Camptothecin Analogues

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Chapter: Essential pharmacology : Anticancer Drugs

Topotecan and Irinotecan are two recently introduced semisynthetic analogues of camptothecin, an antitumour principle obtained from a Chinese tree. They act in a manner similar to etoposide, but interact with a different enzyme (DNA topoisomerase I).


CAMPTOTHECIN ANALOGUES

 

Topotecan and Irinotecan are two recently introduced semisynthetic analogues of camptothecin, an antitumour 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.

 

Topotecan is used in metastatic carcinoma of ovary and small cell lung cancer after primary chemotherapy has failed. The major toxicity is bone marrow depression, especially neutropenia. Other adverse effects are pain abdomen, vomiting anorexia and diarrhoea.

 

Dose: 1.5 mg/m2 i.v. over 30 min daily for 5 days; TOPOTEL 2.5 mg and 4.0 mg inj.

 

Irinotecan

 

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.

 

Individuals expressing the UGT1A1*28 allele of glucuronyl transferase enzyme are more susceptible to irinotecan induced diarrhoea and neutropenia.

 

Dose: 125 mg/m2 i.v. over 90 min, weekly for 4 weeks.

 

IRINOTEL 40 mg (2 ml), 100 mg (5 ml) inj.

 

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