Leflunomide, a novel immunosuppressive agent. The mechanism of inhibition of T cell proliferation

Pyrimidine de novo synthesis inhibition selectively blocks effector but not memory T cell development

Blocking pyrimidine de novo synthesis by inhibiting dihydroorotate dehydrogenase is used to treat autoimmunity and prevent expansion of rapidly dividing cell populations including activated T cells. Here we show memory T cell precursors are resistant to pyrimidine starvation. Although the treatment effectively blocked effector T cells, the number, function and transcriptional profile of memory T cells and their precursors were unaffected. This effect occurred in a narrow time window in the early T cell expansion phase when developing effector, but not memory precursor, T cells are vulnerable to pyrimidine starvation. This vulnerability stems from a higher proliferative rate of early effector T cells as well as lower pyrimidine synthesis capacity when compared with memory precursors. This differential sensitivity is a drug-targetable checkpoint that efficiently diminishes effector T cells without affecting the memory compartment. This cell fate checkpoint might therefore lead to new methods to safely manipulate effector T cell responses.

Effects of metabolic cancer therapy on tumor microenvironment

Targeting tumor metabolism for cancer therapy is an old strategy. In fact, historically the first effective cancer therapeutics were directed at nucleotide metabolism. The spectrum of metabolic drugs considered in cancer increases rapidly - clinical trials are in progress for agents directed at glycolysis, oxidative phosphorylation, glutaminolysis and several others. These pathways are essential for cancer cell proliferation and redox homeostasis, but are also required, to various degrees, in other cell types present in the tumor microenvironment, including immune cells, endothelial cells and fibroblasts. How metabolism-targeted treatments impact these tumor-associated cell types is not fully understood, even though their response may co-determine the overall effectivity of therapy. Indeed, the metabolic dependencies of stromal cells have been overlooked for a long time. Therefore, it is important that metabolic therapy is considered in the context of tumor microenvironment, as understanding the metabolic vulnerabilities of both cancer and stromal cells can guide new treatment concepts and help better understand treatment resistance. In this review we discuss recent findings covering the impact of metabolic interventions on cellular components of the tumor microenvironment and their implications for metabolic cancer therapy.

Leflunomide Inhibits rat-to-Mouse Cardiac Xenograft Rejection by Suppressing Adaptive Immune Cell Response and NF-κB Signaling Activation

Xenotransplantation is a potential solution for the severe shortage of human donor organs and tissues. The generation of humanized animal models attenuates strong innate immune responses, such as complement-mediated hyperacute rejection. However, acute vascular rejection and cell mediated rejection remain primary barriers to xenotransplantation, which limits its clinical application. In this study, we systematically investigated the immunosuppressive effect of LEF using a rat-to-mouse heart xenotransplantation model. SD rat xenogeneic hearts were transplanted into C57BL/6 mice, and survived 34.5 days after LEF treatment. In contrast, BALB/c allogeneic hearts were transplanted into C57BL/6 mice, and survived 31 days after LEF treatment. Compared to normal saline treatment, LEF treatment decreased xenoreactive T cells and CD19⁺ B cells in recipient splenocytes. Most importantly, LEF treatment protected myocardial cells by decreasing xenoreactive T and B cell infiltration, inflammatory gene expression, and IgM deposition in grafts. In vivo assays revealed that LEF treatment eliminated xenoreactive and alloreactive T and B lymphocytes by suppressing the activation of the NF-κB signaling pathway. Taken together, these observations complement the evidence supporting the potential use of LEF in xenotransplantation.

Effects of Paeoniflorin-6'-O-benzene sulfonate on the pharmacokinetics, excretion, and tissue distribution of leflunomide in rats

Paeoniflorin-6'-O-benzene sulfonate (CP-25) is a novel ester derivative of paeoniflorin, which has been shown to have synergistic pharmacodynamic effects with leflunomide (LEF). To determine the effects of CP-25 on the pharmacokinetics of LEF in rats, we developed an UPLC-MS/MS-based method for the determination of levels of teriflunomide (TER, an active metabolite of LEF). This method was used to determine TER concentrations in the plasma, urine, feces, and bile, heart, liver, spleen, lung, kidney, intestinal, brain, and synovial tissues, and peripheral blood mononuclear cells (PBMCs) of rats in the control [LEF (10 mg/kg)] and combined [CP-25(50 mg/kg×7d) plus LEF (10 mg/kg)] groups. TER AUC, T_max , MRT, t_1/2α , and t_1/2β were significantly lower and CL was significantly higher in the combined group than in the control group. Oral CP-25 administration in combination with LEF was found to promote TER excretion in urine, feces, and bile, and to reduce its contents in most tissues and organs, especially in the liver, which may reduce LEF-induced liver injury. CP-25 also increased TER exposure in the synovium and its absorption by PBMCs, and this could explain the synergistic effects of CP-25 and LEF.

Management of ganciclovir resistance cytomegalovirus infection with CMV hyperimmune globulin and leflunomide in seven cardiothoracic transplant recipients and literature review

Cytomegalovirus (CMV) disease caused by genetically resistant CMV poses a major challenge in solid organ transplant recipients, and the development of resistance is associated with increased morbidity and mortality. Antiviral resistance affects 5%-12% of patients following ganciclovir (GCV) therapy, but is more common in individuals with specific underlying risk factors. These include the CMV D+R- serostatus, type of transplanted organ, dose and duration of (Val)GCV ([V]GCV) prophylaxis, peak viral loads, and the intensity of immunosuppressive therapy. Guideline recommendations for the management of GCV resistance (GanR) in solid organ transplant recipients are based on expert opinion as there is a lack of data from controlled trials. Second-line options to treat GanR include foscarnet (FOS) and cidofovir (CDV), but these drugs are often poorly tolerated due to high rates of toxicity, such as renal dysfunction and neutropenia. Here, we report seven cardiothoracic transplant recipients with GCV resistance CMV infection from our centre treated with CMV immunoglobulin (CMVIG) +/- leflunomide (LEF) and reviewed the literature on the use of these agents in this therapeutic setting.

CD147 blockade as a potential and novel treatment of graft rejection

Cluster of differentiation (CD)147 is highly involved in the T cell activation process. High CD147 expression is observed on the surfaces of activated T cells, particularly CD4+ T cells. In organ transplantation, it is important to prevent graft rejection resulting from the excessive activation of T cells, particularly CD4+ T cells, which exhibit a key role in amplifying the immune response. The present study aimed to investigate the effects of CD147 blockade in vitro and in vivo and used a transplant rejection system to assess the feasibility of utilizing CD147 antibody-based immunosuppressant drugs for the treatment of graft rejection. The effects of CD147 antibodies were evaluated on lymphocyte proliferation stimulated by phytohemagglutinin or CD3/CD28 magnetic beads and in a one-way mixed lymphocyte reaction (MLR) system in vitro. For the in vivo analysis, an allogeneic skin transplantation mouse model was used. CD147 antibodies were effective against lymphocytes, particularly CD4+T lymphocytes, and were additionally effective in the one-way MLR system. In the allogeneic skin transplantation mouse model, the survival of transplanted skin was extended in the CD147 antibody-treated group. Furthermore, the level of inflammatory cell infiltration in transplanted skin was reduced. CD147 blockade decreased the serum levels of interleukin (IL)-17 and the proportions of peripheral blood CD4+ and CD8+ memory T cells. The data demonstrated that CD147 blockade suppressed skin graft rejection, primarily by suppressing CD4+T and memory T cell proliferation, indicating that CD147 exhibits great potential as a target of immunosuppressant drugs.

PTEN Regulates Glutamine Flux to Pyrimidine Synthesis and Sensitivity to Dihydroorotate Dehydrogenase Inhibition

Metabolic changes induced by oncogenic drivers of cancer contribute to tumor growth and are attractive targets for cancer treatment. Here, we found that increased growth of PTEN-mutant cells was dependent on glutamine flux through the de novo pyrimidine synthesis pathway, which created sensitivity to the inhibition of dihydroorotate dehydrogenase, a rate-limiting enzyme for pyrimidine ring synthesis. S-phase PTEN-mutant cells showed increased numbers of replication forks, and inhibitors of dihydroorotate dehydrogenase led to chromosome breaks and cell death due to inadequate ATR activation and DNA damage at replication forks. Our findings indicate that enhanced glutamine flux generates vulnerability to dihydroorotate dehydrogenase inhibition, which then causes synthetic lethality in PTEN-deficient cells due to inherent defects in ATR activation. Inhibition of dihydroorotate dehydrogenase could thus be a promising therapy for patients with PTEN-mutant cancers.Significance: We have found a prospective targeted therapy for PTEN-deficient tumors, with efficacy in vitro and in vivo in tumors derived from different tissues. This is based upon the changes in glutamine metabolism, DNA replication, and DNA damage response which are consequences of inactivation of PTENCancer Discov; 7(4); 380-90. ©2017 AACR.See related article by Brown et al., p. 391This article is highlighted in the In This Issue feature, p. 339.

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.

Multiparameter phospho-flow analysis of lymphocytes in early rheumatoid arthritis: implications for diagnosis and monitoring drug therapy

Background

The precise mechanisms involved in the initiation and progression of rheumatoid arthritis (RA) are not known. Early stages of RA often have non-specific symptoms, delaying diagnosis and therapy. Additionally, there are currently no established means to predict clinical responsiveness to therapy. Immune cell activation is a critical component therefore we examined the cellular activation of peripheral blood mononuclear cells (PBMCs) in the early stages of RA, in order to develop a novel diagnostic modality.

Methods and Findings

PBMCs were isolated from individuals diagnosed with early RA (ERA) (n = 38), longstanding RA (n = 10), osteoarthritis (OA) (n = 19) and from healthy individuals (n = 10). PBMCs were examined for activation of 15 signaling effectors, using phosphorylation status as a measure of activation in immunophenotyped cells, by flow cytometry (phospho-flow). CD3+CD4+, CD3+CD8+ and CD20+ cells isolated from patients with ERA, RA and OA exhibited activation of multiple phospho-epitopes. ERA patient PBMCs showed a bias towards phosphorylation-activation in the CD4+ and CD20+ compartments compared to OA PBMCs, where phospho-activation was primarily observed in CD8+ cells. The ratio of phospho (p)-AKT/p-p38 was significantly elevated in patients with ERA and may have diagnostic potential. The mean fluorescent intensity (MFI) levels for p-AKT and p-H3 in CD4+, CD8+ and CD20+ T cells correlated directly with physician global assessment scores (MDGA) and DAS (disease activity score). Stratification by medications revealed that patients receiving leflunomide, systemic steroids or anti-TNF therapy had significant reductions in phospho-specific activation compared with patients not receiving these therapies. Correlative trends between medication-associated reductions in the levels of phosphorylation of specific signaling effectors and lower disease activity were observed.

Conclusions

Phospho-flow analysis identified phosphorylation-activation of specific signaling effectors in the PB from patients with ERA. Notably, phosphorylation of these signaling effectors did not distinguish ERA from late RA, suggesting that the activation status of discrete cell populations is already established early in disease. However, when the ratio of MFI values for p-AKT and p-p38 is >1.5, there is a high likelihood of having a diagnosis of RA. Our results suggest that longitudinal sampling of patients undergoing therapy may result in phospho-signatures that are predictive of drug responsiveness.

Activation of the enhancer of zeste homologue 2 gene by the human papillomavirus E7 oncoprotein

The malignant phenotype of human papillomavirus (HPV)-positive cancer cells is maintained by the activity of the viral E6 and E7 genes. Here, we identified the polycomb group gene enhancer of zeste homologue 2 (EZH2) as a novel downstream target for the viral oncogenes in HPV-transformed cells. EZH2 expression was activated by HPV16 E7 at the transcriptional level via E7-mediated release of E2F from pocket proteins. RNA interference analyses showed that continuous EZH2 expression is required for the proliferation of HPV-positive tumor cells by stimulating cell cycle progression at the G1-S boundary. In addition to its growth-promoting activity, EZH2 also contributed to the apoptotic resistance of cervical cancer cells. Furthermore, we found that HPV-positive dysplastic and tumorigenic cervical lesions were characterized by high levels of EZH2 protein in vivo. We conclude that the E7 target gene EZH2 is a major determinant for the proliferation of HPV-positive cancer cells and contributes to their apoptotic resistance. Moreover, EZH2 may serve as a novel therapeutic target for the treatment of cervical cancer.

