Fungal metabolites. Part 11. A potent immunosuppressive activity found in Isaria sinclairii metabolite

Immunosuppressant FTY720 induces apoptosis by direct induction of permeability transition and release of cytochrome c from mitochondria

FTY720 has immunosuppressive activity in experimental organ transplantation and shows a prompt and protracted decrease of blood T lymphocytes upon oral administration. The blood lymphocyte decrease in vivo was mainly a result of FTY720-induced apoptosis. However, this apoptotic mechanism is not well understood. We examined the mechanism of FTY720-induced apoptosis in lymphoma. Western blotting and fluorescent caspase-specific substrate revealed that caspase-3 is involved in FTY720-induced apoptosis, whereas caspase-1 is not. Apoptotic cell death was inhibited by the pan-caspase inhibitor, Z-VAD-FMK, suggesting that caspase activation is essential for FTY720-induced apoptosis. FTY720 reduced mitochondrial transmembrane potential and released cytochrome c from the mitochondria of intact cells as well as in a cell-free system even in the presence of Z-VAD-FMK. As these mitochondrial reactions occurred before caspase activation, we concluded that FTY720 directly influences mitochondrial functions. The inhibition of mitochondrial permeability transition by Bcl-2 overexpression or by chemical inhibitors prevented all apoptotic events occurring in intact cells and in a cell-free system. Moreover, using a cell-free system, FTY720 did not directly affect isolated nuclei or cytosol. These results indicate that FTY720 directly affects mitochondria and triggers permeability transition to induce further apoptotic events.

Evidence that FTY720 induces T cell apoptosis in vivo

The immunosuppressant FTY720 induces a drastic decrease in blood lymphocytes, especially T cells; a decrease which is assumed to be the immunosuppressive mechanism of this drug. FTY720 causes cell death in vitro in lymphocytes and leukemia cells. However, the deletion mechanism of blood lymphocytes in vivo remains unclear. We investigated whether administration of FTY720 induced lymphocyte apoptosis in blood circulation. A marked decrease in the number of blood lymphocytes was observed within an hour after a single oral administration of FTY720 at doses of 5-10 mg/kg in rats and mice. Experiments using fluorescein isothiocyanate (FITC)-Annexin V and APO-BRDU methods revealed that FTY720 induced blood lymphocyte apoptosis in a dose-dependent manner. On the other hand, lymphocyte homing to Peyer's patches was proposed as the mechanism underlying the blood lymphocyte decrease at these doses. However, similar results were obtained when using aly/aly mice, which lack Peyer's patches and lymph nodes. Thus, we concluded that apoptosis of blood lymphocytes was induced immediately after administration of FTY720, and the cells could be immediately scavenged by phagocytes or the reticuloendothelial system in addition to Peyer's patches homing. We also concluded that T cells were highly sensitive to FTY720, which induced apoptosis in vivo.

Sli2 (Ypk1), a homologue of mammalian protein kinase SGK, is a downstream kinase in the sphingolipid-mediated signaling pathway of yeast

