neuropathic pain and glial upregulation (Viveros-Paredes et al. 2017; Sharma et al. 2015; Navarro et al. 2016). Farnesene has two significant isomers, α and β, the former having four stereoisomers and the last two (Tang et al. 2021). (E)-β-farnesene is a typical volatile component of many higher plants, being also an alarm pheromone in several insect species with the capacity of inducing a flee response in aphids (Bhatia et al. 2015; Zhang et al. 2017). Besides that, neuroprotective effects against H₂O₂-induced cell death in primary cultured cortical neurons have been demonstrated in previous studies (Turkez et al. 2014). Farnesol, an intermediate in the biosynthesis of farnesene, has been shown to inhibit tumorigenesis in animal models suggesting that it functions as a chemopreventative and antitumour agent in vivo (Joo and Jetten 2010). Similarly, β-eudesmol, a hydroxylated sesquiterpene, presented antitumour activity using in vitro and in vivo experimental models (Ma et al. 2008; Srijiwangsa et al. 2018). Moreover, this compound can inhibit angiogenesis, as demonstrated in mice implanted with subcutaneous Matrigel plugs and in mice adjuvant-induced granuloma (Small 1992). Other sesquiterpenes such as helenalin (Arnica sp.), eupalinin (Eupatorium chinense L.), inuviscolide (Inula viscosa L.), angeloylenolin (Centipeda minima L.) were also reported as antitumour agents by promoting autophagy in cancer cells, inhibiting the proliferation by cell cycle arrest and apoptosis (Petrović et al. 2019; Paduch et al. 2007). A similar mechanism of action mediated via G₂/M cell cycle arrest and promotion of apoptosis was reported for germacrone, which showed an antitumour effect on a human hepatoma cell line (He et al. 2019). Bisabolol, a compound widely used in pharmaceutical and cosmetic industries, and its isomers α-bisabolol and epi-α-bisabolol have evidenced antitumour effects against pancreatic cancer by inducing apoptosis and suppressing Akt (Protein kinase B) activation (Seki et al. 2011; Maurya et al. 2014). Moreover, Raut, Karuppayil (Raut and Karuppayil 2014) suggested that α-(-)-bisabolol may be a useful therapeutic candidate for treating skin inflammation. Another study suggests the use of α-bisabolol and nerolidol as potential antifungal agents against Trichophyton spp. (de Medeiros et al. 2021). This compound also shows potential to be incorporated in oral healthcare products since it evidenced antimicrobial activity against Solobacterium moorei, a Gram-positive bacteria associated with halitosis (Forrer et al. 2013).

Besides its antitumoural activity, germacrone also possesses antiviral activity as it was shown to inhibit influenza virus, porcine parvovirus, feline calicivirus, and porcine reproductive and respiratory syndrome virus replication. Germacrone has inhibitory effects on influenza A virus subtype H1N1 (H1N1), Influenza A virus subtype H3N2 (H3N2), and influenza B viruses in the early stages of the viral cycle and can protect mice from a fatal infection (He et al. 2019).

The artemisinins are a class of sesquiterpene lactones extracted from the sweet wormwood plant (Artemisia annua), being a successful example of drugs available in the market (Eckstein-Ludwig et al. 2003). They are potent antimalarial drugs since they can kill all stages of the malaria parasite (Plasmodium falciparum) and are thus widely used worldwide. The action mechanism of the artemisinin is thought to occur through haem-dependent activation of an endoperoxide bridge that occurs within the parasite’s food vacuole (Ismail et al. 2016). Recently, artemisinin and its derivatives have attracted attention. Several in vivo studies confirmed that they have significant therapeutic effects on metabolic diseases, especially diabetes, by promoting insulin secretion and protecting pancreatic ß cells (Jiang et al. 2020).

