Thunbergia is a genus of flowering plant of Acanthaceae family, native to tropical regions of Africa, Madagascar, Australia and South Asia. Thunbergia species include some 100 species of annuals, perennials and shrubs. There are many twinning climbers, as well as some shrubby types in this variable genus. Retizius named the genus Thunbergia in 1780 in honours of Carl Peter Thunberg, a Swedish Physician and Botanist employed in the Dutch East India Company [1,2].

The attractive climbers are a varied group usually with pointed oval to heart-shaped leaves, sometimes lobed or toothed. They occur in many colours, but are most often yellow, orange and purple-blue shades. Many of the Thunbergia species prefer full sun and well-drained soil but can bloom in partial shade also. A sunny location is required as they can receive direct sunlight for six or more hours a day. Regular and deep watering is needed for the plants, allowing the soil to dry slightly between watering (but total dry soil will cause wilting and premature flower drops). Most of the plants of this genus have ornamental value but some of them have medicinal value also.

Class: Equisetopsida

Subclass: Magnoliidae

Order: Lamiales

Family: Acanthaceae

Genus: Thunbergia

Some of the common Thunbergia species are as follows [1,2]:

Thunbergia alata

Thunbergia annua

Tnunbergia battiscombei

Thunbergia coccinea

T. cordata

T. erecta

T. elegans

T. fragrans

T. grandiflora

T. gregorii

T. gibsonii

T. ikbaliana

T. laurifolia

T. lutea

T. mysorensis

T. vogeliana

Species of Thunbergia are different from each other based on their general characteristics such as their geological distribution, height, plant physiology and blooming season mentioned in the tables. Besides these, basic requirement of plant such as soil, water and sun exposure may vary from species to species of Thunbergia plant [2-4].

Thunbergia alata (Table 1 and Figure 1)

Table 1: Thunbergia alata characteristic features.

Figure 1: Thunbergia alata.

Thunbergia coccinea (Table 2 and Figure 2)

Table 2: Thunbergia coccinea characteristic features.

Figure 2: Thunbergia coccinea.

Thunbergia erecta (Table 3 and Figure 3)

Table 3: Thunbergia erecta characteristic features.

Figure 3: Thunbergia erecta.

Thunbergia fragrans (Table 4 and Figure 4)

Table 4: Thunbergia fragrans characteristic features.

Figure 4: Thunbergia fragrans.

Thunbergia grandiflora (Table 5 and Figure 5)

Table 5: Thunbergia grandiflora characteristic features.

Figure 5: Thunbergia grandiflora.

Thunbergia laurifolia (Table 6 and Figure 6)

Table 6: Thunbergia laurifolia characteristic features.

Figure 6: Thunbergia laurifolia.

Antibacterial activity

Jenifer et al. have evaluated the antibacterial activity of Chloroform stem extract of Thunbergia alata against Pseudomonas aeruginosa and significant antibacterial activity of higher concentration of ethanolic leaf extract of Thunbergia alata against Salmonella typhi [20]. Methanolic crude extract of Thunbergia grandiflora leaves against some Gram positive and Gram-negative bacteria showed significant antibacterial activity [21]. Methanolic extract of flower of Thunbergia grandiflora showed antibacterial activity against Escherichia coli, Klebsiella pneumoniae, Staphylococcus aureus, Bacillus cereus, Proteus mirabilis and Streptococcus pyogenes due to the presence of phenols, alkaloids and flavonoids in the flower extract of that plant [22].

Antifungal activity

Chowdhury et al. have evaluated the antifungal activity of methanolic crude extract of Thunbergia grandiflora leaves [21]. It is reported that the decoction of leaves of T. fragrans used in fungal infection [23].

Anthelmintic activity

The anthelmentic activity of methanolic extract of Thunbergia grandiflora on Tubifex tubifex worm due to presence of Proanthocyanidin tannin content was reported [24].

Anti-inflammatory activity

Ethanol and hexane extract of leaves of Thunbergia laurifolia plant showed anti-inflammatory activity against carrageenan-induced paw edema in mice [25]. Rosmarinic acid was found in ethanolic extract of T. laurifolia leaf and was isolated, which showed the anti-inflammatory effect on two mouse models of carrageenan-induced paw edema and cotton pellet-induced granuloma formation [26]. Anti-inflammatory activity of Thunbergia coccinea plant was observed [27].

