Leflunomide and malononitriloamides

Teriflunomide for multiple sclerosis in real-world setting

OBJECTIVES

Teriflunomide 14 mg is a once-daily oral disease-modifying treatment for relapsing-remitting multiple sclerosis. We examined adverse event (AE) profile and efficacy in real life.

MATERIALS AND METHODS

In this observational cohort study, we retrospectively examined 1521 blood samples and data of 102 patients followed for up to 28 months.

RESULTS

The number of female patients starting teriflunomide peaked in the fifth decade, 10 years later compared to male patients (P<.001), reflecting pregnancy concerns. Seventy-six percentages of patients shifted to teriflunomide from treatment with interferon-beta. Expanded disability status scale improved in 11% of patients (18.2±3.6 months follow-up) and remained constant in 67.5% (15±5.3 months follow-up). Of ten relapses, three occurred within 6 months after starting treatment. Seventeen patients (16.5%) discontinued teriflunomide: 53% because of AEs and 29% because of relapse. Levels of alanine aminotransferase (ALT) remained normal in 95.3% of the blood samples and remained below 1.5 times the upper limit of normal in 91% of the 4.7% abnormal samples. One-third of the patients had abnormal ALT values at least once. Haematological abnormalities were found in <4% of the blood samples, but at least one abnormal value was observed in up to 21% of the patients.

CONCLUSIONS

Efficacy and safety of teriflunomide in real-life setting support data obtained by the pivotal trials. Laboratory abnormalities are rare among the large number of samples, but patients may commonly have a single mild, abnormal value if frequently tested.

Changes in Body Mass Related to the Initiation of Disease-Modifying Therapies in Rheumatoid Arthritis

OBJECTIVE

Unintentional weight loss is important and can be predictive of long-term outcomes in patients with rheumatoid arthritis (RA). This study was undertaken to assess how primary therapies for RA may influence changes in body mass index (BMI) in RA patients from a large administrative database.

METHODS

Unique dispensing episodes of methotrexate, prednisone, leflunomide, and tumor necrosis factor inhibitors (TNFi) administered to RA patients were identified from the US Department of Veterans Affairs pharmacy databases. Values for C-reactive protein (CRP) level and BMI closest to the time point within 30 days of the treatment course start date and at follow-up time points were linked. Missing laboratory values were imputed. Weight loss was defined as a decrease in BMI of >1 kg/m(2) . Regression models were used to evaluate changes in BMI during each drug treatment as compared to treatment with methotrexate. To assess the impact of confounding by indication, propensity scores for use of each drug were incorporated in analyses using matched-weighting techniques.

RESULTS

In total, 52,662 treatment courses in 32,859 RA patients were identified. At 6 months from the date of prescription fill, weight gain was seen among patients taking methotrexate, those taking prednisone, and those taking TNFi. On average, compared to methotrexate-treated patients, prednisone-treated patients had significantly more weight gain, while leflunomide-treated patients demonstrated weight loss. In multivariable models, more weight loss (β = -0.41 kg/m(2) , 95% confidence interval [95% CI] -0.46, -0.36; P < 0.001) and a greater risk of weight loss (odds ratio 1.73, 95% CI 1.55, 1.79; P < 0.001) were evident among those receiving leflunomide compared to those receiving methotrexate. Treatment with prednisone was associated with greater weight gain (β = 0.072 kg/m(2) , 95% CI 0.042, 0.10; P < 0.001). These associations persisted in analyses adjusted for propensity scores and in sensitivity analyses.

CONCLUSION

Leflunomide is associated with significantly more, but modest, weight loss, while prednisone is associated with greater weight gain compared to other therapies for RA.

Influence of 5-amino-3-methyl-4-isoxazolecarbohydrazide on selective gene expression in Caco-2 cultured cells

The 5-amino-3-methyl-4-isoxazolecarboxylic acid hydrazide (HIX) is a synthetic isoxazole derivative with a potential for development as an anti-inflammatory drug candidate. The goal of this study was to explore in vitro autoimmune and inflammatory gene modulation by HIX in human Caco-2 cultured cells. The effect of low dose of HIX was tested on the expression level of RNA in 24 h Caco-2 cultures using the QIAGEN Th17 for Autoimmunity & Inflammation RT² Profiler PCR Array. We choose the PCR technology as the most reliable and sensitive gene expression profiling method for analyzing specific gene regulatory networks. In all experiments, Leflunomide (5-methyl-N-[4-(trifluoromethyl)phenyl]-4-isoxazolecarboxamide), an immuno-suppressive disease-modifying antirheumatic drug was used, as a reference to clinical utility of the isoxazole derivatives. Changes in RNA levels were analyzed and differentially expressed genes with at least 2-fold change were identified. For the majority of genes tested, the effects of HIX and Leflunomide were similar, including up-regulation of CX3CL1 and IL-17F, and down-regulation of IL-10 and TLR4. However twelve genes were were differently regulated by the two compounds: interleukins (IL) IL-1B, IL-6 and a chemokine CCL22 were upregulated by HIX and significantly supressed by Leflunomide. In contrary, IL-2 and IL-27 were upregulated by Leflunomide and suppressed by HIX. The network search by Ingenuity Pathway Analysis showed, that majority of differentially expressed genes were involved in cellular inflammatory responses. These results suggest that 5-amino-3-methyl-4-isoxazolecarbohydrazide has a potential for future clinical developments with structure modification as a disease modifying agent in different than Leflunomide applications.

Efficacy of leflunomide for treatment of refractory inflammatory colorectal polyps in 15 Miniature Dachshunds

Inflammatory colorectal polyp (ICRP), common in miniature dachshunds, presents with hematochezia, tenesmus and mucoid feces. Although an 80% response rate has been reported when treated with prednisolone and cyclosporine, effective treatment is needed for the remaining 20% of ICRP dogs. Leflunomide is an immunosuppressive drug reported as effective in several immune-mediated diseases. In the present study, we retrospectively evaluated the efficacy and adverse effects of leflunomide in 15 ICRP dogs that were refractory to treatment with prednisolone and cyclosporine. Treatment efficacy was assessed by endoscopy, clinical symptoms and rectal palpation. Adverse effects were determined by clinical symptoms and blood testing during follow-up. The leflunomide treatment response rate was 93.3%. The median dosage of leflunomide and the median response time were 3 mg/kg (range: 1.7–4.0 mg/kg) and 35 days (range: 20–119 days), respectively. Adverse effects observed included lethargy (3 dogs), anorexia (1 dog), respiratory symptoms (1 dog), leukocytopenia (2 dogs), thrombocytopenia (1 dog), anemia (1 dog) and liver enzyme elevation (8 dogs). Most of the adverse effects improved with symptomatic treatment and leflunomide discontinuation or dosage reduction. In conclusion, leflunomide treatment is effective in ICRP dogs refractory to treatment with prednisolone and cyclosporine. Because several adverse effects were observed, close monitoring is needed during leflunomide treatment follow-up.

