2.1 Recommended Dosage

The recommended dosage of leflunomide is 20 mg once daily. Treatment may be initiated with or without a loading dose, depending upon the patient’s risk of leflunomide-associated hepatotoxicity and leflunomide-associated myelosuppression. The loading dosage provides steady-state concentrations more rapidly.

·            For patients who are at low risk for leflunomide-associated hepatotoxicity and leflunomide-associated myelosuppression the recommended leflunomide loading dosage is 100 mg once daily for 3 days. Subsequently administer 20 mg once daily.

·            For patients at high risk for leflunomide-associated hepatotoxicity (e.g., those taking concomitant methotrexate) or leflunomide-associated myelosuppression (e.g., patients taking concomitant immunosuppressants), the recommended leflunomide dosage is 20 mg once daily without a loading dose [see Warnings and Precautions (5.2, 5.4)]. 

The maximum recommended daily dosage is 20 mg once per day. Consider dosage reduction to 10 mg once daily for patients who are not able to tolerate 20 mg daily (i.e., for patients who experience any adverse events listed in Table 1). 

Monitor patients carefully after dosage reduction and after stopping therapy with leflunomide, since the active metabolite of leflunomide, teriflunomide, is slowly eliminated from the plasma [see Clinical Pharmacology (12.3)]. After stopping leflunomide treatment, an accelerated drug elimination procedure is recommended to reduce the plasma concentrations of the active metabolite, teriflunomide [see Warnings and Precautions (5.3)]. Without use of an accelerated drug elimination procedure, it may take up to 2 years to reach undetectable plasma teriflunomide concentrations after stopping leflunomide[see Clinical Pharmacology (12.3)].

2.2 Evaluation and Testing Prior to Starting Leflunomide

Prior to starting leflunomide treatment the following evaluations and tests are recommended:

·           Evaluate patients for active tuberculosis and screen patients for latent tuberculosis infection [see Warnings and Precautions (5.4)]

·           Laboratory tests including serum alanine aminotransferase (ALT); and white blood cell, hemoglobin or hematocrit, and platelet counts [see Warnings and Precautions (5.2, 5.4)]

·           For females of reproductive potential, pregnancy testing [see Warnings and Precautions (5.1)]

·            Check blood pressure [see Warnings and Precautions (5.10)]

5.1 Embryo-Fetal Toxicity

Leflunomide may cause fetal harm when administered to a pregnant woman. Teratogenicity and embryo-lethality occurred in animal reproduction studies with leflunomide at doses lower than the human exposure level [see Use in Specific Populations (8.1)].

Leflunomide is contraindicated for use in pregnant women [see Contraindications (4)]. Exclude pregnancy before starting treatment with leflunomide in females of reproductive potential [see Dosage and Administration (2.2)]. Advise females of reproductive potential to use effective contraception during leflunomide treatment and during an accelerated drug elimination procedure after leflunomide treatment [see Use in Specific Populations (8.3)]. If a woman becomes pregnant while taking leflunomide, stop treatment with leflunomide, apprise the patient of the potential risk to a fetus, and perform an accelerated drug elimination procedure to achieve non-detectable plasma concentrations of teriflunomide, the active metabolite of leflunomide [see Warnings and Precautions (5.3)]. 

Upon discontinuing leflunomide, it is recommended that all females of reproductive potential undergo an accelerated drug elimination procedure. Women receiving leflunomide treatment who wish to become pregnant must discontinue leflunomide and undergo an accelerated drug elimination procedure, which includes verification that plasma concentrations of the active metabolite of leflunomide, teriflunomide, are less than 0.02 mg/L (0.02 mcg/mL). Based on animal data, human plasma concentrations of teriflunomide of less than 0.02 mg/L (0.02 mcg/mL) are expected to have minimal embryo-fetal risk [see Contraindications (4), Warnings and Precautions (5.3), and Use in Specific Populations (8.1)]. 

