Plants of Medicinal/Toxicological Interest

A summary of the plants being reviewed, their vernacular names, family, traditional uses, and plant parts used can be found in Table S1, and identified compounds, type of bioassay, and main cytotoxic results can be found in Table S2.

Ammocharis coranica Botany and Traditional Use

The species is widespread throughout southern Africa (Fig. 1b), including in the sandy and sun-drenched area known as the Kalahari. It grows from a drought-resistant deciduous bulb spherical in shape, covered in thinly layered tunics, reaching up to 20 cm in diameter. Broad green leaves about 45 cm long with finely toothed margins lay flat on the surface of the soil. After heavy summer rains, beautiful pink trumpet-shaped flowers (Fig. S1a) with an intensely sweet scent can reach heights of 35 cm (Riddles 2017).

The bulb is used by Zulu traditional healers against mental illnesses such as age-related memory loss, dementia, and depression as well as hysteria (Koorbanally et al. 2000). Despite these claims, the active agent has not been identified yet, although it is speculated that crinamine may be the causative agent. The plant is also used as substitute for Boophone disticha (L.f.) Herb., Amaryllidaceae, a well-known hallucinogen and arrow poison (Viladomat et al. 1997). Buphanidrine may also be the causative agent for the central nervous system–related effects as had been described by Koorbanally et al. (2000). Furthermore, traditional healers use cooked scales inserted as enemas to “cleanse blood” and apply them to wounds and boils (Louw et al. 2002).

Chemistry and Bioactivity

Mason et al. (1955) reported the alkaloids lycorine (1), caranine, acetylcaranine, and crinamine, to be present (in this bulbous plant), the best known of which is lycorine, a well-documented compound with multiple bioactivities including anti-inflammatory, acetylcholinesterase inhibition, and mild antibacterial, potent antiviral, and antiparasitic activity (Elisha et al. 2013). However, the compound showed most promise against cancer cells. Roy et al. (2018) reported the compound to be highly specific while being very potent and effective at low concentrations though in some cases being less toxic than first-line chemotherapeutic drugs. A total of 26 cancer cell lines have been treated with this compound, yielding very promising results with an IC50 value of 0.7 µM (0.21 µg/ml) against human multiple myeloma RPMI-8226. Lycorine was also the most prevalent alkaloid in A. coranica with a 0.101% yield based on fresh bulb weight (Wildman 1960).

When the bulbs were extracted using an alkaloid acid–base extraction technique the final chloroform fraction contained a number of isoquinoline alkaloids, namely, 1-O-acetyllycorine, hippadine, 6-alpha-hydroxypowelline, hamayne, and 1-O-acetyl-9-O-demethylpuviine as well as cycloartane triterpenoids, 24-methylenecycloartan-3 beta-ol, cycloeucalenol, cycloeucalenone, and 24-methylenepollinastanone (Koorbanally et al. 2000). Furthermore, the new alkaloids golceptine, 6a-hydroxybuphanidrine, and charisine were reported from a new location (Raghoo et al. 2021). Consequently, A. coranica is alluring for prospective anticancer lead compounds, NCEs, and maybe even market-available drugs, with lycorine alone warranting further investigation into this plant. This species may yet harbor many other bio-active compounds.

Artemisia afra Botany and Traditional Use

With its silver-grey foliage, this perennial shrub is as popular in South African herb gardens as it is in the research community. The thick woody stems carry bushy, untidy clumps of almost fern-like leaves and can grow up to 2 m. This species is found across southern Africa (Fig. 1c) and grows in sand, clay, or loam soils (Van Wyk et al. 1997). The tips of branches produce creamy yellow flowers modest in size in late summer and autumn (Van der Walt 2004).

Watt and Breyer-Brandwijk (1962) reported multiple uses in traditional African medicine but steaming the plant for airway diseases seemed to be the most prevalent. This could be due to volatile compounds as listed by Liu et al. (2009). It is also used by Zulu and Xhosa traditional healers as a remedy for influenza, colds, and coughs. Sesotho and Meru traditional healers use it to treat intestinal worms and reproductive problems (Hutchings 1989; Moteetee and Van Wyk 2011; Mwangi et al. 2017; Moteetee et al. 2019). Furthermore, the dried plant material is often mixed with other medicinal plants to be used by Bapedi traditional healers for the treatment of tuberculosis and opportunistic infections (Semenya and Mayori 2019). Reinforcing the antibacterial effects of this plant, a concoction is used against acne and boils (Mabona and Van Vuuren 2013). It is further said that the leaves are used in an infusion to treat child diseases like measles (Ndhlovu et al. 2021). Finally, prostrate problems were added to the list of treated diseases (Kose et al. 2015).

