**2. Traditional Uses of Selected Plants**

A total of 27 anticancer traditional medicinal plants that belong to 18 botanical families and 27 genera are identified in this review. The botanical families Euphorbiaceae and Cucurbitaceae were the most dominant, represented with 15% and 11% of the selected plant species, respectively (Figure 1). All of the reviewed plants have direct traditional uses for treating either ailments with cancer-like symptoms (determined by traditional practitioner) or for laboratory-confirmed cancer cases. Besides treating cancer, the plants selected in this review are also cited for their various traditional uses, including for the treatment of eczema, leprosy, rheumatism, gout, ringworm, diabetes, respiratory complaints, warts, hemorrhoid, syphilis, and skin diseases (Table 1). The output calls for the need for further phytochemical and pharmacological investigation giving priority to those plants which have been cited most for their use to treat cancer.

**Figure 1.** Major plant families (in %) of reviewed plants species vegetation zone of Ethiopia [18] (the unmarked blocks are other species).


**Table 1.** General traditional use of selected Ethiopian medicinal plants.

#### **3. Phytochemistry of Ethiopian Anticancer Plants**

The present review reports secondary metabolites isolated from 27 plants that are traditionally used to treat different types of cancer in Ethiopia. Phytochemical investigations of traditionally used Ethiopian anticancer plants have led to the isolation of compounds that belong to different classes of natural products [10,57]. In this review, we have not included plants those displayed compounds with very low cytotoxic/antiproliferative activity (i.e., IC50 (Concentration that inhibited cell proliferation by 50%)/ED50 (Effective dose for 50% of the population) > 50 μg/mL or > 100 μM, in most cases, except few where compounds tested against a panel of cell lines) or plants from which no anticancer compounds were isolated/reported. This review compiled and discussed the potential anticancer/antiproliferative agents based on the types of secondary metabolites, such as terpenoids, phenolic compounds, alkaloids, steroids, and lignans.

#### *3.1. Terpenoids*

Terpenoids are classified according to the number of their isoprene unit as hemi-, mono-, di-, tri-, tetra-, and polyterpenes [58]. Various studies reported that the anticancer activity of terpenoids is due to the inhibition of inflammation, cancer cell proliferation, angiogenesis and metastasis, and induction of programmed cell death [59]. Triterpenoids are one important class of terpenoids, which contain isopentenyl pyrophosphate oligomers [60]. They are biosynthesized by plants through cyclization of 30-carbon intermediate squalene and include various structural subclasses [61]. Several triterpenoids have been shown to have anticancer activity.

Among the different types of triterpenoids, pentacyclic triterpnoids display the most potent anti-inflammatory and anticancer activity [62]. Addo et al. [63] reported the isolation of two new nagilactones along with seven known from the root of *Podocarpus falcatus* (Thunb.) collected from Berga forest, Addis Alem, central Ethiopia. *P. falcatus*is traditionally used to treat jaundice, gastritis, and amoeba [6]. Among the isolated compounds 16-hydroxynagilactone F (**1**), 2β,16-dihydroxynagilactone F (**2**), 7β-hydroxymacrophyllic acid, nagilactone D (**3**), 15-hydroxynagilactone (**4**), and nagilactone I (**5**) (Figure 2) showed potent antiproliferative activity against HT-29 cell line (IC50 < 10 μM) (Table 2). *Premna schimperi*, another traditionally used Ethiopian plant, also showed cytotoxic activity against L929, RAW264.7, and SK.N.SH with IC50 values of 11 ± 2.3, 10 ± 2.3, and 1.5 ± 0.3 μg/mL, respectively [57]. The methanolic extract of another commonly used Ethiopian plant, *Croton macrostachyus*, was also shown to possess cytotoxic activity against HTC116 cell line [64]. A diterpenoid compound methyl 2-(furan-3-yl)-6α,10β-dimethy-l4-oxo-2,4,4α,5,6,6α,10α,10β-octahydro-1*H*-benzo[f]isochromene-7 carboxylate) (**6**), demonstrated a moderate cytotoxic activity (IC50 = 50 μg/mL). The compound was shown to trigger caspase mediated apoptotic cell death. 3β-Hydroxylup-20(29)-ene-27,28-dioic acid dimethyl ester (**7**), isolated from root of *Plumbago zeylanica* collected from India, also exhibited anti-proliferative and anti-migration activity against triple-negative breast cancer cell lines at IC50 value of 5 μg/mL [65].