Antipyretic and analgesic activity

Both analgesic and antipyretic activity of methanolic extract of leaves of Thunbergia coccinea Wall was observed in rats [28].

Antidiabetic potency

Administration of aqueous extract of dried Thunbergia laurifolia leaves in alloxan-induced diabetic rats showed hypoglycemic effect [29].

Antidote and detoxification activity

Leaf extract of Thunbergia laurifolia is reported as an antidote against poisonous insecticide [30]. Tejasen et al. also demonstrated that the cholinergic effect on rats is reduced after administration of aqueous extract of T. laurifolia and mortality rate of rats treated with folidol (an organophosphate insecticide) is decreased due to detoxification property of T. laurifolia [29]. Aqueous leaf extract of T. laurifolia supplied to rats in drinking water showed significant protectivity against Cadium (heavy metal) toxicity from structural damage of kidney and liver tissues of rats [30].

Tetrodotoxin, a potent heat stable neurotoxin, which blocks the Na⁺ channel, and cause paralysis of diaphragm and cause death due to respiratory failure. A significant anti-tetrodotoxin effect of aqueous extract of T. laurifolia was studied in ICR mice [31].

Wutythamawech et al., have reported the use of crude extract of T.laurifolia leaves to treat drug-addicted people [32] and methanolic extract of Thunbergia laurifolia leaf increases the cerebral activity in drug-addicted patients [33].

Pb (lead), is a neurotoxicant, causes cognitive and behavioral defects, and inhibits acetyl-cholinesterase activity of brains. Aqueous leaf extract of T.laurifolia showed protection against Pb induced acetylcholinesterase dysfunction of brain in mice [34].

Antitumor activity

Aqueous extract of Thunbergia laurifolia exhibited inhibitory and antitumor or antiproliferative effects on MCF-7 breast cancer cells of uman using multiple cellular and molecular approaches [35].

Antioxidant activity

Ethanolic extract of Thunbergia laurifolia leaf have antioxidant activity [36]. Microwave dried leaf extract of Thunbergia laurifolia showed more antioxidant activity [37]. Oonsivilai et al. 2008 evaluated the antioxidant activities and total phenolic content of Thunbergia laurifolia extracts using free radical scavenging, ferric reducing antioxidant power assay (FRAP) and the Folin-Ciocalteu methods. Aqueous extract of T. laurifolia leaf exhibited highest antioxidant activity, while ethanol and acetone extracts of T. laurifolia leaves showed moderate antioxidant activity. All of these mentioned extract have high phenolic acids content as caffeic acid, gallic acid, protocatechuic acid and flavonoids as apigenin showing antioxidant activity [38]. Rosmarinic acid isolated from 95% ethanolic extract of dried Thunbergia laurifolia leaves has antioxidant activity [39]. Potential antioxidant activity of methanolic extract of Thunbergia coccinea leaves was observed [40].

Cytotoxicity

The aqueous extract of T. laurifolia leaf showed low cytotoxicity (>200 ug/ml concentration) to L929, BHK (21) C13, Caco-2 and HepG2 cell lines when compared between other extracts such as ethanol and ether and this report suggested the application or use of aqueous extract of T. laurifolia leaf due to its low cytotoxicity to kidney cells and human liver cell lines [38].

Hepatoprotective activity

The hepatoprotective activity of aqueous extract of Thunbergia laurifolia against ethanol induced liver injury in rat hepatocytes was observed by Pramyothin et al. [41].

Antinociceptive activity

Administration of Rosmarinic acid isolated from ethanolic extract of T. laurifolia showed significant antinociceptive activity in the hotplate test and rosmarinic acid at 50 and 100 mg/kg significantly showed its reducing activity on acetic acid induced writhing by 52% (p<0.01) and 85% (p<0.001) respectively and also on formalin-induced pain in the early and late phases. These results indicated both central and peripheral antinociceptive activities of rosmarinic acid [38].

