Topoisomerase inhibitor I/II

Question 1

Describe the function of type 1 and type 2 topoisomerase enzymes

Answer 1

Type 1:

• Temporarily cut ONE strand of DNA
• Swivelling occurs around the intact strand
• alters the number of twists in the DNA double helix

Type 2:

• Temporarily cut BOTH strands of DNA at once
• pass another double strand (or another section of the same molecule), through the gap
• Alters the number of twists, disentangle knots

Question 2

Example drugs and corresponding tumor types of topoisomerase inhibitors (I and II)

Answer 2

Type 1: Topotecan

• Ovarian cancer
• SCLC
• Cervical cancer
• CRC

Type 2: 
Antibiotic (anthracyclin intercalating agents) - Doxorubicin
- Breast cancer
- Bladder cancer
- Lymphoma
- Sarcoma
Non-intercalating - Etoposide
- AML, Lymphomas
Testicular cancer
Lung cancer
Paediatric cancers

Question 3

Mechanism of action of topoisomerase inhibitors (I and II)

Answer 3

Type 1:
Bind to complex of Topoisomerase 1 and DNA and therefore prevent re-ligation of DNA strand break. This triggers DNA damage and cell death

Type 2:
After formation of double strand breaks, stabilisation of topoisomerase II complex (doxorubicin) or prevention of DNA strand ligation (etoposide). The generation of high levels of Top2 DNA covalent complexes has profound effects on cell physiology. Top2 poisons effectively block transcription and replication. DNA strand breaks are rapidly detected following treatment with Top2 poisons, and most of the strand breaks are protein linked, as expected. Cells subsequently commit to apoptosis, in fact etoposide is a very commonly used agent to study apoptotic processes

Question 4

Drug resistance of type 2 topoisomerase inhibitors

Answer 4

Type 2:
1. Decreased drug accumulation: Because some topoisomerase inhibitors (e.g. etoposide, doxorubicin) are substrates for P-glycoprotein (Pgp) (MDR1), cells that express this efflux pump protein will display resistance to these agents due to decreased drug accumulation.
2. Mutation or decreased expression of topoisomerase II
Decreased apoptosis due to mutation of p53

Question 5

Toxicity of topoisomerase inhibitors

Answer 5

Short-term toxicity includes myelosuppression and gastrointestinal toxicity. Long-term survivors are at risk of cardiac toxicity and secondary leukaemia