In vitro antiplasmodial activity of 4-phenylcoumarins from Exostema mexicanum

The stem bark of Exostema mexicanum (Rubiaceae) is used in Latin American f olk medicine as a quinine substitute for malaria treatment. Bioassay-guided fractionation of lipophilic and hydrophilic extracts from the stem bark an d branches yielded two previously undescribed 4-phenylcoumarins: 4:8-dihydr oxy-5,7-dimethoxy-4-phenylcoumarin (exomexin A) and 3 :4'-dihydroxy-5,7,8-t rimethoxy-4-phenylcoumarin B). Together with five known derivatives the in vitro activities against a chloroquine-sensitive strain (poW) and a chloroq uineresistant strain (Dd2) of Plasmodium falciparum have been evaluated. Th e most lipophilic compound, 4:5,7,8-tetramethoxy-4-phenylcoumarin (O-methyl exostemin) revealed the strongest antiplasmodial activity (IC50 values: 3.6 mug/ml [poW], 1.6 mug/ml [Dd2]).