A Narrative Review of the Antitumor Activity of Monoterpenes from Essential Oils: An Update

Monoterpenes are a group of natural products that have been widely studied due to their therapeutic potential against various pathologies. These compounds are abundant in the chemical composition of essential oils. Cancer is a term that covers more than 100 different types of malignant diseases and is among the leading causes of death in the world. Therefore, the search for new pharmacotherapeutic options applicable to cancer is urgent. In this review, studies on the antitumor activity of monoterpenes found in essential oils were selected, and botanical, chemical, and pharmacological aspects were discussed. The most investigated monoterpenes were carvacrol and linalool with highly significant in vitro and in vivo tumor inhibition in several types of cancers. The action mechanisms of these natural products are also presented and are wildly varied being apoptosis the most prevalent followed by cell cycle impairment, ROS production, autophagy, necroptosis, and others. The studies reported here confirm the antitumor properties of monoterpenes and their anticancer potential against various types of tumors, as demonstrated in in vitro and in vivo studies using various types of cancer cells and tumors in animal models. The data described serve as a reference for the advancement in the mechanistic studies of these compounds and in the preparation of synthetic derivatives or analogues with a better antitumor profile.


Introduction
Cancer is a leading cause of death worldwide and so an important public health issue. Aging, population growth, and risk factors related to socioeconomic development are the main causes of cancer boost. In 2020, it was estimated that there were 19.3 million new cancer cases and nearly 10.0 million cancer deaths, with female breast, lung, colorectal, prostate, and stomach cancers topping the incidence list [1]. Chemotherapy represents the main conduct in the treat-ment of cancer. However, many chemotherapeutics not only kill cancer cells but also affect normal cells and tissues, which demonstrate a need to search for more selective active compounds that develop fewer side effects [2,3].
Several drugs used in cancer treatment are natural products or derivatives of them, such as taxol, isolated from Taxus baccata and the analogs docetaxel (Taxotere) and cabazitaxel (Jevtana) in addition to vincristine and vinblastine (Velban) derived from Catharanthus roseus and their analogues vindesine (Eldisine) and vinorelbine (Navelbine) [3,4]. In addition, of 175 molecules approved for antitumor therapy between 1940 and 2014, approximately 50% were derived from natural products or their derivatives [5,6]. Thus, natural products represent a significant source of antitumor agents whose pharmacological action and clinical application has demonstrated success in cancer therapy [2]. The mixture of volatile compounds from plants known as essential oil plays a significant role in plant defense. The oil is made up of hundreds of compounds that are mainly represented by terpenoids. Essential oils have antitumor activity against several types of tumors, such as hepatocellular carcinoma, triple negative breast cancer, and acute myeloid leukemia (AML), in addition to presenting low toxicity [7][8][9][10][11]. The essential oil of leaves from Zornia brasiliensis and Guatteria friesiana (W. A. Rodrigues), plants used in folk medicine in Brazil and Colombia, have terpenoids as its main components and are highly cytotoxic and inhibit tumor growth from several types of cancer both in vitro and in vivo [12,13]. The anticancer potential of monoterpenes found in essential oils has also been reported [7,14]. Essential oil from the leaves of Croton regelianus Muell. Arg., a native plant from Northeast Brazil used in folk medicine, has ascaridole, a monoterpene, as one of the main constituents with high antitumor activity against leukemia, glioblastoma, and sarcomas in mice models [15]. Furthermore, monoterpenes (-)-α-pinene, (-)-β-pinene, and limonene from essential oils from Piper rivinoides Kunth were active against tumor cell lines of oral squamous cell carcinoma (SCC9 and SCC25), showing greater selectivity in relation to carboplatin, the drug of choice in the clinic [16]. Thus, as described in Method, the present study performed an up-to-date review of essential oil monoterpenes with antitumor activity.

Method
The present review was based in data search performed in the scientific literature database PubMed, the most accessed for biomedical (medicine and health) literature worldwide. The eligibility criteria and selection of the studies were based on the following keywords: monoterpene, essential oils, antitumoral, anticancer, and cytotoxicity, and covered a period from January 2014 to December 2020, that way, performing an update on our previous review published in 2014 [14]. The scientific publications were selected from studies published in English and discussed in this manuscript.