Systemic treatments for noninfectious vitreous inflammation

Vitreous inflammation, or vitritis, may result from many causes, including both infectious and noninfectious, including rheumatologic and autoimmune processes. Vitritis is commonly vision threatening and has serious sequelae. Treatment is frequently challenging, but, today, there are multiple methods of systemic treatment for vitritis. These categories include corticosteroids, antimetabolites, alkylating agents, T-cell inhibitors/calcineurin inhibitors, and biologic agents. These treatment categories were reviewed last year, but, even over the course of just a year, many therapies have made progress, as we have learned more about their indications and efficacy. We discuss here discoveries made over the past year on both existing and new drugs, as well as reviewing mechanisms of action, clinical dosages, specific conditions that are treated, adverse effects, and usual course of treatment for each class of therapy.

Teriflunomide versus subcutaneous interferon beta-1a in patients with relapsing multiple sclerosis: a randomised, controlled phase 3 trial

BACKGROUND

In previous studies, teriflunomide significantly reduced the annualised relapse rate (ARR) and disability progression.

OBJECTIVE

This phase 3, rater-blinded study (NCT00883337) compared teriflunomide with interferon-beta-1a (IFNβ-1a).

METHODS

Patients with relapsing multiple sclerosis were randomised (1:1:1) to oral teriflunomide 7-or 14 mg, or subcutaneous IFNβ-1a 44 µg. The primary composite endpoint was time to failure, defined as first occurrence of confirmed relapse or permanent treatment discontinuation for any cause. Secondary endpoints included ARR, Fatigue Impact Scale (FIS) and Treatment Satisfaction Questionnaire for Medication (TSQM). The study was completed 48 weeks after the last patient was randomised.

RESULTS

Some 324 patients were randomised (IFNβ-1a: 104; teriflunomide 7 mg: 109; teriflunomide 14 mg: 111). No difference in time to failure was observed. There was no difference in ARR between teriflunomide 14 mg and IFNβ-1a, but ARR was significantly higher with teriflunomide 7 mg. FIS scores indicated more frequent fatigue with IFNβ-1a, though differences were only significant with teriflunomide 7 mg. TSQM scores were significantly higher with teriflunomide. There were no unexpected safety findings.

CONCLUSION

Effects on time to failure were comparable between teriflunomide and IFNβ-1a. There was no difference between teriflunomide 14 mg and IFNβ-1a on ARR, though ARR was higher with teriflunomide 7 mg. The teriflunomide safety profile was consistent with previous studies.

Teriflunomide efficacy, safety and tolerability in patients with relapsing forms of multiple sclerosis

SUMMARY Teriflunomide is a once-daily, orally administered, disease-modifying therapy for the treatment of relapsing forms of multiple sclerosis. The clinical development program for teriflunomide is extensive, including four Phase III studies. Pivotal trials have demonstrated the effectiveness of teriflunomide 7- and 14-mg doses on relapse, disability progression and MRI activity, and long-term follow-up has revealed no new or unexpected safety signals. The multiple sclerosis treatment landscape is shifting; oral medications offer improved convenience and tolerability over disease-modifying therapy injections, and patients are being treated earlier in the disease course to prevent progression. Clinical knowledge to date indicates that teriflunomide is a viable treatment option to meet individual patient needs. Additional insight is expected in the postmarket setting.

Systemic treatment of vitreous inflammation

Non infectious vitreous inflammation is often vision threatening and can be associated with potentially life-threatening systemic conditions. Treatment is often challenging as it involves systemic medications that can be associated with adverse effects. The classes of drugs are ever expanding and include corticosteroids, antimetabolites, alkylating agents, T-cell and calcineurin agents, biologic agents, and interferons. Each class of systemic therapy for non-infectious vitreous inflammation is reviewed. We discuss the mechanisms of action, usual clinical dosages, the specific conditions that are treated, the adverse effects, and usual course of treatment for each class of therapy.

Leflunomide: a drug with a potential beyond rheumatology

Leflunomide, an inhibitor of the dihydroorotase dehydrogenase and thereby pyrimidine synthesis, was introduced and licensed for the treatment of rheumatoid arthritis in 1998. In the following years, its antiviral properties were discovered and the drug was used in solid organ transplantation for polyomavirus type BK or cytomegalovirus infection. Owing to its long half-life and weak interaction with the cytochrome system, special considerations apply in the use of this drug. This article summarizes the clinical experience with leflunomide in rheumatology and in the evolving field of transplantation.

Efficacy of leflunomide for treatment of immune-mediated polyarthritis in dogs: 14 cases (2006-2008)

OBJECTIVE

To evaluate efficacy and adverse effects of leflunomide for the treatment of naturally occurring immune-mediated polyarthritis (IMPA) in dogs.

DESIGN

Retrospective case series.

ANIMALS

14 dogs with cytologically confirmed IMPA.

PROCEDURES

Medical records were used to identify dogs with a diagnosis of IMPA that were treated with leflunomide. Signalment, radiographic findings, laboratory data, dosage of leflunomide, treatment duration, treatment response, and occurrence of adverse effects were determined from medical records.

RESULTS

Mean +/- SD initial dosage of leflunomide was 3.0 +/- 0.5 mg/kg (1.4 +/- 0.2 mg/lb) PO once daily. Treatment duration for the initial starting dosage ranged from 1 to 6 weeks. Of the 14 dogs treated with leflunomide, 8 had complete resolution of clinical signs of IMPA initially, 5 had partial response to treatment, and 1 had minimal response to treatment. Adverse effects from treatment with leflunomide were not observed during the treatment period.

CONCLUSIONS AND CLINICAL RELEVANCE

Oral administration of leflunomide was a safe and effective alternative to oral administration of corticosteroids for treatment of IMPA in dogs. On the basis of findings in this study, a starting dosage for leflunomide of 3 to 4 mg/kg (1.4 to 1.8 mg/lb) PO once daily for at least 6 weeks before making dose adjustments is recommended. Dose adjustments should be based on cytologic evaluation of synovial fluid and clinical signs of IMPA. Hematologic variables, serum biochemical analysis results, and clinical signs of IMPA should be monitored for evidence of adverse effects to treatment with leflunomide.

Liver regeneration is impaired by FK778 in partially hepatectomized rats, while supplemental uridine restores both liver growth and hepatocyte proliferation

AIM

The impact of mandatory immunosuppression on liver regeneration after segmental liver transplantation is of clinical importance. FK778, a novel immunosuppressant, inhibits pyrimidine biosynthesis and prevents rejection after organ transplantation in a dose-dependent manner. We investigated the effect of FK778 at a highly effective dose on liver regeneration in a small animal model.

