U01.11.078 Leflunomide

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Learning Objectives

Master the mechanism of action of Leflunomide as a Disease-Modifying Antirheumatic Drug (DMARD). Understand how its inhibition of pyrimidine synthesis specifically targets rapidly dividing immune cells, and recognize its critical teratogenic and hepatic contraindications.

1. Mechanism: Pyrimidine Synthesis Inhibition

Leflunomide is a prodrug that is converted into its active metabolite, which reversibly inhibits dihydroorotate dehydrogenase. This enzyme is essential for the de novo synthesis of uridine monophosphate (UMP), a precursor for pyrimidines.

Biochemical Target Cellular Consequence
Dihydroorotate Dehydrogenase Arrests the cell cycle in the G1 phase.
T-Cell Proliferation Suppressed; lymphocytes are unable to replicate their DNA without pyrimidines.

2. Clinical Use: Chronic Inflammatory Arthritis

Leflunomide is primarily utilized as a long-term treatment to slow the progression of joint destruction in autoimmune conditions. It is often used as an alternative for patients who do not tolerate Methotrexate.

Indication Clinical Goal
Rheumatoid Arthritis Reduces signs/symptoms and inhibits structural damage.
Psoriatic Arthritis Management of both skin manifestations and joint inflammation.

3. Adverse Effects and Toxicity

Leflunomide has a very long half-life (several weeks) due to enterohepatic circulation, making its adverse effects and teratogenicity particularly dangerous.

Adverse Effect Monitoring / Management
Hepatotoxicity Regular monitoring of LFTs is required.
Teratogenicity Strictly contraindicated in pregnancy. Requires a washout (Cholestyramine) if pregnancy is desired.
Hypertension Requires regular blood pressure checks.
Diarrhea Most common gastrointestinal side effect.

Activity:

High-Yield Mnemonics & Tips:

  • Mechanism: Leflunomide = Left-side of the pyrimidine pathway (it blocks an early step).
  • Washout: Because of the long half-life, patients who want to become pregnant must take Cholestyramine to clear the drug from their system quickly.
  • Step 1 Context: Compare this to Methotrexate (blocks dihydrofolate reductase) and Mycophenolate (blocks IMP dehydrogenase for purines). Leflunomide is the Pyrimidine equivalent.

Activity: