Leflunomide: A Disease-Modifying Antirheumatic Drug – Pharmacology, Clinical Use, and Safety Profile

Leflunomide is an oral disease-modifying antirheumatic drug (DMARD) approved for the treatment of rheumatoid arthritis (RA) and psoriatic arthritis. It was first introduced in the late 1990s and remains a valuable second-line agent for patients who have not responded adequately to methotrexate or other conventional synthetic DMARDs. This report provides a concise overview of leflunomide’s pharmacology, therapeutic indications, dosing, adverse effects, and important safety considerations.



Mechanism of Action

Leflunomide acts as an inhibitor of dihydroorotate dehydrogenase (DHODH), a key enzyme in the de novo synthesis of pyrimidine ribonucleotides. Rapidly dividing immune cells, particularly activated lymphocytes, depend on this pathway for proliferation. By blocking DHODH, leflunomide reduces pyrimidine pools, thereby suppressing T- and B-cell expansion and modulating the inflammatory response in autoimmune arthritis. The active metabolite, teriflunomide, is responsible for the drug’s therapeutic effects and has a long half-life of approximately 2 weeks, allowing once-daily dosing.



Pharmacokinetics

After oral administration, leflunomide is rapidly converted to its active form, teriflunomide, primarily in the liver and gastrointestinal mucosa. Teriflunomide is highly protein-bound (>99%) and undergoes enterohepatic recirculation, contributing to its prolonged elimination half-life. Without a washout procedure, it may take up to 2 years for plasma levels to become undetectable. This long half-life is clinically relevant for managing toxicity and for women of childbearing potential who require rapid drug elimination before pregnancy.



Clinical Use and Efficacy

Leflunomide is indicated for adults with active rheumatoid arthritis to reduce signs and symptoms, inhibit structural joint damage, and improve physical function. It is also approved for psoriatic arthritis. In clinical trials, leflunomide demonstrated efficacy comparable to methotrexate and sulfasalazine, with significant reductions in swollen and tender joint counts, C-reactive protein, and radiographic progression. It is often used as monotherapy or in combination with other DMARDs such as methotrexate, although combination therapy increases the risk of hepatotoxicity.



Dosing

The typical loading dose is 100 mg once daily for three days, followed by a maintenance dose of 20 mg once daily. The loading dose accelerates achievement of steady-state concentrations. For patients who cannot tolerate the 20 mg dose, a reduction to 10 mg daily may be attempted while monitoring for loss of efficacy. Dose adjustments are necessary in patients with mild hepatic impairment, but the drug is contraindicated in significant liver disease.



Adverse Effects

The most common adverse effects include diarrhea, nausea, alopecia, skin rash, and elevated liver enzymes. Diarrhea often resolves with continued use but may necessitate dose reduction. Hepatotoxicity is a serious concern; elevations in transaminases occur in up to 10% of patients, and rare cases of severe liver injury have been reported, particularly in patients with pre-existing liver disease or concomitant use of other hepatotoxic drugs. Monitoring of liver function tests monthly for the first six months and every 6–8 weeks thereafter is recommended.



Other notable adverse effects include hypertension, which may develop or worsen in some patients, necessitating regular blood pressure monitoring. Bone marrow suppression (leukopenia, thrombocytopenia) occurs infrequently, and complete blood counts should be performed periodically. Peripheral neuropathy, interstitial lung disease, and severe skin reactions (e.g., Stevens-Johnson syndrome) are rare but serious.



Drug Interactions

Leflunomide interacts with several drugs. Cholestyramine and activated charcoal are used to accelerate elimination in cases of overdose or toxicity. Rifampin increases the formation of teriflunomide, potentially enhancing toxicity. Methotrexate combination increases hepatotoxicity risk. Warfarin effects may be potentiated, requiring INR monitoring. Also, leflunomide may increase the levels of drugs metabolized by CYP2C8 and CYP1A2, although clinical significance varies.



Contraindications and Precautions

Leflunomide is contraindicated in pregnancy because of teratogenic effects observed in animal studies and human case reports. Women of childbearing potential must use effective contraception and undergo a drug elimination procedure (cholestyramine 8 g three times daily for 11 days) if planning pregnancy. Men attempting to father a child should also consider elimination due to potential exposure via seminal fluid. The drug is also contraindicated in severe hepatic impairment, severe immunodeficiency, and in patients with serious infections (e.g., tuberculosis, hepatitis B or C). Vaccinations with live attenuated vaccines should be avoided during therapy.



Special Populations

In elderly patients, no dose adjustment is necessary, but careful monitoring for liver toxicity and bone marrow suppression is warranted. For patients with mild renal impairment, no dose adjustment is needed, but data in moderate to severe renal impairment are limited. Leflunomide is excreted in human milk and is not recommended during breastfeeding.



Monitoring Recommendations

Before initiating leflunomide, baseline evaluation should include complete blood count, - https://farmaciacucchiara.it/, liver enzymes, serum creatinine, and blood pressure. Screening for latent tuberculosis and viral hepatitis is advised. During therapy, liver enzymes and complete blood counts are monitored every 4 weeks for 6 months, then every 8 weeks thereafter. Blood pressure should be checked regularly. If liver enzymes rise to more than 2–3 times the upper limit of normal, dose reduction or discontinuation and accelerated elimination should be considered.



Conclusion

Leflunomide is an effective and generally well-tolerated DMARD for rheumatoid arthritis and psoriatic arthritis. Its unique mechanism of action distinguishes it from other conventional synthetic DMARDs. However, its long half-life requires careful patient selection, vigilant monitoring for hepatic and hematologic toxicity, and thorough counseling regarding pregnancy avoidance. With appropriate use and adherence to safety protocols, leflunomide remains a useful therapeutic option for patients with inflammatory arthritis who have not responded to or cannot tolerate first-line therapies.