Anti-infectious activity in plants of the genus Tabebuia

Infectious diseases are a worldwide public health problem. There is growing research in the field of new plant-based drugs for treating such diseases. Our objective was to perform a systematic literature review to evaluate the anti-infectious activity (antibacterial, antifungal, antiviral and antiparasitic) attributed to plants of the Tabebuia (Bignoniaceae) genus. We conducted a search for the period of 2000-2013 in ScienceDirect, Scopus, PubMed, Embase, Napralert and SciELO databases using the following MeSH terms: Tabebuia , biological activity, bioactive compounds, chemical compounds, diseases, traditional medicine, tropical infections, infections and treatment. We found ethnobotanical and experimental ( in vitro ) evidence supporting the use of Tabebuia species for treating infectious diseases. In addition, the compounds responsible for their antimicrobial activity have been isolated, and their structures have been elucidated, emphasizing among them naphthoquinones such as lapachol. Natural products isolated from Tabebuia plants may be an alternative for developing new anti-infectious agents.


Introduction
Plants produce large amounts of compounds known as phytochemicals, and each plant synthesizes a vast variety of these phytochemicals. Phytochemicals not only maintain the plant's physiological activities, but they also protect it against foreign agents such as bacteria, fungi, insects and animals that feed on them (Dixon 2001, Schultz 2002. Since ancient times, phytochemicals have been used as treatments to cure various diseases. Presently, multiple pharmaceutical agents contain natural compounds, including drugs that contain variations of these natural molecules (Kinghorn 2001). Many plants have been used as Anti-infectious activity in plants of the genus Tabebuia mini-review the base or as precursors for developing several synthetic or semi-synthetic drugs (Wessjohann 2000, Newman et al. 2003. In fact, studies by Newman clearly demonstrate that 61% of all new small-molecule drugs introduced during 1981-2002 have been produced using natural products (7% natural products, 27% natural-product derivatives, 5% synthetic derivatives from natural products and 23% synthetic compounds designed from a natural product). Natural products are an innovative source of therapeutic agents for treating infectious diseases, and other ailments (Altmann 2001).
The decreased susceptibility of infectious agents to antimicrobials has warranted the need to increase the therapeutic arsenal of anti-infectious agents, emphasizing antibacterial, antiparasitic and antifungal agents (Gould 2008, Pitman et al. 2011, Wise 2011. The scientific community and pharmaceutical companies have given medicinal plants special attention in the last years because of their promising potential to be used to develop innovative anti-infectious agents of natural origin (Osbourn 1996, Tagboto & Townson 2001, Ginsburg & Deharo 2011. The Tabebuia genus includes approximately 100 species and is the largest genus in the Bignoniaceae family. This plant family is distributed from the southwestern United States to the northern regions of Argentina and Chile (Dvorkin-Camiel & Whelan 2008), where almost one-half of its genus and species are located (Olmstead et al. 2009).
Species of the Tabebuia genus have been used empirically as anti-inflammatory, anti-cancer and anti-microbial agents in rural areas of Colombia, Bolivia, Brazil and other Latin-American countries (Bueno et al. 2001, Agra et al. 2007, Negrelle & Fornazzari 2007, Gomez-Estrada et al. 2011, Hajdu & Hohmann 2012; the Tabebuia genus is commonly recognized as a therapeutic alternative by rural or remote populations. The results of ethnobotanical and ethnopharmacological studies indicating the potential use of these plants to treat a large variety of diseases has encouraged the search of new phytotherapeutic drugs using plant biodiversity . The extensive use of Tabebuia species for the treatment of infectious diseases in traditional medicine in Latin America, and the lack of reviews in the field motivated this systematic review of literature summarizing the relevant information published in the last 13 years. To our knowledge, this is the first literature review concerning the anti-infectious activities of plants of the genus Tabebuia. Several Tabebuia species have been used in traditional medicine to treat infectious diseases; Lapachol, for example, (which was first isolated from Tabebuia avellanedae) has antibacterial, antiviral, antiparasitic and antifungal activities (like other napthoquinones). However, it is necessary to clearly identify compounds other than lapachol that have anti-infectious activity and the mechanisms of action responsible for these activities.

Materials and Methods
We performed a systematic review in the databases of Index Medicus/MEDLINE (www.pubmed. com), Scopus (www.scopus.com), ScienceDirect (www.sciencedirect.com), Embase (www.embase. com), Napralert (www.napralert.com) and SciELO (www.scielo.org) using the following combination of MeSH (Medical Subject Headings) terms: "Tabebuia" OR "biological activity" OR "bioactive compounds" OR "chemical compounds" OR "diseases" OR "traditional medicine" OR "tropical infections" OR "infections" AND "treatment". To uncover the prominence of Tabebuia plants in the development of new anti-infectious agents, we included all articles published in English, Spanish or Portuguese during the years 2000-2013.
Completed the search, we identified 264 articles and selected 36 based on their titles, abstracts and review of the full text. The selected articles were grouped according to the associated biological (i.e., antibacterial, antifungal, antiviral and antiparasitic) activity; we also considered all complementary general references related to the review. Because the objective of this review was to show a general context of the ethnopharmacology of the genus Tabebuia and the compounds related with the antiinfectious activity, we excluded investigations related to phytochemical, chemical and genetic studies.