ISP-1 is a new type of immunosuppressant, the structure of which is homologous to that of sphingosine. In a previous study, ISP-1 was found to inhibit mammalian serine palmitoyltransferase, the primary enzyme involved in sphingolipid biosynthesis, and to reduce the intracellular pool of sphingolipids. ISP-1 induces the apoptosis of cytotoxic T cells, which is triggered by decreases in the intracellular levels of sphingolipids. In this study, the inhibition of yeast (Saccharomyces cerevisiae) proliferation by ISP-1 was observed. This ISP-1-induced growth inhibition was also triggered by decreases in the intracellular levels of sphingolipids. In addition, DNA duplication without cytokinesis was detected in ISP-1-treated yeast cells on flow cytometry analysis. We have cloned multicopy suppressor genes of yeast which overcome the lethal sphingolipid depletion induced by ISP-1. One of these genes, SLI2, is synonymous with YPK1, which encodes a serine/threonine kinase. Kinase-dead mutants of YPK1 did not show any resistance to ISP-1, leading us to predict that the kinase activity of the Ypk1 protein should be essential for this resistance to ISP-1. Ypk1 protein overexpression had no effect on sphingolipid biosynthesis by the yeast. Furthermore, both the phosphorylation and intracellular localization of the Ypk1 protein were regulated by the intracellular sphingolipid levels. These data suggest that the Ypk1 protein is a downstream kinase in the sphingolipid-mediated signaling pathway of yeast. The Ypk1 protein was reported to be a functional homologue of the mammalian protein kinase SGK, which is a downstream kinase of 3-phosphoinositide-dependent kinase 1 (PDK1). PDK1 phosphotidylinositol (PI) is regulated by PI-3,4,5-triphosphate and PI-3,4-bisphosphate through the pleckstrin homology (PH) domain. Overexpression of mammalian SGK also overcomes the sphingolipid depletion in yeast. Taking both the inability to produce PI-3,4, 5-triphosphate and PI-3,4-bisphosphate and the lack of a PH domain in the yeast homologue of PDK1, the Pkh1 protein, into account, these findings further suggest that yeast may use sphingolipids instead of inositol phospholipids as lipid mediators.

FTY720 immunosuppression impairs effector T cell peripheral homing without affecting induction, expansion, and memory

FTY720 (2-amino-2-(2-[4-octylphenyl]ethyl)-1,3-propanediol hydrochloride) prolongs survival of solid organ allografts in animal models. Mechanisms of FTY720 immunomodulation were studied in mice infected with lymphocytic choriomeningitis virus (LCMV) to assess T cell responses or with vesicular stomatitis virus to evaluate Ab responses. Oral FTY720 (0.3 mg/kg/day) did not affect LCMV replication and specific CTL and B cells were induced and expanded normally. Moreover, the anti-viral humoral immune responses were normal. However, FTY720 treatment showed first a shift of overall distribution of CTL from the spleen to peripheral lymph nodes and lymphocytopenia was observed. This effect was reversible within 7-21 days. Together with unimpaired T and B cell memory after FTY720 treatment, this finding rendered enhancement of lymphocyte apoptosis by FTY720 in vivo unlikely. Secondly, the delayed-type hypersensitivity reaction to a viral MHC class I-presented peptide was markedly reduced by FTY720. These results were supported by impaired circulation of LCMV specific TCR transgenic effector lymphocytes in the peripheral blood and reduced numbers of tissue infiltrating CTL in response to delayed-type hypersensitivity reaction. Thirdly, in a CD8+ T cell-mediated diabetes model in a transgenic mouse expressing the LCMV glycoprotein in the islets of the pancreas, FTY720 delayed or prevented disease by reducing islet-infiltrating CTL. Thus, FTY720 effectively reduced recirculation of CD8+ effector T cells and their recruitment to peripheral lesions without affecting the induction and expansion of immune responses in secondary lymphoid organs. These properties may offer the potential to treat ongoing organ-specific T cell-mediated immunopathologic disease.

Specificity of inhibitors of serine palmitoyltransferase (SPT), a key enzyme in sphingolipid biosynthesis, in intact cells. A novel evaluation system using an SPT-defective mammalian cell mutant

