Alkylating agents: They are cell-cycle independent drugs, means they can work at any point in the cell cycle. They bind covalently to DNA with the help of their alkyl groups. The cell becomes unable to replicate crosslinked DNA. This leads to apoptosis.
Antimetabolites: – Unlike alkylating agents, anti-metabolites are cell cycle dependent. Many of them have a similar structure to the building blocks of DNA and RNA. Anti-metabolites resemble either nucleobases or nucleosides (a nucleotide without the phosphate group) but have altered chemical groups. These drugs exert their effect by either blocking the enzymes required for DNA synthesis or becoming incorporated into DNA or RNA. By inhibiting the enzymes involved in DNA synthesis, they prevent mitosis because the DNA cannot duplicate itself. Also, after misincorporation of the molecules into DNA, DNA damage can occur and programmed cell death (apoptosis) is induced.
Antibiotics: Antibiotics which have therapeutic effects include anthracyclines, actinomycin, bleomycin, mitomyosin etc. Anthracyclins intercalate between DNA strands. It also generates free radicals that damage DNA Actinomycin is a complex molecule that intercalates DNA and prevents RNA synthesis. Bleomycin also intercalates DNA but produces free radicals that damage DNA. Mitomycin is a cytotoxic antibiotic with the ability to alkylate DNA.
Plant alkaloids: have two effects:
- First, they prevent microtubule function. Vinca alkaloids and taxanes are the two main groups of anti-microtubule agents. The vinca alkaloids prevent the formation of the microtubules, whereas the taxanes prevent the microtubule disassembly. By doing so, they prevent the cancer cells from completing mitosis. Following this, cell cycle arrest occurs, which induces programmed cell death.
- Second, both these drugs can affect blood vessel growth; an essential process that tumors utilize in order to grow and metastasize.
Others: Some other drugs like topoisomerase inhibitors are also known. These drugs do not allow normal unwinding of DNA during the replication process, examples are, irinotecan, topotecan.