General Mechanism of Action
Antimetabolites act as cellular saboteurs. Their pharmacodynamics rely on three main principles:
- Synthesis inhibition: drugs block the formation of natural DNA and RNA precursors.
- Competitive antagonism: substances directly compete with normal metabolites for enzyme binding.
- Synthesis of "false" nucleotides: incorporating into the growing nucleic acid chain instead of a normal component, terminating further synthesis at a certain stage.
A crucial feature of this group is cell-cycle phase specificity. They act exclusively during the S phase of the cell cycle. Quiescent cells (outside the proliferation phase) are insensitive to them. The ultimate result of antimetabolite action is an acute nucleotide deficit, loss of replication capacity, and the triggering of apoptosis (programmed cell death).
Folic Acid Antagonists: Methotrexate
Folic acid is not synthesized de novo in the human body, but it is critically important for purine and thymidine production. Methotrexate acts as its structural analogue and antagonist.
- Target: dihydrofolate reductase enzyme. Methotrexate has a significantly higher affinity for it than the natural substrate.
- Effect: the conversion of dihydrofolates to active tetrahydrofolic acid is blocked. A coenzyme deficit develops, causing a sharp drop in adenine, guanine, and thymidine production.
- Clinical use: as part of combination therapy for malignancies (leukemias, carcinomas), and in low doses (monotherapy) for rheumatoid arthritis, severe psoriasis, and Crohn's disease. Systemically administered drug does not cross the blood-brain barrier, so it is administered intrathecally to target the CNS.
- Toxicity: myelosuppression (corrected by leucovorin rescue), nephrotoxicity (requires preventive urine alkalinization), and risk of liver cirrhosis. Pregnancy is an absolute contraindication.
Purine Antagonists
This group includes mercaptopurine (a thiol analogue of hypoxanthine) and its precursor, azathioprine.
Inside the cytoplasm, mercaptopurine is transformed into the active metabolite 6-thioinosinic acid. It acts in two ways: it physically terminates the purine production chain and creates a "false feedback" effect. The cell perceives this metabolite as an excess of adenosine and completely stops endogenous synthesis.
- Metabolic features: the drug is inactivated by the enzyme xanthine oxidase. If the patient concurrently takes an inhibitor of this enzyme (allopurinol), metabolism slows down. To avoid severe toxicity, the dose of mercaptopurine must be reduced by 75%.
- Resistance: most commonly associated with decreased activity of the enzyme HGPRT (hypoxanthine-guanine phosphoribosyltransferase), which is required to activate the drug. Deficiency of this enzyme is also seen in Lesch-Nyhan syndrome.
- Comparison: azathioprine has a more pronounced immunosuppressive effect, making it the drug of choice when an immunosuppressant is required.
Pyrimidine Antagonists
This subgroup includes drugs that disrupt pyrimidine base metabolism:
- Fluorouracil (5-fluorouracil): a uracil analogue in which a hydrogen atom is replaced by fluorine. Administered intravenously or topically (avoiding the oral route due to high GI mucosal toxicity). It blocks thymidylate synthase and incorporates into RNA structure. Used mainly for solid tumors of the GI tract and breast. A specific side effect with prolonged use is palmar-plantar erythrodysesthesia ("hand-foot syndrome").
- Cytarabine: a 2-deoxycytidine analogue (contains arabinose instead of ribose). Potently inhibits DNA polymerase and causes DNA chain elongation termination. Used primarily for hematologic malignancies. In high doses, it can cause severe neurotoxicity (encephalopathy, paralysis).
- Gemcitabine: a fluorinated analogue of deoxycytidine. Competes with cytidine triphosphate and inhibits ribonucleotide reductase. Specific side effects include a flu-like syndrome and nephrotoxicity.