In the present study, we demonstrate a model cell system for evaluating the specificity of inhibitors of serine palmitoyltransferase (SPT), the enzyme that catalyzes the first step of sphingolipid biosynthesis. The LY-B strain is a Chinese hamster ovary (CHO) cell mutant defective in SPT, and the LY-B/cLCB1 strain is a genetically corrected revertant of the mutant. Although LY-B cells grew only slightly in sphingolipid-deficient medium, their growth was restored to the level of LY-B/cLCB1 cells under sphingosine-supplied conditions, indicating that, in CHO cells, the growth inhibition caused by SPT inactivation was rescued almost fully by the metabolic complementation of sphingolipids. Cultivation of LY-B/cLCB1 cells in sphingolipid-deficient medium in the presence of 10 microM sphingofungin B and ISP-1 (myriocin, thermozymocidin), potent inhibitors of SPT activity, caused severe growth inhibition with approximately 95% inhibition of de novo sphingolipid synthesis. The growth inhibition by sphingofungin B and ISP-1 was rescued substantially by exogenous sphingosine, whereas the cytotoxicity of two other types of SPT inhibitor, L-cycloserine and beta-chloro-L-alanine, was hardly rescued. Similar cytotoxic patterns of these inhibitors also were observed on the growth of SPT-defective LY-B cells cultured under sphingosine-supplied conditions. The SPT inhibitors did not affect metabolic conversion of exogenous [(3)H]sphingosine to complex sphingolipids. Thus, the cytotoxicity of sphingofungin B and ISP-1, but not L-cycloserine or beta-chloro-L-alanine, is due largely to inhibition of sphingolipid synthesis by inhibiting the SPT activity.

Effect of FTY720, a novel immunosuppressant, on adjuvant- and collagen-induced arthritis in rats

The anti-arthritic effect of FTY720, 2-amino-2-[2-(4-octylphenyl)ethyl]propane-1,3-diol hydrochloride, a novel immunosuppressant which induces peripheral lymphocyte homing to peripheral lymph nodes, was compared with those of anti-rheumatic compounds, mizoribine and prednisolone in rat adjuvant-induced arthritis (AA) and collagen-induced arthritis (CIA). FTY720 at doses of 0.03-0.3 mg/kg, mizoribine at 3-30 mg/kg, and prednisolone at 1-10 mg/kg were orally administered to rats for 21 days from the day of inoculation with heat-killed Mycobacterium tuberculosis or type II collagen. Efficacy of FTY720 at 0.3 mg/kg was almost equal or higher as compared with those of mizoribine and prednisolone in both AA and CIA models. FTY720, but not mizoribine and prednisolone, decreased selectively lymphocyte counts in the peripheral blood in both models below the levels of the normal rats. Although FTY720 gave no other abnormal signs resulting in side effects, mizoribine was lethal to rats at 30 mg/kg and prednisolone inhibited body weight gain at 10 mg/kg, indicating that FTY720 has a wider margin of safety compared with these reference compounds. FTY720 also inhibited the production of anti-collagen antibody in CIA model, while neither mizoribine nor prednisolone did it. These results suggest that FTY720 is a promising compound for the treatment of arthritis with a unique profile.

Antibody against a novel, myriocin (ISP-I)-based immunosuppressant, FTY720

An antibody was prepared by immunizing rabbits with an ovalbumin conjugate of 2-amino-2-(2-(4-(4-mercaptobutyl)phenyl)ethyl)propane-1,3-diol HCl (AMPD-4), which contains the essential structure of the novel immunosuppressant FTY720. As the antibody reacted to not only AMPD-4, but also FTY720, it should be useful for immunoassay of FTY720 in body fluids, tissues and cells.

The FTY720 story

The chemical 2-amino-2[2-(4-octylphenyl)ethyl]-1,3,propane diol is one of a class of small-molecule immunosuppressive agents. Better known as FTY720, this compound was chemically synthesized in an effort to minimize the toxic in vivo properties of a structurally related and highly potent immunosuppressive agent, myriocin. FTY720's mechanism of action, although not fully characterized, appears to be unique among immunosuppressants. Whereas the most well known biochemical characteristic of myriocin is its ability to inhibit serine palmitoyl transferase, the enzyme that initiates the biosynthetic pathway that leads to sphingosine, FTY720 is ineffective in this regard. In vivo, FTY720 induces a significant reduction in the number of circulating lymphocytes. It is thought to act by altering lymphocyte trafficking/homing patterns through modulation of cell surface adhesion receptors and ligands in a manner that has yet to be elucidated. Although much research has yet to be done to unravel the nature of the mechanism of action of FTY720, its efficacy has been sufficiently proven in numerous animal models, especially when administered in combination with cyclosporine. The agent is now progressing through human clinical trials, with the results of phase 1 clinical trials showing safety and tolerability in adult recipients of renal transplants. It is hoped that FTY720 will eventually prove to be an efficacious new weapon in the immunosuppressive armamentarium.

