Background: Due to the high prevalence of viral infections having no specific treatment and the constant appearance of resistant viral strains, the development of novel antiviral agents is essential. The aim of this study was to evaluate the antiviral activity against bovine viral diarrhea virus, herpes simplex virus type 1 (HSV-1), poliovirus type 2 (PV-2) and vesicular stomatitis virus of organic (OE) and aqueous extracts (AE) from: Baccharis gaudichaudiana, B. spicata, Bidens subalternans, Pluchea sagittalis, Tagetes minuta and Tessaria absinthioides. A characterization of the antiviral activity of B. gaudichaudiana OE and AE and the bioassay-guided fractionation of the former and isolation of one active compound is also reported. Methods: The antiviral activity of the OE and AE of the selected plants was evaluated by reduction of the viral cytopathic effect. Active extracts were then assessed by plaque reduction assays. The antiviral activity of the most active extracts was characterized by evaluating their effect on the pretreatment, the virucidal activity and the effect on the adsorption or post-adsorption period of the viral cycle. The bioassay-guided fractionation of B. gaudichaudiana OE was carried out by column chromatography followed by semipreparative high performance liquid chromatography fractionation of the most active fraction and isolation of an active compound. The antiviral activity of this compound was also evaluated by plaque assay. Results: B. gaudichaudiana and B. spicata OE were active against PV-2 and VSV. T. absinthioides OE was only active against PV-2. The corresponding three AE were active against HSV-1. B. gaudichaudiana extracts (OE and AE) were the most selective ones with selectivity index (SI) values of 10.9 (PV-2) and >117 (HSV-1). For this reason, both extracts of B. gaudichaudiana were selected to characterize their antiviral effects. Further bioassay-guided fractionation of B. gaudichaudiana OE led to an active fraction, F_C (EC₅₀=3.1 μg/ml; SI= 37.9), which showed antiviral activity during the first 4 h of the viral replication cycle of PV-2 and from which the flavonoid apigenin (EC₅₀ = 12.2 ± 3.3 μM) was isolated as a major compound. Conclusions: The results showed that, among the species studied, B. gaudichaudiana seemed to be the most promising species as a source of antiviral agents.

Bioassay-guided fractionation of the organic extract of Ambrosia tenuifolia Sprengel (Asteraceae) led to the isolation of two bioactive sesquiterpene lactones with significant trypanocidal and leishmanicidal activities. By spectroscopic methods ( ¹ H- and ¹³ C-nuclear magnetic resonance, distortionless enhancement by polarization transfer, correlated spectroscopy, heteronuclear multiple-quantum coherence, electron impact-mass spectrometry, and infrared spectroscopy), these compounds were identified as psilostachyin and peruvin. Both compounds showed a marked in vitro trypanocidal activity against Trypanosoma cruzi epimastigotes with 50% inhibitory concentration (IC ₅₀ ) values of less than 2 μg/ml. Psilostachyin exerted a significant in vitro activity against the trypomastigote forms of T. cruzi (IC ₅₀ , 0.76 μg/ml) and was selected for in vivo testing. Psilostachyin-treated mice had a survival of 100% and lower parasitemia values than control mice. Both compounds were also tested on Leishmania sp. promastigotes: psilostachyin (IC ₅₀ , 0.12 μg/ml) and peruvin (IC ₅₀ , 0.39 μg/ml) exerted significant leishmanicidal activities. This is the first time that the trypanocidal and leishmanicidal activities of these compounds have been reported. The selectivity index (SI) was employed to evaluate the cytotoxic effect of lactones on T lymphocytes. Although the SIs of both compounds were high for T. cruzi epimastigotes, psilostachyin was more selective against trypomastigotes (SI, 33.8) while peruvin showed no specificity for this parasite. Both compounds presented high selectivity for Leishmania spp. The results shown herein suggest that psilostachyin and peruvin could be considered potential candidates for the development of new antiprotozoal agents against Chagas' disease and leishmaniasis.

In vitro trypanocidal and leishmanicidal activities of the flavonoids hispidulin, from Ambrosia tenuifolia, and santin, from Eupatorium buniifolium, are reported. A sensitive technique that takes advantage of (³H)thymidine uptake by dividing trypanosomatids has been adjusted for quantification of the parasiticidal effect of the natural products. The IC₅₀ values for hispidulin and santin on Trypanosoma cruzi epimastigotes were 46.7 and 47.4 μM, respectively. On trypomastigotes, the IC₅₀ values were 62.3 μM for hispidulin and 42.1 μM for santin. Hispidulin was more active than santin on promastigotes of Leishmania mexicana (IC₅₀ = 6.0 μM versus 32.5 μM). No cytotoxic activity was observed on lymphoid cells, making hispidulin and santin potential lead compounds for the development of new natural drugs. This is the first report on the trypanocidal and leishmanicidal activities of these flavonoids and on the presence of santin in E. buniifolium.

Anti-inflammatory activity was detected in the CH₂Cl₂ extract of the aerial parts of Eupatorium buniifolium using the TPA-mouse ear model. Three compounds isolated from this extract, by bioassay-guided fractionation, significantly inhibited the inflammatory response. The compounds were identified as 5,7,5′-trihydroxy-3,6,2′,4′-tetramethoxyflavone (1), scopoletin (2) and centaureidin (3) which inhibited the edema by 67.3 %, 59.8 % and 49.7 %, respectively, at a dose of 1 mg/ear.