indication

For the treatment of metastatic colorectal cancer (first-line therapy when administered with 5-fluorouracil and leucovorin). Also used in combination with cisplatin for the treatment of extensive small cell lung cancer. Irinotecan is currently under investigation for the treatment of metastatic or recurrent cervical cancer.

pharmacology

Irinotecan is an antineoplastic enzyme inhibitor primarily used in the treatment of colorectal cancer. Irinotecan is a semisynthetic derivative of camptothecin. Camptothecins interact specifically with topoisomerase I, an enzyme in the cell nucleus that regulates DNA topology and facilitates nuclear processes such as DNA replication, recombination, and repair. During these processes, topoisomerase I relieves torsional strain in DNA by inducing reversible single-strand breaks, allowing single DNA strands to pass through the break. The 3'-DNA terminus of the broken DNA strands bind covalently with the topoisomerase enzyme to form a catalytic intermediate called a cleavable complex. After the DNA is sufficiently relaxed and the strand passage reaction is complete, DNA topoisomerase reattaches the broken DNA strands to form the chemically unaltered topoisomers that allow transcription to proceed. Irinotecan and its active metabolite SN-38 bind to the topoisomerase I-DNA complex and prevent religation of these single-strand breaks. Current research suggests that the cytotoxicity of irinotecan is due to double-strand DNA damage produced during DNA synthesis when replication enzymes interact with the ternary complex formed by topoisomerase I, DNA, and either Irinotecan or SN-38. Mammalian cells cannot efficiently repair these double-strand breaks. The precise contribution of SN-38 to the activity of irinotecan in humans is not known. Irinotecan is cell cycle phase-specific (S-phase).

mechanism of action

Irinotecan inhibits the action of topoisomerase I. Irinotecan prevents religation of the DNA strand by binding to topoisomerase I-DNA complex. The formation of this ternary complex interferes with the moving replication fork, which induces replication arrest and lethal double-stranded breaks in DNA. As a result, DNA damage is not efficiently repaired and apoptosis (programmed cell death) occurs.

toxicity

Gastrointestinal complications, such as nausea, vomiting, abdominal cramping, diarrhea, and infection.

biotransformation

Hepatic

absorption

100%

half life

6-12 hours

route of elimination

The cumulative biliary and urinary excretion of irinotecan and its metabolites (SN-38 and SN-38 glucuronide) over a period of 48 hours following administration of irinotecan in two patients ranged from approximately 25% (100 mg/m2) to 50% (300 mg/m2).

drug interactions

Aprepitant: Aprepitant may change levels of the chemotherapy agent, irinotecan.

Atazanavir: Increases levels/effect of irinotecan

Fosphenytoin: The hydantoin decreases the effect of irinotecan

Ketoconazole: Ketoconazole increases the effect and toxicity of irinotecan

Phenytoin: The hydantoin decreases the effect of irinotecan

Telithromycin: Telithromycin may reduce clearance of Irinotecan. Consider alternate therapy or monitor for changes in the therapeutic/adverse effects of Irinotecan if Telithromycin is initiated, discontinued or dose changed.

Thiotepa: Thiotepa, a strong CYP2B6 inhibitor, may decrease the metabolism and clearance of Irinotecan, a CYP2B6 substrate. Consider alternate therapy or monitor for changes in the therapeutic and adverse effects of Irinotecan if Thiotepa is initiated, discontinued or dose changed.

Trastuzumab: Trastuzumab may increase the risk of neutropenia and anemia. Monitor closely for signs and symptoms of adverse events.

Voriconazole: Voriconazole, a strong CYP3A4 inhibitor, may increase the serum concentration of irinotecan by decreasing its metabolism. Monitor for changes in the therapeutic and adverse effects of irinotecan if voriconazole is initiated, discontinued or dose changed.