Results and Discussion
This discussion focused on the in vitro and in vivo antitumoral role of essential oil monoterpenes was based on studies indexed in the biomedical database PubMed and as described in Method.
Chemical structures and names of antitumor monoterpenes are provided in Figure 1 , which showed cytotoxicity against A2780 cells comparable with that of doxoru-bicin, enhanced apoptosis in A2780 cell, and caused a dosedependent increase in preG1 cell cycle events [17]. Furthermore, 1,8-cineole induced the apoptosis and G2/M phase arrest of cell cycle in A431 cells by increasing the expression of p53. Also, there was the expression of apoptotic proteins, such as Bax/Bcl-2, Cyt-c, caspase-9, and caspase-3. Molecular docking simulations predicted the hydrophobic interaction between 1,8-cineole with PARP1 receptor and Bcl-2 [18]. 1,8-Cineole was selective, and it did not affect cell viability of normal lung WI-38 cells and inhibited A549 cell migration but did not induce apoptosis [19].
3.1.2. α-Thujone. α-Thujone has demonstrated several pharmacological effects, such as antitumor, analgesic, and insecticide. It was seen that α-thujone has not shown cytotoxicity to HCT116 and SW620 cells; however, it increased the proliferation of CD3AK immune cells and its cytotoxicity against cancer cells by improving the expression of CD107a, p-Akt, and p-ERK1/2 [20]. Furthermore, α-thujone also exerted effect on GBM (glioblastoma) cells. The observed effects were induction of apoptosis, considerable inhibition of cell motility, induction of oxidative stress, and autophagy in α-thujone-treated tumor cells. Meanwhile, normal astrocytes showed lower sensitivity to α-thujone treatment [21].
3.1.3. Bornyl Acetate. The combination of bornyl acetate with 5-fluorouracil potentiated the anticancer activity of 5fluorouracil in human gastric cancer (SGC-7901) cells. The results of this combination were induction of apoptosis, DNA fragmentation, G2/M cell cycle arrest, cell morphology alterations, and potentiated effect of cell growth inhibition, indicating a synergistic enhancement in the anticancer activity of 5-fluorouracil [22].

β-Pinene.
(-)-β-Pinene has demonstrated a cytotoxic effect against tumor cells. It was observed in human oral tongue cancer cells that (-)-β-pinene showed cytotoxicity, altered cell morphology, large amounts of pyknotic nuclei, membrane blebs, and cell shrinkage, and caspase inhibitors ZVAD (pancaspase) and ZDEVD (caspase-3) reduced cell death. In addition, (-)-β-pinene also showed selectivity in primary normal human gingival fibroblast [16]. β-Pinene has also been evaluated in association with an already established drug, paclitaxel, in non-small-cell lung cancer cells. The results revealed that the combination of paclitaxel with β-pinene showed a substantial synergistic effect. Furthermore, paclitaxel+β-pinene showed morphological change characteristic of apoptosis like chromatin condensation and fragmentation of the nucleus [23].
3.1.5. Carvone. Carvone is a monocyclic monoterpene that has been reported to have antioxidant, antimicrobial, anticonvulsant, and antitumor activities. Regarding the antioxidant effect, it was seen that carvone treatment increased levels of total antioxidant capacity in cultured primary rat neuron cells. In addition, it increased levels of total oxidative stress in primary rat neuron cultures and rat brain NB cell line N2a. There was also a reduction in cell viability rates in both cell types [24].

