Irinotecan Amaxa is a semisynthetic substance derived from camptothecin. It is an antitumor drug that acts as a specific inhibitor of DNA topoisomerase I. Under the action of carboxylesterase in most tissues, the drug is metabolized to the compound SN-38, which, compared with irinotecan, is more active against purified topoisomerase I and more cytotoxic against a number of human and mouse tumor cell lines. Inhibition of DNA topoisomerase I by irinotecan or SN-38 leads to single-strand DNA damage, which blocks the DNA replication fork and leads to cytotoxic action. It was found that this cytotoxic effect is time-dependent and specific for the S-phase of the cell cycle.