MW: 623.14. Irinotecan is a topoisomerase 1 inhibitor, which prevents DNA from unwinding. Chemically it is a semisynthetic analogue of the natural alkaloid camptothecin. It is activated by hydrolysis to SN-38 which is then inactivated by glucuronidation by UGT1A1. This inactivation eventually leads to inhibition of both DNA replication and transcription (1, 2).