Tabebuia impetiginosa, a plant native to the Amazon rainforest and other parts of Latin America, is traditionally used for treating fever, malaria, bacterial and fungal infections, and skin diseases.
Historically, people have used natural products such as plants, animals, microorganisms, and other biological resources to assuage and cure diseases . Many of the commercial drugs (such as atropine, teniposide, aescin, digoxin, silymarin, and so on) available today were initially developed from plants and other biological resources used in traditional medicines . Therefore, knowledge of the traditional use of natural products plays a large role in drug discovery and development.
Tabebuia impetiginosa (Mart. Ex DC. Mattos) is a plant belonging to the family Bignoniaceae, which is mainly distributed in the Amazon rainforest and other tropical regions of Central and Latin America . It is not only a decorative plant but also has high pharmaceutical value. T. impetiginosa has been used as a traditional medicine to treat various diseases and has antinociceptive, anti-edematogenic, antibiotic, and antidepressant effects . Moreover, the inner bark of this tree can be made into poultice or concentrated tea to treat various skin inflammatory diseases . Several categories of compounds have been isolated and identified from T. impetiginosa, principally quinones, flavonoids, naphthoquinones, and benzoic acids . In recent years, many investigations have demonstrated that extracts or compounds isolated from T. impetiginosa reveal an extensive range of pharmacological activities such as anti-obesity, antifungal, anti-psoriatic, antioxidant, anti-inflammatory, and anti-cancer activities . It is particularly prominent in immunopharmacology. Typically, the mechanism of anti-inflammatory activity of extract from the inner bark of Tabebuia was studied through a molecular biological approach. Nevertheless, the clinical applications of T. impetiginosa have been poorly researched, and there is a void of information on its mechanisms of action.
T. impetiginosa has been used traditionally to treat cancer , obesity , depression , viral, fungal, and bacterial infections , and inflammatory symptoms such as pain , arthritis , colitis , and prostatitis since the Inca civilization. The Callawaya Tribe makes a concentrated tea out of the tree’s inner bark for treating skin inflammatory diseases . Moreover, it can be used as an astringent and diuretic . Caribbean folk healers utilize the bark and leaves of T. impetiginosa to cure toothaches, backaches, and sexually transmitted diseases . Latino and Haitian populations were also reported to use this plant for the treatment of infectious disease . Brazilian people have traditionally used this plant for anti-inflammatory, analgesic, and antiophidic purposes against snake venom . Traditional healers in Brazil prescribed T. impetiginosa for cancer and tumor prevention or treatment; 69.05% for the treatment of tumors and cancer in general and 30.95% for specific tumors or cancers . Such ethnomedicinal uses of T. impetiginosa led us to pay attention to it for a full understanding of its immunopharmacological properties for the future development of an effective drug against ethnopharmacologically targeted diseases with this plant.
Several categories of phytochemicals have been identified in the leaves, bark, and wood of T. impetiginosa. From T. impetiginosa bark, 19 glycosides comprised of four iridoid glycosides, two lignan glycosides, two isocoumarin glycosides, three phenylethanoid glycosides, and eight phenolic glycosides were methanol-extracted . Major constituents of T. impetiginosa are furanonaphthoquinones, naphthoquinones, anthraquinones (e.g., anthraquinone-2-carboxylic acid (Compound 1 in Figure 1)), quinones, benzoic acid, flavonoids, cyclopentene dialdehydes, coumarins, iridoids, and phenolic glycosides . The presence of naphthoquinones attracted scientific attention, with lapachol (2) and β-lapachone (3) especially piquing the interest of professionals in the medical field. Lapachol inhibits proliferation of tumor cells, while β-lapachone exhibits strong toxicity in murine and human cells. Lapachol has been shown to reduce the number of tumors caused by doxorubicin in Drosophila melanogaster heterozygous for the tumor suppressor gene. Lapachol can also decrease the invasion of HeLa cells, which could represent an interesting scaffold for the development of novel antimetastatic compounds .
