Dando continuidade as pesquisas de identificação de metabólitos secundários com propriedades anti-infecciosas potenciais a partir de espécies de plantas dos biomas brasileiros, dois alcaloides piperidínicos ( - )-cassina e ( - )-espectalina foram isolados das flores de Senna spectabilis (sin. Cassia spectabilis). As estruturas destes compostos foram elucidadas a partir de dados espectroscópicos e espectrométricos. Adicionalmente, esses alcaloides foram acetilados, resultando nos derivados ( - )-3-O-acetilcassina e ( - )-3-O-acetilespectalina. Todas as substâncias foram submetidas ao bioensaio empregando culturas de eritrócitos, infectadas com Plasmodium falciparum, um teste específico para avaliação antimalárica. Dentre as substâncias avaliadas, os dois primeiros alcaloides (IC50 1,82 µM e IC50 2,76 µM) foram mais potentes que os derivados (IC50 24,47 µM e IC50 25,14 µM) em comparação com a cloroquina (IC50 0,30 µM), utilizada como padrão. Estes dados mostram que os alcaloides piperidínicos constituem uma classe de produtos naturais que apresenta amplo espectro de atividades biológicas, sendo portanto, importantes modelos para o planejamento de fármacos, incluindo os antimaláricos.

In our continuing work looking for new anti-infective lead compounds from Brazilian biomes, the two known piperidine alkaloids ( - )-cassine and ( - )-spectaline were isolated from the flowers of Senna spectabilis (syn. Cassia spectabilis). Their structures were elucidated using a combination of spectroscopic and spectrometric data analysis. Further, these compounds were acetylated yielding the derivatives ( - )-3-O-acetylcassine and ( - )-3-O-acetylspectaline. All compounds were screened against P. falciparum-infected red blood cells (RBC) in culture, aiming to identify antimalarial prototypes. Among all compounds screened, the first two alkaloids (IC50 1.82 µM and IC50 2.76 µM) were more effective than the derivatives (IC50 24.47 µM and IC50 25.14 µM) in comparison to the standard compound chloroquine (IC50 0.30 µM). These data show that piperidine alkaloids constitute a class of natural products that feature a broad spectrum of biological activities, and are, therefore, important templates for drug design, including antimalarial.

A mistura dos derivados semissintéticos cloridrato da (-)-3-O-acetil-cassina e cloridrato da (-)-3-O-acetil-espectalina, preparada a partir da mistura dos alcalóides (-)-cassina e (-)-espectalina (4:1) obtida de Senna spectabilis, é um potente inibidor da acetilcolinesterase (AChE), assim justificando mais estudos moleculares. Neste sentido, estudos de docking e dinâmica moleculares foram conduzidos neste trabalho com o objetivo de adquirir uma compreensão mais profunda de todos os aspectos estruturais das moléculas cloridratos da (-)-3-O-acetil-cassina e (-)-3-O-acetil-espectalina, as quais diferem em seus potenciais inibidores de AChE. Os dois derivados em estudo apresentaram diversas interações com o sítio periférico aniônico dentro da cavidade catalítica de AChE de Torpedo californica. Entretanto, somente o composto majoritário (-)-3-O-acetil-cassina mostrou interação com a tríade catalítica de maneira significativa. As simulações de dinâmica molecular utilizando água como solvente foram importantes para compreender as interações hipotéticas entre cloridratos da (-)-3-O-acetil-cassina e (-)-3-O-acetil-espectalina com AChE. Os dados obtidos indicam que o composto (-)-3-O-acetil-cassina é o inibidor da enzima mais potente possivelmente devido às suas interações favoráveis com a proteína, com menor custo de dessolvatação. Estes resultados sugerem que o tamanho da cadeia lateral influencia no potencial inibitório das moléculas avaliadas e podem representar o ponto de partida para o desenvolvimento de novos derivados de (-)-3-O-acetil-cassina, objetivando a descoberta de inibidores de AChE mais eficazes.

The mixture of semi-synthetic derivatives (-)-3-O-acetyl-cassine hydrochloride and (-)-3-O-acetyl-spectaline hydrochloride, prepared from the mixture of natural alkaloids (-)-cassine and (-)-spectaline (4:1) isolated from Senna spectabilis, has been shown to be a potent acetylcholinesterase (AChE) inhibitor, thereby prompting further molecular studies. In this sense, docking and dynamic molecular studies were carried out in this work, aiming to acquire a deeper understanding about all the structural aspects of molecules (-)-3-O-acetyl-cassine and (-)-3-O-acetyl-spectaline hydrochlorides, which differ with respect to their AChE inhibitory potentials. Both molecules establish important interactions with the peripheral anionic site within the catalytic gorge of Torpedo californica AChE. However, only the major compound (-)-3-O-acetyl-cassine hydrochloride significantly interacts with the catalytic triad. Explicit-solvent molecular dynamic simulations were conducted in order to gain better understanding about the hypothetical interactions taking place between the semi-synthetic alkaloid molecules (-)-3-O-acetyl-cassine and (-)-3-O-acetyl-spectaline hydrochlorides and AChE. The data obtained in this study indicated that (-)-3-O-acetyl-cassine hydrochloride is the most potent inhibitor of AChE possibly due to the favorable interactions of this molecule with the target protein, with lower desolvation cost. These results suggested that the size of the side chain has an effect on the inhibitory potential of the evaluated molecules and may represent the starting point for the development of new derivatives of (-)-3-O-acetyl-cassine hydrochloride, with a view to the discovery of new effective AChE inhibitors.

The use of natural products has definitely been the most successful strategy in the discovery of novel medicines. Secondary metabolites from terrestrial and marine organisms have found considerable use in the treatment of numerous diseases and have been considered lead molecules both in their natural form and as templates for medicinal chemistry. This paper seeks to show the great value of secondary metabolites and emphasize the rich chemical diversity of Brazilian biodiversity. This natural chemical library remains understudied, but can be a useful source of new secondary metabolites with potential application as templates for drug discovery.