Table 5.3 Relevant sesquiterpenes and their bioactivities

4.3 Diterpenes with Relevant Pharmacological Activities

Fig. 5.7

Example of diterpenes with relevant biological activity described in the literature

The chemical structures of some relevant dizer-enes are shown in Fig. 5.7. This class of compounds shows significant biological activities, including anti-inflammatory, anticancer, analgesic, antimicrobial, and antifungal activities. Some diterpenes also have cardiovascular activity, such as forskolin (Salehi et al. 2019), marrubenol (El Bardai et al. 2004), and kaurene (Ambrosio et al. 2006). They cause relaxation of smooth muscles in the walls of blood vessels by inhibiting Ca²⁺ channels (Salehi et al. 2019). Likewise, tanshinone IIA, an active ingredient isolated from the rhizome of Salvia miltiorrhiza Bunge, can act on Ca²⁺ influx and increase endothelial nitric oxide synthase protein expression and phosphorylation, leading to vasodilatation and blood pressure reduction (Shang et al. 2012). Recently, it has also been suggested that tanshinone IIA can prevent the formation of atherosclerosis and the damage and hypertrophy of the heart (Wang et al. 2017; Chen and Xu 2014). Due to the low intestinal absorption of this lipophilic compound, sodium tanshinone IIA sulfonate has been developed, aiming to increase its bioavailability (Shang et al. 2012).

Kaurenoic and pimaradienoic acids, isolated from the roots of Viguiera sp., have been evaluated on vascular smooth muscle contractility of rat aorta and showed pronounced antispasmodic and relaxant activity (Tirapelli et al. 2005). Resiniferatoxin, a vanilloid isolated from the Euphorbia resinifera latex, is used in clinical trials for bladder hyperreflexia and diabetic neuropathy (Appendino and Szallasi 1997).

Andrographolide, a diterpene-lactone isolated from herbaceous plant Andrographis paniculata, has promissory effects for treating diabetes (Nugroho et al. 2013). It also has potent antibacterial activity against most Gram-positive bacteria, showing interesting inhibitory properties on P. aeruginosa and S. aureus biofilms (Zhang et al. 2020).

The diterpenoid stevioside is an active ingredient isolated from Stevia rebaudiana (Bertoni) Hemsl. It possesses insulinotropic, glucagonostatic, and antihyperglycaemic effects, therefore acting as an antihyperglycaemic agent and inducing blood pressure reduction (Gregersen et al. 2004; Jeppesen et al. 2000). It has been shown that stevioside and the aglucon steviol potentiate insulin secretion in a glucose-dependent way in vitro (Gu et al. 2019). A clinical trial using stevia supplementation on glycaemic control improved cardiometabolic risk in diabetic patients (Rashad et al. 2019).

Paclitaxel is a tetracyclic diterpenoid isolated from Taxus sp, a known anticancer drug currently used in medicine to treat several kinds of tumours. Paclitaxel and its analogue docetaxel act as an antimitotic agent by binding to the microtubules (Yared and Tkaczuk 2012; Galletti et al. 2007).

The abietane-type diterpenoid ferruginol also displays antitumour properties in human ovarian cancer and malignant melanoma cell models by inhibiting cell migration and inducing apoptosis. Nevertheless, up until now, there is only one commercial drug composed of abietane-derived diterpenoids, ecabet sodium, which is used to treat reflux oesophagitis and peptic ulcer disease (González-Cardenete et al. 2021). In addition, abietane analogues such as ferruginol and dehydroabietinol exhibit inhibitory activity in vitro towards some Mycobacterium species and have been shown to have antiparasitic activity in the antipromastigote assay (González-Cardenete et al. 2021). Triptolidenol, triptolide, tripterine, and triptonide, which are isolated from Tripterygium wilfordii, are effective in inflammation and auto-immune disorders. Its primary mechanism consists in inhibiting the production of inflammatory cytokines (Jin et al. 2021).