5.2 Hepatotoxicity

Severe liver injury, including fatal liver failure, has been reported in some patients treated with leflunomide. Patients with pre-existing acute or chronic liver disease, or those with serum alanine aminotransferase (ALT) of greater than twice the upper limits of normal (>2xULN) before initiating treatment, should not be treated with leflunomide. Use caution when leflunomide is given with other potentially hepatotoxic drugs. Monitoring of ALT levels is recommended at least monthly for six months after starting leflunomide, and thereafter every 6 to 8 weeks. If ALT elevation > 3 fold ULN occurs, interrupt leflunomide therapy and investigate the cause. If likely leflunomide-induced, perform the accelerated drug elimination procedure and monitor liver tests weekly until normalized [see Warnings and Precautions (5.3)]. If leflunomide-induced liver injury is unlikely because some other cause has been found, resumption of leflunomide therapy may be considered.

If leflunomide and methotrexate are given concomitantly, follow the American College of Rheumatology (ACR) guidelines for monitoring methotrexate liver toxicity with ALT, AST, and serum albumin testing.

5.3 Procedure for Accelerated Elimination of Leflunomide and its Active Metabolite

The active metabolite of leflunomide, teriflunomide, is eliminated slowly from the plasma [see Clinical Pharmacology (12.3)].

Use of an accelerated drug elimination procedure will rapidly reduce plasma concentrations of leflunomide and its active metabolite, teriflunomide. Therefore, an accelerated elimination procedure should be considered at any time after discontinuation of leflunomide, and in particular, when a patient has experienced a severe adverse reaction (e.g., hepatotoxicity, serious infection, bone marrow suppression, Steven Johnson Syndrome, toxic epidermal necrolysis, peripheral neuropathy, interstitial lung disease), suspected hypersensitivity, or has become pregnant. It is recommended that all women of childbearing potential undergo an accelerated elimination procedure after stopping leflunomide treatment.

Without use of an accelerated drug elimination procedure, it may take up to 2 years to reach plasma teriflunomide concentrations of less than 0.02 mg/L, the plasma concentration not associated with embryo-fetal toxicity in animals.

Elimination can be accelerated by the following procedures:

1)         Administer cholestyramine 8 grams orally 3 times daily for 11 days.

2)         Alternatively, administer 50 grams of activated charcoal powder (made into a suspension) orally every 12 hours for 11 days.

Verify plasma teriflunomide concentrations of less than 0.02 mg/L (0.02 mcg/mL) by two separate tests at least 14 days apart. If plasma teriflunomide concentrations are higher than 0.02 mg/L, repeat cholestyramine and/or activated charcoal treatment.

The duration of accelerated drug elimination treatment may be modified based on the clinical status and tolerability of the elimination procedure. The procedure may be repeated as needed, based on teriflunomide concentrations and clinical status.

Use of the accelerated drug elimination procedure may potentially result in return of disease activity if the patient had been responding to leflunomide treatment.

5.4 Immunosuppression, Bone Marrow Suppression, and Risk of Serious Infections

Leflunomide is not recommended for patients with severe immunodeficiency, bone marrow dysplasia, or severe, uncontrolled infections. If a serious infection occurs, consider interrupting leflunomide therapy and initiating the accelerated drug elimination procedure [see Warnings and Precautions (5.3)]. Medications like leflunomide that have immunosuppression potential may cause patients to be more susceptible to infections, including opportunistic infections, especially Pneumocystis jiroveci pneumonia, tuberculosis (including extra-pulmonary tuberculosis), and aspergillosis. Severe infections including sepsis, which may be fatal, have been reported in patients receiving leflunomide, especially Pneumocystis jiroveci pneumonia and aspergillosis. Most of the reports were confounded by concomitant immunosuppressant therapy and/or comorbid illness which, in addition to rheumatoid arthritis, may predispose patients to infection.