Several terpenoids have been isolated from Ethiopian plants that have claims of having anticancer activity, although these plants may have been collected from other sources. For example, sonhafouonic acid (**8**) from *Zehneria scabra*, collected from Cameroon, demonstrated potent cytotoxicity against brine shrimp assay [66], while Lin et al. [67] showed the antiproliferative activity of euphol (**9**), isolated from *Euphorbia tirucalli* from Taiwan against human gastric cancer cells. Euphol selectively promotes apoptosis by mitochondrial-dependent caspase-3 activation and growth arrest through induction of p27kip1 and inhibition of cyclin B1 in human gastric CS12 cancer cells. It also showed a selective and strong cytotoxicity against other groups of human cancer cell lines such as glioblastoma (the most frequent and aggressive type of brain tumor) [67,68]. The molecular mechanism of action of another anticancer triterpenoid, maslinic acid (**10**), isolated from the leaves of *Olea europaea* has been studied, which induced apoptosis in HT29 human colon cancer cells by directly inhibiting the expression of Bcl-2, increasing that of Bax, releasing cytochrome-C from the mitochondria and activating caspase-9 and then caspase-3 [69]. Similarly, the leaf extract of *Ricinus communis* collected from Malta was also reported for its cytotoxicity against several human tumor cells and induction of apoptosis against

human breast tumors, SK-MEL-28. The monoterpenoids 1,8-cineole, camphor and α-pinene, and the sesquiterpenoid β-caryophyllene, isolated from *R. communis*, also showed cytotoxicity against similar cell lines in a dose-dependent manner [70].

**Figure 2.** *Cont.*

Curcusone D (**16**) CH3 OH

**Figure 2.** Structures of anticancer terpenoids reported from plants available in Ethiopia.

*Jatropha curcas* is a medicinal plant traditionally used to treat a variety of ailments in different parts of the world including Ethiopia [71]. Investigation of *J. curcas*, collected from China, resulted in the isolation of twelve phorbol esters (diterpenoids) including jatrophalactone (**11**), curcusecon A–J, 4-epi-curcusecon E, curcusone E, 3-dehydroxy-2-epi-caniojane (**12**), curcusone A (**13**), curcusone B (**14**), curcusone C (**15**), curcusone D (**16**), jatrogrosidone, 2-epi-jatrogrossidone, and 4*E*-jatrogrossidentadion (**17**) [72]. Most of these compounds showed potent cytotoxicity with IC50 values ranging from 0.084 to 20.6 μM against HL-60, SMMC-7721, A-549, MCF-7, SW480, and HEPG2 cell lines [72,73].

The pentacyclic triterpenoid oleanonic acid (**18**), isolated from *Ekebergia capensis* [74], exhibited potent cytotoxic activity against human epithelial type 2 (HEp2) and murine mammary carcinoma (4T1) cell with IC50 values of 1.4 and 13.3 μM, respectively. Another pentacyclic triterpenoid, asiatic acid (**19**), isolated from *Centella asiatica*, also showed 80% growth inhibition of human colorectal (SW480), human stomach (SNU668), and murine colorectal adenocarcinoma (CT26) cell lines with IC50 values of 20 μg/mL [75]. The fresh fruit of *Cucumis prophetarum* from Saudi Arabia yielded a series of cucurbitacin and analogs (cucurbitacin E (**20**), cucurbitacin B (**21**), cucurbitacin D (**22**), cucurbitacin F 25-*O*-acetate, cucurbitacin E glucoside (**23**), dihydrocucurbitacin D, hexanor-cucurbitacin D, and isocucurbitacin D (**24**)), of which compounds **20**–**24** showed cytotoxic activity against MCF-7, MDA MB 231, A2780, A2780 CP, HepG2, and HCT-116 with IC50 values ranging from 1 to 27.3 μM [76].






**Table 2.** *Cont.*

breast cancer, A2780 = Human ovarian carcinoma, A2780 CP =

proliferation by 50%. \* Plant material collected from Ethiopi.

cisplatin-resistant

 ovarian carcinoma, HCT116 = Human colorectal carcinoma. IC50 =

Concentration

 that inhibited cell