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In addition to the antioxidant effect, carvone also showed antitumor activities. Carvone inhibited proliferation of MCF 7 and MDA MB 231 cells and inhibited the migration of these breast cancer cells. In addition, it induced apoptosis, as was seen by the fragmentation of nuclei and the presence of apoptotic bodies, and it also arrested MCF 7 cells in the S phase of the cell cycle, caused DNA damage and ROS production, and increased levels of p53, Bad, cleaved caspase-3, and cleaved PARP [25]. Carvone also exerted anticancer effects on myeloma cancer cells (KMS-5) with antiproliferative effects that were associated with the induction of apoptosis and arrest of the G2/M cell cycle. It has also been seen that carvone can inhibit cell invasion and protein expression p-P38 at IC50 [26]. Another effect observed was with carvone associated with doxorubicin, in which carvone showed a synergistic anticancer effect with doxorubicin on the MCF 7 cell line, in addition to decreasing the toxicity of doxorubicin on a normal heart cell line. In BALB/c mice, carvone protected the heart from doxorubicin toxicity. The protective effect of carvone was due to an increase in catalase activity [27]. Carvone also prevented skin carcinogenesis and reduced levels of phase I enzymes (Cyt P450 e-Cyt b5) with increased levels of phase II enzymes (GR, GST, and GSH). There was also increase in the expression of Bax, caspase-3, and caspase-9 with decreased expression of mutated p53 and Bcl-2 in Swiss albino mice treated with DMBA (7, 12-dimethylbenz [a] anthracene) and D-carvone [28].

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3.1.6. Limonene. Limonene effects have recently been investigated on T24 human bladder cancer cells presenting an IC50 of 9 μM. Ye et al. demonstrated that it presented the antitumor capacity to induce cell cycle arrest, suppression of cell migration and invasion, and apoptosis with observation of nuclear fragmentation, chromatin condensation, splitting of the nucleus, increase of Bax and caspase-3, and decrease of Bcl-2 expressions [29]. D-limonene showed lung antitumor activity in vivo and in vitro by preventing the growth of lung cancer cells and inducing apoptosis through mechanisms involving autophagy. There was an increase in Bax and cleaved PARP during treatment, which may be related to induce death of lung cancer cells. Increases were also found in Atg-5, presuming that Atg5 overload may be partially involved in D-limonene-induced apoptosis [30].
3.1.7. Thymoquinone. It has recently been demonstrated that thymoquinone induces apoptosis of 786-O human renal carcinoma cells. In more details, it decreases cell viability in a concentration-and time-dependent manner with an IC50 of 3.8-12.9 μM, increases intracellular ROS levels and the sub-G1 population, and decreases the migration and invasiveness potential of tumor cells [31]. Thymoquinone reduced viability and increased cell death due to apoptosis in human lung tumor cells A549 presenting the highest IC50 against cancer cells at 47 ± 0:09 μM. The approach with this monoterpene significantly increased the Bax/Bcl-2 ratio, positively regulated the expression of p53, and activated caspase-dependent apoptosis by the activation of caspases-3 and -9 [32].

Monoterpenes That Induced Apoptosis and Tumor
Inhibition In Vivo 3.2.1. α-Pinene. The α-pinene is a natural compound that has demonstrated anticancer activity. The α-pinene inhibited liver cancer cell growth. In nude mice with hepatocellular carcinoma, Chk1 and Chk2 levels were upregulated, Cyclin B, CDC25, and CDK1 levels were downregulated, and the average tumor size was significantly smaller in mice treated with α-pinene. The average tumor was significantly lighter than the control, and histology examination of xenografts showed more slow-growing cells and dead cells. α-Pinene also caused marked accumulation of a G2/M population in BEL-7402 cells [19]. In human cancer ovary (PA-1) cells, α-pinene inhibited the cycle progression at G2 to M phase and markedly increased caspase-3-dependent apoptotic cell death [33]. It was also observed that HepG2 cells treated with α-pinene exhibited growth inhibition, G2/Mphase cell cycle arrest, triggered oxidative stress, and induced apoptosis. The cell cycle arrest was associated with downregulated cyclin-dependent kinase 1 (CDK1) and miR-221 levels. There were also upregulated levels of CDKN1B/p27, γ-H2AX, phosphorylated ATM, phosphorylated Chk2, and phosphorylated p53 [34]. The combination of paclitaxel with α-pinene showed a substantial synergistic effect, showing consequences on cell cycle distributions of A549 cells, in which the percentage of sub-G0/G1-phase cells was decreased on the addition of α-pinene to paclitaxel, and the population of G0/G1 cells was increased presenting morphological change characteristic of apoptosis [23]. α-Pinene also demonstrated protective effect against ultraviolet Ainduced cellular damages in human skin epidermal keratinocytes (HaCaT cells). α-Pinene showed a probable antioxidant property, preventing UVA-induced cytotoxicity, generation of ROS, lipid peroxidation, and DNA stand breaks. Furthermore, α-pinene inhibited inflammatory mediators such as NF-κB, TNF-α, and IL-6 expression and also modulated nucleotide excision repair proteins via activation of p53 and p21 that prevent the formation of UVAinduced cyclobutane pyrimidine dimers [35].
3.2.2. Borneol. Natural borneol (NB) has been used as a promoter of drug absorption; thus, it was seen that borneol could potentiate the cellular uptake of bisdemethoxycurcumin (possible anticancer activity) [36]. Besides that, borneol can also act as an "upper guiding drug," because it has a potential autophagic inhibitor activity that may guide luteolin (anticancer activity) in the ubiquitin-proteasome pathway and the ubiquitin-signal autophagic degradation [37]. It was also seen that borneol increased the blood-brain barrier permeability and intracellular uptake of doxorubicin (DOX), which potentiated DOX-induced G2/M cell cycle arrest and inhibited U251 human glioma xenograft growth in vivo through combined treatment of DOX with natural borneol [38]. The effect of borneol was also seen in combination with cisplatin, and it was seen that borneol synergistically enhanced the anticancer activity of cisplatin in human glioma cells. Cotreatment of cisplatin inhibited U251 cell viability and enhanced cisplatin-induced apoptosis with caspase activation and reactive oxygen species overproduction, and borneol also increased cisplatin-induced cell growth in a ROS-dependent manner [39].