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.

Differential effects of leflunomide and methotrexate on cytokine production in rheumatoid arthritis

BACKGROUND

T cells have a pivotal role in RA. Leflunomide inhibits pyrimidine biosynthesis, to which T cells are especially susceptible, and therefore may have a different cytokine profile than methotrexate.

MATERIALS AND METHODS

Serum samples of 100 patients with RA, treated with leflunomide (n = 50) or methotrexate (n = 50), were collected at baseline, after 16 weeks and after 1 year's treatment. Serum levels of interleukin 6 (IL6), and interferon (IFN) gamma were determined by ELISA. Additionally, peripheral blood mononuclear cells (PBMC) of five healthy volunteers and three patients with RA were isolated and the effects of the active metabolite of leflunomide (A77-1726, 0-200 mmol/l) on cell proliferation and on IL6 and IFNgamma production were determined by ELISA. In peripheral blood lymphocytes (PBL) and monocytes (PBM) from two healthy volunteers the effects of A77-1726 on IL6 production were measured by ELISA and PCR.

RESULTS

Mean (SEM) serum levels of IFNgamma were significantly reduced after leflunomide treatment (baseline 43 (10) pg/ml; 1 year 29 (7) (p = 0.015), but there was no change in IL6 levels (baseline 158 (41), 1 year 151 (48)). Both IFNgamma and IL6 levels were significantly reduced after methotrexate treatment. This observation was supported by in vitro experiments. The production of IFNgamma by PBL was inhibited by A77-1726, but IL6 production by PBM was not inhibited.

CONCLUSION

The differential effect on IFNgamma and IL6 production supports the hypothesis that activated T cells are preferentially inhibited by leflunomide. An explanation may be either inhibition of uridine synthesis or effects on signal transduction pathways.

Simultaneous determination of leflunomide and its active metabolite, A77 1726, in human plasma by high-performance liquid chromatography

The isoxazol derivative leflunomide [N-(4'-trifluoromethylphenyl)-5-methylisoxazole-4-carboxamide] is an inhibitor of de novo pyrimidine synthesis used for the treatment of rheumatoid artrithis. In the present study, a liquid-liquid extraction-based reversed-phase HPLC method with UV detection was validated and applied for the analysis of leflunomide and its active metabolite, A77 1726, in human plasma. The analytes were separated using a mobile phase, consisting of acetonitrile, water and formic acid (40/59.8/0.2, v/v), at a flow rate of 0.5 mL/min, and UV detection at 261 nm. The retention times for A77 1726, leflunomide and warfarin (internal standard) were 8.2, 16.2 and 12.2 min, respectively. The validated quantification range of the method was 0.05-100 micro g/mL for leflunomide and 0.1-100 micro g/mL for A77 1726. The developed procedure was applied to assess steady-state plasma concentrations of A77 1726 in patients with rheumatoid arthritis treated with 10 or 20 mg leflunomide per day.

CD4+ T cell matters in tumor immunity

CD4+ T cells have been shown to be able to affect tumor growth through both direct and indirect means. In addition, a requirement has been demonstrated for CD4+ T cells in the regulation and induction of T cell memory, and CD4+ suppressor T cells have been identified, stressing a role for CD4+ T cells in the induction and maintenance of antitumor immune responses. A review of the involvement of CD4+ T cells at different stages of tumor immunity is provided, and based on these data we discuss how CD4+ T cell response induction could be incorporated into tumor immunotherapy strategies.

Mechanism of leflunomide-induced proliferation of mitochondria in mammalian cells

Leflunomide (LFM) is an inhibitor of mitochondrial enzyme dihydroorotate dehydrogenase (DHODH) that catalyzes the conversion of dihydroorotate to orotate coupled with the generation of reactive oxygen species (ROS) from mitochondria. We demonstrate here that LFM causes an unrestrained proliferation of mitochondria both in human osteosarcoma cell line 143B cells and rat liver derived RL-34 cells. Increases in the total mass of mitochondria per cell in LFM-treated cells were evidenced by the application of Green FM or 10-n-nonyl acridine orange to flow cytometry, an enhanced replication of mtDNA and electron microscopy. Externally added uridine improved the disturbance in cell cycle progression in LFM-treated cells, but failed to suppress such unrestrained mitochondrial proliferation. On the contrary, lapacol and 5-fluoroorotate, inhibitors of DHODH besides LFM, suppressed the biogenesis of mitochondria during the cell cycle progression. LFM, but not lapacol or 5-fluoroorotate, caused increases of the intracellular level of acetylated alpha-tubulin. These data suggest that the inhibition of DHODH may not be at least primarily related to the LFM-induced abnormal proliferation of mitochondria, and support our recent published observation that changes in the physicochemical properties of microtubules may be in someway concerned with the biogenesis of mitochondria.