Cases of tuberculosis were observed in clinical studies with teriflunomide, the metabolite of leflunomide. Prior to initiating leflunomide, all patients should be screened for active and inactive (“latent”) tuberculosis infection as per commonly used diagnostic tests. Leflunomide has not been studied in patients with a positive tuberculosis screen, and the safety of leflunomide in individuals with latent tuberculosis infection is unknown. Patients testing positive in tuberculosis screening should be treated by standard medical practice prior to therapy with leflunomide and monitored carefully during leflunomide treatment for possible reactivation of the infection.

Pancytopenia, agranulocytosis and thrombocytopenia have been reported in patients receiving leflunomide alone. These events have been reported most frequently in patients who received concomitant treatment with methotrexate or other immunosuppressive agents, or who had recently discontinued these therapies; in some cases, patients had a prior history of a significant hematologic abnormality.

Patients taking leflunomide should have platelet, white blood cell count and hemoglobin or hematocrit monitored at baseline and monthly for six months following initiation of therapy and every 6- to 8 weeks thereafter. If used with concomitant methotrexate and/or other potential immunosuppressive agents, chronic monitoring should be monthly. If evidence of bone marrow suppression occurs in a patient taking leflunomide, stop treatment with leflunomide, and perform an accelerated drug elimination procedure to reduce the plasma concentration of the leflunomide active metabolite, teriflunomide [see Warnings and Precautions (5.3)].

In any situation in which the decision is made to switch from leflunomide to another anti-rheumatic agent with a known potential for hematologic suppression, it would be prudent to monitor for hematologic toxicity, because there will be overlap of systemic exposure to both compounds.

5.5 Stevens-Johnson Syndrome, Toxic Epidermal Necrolysis, and Drug Reactions with Eosinophilia and Systemic Symptoms

Rare cases of Stevens-Johnson syndrome and toxic epidermal necrolysis, and drug reaction with eosinophilia and systemic symptoms (DRESS) have been reported in patients receiving leflunomide. If a patient taking leflunomide develops any of these conditions, stop leflunomide treatment and perform an accelerated drug elimination procedure [see Warnings and Precautions (5.3)].

5.6 Malignancy and Lymphoproliferative Disorders

The risk of malignancy, particularly lymphoproliferative disorders, is increased with the use of some immunosuppression medications. There is a potential for immunosuppression with leflunomide. No apparent increase in the incidence of malignancies and lymphoproliferative disorders was reported in the clinical trials of leflunomide, but larger dosages and longer-term studies would be needed to determine whether there is an increased risk of malignancy or lymphoproliferative disorders with leflunomide.

5.7 Peripheral Neuropathy

Cases of peripheral neuropathy have been reported in patients receiving leflunomide and in clinical studies with teriflunomide, the active metabolite of leflunomide. Most patients recovered after discontinuation of treatment, but some patients had persistent symptoms. Age older than 60 years, concomitant neurotoxic medications, and diabetes may increase the risk for peripheral neuropathy. If a patient taking leflunomide develops a peripheral neuropathy, consider discontinuing leflunomide therapy and performing an accelerated drug elimination procedure [see Dosage and Administration (5.3)]. 

5.8 Interstitial Lung Disease

Interstitial lung disease and worsening of pre-existing interstitial lung disease have been reported during treatment with leflunomide and has been associated with fatal outcomes [see Adverse Reactions (6.2)]. The risk of leflunomide-associated interstitial lung disease is increased in patients with a history of interstitial lung disease. Interstitial lung disease is a potentially fatal disorder that may occur acutely at any time during therapy and has a variable clinical presentation. New onset or worsening pulmonary symptoms, such as cough and dyspnea, with or without associated fever, may be a reason for discontinuation of leflunomide therapy and for further investigation as appropriate. If discontinuation of leflunomide is necessary, consider performing an accelerated drug elimination procedure [see Warnings and Precautions (5.3)]. 