Chemistry and Bioactivity

The phytochemistry of A. afra was comprehensively reviewed by Liu et al. (2009), Du Toit and van der Kooy (2019), and Shinyuy et al. (2023). Most cytotoxicity studies conducted on A. afra remain to be on extract/fraction level and only isoalantolactone (2) was tested against cancer cell lines.

Fouche et al. (2008) reported moderate activity of CH2Cl2:MeOH extracts of the leaves against renal (TK10), breast (MCF7), and melanoma (UACC62) cancer cells with total growth inhibition (TGI) at 26.62, 15.00, and 9.73 µg/ml, respectively, compared to the positive control, etoposide with a TGI at 27, > 100, and 26.2 µg/ml, respectively. Moreover, the extract displayed good activity against non-small cell lung carcinoma (NCI-H522), melanoma (SK-MEL-5), and colon (HT29) cancer cell lines, with TGI at 13.49, 13.49, and 14.13 µg/ml, respectively.

Ethanol extracts were found to induce apoptosis of promonocytic leukemia (U937) and cervical (HeLa) cancer cells with IC50 values of 18.21 and 31.88 µg/ml, respectively (Spies et al. 2012). This apoptosis was observed after a 24-h treatment with plant extract as well as the positive control melphalan (IC50 value not reported), further highlighting the possible anticancer characteristics of the plant.

In addition, isoalantolactone was tested against cervical HeLa cancer cells delivering a low IC50 value of 8.15 μM (1.89 μg/ml) (Venables et al. 2015). Van Loggenberg et al. (2022) tested infusions prepared from A. afra against several lung cancer cell lines and reported IC50 values as low as 6 µg/ml, with selectivity index values as high as 10 when tested in Vero non-cancerous cell lines. Similarly, Vogel et al. (2023) screened A. afra extracts against various breast cancer cell lines and reported IC50 values as low as 4.5 µg/ml. Much more information is needed about this interesting plant, especially following such promise early on in anticancer studies (Taleghani et al. 2020).

Dipcadi glaucum Botany and Traditional Use

This hardy annual herb has mostly eluded the interest of researchers. Distributed across Angola, Namibia, Botswana, Zambia, Zimbabwe, and South Africa (Fig. 1d), the species grows from a bulb of about 4 cm in diameter and can reach a height of 120 cm (Vahrmeijer 1981). The blue-green leaves, covered in a waxy powder, are strap-shaped and can grow to be 45 cm long with a width of 4 cm (Fig. S1c). The flowers are green with long pedicels spiralled on the peduncle.

The plant’s influence on wildlife and animals in agriculture, however, is so significant, it deserves some attention, exerting its toxic effects on the central nervous system and digestive system of livestock. The plant appears to be highly toxic as its vernacular name in Afrikaans is known as malkopui directly translating to “mad-head-onion.” Other studies support this but admits the toxin and by extent its mode of action is unknown. Cardiac-glycosides, usually responsible for these effects, are reportedly not present in this species (Botha and Penrith 2008). In this review, closely related species will be covered as very little research has been conducted on D. glaucum with ScienceDirect listing only seven scientific papers in total. A decoction prepared from D. polyphyllum Bak. and D. umbonatum Bak. is used as a gonorrhea treatment by the Sotho people while the ashed plant is rubbed between the fingers to improve accuracy during hunting and fighting. It is also used as a treatment of pimples. Dipcadi viride Moench. is also used as a vegetable by the Sotho people.

Chemistry and Bioactivity

The chemistry of D. glaucum is poorly investigated. It is only reported that it contains no significant levels of alkaloids or glycosides (Al-Najjar 2020). Until recently, the cytotoxicity was investigated neither in vitro nor in vivo. Notably, given the impact of this plant species’ toxicity on ruminant livestock, which is well known, it is surprising that so little research has been conducted to date. The leaves of Dipcadi serotina (L.) Medicus contain quercetin and kaempferol while those of Dipcadi viride (L.) Moench contain cardiac-glycosides (Williams et al. 1988). Marzouk et al. (2019) identified 22 compounds for the first time in Dipcadi erythraeum Webb & Berthel including cardiac-glycosyl flavonoids and derivates of phenolic acid, adding to the list of seven flavonoids previously isolated by El-Shabrawy et al. (2016). Another species, D. krishnadevarayae, produces high levels of saponins and tannins, possibly explaining its mild antihelminthic activity (Jyothi et al. 2018).