The diterpene pepluanone, isolated from Euphorbia peplus, reduced inflammatory processes by inhibiting cell signalling (Barile et al. 2007), while putranjivain A, isolated from Euphorbia jolkini, demonstrated an antiviral effect against Herpes simplex virus-2 (HSV-2) in Vero cells by inhibiting cell penetration (Cheng et al. 2004). Andrographolide also showed promising antiviral activity due to inhibiting Chikungunya virus infection and reduced virus production (Wintachai et al. 2015). Other diterpenes with very interesting biological activity are the ginkgolides which show antagonistic activity towards platelet-activating factors, resulting in antithrombotic effects (Cui et al. 2019). A clinical study showed that ginkgolide effectively improved neurological deficit after recombinant tissue plasminogen activator therapy in acute ischaemic stroke patients (Zhang et al. 2021).

Table 5.4 Relevant diterpenes and their bioactivities

4.4 Triterpenes with Relevant Pharmacological Activities

Fig. 5.8

Example of triterpenes with relevant biological activity described in the literature

The chemical structures of some relevant triterpenes are shown in Fig. 5.8. The majority of triterpenes found in nature are cyclic triterpenes that can be further converted to various metabolites, including saponins. Saponins are not as abundant as most low molecular weight terpenes. However, this compound class presents a large diversity since they present different active sites allowing for the glycosylation of saponin aglycones (Dinda et al. 2010). Among saponins for which broad-spectrum pharmacological bioactivities have been reported, cucurbitacins are the most widely known (Alghasham 2013). Different studies have shown that these compounds, present in plants from the Cucurbitaceae family and used in traditional Chinese medicine, have different biological properties with particular relevance for their anticancer, anti-inflammatory, and hepatoprotective activity (Jing et al. 2020; Liu et al. 2021). Cucurbitacins exhibit anticancer activity through a variety of mechanisms, including the promotion of apoptosis through the Janus kinase/signal transducer and activator of transcription (JAK/STAT3) and Mitogen-Activated Protein Kinase (MAPK) pathways, the promotion of cell cycle arrest by inhibiting cyclins and by inhibiting the invasion and migration of cancer cells (Jing et al. 2020). These highly oxidised tetracyclic triterpenes have shown synergistic effects when combined with some drugs and also have been proposed as potential chemotherapy before the onset of lung cancer (Jing et al. 2020; Liu et al. 2021).

Lupeol, a compound found in a wide range of fruits, vegetables, and medicinal herbs, manifests chemopreventive effects and, at the same time, shows direct antioxidant activity in vitro and on animal models due to decreased lipid peroxidation and increased enzymatic and non-enzymatic antioxidants (Chaturvedi et al. 2008; Palanimuthu et al. 2012).

Ursolic acid can induce tumour cell apoptosis and has apparent antitumour effects on the NCI-H292 human lung cancer cells in vitro (Chen et al. 2019). In another in vitro study with human osteosarcoma (143B cells), apoptosis was induced by inactivating Wnt/β-catenin signalling through upregulating p53 (Zhang et al. 2016b).

Another natural triterpenoid, pomolic acid (isolated from Chrysobalanus icaco and Licania tomentosa), is cytotoxic against K562 cell line (human erythroleukaemia) and possesses anti-multidrug resistance properties (Fernandes et al. 2003).

Jenisseensosides (glucosides of quillaic acid) isolated from the plant Silene jenisseensis and Silene fortunei, can stimulate the proliferation of the Jurkat cell line when in low concentrations, while in higher doses, they regress to apoptosis (Lacaille-Dubois et al. 1997; Gaidi et al. 2002). Avicins, saponins isolated from Acacia victoriae, have preventive effects against H-ras gene mutation and changes in carcinogenesis (Hanausek et al. 2001).