Antibacterial activity:
The most extensive studies conducted have been on the antibacterial activity of extracts obtained from Tabebuia plants; this includes ethnobotanical and ethnopharmacological studies (Negrelle & Fornazzari 2007, Gomez-Estrada et al. 2011). The ethyl acetate extract obtained from the inner bark of Tabebuia ochracea and Tabebuia rosea inhibits Staphylococcus aureus growth at concentrations ranging between 1.25 and 10 mg/well. This characteristic may be attributed to the presence of quinone-type compounds that have displayed activity against Staphylococcus aureus strains (Riffel et al. 2002). However, no inhibitory activity was found against Escherichia coli and Pseudomonas aeruginosa by Tabebuia ochracea and Tabebuia rosea in the ethyl acetate extract (Franco Ospina et al. 2013).
The ethanol extracted from the leaf of Tabebuia rosea has also been evaluated; at concentrations between 50 and 300 mg/mL (50,000 and 30,0000 mg/L) it can inhibit the growth of Klebsiella pneumoniae (Sathiya & Muthuchelian 2008). This antibacterial activity can be associated with the presence of various active principles or phytoconstituents such as phenolic compounds, quinoids and flavonoids present in the leaf ethanol extract of Tabebuia rosea (Joselin et al. 2013).
The leaf methanolic extract from Tabebuia chrysantha inhibited the growth of Staphylococcus aureus at a concentration of 125 mg/mL (125,000 mg/L). This extract was more active than the chloroform and ether extracts used at the same concentration (Pérez et al. 2007). These results suggest that the polar compounds found in the methanolic extract could be responsible for this activity; however, the antibacterial activity found was small. In addition, the authors found that none of the extracts obtained from Tabebuia chrysantha inhibited the growth of Escherichia coli, Proteus vulgaris and Pseudomona aeruginosa. In a more recent study, the antibacterial activity of 21 herbal extracts and four essential oils against Helicobacter pylori and Campylobacter jejuni was evaluated. Tabebuia impetiginosa hydro-alcoholic extract inhibited 36% of Helicobacter pylori growth but had no effect on Campylobacter jejuni (Cwikla et al. 2010), or on the adhesion of Campylobacter jejuni to human colon epithelial cells, demonstrated by IC 50 values less than 3 mg/mL (3,000 mg/L) (Bensch et al. 2011). The effect of Tabebuia impetiginosa inner-bark compounds on a group of human intestinal bacteria was also evaluated; anthraquinone-2-carboxylic acid efficiently inhibited Clostridium paraputrificum growth at a dose of 1 µg/disc, while lapachol only moderately inhibited bacterial growth. Both compounds slightly inhibit the growth of Clostridium perfringes and Escherichia coli at a dose of 100 µg/disc (Park et al. 2005). These results suggest that Tabebuia impetiginosa compounds may be valuable in the development of new effective agents against the growth of damaging intestinal bacteria such as Clostridium spp.
For the period evaluated in this review, we found only one report related to the antimycobacterial activity of a medicinal drink prepared with the hydro-alcoholic extract from the bark of Tabebuia avellanedae, which inhibited the growth of Mycobacterium tuberculosis H37Rv (Oliveira et al. 2007).
Natural products have also been used to inhibit the development of periodontal disease, as an adjunct to conventional oral-hygiene methods. For this reason, the antibacterial activity of a hydroalcoholic extract formulation consisting of various plant species, including Tabebuia impetiginosa, was tested; this formulation inhibits the growth of Staphylococcus aureus and Bacillus subtilis, with a MIC of 312.5 µL/mL (Cordeiro et al. 2006).