Prolongation of canine liver allograft survival by a novel immunosuppressant, FTY720: effect of monotherapy and combined treatment with conventional drugs

BACKGROUND

The immunosuppressive effect and other properties of a novel immunosuppressant, FTY720, have been studied mostly in the experimental transplantation of various extrahepatic organs. In this experiment, we evaluated the antirejection potency and adverse effects of this agent on liver grafts using a canine liver transplantation model.

METHODS

Forty-eight orthotopic liver transplantations were performed by the standard technique under a veno-venous bypass. Liver recipients were divided into two studies: a single-dose study with FTY720 at various doses and a combined dose study with conventional immunosuppressants (cyclosporine or tacrolimus) alone and combined with FTY720. Survival, biochemical and hematological tests, blood levels of immunosuppressants, and postmortem histology were determined.

RESULTS

The median survival of untreated control animals was 9 days, whereas treatment with FTY720 at a dose of 0.1 mg/kg/day prolonged graft survival to 49.5 days. FTY720 at 1 mg/kg/day showed a slight but insignificant prolongation to 16 days, but when the dose was increased to 5 mg/kg/day, the graft was rejected at 10 days. The combination of FTY720, 0.1 mg/kg/day, with a subtherapeutic dose of cyclosporine, 5 mg/kg/ day, prolonged median animal survival from 40 days with cyclosporine alone to 74 days. A combination of FTY720 (0.1 mg/kg/day) with tacrolimus (0.5 mg/kg/ day) compromised animal survival, reducing survival from 83.5 days with tacrolimus alone to 30.5 days due to infectious complication and emaciation by overimmunosuppression. No evident drug-induced side effects were observed.

CONCLUSIONS

FTY720 has a potent immunosuppressive effect when used alone at 0.1 mg/kg/day in canine liver transplantation. FTY720 is a promising candidate for future clinical application in orthotopic liver transplantation.

Effects of FTY720, a novel immunosuppressant, on experimental autoimmune uveoretinitis in rats

The immunosuppressive properties of FTY720, a novel immunosuppressant obtained by structural modification of ISP-I isolated from the fermentation broth of Isaria sinclairii, were studied in experimental autoimmune uveoretinitis (EAU) in rats. Lewis rats were immunized with S-antigen and treated with FTY720 (0. 03, 0.06, 0.1 mg kg(-1)day(-1)) or distilled water for 16 days after the immunization. FTY720 suppressed the incidence and intensity of EAU in a dose-dependent manner as demonstrated by clinical and histological examinations. The drug significantly suppressed the serum levels of antibodies to S-antigen and antigens-specific lymphocyte proliferation. The number of peripheral lymphocytes, but not neutrophils, was markedly reduced by FTY720 treatment. FTY720 also suppressed the intensity of EAU when it was given from the day of EAU onset. These results indicate that FTY720 has intense immunosuppressive effects on EAU in rats and may be a potential candidate for use in the treatment of patients with autoimmune uveitis.

Peptidic immunosuppressants from the fungus Trichoderma polysporum

Three new 10-residue lipopeptaibols, trichopolyns III-V have been isolated from the fungus Trichoderma polysporum together with the known trichopolyns I and II. Structure determination has been achieved on the basis of spectroscopic and chemical evidence. These peptides have been shown to suppress the proliferation of lymphocytes in mouse allogeneic mixed lymphocyte reaction.