METHODS

Inbred Lewis rats were subjected to 70% partial hepatectomy (PH) and treated with saline (n = 28), uridine (n = 16), FK778 alone (n = 28) or in combination with uridine (n = 16). FK778 was given intravenously daily at a dose of 25 mg/kg bodyweight (bw) and uridine was given daily intraperitoneally at a dose of 250 mg/kg bw. Liver bodyweight ratio (LBR), hepatocyte proliferation index (PI), blood chemistry and morphological analysis were incorporated. PI was determined by Ki-67 immunostaining. De Ritis ratio was calculated to assess the extent of liver damage.

RESULTS

In FK778-treated animals PI was decreased at 24 h and 72 h and LBR was lower at 48 h and 72 h (P < 0.05) after the PH. In addition, morphological analysis showed confluent central lobular necrosis at 72 h in four of seven animals. Uridine supplementation restored PI, LBR and the de Ritis ratio in FK778-treated animals and no confluent necroses were observed.

CONCLUSION

FK778 is antihepatotrophic as well as antiproliferative during rat liver regeneration. Both liver growth and hepatocyte proliferation are completely restored by supplementation with uridine. In addition, supplemental uridine markedly reduces the severity of morphological abnormalities consistent with FK778 toxicity.

Introducing the first polymer-free leflunomide eluting stent

BACKGROUND

We here describe the pharmacological characteristic, in vivo efficacy, and in vitro mechanisms of a polymer-free leflunomide eluting stent in comparison to its rapamycin-coated equivalent.

METHODS

Stents were coated with 40 mM solutions of leflunomide (L) or rapamycin (R) or were left uncoated (BM). Neointima formation was assessed 6 weeks after implantation into Sprague Dawley rats by optical coherence tomographies (OCT) and histopathology. In vitro proliferation assays were performed using isolated endothelial and smooth-muscle-cells from Sprague Dawley rats to investigate the cell-specific pharmacokinetic effect of leflunomide and rapamycin.

RESULTS

HPLC-based drug release kinetics revealed a similar profile with 90% of the drug being released after 12.1+/-0.2 (L) and 13.0+/-0.2 days (R). After 6 weeks, OCTs showed that in-stent luminal obliteration was less for the coated stents (L:12.0+/-9.4%, R:13.3+/-13.1%) when compared to identical bare metal stents (BM:26.4+/-4.7%; p<or=0.046). Histology with computer-assisted morphometry was performed and demonstrated reduced in-stent I/M thickness ratios (L:2.5+/-1.2, R:3.7+/-3.3, BM:6.7+/-2.3, p<or=0.049 for L and R vs. BM) and neointimal areas (L:0.6+/-0.3, R:0.7+/-0.2, BM:1.3+/-0.4, p<or=0.039 for L and R vs. BM) with stent coating. No differences were found for injury and inflammation scores (L and R vs. BM; p=NS). In vitro SMC proliferation was dose-dependently and similarly inhibited by L and R at 1-100 nM (p=NS L vs. R). Interestingly, human EC proliferation at 10-100 nM was significantly inhibited only by R (p<0.001), but not by L (p=NS).

CONCLUSIONS

The diminished inhibition of EC proliferation may improve arterial healing and contribute to the safety profile of the leflunomide stent.

ATP competitive inhibitors of D-alanine-D-alanine ligase based on protein kinase inhibitor scaffolds

D-Alanine-D-alanine ligase (DDl) is an essential enzyme in bacterial cell wall biosynthesis and an important target for developing new antibiotics. Here, we describe a new approach to identify new inhibitor scaffolds for DDl based on similarity in the ATP binding region of different kinases and DDl. After an initial screening of several protein kinase inhibitors, we found that the Brutons's tyrosine kinase inhibitor LFM-A13, an analog of the Leflunomide metabolite A771726, inhibits DDl with a K(i) of 185 microM. A series of malononitrilamide and salicylamide derivatives of LFM-A13 has been synthesized to confirm the validity of this scaffold as an inhibitor of DDl.

In vitro effects of the active metabolite of leflunomide, A77 1726, on feline herpesvirus-1

OBJECTIVE

To determine whether the active metabolite of leflunomide, A77 1726 (A77), inhibits replication of feline herpesvirus-1 (FHV-1) in cell culture.

STUDY POPULATION

Crandell Rees feline kidney (CRFK) cell cultures.

PROCEDURES

Cell cultures were inoculated with FHV-1 and treated simultaneously with concentrations of A77 ranging from 0 to 200microM. The antiviral effect of A77 was determined by use of conventional plaque reduction assays. The effect of A77 on viral load was determined via real-time PCR analysis, and transmission electron microscopy was used to evaluate the effect of A77 on viral morphology. To determine whether the antiviral effect was attributable to alterations in CRFK cell viability and number, CRFK cells were treated with various concentrations of A77 and stained with Annexin V and propidium iodide to assess apoptosis and a mitochondrial function assay was used to determine cell viability.

RESULTS

Concentrations of A77 > or = 20microM were associated with substantial reduction in plaque number and viral load. Concentrations > or = 100microM were associated with complete suppression of plaque formation. At low concentrations of A77, clusters of intracytoplasmic virus particles that appeared to lack tegument and an external membrane were detected. Treatment of uninfected CRFK cell monolayers with A77 was associated with reduction in mitochondrial function with minimal evidence of apoptosis.

CONCLUSIONS AND CLINICAL RELEVANCE

Leflunomide may be an alternative to current calcineurin-based immunosuppressive protocols used in feline organ transplantation because of its antiherpesviral activity.

Impairment of T cell interactions with antigen-presenting cells by immunosuppressive drugs reveals involvement of calcineurin and NF-κB in immunological synapse formation

A stable supramolecular cluster in T cells at the contact site of APCs, the immunological synapse (IS), is essential for full T cell activation. Failure of IS maturation, as determined by defective relocalization of the TCR/CD3 complex at the T cell/APC contact site, is linked with T cell hyporesponsiveness. The effects of clinically used immunosuppressants on these critical events, however, are undefined. Here, we show that treatment of T cells with cyclosporin A, FK506, and dexamethasone, which are known to inhibit calcineurin and NF-kappaB, respectively, but not rapamycin, the inhibitor of mammalian target of rapamycin, selectively prevented TCR/CD3 relocalization into the IS, while relocalization of adhesion and cytoskeletal proteins as well as T cell/APC conjugate formation remained unaltered. The involvement of calcineurin and NF-kappaB in IS maturation was confirmed by using specific inhibitors of these molecules (FR901725, gossypol, SN50). FK778, as an inhibitor of DNA replication and also TCR/CD3-activated tyrosine kinases, globally abrogated cytoskeletal, adhesion, and signaling molecule relocalization, thereby preventing formation of an IS at an earlier, immature stage along with impaired, antigen-specific T cell/APC conjugate formation. Collectively, blocking IS formation at distinct stages may mediate effects on T cell activation of currently used immunosuppressants, apart from their capacity to block gene transcription, cytokine signaling, and DNA replication. Furthermore, these data imply novel functions of calcineurin and NF-kappaB for successful IS maturation.

