In vitro antitumor activity of the novel marine agent, ecteinascidin-743 (ET-743, NSC-648766) against human tumors explanted from patients

High antitumour activity of ET743 against human tumour xenografts from melanoma, non-small-cell lung and ovarian cancer

BACKGROUND

Ecteinascidin-743 (ET743) is a novel antitumour agent originating from the Caribbean tunicate Ecteinascidia turbinata. It has potent cytotoxic and antitumour activity and a potential new mechanism of action. The aim of the present study was to further explore the antitumour activity of ET743 in human tumour xenografts from melanoma, non-small-cell lung and ovarian cancer.

DESIGN

As the antitumour profile of ET743 was largely unknown a chemo-sensitive and a marginal chemo-resistant human tumour xenograft were selected for each tumour type. ET743 was administered intravenously using two administration schedules (days 0, 4, 8 and 0-2, 13-15).

RESULTS

ET743 was very active at the maximum tolerated dose (MTD) in the chemo-sensitive xenograft melanoma MEXF 989, non-small-cell lung cancer LXFL 529, and ovarian cancers HOC22 and (marginally resistant to cisplatin) HOC18. Activity was also seen at 1/2 MTD. Apart from HOC18, ET743 caused complete remissions in the responding xenografts. The compound was inactive in the chemo-resistant xenograft melanoma MEXF 514 and non-small-cell lung cancer LXFA 629. In terms of antitumour activity the days 0, 4, 8 schedule had advantages over the days 0-2, 13-15 schedule.

CONCLUSIONS

ET743 is a very effective agent in chemo-sensitive and marginal chemo-resistant xenografts, but inactive in chemo-resistant tumour xenografts. The activity of ET743 in the marginally cisplatin-resistant ovarian cancer HOC18 might indicate absence or incomplete cross-resistance against cisplatin. It is recommended to include melanoma, non-small-cell lung cancer, and ovarian cancer in phase II clinical trials and to use an intermittent schedule.

Poisoning of human DNA topoisomerase I by ecteinascidin 743, an anticancer drug that selectively alkylates DNA in the minor groove

Ecteinascidin 743 (Et743, National Service Center 648766) is a potent antitumor agent from the Caribbean tunicate Ecteinascidia turbinata. Although Et743 is presently in clinical trials for human cancers, the mechanisms of antitumor activity of Et743 have not been elucidated. Et743 can alkylate selectively guanine N2 from the DNA minor groove, and this alkylation is reversed by DNA denaturation. Thus, Et743 differs from other DNA alkylating agents presently in the clinic (by both its biochemical activities and its profile of antitumor activity in preclinical models). In this study, we investigated cellular proteins that can bind to DNA alkylated by Et743. By using an oligonucleotide containing high-affinity Et743 binding sites and nuclear extracts from human leukemia CEM cells, we purified a 100-kDa protein as a cellular target of Et743 and identified it as topoisomerase I (top1). Purified top1 was then tested and found to produce cleavage complexes in the presence of Et743, whereas topoisomerase II had no effect. DNA alkylation was essential for the formation of top1-mediated cleavage complexes by Et743, and the distribution of the drug-induced top1 sites was different for Et743 and camptothecin. top1-DNA complexes were also detected in Et743-treated CEM cells by using cesium chloride gradient centrifugation followed by top1 immunoblotting. These data indicate that DNA minor groove alkylation by Et743 induces top1-mediated protein-linked DNA breaks and that top1 is a target for Et743 in vitro and in vivo.