IRINOTECAN HYDROCHLORIDE IS AN ANTINEOPLASTIC AGENT OF THE TOPOISOMERASE I INHIBITOR CLASS. IRINOTECAN IS A DERIVATIVE OF CAMPTOTHECIN. CAMPTOTHECINS INTERACT SPECIFICALLY WITH THE ENZYME TOPOISOMERASE I WHICH RELIEVES TORSIONAL STRAIN IN DNA BY INDUCING REVERSIBLE SINGLE-STRAND BREAKS. IRINOTECAN AND ITS ACTIVE METABOLITE SN-38 BIND TO THE TOPOISOMERASE I-DNA COMPLEX AND PREVENT RELIGATION OF THESE SINGLE-STRAND BREAKS. CURRENT RESEARCH SUGGESTS THAT THE CYTOTOXICITY OF IRINOTECAN IS DUE TO DOUBLE-STRAND DNA DAMAGE PRODUCED DURING DNA SYNTHESIS WHEN REPLICATION ENZYMES INTERACT WITH THE TERNARY COMPLEX FORMED BY TOPOISOMERASE I, DNA, AND EITHER IRINOTECAN OR SN-38. MAMMALIAN CELLS CANNOT EFFICIENTLY REPAIR THESE DOUBLE-STRAND BREAKS.