Immunotherapy for De Novo Renal Transplantation

Immunosuppressive drugs have been traditionally developed to prevent acute rejection and to improve short-term kidney transplant outcomes. There is still a medical need to improve outcomes among subgroups of patients at higher risk for graft loss and to reduce cardiovascular, infectious and malignancy-associated morbidity and mortality, and improve long-term adherence. Several new immunosuppressive agents and formulations are undergoing clinical investigation and are discussed in this review.A modified release tacrolimus formulation (MR4) for once-daily administration is undergoing phase III trials. It has been developed to be administered de novo or for maintenance using the same therapeutic target tacrolimus trough concentrations as for the original formulation. Belatacept (LEA29Y), a second generation cytotoxic-T-lymphocyte-associated antigen immunoglobulin (CTLA4-Ig), blocks the interaction between CD80/86 and CD28 costimulatory pathways. In phase II trials, belatacept was as effective as ciclosporin (cyclosporine) when administered in combination with basiliximab, mycophenolate mofetil (MMF) and corticosteroids. Currently, belatacept is undergoing phase III trials including one study in recipients of organs from expanded criteria donors. Inhibitors of the Janus protein tyrosine kinase (JAK)-3 show some selectivity for cells of the lymphoid lineage and have been shown to be effective in late preclinical transplant models. The most frequent adverse effects have been related to nonspecific binding to JAK2 kinases. CP-690550, a JAK3 inhibitor is currently in phase II clinical trials.FK778, is a synthetic malononitrilamide that targets the critical enzyme of the de novo pyrimidine synthesis, dihydroorotic acid dehydrogenase, and receptor-associated tyrosine kinases has completed phase II trials. FK778 also shows antiviral activities that have been tested in patients with polyomavirus nephropathy. Fingolimod (FTY720), a synthetic sphingosine phosphate receptor modulator that reduces the recirculation of lymphocytes to blood and peripheral tissues including inflammatory lesions and graft sites is undergoing phase III trials. Although the efficacy of fingolimod is similar to MMF in patients receiving full doses of ciclosporin, safety issues such as a negative chronotropic effect, macular oedema, pulmonary adverse reactions and graft function resulted in premature discontinuation of the development programme for kidney transplantation. Because there was no clear clinical benefit over treatment options, the clinical development programme of FK778 was discontinued.Finally, a new evolving strategy with powerful induction-induced prolonged T-cell depletion followed by low-dose immunosuppressive monotherapy is showing promising results.

Assessment of an LC-MS method for plasma quantification of the new immunosuppressant FK778 through comparison with HPLC-UV

FK778 is a new immunosuppressive agent, derived from the leflunomide-active metabolite A77 1726. It inhibits de novo pyrimidine nucleotide synthesis showing efficacy in the prevention and treatment of rejection in experimental transplant models. The aim of this work was to develop an HPLC-MS method to measure FK778 in plasma for pharmacokinetic studies. The equipment used for mass evaluation was an HLPC coupled to an ion trap analyzer through an electrospray source. After precipitation of plasma proteins with acetonitrile, the supernatant was injected onto an analytical RP-C18 column. Chromatographic separation was performed under isocratic conditions, using a mobile phase consisting of ammonium acetate buffer and acetonitrile (55:45. vol/vol). MS detection was performed in the negative ionization mode by monitoring the molecular ion of FK778 (m/z 307) and IS (m/z 269), using selected ion monitoring for both. However, we observed peaks corresponding to dimers, trimers, and tetramers of FK778 (m/z 637, m/z 945, m/z 1274). The HPLC-MS method was applied to pharmacokinetics in animal models showing comparable results to those obtained by an HPLC-UV assay at 290 nm. Good agreement was observed in the plasma FK778 concentration versus time curves. The rapid preparation of samples and the short run-time make this method attractive for use in clinical practice.

In vitro metabolism studies on the isoxazole ring scission in the anti-inflammatory agent lefluonomide to its active alpha-cyanoenol metabolite A771726: mechanistic similarities with the cytochrome P450-catalyzed dehydration of aldoximes

The 3-unsubstituted isoxazole ring in the anti-inflammatory drug leflunomide undergoes a unique N-O bond cleavage to the active alpha-cyanoenol metabolite A771726, which resides in the same oxidation state as the parent. In vitro studies were conducted to characterize drug-metabolizing enzyme(s) responsible for ring opening and to gain insight into the mechanism of ring opening. Under physiological conditions, leflunomide was converted to A771726 in rat and human plasma (rat plasma,t1/2 = 36 min; human plasma, t1/2 = 12 min) and whole blood (rat blood, t1/2 = 59 min; human blood, t1/2 = 43 min). Human serum albumin also catalyzed A771726 formation, albeit at a much slower rate (t1/2 = 110 min). Rat and human liver microsomes also demonstrated NADPH-dependent A771726 formation (human liver microsomes, Vmax = 1797 pmol/min/mg and Km = 274 microM). Leflunomide metabolism in microsomes was sensitive to furafylline treatment, suggesting p4501A2 involvement. 3-Methylleflunomide, which contained a 3-methyl substituent on the isoxazole ring, was resistant to ring opening in base, plasma, blood, and liver microsomes. In microsomes, two monohydroxylated metabolites were formed, and metabolite identification studies established the 3- and the 5-methyl groups on the isoxazole ring as sites of hydroxylation. These results indicate that the C3-H in leflunomide is essential for ring opening. Although A771726 formation in human liver microsomes or recombinant p4501A2 required NADPH, its formation was greatly reduced by oxygen or carbon monoxide, suggesting that the isoxazole ring opening was catalyzed by the p450Fe(II) form of the enzyme. A mechanism for the p450-mediated ring scission is proposed in which the isoxazole ring nitrogen or oxygen coordinates to the reduced form of the heme followed by charge transfer from p450Fe(II) to the C=N bond or deprotonation of the C3-H, which results in a cleavage of the N-O bond.