Application of LC/MS/MS in the quantitation of SU101 and SU0020 uptake by 3T3/PDGFr cells

SU101 or leflunomide, has been studied extensively because of its anti-cancer and immunomodulating properties. The parent isoxazole compound is converted in vitro and metabolized in vivo to an open ring isomeric form, SU0020. Several pharmacological activities have been reported for the parent and metabolite compounds including inhibition of platelet-derived growth factor (PDGF)-mediated signaling for the parent compound and inhibition of de novo pyrimidine biosynthesis for the metabolite. The inhibition of PDGF-mediated signaling and the anti-tumor properties have been ascribed to the parent compound. In spite of its short plasma half-life of the parent molecule, SU101 can be administered intermittently in animal tumor models and retain efficacy. Therefore, the relationship between plasma levels of SU101 and its efficacy in tumor-implanted immuno-compromised mice is not well established. This study was conducted to assess the concentration of SU101 in 3T3/PDGFr alpha and beta cells (NIH3T3 mouse fibroblasts engineered to overexpress human PDGFr alpha or beta) to better understand the cellular levels of SU101 and SU0020. Two strains of 3T3/PDGFr cells (alpha and beta) were incubated with 1, 25, and 100 microM concentrations of SU101 for 1, 6, 24, and 48 hours. Quantitation of SU101 and SU0020 in these cell lines was achieved by a specific and sensitive liquid chromatography-tandem mass spectrometry (LC/MS/MS) method. Interestingly, in both alpha and beta cell lysates SU101 was much more concentrated than SU0020. The greater concentration of SU101 versus SU0020 that was observed may be due to the preferential partitioning of SU101 into the cells and this shows that significant levels of the parent drug can reach the pharmacological site of action for inhibition of PDGF receptors. The data suggest that the conversion of SU101 to SU0020 is much slower in these cells than in the incubation media.

Experiences with leflunomide in solid organ transplantation

BACKGROUND

Leflunomide (Arava), a drug widely used for treatment of rheumatoid arthritis, has a very promising background in experimental transplantation. Its activity in experimental models of chronic rejection, its synergy with calcineurin phosphatase inhibitors, and its inhibitory effects on herpes virus replication are compelling reasons to pursue its clinical evaluation in transplantation. We report the use of this drug over the past 3 years in various clinical situations.

METHODS

A retrospective review was performed in 53 liver and kidney transplant recipients receiving Arava. A single-dose pharmacokinetic (PK) study was first performed in stable, renal transplant recipients, and an initially targeted serum level of 100 microg/mL (300 microM) was calculated to require a loading dose of 1200-1400 mg over a 7-day period. We correlate the appearance of toxicity with serum levels of active drug and review the outcomes in patients whose clinical condition required dose reductions of conventional immune suppressive drugs.

RESULTS

Fifty-three patients received leflunomide from 5 days to more than 430 days, and 37 patients received the drug for more than 60 days. The primary toxicity was anemia in the renal transplant patients and elevation of liver enzymes in the liver transplant patients. At comparable oral doses, serum levels were substantially lower and anemia more common in patients with serum creatinine >3 mg/dL. In liver and renal recipients with serum creatinine <3 mg/dL, the drug was well tolerated and dose-limiting side effects occurred in less than 15% when drug serum levels were less than 80 microg/ml. Patients with serum creatinine >3 mg/dL often required serum levels of active drug reduced to <60 microg/mL. In 12 of 18 renal patients treated for 200 days or more, the dose of cyclosporine or Prograf was reduced by a mean of 38.5% and stopped in one patient. The prednisone dose was reduced by a mean of 25% in these same 13 patients. Cyclosporine or FK506 was stopped completely in four liver recipients and reduced by 65% in another patient. No evidence of acute rejection developed in any of these liver or kidney transplant patients.

CONCLUSION

Leflunomide seems to possess substantial immune suppressive potency in renal and liver transplant recipients and may be safely dosed for more than 300 days. The data suggest that calcineurin phosphatase inhibitors and prednisone can be safely reduced in patients with serum levels of active drug above 50 microg/mL. Because of a wide inter-patient range of active metabolite terminal half-life (>300%), monitoring of serum levels would seem to be an important part of its evaluation.

The serine phosphatases PP1 and PP2A associate with and activate the actin-binding protein cofilin in human T lymphocytes

Cofilin, an actin-depolymerizing protein, is essential for the functional dynamics of the actin cytoskeleton and for cell viability. In unstimulated human peripheral blood T lymphocytes cofilin is phosphorylated and localized in the cytoplasm. Following co-stimulation through accessory receptors (e.g. CD2 or CD28) - however, not following TCR/CD3 stimulation alone - cofilin undergoes dephosphorylation. The subcellular localization as well as the actin-binding activity of cofilin are regulated by the phosphorylation state of serine-3. Thus, only the dephosphorylated form of cofilin associates with the actin cytoskeleton and possesses the capability to translocate into the nucleus. Recently, LIM-kinase 1 was shown to inactivate cofilin through phosphorylation. Here, we have identified the functional counterparts of LIM-kinase 1: the serine/threonine phosphatases of type 1 and type 2A not only associate with cofilin but also dephosphorylate this 19-kDa protein and thereby mediate cofilin activation. In malignant T lymphoma cells, activation of these phosphatases occurs spontaneously, independent of external stimuli. In untransformed human peripheral blood T lymphocytes, these phosphatases function through a cyclosporin A/FK506-resistant co-stimulatory signaling pathway which is common for the accessory receptors CD2 and CD28. This co-stimulatory signaling pathway is also not affected by a series of other clinically established immunosuppressive drugs (i.e. rapamycin, dexamethasone, leflunomide or mycophenolic acid).

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.

Novel mechanism of inhibition of cytomegalovirus by the experimental immunosuppressive agent leflunomide

BACKGROUND

Despite progress in antiviral chemotherapy, cytomegalovirus (CMV) remains a major cause of morbidity and mortality among pharmacologically immunosuppressed organ transplant recipients, frequently engaging the clinician in a struggle to balance graft preservation with control of CMV disease. Leflunomide, an inhibitor of protein kinase activity and pyrimidine synthesis, is an experimental immunosuppressive agent effective against acute and chronic allograft rejection in animal models. Because a number of CMV proteins are known to be phosphorylated, we tested the hypothesis that this agent might exert inhibitory activity against CMV.

METHODS AND RESULTS

Plaque assays demonstrated dramatic dose-dependent attenuation of production of multiple clinical CMV isolates in leflunomide-treated human fibroblasts and endothelial cells, common targets for CMV infection in vivo. As shown by Northern blot analysis and immunohistochemical staining, leflunomide neither interferes with transcription of immediate early or late viral genes, nor with expression of corresponding proteins. CMV-specific DNA dot blots and biochemical enzyme assays indicated that, in contrast to currently approved anti-CMV drugs, leflunomide exerts no inhibitory effect on the accumulation of viral DNA in infected cells, or on viral DNA polymerase activity. Rather, as visualized by transmission electron microscopy, this agent appears to act at a late stage in virion assembly by preventing tegument acquisition by viral nucleocapsids. Finally we have demonstrated equivalent inhibitory activity of leflunomide against multi-drug-resistant CMV isolates.