Camphene.
Camphene is a bicyclic monoterpene and a component of essential oils derived from plants such as rosemary, turmeric, pine, and ginger that has high antioxidant, anti-inflammatory, and antimicrobial activity [40,41]. The in vitro activity of camphene induced apoptosis by the intrinsic pathway in melanoma cells causing endoplasmic reticulum stress. Moreover, there were also release of Ca2þ together with calreticulin and HmgB1, upregulation of caspase-3 activity, and loss of mitochondrial membrane potential. Camphene was also evaluated in vivo, in male C57Bl/6 mice, inhibiting subcutaneous tumor growth of highly aggressive melanoma cells in a syngeneic model [42].

3.2.4.
Carvacrol. Carvacrol, when tested on AGS (human gastric adenocarcinomas) and WS-1 (normal human fibroblastic cells), had toxic effects but was more effective in cancer cells. One possible mechanism was apoptosis induced by DNA damage [43]. Carvacrol also induced apoptosis in different cells, such as A549 cells, DU145, human colon cancer cell lines, HL-60, Jurkat, and PC-3 [44][45][46][47][48]. Apoptosis occurred in A549 cells through activation of key regulators of apoptosis, such as p-JNK and Bax, and reduction of Bcl2, release of cytochrome C, activation of the caspase cascade, and production of reactive oxygen species (ROS) [44].
In DU145, apoptosis occurred with increased generation of ROS, disruption in the mitochondrial membrane potential, and arrest in the G0/G1 phase of the cell cycle [45]. In human colon cancer cell lines, there was also cell apoptosis with cell cycle arrest in the G2/M phase, decrease in expression of cyclin B1, downregulation in the expression of Bcl-2, induction in the phosphorylation of extracellular regulated protein kinase and protein kinase B (p-Akt), and upregulation in the expression of Bax N-terminal kinase and c-Jun [46]. In HL-60 (human acute promyelocytic leukemia cells) and Jurkat (human T lymphocyte cells), apoptosis occurred with the formation of free radicals, reduced levels of antioxidants such as catalase and superoxide dismutase, increased Bax expression, and decreased Bcl-2 expression, and apoptosis by carvacrol was mediated by caspase-3 [47]. In addition, apoptosis acted in reducing the cell viability of PC-3 cells (prostate cancer cell line), showing high levels of ROS, disruption of the mitochondrial membrane potential, prevented cell cycle in G0/G1, decline in the expression of cyclin D1 and cyclin-dependent kinase 4 (CDK4), and increased expression of the CDK inhibitor p21 [48].
Cisplatin (CP) and carvacrol showed dose-dependent cytotoxicity and activated ERK1/2. The MEK inhibitor PD325901 suppressed ERK expression, increased cytotoxicity of carvacrol, increased viability of cells by modulating apoptosis, and increased microtubule-associated protein 1A/1B-light chain 3 beta expression in cisplatin treatment. Cotreatment with cisplatin and carvacrol increased the viability of cancer cells compared to treatment with CP, due to the suppression of apoptosis. MEK inhibition decreased cell viability, without causing changes in apoptosis. The carvacrol also increased cisplatin-induced expression of light chain 3 beta. It induced CP resistance in HeLa cells through ERK1/2-independent suppression of apoptosis and ERK1/2dependent modulation of autophagy [49].
Other effects were observed, as alteration of soluble factors in HCT-116 and HT-29 (human colorectal carcinoma) [50]. Carvacrol also suppressed cell proliferation and migration, and its inhibitory effect was attenuated in NSCLC (non-small-cell lung cancer) cells with overexpression of AXL, in which the treatment promoted downregulation of AXL expression and inhibited AXL phosphorylation after ligand stimulation [51]. It was able to inhibit cell proliferation, prevent metastasis in hepatocellular carcinogenesis, and suppress the elevation of serum tumor marker enzymes, carcinoembryonic antigen, and alpha-feto protein induced by diethylnitrosamine [52]. It did not show mutagenic effects on human lymphocytes but showed that at concentrations above 100 mg/L there was a decrease in cell viability. Furthermore, it caused statistically significant increases in the levels of TAC (total antioxidant capacity) and TOS (total oxidative stress) on human lymphocytes [53]. In vivo, changes in body weight and oxidative stress index were found in plasma and stomach tissues of treated Wistar rats [43].