Antifungal activity: Powerful antifungal
drugs currently exist; however, the development of fungal resistance to these drugs, drug toxicity and limitations in the bioavailability have created a need for the development of new drugs to control evolving fungal infections. Natural products might be of great utility in this field.
Fourteen plants commonly used in Paraguayan traditional medicine, including Tabebuia avellanedae, have been evaluated; dichloromethane extract from the bark of Tabebuia avellanedae was found to exerts a broad antifungal action particularly against Aspergillus fumigatus, Candida albicans, Cryptococcus neoformans, Microsporum gypseum, Penicillium purpurogenum, Saccharomyces cerevisiae and Trichophyton mentagrophytes. These aqueous and methanolic extracts displayed great inhibitory activity against Cryptococcus neoformans, Microsporum gypseum, Penicillium purpurogenum and Trichophyton mentagrophytes (Portillo et al. 2001).
A study of 11 plant species traditionally used in the Cerrado region of Brazil revealed that only four extracts, including the ethanol extract from Tabebuia caraiba, inhibited the growth of Candida albicans at a dose of 20 mg/mL (20,000 mg/L). In a research center in Campinas, Brazil, six plants were selected for evaluation of their potential activity against ten Candida species. The results revealed that the methanol extract from Tabebuia avellanedae exerts an inhibitory activity against Candida albicans, Candida dubliniensis, Candida parapsilosis, Candida tropicalis, Candida guilliermondii, Candida utilis, Candida krusei, Candida lusitaniae, Candida glabrata and Candida rugosa, with MIC values in the range of 0.06 to 0.0001 mg/mL (60 to 0.1 mg/L). The dichloromethane extract has inhibitory activity only against Candida krusei, with an MIC value of 0.06 mg/mL (60 mg/L) (Hofling et al. 2010).

Antiviral activity:
We found only two studies evaluating the antiviral activity of extracts from Bignoniaceae species despite the urgency to develop new antiviral agents to combat the resistance and toxicity issues encountered when using existing antiviral drugs.
The results revealed no activity of Tabebuia aurea stem ethanol extracts and Tabebuia cassinoides stem and leaf ethanol extracts against encephalomyocarditis virus (EMCV), human herpes virus 1 (HHV-1) and vaccinia virus (Western Reserve strain, [VACV-WR]). This lack of activity was most likely caused by the high cytotoxicity of naphthoquinones present in the extracts (Brandão et al. 2010a). A second study evaluated the antiviral activity of Tabebuia impetiginosa, Tabebuia serratifolia and Tabebuia stenocalyx stem ethanol extracts and Tabebuia stenocaly leaf ethanol extract against EMCV, HHV-1 and VACV-WR viruses.
The results demonstrated that only the Tabebuia impetiginosa extract exerted activity against HHV-1, with a one-half maximal effective concentration (EC 50 ) of 166.6 µg/mL (Brandão et al. 2010b).
Antiparasitic activity: Despite the high prevalence of parasitic diseases in tropical and subtropical regions, their pharmacological treatment is difficult, especially the cases of neglected tropical diseases (NTDs) such as Leishmaniasis, Chagas disease and Malaria. The ineffectiveness of pharmacological treatments in the chronic phase of the majority of these pathologies, the drug's toxic effects, the presence of secondary effects and the parasitic resistance to these agents have stimulated interest in identifying new antiparasitic agents.
Lapachol, although structurally related to Atovaquone, exhibits low activity against Plasmodium berghei in mice and against Plasmodium falciparum in vitro. This fact has led to the synthesis of phenazines from lapachol, β-lapachone and their derivatives, thereby allowing the synthesis of compounds, such as 3-sulfonic acid-beta-lapachone-derived phenazine. The latter compound achieves 98% inhibition of the Plasmodium berghei-induced parasitemia in mice (De Andrade-Neto et al. 2004). Tabebuia billbergii inner bark and wood extracts have been categorized based on their bioactivity; this has allowed the isolation and identification of several compounds. Naphthofurandiones, for example, display significant antimalarial activity in vitro against Plasmodium berghei (Gómez-Estrada et al. 2012); in particular, 2-acetyl-naphtho-[2,3b]-furan-4,9dione (10) (Figure 3) modifying the naphthoquinones, especially their imidazole ring, could produce compounds with high trypanocidal activity.
Hexane, chloroform and hydro-alcohol extracts obtained from the bark of Tabebuia serratifolia, a plant used in traditional Peruvian medicine for treating cutaneous leishmaniasis, were recently tested for their trypanocidal and anti-leishmanial activity (Gonzalez-Coloma et al. 2012).

Conclusion
The resistance of several microorganisms to antimicrobial treatment has increased in the last years; natural products may potentially be a source to produce new drugs to treat infectious diseases. Various Tabebuia species have been used in traditional medicine to treat bacterial, fungal, viral and parasitic diseases, and other ailments. Tabebuia avellanedae, also known as Tabebuia impetiginosa, is the species that has been most extensively studied. In these studies, various compounds exhibiting biological activities have been isolated, naphthoquinones such as lapachol and β-lapachone being the most significant.
Compounds isolated from different Tabebuia species have been characterized and utilized to synthesize derivatives. These derivatives can potentially be used as a source for new antimicrobial agents; however, it is essential we evaluate their mechanisms of action. Because the experimental methods employed in the evaluated literature differ significantly, it is impossible to conduct a side-by-side comparison of the results reported for each group of microorganisms. By understanding the traditional use of these plants, and conducting phytochemical research and investigations of their biological activities (in vivo and in vitro), we will attain the knowledge necessary for future discovery of new compounds for the control of infectious diseases.