Leflunomide analogue FK778 is vasculoprotective independent of its immunosuppressive effect: potential applications for restenosis and chronic rejection

BACKGROUND

Leflunomide (LFM) inhibits experimentally both acute and chronic allograft rejection. The inhibition of dihydroorotate dehydrogenase (DHODH) in pyrimidine synthesis is suggested to be the major immunosuppressive mechanism. The mechanism of its vasculoprotective effect is not known, although it may be linked to inhibition of receptor tyrosine kinases (RTK). Here, we have investigated whether sufficient vasculoprotective effect could be obtained upon administration of FK778, a LFM analogue with shorter half-life, and compared the dose response with that of a known platelet-derived growth factor RTK inhibitor, imatinib, after endothelial injury in vivo.

METHODS AND RESULTS

Wistar rats were used for aorta denudations. The rats remained untreated or received either FK778 or imatinib (STI571) at decreasing oral doses from 10 mg/kg per day. Half of the animals in both treatment groups also received uridine to reverse DHODH activity. Morphometric analysis was done after 14 day follow-up. In the untreated group, moderate neointima formation was detected. FK778 almost completely inhibited intimal formation, with or without uridine addition (P<0.05). Imatinib also inhibited neointima formation (P<0.05), whereas exogenous uridine reversed its effect.

CONCLUSIONS

Our results demonstrate that FK778 inhibits neointima formation by way of a mechanism that is independent of DHODH inhibitory activity on vascular smooth muscle cell. Interestingly, the effect of imatinib was inhibited by uridine, suggesting that part of its action on vascular stenosis could be mediated through inhibition of pyrimidine synthesis.

Clinical pharmacokinetics of leflunomide

Leflunomide is the first disease-modifying antirheumatic drug to be approved for rheumatoid arthritis in the past 10 years. Orally administered leflunomide is almost completely converted into its active metabolite A77 1726 (hereafter referred to as M1). M1 displays linear pharmacokinetics at the dosages of leflunomide used in clinical practice. It has a long elimination half-life (approximately 2 weeks), reaching a steady state after approximately 20 weeks. M1 is highly bound to plasma proteins. The pharmacokinetics of M1 are not affected by food intake, and dosage requirements are not influenced by age or gender. Approximately 90% of a single dose of leflunomide is eliminated, 43% in urine, primarily as leflunomide glucuronides and an oxalinic acid derivative of M1, and 48% in faeces, primarily as M1. Elimination can be dramatically increased by using charcoal or cholestyramine. In vitro studies have shown no major influence of leflunomide on the metabolism of analgesics, nonsteroidal anti-inflammatory drugs and methotrexate, drugs usually used in the treatment of rheumatoid arthritis. In clinical studies with a limited number of patients using these drugs concomitantly, no safety problems appeared. Nonspecific inducers of cytochrome P450 (CYP) and some drugs metabolised by CYP2C9 affect the metabolism of M1, and caution should be used in patients cotreated with them. Additional in vitro and in vivo pharmacokinetic studies are needed to better understand the nonenzymatic and enzymatic metabolism of leflunomide. Additional clinical trials should be performed in order to find new indications for leflunomide in other autoimmune diseases, and new combination therapeutic strategies in rheumatoid arthritis. This review is a summary of current knowledge of the pharmacokinetics of leflunomide, focusing primarily on humans and in particular on patients with rheumatoid arthritis.

Leflunomide: mode of action in the treatment of rheumatoid arthritis

Leflunomide is a selective inhibitor of de novo pyrimidine synthesis. In phase II and III clinical trials of active rheumatoid arthritis, leflunomide was shown to improve primary and secondary outcome measures with a satisfactory safety profile. The active metabolite of leflunomide, A77 1726, at low, therapeutically applicable doses, reversibly inhibits dihydroorotate dehydrogenase (DHODH), the rate limiting step in the de novo synthesis of pyrimidines. Unlike other cells, activated lymphocytes expand their pyrimidine pool by approximately eightfold during proliferation; purine pools are increased only twofold. To meet this demand, lymphocytes must use both salvage and de novo synthesis pathways. Thus the inhibition of DHODH by A77 1726 prevents lymphocytes from accumulating sufficient pyrimidines to support DNA synthesis. At higher doses, A77 1726 inhibits tyrosine kinases responsible for early T cell and B cell signalling in the G(0)/G(1) phase of the cell cycle. Because the immunoregulatory effects of A77 1726 occur at doses that inhibit DHODH but not tyrosine kinases, the interruption of de novo pyrimidine synthesis may be the primary mode of action. Recent evidence suggests that the observed anti-inflammatory effects of A77 1726 may relate to its ability to suppress interleukin 1 and tumour necrosis factor alpha selectively over their inhibitors in T lymphocyte/monocyte contact activation. A77 1726 has also been shown to suppress the activation of nuclear factor kappaB, a potent mediator of inflammation when stimulated by inflammatory agents. Continuing research indicates that A77 1726 may downregulate the glycosylation of adhesion molecules, effectively reducing cell-cell contact activation during inflammation.

Mycophenolic acid-induced GTP depletion also affects ATP and pyrimidine synthesis in mitogen-stimulated primary human T-lymphocytes

BACKGROUND

Mycophenolate mofetil (MMF) is an effective immunosuppressant developed for use in organ transplantation. It specifically targets lymphocyte purine biosynthesis. However, side effects do occur. Understanding how the active metabolite of MMF, mycophenolic acid (MPA) affects the normally integrated interaction between intracellular purine and pyrimidine pathways might aid the development of improved therapeutic regimes.

METHODS

We used a primary human T-lymphocyte model to study how preincubation with MPA (0.1-50 microM) affected normal ribonucleotide pool responses to phytohemagglutinin using radiolabeled precursors.

RESULTS

MPA not only restricted the mitogen-induced expansion of GTP pools, but actually induced a severe drop in both GTP (10% of unstimulated cells) and GDP-sugar pools, with a concomitant fall in ATP (up to 50%). These effects were concentration dependent. By contrast, uridine pools expanded whereas CTP pools remained at resting levels. These changes were confirmed by the altered incorporation of [14C]-bicarbonate and [14C]-glycine into nucleotides. Restriction of [14C]-hypoxanthine incorporation and reduction of [14C]-uridine uptake comparable to that of unstimulated cells indicated that MPA also inhibited both salvage routes of nucleotide synthesis.

CONCLUSION

MPA affects pyrimidine as well as purine responses to mitogens in T-lymphocytes, but not in an integrated way. The molecular mechanisms underlying these disproportionate changes can best be explained by MPA-related inhibition of amidophosphoribosyltransferase, catalysing the first step in purine biosynthesis. This would increase phosphoribosylpyrophosphate availability, thereby stimulating UTP biosynthesis. Such imbalances, coupled with ATP-depletion, could underlie reported side effects and might be overcome by appropriate combination therapies.

