Abstract
A methanolic crude extract of the plant Garuga pinnata Roxb. (Burseraceae) showed promising cytotoxic activity against a panel of human tumor cell lines in vitro, including KB and its drug-resistant sublines (Ferguson et al. Cancer Res. 1988, 48, 5956). Pheophorbide- a and- b methyl esters ( 3, 4) were isolated as active principles with broad photo-dependent cytotoxic activities in the micromolar range. These findings prompted SAR studies of known and novel pheophorbide- a derivatives as photo-dependent and photo-independent cytotoxic agents. The results showed that zinc-protoporphyrin IX ( 10), zinc 13 2 ( R)-hydroxypheophorbide- a methyl ester ( 22), and zinc chlorin- e6 trimethyl ester ( 13) possessed photo-independent cytotoxic activity. Compounds 13 and 22 were the most active cytotoxic agents of the series (mean ED 50 4.6±1.0 μM and 5.7±0.7 μM, respectively) against KB cells incubated in the dark.
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