Effects of leflunomide on glomerulonephritis induced by antibasement membrane antibody in rats

Methods for Testing Immunological Factors

Hypersensitivity reactions can be elicited by various factors: either immunologically induced, i.e., allergic reactions to natural or synthetic compounds mediated by IgE, or non-immunologically induced, i.e., activation of mediator release from cells through direct contact, without the induction of, or the mediation through immune responses. Mediators responsible for hypersensitivity reactions are released from mast cells. An important preformed mediator of allergic reactions found in these cells is histamine. Specific allergens or the calcium ionophore 48/80 induce release of histamine from mast cells. The histamine concentration can be determined with the o-phthalaldehyde reaction.

Leflunomide and malononitriloamides

Leflunomide is a new immunomodulatory drug effective in experimental models of autoimmune diseases and allo- or xenotransplantation. In a Phase II clinical trial leflunomide has shown high tolerability and efficacy in patients with advanced rheumatoid arthritis. The immunomodulatory activity of leflunomide is attributed to its primary metabolite, A77 1726, a malononitriloamide. The in vitro and in vivo mechanisms of action of this class of compounds remain to be completely defined. A77 1726 and several malononitriloamide analogues inhibit T- and B-cell proliferation, suppress immunoglobulin production, and interfere with cell adhesion. While no one central molecular mechanism of action has been proposed to explain all the effects of the malononitriloamides, inhibition of de novo pyrimidine biosynthesis and inhibition of cytokine- and growth factor-receptor associated tyrosine kinase activity are leading hypotheses for the effects of A77 1726 on T- and B-cell proliferation and function. Leflunomide is effective when administered at daily doses of 10 and 25 mg to patients with active rheumatoid arthritis. The improved efficacy at the 25 mg dose is associated with a higher incidence of adverse effects (gastrointestinal symptoms, weight loss, allergic reactions, skin rash, and reversible alopecia). Due to the long plasma half-life of A77 1726 (11-16 days), loading doses are required to achieve steady-state concentrations. Phase III randomised, placebo-controlled trials using daily doses of 10 or 20 mg are underway in the US and Europe to confirm and extend the results of the Phase II study. Malononitriloamide analogues of A77 1726 are being evaluated for immunosuppressive efficacy in preclinical models of transplantation, because these compounds have a shorter half-life in animals than A77 1726. If these analogues show efficacies similar to leflunomide in these models and have shorter half-lives than A77 1726 in Phase I trials, the preclinical and Phase I data will be used to select the analogues for Phase II trials in organ transplant recipients.

Leflunomide: an immunomodulatory drug for the treatment of rheumatoid arthritis and other autoimmune diseases

Leflunomide (Arava) has recently been approved by the Food and Drug Administration for the treatment of rheumatoid arthritis (RA). The drug, due to its protective effects on structural joint damage, has been classified as a disease modifying anti-rheumatic drug (DMARD). Leflunomide is structurally dissimilar from other drugs currently used to treat RA and exhibits a different mechanism of action. It has shown to be protective in a variety of animal models of arthritis and autoimmunity based on its immunomodulatory activity. Leflunomide is rapidly converted in vivo to its pharmacologically active metabolite A77 1726. This metabolite is a potent non-cytotoxic inhibitor of the enzyme dihydroorotate dehydrogenase (DHODH), a key enzyme in the de novo synthesis of uridine monophosphate (UMP). Activated lymphocytes depend on the pyrimidine de novo syntheses to fulfill their metabolic needs for clonal expansion and terminal differentiation into effector cells. De novo synthesis of pyrimidines is not only essential to provide precursors for new RNA and DNA synthesis, but also for phospholipid synthesis and the pyrimidine sugars necessary for protein glycosylation, which support the massive expansion in membrane biosynthesis to form daughter cells. This mechanism likely contributes to leflunomide's action as a DMARD in RA and other autoimmune diseases. This review is a summary of current in vivo and in vitro data, focussing primarily on the mechanism of action of leflunomide in RA.

Methotrexate and leflunomide: biochemical basis for combination therapy in the treatment of rheumatoid arthritis

OBJECTIVES

Methotrexate is currently one of the most widely prescribed disease-modifying antirheumatic drugs (DMARDs) for the treatment of rheumatoid arthritis (RA). Combination therapy of methotrexate with other DMARDs increases the clinical success of low-dose methotrexate treatment. Leflunomide is a new DMARD that may have a high potential for success in combination therapy with methotrexate. This review compares the mode of action of methotrexate and leflunomide and speculates on how this contributes to therapeutic efficacy in RA when these agents are used singly or in combination.