5.9 Vaccinations

No clinical data are available on the efficacy and safety of vaccinations during leflunomide treatment. Vaccination with live vaccines is, however, not recommended. The long half-life of the active metabolite of leflunomide should be considered when contemplating administration of a live vaccine after stopping leflunomide.

5.10 Blood Pressure Monitoring

In placebo-controlled studies with the active metabolite of leflunomide, teriflunomide, elevations in blood pressure were observed in some subjects. Blood pressure should be checked before starting treatment with leflunomide and monitored periodically thereafter [see Adverse Reactions (6.1)].

6.1 Clinical Trials Experience

Because clinical studies are conducted under widely varying conditions, adverse reaction rates observed in the clinical studies of a drug cannot be directly compared to rates in the clinical studies of another drug and may not reflect the rates observed in practice.

In clinical studies (Trials 1, 2, and 3), 1,865 patients were treated with leflunomide administered as either monotherapy or in combination with methotrexate or sulfasalazine. Patients ranged in age from 19 to 85 years, with an overall median age of 58 years. The mean duration of RA was 6 years ranging from 0 to 45 years.

Elevation of Liver Enzymes

Treatment with leflunomide was associated with elevations of liver enzymes, primarily ALT and AST, in a significant number of patients; these effects were generally reversible. Most transaminase elevations were mild (≤ 2-fold ULN) and usually resolved while continuing treatment. Marked elevations (>3-fold ULN) occurred infrequently and reversed with dose reduction or discontinuation of treatment. Table 1 shows liver enzyme elevations seen with monthly monitoring in clinical trials Trial 1 and Trial 2. It was notable that the absence of folate use in Trial 3 was associated with a considerably greater incidence of liver enzyme elevation on methotrexate.

Table 1. Liver Enzyme Elevations >3-fold Upper Limits of Normal (ULN) in Patients with RA in Trials 1, 2, and 3*

MTX = methotrexate,PL = placebo,SSZ = sulfasalazine, ULN = Upper limitofnormal

*Only10% ofpatients in Trial 3  receivedfolate.All patients in Trial 1 receivedfolate.

In a 6 month study of 263 patients with persistent active rheumatoid arthritis despite methotrexate therapy, and with normal LFTs, leflunomide was administered to a group of 130 patients starting at 10 mg per day and increased to 20 mg as needed. An increase in ALT greater than or equal to three times the ULN was observed in 3.8% of patients compared to 0.8% in 133 patients continued on methotrexate with placebo.

Most Common Adverse Reactions

The most common adverse reactions in leflunomide-treated patients with RA include diarrhea, elevated liver enzymes (ALT and AST), alopecia and rash. Table 2 displays the most common adverse reactions in the controlled studies in patients with RA at one year (≥ 5% in any leflunomide treatment group).

Table 2. Percentage Of Patients With Adverse Events ³ 5% In Any LeflunomideTreated Group in all RA Studies in Patients with RA

MTX = methotrexate, PL = placebo, SSZ = sulfasalazine

1Only 10% of patients in Trial3 received folate. All patients in Trial 1 received folate; none in Trial 2 received folate.

2 Includes all controlled and uncontrolled trials with leflunomide (duration up to 12 months).

3Hypertension as a preexisting condition was overrepresented in all leflunomide treatment groups in phase III trials.

Adverse events during a second year of treatment with leflunomide in clinical trials were consistent with those observed during the first year of treatment and occurred at a similar or lower incidence.

Less Common Adverse Reactions

In addition, in controlled clinical trials, the following adverse events in the leflunomide treatment group occurred at a higher incidence than in the placebo group. These adverse events were deemed possibly related to the study drug.

