Cytotoxic phenanthroindolizidine alkaloids in Tylophora tanakae against human gastric carcinoma cells

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We have noticed the presence of cytotoxic activity in the body fluid of a butterfly, Ideopsis similis, in family Danaidae. The cytotoxic principles were purified and identified to be phenanthroindolizidine alkaloids. In addition, the cytotoxic activity was observed in the leaves of Tylophora tanakae, which is a host plant for caterpillars of I. similis. In the present study, cytotoxic principles in methanol extracts of the leaves against human gastric carcinoma TMK-1 cells were purified by acid-base partition, repeated HPLC and preparative TLC, and two cytotoxic compounds were isolated. From spectroscopic analysis of their structures, they were identified as phenanthroindolizidine alkaloids, trans-(+)-3, 14α dihydroxy-4, 6, 7-trimethoxyphenanthroindolizidine N-oxide and (-)-7-hydroxy-2, 3, 6-trimethoxyphenanthroindolizidine. The amounts of two compounds isolated from 200g of dried leaves were 3.5 and 4.0mg, respectively. These alkaloids are different from those in the fluid of I. similis. When TMK-1 cells were treated with these phenanthroindolizidine alkaloids, strong cytotoxic effects were observed, doses of 6ng/ml for trans-(+)-3, 14α-dihydroxy-4, 6, 7-trimethoxyphenanthroindolizidine N-oxide and 3ng/ml for (-)-7-hydroxy-2, 3, 6-trimethoxyphenanthroindolizidine inducing 50% cell growth inhibition (IC50).
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