Citral.
Citral had an antiproliferative effect on several cancer cells. Apoptosis was observed in human stomach cancer cells [54]; in prostate cancer cells (PC3 cells) through upregulating BAX and downregulating Bcl-2 expression [55]; and in HCT116 and HT29 (colorectal cancer cell lines), in which it induced mitochondrial-mediated apoptosis via increased intracellular ROS and phosphorylation of p53 protein and the expression of Bax and decreased expression of Bcl-2 and Bcl-xL which promoted the cleavage of caspase-3 [56]; and citral also showed cytotoxicity in Burkitt's lymphoma cell line human and additively increased the cytotoxic and apoptotic effects of doxorubicin [57]. The combination of citral and doxorubicin increased the expression of the proapoptotic protein BAK but decreased the expression of the antiapoptotic protein BCL-XL compared to cells just treated with doxorubicin [57].
In addition to apoptosis, citral also induced cytotoxicity by regulating several genes involved in signaling pathways, inhibiting metastases, inhibiting colony formation, and interrupting migration of colonies of cancer cells [54]. Furthermore, it was able to block the growth of breast tumor mediated by ALDH1A3 through its inhibition. Inhibition of ALDH1A3 blocked the colony formation and activity regulating gene expression [58]. Other effects caused by citral were the damage to the clonogenic property and changes in morphology of cancer cells and suppression of lipogenesis of prostate cancer cells [54].
Studies also have suggested that citral binds to MARK4, inhibiting its enzyme activity, which is associated with the cell cycle and, therefore, cancer. Moreover, it also inhibited proliferation of the breast cancer cell line MCF-7 [59]. It was also seen that treatment with citral reduced the size and number of cells with ALDH+ activity of the tumors in BALB/c mice challenged with 4T1. Furthermore, smaller tumors and delayed tumorigenicity were observed in mice treated with citral after undergoing primary tumor cell reimplantation [60].
3.2.6. Citronellol. Citronellol has been described with antitumor activity against lung [61] and breast cancers [62], inducing necroptosis and apoptosis, respectively. For lung cancer, IC50 was found to be 49.74 μg/mL and necroptosis was confirmed by an upregulation of TNF-α pathway and downregulation of caspase-3 and -8 activities. Besides, citronellol at a dose of 50 mg/kg inhibited 80% of subcutaneous tumor growth previously induced by intraperitoneal injection of NCI-H1299 in nude mice. For breast cancer, IC50 was found to be 35 and 80 μM, using two different tumor cell lines, and apoptosis was validated by the loss of cell viability, increase in ROS generation, altered mitochondrial membrane potential, enhanced DNA damage, and modulation of the expression of apoptotic proteins (inhibition of Bcl-2 with upregulation of Bax and caspase-9 and -7) in MCF-7 and MDA-MB-231 cells.  [63]. However, geraniol presents antitumor activity by several other mechanisms, as observed in the past years. In an oral carcinogenesis model using a 200 mg/kg dose, geraniol downregulates the activation of NF-κB, reducing the expression of TNF-α, IL-1β, COX-2, and iNOS [64]. On PC-3 prostate cancer cells, using microarray, it was observed that geraniol downregulates the transcription factor E2F8 suppressing cell growth and inducing G2/M arrest [65]. On lung adenocarcinoma cancer cells, with an IC50 of 797.2 μM and in vivo doses of 50 and 75 mmol G/kg, it inhibits the mevalonate pathway resulting in growth inhibition of A549 cells and in vivo subcutaneous tumor growth besides promotion of apoptosis [66]. On Ishikawa endometrial cancer cells, with an IC50 of 140.929 μM, geraniol induces apoptosis with the involvement of the mitochondrial pathway, observed by a decrease in Bcl-2 and increase in Bax staining and TUNEL-positive cells, as well as an increase in the mRNA levels of Bax, caspase-3 and -8, cytochrome C, and Fas and a decrease in the Bcl-2 gene [67].