Blood and Lymphatic System: leukocytosis, thrombocytopenia;

Cardiovascular: chest pain, palpitation, thrombophlebitis of the leg, varicose vein;

Eye: blurred vision, eye disorder, papilledema, retinal disorder, retinal hemorrhage;

Gastrointestinal: alkaline phosphatase increased, anorexia, bilirubinemia, flatulence, gamma-GT increased, salivary gland enlarged, sore throat, vomiting, dry mouth;

General Disorders: malaise;

Immune System: anaphylactic reaction; Infection: abscess, flu syndrome, vaginal moniliasis;

Nervous System: dizziness, headache, somnolence; Respiratory System: dyspnea;

6.2 Post Marketing Experience

The following additional adverse reactions have been identified during postapproval use of leflunomide. Because these reactions are reported voluntarily from a population of uncertain size, it is not always possible to reliably estimate their frequency or establish a causal relationship to drug exposure.

Blood and Lymphatic System: agranulocytosis, leukopenia, neutropenia, pancytopenia;

Infection: opportunistic infections, severe infections including sepsis;

Gastrointestinal: acute hepatic necrosis, colitis, including microscopic colitis,  hepatitis, jaundice/cholestasis, pancreatitis; severe liver injury such as hepatic failure

Immune System: angioedema;

Nervous System: peripheral neuropathy;

Respiratory: interstitial lung disease, including interstitial pneumonitis and pulmonary fibrosis, which may be fatal; pulmonary hypertension;

Skin and Appendages: erythema multiforme, Stevens-Johnson syndrome, toxic epidermal necrolysis, vasculitis including cutaneous necrotizing vasculitis, cutaneous lupus erythematosus, pustular psoriasis or worsening psoriasis.

8.1 Pregnancy

Pregnancy Exposure Registry

There is a pregnancy exposure registry that monitors pregnancy outcomes in women exposed to leflunomide during pregnancy. Health care providers and patients are encouraged to report pregnancies by calling 1-877-311-8972 or visit http://www.pregnancystudies.org/participate-ina-study/.

Risk Summary

Leflunomide is contraindicated for use in pregnant women because of the potential for fetal harm. In animal reproduction studies, oral administration of leflunomide during organogenesis at a dose of 1/10 of and equivalent to the maximum recommended human dose (MRHD) based on AUC, respectively in rats and rabbits, caused teratogenicity (rats and rabbits) and embryo-lethality (rats) [see Data]. Pregnancy exposure registry data are not available at this time to inform the presence or absence of drug-associated risk with the use of leflunomide during pregnancy. The background risk of major birth defects and miscarriage for the indicated populations is unknown. The background risk in the U.S. general population of major birth defects is 2 to 4% and of miscarriage is 15 to 20% of clinically recognized pregnancies. If this drug is used during pregnancy, or if the patient becomes pregnant while taking this drug, stop treatment with leflunomide, apprise the patient of the potential hazard to a fetus, and perform the accelerated drug elimination procedure to achieve teriflunomide concentrations of less than 0.02 mg/L (0.02 mcg/mL) [see Warnings and Precautions (5.3)]. 

Clinical Considerations

Fetal/Neonatal adverse reactions  

Lowering the plasma concentration of the active metabolite, teriflunomide, by instituting an accelerated drug elimination procedure as soon as pregnancy is detected may decrease the risk to the fetus from leflunomide. The accelerated drug elimination procedure includes verification that the plasma teriflunomide concentration is less than 0.02 mg/L [see Warnings and Precautions (5.3) and Clinical Pharmacology (12.3)].

Data  

Animal Data  

In an embryofetal development study, pregnant rats administered leflunomide during organogenesis from gestation days 7 to 19 at a dose approximately 1/10 of the MRHD (on an AUC basis at a maternal oral dose of 15 mg/kg), teratogenic effects, most notably anophthalmia or microophthalmia and internal hydrocephalus, were observed. Under these exposure conditions, leflunomide also caused a decrease in the maternal body weight and an increase in embryolethality with a decrease in fetal body weight for surviving fetuses. In an embryofetal development study, pregnant rabbits administered leflunomide during organogenesis from gestation days 6 to 18 at a dose approximately equivalent to the MRHD (on an AUC basis at a maternal oral dose of 10 mg/kg), a teratogenic finding of fused, dysplastic sternebrae was observed. Leflunomide was not teratogenic in rats and rabbits at doses approximately 1/150 and 1/10 of the MRHD, respectively (on an AUC basis at maternal oral dose of 1 mg/kg in both rats and rabbits).

