## 2 Biological Activities of Monoterpenes
### Acyclic Monoterpenes
Geranyl acetate (**2c**, Figure 3) is a major component of various essential oils and plant extracts that have shown biological activity. A recent study described the cytotoxicity of this compound from the essential oils of kaffir lime callus [29]. This study conducted MTT assays on the breast cancer cell line, T47D and non-cancerous cells Sel Vero. The resultsof these assays showed a low cytotoxicity in both cancerous and noncancerous cells. The determined IC\({}_{50}\) values exceeded 100 \(\upmu\)g/mL in both cases [29]. These findings suggest that the acetate group makes this compound much more cell permeable. Geranylacetone (**2d**, Figure 3) has been identified in a number of essential oils and plant extracts that have demonstrated anticancer activities. Recently, geranylacetone was identified as a major component of _Lithocarpus polystachyus_ [PERSON]. a plant with documented anticancer properties [30]. This monoterpene was shown as a major constituent of the volatile compound mixture, present at a concentration of about 283 \(\upmu\)g/mL in the distilled sample [30]. An independent study identified geranylacetone as a major component of _Dianella estifolia_ (6% GC-MS) [31]. This study conducted a series of in vitro experiments to understand the antioxidant potential of this oil. A series of biological assays (DPPH, ABTS, and FRAP) were conducted to determine the antioxidant properties of this essential oil (EO) in comparison with positive control BHT (butylated hydroxytoluene). Geranylacetone is also able to react as a free radical scavenger. Both geraniol and geranylacetone have been demonstrated to serve as free radical scavengers, albeit at high concentrations of 4-8.0 mM. DPPH radical scavenging assay showed that geranylacetone after 15 min indicated a modest dose-dependent effect [32]. Halomon (**6**, Figure 3), a halogenated acyclic monoterenoid, has been reported to be active against the non-small cell lung carcinoma cell model (NSCLCN6-L16) and colon cancer cell model (HCT-116) in vitro. Furthermore, in vivo studies have demonstrated the anticancer activity of this compound against U251 brain cancer. Halomon showed cytotoxicity against many cancer subtypes, including renal-, brain-, and colon-derived solid tumor cell lines. It also showed that this monoterenoid acts as an inhibitor for DNA methyltransferase-1 with an IC\({}_{50}\) value of 1.25 \(\upmu\)M [33], which decreases the expression of tumor suppressor genes. Preliminary in vivo studies evaluated the biological effect of Halomon via intraperitoneal injections on U251 brain cancer xenograft model. This model involves the implantation of cancer cells into immunocompromised rats; the tumor is allowed to grow, and then the effects of a treatment are evaluated in vivo. It was demonstrated that five daily doses of 50 mg/kg had a 40% success rate in treatment [33]. Anticancer and antioxidant activity have also been demonstrated with lavandulyl acetate (**5**, Figure 3) which is found in essential oils and plant extracts such as _Lavandula angusifolia_ (lavender oil). GC-MS indicated that lavandulyl acetate is found in 23 percent (_w/w_) of the overall mixture on average of this essential oil [34]. The essential oil was evaluated on several assays including cytokine cellular response, LPS-stimulated HaCaT Keratinocytes, and LPS-prestimulated human monocyte derived macrophages (hMDMs). A dose dependency was observed between lavandulyl acetate in the oil and a potent anti-inflammatory and pro-regenerative response [34]. The studies indicate that this compound might hit several targets as these biological processes involve different pathways. The pharmacological effects of lavender oil depend on the concentration of lavandulyl acetate as independent studies have also shown other biological activities for this essential oil [31]. Linalool (**3a**, Figure 3), a chiral acyclic monoterenoid, has demonstrated anticancer and antioxidative properties [35; 36].