3.2.8. Linalool. Linalool has been a major topic of investigation over the last years. Its acts against several types of cancer mainly through induction of cell cycle arrest and oxidative stress. On A549 lung adenocarcinoma cells, linalool suppresses cell growth through cell cycle arrest, and oxidative stress also acts on the mitochondrial membrane potential depolarization and inhibition of cell proliferation, besides preventing cell migration [19]. On OECM 1 oral cancer cells, with an IC50 of 10 μM, it inhibits the viability of cells and induces cell cycle arrest and apoptosis with a decrease in the expression of p-PI3K, p-AKT, and Bcl-2 and a rise in the expression of Bax [68]. On HCT 116 colon cancer cells, linalool promotes apoptosis via lipid peroxidation observed in vitro and in vivo; moreover, it induces a reduction of 55% in mean xenograft subcutaneous tumor weight in a 200 mg/kg dose [69]. On T-47D breast, SW 620 colorectal and HepG2 liver cancer cells, with IC50 of 224, 222, and 290 μM, linalool induced a concentration of cells in the G1 phase and by cytokine array analysis observed a stimulation of IFN-γ, IL-13, IL-2, IL-21, IL-21R, IL-4, IL-6sR, and TNF-α secretion, suggesting it could also induce Th1 cellular immune response [70]. On HepG2 hepatocellular carcinoma cells, it induces cell cycle arrest and apoptosis with the involvement of Ras, MAPKs, and Akt/mTOR pathways, besides ROS production influencing cytotoxicity [71]. On U937 leukemia and HeLa cervical cancer cells, with an IC50 of 2.59 and 11.02 μM, linalool induces cell cycle arrest facilitating the expression of p53, p21, p27, p16, and p18 [72]. On sarcoma-180 cells in vitro and in a solid tumor model, linalool appeared to be selectively cytotoxic towards tumor cells in contrast to conventional chemotherapeutic drug, presenting reduction in cell viability, tumor volume, tumor weight, and tumor cell count data; furthermore, it induces apoptosis and cell cycle arrest in tumor cells and extensive necrosis and reduced viable tissue mass in vivo [73].
Perillyl alcohol presents an IC50 of 1.8 and 2 mM on U87 and U251 glioblastoma cells and an apoptosis induction through a signaling mechanism mediated by Na/K-ATPase [74]. Also, it has an antitumor activity by inhibiting HIF-1 in HeLa cervical, SK-Hep1 hepatic, and HCT116 colon cancer cells, which was mediated by the inhibition of the mTOR/4E-BP1 signaling pathways; moreover, in a xenograft tumor model using HCT116 cells, perillyl alcohol led to a tumor inhibition of 64.11% [75]. It was also demonstrated to induce apoptosis in an elaborated work using a chemoprevention gene therapy approach with perillyl alcohol and a replication incompetent adenovirus to deliver melanoma differentiation associated gene-7/Interleukin-24 (mda-7/IL-24) with a resulting enhanced conversion of mda-7/IL-24 mRNA into protein in AsPC-1, PANC-1, MIA PaCa-2, and BxPC-3 pancreatic cancer cells [76]. Andrade et al. showed that perillaldehyde 8,9-epoxide inhibits up to 58.7% of tumor growth in a sarcoma mouse model [77] and induces apoptosis and necrosis in OVCAR-8 ovarian, HCT-116 colon, SF-295 brain, and HL-60 leukemia tumor cells with IC50 values of 0.64-1.75 μL/mg [78]. Regarding dehydroperillic acid, it was proved to inhibit DNA synthesis and promote apoptosis, being highly cytotoxic (IC50 = 125 μg/mL) and selective (selective index = 400) in A549 lung cancer cells [79].
3.2.10. Terpineol. Terpineol has been described as able to suppress cell migration and induce cell cycle arrest and apoptosis in HepG2 hepatic cancer cells with IC50 of 19.5 μM and observation of DNA fragmentation; besides, the doses of 10 and 20 mg/kg in subcutaneous tumors promoted a reduction in the tumor weight and volume [80]. Terpineol cytotoxic role was also investigated by screening several cell lines (HT29, HCT116, COLO320, DLD1, AGS, COLO357, Panc-1, MIA-PACA, DU145, and CL-1) of colorectal, gastric, pancreatic, and prostate cancer cells showing a significant growth inhibition in all the different cancer cells. Besides inducing a decrease in subcutaneous tumor volume, importantly, it was shown to improve the effect of several anticancer agents [81]. Terpineol was also investigated on BEL-7402 liver cancer cells, presenting an IC50 of 0.32 mg/ mL, inhibition of cell growth, and induction of apoptosis, with observation of cell shrinkage, deformation and vacuolization of mitochondria, nuclear chromatin condensation and fragmentation, formation of apoptotic bodies, and accumulation of cells at G1 or S phase [82]. Table 1 shows the monoterpenes and their antitumor effects on specific experimental models.  [83,84]. It was also studied on T24, SW280, and J28 bladder cancer cells presenting IC50 of 90.1-130.5 μM, cell cycle arrest, and mitochondria-related apoptosis with activation of the ROS-JNK/p38 pathway [85]. On Cal27, SCC4, and SCC9 oral 6 BioMed Research International  [18] α-Phellandrene Active (altered gene expression)