In a pre- and post-natal development study, when female rats were treated leflunomide at a dose that was approximately 1/100 of the MRHD (on an AUC basis at a maternal dose of 1.25 mg/kg) beginning 14 days before mating and continuing until the end of lactation, the offspring exhibited marked (greater than 90%) decreases in postnatal survival.

8.2 Lactation

Risk Summary

Clinical lactation studies have not been conducted to assess the presence of leflunomide in human milk, the effects of leflunomide on the breastfed child, or the effects of leflunomide on milk production. Because of the potential for serious adverse reactions in a breastfed infant from leflunomide, advise a nursing woman to discontinue breastfeeding during treatment with leflunomide.

8.3 Females and Males of Reproductive Potential

Leflunomide may cause fetal harm when administered during pregnancy. Advise females of the potential risk to the fetus. Advise females to notify their healthcare provider immediately if pregnancy occurs or is suspected during treatment [see Use in Specific Populations (8.1)]. Women receiving leflunomide treatment who wish to become pregnant should discontinue leflunomide and undergo an accelerated drug elimination procedure to achieve plasma teriflunomide concentrations of less than 0.02 mg/L (0.02 mcg/mL) [see Warnings and Precautions (5.3)].

Pregnancy Testing

Exclude pregnancy in females of reproductive potential before starting treatment with leflunomide.

Contraception  

Females  

Advise females of reproductive potential to use effective contraception during treatment with leflunomide and while undergoing a drug elimination procedure until verification that the plasma teriflunomide concentration is less than 0.02 mg/L [see Warnings and Precautions (5.3)].

8.4 Pediatric Use

The safety and effectiveness of leflunomide in pediatric patients have not been established.

The safety and effectiveness of leflunomide in the treatment of polyarticular course juvenile idiopathic arthritis (JIA) was evaluated in a single multicenter, double-blind, active-controlled trial in 94 pediatric patients (1:1 randomization) with polyarticular course juvenile idiopathic arthritis (JIA) as defined by the American College of Rheumatology (ACR). In this population, leflunomide treatment was found not to be effective.

The safety of leflunomide was studied in 74 patients with polyarticular course JIA ranging in age from 3 to 17 years (47 patients from the active-controlled study and 27 from an open-label safety and pharmacokinetic study). The most common adverse events included abdominal pain, diarrhea, nausea, vomiting, oral ulcers, upper respiratory tract infections, alopecia, rash, headache, and dizziness. Less common adverse events included anemia, hypertension, and weight loss. Fourteen pediatric patients experienced ALT and/or AST elevations, nine between 1.2 and 3-fold the upper limit of normal, five between 3 and 8-fold the upper limit of normal.

8.5 Geriatric Use

 Of the total number of subjects in controlled clinical trials (Trials 1, 2, and 3) of leflunomide, 234 subjects were 65 years and over [see Clinical Studies (14)]. No overall differences in safety or effectiveness were observed between these subjects and younger subjects, and other reported clinical experience has not identified differences in responses between the elderly and younger patients, but greater sensitivity of some older individuals cannot be ruled out. No dosage adjustment is needed in patients over 65.

8.6 Hepatic Impairment

Dedicated studies of the effect of hepatic impairment on leflunomide pharmacokinetics have not been conducted. Given the need to metabolize leflunomide into the active species, the role of the liver in drug elimination/recycling, and the possible risk of increased hepatic toxicity, the use of leflunomide in patients with hepatic impairment is not recommended.

8.7 Renal Impairment

Dedicated studies of the effect of renal impairment on leflunomide pharmacokinetics have not been conducted. Given that the kidney plays an important role in drug elimination, caution should be used when leflunomide is administered to these patients.