Figure 1. Chemical structures of Tabebuia impetiginosa-derived components.
Fatty acids, especially oleic acid (4), palmitic acid (5), and linoleic acid (6), are found in the bark of T. impetiginosa. Free sugars also were identified in the bark, with glucose being the most abundant, followed by fructose and sucrose. Organic acids, especially oxalic acid (7), are present, as well as the fat-soluble alcohols α-tocopherol (8) and γ-tocopherol (9). α-Tocopherol can reduce cardiovascular disease risk and neurodegenerative disorders . In addition, T. impetiginosa has some volatile constituents that exhibit antioxidant activity. The major volatile constituents in T. impetiginosa include 4-methoxybenzaldehyde (10), 4-methoxyphenol (11), 5-allyl-1,2,3-trimethoxybenzene (12), 1-methoxy-4-(1E)-1-propenylbenzene (13), and 4-methoxybenzyl alcohol (14) .
Cyclopentene derivatives are secondary metabolites of plants, and this constituent from T. impetiginosa contained six known cyclopentenyl esters (avallaneine A–F (15–20)), two new cyclopentyl esters (avallaneine G (21) and H (22)), and two known cyclopentenyl esters. These cyclopentene derivatives may provide a significant anti-inflammatory effect on the lipopolysaccharide (LPS)-mediated inflammatory response by blocking the production of NO and PGE2; therefore, it is important to determine the molecular mechanism whereby cyclopentenyl esters from T. impetiginosa inhibit inflammatory responses . Moreover, Koyama et al.  isolated two cyclopentene dialdehydes, 2-formyl-5-(4′-methoxybenzoyloxy)-3-methyl-2-cyclopentene-1-acetaldehyde (23) and 2-formyl-5-(3′,4′-dimethoxybenzoyloxy)-3-methyl-2-cyclopentene-1-acetaldehyde (24), that exert anti-inflammatory activity in human leukocytes. Thus, it is necessary to further investigate their activities.
Previous research has indicated various pharmacological effects of T. impetiginosa and its crude extracts and chemical compounds in a series of in vitro and animal models. It exhibits antibacterial, antioxidant, antifungal, antinociceptive, antidiabetic, anti-edema, anti-inflammatory, and anti-cancer activities at different concentrations or doses. The main pharmacological activities of extracts or compounds isolated from T. impetiginosa reported in in vitro and in vivo studies are briefly summarized in Table 1 and described in detail in the following subsections.
Table 1. Immunopharmacological effects of Tabebuia impetiginosa.
T. impetiginosa has been used as a traditional medicine in Central and South America to treat edema, arthritis, diuretic, and infections. Based on its traditional use, in vivo and in vitro experiments examining its pharmacological potential have been conducted. In vivo experiments were conducted using edema, osteoarthritis, animal paw edema, and writhing (and other) models to screen effects of T. impetiginosa. Moreover, there are numerous studies confirming that extracts or compounds isolated from T. impetiginosa have various pharmacological activities such as anti-obesity, antibacterial, antifungal, antiviral, anti-psoriatic, antioxidant, anti-inflammatory, and anti-cancer activities.
Currently, substantial progress has been made in exploration of the phytochemistry and pharmacological activity of T. impetiginosa. Nonetheless, there are still challenges and gaps in published research papers that should be further explored to establish its clinical application value. Firstly, the extracts and compounds isolated from T. impetiginosa possess multiple pharmacological activities, though most functional mechanisms remain unclear and need to be further explored through in vivo and in vitro experiments. Furthermore, most studies on T. impetiginosa are still in the in vitro and in vivo mouse model stages. Toxicological research can be conducted on other animals such as rabbits in the future to evaluate its safety, which will pave the way for further clinical trials. In addition, further comprehensive experiments are needed to enrich the data and discover other pharmacological uses of T. impetiginosa and to find the exact mechanisms by which its extracts bind to target proteins.