Simultaneous separation by high-performance liquid chromatography of carbamoyl aspartate, carbamoyl phosphate and dihydroorotic acid

Leflunomide is an immunomodulatory drug which acts by inhibiting dihydroorotic acid dehydrogenase, the fourth enzyme of pyrimidine biosynthesis. We modified our high-performance liquid chromatography method to demonstrate that the principal metabolite in mitogen-stimulated human T-lymphocytes incubated with leflunomide was not dihydroorotic acid, but carbamoyl aspartate. Identification involved preparation of [14C]carbamoyl aspartate from [14C]aspartic acid and mammalian aspartate transcarbamoylase. Accumulation of carbamoyl aspartate indicates that under these conditions the equilibrium constant for dihydroorotase favours the reverse reaction. This HPLC method, enabling simultaneous separation of the first four intermediates in the de novo pyrimidine pathway may be of use in a variety of experimental situations.

In vivo activity of leflunomide: pharmacokinetic analyses and mechanism of immunosuppression

BACKGROUND

Leflunomide is an experimental drug with demonstrated ability to prevent and reverse acute allograft and xenograft rejection. The two biochemical activities reported for the active metabolite of leflunomide, A77 1726, are inhibition of tyrosine phosphorylation and inhibition of dihydroorotate dehydrogenase, an enzyme necessary for de novo pyrimidine synthesis. These activities can be distinctly separated in vitro by the use of uridine, which reverses the anti-proliferative effects of A77 1726 caused by inhibition of de novo pyrimidine synthesis. We report the effect of uridine on the in vivo immunosuppressive activities of leflunomide.

METHODS

We first quantified the serum levels of A77 1726, the active metabolite of leflunomide, after a single treatment of leflunomide (5, 15, and 35 mg/kg). Additionally, we quantified the levels of serum uridine and of nucleotide triphosphates in the liver, spleen, and lymph nodes of Lewis rats after the administration of a single dose of uridine (500 mg/kg; i.p.). Lewis rats heterotopically transplanted with brown Norway or Golden Syrian hamster hearts were treated for 50 or 75 days with leflunomide (5, 15, and 35 mg/kg/day; gavage) alone or in combination with uridine (500 mg/ kg/day; i.p.). Hematocrits were determined and the levels of alloreactive or xenoreactive immunoglobulin (Ig)M and IgG were determined by flow cytometric analysis. The allograft and xenografts, small bowel, liver, kidney, and spleen were subjected to pathological examination.

RESULTS

A linear relationship was observed between the serum A77 1726 concentrations in Lewis rats and the dose of leflunomide administered. Peak A77 1726 concentrations were 20.9, 71.8 and 129.3 mg/l (77.5, 266.1 and 478.8 microM) for the 5, 15, and 35 mg/kg doses of leflunomide, respectively. The concentration of uridine in the serum of normal Lewis rats is 6.5 microM; after i.p. administration of 500 mg/kg uridine, the serum uridine concentrations peaked at 384.1 microM in 15-30 min. The rapid elimination of uridine was not reflected in the lymphoid compartments, and the pharmacokinetics of pyrimidine nucleotides in the spleen resembled that of A77 1726. This dose of uridine, when administered daily (500 mg/kg/day, i.p.), weakly antagonized the immunosuppressive activities of leflunomide (5, 15, and 35 mg/kg/day) in the allotransplantation model. In contrast, in the xenotransplantation model, the same concentration of uridine completely antagonized the immunosuppressive activities of low-dose leflunomide (15 mg/kg/day) and partially antagonized the immunosuppressive activities of high-dose leflunomide (35 mg/kg/day). Toxicities associated with high-dose leflunomide (35 mg/kg/day) were anemia, diarrhea, and pathological changes in the small bowel and liver. These toxicities were significantly reduced by uridine co-administration.

CONCLUSION

These studies reveal that the blood levels of A77 1726 in Lewis rats satisfy in vitro requirements for both inhibition of de novo pyrimidine synthesis and protein tyrosine kinase activity. Our data also illustrate that the in vivo mechanism of immunosuppression by leflunomide is complex and is affected by at least the following four factors: type and vigor of the immune response, availability of uridine for salvage by proliferating lymphocytes, species being investigated, and concentration of serum A77 1726.

New concepts in treatment protocols for severe systemic vasculitis

Glucocorticosteroids (GCs) are the drug of choice in all clinical types of giant cell arteritis (GCA); a study delineated that an unexpectedly high percentage of patients required long-term GCs, with the consequence of significant complications attributable to GC therapy. Azathioprine and methotrexate are recommended as GC-sparing drugs. Cyclosporin A was found to confer no additive effect versus GC treatment alone. Depot GCs intramuscularly every 3 weeks decreased the cumulative GC dose and were associated with fewer bone fractions compared with daily oral GCs. Pulse cyclophosphamide has been shown to be as effective as the standard therapy in necrotizing vasculitides; however, an alarmingly high rate of infections was observed in this study in both arms possibly related to the high dosage of GCs. New drugs such as mycophenolate mofetil and leflunomide appear as alternatives as maintenance therapy in antineutrophil cytoplasm autoantibody-associated vasculitides in pilot studies. Interferon-alpha (IFN-alpha) has been shown to be effective in treatment-resistant Churg-Strauss syndrome, and IFN-alpha or ribavirin can be used successfully in essential mixed cryoglobulinemia (induced by hepatitis C virus). Thalidomide was shown to be effective for treating oral and genital ulcers and follicular lesions in Behçet's syndrome; severe refractory Behçet's syndrome uveitis responded to treatment with IFN-alpha.

Species-related inhibition of human and rat dihydroorotate dehydrogenase by immunosuppressive isoxazol and cinchoninic acid derivatives