Oleanolic acid and its isomer, ursolic acid, are pentacyclic triterpenoid compounds isolated from plants that frequently occur simultaneously in nature, either in free acid form or as an aglycone precursor of triterpenoid saponins. Because they share similar structural features, they show similar pharmacological activities (Jesus et al. 2015). Oleanolic and ursolic acid have shown immunomodulatory activity induced by activating macrophages infected in vitro with Mycobacterium tuberculosis as they passed from an M2-like (macrophages anti-inflammatory) phenotype to a M1-like phenotype (macrophages pro-inflammatory) characterised by high production of TNF-α and reactive oxygen species (ROS), and low production of Transforming Growth Factor Beta (TGF-β), resulting in the control and elimination of the intracellular mycobacteria (López-García et al. 2015). In another study, a clinical trial was performed with adolescents that ingested olive oil enriched with oleanolic acid to obtain postprandial triglyceride-rich lipoproteins that were further incubated with THP-1 (monocyte-like cell line) derived macrophages, showing that this compound has a high potential to prevent metabolic syndrome due to its anti-inflammatory activity (Fernández-Aparicio et al. 2021).

Ginsenosides are a group of glycosylated triterpenes constituents of ginseng that show anti-inflammatory activity both in vitro and animal models. Its mechanism of action is related to the inhibition of the NF-κB signalling pathway, suppressing the production of pro-inflammatory mediators and enzymes such as TNF-α, inducible Nitric Oxide Synthase (iNOS), and cyclooxygenase-2 (COX2) (Kim et al. 2017).

Studies have revealed that ginsenosides can play a relevant role in treating diabetes and ameliorate insulin resistance in the liver. Experimental and clinical data emerge to support the antidiabetic efficacy of ginsenosides attributed to their anti-inflammatory, antioxidant, and antihyperglycaemic activities, therefore, supporting the use of Panax ginseng in traditional medicine as an adjuvant to treat diabetes mellitus (Shao et al. 2020). In addition to their anti-inflammatory effects, ginsenosides also have an antioxidant effect. They can protect cardiomyocytes from oxidative damage caused by internal and external oxidants (Sarhene et al. 2021) and act on other cardiovascular diseases by regulating vascular function, inhibiting cardiomyocyte hypertrophy, and inhibiting thrombosis (Lee and Kim 2014).

Anti-inflammatory activity has also been ascribed to α-amarin, a pentacyclic terpene that exhibited this activity in rat models by reducing neutrophil infiltration and pro-inflammatory cytokine production of acute periodontitis (Holanda Pinto et al. 2008).

Other pentacyclic triterpenoids, such as boswellic acids isolated from the gum resin of Boswellia sp. (in particular cetyl-11-keto-ß-boswellic acid and 11-keto-ß-boswellic acid), have also evidence the same activity (Ammon 2016). Among other mechanisms of action, boswellic acids can act by inhibiting leukotriene synthesis and, to a less extent, prostaglandin synthesis. Various preclinical and clinical studies have established that these compounds exhibit substantial potential in managing inflammatory such as diminishing inflammation in osteoarthritis and colitis and helping control the brain oedema associated with radiotherapy of cerebral tumours (Takahashi et al. 2012; Hamidpour et al. 2013). In addition, it has been demonstrated that boswellic acids have an inhibitory and apoptotic effect in vitro against the cellular growth of leukaemia HL-60 cells, inhibiting the synthesis of DNA and RNA in a dose-dependent manner (Shao et al. 1998).

Different triterpenes also play a role in microorganism growth inhibition or elimination. Oleanolic acid and its derivatives present a specific inhibitory effect on Staphylococcus aureus, including methicillin-resistant strains, Streptococcus mutans, Listeria monocytogenes, Enterococcus faecium, and Enterococcus faecalis by destroying the cell membranes of the bacteria (Blanco-Cabra et al. 2019; Kim et al. 2015b). They were also active against species of mycoplasma, and Gram-positive and Gram-negative bacteria also have anti-human immunodeficiency virus (anti-HIV) activity (Khwaza et al. 2018).