Monoterpenes That Induced Other Antitumor
Mechanisms and Tumor Inhibition In Vivo 3.3.1. Cuminaldehyde. Cuminaldehyde has been shown to possess the ability to inhibit topoisomerase I and II activities and to upregulate lysosomal vacuolation on different cancer cells, exercising antitumor activity both in vitro and in vivo [93,94]. Using COLO 205 colorectal adenocarcinoma cells, Cherng's group found an IC50 of 16.31 μM and a tumor growth inhibition of 69.4% in subcutaneous tumors with a dose of 20 mg/kg. The same group also investigated the effect of cuminaldehyde in A549 lung adenocarcinoma cells finding similar results with an IC50 of 18.33 μM and a subcutaneous tumor growth inhibition of 50% using a dose of 10 or 20 mg/kg.

3.3.2.
Fenchone. Fenchone, a major component of Mesosphaerum sidifolium (Lamiaceae) oil, at a dose of 60 mg/kg, induced in vivo cell growth inhibition (total viable cancer cells, tumor volume, and mass) on Ehrlich ascites carcinoma model through stimulation of cell cycle arrest [95]. This bicyclic monoterpene is also present in the essential oil of fennel (Foeniculum vulgare) and has antioxidant, antiinflammatory, antinociceptive, antifungal, and acaricidal properties [96,97].

Myrtenal.
Myrtenal has been shown to cause cell death, reduce migration and invasion of B16F0, B16F10, and SkMel-5 melanoma cells in vitro, and significantly decrease metastasis promoted by B16F10 cells in vivo, using a 15 mg/kg dose, through inhibition of V-ATPase [98]. Farrag et al. demonstrated the chemopreventive efficacy of myrtenal against the bladder carcinogenesis of rats. The downregulation in the expressions of COX-2, NF-κB, and STAT-3 correlated with the suppression of the levels of inflammatory cytokines of TNF-α and IL-6 and biomarkers of oxidative damage (MDA and NO). In addition, a significant increase in caspase-A3 activity and Bax/Bcl-2 ratio indicate that the anti-inflammatory effect and the induction of apoptosis contributed to this activity [99].

