The pyrimidine antagonists act to block the synthesis of pyrimidine containing nucleotides (C and T in DNA; C and U in RNA). The drugs used to block the construction of these nucleotides have structures that are similar to the natural compound. By acting as 'decoys', these drugs can prevent the production of the finished nucleotides. They may exert their effects at different steps in that pathway and may directly inhibit crucial enzymes. The pyrimidine antagonist may also be incorporated into a growing DNA chain and lead to termination of the process.

For a cell to reproduce, it must first faithfully replicate all of the DNA in its genome. During DNA synthesis, pyrimidine and purine molecules must be made available to allow for the synthesis of the nucleotide building blocks and ultimately the new DNA molecules. A reduction in the availability of the raw materials needed to build DNA, which can be caused by pyrimidine antagonists, leads to stoppage of DNA synthesis and inhibition of cell division.

Cancer cells are often quite rapidly dividing and therefore engaged in DNA synthesis. RNA synthesis is necessary for protein production. The pyrimidine antagonists inhibit the normal processes of DNA and/or RNA synthesis.

Some pyrimidine antagonists used in cancer therapy are:

A Closer Look at Pyrimidine Antagonists: 5-FU

In addition to their effects on DNA synthesis, pyrimidine antagonists that block the function of uracil, such as 5-FU, are able to block the synthesis of RNA molecules. RNA molecules are formed in the transcription process (described in detail in the section on transcription. The RNA message (mRNA) may be used to guide the production of proteins necessary for cellular activities. By blocking RNA formation, 5-FU may induce cell death. Studies have also shown that 5-FU affects the processing of other forms of RNA such as ribosomal RNA (rRNA), transfer RNA (tRNA), and small nuclear RNA (snRNA).⁽¹⁾

A final way in which 5-FU may inhibit cell growth is through its ability to prevent the synthesis of thymine nucleotides from uracil nucleotides.