Likewise, many studies have shown that glycyrrhizin, a saponine from liquorice (Glycyrrhiza glabra) has potent effects in inhibiting Gram-positive and Gram-negative bacteria, such as Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and Bacillus subtilis as well as the yeast Candida albicans (Karahan et al. 2016; Wang et al. 2015; Martins et al. 2016b). Glycyrrhizin has also demonstrated antiviral activity, with the possible mechanism of action being the decrease of the expression of genes of virulence. Glycyrrhizin has anti-HSV-1 and anti-HSV-2 activities in vivo and in vitro; therefore, can significantly inhibit virus replication and reduce the number of infectious virus particles (Huan et al. 2021). It also effectively inhibited the Severe Acute Respiratory Syndrome-Associated Coronavirus (SARS-CoV) virus replication through cellular signalling pathways such as protein kinase C (Cinatl et al. 2003). Moreover, it is proposed that the long-term administration of glycyrrhizin protects patients with chronic hepatitis C from developing hepatocellular cancer. It improves the outcomes of interferon administration due to a synergistic effect (Ashfaq et al. 2011).

Antiviral activity has also been evidenced by moronic acid and betulonic acid, two compounds extracted from the plant Rhus javanica that showed a potent in vitro inhibitory effect on Herpes Simplex Virus-1 (HSV-1). Betulinic acid and its derivatives also have anti-human immunodeficiency virus activity (Huang et al. 2018) being considered the earliest discovered pentacyclic triterpenoid compound with anti-HIV activity (Kumar et al. 2018; Aiken and Chen 2005). Additionally, they have been shown to inhibit the replication of vesicular stomatitis virus and encephalomyocarditis virus (Cavalcante et al. 2020; Meira et al. 2019).

Notoginsenoside ST-4i, isolated from the Chinese herb Panax notoginseng, demonstrated in vitro inhibitory activities against HSV-1 and Herpes Simplex Virus-2 (HSV-2) (Pei et al. 2011).

Lupenone and its saponin showed in vitro viral plaque inhibitory effects against HSV-1 and HSV-2 (Xu et al. 2018b). Lupenone isolated of rhizoma musae (the root of Musa basjoo Sied. et Zucc.) also has antidiabetic activity by inhibition of α-amylase and α-glucosidase in diabetic mice (Xu et al. 2021). Table 5.5. describes the activities and examples of plant sources for relevant triterpenes.