12.1 Mechanism of Action

Leflunomide is an isoxazole immunomodulatory agent that inhibits dihydroorotate dehydrogenase (a mitochondrial enzyme involved in de novo pyrimidine synthesis) and has antiproliferative activity. Several in vivo and in vitro experimental models have demonstrated an anti-inflammatory effect.

12.3 Pharmacokinetics

Following oral administration, leflunomide is metabolized to an active metabolite, teriflunomide, which is responsible for essentially all of leflunomide’s in vivo activity. Plasma concentrations of the parent drug, leflunomide, have been occasionally seen at very low concentrations. Studies of the pharmacokinetics of leflunomide have primarily examined the plasma concentrations of the active metabolite, teriflunomide.

 Absorption 

Following oral administration, peak teriflunomide concentrations occurred between 6 to 12 hours after dosing. Due to the very long half-life of teriflunomide (18 to 19 days), a loading dose of 100 mg for 3 days was used in clinical studies to facilitate the rapid attainment of steady-state teriflunomide concentrations. Without a loading dose, it is estimated that attainment of steady-state plasma concentrations would require about two months of dosing. The resulting plasma concentrations following both loading doses and continued clinical dosing indicate that plasma teriflunomide concentrations are dose proportional. 

 Effect of Food 

            Co-administration of leflunomide tablets with a high fat meal did not have a  significant impact on teriflunomide plasma concentrations. 

Distribution  

Teriflunomide is extensively bound to plasma protein (>99%) and is mainly distributed in plasma. The volume of distribution is 11 L after a single intravenous (IV) administration. 

Elimination  

Teriflunomide, the active metabolite of leflunomide, has a median half-life of 18 to 19 days in healthy volunteers. The elimination of teriflunomide can be accelerated by administration of cholestyramine or activated charcoal. Without use of an accelerated drug elimination procedure, it may take up to 2 years to reach plasma teriflunomide concentrations of less than 0.02 mg/L, due to individual variation in drug clearance [see Warnings and Precautions (5.3)]. After a single IV administration of the metabolite (teriflunomide), the total body clearance of teriflunomide was 30.5 mL/h.  

            Metabolism

            In vitro inhibition studies in human liver microsomes suggest that cytochrome P450   (CYP) 1A2, 2C19 and 3A4 are involved in leflunomide metabolism. In vivo,    leflunomide is metabolized to one primary (teriflunomide) and many minor metabolites. In vitro, teriflunomide is not metabolized by CYP450 or flavin monoamine oxidase enzymes. The parent compound is rarely detectable in plasma.         

            Excretion  

            Teriflunomide, the active metabolite of leflunomide, is eliminated by direct biliary excretion of unchanged drug as well as renal excretion of metabolites. Over 21 days,     60.1% of the administered dose is excreted via feces (37.5%) and urine (22.6%).  After an accelerated elimination procedure with cholestyramine, an additional 23.1% was recovered (mostly in feces).          

             Studies with both hemodialysis and CAPD (chronic ambulatory peritoneal dialysis)indicate that teriflunomide is not dialyzable.  

Specific Populations 

            Gender. Gender has not been shown to cause a consistent change in the in vivo pharmacokinetics of teriflunomide.  

            Smoking. A population based pharmacokinetic analysis of the clinical trial data indicates that smokers have a 38% increase in clearance over non-smokers; however,  no difference in clinical efficacy was seen between smokers and nonsmokers.  

Drug Interaction Studies 

Drug interaction studies have been conducted with both leflunomide and with its active metabolite, teriflunomide, where the metabolite was directly administered to the test subjects. 

The Potential Effect of Other Drugs on Leflunomide 

·         Potent CYP and transporter inducers:  

Following concomitant administration of a single dose of leflunomide to subjects receiving multiple doses of rifampin, teriflunomide peak concentrations were increased (~40%) over those seen when leflunomide was given alone [see Drug Interactions (7)].  