METHODS

A literature review of the biochemical mechanisms considered to be the basis for the therapeutic efficacy of methotrexate and leflunomide in treating RA is presented.

RESULTS

Low-dose methotrexate inhibits cytokine production, purine biosynthesis, and, in an animal model, causes the release of adenosine, a potent antiinflammatory agent. Leflunomide, through inhibition of de novo pyrimidine biosynthesis, can regulate lymphocyte proliferation.

CONCLUSIONS

The biochemical mechanisms underlying the therapeutic efficacy of low-dose methotrexate and leflunomide in the treatment of RA are quite different. The potentially complementary mechanisms of action of these two effective DMARDs should provide a rationale for their use in combination therapy for patients whose condition no longer responds to methotrexate alone.

Leflunomide protects mice from multiple low dose streptozotocin (MLD-SZ)-induced insulitis and diabetes

In certain animal models of autoimmunity the isoxasol derivative leflunomide has been reported to exert a protective effect against autodestruction. In the present study, the immunomodulatory potential of the main metabolite of leflunomide, A77 1726, in experimentally induced autoimmune diabetes was investigated. The disease was induced in genetically susceptible CBA/H mice by multiple low doses of streptozotocin (MLD-SZ, 40 mg/kg per day, given intraperitoneally for 5 consecutive days). Effects of leflunomide were evaluated by two treatment protocols: mice treated with MLD-SZ were injected intraperitoneally with A77 1726 for 10 consecutive days, either during the first 10 days of the disease (early treatment), or starting from day 10 after disease induction (late treatment). Disease manifestations defined by hyperglycaemia, mononuclear infiltration into pancreas, expression of interferon-gamma (IFN-gamma) and inducible nitric oxide synthase (iNOS) and destruction of the islets of Langerhans were reduced in a dose-dependent fashion after early treatment with A77 1726 (dose range of 5-35 mg/kg per day). Moreover, late treatment with the high dose of the drug (25 mg/kg per day), started when the autoimmune disease was already apparent, arrested progression of ongoing inflammatory response. Analysis of the effects of A77 1726 on the adhesive interactions of spleen-derived or peripheral blood-derived mononuclear cells from MLD-SZ-treated and normal mice demonstrated that the drug inhibits both ex vivo and in vitro spontaneous mononuclear cell aggregation, thus suggesting that an important component of leflunomide's immunomodulatory action is suppression of adhesive interactions. These results demonstrate both preventive and therapeutic effects of leflunomide in a model of MLD-SZ-induced diabetes and suggest that the drug may be considered a potent therapeutic tool for autoimmune inflammatory disorders, including diabetes.

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.

Use of type I and type IV hypersensitivity responses to define the immunopharmacological profile of drugs

Administration of antigen suspended in incomplete Freund's adjuvant supplemented with either heat-killed Mycobacterium tuberculosis (complete Freund's adjuvant, CFA) or Bordetella pertussis toxin sensitizes animals so that subsequent antigen challenge leads to delayed-type (DTH) or immediate type hypersensitivity (ITH) responses, named type IV and type I, respectively. Appropriate timing of administration of drugs with respect to immunization or antigen challenge allowed to detect predominantly immunosuppressive, antiinflammatory or antianaphylactic activities. Among the reference drugs tested, only cyclosporin A (CsA) and dexamethasone (Dex) markedly inhibited DTH reaction, due to their immunosuppressive and antiinflammatory activities, respectively, whereas leflunomide and indomethacin resulted less potent. On the other hand, only dexchlorpheniramine, a histamine-receptor antagonist, afforded significant protection against anaphylactic shock, a form of ITH. Two new chemical entities were studied according to this protocol: ITF 1697, a chemically stabilized C-reactive protein-derived tetrapeptide, and ITF 2018, a leflunomide analogue. Data obtained with these new compounds showed that ITF 1697 has antianaphylactic activity, while ITF 2018 is endowed, mainly, with antiinflammatory activity. These results show that, through appropriate timing of administration, established in vivo models of immunologically mediated disease states allow an accurate profiling of the effects of pharmacologically active molecules and the detection of unsuspected activities for new drugs.

Leflunomide, a novel immunomodulating drug, inhibits homotypic adhesion of peripheral blood and synovial fluid mononuclear cells in rheumatoid arthritis

OBJECTIVE AND DESIGN

A novel immunomodulating drug, leflunomide has been shown recently to be effective and well tolerated in patients suffering from rheumatoid arthritis (RA). The present study evaluated the effect of the drug on cell adhesion in RA.