Antineoplastic activity has been reported from a methanol extract of D. erythraeum showing mild activity against MCF7 and colon HCT116 cell lines with cell viabilities of 43.6% and 48.4% when treated at a concentration of 100 µg/ml, but no activity against lung A549 or liver HepG2 cell lines (Marzouk et al. 2019). It highlights that scientific information about D. glaucum’s general bioactivity, and more specifically its cytotoxicity, is scarce to non-existent.

Elephantorrhiza elephantina Botany and Traditional Use

This perennial sub-shrub has been described as an “underground tree.” Aerial stems up to 90 cm tall are unbranched and unarmed harboring dull green, bipinnate compound leaves (Fig. S1d). The dark reddish-brown bark, trunk, and roots grow underground in sandy soil, with the hot, dry Kalahari being the perfect habitat. In fact, this plant is extensively distributed all over southern Africa (Fig. 1e). Yellow to white flowers can be seen between September and November regardless of rainfall and are arranged in axillary, solitary, or clustered racemes (Grobler 2010).

Being a plant of great ethnomedicinal importance, Kose et al. (2015) reported instances of over-harvesting. The roots and rhizomes are used traditionally to treat acne and other skin ailments, even to lighten skin and in the treatment of diarrhea and dysentery (Hutchings 1989; Mabona et al. 2013; Mhlongo and Van Wyk 2019). Along with the application against a myriad of diseases, it is also used in the treatment of breast cancer, tuberculosis and pneumonia, piles, infertility, and even syphilis and herpes (Kose et al. 2015; Maroyi 2017). It should further be noted that the leaves proved to have anti-diabetic and anti-inflammatory activity while remaining non-cytotoxic (Olaokun et al. 2020).

Chemistry and Cytotoxicity

Using gas chromatography-mass spectrometry (GC–MS) with petroleum ether as an extraction solvent, pregnenolone, α-sitosterol, lupeol, and cycloeucalenol acetate were dereplicated and with methanol as extraction solvent, 2-methylpentanoic acid, 4-oxopentanoic acid, α-methyl-1H-imidazole-4-ethanamine, benzothiazole, and but-3-yn-1-yl heptadecylester carbonic acid, where identified (Asong et al. 2019). A review paper further documented the chemical constituents which included anthraquinones, fatty acids, esters, flavonoids, glycosides, phenolic compounds, phytosterols, sugars, saponins, and triterpenoids (Maroyi 2017). Diosgenin and oleanolic acid were isolated from the plant as recently as 2014 by Mpofu et al. (2014), with the compound (−)-epicatechin giving interesting synergistic activity in combination with palmitic acid. The leaves contain alkaloids, triterpenes, phytosterols, and saponins, while for the roots, terpenoids, phlabotannins, saponins, and alkaloids could not be identified (Kudumela and Masoko 2018; Olaokun et al. 2020).

Leaves are used traditionally in the treatment of breast cancer (Raimi et al. 2020). A cytotoxicity study using African green monkey (Vero) cells testing acetone extracts of the entire plant showed a LC50 of 416.4 µg/ml, which falls far outside the cutoff point of LC50 < 200.0 μg/ml suggested by the NCI for plant extracts to be considered cytotoxic (Kudumela et al. 2018). A follow-up study was conducted the following year, using the same Vero cell line, and it was found that methanol extracts of the rhizomes had an LC50 value of 9.4 µg/ml which falls well within the range of the NCI (Asong et al. 2019). By means of the brine shrimp assay, methanolic extracts of the plant showed great cytotoxic activity with an LC50 of 1.8 µg/ml, whereas an aqueous extract exhibited only mild toxicity (Mpofu et al. 2014).

Cell line assays were carried out with the acetone, ethanol, cold water, and hot water extract of the leaves, against H4IIE hepatoma, a tumor cell line as well as C2C12 myocytes and myotubules, stages of muscle cells, delivering IC50’s between 697 and > 1000 µg/ml, between 87 and 256 µg/ml, and > 100 µg/ml, respectively, compared to the positive control doxorubicine with 15.53, 2.75, and 176 µg/ml, respectively. In summary, while extracts were not active against hepatoma cells, the highest toxicity recorded for the cold water extract against myocytes was an IC50 of 87 µg/ml (Olaokun et al. 2020).

