What is the mechanism of action (MOA) of irinotecan?

Irinotecan's mechanism of action involves the inhibition of topoisomerase I. Topoisomerase I is an enzyme that plays a crucial role in DNA replication and transcription by causing reversible breaks in one strand of the DNA double helix, allowing the DNA to unwind and relieve torsional strain. When irinotecan binds to the topoisomerase I-DNA complex, it stabilizes this complex, preventing the DNA strand from re-ligating and ultimately leading to DNA damage and cell death, particularly in rapidly dividing cancer cells. This is particularly effective in the treatment of various cancers, including colorectal cancer.

The other mechanisms mentioned in the choices, such as inhibiting topoisomerase II, interfering with DNA synthesis, or inhibiting microtubule formation, do not accurately describe irinotecan's action and are associated with other classes of chemotherapeutic agents. Topoisomerase II is targeted by drugs like doxorubicin and etoposide, while inhibitors of DNA synthesis are typically antimetabolites. Similarly, drugs that inhibit microtubule formation, such as paclitaxel or vincristine, operate through a different mechanism, focusing on disrupting mitotic spindle function. Understanding the specific action of