Aporphine alkaloids with antitrypanosomal activity from Ocotea puberula (Rich.) Nees

American trypanosomiasis is a tropical disease caused by Trypanosoma cruzi. It is estimated that around six million people are infected in Latin America, with a mortality rate of about fourteen thousand people per year. In Brazil the treatment of this disease is done by only one drug discovered about 50 years ago – benznidazole – that has low efficacy and strong side effects.1 As part of our continuous study concerning the prospection of bioactive metabolites from plant species, the leaves of O. puberula were subjected to acid/base extraction procedure to afford an alkaloid fraction. This fraction displayed activity against trypomastigote forms of T. cruzi (100% of parasite death at 300 µg/mL) and was subjected to a bioactivity-guided fractionation. After successive chromatographic procedures using silica-gel and Sephadex LH-20, five pure alkaloids were obtained. Analysis of NMR and MS spectra and comparison with literature data2-4 allowed the identification of dehydrodicentrine (1), dicentrine (2), predicentrine (3), N-methyllaurotetanine (4) and dicentrinone (5) as showed in figure 1. This is the first occurrence of alkaloids 1 and 5 in O. puberula.
Previous reports described the effects of compounds 3 and 5 against epimastigote and trypomastigote forms of T. cruzi, respectively.5,6 The antitrypanosomal effect of alkaloids 1, 2 and 4 has been described here for the first time. The pure alkaloids 1, 2 and 5 were then evaluated against trypomastigote forms of T. cruzi, showing EC50 values of 28.4, 76.8 and 16.4 µM, respectively. Similarly, the fraction containing a mixture of related alkaloids 3 and 4 displayed EC50 value of 55.7 µM, also against trypomastigotes. Alkaloids 1, 3 – 5 displayed no toxicity against NCTC cells (CC50 > 200 µM). Although aporphines 1 – 4 have similar structures, a higher activity was detected in the oxoaporphine derivative 5, suggesting that the presence of carbonyl group and heteroaromatic ring are crucial to the activity against trypomastigotes.