Table 5.5 Triterpenes and their bioactivities

5 Terpene Applications, Challenges, and Future Perspectives

5.1 Application

Besides playing a fundamental role in plants, terpenes and their derivatives find diverse applications in several industries, such as food, pharmaceutical, flavours, fragrances, and biofuels (Bicas et al. 2009; Tetali 2019; Harman-Ware 2020b). Herbs and spices have been used since ancient times to preserve food due to their antimicrobial and insecticidal properties ascribed to their composition in terpenes (Gottardi et al. 2016). These properties were probably the basis of their initial use in ancient medicine and supported their continued use of mouthwashes, cough medicines, disinfectants, and insect repellents. More complex structural terpenes frequently show other properties of interest that have been taken into an advantage for their health benefits (Guimarães et al. 2019; Papada et al. 2018; Bhavaniramya et al. 2019). In this regard, taxol and vinblastine are used as drugs in cancer treatment. In contrast, others mimic animal hormones such as diosgenin, a sterol synthesised from terpenoid precursors and present in the Mexican yam (Dioscorea mexicana), has been used to produce progesterone for birth control pills and other medicinal steroids (Demain and Vaishnav 2011; Jesus et al. 2016). The most important use of terpenes by humans is for consumption since they are required to produce essential molecules, such as vitamin A synthesised from β-carotene, an abundant plant terpene (Wagner and Elmadfa 2003; Grassmann 2005). More commonly, terpenes in foods affect our eating experience. Terpenoid pigments, such as astaxanthin and lycopene, are heavily used in the food industry as natural colours, for they have a large influence on the acceptability of many foods (Mata-Gómez et al. 2014). Volatile terpenoid compounds impart flavour to foods via their detection by the olfactory system (Schieber and Wüst 2020). For example, the ginger flavour is caused by nootkatone (Srinivasan 2017), and zingiberene imparts a grapefruit flavour (El Hadi et al. 2013). Many herbs used as fresh plant material contain volatile organic compounds (VOCs) that are used in the preparation of flavours in beverages. For example, a soda contains more than one citrus oil (e.g., lemon, lime, orange, neroli) (Ameh and Obodozie-Ofoegbu 2016), already in the preparation of cola flavours are used significant amounts of spice oils such as cinnamon (Buglass and Caven-Quantrill 2012). In terms of alcoholic spirits, specifically for gin, the aroma stems from the use of botanicals during the distillation process, whereby individual aroma notes are imparted by constituent odour-active VOCs predominant compounds, whereby monoterpenes were the most frequently identified compounds, assigned mainly to juniper and coriander oil (Buck et al. 2020). More recently, studies have reported that some terpenes have adequate properties, such as viscosity, flash point, high energy density, among others, that make them good candidates for speciality biofuels, capable of supplementing or even replacing current petroleum-derived fuels (Mewalal et al. 2017; Donoso et al. 2022; Donoso et al. 2021). In the last years, the most promising examples include the monoterpenes limonene and β-phellandrene (Lei et al. 2021), the sesquiterpenes farnesene and bisabolene (Gupta and Phulara 2015) and the diterpenes phytene and cambrene (Scown et al. 2021). Although plant terpenes represent a potential source of alternative and sustainable energy, their implementation as commercial biofuels is compromised by the low yield of specific terpenes in plants (Jiang et al. 2016). Nevertheless, commercial-scale recovery of plant terpenes from some plants such as pine, citrus, and eucalyptus is already taking place and together with the recent advances of biotechnological tools such as genome editing, can pave the way for effective use of terpenes as a feedstock for alternative biofuels (Beller et al. 2015).

5.2 Challenges and Future Perspectives in the Terpenes Industry

Terpenes are compounds of great industrial interest with a wide range of applications. However, there is still a long way between the characterisation and isolation of terpenes and their extraction and application on an industrial scale (Jiang et al. 2016). There are resources throughout nature that could be discovered with applications of human interest. Secondary metabolites from plant matrices, especially terpenes, are still an underexplored source of compounds, with only a small proportion being investigated. Diverse genetic and climatic factors influence the synthesis of secondary plant metabolites (Pang et al. 2021). Biotyping factors such as plant competition, pollinators, viruses, bacteria and predators, and abiotic conditions such as soil type, hydric or climatic stress, high light intensity, among others, alter the production of terpenes in plants. Besides climatic and biotyping factors, there are also difficulties related to botanical species' slow growth, low yield, and incomplete knowledge of biosynthesis mechanisms (Dudareva et al. 2013). Therefore, developing a model that can standardise the production of terpenes in plants under different environmental conditions is still a challenge for the industry. However, the biosynthesis of many natural products is still poorly understood, and this hampers the ability to engineer microbial hosts to produce these compounds (Beutler 2009). The anticancer agent paclitaxel (taxol) biosynthesis is an excellent example of this problem. The compound is derived from the bark of the slow-growing evergreen Pacific yew tree (Taxus brevifolia), and the bark is the only part of the plant that is used commercially. The bark is harvested sustainably, but it is still a limited resource. In addition, the complex biosynthesis of paclitaxel makes it challenging to produce, resorting to recombination techniques and microbial fermentation engineering. A potential solution to this problem may be genetically modified plants (Croteau et al. 2006; Zhu and Chen 2019). This technology would involve constructing a metabolic pathway to produce the natural product in the microbial frame and incorporate the necessary genes into the organism's genome (Heskes et al. 2018). From a future perspective, a better understanding between the action of molecules, the synergy or antagonism of compounds, and the proof of the biological activity in vivo is of paramount importance, with the synthetic production of terpenes being a potentially viable solution in the future for terpenes supply and the development of industries that have these compounds as raw material.