Glucosides

Damtoft et al. have isolated two iridoid glucosides named Thunaloside and alatoside from Thunbergia alata along with the known iridoid glycosides, stilbericoside, 6-epi-stilbericoside and thunbergioside [42]. Ismail et al. have isolated iridoid glucosides such as isounedoside, which contains C-10 as a carboxylic acid group and grandifloric acid, which contains a rare 6,7-epoxide functional group from Thunbergia grandiflora [43]. Kanchanapoom et al. 2002 have isolated two new iridoid glucosides; 8-epi-grandifloric acid and 3’-O-beta-glucopyranosyl-stilbericoside from the methanolic extract of the aerial part of the Thunbergia laurifolia plant along with these known compounds such as benzyl-2’-O-β-glucopyranosyl glucopyranoside, grandifloric acid, E-2-hexenyl-β-glucopyranoside, hexanol-β- glucopyranoside and they have also determined the structure of these compounds [44]. Subramanian et al., have reported to contain luteolin-7-glucoside, malvidin-3, 5-di-glucoside and stilbericoside in flowers of Thunbergia grandiflora [45].

Phenolic compounds and others

Plant phenolic compounds include tannin, phenolic acids, flavonoids, naphthalene and other. Kabir et al. have evaluated the total tannin content in Thunbergia grandiflora, which indicate the presence of Proanthocyanidin, a condensed tannin compound [24]. Purnima et al. and Kanchanapoom et al. have reported the presence of some bioactive phenolic compounds like delpinidine-3,5-di-O-β-glucopyranoside, apigenin and apigenin-7-O-β-D-glucopyranoside and chlorogenic acid, 6-C-glucopyranosyl apigenin and 6,8-di-C-glucopyranosyl apigenin in the flower and leaf extract of Thunbergia laurifolia plants respectively [44,46]. Jansen et al. has reported that phenolic compounds such as caffeoylmalic acid, feruloylmalic acid and p-coumaroylmalic acid have found in the leaves of Thunbergia alata [47]. Oonsivilai et al. have extracted other phenolic acids such as gallic acid, caffeic acid and protocatechuic acid from the leaf extract of Thunbergia laurifolia plants [48]. Chowdhury et al. have purified flavonoid named 5- hydroxy-4’, 6,7-trimethoxy flavone, C₁₈H₁₆O₆ [21]. Subramanian et al. have reported to contain flavonoids such as apigenin-7-glucronide, luteolin in flowers of Thunbergia grandiflora [45]. Hung et al. have purified and identified eight compounds. The identified compounds are 8-Hydroxy-8-methyl- 9-methene-cyclopentane- [7,11]-1, 4, 6-trihydronaphthalene-12-one, named as Thunbergia A (1); 3,4-dihydro-4, 5, 8-trihydroxy-2-(3- methyl-2-butenyl) –naphtha [2,3-b] oxiren-1 (2H)-one (2); 8-(betaglucopyranosyloxy) -3, 4-dihydro-2-(3-methyl-2-butenyl) naphtha [2,3-b] oxiren-1 (2H)-one (3); galangin (4); quercetin (5); luteolin (6); 5, 6, 3’, 4’-tetrahydroxy-3, 7-dimethoxy- flavone (7) and upeol (8) [49]. Rosmarinic acid is present in the ethanolic extract of Thunbergia laurifolia [39].

Purification, identification and characterization of unexplored bioactive compounds showing pharmacological activities of these species will be the area of future work. Production of bioactive compounds from these plant species can be increased since they possess pharmacological activities like antibacterial, antifungal, antidiabetic, antipyretic, anti inflammatory, anthelmentic, antioxidant, antinociceptive, anti-drug, antidote, antimutagenic, detoxifying, cytotoxic and hepatoprotective which may be advantageous for pharmaceutical industry to explore effective drugs from different species of the Thunbergia plant.

This review article reports about the medicinal importance of the most of the species of Thunbergia. According to this review article, Thunbergia, having the medicinal properties such as antibacterial, antifungal, antidiabetic, antipyretic, anti-inflammatory, anthelmentic, antioxidant, antinociceptive, anti-drug, antidote, antimutagenic, detoxifying, cytotoxic and hepatoprotective activities, can be considered as potential alternative herbal medicine to treat different types of diseases.

I want to acknowledge Dr. Indrani Chandra and Dr. Sabyasachi Chatterjee of the Department of Biotechnology in the University of Burdwan for giving me the opportunity to make this review on Thunbergia species under their guidance. I am very grateful to them for supporting me. I would like to thank my family for their full support.