## 2 Sesquietpene Lactones with Significant In Vivo Activity
### Costunolide  
Costunolide (**3**) (Figure 4), has the same chemical formula as alantolactone (**1**) (C\({}_{15}\)H\({}_{20}\)O\({}_{2}\)). It is a member of the germacranolide subclass and was first isolated from _Saussurea costus_ (Falc.) Lipsch. roots in 1960 [82]. It is present in many Asteraceae genera such as _Inula_[24], _Lactuca_ and _Helianthus_[83], but also those from other families like Magnoliaaceae [84]. Biosynthesis of costunolide (**3**) is well documented and occurs through the mevalonate pathway [85]. Briefly, the process starts with the cyclization of farnesyl pyrophosphate, forming germacrene A. Next, the isoprenyl side chain of germacrene A undergoes hydroxylation by germacrene A hydroxylase, followed by oxidation to germacrene acid. It is finally synthesized after oxidation and cyclization of germacrene acid [85]. Some species are rich sources of costunolide (**3**), such as essential oil of _Saussurea lappa_ roots (yield \(\sim\)3%), of which 52% was identified as costunolide (**3**) [86]. The total synthesis of costunolide (**3**) has been described using different methods/strategies [87; 88; 89]. In a recent review [90], [PERSON] and [PERSON] detail the activities of customolide (**3**) and discuss its therapeutic potential. Like alantolactone (**1**), compound **3** also exhibits anticancer bioactivity against different cancer cells via various routes. In fact, it acts as an apoptosis inducer, cell cycle regulator, angiogenesis and metastasis inhibitor and can also reverse the drug resistance mechanism [91]. There are several recent publications describing in vivo costunolide (**3**) studies, such as the work by [PERSON] et al. [92] on costunolide's anticancer effect on osteosarcoma xenografted mice. Results showed a daily 20 mg/kg (b.w.) dose was enough to significantly reduce tumor weight by about half, as well as reduce the number of lung metastases to around one third of that in the control group. Western blot analysis of the tissues revealed this effect could be attributed to costunolide (**3**) inhibition of STAT3 transcription, a factor that is widely known to be linked to oncogenesis and cancer proliferation [93]. Figure 4: Structure of costunolide (**3**). It is interesting to note the similarity between previously mentioned STAT3 inhibitory activity by alantolactone (**1**) [45]. To highlight another similarity, costunolide (**3**) also exhibits activity against gastric adenocarcinoma in xenografted mice [94]. Results from this study showed costunolide (**3**) induced caspase-mediated mitochondrial apoptosis in cancer cells and a 50 mg/kg (b.w.) dose on alternate days significantly reduced tumor size to about half. It achieved very similar results to the positive control cisplatin, a chemotherapeutic agent used in clinical treatment [94]. Interesting to note is costunolide's anticancer effect through telomerase reverse transcriptase inhibition [95], a mechanism not known in alantolactone (**1**). As reported by the researchers, a 5 mg/kg (b.w.) dose injected on alternate days in glioma xenografted mice significantly reduced tumor size by over 50%. This inhibition was associated with reduced telomerase activity, which leads to ROS-associated apoptosis [95]. A subsequent work by the same research group also showed the same dose affects lipid metabolism in glioma xenograft tumors, by lowering expression of FASN, SREBP-1, and PGC-1\(\alpha\)[96], which are key genes targeted for cancer treatment [97; 98; 99]. Beyond costunolide's anticancer effect, it has also proven to be a potent anti-inflammatory agent [90], as well as an antidiabetic, antihelmth, antimicrobial, antiulcer and antioxidant [91]. Its anti-inflammatory action has been well documented in vitro [84; 100] and in vivo, appearing to be linked to NF-kB pathway inhibition [101]. For example, costunolide (**3**) is able to suppress inflammatory angiogenesis [102], alleviating gastric ulcers [103], and acute lung and liver damage [104; 105; 106; 107].