This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Chemotherapy, Subject name: Pharmacology
Which of the following drugs is topoisomerase 1 inhibitor? 
Select and explain. 
A. Doxorubicin, B. Irinotecan, C. Etoposide, D. Vincristine
B: Irinotecan
Answer: B: Irinotecan 
Explanation: CAM PTOTHECIN ANALOGUES Topotecan and Irinotecan are two recently introduced semisynthetic analogues of camptothecin, an anti tumour principle obtained from a Chinese tree. They act in a manner similar to etoposide, but interact with a different enzyme (DNA topoisomerase I). Their binding to this nuclear enzyme allows single strand breaks in DNA, but not its resealing after the strand has untwisted. They damage DNA during replication; act in the S phase and arrest cell cycle at G2 phase. lrinotecan It is a prodrug; is decarboxylated in liver to the active metabolite. Cholinergic effects are produced in some patients because it inhibits AChE. These effects can be suppressed by prior atropinization. Irinotecan is indicated in metastatic/ advanced colorectal carcinoma, cancer lung/ cervix/ ovary, etc. Dose limiting toxicity is diarrhoea; neutropenia, thrombocytopenia, haemorrhage, bodyache and weakness are the other adverse effects. ESSENTIALS OF MEDICAL PHARMACOLOGY K.D.TRIPATHI SIXTH EDITION PAGE NO:825,826