CONCLUSIONS

These findings imply that leflunomide, an effective immunosuppressive agent, shows potential to concurrently attenuate a major complication of immunosuppression, CMV disease, by a novel mechanism of antiviral activity.

Malononitrilamides and tacrolimus additively prevent acute rejection in rat cardiac allografts

The novel immunosuppressive agents malononitrilamides (MNA) 279 and 715 are derivatives of A77 1726, the primary metabolite of leflunomide. The effects of these agents have been previously demonstrated in rat skin and cardiac allo- and xenotransplant models. The aim of this study was to evaluate the combination of MNA and tacrolimus in a high-responder rat cardiac allotransplant model. Graft survival was evaluated following 10 days of post-transplant oral therapy of MNA or tacrolimus alone and the drugs combined in PVG recipients of DA grafts. An iso-effect curve of single and combined drugs was constructed. Histological changes in grafts were evaluated at 10 days. MNA (20 mg/kg) or tacrolimus (2.4 mg/kg) alone prolonged graft survival with median survival of 14 and 13.5 days, respectively. Combined therapy of MNA (10 mg/kg) and tacrolimus 1.6 mg/kg likewise resulted in a median survival of 13.25 days and an iso-effect curve for these doses was constructed. Another iso-effect curve for median graft survival of 18-19 days, including MNA (10 mg/kg) + tacrolimus (3.2 mg/kg) in combination, MNA (30 mg/kg) alone and tacrolimus (4.8 mg/kg) alone, was constructed and both isoboles showed a straight line, demonstrating additive effects (zero interaction). In addition, histological analysis of grafts confirmed the benefit of the drug combination. No additional toxicity was noted with combined therapy. Optimal doses of MNA or tacrolimus had comparative effects on graft survival and histological changes, and a combination of the two drugs was beneficial with respect to both these parameters. The iso-effect curves verified additive effects of the drug combination.

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.

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.

Newer immunosuppressive drugs: a review

In recent years, many new immunosuppressive drugs have been discovered and developed for clinical use in transplantation. This review focuses on those drugs (leflunomide, mycophenolate mofetil, sirolimus, tacrolimus) that have been shown to have immunosuppressive activity in patients. Different anti-interleukin-2 receptor antibodies are also reviewed as an example of a resurgence of development in the area of monoclonal antibodies. The price for reducing the incidence of allograft rejection by improved immunosuppression was thought to be a proportional increase in the incidence of infection and malignancy. Data from Phase III clinical trials of new immunosuppressants, however, show a statistically significant reduction in the incidence of acute rejection produced by these new drugs, which has not been accompanied by increases in infection and malignancy rates. The wide array of new drugs offers the opportunity to use combinations that block different pathways of immune activation while at the same time selecting drug combinations with nonoverlapping toxicity profiles so that doses of each single drug can be reduced below toxicity levels. The immunosuppressive therapy for patients can be tailored according to their individual needs.

Inhibition of murine IgE and immediate cutaneous hypersensitivity responses to ovalbumin by the immunomodulatory agent leflunomide

Leflunomide has been identified as an immunoregulatory and anti-inflammatory compound. Allergic disease is characterized by elevated serum IgE levels, production of allergen-specific IgE and the release of inflammatory mediators from mast cells and granulocytes. Here we demonstrate, using an in vivo murine model, the ability of leflunomide to down-regulate levels of total and allergen-specific serum IgE production. Mice receiving leflunomide (45 mg/kg) orally at the time of primary immunization with ovalbumin adsorbed to aluminium hydroxide adjuvant, showed a reduction in total serum IgE levels of 95%, 41% and 32% following primary, secondary and tertiary immunizations, respectively (P < 0.05). When leflunomide was administered both at the time of primary and subsequent immunizations, reductions in total and specific serum IgE levels of > 80% and > 38%, respectively, were observed (P < 0.05). Administration of leflunomide to mice which had already developed an IgE response resulted in reductions in total and specific serum IgE levels of > 80% and > 45%, respectively (P < 0.05). Following leflunomide treatment, animals failed to develop immediate cutaneous hypersensitivity responses when challenged intradermally with allergen. Down-regulation of immunoglobulin production was not restricted to IgE, since levels of allergen-specific IgG1 and IgG2a in serum were also reduced. The finding of significant reductions in total and allergen-specific IgM suggests that the mechanism of action does not involve selective inhibition of immunoglobulin class switching. A loss in production of the T helper cell-derived B cell differentiation factor IL-5 may account for the reduction in immunoglobulin levels. In adoptive transfer experiments leflunomide did not induce tolerance in allergen-reactive Th2 populations, contrary to animal disease models of transplantation and autoimmunity, where leflunomide was shown to induce tolerance in the effector T cell population.

FK506 treatment in combination with leflunomide in hamster-to-rat heart and liver xenograft transplantation

BACKGROUND

In the experiment described here, we investigated the effects of the immunosuppressants FK506 and leflunomide (Lef) on the survival of hamster hearts and liver xenografts in Lewis rats.

METHODS

Lewis rats were used as recipients of hamster heart or liver grafts using different regimens of FK506 and Lef. Donor-matched heart grafts were transplanted into long-term surviving Lewis rat recipients of hamster xenografts to test donor-specific prolongation of xenograft survival. Hyperimmune, late xenograft rejection, and naive sera were transferred into long-term surviving Lewis rat recipients of hamster heart xenografts to determine whether these sera could inhibit the efficacy of donor-specific long-term survival. Anti-donor-specific antibodies were analyzed by flow cytometry.