The isoxazol leflunomide (N-(4-trifluoromethylphenyl)-5-methylisoxazol-4-carboxamide) and its active metabolite A77-1726 (N-(4-trifluoromethyl)-phenyl-2-cyano-3-hydroxy-crotonic acidamide) are promising disease-modifying antirheumatic drugs now in clinical trials. The malononitrilamides MNA279 (2-cyano-3-cyclopropyl-3-oxo-(4-cyanophenyl)propionamide) MNA715(N-(4-trifluoromethyl)-phenyl-2-cyano-3-hydroxy-hept-2-en-6- in-carboxylic acidamide) and HR325 (1(3-methyl-4-trifluoro methylphenyl-carbamoyl)-2-cyclopropyl-2oxo-propionitrile) were shown to block rejection after allograft and xenograft transplantation in animals. Brequinar and other cinchoninic acid derivatives have also been evaluated as immuno-suppressive agents. A77-1726, HR325 and brequinar have been shown to have strong inhibitory effects on mitochondrial dihydroorotate dehydrogenase [EC 1.3.99.11], the fourth enzyme of pyrimidine de novo synthesis, with concomitant reduction of pyrimidine nucleotide pools. Pyrimidine nucleotides are essential for normal immune cell functions. Because most investigations had been carried out with cells, cell homogenates or mitochondrial fractions, it was the rationale of the present study to differentiate, under standardized conditions, the effect of leflunomide, A77-1726, MNA279, MNA715, HR 325 and brequinar on the recombinant rat and human enzymes, which were purified in our laboratory. Whereas leflunomide was a relatively weak inhibitor of the rat (IC50 = 6.3 microM) and human (IC50 = 98 microM) dihydroorotate dehydrogenase, the influence of A77-1726, MNA 279, MNA715 and HR325 was of comparable efficacy for either the rat (range of IC50, 19-53 nM) or the human enzyme (range of IC50, 0.5-2.3 microM). From the IC50 values, it was deduced that brequinar was a more potent inhibitor of the human dihydroorotate dehydrogenase activity (IC50 = 10 nM) than of the rat enzyme (IC50 = 367 nM). The rat enzyme was influenced by all isoxazol derivatives to a greater extent (IC50 = 19 nM A77-1726) than the human enzyme (IC50 = 1.1 microM A77-1726). These results may provide a plausible explanation for the findings of other laboratories with cultured cell lines and lymphocytes: in comparison to cells derived from human tissues, rat and other rodent cells were more susceptible to the isoxazol derivatives and less susceptible to brequinar. Our detailed kinetic investigations of the bisubstrate reaction catalyzed by rat dihydroorotate dehydrogenase revealed a noncompetitive type of inhibition by A77-1726 with respect to the substrate dihydroorotate and the cosubstrates ubiquinone or decylubiquinone. For brequinar, the inhibition was noncompetitive with respect to the substrate dihydroorotate, whereas with the quinone it was found to follow the "mixed typed" inhibition. In addition, brequinar acted as a "slow-binding" inhibitor of the human dihydroorotate dehydrogenase, a feature that might be of consequence for the reversibility of the reaction with the target.

Structural and functional comparison of agents interfering with dihydroorotate, succinate and NADH oxidation of rat liver mitochondria

Mitochondrially bound dihydroorotate dehydrogenase (EC 1.3.99.11) catalyses the fourth sequential step in the de novo synthesis of uridine monophosphate; this enzyme uses ubiquinone as the proximal and cytochrome oxidase as is the ultimate electron transfer system. Here, seven compounds with proven antiproliferative activity and in vitro antipyrimidine effects were investigated with isolated functional mitochondria of rat tissues in order to differentiate their anti-dihydroorotate dehydrogenase potency versus putative effects on the respiratory chain enzymes. Ten microM of brequinar sodium, the leflunomide derivatives A77-1726, [2-cyano-3-cyclopropyl-3-hydroxy-enoic acid (4-trifluoromethylphenyl)-amide], MNA 279, (2-cyano-N-(4-cyanophenyl)-3-cyclopropyl-3-oxo-propanamide), MNA715 (2-cyano-3-hydroxy-N-(4-(trifluoromethyl)-phenyl-6-heptanamide), HR325 (2-cyano-3-cyclopropyl-3-hydroxy-N-[3'-methyl-4'-(trifluoromethyl)phenyl ]-propenamide), and the diazine toltrazuril completely inhibited the dihydroorotate-induced oxygen consumption of liver mitochondria. Succinate and NADH oxidation were found to be influenced only at elevated drug concentration (100 microM), with the exception of HR325, 10 microM of which caused a 70% inhibition of NADH and 50% inhibition of succinate oxidation. This was comparable to the effects of toltrazuril, which caused an approximate 75% inhibition of NADH oxidation. Ciprofloxacin was shown here to have only marginal effects on the redox activities of the inner mitochondrial membrane. This differentiation of drug effects on mitochondrial functions will contribute to a better understanding of the in vivo pharmacological activity of these drugs, which are presently in clinical trials because of their immunosuppressive, cytostatic or anti-parasitic activity. A comparison of the influence of A77-1726, HR325, brequinar and 2,4-dinitrophenol on energetically coupled rat liver mitochondria revealed only a weak uncoupling potential of A77-1726 and brequinar. In addition, a modeling study was raised to search for common spatial arrangements of functional groups essential for binding of inhibitors to dihydroorotate dehydrogenase. From the structural comparison of different metabolites and inhibitors of pyrimidine metabolism, a 6-point model was obtained by conformational analysis for the drugs tested on mitochondrial functions, pharmacophoric perception and mapping. We propose our model in combination with kinetic data for a rational design of highly specific inhibitors of dihydroorotate dehydrogenase.

Pyrazole bioisosteres of leflunomide as B-cell immunosuppressants for xenotransplantation and chronic rejection: scope and limitations

T-cell immunosuppressant-based therapies efficiently control early graft rejection in allotransplantation settings. They fail, however, to prevent those rejection events which are mediated by transplant-induced antibody (Ab) responses such as those involved in xenograft and chronic allograft rejection. This is mainly due to their inability to block T-cell-independent Ab production against the transplanted organs. The bioactive metabolite 2(Z) of leflunomide (1) inhibits the formation of such Ab, but the drug has pharmacokinetic properties and a therapeutic window incompatible with transplantation indications. Pyrazole 3, a constrained analogue of 2(Z), was designed and shown to be conformationally and biologically similar to 2(Z). Further investigations with derivatives of 3 demonstrated that the pyrazoles had very tight structure-activity relationships, the only equipotent compound being 3o. However, in contrast to 2(Z), both 3 and 3o were inactive in vivo due to short half-life and drug concentrations lower than the in vitro obtained IC50 values. Compound 3o inhibits T-cell-independent Ab production by a different biochemical mechanism from that of 2(Z) and 3 and may therefore represent a valuable tool for the identification of new targets for B-cell inhibition.

