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Anticancer Drugs (Nucleotide Synthesis Inhibitors)

For medical students2 min readUpdated 2026-10-10

Anticancer and antiviral drugs in this group are structural analogs of purines, pyrimidines, or their metabolites. By incorporating into cellular processes as pseudosubstrates or blocking key enzymes, they halt nucleotide synthesis, leading to DNA and RNR chain termination and the death of rapidly dividing cells.

5-FluorouracilA suicide inhibitor of thymidylate synthase that blocks dTMP synthesis.

MethotrexateA folic acid analog that inhibits dihydrofolate reductase.

AcyclovirA guanine analog that terminates DNA synthesis in herpes viruses.

AzaserineA glutamine analog that disrupts de novo purine ring synthesis.

General Principle of Action

The action of these drugs is based on the use of chemical structures similar to monosaccharides, purines, and pyrimidines, but with modifications in the heterocyclic ring or carbohydrate component. Once inside the cell, these compounds exert a toxic effect through two main pathways:

Inhibition of enzymes required for the synthesis of nucleotides or nucleic acids.
Incorporation into DNA or RNA structure as pseudosubstrates. This disrupts proper complementary base pairing and halts the elongation process of the polynucleotide chain.

Blockade of dTMP Synthesis: Thymidylate Synthase and Dihydrofolate Reductase

The synthesis of deoxythymidine monophosphate (dTMP) from deoxyuridine monophosphate (dUMP) is a critical step for DNA replication prior to cell division. This reaction is catalyzed by the enzyme thymidylate synthase, with a folate derivative ($N^5,N^{10}$-methylene-tetrahydrofolate) serving as the methyl group donor.

During the reaction, tetrahydrofolate is oxidized to dihydrofolate. To prevent the cycle from stalling, dihydrofolate must be reduced back by the enzyme dihydrofolate reductase.

Key chemotherapeutic agents target these two enzymes:

5-Fluorouracil selectively blocks thymidylate synthase.
Methotrexate inhibits the activity of dihydrofolate reductase, depleting active folate stores within the cell.

Mechanisms of Action of Major Drugs

Drug	Target / Blocked Process	Clinical Application
5-Fluorouracil	Inhibits thymidylate synthase after conversion to 5-F-dUMP (suicide inhibitor). Stops dTMP production.	Solid tumors (stomach, lung, breast).
Methotrexate	Competitive inhibitor of dihydrofolate reductase. Disrupts purine synthesis and the conversion of dUMP to dTMP due to folate deficiency.	Tumor chemotherapy.
Acyclovir	Guanine analog. In its triphosphate form, it incorporates into viral DNA and terminates its synthesis.	Herpes virus infections.
Azidothymidine (AZT)	As AZT-triphosphate, it blocks human immunodeficiency virus replication.	HIV/AIDS therapy.

Suicide Inhibition by 5-Fluorouracil

In the body, 5-fluorouracil (5-F-U) is metabolized into its active form, 5-fluoro-deoxyuridine monophosphate (5-F-dUMP). This metabolite binds to the active site of thymidylate synthase and its coenzyme, attempting to enter the normal methyl transfer reaction.

However, unlike hydrogen in the normal substrate, the fluorine atom is tightly bound to the carbon ring and cannot be cleaved. This results in an irreversible covalent complex, permanently locking the enzyme via a 'suicide' mechanism (suicide inhibition). This leads to 'thymineless death' in cancer cells that are unable to synthesize DNA.

Amino Acid Analogs: Azaserine

Certain antibiotics also interfere with nucleotide synthesis. For example, azaserine is a structural analog of the amino acid glutamine. It acts as a competitive inhibitor of enzymes for which glutamine serves as a nitrogen source.

During de novo purine ring synthesis, glutamine donates nitrogen atoms at positions N3 and N9 and participates in the formation of the GMP amino group. Azaserine blocks these steps by inhibiting the activity of the corresponding enzymes (PRPP glutamine amidotransferase, FGAM synthetase, and GMP synthetase).

Mnemonic

To remember the drug targets: 5-Fluorouracil hits Thymidylate synthase (FUT), and Methotrexate hits DihydroFolate Reductase (MDFR).

Frequently asked questions

Which enzyme performs the initial phosphorylation of acyclovir in an infected cell?

The initial phosphorylation of acyclovir in an infected cell is performed by viral thymidine kinase. This enzyme catalyzes the conversion of the drug into acyclovir monophosphate. This primary activation reaction occurs exclusively inside infected cells, ensuring high selectivity and safety for the host's healthy cells. Subsequently, cellular kinases complete the phosphorylation cascade to form the final active form, acyclovir triphosphate.

What are the main adverse effects of methotrexate therapy?

Methotrexate therapy is associated with adverse reactions such as drug-induced liver injury, including hepatotoxicity and elevated serum transaminases; cytopenias; and oral mucositis. To prevent adverse reactions, including hepatotoxicity, folate supplementation (folic acid/leucovorin) is recommended for patients receiving methotrexate.

Which specific reactions of purine ring synthesis are disrupted by folate deficiency caused by methotrexate?

Folate deficiency caused by methotrexate disrupts carbon atom incorporation into the purine ring structure during purine nucleotide synthesis. Due to the blockade of coenzyme folate forms, the following steps are impaired:

Incorporation of a carbon atom at position 8 of the purine ring — a reaction requiring $N^5,N^{10}$-methenyl-$H_4$-folate.
Incorporation of a carbon atom at position 2 of the purine ring — a reaction requiring $N^{10}$-formyl-$H_4$-folate.

This leads to the blockade of inosine monophosphate (IMP) synthesis and a reduction in AMP and GMP production.

Why do folic acid analogs block cell proliferation?

As 5-F-dUMP, the drug binds to thymidylate synthase, forming a stable ternary complex that cannot dissociate due to the uncleavable fluorine atom. The enzyme is irreversibly inactivated.

Which processes in purine synthesis are affected by azaserine?

As a glutamine analog, azaserine blocks the incorporation of nitrogen atoms into positions N3 and N9 of the purine ring, as well as the formation of GMP from xanthylic acid.

How does acyclovir work?

It is a structural analog of guanine. Once phosphorylated in the cell, it incorporates into the elongating viral DNA chain instead of a normal nucleotide, causing chain termination.

Go deeper

Detailed folate methylation and regeneration cycle.
Biochemical role of serine hydroxymethyltransferase in one-carbon metabolism.
Steps of de novo purine synthesis: origins of purine ring atoms.
Role of phosphoribosyl pyrophosphate (PRPP) in the metabolic activation of 5-fluorouracil.