RESULTS

After a short induction with FK506 plus Lef, maintenance treatment with FK506 alone was sufficient to prolong survival of hamster xenografts. All hamster heart and four of six hamster liver xenografts survived for more than 3 months. Second hamster hearts were permanently accepted by Lewis rats bearing long-term surviving hamster heart xenografts when rats were treated with FK506 monotherapy (mean survival time >60 days, n=4). Long-term surviving hamster heart grafts were rejected after transfer of hyperimmune serum but not late xenograft rejection serum or naive serum. Lef and FK506 significantly reduced the production of anti-donor-specific antibodies in Lewis rats transplanted with hamster liver and heart xenografts.

CONCLUSION

Long-term survival of hamster liver and heart xenografts in Lewis rats could be induced by a regimen of short-term FK506 in combination with Lef followed by FK506 monotherapy. The acquired sensitivity of late xenoreactivity to FK506 reflects primarily a modification in the host immune response to the hamster graft.

Malononitrilamides 715 and 279 prolong rat cardiac allograft survival, reverse ongoing rejection, inhibit allospecific antibody production and interact positively with cyclosporin

A77 1726 is a malononitrilamide (MNA) and the active metabolite of leflunomide, which has been extensively investigated and shown to be a potent immunosuppressive drug. However, the half-life of A77 1726 is about 15-18 days in humans and leflunomide is therefore currently being developed for the treatment of autoimmune disease and not for transplantation. Search for analogues has led to the discovery of MNA 715 and 279, derivatives of A77 1726. Previous experimental experience of these compounds is still limited. The aim of the present study was to verify the efficacy of these MNAs concerning prevention and reversal of rejection, inhibition of antibody production and interaction with cyclosporin A (CsA). Heterotopic cardiac transplantation in DA to PVG rats was used. Subgroups of rats were given either CsA, MNA 715 or MNA 279 for 10 days, starting at either day 0 or day 4, or received no treatment. Titres of allospecific immunoglobulin M (IgM) and immunoglobulin G (IgG) were quantified by flow cytometry. Ten days of induction with MNA 715 or 279 produced significantly longer graft survival than in controls. Treatment from day 4 onwards, when acute rejection was established, rescued all grafts. Allospecific production of IgM or IgG was absent during MNA induction and was suppressed in animals receiving a rescue course of MNA. The transplant model was potentiated by addition of the immunomodulator quinolone-3-carboxamide (Linomide), which eliminates the effect of CsA and other immunosuppressants. The combined treatment with MNA and CsA was successful in overcoming the challenge of Linomide, demonstrating the additive effects of the two drugs. In conclusion, MNA 715 and 279 were shown to be potent immunosuppressants, preventing and reversing acute allograft rejection, inhibiting and suppressing allospecific antibody production, and the drugs interacted positively with CsA.

Dose-dependent enhancing and inhibitory effects of A77 1726 (leflunomide) on cytotoxic T lymphocyte induction

Leflunomide is an immunosuppressive prodrug which prevents allograft rejection in several animal model systems and may, therefore, have clinical application in organ transplant recipients. Although cytotoxic T lymphocytes (CTL) are an important component of the allograft rejection response, the effect of leflunomide on CTL development has not been thoroughly explored. In this study we have determined the effect of A77 1726, the active metabolite of leflunomide, on CTL induction in C57BL/6 mouse T cell cultures stimulated with anti-CD3 monoclonal antibody. Conjugate formation with P815 target cells, granzyme B enzymatic activity in CTL lysates, and P815 cytolysis in a 51Cr-release assay were used as determinants of in vitro CTL function. At high concentrations (10-20 microM), A77 1726 strongly inhibited CTL generation. In contrast, a low concentration (0.5 microM) of A77 1726 promoted CTL development. These dose-dependent opposing effects of A77 1726 on CTL induction could not be attributed to alterations in CD8+ lymphocyte percentages, interleukin-2 or CD25 expression, or the ability to conjugate with P815 target cells. However, both interferon-gamma and granzyme B expression were significantly decreased when CTL were induced in the presence of 10-20 microM A77 1726, and were slightly, but not always significantly, elevated in the presence of 0.5 microM A77 1726. We conclude that at high concentrations A77 1726 is a potent inhibitor of CTL induction, but a low concentration of A77 1726 enhances CTL development.

Malononitrilamides 715 and 279 prevent accelerated cardiac allograft rejection synergistically with cyclosporin A in presensitized rats

A77 1726 is a malononitrilarnide (MNA) and the active metabolite of leflunomide. Leflunomide has been extensively investigated and shown to be a potent immunosuppressive drug. However, the half-life of A77 1726 is about 15-18 days in humans and the leflunomide is, therefore, currently being evaluated for the treatment of autoimmune disease and not for transplantation. The search for analogues has led to the development of MNA 715 and 279, derivatives of A77 1726. Previous limited experimental experience has shown these MNAs to prevent skin allograft and xenograft rejection and graft-versus-host disease in rodents, and to reverse ongoing allograft rejection. The aim of the present study was to verify the efficacy of these MNAs in a cardiac retransplant model of sensitized rats, concerning prevention of accelerated rejection, inhibition of antibody production and interaction with cyclosporin A (CsA). Heterotopic cardiac transplantation and retransplantation in Dark Agouti (DA) to Piebald Virol Glaxo (PVG) rats was used. Subgroups of rats were given either CsA, MNA 715, MNA 279 or combined CsA/MNA for 10 days starting either day 0 or day -1, as of regrafting or no treatment. Titres of allospecific IgM and IgG were quantitated by flow cytometry. Ten days of MNA 715 or 279 from day -1 prevented accelerated rejection of the retransplant, as did CsA. Neither treatment given from day 0 prevented rejection within 24 h. However, a combination of MNA 715 or 279 and CsA from day 0 effectively prevented accelerated regraft rejection. Production of specific alloantibodies was reduced in all immunosuppressed subgroups, IgG titres at day 7 in MNA-treated subgroups being significantly lower compared with those in the CsA-treated subgroup. In conclusion, MNA 715 and 279 were shown to be potent immunosuppressants with the capacity to prevent accelerated regraft rejection in rat cardiac transplants, most efficiently in combination with CsA, and to suppress specific alloantibody production.

