Mechanism of Action of Irinotecan

Introduction

Irinotecan is a semi-synthetic camptothecin derivative used as an antineoplastic agent, primarily in the treatment of colorectal cancer and other solid tumors. It is a prodrug that requires metabolic activation to exert its cytotoxic effects. Irinotecan interferes with DNA replication in rapidly dividing cancer cells, leading to cell death.

Mechanism of Action (Step-wise)

1. Prodrug Activation
   Irinotecan is converted in the liver by carboxylesterase enzymes into its active metabolite, SN-38.
2. Topoisomerase I Inhibition
   SN-38 inhibits DNA topoisomerase I, an enzyme responsible for relieving torsional strain during DNA replication.
3. Stabilization of DNA-Enzyme Complex
   It stabilizes the transient DNA-topoisomerase I complex, preventing re-ligation of single-strand breaks.
4. Accumulation of DNA Damage
   During DNA replication, these single-strand breaks convert into double-strand breaks.
5. Cell Cycle-Specific Action
   The cytotoxic effect is most pronounced during the S-phase of the cell cycle.
6. Induction of Apoptosis
   Accumulated DNA damage triggers apoptosis in rapidly dividing tumor cells.

Pharmacokinetics

• Administration: Intravenous
• Activation: Hepatic conversion to SN-38
• Metabolism: SN-38 is inactivated by UGT1A1-mediated glucuronidation
• Half-life: Irinotecan ~6–12 hours; SN-38 longer
• Excretion: Biliary and fecal (major), renal (minor)

Clinical Uses

• Metastatic colorectal cancer
• Combination chemotherapy regimens (e.g., FOLFIRI)
• Other solid tumors (off-label uses)

Adverse Effects

• Severe diarrhea (early cholinergic and delayed secretory type)
• Myelosuppression (neutropenia)
• Nausea and vomiting
• Alopecia
• Fatigue

Comparative Analysis

Irinotecan differs from topotecan mainly in its prodrug nature and clinical indications. Compared to cisplatin, which causes DNA cross-linking, irinotecan specifically targets topoisomerase I and is cell cycle-specific.

MCQs

1. Irinotecan is a prodrug converted into:
   a) Cisplatin
   b) SN-38
   c) Methotrexate
   d) Doxorubicin
   Answer: b) SN-38
2. Target enzyme of irinotecan:
   a) DNA polymerase
   b) Topoisomerase II
   c) Topoisomerase I
   d) RNA polymerase
   Answer: c) Topoisomerase I
3. Phase of cell cycle affected:
   a) G0
   b) G1
   c) S-phase
   d) M-phase
   Answer: c) S-phase
4. Mechanism involves:
   a) DNA cross-linking
   b) Inhibition of DNA repair
   c) Stabilization of DNA breaks
   d) RNA synthesis inhibition
   Answer: c) Stabilization of DNA breaks
5. Major adverse effect:
   a) Nephrotoxicity
   b) Cardiotoxicity
   c) Severe diarrhea
   d) Ototoxicity
   Answer: c) Severe diarrhea
6. Enzyme responsible for activation:
   a) CYP3A4
   b) Carboxylesterase
   c) UGT1A1
   d) MAO
   Answer: b) Carboxylesterase
7. SN-38 is inactivated by:
   a) Oxidation
   b) Glucuronidation
   c) Reduction
   d) Hydrolysis
   Answer: b) Glucuronidation
8. Used primarily in:
   a) Breast cancer
   b) Lung cancer
   c) Colorectal cancer
   d) Prostate cancer
   Answer: c) Colorectal cancer
9. Type of DNA damage caused:
   a) Cross-links
   b) Single-strand breaks
   c) Methylation
   d) Deamination
   Answer: b) Single-strand breaks
10. Delayed diarrhea is due to:
    a) Infection
    b) Secretory mechanisms
    c) Bleeding
    d) Hypersensitivity
    Answer: b) Secretory mechanisms

FAQs

1. What is the mechanism of action of irinotecan?
   It inhibits topoisomerase I after conversion to SN-38, preventing DNA repair.
2. Why is irinotecan considered a prodrug?
   Because it must be converted into its active metabolite SN-38.
3. What is the major toxicity of irinotecan?
   Severe diarrhea and neutropenia.
4. What is the role of UGT1A1?
   It inactivates SN-38 via glucuronidation.
5. Why is irinotecan S-phase specific?
   It affects DNA replication processes occurring during S-phase.
6. What cancer is irinotecan commonly used for?
   Metastatic colorectal cancer.

References

• Goodman & Gilman’s The Pharmacological Basis of Therapeutics – Anticancer Drugs
• Katzung: Basic and Clinical Pharmacology – Cancer Chemotherapy
• Tripathi: Essentials of Medical Pharmacology
• Harrison’s Principles of Internal Medicine – Oncology

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