topoisomerase inhibitor

any one of a class of CYTOTOXIC DRUG that work by blocking the action of topoisomerase I, an enzyme that promotes the uncoiling of the DNA double helix, which is a necessary preliminary to replication. These drugs include irinotecan (Campto), used for treating advanced colorectal cancer; and topotecan (Hycamtin), for treating advanced ovarian cancer. They are administered by intravenous infusion; side-effects include delayed but severe diarrhoea and reduction in blood-cell production by the bone marrow.