Geigeria ornativa Botany and Traditional Uses

Common in southern Africa (Fig. 1f), this perennial herb grows low and keeps close to the ground (Vahrmeijer 1981). The stem is scarce and usually branched, angular, flattened, and ribbed as it ascends. The leaves are shallowly toothed and located along the stem while forming a basal rosette, protruding up to 10 cm. Flowers are bright yellow and usually located where stems branch (Fig. S1b). No published literature is available regarding the traditional uses of G. ornativa. According to Watt and Breyer-Brandwijk (1962), the Sotho people used G. africana Gr. as an antiparasitic agent while a decoction of G. aspera Harv. is used for a remedy of giddiness.

Chemistry and Cytotoxicity

Ingestion of the plant result in vermeersiekte (vomiting disease) and is marked by hepatotoxicity and photosensitivity (Mbaveng et al. 2014). It is believed that the toxin is furanosesquiterpenoids targeting striated muscle, but these compounds have not yet been identified in this species (Botha and Penrith 2008). The plant has been neglected in the research world, but other species from the genus Greigeria have enjoyed some attention. A good example is Greigeria burkei (Benth.) S.A.O’Donnell & G.P.Lewis delivering the sesquiterpine lactones geigerin, vermeeric acid, and vermeerin. Biological effects are reported against emesis, respiratory suppression, and bacteria (Zdero and Bohlmann 1989; Bohlmann et al. 1982; Coleman et al. 1984; Awouafack et al. 2013; Ndhlala et al. 2013). Geigerin showed a mild apoptotic affect against murine myoblast cells (C2C12) with cell viability of 31.2% after 72 h at 5314 mM/1321.6 μg/ml (Botha et al. 2017).

Prosopis juliflora Botany and Traditional Use

This drought-resistant deciduous shrub is an invasive, but naturalised, species in southern Africa, flourishing in semi-arid and arid tracts of tropical and sub-tropical areas, including the Kalahari region of South Africa (Fig. 1g) (Sawal et al. 2004; Heshmati et al. 2019). Reaching a height of up to 15 m, its drooping branches carry feathery foliage on straight, paired spined twigs (Fig. S1f). This plant has enjoyed much attention from medicinal researchers until the 1980s, followed by a slight hiatus, but is gaining momentum once again (Ukande et al. 2019). Popular among Indian Ayurvedic traditional healers, it is known for its use in inflammatory illnesses such as rheumatism and as a remedy for scorpion stings and snake bites (Ahmad et al. 1989).

Chemistry and Cytotoxicity

Two piperadine alkaloids were first isolated from the leaves in 1989, juliprosinene and juliflorinine, adding to the already known juliflorine, juliflorocine, julifloridine, and juliprosipine (Ahmad et al. 1989). Dhivya et al. (2018) reported six new compounds and several known compounds from the hexane extracts of the seed pods, namely N-hexadecanoic acid, 9,12-octadecadionoic acid methyl ester, 9,12-octadecadienoic acid methyl ester, 12-tridecynoic acid methyl ester, 9-octadecyne, and squalene. Other compounds identified include prosoflorine, juliprosine, (−)-mesquitol, 7,3,4-trihydroxy-3-methoxyflavone, catechin, dehydroabietic acid, patulitrin, schaftoside, 24-methylencycloartan-3-one, zerumbone, N-β-chloropropionyltryptamine, cassine, prosophyline, tryptamine, β-phenylthylamine, indolizidine, myo-inositol-4C-methyl, and linoleic acid (Ukande et al. 2019).

Malik et al. (2018) reported 1-methoxy-2-propyl acetate, fluoro-ethyne, cyclobutanol, 1-methyldecylamine, 3-hydroxybutanal, cyclopropaneoctanoic acid, methyl 5-(2-undecylcyclopropyl) pentanoate, ergosterol acetate, maymyrsine, cycloartenol, carpesterol benzoate, and 9-(2-oxiranyl)-1-nonanol. The authors summarized compounds with “confirmed” antineoplastic effects as pentanal, butyramide, N-hexadecanoic acid, and hydroxyurea. The authors did not test these compounds for bioactivity but instead referred to a terpenoid database that could not be found.

Patulitrin, a flavonoid, was isolated from the fruit (Wassel et al. 1972), whereas alkaloid extracts from the leaves exhibited IC50 values of 90.5, 42.5, and 20 μg/ml after 24, 48, and 72 h, respectively, against the human lymphoblast MOLT-4 cell line, while an IC50 could not be determined against human normal mitogen stimulated T-lymphocytes, due to very low growth inhibition, indicating high selectivity (Sathiya and Muthuchelian