Leflunomide inhibits pyrimidine de novo synthesis in mitogen-stimulated T-lymphocytes from healthy humans

The mode of action of Leflunomide, an immunomodulatory drug used in rheumatoid arthritis, is debated. This study, using 14C-labeled de novo purine and pyrimidine synthesis precursors, proves conclusively that the prime target in proliferating human T-lymphocytes is pyrimidine biosynthesis at the level of dihydroorotic-acid dehydrogenase. Leflunomide (25 and 50 microM), like Brequinar (0.5 and 1 microM), a demonstrated dihydroorotic-acid dehydrogenase inhibitor, was cytostatic, not cytotoxic, with proliferation being halted in the G1 phase. Both drugs restricted the normal 4-8-fold mitogen-induced expansion of pyrimidine pools over 72 h to concentrations found in nonstimulated T-cells and [14C]bicarbonate incorporation into UTP, ATP, and GTP. Uridine (50 microM) restored expansion of all pools, but [14C]bicarbonate incorporation into ATP and GTP only, not UTP. [14C]Hypoxanthine salvage was also restricted, indicating that purine salvage pathways are compromised likewise by both inhibitors. [14C]Glycine studies confirmed that restriction of de novo purine synthesis occurred secondary to inhibition of proliferation since this was reversed by uridine rescue, except at 100 microM Leflunomide. 100 microM Leflunomide markedly depleted ATP and GTP pools also, which would have serious consequences for ATP-dependent enzymes essential to the immune response, thereby explaining non-pyrimidine-related effects reported for Leflunomide at 100 microM and above.

Isoxazolylthioamides as potential immunosuppressants a combinatorial chemistry approach

A library of thioamide derivatives of leflunomide 1a and of its bioactive metabolite 1b has been synthesised on solid phase. Thus, para-substituted phenylacetic acids were coupled to TentaGel and were subsequently reacted with aromatic isothiocyanates. Treatment of the resulting enaminothioamides with hydroxylamine led to their simultaneous cyclisation and cleavage from the resin affording 2-25. Their in vitro profiling demonstrated that the amide-thioamide isologous substitution was detrimental of the biological activity.

Blockade of induced xenoantigen expression prevents rejection after retransplantation of accommodated hamster-to-rat heart xenografts

BACKGROUND

We have shown previously that a 2-week course of leflunomide (LF) together with a maintenance therapy of cyclosporine (CsA) rendered hamster-to-rat heart xenografts (Xg) resistant against anti-hamster IgM xenoantibody (XAb)-mediated rejection, a state compatible with the notion of accommodation. Our aim in this study was to investigate the mechanism underlying this Xg accommodation.

METHODS

"Accommodated" Xgs were retransplanted to CsA-treated naive rats in the presence or absence of additional LF treatment or anti-hamster IgM serum injection. Immunohistopathology and fluorescence-activated cell sorting was performed to detect IgM and complement (C) deposition in Xgs, and endothelial cell (EC) expression of P- and E-selectin, ICAM-1, and VCAM-1 in vivo and in vitro.

RESULTS

Retransplanted accommodated Xgs were rejected in CsA-treated naive rats and elicited IgM XAbs. Passive transfer of IgM XAbs provoked hyperacute rejection of both control and retransplanted Xgs. Addition of a 5-day course of LF prevented the rejection of only accommodated Xgs. Adoptively transferred IgM XAbs were deposited in rejected control and accommodated Xgs, but not in accommodated Xgs accepted by LF-treated rats. LF blocked the EC induction of P- and E-selectins in both control fresh and accommodated Xgs. Hence, after retransplantation accommodated Xgs express mainly induced xenoantigens (XAgs), such as P- and E-selectins, that can entirely be suppressed by LF. In contrast, control hamster Xgs express additional XAgs and remain susceptible to XAb-mediated rejection. These findings are in agreement with in vitro studies showing that LF totally suppressed induced EC antigens (e.g., P-selectin and E-selectin), but not constitutively expressed antigens (e.g., ICAM-1).

CONCLUSION

Accommodated Xgs show a down-regulation of constitutive XAgs, but may be rejected after retransplantation by a mechanism involving EC expression of inducible XAgs. LF is able to block this latter XAg induction.

Modification of humoral responses by the combination of leflunomide and cyclosporine in Lewis rats transplanted with hamster hearts

BACKGROUND

Vigorous antibody-mediated responses prevent the successful engraftment of hamster hearts transplanted into Lewis rats. Early antibody responses mediating acute rejection of the xenograft are T cell-independent and resistant to the T-cell immunosuppressant, cyclosporine (CsA). Immunosuppression with the combination of leflunomide plus CsA completely prevents xenograft rejection, but when such immunosuppression is stopped the hamster heart is rejected by a process that we term late xenograft rejection. We report here on some of the immunological features of late xenograft rejection.

METHODS

Lewis rats transplanted with hamster hearts were treated with leflunomide (5 mg/kg/day by gavage) for 14-21 days and CsA (20 mg/kg/day by gavage) continuously from the day of transplant. Serum was harvested and the functional activities of the xenoreactive antibodies were quantitated by in vivo passive transfer of sera, flow cytometry, in vitro C3 deposition assays, and Western blotting.

RESULTS

CsA alone prevented late xenograft rejection and the accompanying production of xenoreactive antibodies. The xenoreactive antibodies accompanying acute or late xenograft rejection were predominantly IgM, but only serum from rats undergoing acute xenograft rejection was able to induce hyperacute rejection. The ability of serum to induce hyperacute rejection correlated with its ability to induce C3 deposition on hamster lymphocytes in vitro. The repertoire of hamster antigens recognized by IgM in the serum of rats undergoing late xenograft rejection is more restricted than that of IgM in the serum of rats undergoing acute xenograft rejection. We additionally demonstrate that long-term graft survival is not dependent on graft accommodation.

CONCLUSIONS

These studies demonstrate that a brief treatment with the combination of leflunomide and CsA profoundly modifies the humoral xenoreactivity in the recipient, converting it from a T-independent into a T cell-dependent response. Differences in functional activity of sera from acute or late xenograft rejection suggest that antigenic specificity defines the ability of IgM to induce complement activation and hyperacute rejection.

A new rational hypothesis for the pharmacophore of the active metabolite of leflunomide, a potent immunosuppressive drug

Leflunomide is one of the most promising disease-modifying antirheumatic drug now in clinical trials for the treatment of rheumatoid arthritis. Metabolic studies have indicated that leflunomide is rapidly processed in vivo to an active metabolite, A771726 (2). To identify the chemical characteristics necessary for the immunosuppressive activity of 2, configurational and conformational studies were carried out on the latter and its inactive analogues (ethyl 3-hydroxy-2-((4-(trifluoromethyl)phenyl)carbamoyl)but-2-enoate, 3a, and 3-hydroxy-2-nitro-N-(4-(trifluoromethyl)phenyl)but-2-enamide, 3b). These studies suggested that the pharmacophore responsible for the immunosuppressive activity of 2 is a beta-keto amide with the enolic hydroxy group cis to the amidic moiety. To verify this hypothesis, a new class of immunosuppressive agents was designed and synthesized. Their testing in vitro and in vivo identified compounds which were more potent than both leflunomide and 2 and above all confirmed our hypothesis as to the key structural and chemical determinants for the immunosuppressive properties of 2 and our compounds.