Monoterpenes That Induced Other Antitumor
Mechanisms In Vitro 3.4.1. α-Phellandrene. α-Phellandrene showed antitumor activity, which decreased the cell viability, promoted the cell cycle distribution, significantly increased reactive oxygen species levels, decreased mitochondrial membrane potential levels, increased the necrotic cell number, and increased NO production, LDH leakage, and ATP depletion on human liver cancer cells (J5) [100].
In Lin et al. [102], mice were injected with mouse leukemia WEHI-3 cells and subsequently treated orally with or without α-PA. α-PA influences the murine WEHI-3 leukemia model in vivo by increasing the percentage of CD3 (Tcell marker), CD19 (B-cell marker), and MAC3 (macrophages) markers but reduced the percentage of CD11b (monocytes) cell surface markers. α-PA increased phagocytosis by macrophages and promoted natural killer cell activity. α-PA also increased B-and T-cell proliferation.

3.4.2.
Cymene. p-Cymene inhibits MMP-9 expression and increases TIMP-1 production besides inhibiting the ERK1/ 2 and p38 MAPK signal pathways on HT-1080 human fibrosarcoma cells, presenting a robust 87% inhibition of in vitro invasiveness [103]. p-Cymene is a natural antioxidant found in several plant species and is a constituent of fruits, wines, and spices, such as Origanum vulgare (oregano) and Thymus vulgaris (thyme). This compound has several pharmacological activities, in addition to an antinociceptive effect on cancer pain through inhibitory pathways and modulation of calcium currents [104][105][106].
3.4.3. Rotundifolone. Rotundifolone, also known as piperitenone is a naturally occurring oxygenated monoterpene and represents one of the main components of the essential oil of many species of Mentha [107,108]. Rotundifolone has 14 BioMed Research International recently been reported as presenting antioxidant and antiproliferative antitumor activities on U87MG glioblastoma cells with IC50 of 30 mg/L and alteration of PTEN/PI3K/ AKT/NF-κB signaling pathways [109]. The data described here may have implications for future research, possibly being used as a reference for the advancement in the mechanistic studies of these compounds, as well as in the preparation of synthetic derivatives or analogues with a better antitumor profile. Indeed, chemical modification in natural products is a current approach to improve drug actions against several diseases, including cancer [111].

Conclusions
The medicinal properties of essential oils have been evidenced through scientific investigations using experimental models. Considering that monoterpenes are commonly the main constituents of these oils, these natural products should contribute to pharmacological actions. The studies reported in this review confirm the antitumor properties of monoterpenes and their anticancer potential against various types of tumors, as demonstrated in in vitro and in vivo studies using various types of cancer cells and tumors in animal models. Since the last revision of this subject, several mechanisms of action of these compounds were elucidated, and deeper understanding of the use of theses monoterpenes in in vivo models was gained [14]. The main antitumor mechanism was through induction of apoptosis through several different pathways as inhibition of antiapoptotic proteins Bcl2 and BclXL or upregulation of cytochrome C release channel as BAX and BAK. Increase in ROS production and cell cycle impairment, mainly by disruption of cyclin D expression or CDK4 and its regulator p21, was also a common antitumor mechanism. Less common but not less interesting and important were autophagy induced by limonene and necroptosis by citronellol. Several tumor-related pathways were also regulated by these compounds inducing antiproliferative, death induction, decreased migration, or other phenotypes such as the oncogenic proteins JNK, Ras/ MAPK, ERK1/2, p38, and PI3K; tumor suppressor proteins such as p53 and Chk1/2; and CDC25 and inflammationrelated NF-κB, TNF-α, IL1-β, COX2, and iNOS. The data described serve as a reference for the advancement in the mechanistic studies of these compounds and in the preparation of synthetic derivatives or analogues with a better antitumor profile.

Data Availability
The data in this review article were obtained from the PubMed database.