MATERIAL AND TREATMENT

Peripheral blood and synovial fluid mononuclear cells were obtained from a clinical trial, undertaken primarily to evaluate the efficacy and pharmacokinetic profile of multiple-dose pulsing leflunomide therapy in RA patients. PB MNC and corresponding synovial fluid (SF) MNC for in vitro homotypic aggregation (HA) assay were obtained from healthy volunteers and RA patients with active disease not treated with leflunomide in vivo.

METHODS

Expression of activation antigens (CD25, CD54, CD69, CD71, HLA-DR) on peripheral blood mononuclear cells (PB MNC), as well as ex vivo ability of cells to aggregate spontaneously were determined in patients before entering into the clinical trial and at the end of 6 months treatment. HA was measured by aggregation in vitro. Data were compared by Student's t-test.

RESULTS

There was a decreased expression of activation antigens and decreased spontaneous MNC clustering after leflunomide therapy. We found in the in vitro study that HA of PB and SF MNC was mainly mediated through beta 2-integrin molecules. The active metabolite of leflunomide, A77 1726, effectively suppressed both spontaneous and phorbol-ester (PMA)-induced HA. Disruption of cell aggregates by A77 1726 was dose-dependent and, most likely, unrelated to the quantitative modulation of integrin receptors.

CONCLUSIONS

Results from this study support the idea that leflunomide elicits its immunomodulatory action, at least partially, by modulating the adhesion process.

Effects of leflunomide and other immunosuppressive agents on T cell proliferation in vitro

Leflunomide and its active metabolite, A771726, are structurally unrelated to immunosuppressive agents currently under investigation. Previous in vitro studies have revealed that leflunomide primarily inhibits interleukin-2-stimulated T cell proliferation. In the current study, we have extended our previous work and demonstrate that leflunomide prevents T cell progression induced by phytohemagglutinin into the S phase of the cell cycle. To discriminate further the action on T cells of leflunomide from other immunosuppressive agents, we performed kinetic studies where leflunomide was added either after the initiation of mixed lymphocyte cultures (MLC) or after interleukin-2 stimulation of CTLL-4 cell proliferation. These studies revealed that leflunomide acted comparably to rapamycin, but was distinct from brequinar sodium in the MLC, and from cyclosporine and mycophenolic acid in both MLC and CTLL-4. Although previous biochemical studies indicated that leflunomide can inhibit src-family tyrosine kinase activity, more recent studies have suggested that leflunomide can also inhibit pyrimidine synthesis. Our data demonstrate that the ability of leflunomide (25-100 microM) to inhibit MLC and CTLL-4 cell proliferation is partially antagonized by uridine (25-100 microM), and support the hypothesis that leflunomide inhibits pyrimidine synthesis in T cells. Unique molecular mechanisms of immunosuppression suggest that drug combinations may result in synergistic immunosuppression. Our in vitro studies revealed synergistic inhibition of T cell proliferation with the combinations of leflunomide with cyclosporine or with rapamycin. We have extended those studies to quantitate inhibition of MLC by the combinations of leflunomide and brequinar sodium or mycophenolic acid.

Leflunomide (HWA 486), a novel immunomodulating compound for the treatment of autoimmune disorders and reactions leading to transplantation rejection

Leflunomide has been shown to be very effective in preventing and curing several autoimmune animal diseases. Further, this agent is as effective as cyclosporin A in preventing the rejection of skin and kidney transplants in rats. Preliminary results from patients suffering from severe cases of rheumatoid arthritis demonstrated that clinical and immunological parameters could be improved with leflunomide therapy. Mode of action studies revealed that this substance antagonizes the proliferation inducing activity of several cytokines and is cytostatic for certain cell types. In this light, we could show that tyrosine phosphorylation of the RR-SRC peptide substrate and the autophosphorylation of the epidermal growth factor (EGF) receptor were, dose dependently, inhibited by leflunomide. EGF activates the intrinsic tyrosine kinase of its receptor, which stimulates the phosphorylation of a variety of peptides, the amino acid residue in all cases is tyrosine. These results indicate that much of leflunomide's activity could be due to the inhibition of tyrosine-kinase(s), which is an important general mechanism for the proliferation of various cell types. Thus, leflunomide, which is effective against autoimmune diseases and reactions leading to graft rejection, would seem to have a mode of action separating it from known immunosuppressive drugs.