Structure–Activity Relationship Analysis of Flavonoids Isolated from the Leaves of Erythroxylum rimosum O. E. Schulz

The flavonoids constitute a class of secondary metabolites distributed in plants, occurring in free form (aglycone) or conjugated sugars (glycosides), and are notable for their recognized pharmacological potential. This study investigated flavonoids isolated from the leaves of Erythroxylum rimosum, correlating their chemical structures with biological activities evaluated in vitro assays. The flavonoids did not show significant cytotoxicity in mammalian cells (CC50 > 100 μM), while flavonoids 4 and 5, glycosylated with six-carbon sugar, inhibited the promastigote forms of L. braziliensis with IC50 of 10.34 ± 0.70 μM and 10.14 ± 0.50 μM, respectively. In intracellular infection models, they reduced ~43% of parasitized cells and ~40% of amastigotes. Scanning electron microscopy demonstrated changes in promastigotes, including loss of membrane integrity and flagellum shortening. A molecular docking study with L. mexicana arginase suggested enzyme inhibition as a possible mechanism of action, influenced by sugar chirality. In the antioxidant evaluation, using the DPPH and β-carotene methods, quercetin showed the best inhibitory concentration value (IC50 = 0.22 ± 0.94 mg/mL), followed by glycoside 5 (IC50 = 0.42 ± 0.09 mg/mL). In addition, the flavonoids glycosylated with five-carbon sugar inhibited the acetylcholinesterase enzyme.