Mechanism of Action of Leflunomide
Leflunomide is an immunomodulatory agent that works primarily by inhibiting dihydroorotate dehydrogenase (DHODH), a key mitochondrial enzyme in the de novo pyrimidine synthesis pathway, thereby blocking lymphocyte proliferation. 1
Primary Mechanism: Inhibition of De Novo Pyrimidine Synthesis
Leflunomide functions as a prodrug that is rapidly metabolized to its active metabolite A77 1726 (M1), which is responsible for essentially all of its pharmacologic activity in vivo 1. This active metabolite inhibits DHODH at concentrations of approximately 600 nmol/L, which are readily achieved during therapeutic dosing 2, 3.
How This Affects Immune Cells
- Activated lymphocytes require an 8- to 16-fold increase in ribonucleotide levels to progress from G1 to S phase of the cell cycle 4
- These cells depend heavily on de novo pyrimidine synthesis (rather than salvage pathways) to meet the metabolic demands of clonal expansion and differentiation into effector cells 5
- By blocking DHODH, leflunomide prevents adequate production of uridine monophosphate (UMP), a critical pyrimidine precursor 6
- This triggers the p53 "sensor" protein, which arrests stimulated lymphocytes at the G1 phase, preventing their proliferation 2, 3, 4
Why Non-Lymphoid Cells Are Spared
The relative lack of toxicity to non-lymphoid cells occurs because these cells can fulfill their ribonucleotide requirements through the salvage pyrimidine pathway, making them less dependent on de novo synthesis 2, 3. This explains the drug's selective immunosuppressive effect.
Secondary Mechanism: Tyrosine Kinase Inhibition
A77 1726 also inhibits protein tyrosine kinases, though this occurs at concentrations 10- to 500-fold higher than those required for DHODH inhibition 4, 7. While initially considered secondary, some evidence suggests this mechanism may contribute to the drug's anti-inflammatory effects in vivo, particularly through reduction of tyrosine phosphorylated proteins in lymphoid tissues 7.
Clinical Implications
The mechanism translates to:
- Antiproliferative activity against rapidly dividing immune cells 1
- Anti-inflammatory effects demonstrated in multiple experimental models 1, 5
- Disease-modifying properties in rheumatoid arthritis, as evidenced by its ability to slow radiographic joint damage 6
Important Caveat
The drug's long half-life of approximately 2 weeks necessitates a loading dose (100 mg daily for 3 days) to achieve therapeutic levels rapidly; without this, steady-state concentrations would require nearly two months 1. This pharmacokinetic property is critical for both efficacy and safety considerations, particularly regarding the need for drug elimination procedures in cases of toxicity or pregnancy.