·         An in vivo  interaction study with leflunomide and cimetidine (non-specific weak CYP inhibitor) has demonstrated a lack of a significant impact on teriflunomide exposure. 

The Potential Effect of Leflunomide   on Other Drugs

·         CYP2C8 Substrates 

There was an increase in mean repaglinide Cmax and AUC (1.7-and 2.4-fold, respectively), following repeated doses of teriflunomide and a single dose of 0.25 mg repaglinide, suggesting that teriflunomide is an inhibitor of CYP2C8 in vivo. The magnitude of interaction could be higher at the recommended repaglinide dose [see Drug Interactions (7)].  

·         CYP1A2 Substrates 

Repeated doses of teriflunomide decreased mean Cmax and AUC of caffeine by 18% and 55%, respectively, suggesting that teriflunomide may be a weak inducer of CYP1A2 in vivo. 

·         OAT3 Substrates  

There was an increase in mean cefaclor Cmax and AUC (1.43-and 1.54-fold, respectively), following repeated doses of teriflunomide, suggesting that teriflunomide is an inhibitor of organic anion transporter 3 (OAT3) in vivo[see Drug Interactions (7)]. 

·         BCRP and OATP1B1/1B3 Substrates  

There was an increase in mean rosuvastatin Cmax and AUC (2.65-and 2.51-fold, respectively), following repeated doses of teriflunomide, suggesting that teriflunomide is an inhibitor of BCRP transporter and organic anion transporting polypeptide 1B1 and 1B3 (OATP1B1/1B3) [see Drug Interactions (7)]. 

·         Oral Contraceptives  

There was an increase in mean ethinylestradiol Cmax and AUC0-24 (1.58-and 1.54-fold, respectively) and levonorgestrel Cmax and AUC0-24 (1.33-and 1.41-fold, respectively) following repeated doses of teriflunomide [see Drug Interactions (7)]. 
. Teriflunomide did not affect the pharmacokinetics of bupropion (a CYP2B6 substrate), midazolam (a CYP3A4 substrate), S-warfarin (a CYP2C9 substrate), omeprazole (a CYP2C19 substrate), and metoprolol (a CYP2D6 substrate).

13.1 Carcinogenesis, Mutagenesis, Impairment of Fertility

No evidence of carcinogenicity was observed in a 2-year bioassay in rats at oral doses of leflunomide up to the maximally tolerated dose of 6 mg/kg (approximately 1/40 the maximum human teriflunomide systemic exposure based on AUC). However, male mice in a 2-year bioassay exhibited an increased incidence in lymphoma at an oral dose of 15 mg/kg, the highest dose studied (1.7 times the human teriflunomide exposure based on AUC). Female mice, in the same study, exhibited a dose-related increased incidence of bronchoalveolar adenomas and carcinomas combined beginning at 1.5 mg/kg (approximately 1/10 the human teriflunomide exposure based on AUC). The significance of the findings in mice relative to the clinical use of leflunomide is not known.

Leflunomide was not mutagenic in the Ames assay, the unscheduled DNA synthesis assay, or in the HGPRT gene mutation assay. In addition, leflunomide was not clastogenic in the  in  vivo mouse micronucleus assay or in the in vivo Chinese hamster bone marrow cell cytogenic test. However, 4-trifluoromethylaniline (TFMA), a minor metabolite of leflunomide, was mutagenic in the Ames assay and in the HGPRT gene mutation assay, and was clastogenic in the in vitro Chinese hamster cell chromosomal aberration assay. TFMA was not clastogenic in the in vivo mouse micronucleus assay or in the in vivo Chinese hamster bone marrow cell cytogenic test.

Leflunomide had no effect on fertility or reproductive performance in either male or female rats at oral doses up to 4 mg/kg (approximately 1/30 the human teriflunomide exposure based on AUC) [see Use in Specific Populations (8.1, 8.6)].