Cladribine

Mechanism of action

Cladribine is deoxyadenosine with a chlorine atom, which makes it resistant to breakdown by adenosine deaminase. It enters cells passively and is phosphorylated by deoxycytidine kinase. Lymphocytes and monocytes have a lot of this kinase and little of the nucleotidase that reverses it, so cladribine nucleotides accumulate there, damage DNA synthesis and repair, and kill both dividing and resting cells. The result is a deep, long-lasting fall in lymphocytes.

1. Chlorine protects cladribine from adenosine deaminase
2. Deoxycytidine kinase phosphorylates it inside lymphocytes
3. Nucleotides accumulate and break DNA synthesis and repair
4. Dividing and resting lymphocytes die

Indications

• Active hairy cell leukaemia with significant cytopenias or symptoms (single intravenous course).
• Relapsing forms of multiple sclerosis (tablets), usually after another treatment has failed or was not tolerated. Multiple sclerosis →

Adverse effects

Common

• Fever, often in the first month and usually in neutropenic patients
• Neutropenia, anaemia, thrombocytopenia
• Fatigue, rash, nausea, headache
• Long-lasting low CD4 lymphocytes

Contraindications

• Hypersensitivity to cladribine (injection)
• Oral MS form: current malignancy, pregnancy, HIV infection and other conditions listed on its label

Key interactions

Other nephrotoxic drugs

Kidney injury reported with high-dose cladribine combined with nephrotoxic agents.

Other myelosuppressive or immunosuppressive drugs

Additive cytopenias and infection risk.

Live vaccines

Risk of vaccine infection while lymphocytes are low.

What is monitored

• Blood counts, especially in the first months
• Temperature and signs of infection
• Kidney and liver function
• Lymphocyte counts before each MS treatment course

Administration and safety

• For hairy cell leukaemia it is given as a continuous intravenous infusion over several days, prepared with a filter.
• The diluent for continuous infusion contains benzyl alcohol, which is not used in neonates.
• Hazardous drug: chemotherapy handling precautions.

Pregnancy and breastfeeding

Can cause fetal harm; the oral multiple sclerosis form is contraindicated in pregnancy and requires effective contraception.

Nursing care

• Measure temperature regularly: fever is common, but most episodes happen during neutropenia and need assessment.
• Infection precautions while lymphocytes stay low.
• Check the infusion line and pump for continuous infusions.
• Teaching: contraception during and after treatment; report fever or new symptoms.

Pharmacokinetics

Absorption

Intravenous infusion for hairy cell leukaemia; oral tablets for multiple sclerosis.

Distribution

Concentrates as nucleotides in lymphocytes and monocytes.

Metabolism

Phosphorylated inside cells; the effect of kidney or liver impairment has not been well studied, so it is used with caution there.

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Connections

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: DailyMed - CLADRIBINE injection, solution (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - CLADRIBINE tablet (DailyMed (U.S. National Library of Medicine), 2026); Cladribine Injection: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

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