Immunosuppression preventing concordant xenogeneic islet graft rejection is not sufficient to prevent recurrence of autoimmune diabetes in nonobese diabetic mice

BACKGROUND

We and others have reported previously that the immunosuppressant, leflunomide (Lef), can prevent allogeneic and xenogeneic islet graft rejection in streptozocin (STZ)-induced diabetic animals. However, whether Lef required to prevent islet graft rejection is sufficient to prevent the recurrence of autoimmune diabetes has not been addressed.

METHODS

The effect of Lef on concordant xenogeneic islet graft in STZ-induced diabetic mice and autoimmune nonobese diabetic (NOD) mice were studied. Then, whether Lef prevents the onset of spontaneous diabetes in young NOD mice and the recurrence of diabetes after major histocompatibility complex (MHC)-matched islet transplantation in diabetic NOD mice were investigated.

RESULTS

In STZ-induced diabetic BALB/c mice, Lef treatment significantly prolonged rat islet graft survival. However, Lef could not significantly prolong rat islet graft survival in autoimmune diabetic NOD mice. For prevention studies, treatment with Lef at 30 mg/ kg/day from 4 weeks to 20 weeks of age significantly reduced the incidence of spontaneous diabetes in NOD mice. However, when the NOD mice were treated from 8 to 24 weeks of age, the incidence of spontaneous diabetes was not significantly reduced as compared to the incidence of diabetes in the untreated female NOD mice at 28 weeks of age. Finally, in the MHC-matched islet transplant model, Lef could not significantly prolong MHC-matched nonobese diabetes-resistant mice islet graft survival in NOD mice.

CONCLUSIONS

Lef preventing concordant xenogeneic islet graft rejection is not sufficient to prevent the recurrence of autoimmune diabetes in NOD mice. We believe that controlling autoimmunity after islet transplantation will lead the way to promote successful clinical islet transplantation in the future.

Molecular cloning of Saccharomyces cerevisiae MLF4/SSH4 gene which confers the immunosuppressant leflunomide resistance

Immunosuppressant leflunomide inhibits the growth of cytokine-stimulated lymphoid cells in vitro and also inhibits the growth of eukaryotic microorganism Saccharomyces cerevisiae. To elucidate the molecular mechanism of the action of the drug, a yeast gene which suppresses the anti-proliferative effect when in increased copy number was cloned and designated MLF4 for multicopy suppressor of leflunomide sensitivity. DNA sequencing analysis indicates that the MLF4 gene is identical to the SSH4 gene which suppresses the shr3 mutation. Excess of amino acids overcame the anti-proliferative activity of leflunomide. Thus, leflunomide is suggested to affect amino acid transport by interacting with Shr3 chaperon-like protein.

Inhibition of anti-CD3 antibody-induced mouse T cell activation by pentoxifylline in combination with rapamycin or A77 1726 (leflunomide)

Pentoxifylline (PTX), rapamycin (RAP), and leflunomide are potent immunomodulatory drugs with differing modes of action. In order to develop new drug combinations for immunotherapy, we tested the effects of PTX in combination with RAP or A77 1726 (the active metabolite of leflunomide) on in vitro T cell activation in a mouse model system. T lymphocytes in spleen cell preparations were stimulated with anti-CD3 monoclonal antibody alone, or in the presence of PTX (25-200 microg/ml), RAP (0.5-5.0 ng/ml), A77 1726 (2.5-10.0 microM), PTX/RAP (25-200 microg/ml and 0.5-5.0 ng/ml, respectively), or PTX/A77 1726 (25-200 microg/ml and 2.5-10.0 microM, respectively). Anti-CD3-induced T cell proliferation was inhibited in a dose-dependent fashion by the individual drugs. An additive inhibitory effect was observed in cultures treated with PTX/RAP or PTX/A77 1726. The effects of PTX, RAP, A77 1726, PTX/RAP, or PTX/A77 1726 (at concentrations approximating the IC50 of individual drugs for inhibition of lymphoproliferation) on anti-CD3-activated killer (AK) cell induction, CD25 expression, and interleukin (IL)-2 synthesis in anti-CD3-activated spleen cell cultures were also determined. Alone, each drug was able to suppress AK cell induction to varying degrees. PTX plus RAP exhibited strong synergism, while the combination of PTX and A77 1726 had an additive inhibitory effect on AK cell induction. CD25 expression was only weakly inhibited by A77 1726, but the percentage of CD25-expressing cells was greatly reduced in cultures treated with PTX or RAP. The combination of PTX and RAP had an additive inhibitory effect on CD25 expression while PTX and A77 1726 together had an effect equivalent to PTX alone. IL-2 synthesis was inhibited by PTX but was unaffected by RAP or A77 1726. Treatment with PTX plus RAP led to a further reduction in IL-2 production but co-treatment with PTX and A77 1726 approximated the inhibitory effect of PTX alone. We conclude that the combination of PTX and RAP is noteworthy for its potent immunomodulatory activity and may be of use in clinical situations where it is desirable to prevent T cell activation.

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.

Suppression of T-independent IgM xenoantibody formation by leflunomide during xenografting of hamster hearts in rats

BACKGROUND

It was recently shown that leflunomide (LF) delayed xenoantibody (XAb) formation and xenograft (Xg) rejection in a hamster-to-rat heart transplantation model. Our aim in this study was further investigation of the mechanism of LF-mediated suppression of XAb formation.

METHODS

Hamster hearts were heterotopically transplanted to euthymic or nude rats receiving LF and/or cyclosporine (CsA). Second hamster hearts were transplanted at the time of first Xg rejection. Serum from rejecting rats was transferred to naive rats receiving a hamster heart Xg. The isotype of XAbs was examined by fluorescence-activated cell sorting. Tissue deposition of XAbs and complement was determined by immunofluorescence. XAb formation and its response to LF were also investigated in severe combined immunodeficient mice reconstituted with purified CD5+ or CD5- rat B cells.

