A 55 year old male patient complains of abnormal growth on right cheek. Patient has a habit of tobacco chewing since 30 years. Radiologic findings reveal 8x8 cm fungating mass, invading maxilla and mandible on right side. Histologic findings are suggestive of squamous cell carcinoma. Which of the following is not true regarding the treatment of this condition?
Majority of the drugs used for cancer therapy are designed to block DNA replication or inhibit the enzymes that participate in replication (directly or indirectly). 
Methotrexate (inhibits dihydrofolate reductase) and 5-fluorouracil (inhibits thymidylate synthase) block nucleotide synthesis.
In recent years, topoisomerase inhibitors are being used. They block the unwinding of parental DNA  strands and prevent replication.

Key Concept:
Topoisomerase inhibitors block the unwinding of parental DNA strands and hence preventing replication.
The drugs used are designed to block DNA replication
Methotrexate and 5-fluorouracil block nucleotide synthesis
Topoisomerase inhibitors promote the unwinding of parental DNA  strands and prevent replication
The drugs used for cancer therapy inhibit the enzymes that participate in replication