RESULTS

After xenografting, untreated PVG rats developed high titers of anti-hamster IgM XAbs that appeared T-independent (T-I) as they could not be suppressed by CsA and also occurred in athymic nude rats. A second Xg transplanted in control or CsA-treated rats rejecting a first Xg was subject to hyperacute rejection. Hyperacute rejection also occurred in naive rats after adoptive transfer of serum from rejecting rats. Monotherapy with LF resulted in a suppression of early IgM XAb formation and in a delay of Xg rejection, which was associated with predominantly IgG anti-hamster XAbs. These XAbs were T-dependent, as they did not occur in nude rats and were suppressed by CsA. CD5+ B lymphocytes appeared to contribute to T-I IgM XAb formation, as LF reduced the percentage of peripheral blood CD5+ B lymphocytes and severe combined immunodeficient mice reconstituted with purified CD5+ B cells, but not with CD5- B cells, produced anti-hamster IgM which were suppressed by LF but not CsA.

CONCLUSIONS

In rats, T-I XAb formation is a first step leading to hamster Xg rejection and is suppressed by LF leading to prolonged Xg survival.

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.

Prevention of transplant rejection: current treatment guidelines and future developments

In the past 2 decades, progressive improvements in the results of organ transplantation as a therapeutic strategy for patients with end-stage organ disease have been achieved due to greater insight into the immunobiology of graft rejection and better measures for surgical and medical management. It is now known that T cells play a central role in the specific immune response of acute allograft rejection. Strategies to prevent T cell activation or effector function are thus all potentially useful for immunosuppression. Standard immunosuppressive therapy in renal transplantation consists of baseline therapy to prevent rejection and short courses of high-dose corticosteroids or monoclonal or polyclonal antibodies as treatment of ongoing rejection episodes. Triple-drug therapy with the combination of cyclosporin, corticosteroids and azathioprine is now the most frequently used immunosuppressive drug regimen in cadaveric kidney recipients. The continuing search for more selective and specific agents has become, in the past decade, one of the priorities for transplant medicine. Some of these compounds are now entering routine clinical practice: among them are tacrolimus (which has a mechanism of action similar to that of cyclosporin), mycophenolate mofetil and mizoribine (which selectively inhibit the enzyme inosine monophosphate dehydrogenase, the rate-limiting enzyme for de novo purine synthesis during cell division), and sirolimus (rapamycin) [which acts on and inhibits kinase homologues required for cell-cycle progression in response to growth factors, like interleukin-2 (IL-2)]. Other new pharmacological strategies and innovative approaches to organ transplantation are also under development. Application of this technology will offer enormous potential not only for the investigation of mechanisms and mediators of graft rejection but also for therapeutic intervention.

Leflunomide and malononitrilamides

Leflunomide is a new immunomodulatory drug that is effective in experimental models of autoimmune diseases and in allo or xenotransplantation. In a phase II clinical trial, leflunomide showed high tolerability and efficacy in patients with advanced rheumatoid arthritis. The immunomodulatory activity of leflunomide is attributed to its primary metabolite A77 1726, which is a malononitrilamide. The in vitro and in vivo mechanisms of action of this class of compounds are not defined completely. Several malononitrilamide analogues and A77 1726 inhibit T- and B-cell proliferation, suppress immunoglobulin production, and interfere with cell adhesion. Although no central molecular mechanism of action has been proposed to explain all the effects of the malononitrilamides, the inhibition of de novo pyrimidine biosynthesis and 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 in daily dosages of 10 mg and 25 mg to patients with active rheumatoid arthritis. The improved efficacy of a 25 mg dose is associated with a higher incidence of adverse effects (gastrointestinal symptoms, weight loss, allergic reactions, skin rash, and reversible alopecia). Because of the long plasma half-life of A77 1726 (11 to 16 days), loading doses are necessary to achieve steady state concentrations. Phase III randomized, placebo-controlled trials that use daily dosages of 10 mg or 20 mg are under way in the United States and Europe to confirm and extend the results of the phase II study. Malononitrilamide analogues of A77 1726 are being evaluated for immunosuppressive efficacy in preclinical models of transplantation. If these analogues show efficacies and therapeutic indexes that are similar to leflunomide in these models and that 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.

Prolongation of rat islet allograft survival by the immunosuppressive agent leflunomide

The purpose of this study was to investigate the effect of Leflunomide (Lef), alone or in combination with a suboptimal dose of cyclosporine (CsA), on rat allogeneic islet transplantation. Two thousands islets were transplanted under the left kidney capsule of a streptozocin-induced diabetic Lewis recipient. In the ACI to Lewis combination, the mean survival time (MST) of the untreated group was 5.2 +/- 0.8 days. Lef at 2.5, 5, and 10 mg/kg/day for 14 days significantly prolonged MSTs to 19.0 +/- 1.6, 29.8 +/- 3.7, and 29.0 +/- 5.3 days (P<0.01), respectively. CsA at 5 mg/kg/day also prolonged graft survival to 21 +/- 3.5 days. When CsA (5 mg/ kg/day) was combined with Lef (5 or 10 mg/kg/day) and administered for 14 days, the survival rate of the islet allografts was further increased to 34.8 -/+ 4.7 and 36.0 -/+ 6.6 days, respectively. When Lef or CsA monotherapy was extended to 28 days at a dose of 5 mg/kg/ day, MSTs were further increased to 45.8 -/+ 8.8 or 37.4 -/+ 4.7 days, respectively. Graft MST was 56.4 -/+ 9.9 days when Lef and CsA combination therapy was administered for 28 days. In the Brown-Norway to Lewis combination, MST of the allogeneic islets in untreated rats was 6.2 -/+ 0.8 days. When Lef or CsA alone, at 5 mg/kg/day, was administered for 28 days, two of seven Lef-treated rats remained normoglycemia for more than 100 days. Graft survival longer than 100 days occurred in one of five CsA-treated rats, and in five of eight rats treated with the combination of Lef and CsA. The graft-bearing left kidney was removed after 100 days in rats with functional islet allografts, and a second Brown-Norway islet graft was transplanted into the right kidney. In all recipients, the second graft was rejected by 9.8 -/+ 1.5 days. In summary, our findings demonstrate that Lef prolonged allogeneic islet graft survival, and its immunosuppressive effect was improved when combined with CsA.