Effect of pretreatment with FTY720 and cyclosporin on ischaemia-reperfusion injury of the liver in rats

The effect of pretreatment with FTY720 (2-amino-2-[2-(4-octylphenyl)ethyl]-1,3-propanediol hydrochloride) or cyclosporin, or both, on neutrophil-mediated injury has been examined by use of a rat model of transient clamping of hepatic flow. Pretreatment with FTY720 alone or with cyclosporin induced a marked reduction of circulatory lymphocytes, whereas the use of these drugs in combination was very effective at suppressing the elevation of the number of peripheral polymorphonuclear neutrophils (PMN) after reperfusion. Pretreatment with cyclosporin, with or without FTY720, significantly reduced hepatic damage, whereas FTY720 alone tended to prolong hepatic damage. Pretreatment of cyclosporin alone, but not in combination with FTY720, significantly reduced the accumulation of PMN and led to lower myeloperoxidase levels in the damaged liver. In conclusion, pretreatment with cyclosporin, with or without FTY720, reduced hepatic damage after warm ischaemia-reperfusion, whereas pretreatment with FTY720 alone tended to prolong this damage.

Recent advances in immunosuppressants

In recent years, a large number of structurally diverse immunosuppressants have been discovered that are effective for the treatment of organ transplantation. Some of them are undergoing clinical trials and may soon enter into routine clinical practice. These compounds are either chemical entities obtained from natural sources/synthetic means or biomaterials such as monoclonal antibodies/gene products/proteins. They have been found to interfere at different stages of T cell activation and proliferation, and can be identified as inhibitors of nucleotide synthesis, growth factor signal transduction and differentiation. Newer strategies involving combination of new agents with traditional immunosuppressants, monoclonal antibodies and gene therapy offer enormous potential, not only for the investigation of mechanisms pertaining to graft rejection, but also for its therapeutic prevention.

Serine palmitoyltransferase inhibition reverses anti-proliferative effects of ceramide synthase inhibition in cultured renal cells and suppresses free sphingoid base accumulation in kidney of BALBc mice

The purpose of this study was to determine the ability of the fungal serine palmitoyltransferase (SPT) inhibitor, myriocin, to prevent the anti-proliferative and cytotoxic effects of fumonisin B(1) in cultured pig kidney epithelial cells, LLC-PK(1). In an earlier study with LLC-PK(1) cells, β-chloroalanine (a nonspecific SPT inhibitor) was found to inhibit the fumonisin-induced accumulation of free sphinganine by >90% but only partially reversed (50-60%) fumonisin's antiproliferative and cytotoxic effects. β-Chloroalanine is not the ideal SPT inhibitor for this type of study because it also inhibits other pyridoxal 5'-phosphate-dependent enzymes. A potent and selective fungal SPT inhibitor (myriocin) was partially purified from liquid cultures of Isaria (=Cordyceps) sinclairii by a combination of organic extraction and column chromatography. The various fractions were bioassayed for their ability to inhibit fumonisin-induced sphinganine accumulation in LLC-PK(1) cells. The activity in partially purified material was compared to the activity of highly purified myriocin and the results expressed as myriocin equivalents. The estimated IC(50) and IC(95) for inhibition of fumonisin-induced sphinganine accumulation were approximately 1.8 and 22 nM, respectively. The IC(95) concentration of the fungal SPT inhibitor reversed the antiproliferative effects and prevented fumonisin-induced apoptosis after 48 h exposure to 50 μM fumonisin B(1). The SPT inhibitor was also effective at reducing free sphinganine in vivo. Free sphinganine concentration was reduced 60% in kidney of mice injected i.p. with SPT inhibitor plus fumonisin B(1) when compared to fumonisin B(1) alone. The ability of SPT inhibition to reduce fumonisin B(1)-induced sphinganine accumulation in vivo may be useful in the development of therapeutic agents for treatment of animals suspected to have been exposed to toxic levels of fumonisin in feeds.

The identification of myriocin-binding proteins

BACKGROUND

Myriocin is a natural product that potently induces apoptosis of a murine cytotoxic T lymphocyte cell line (CTLL-2) and inhibits a serine palmitoyltransferase (SPT) activity that has been detected in cell extracts and is thought to initiate sphingolipid biosynthesis. Because SPT has never been biochemically purified and a comprehensive appraisal of myriocin-binding proteins has not been conducted, we isolated specific targets using myriocin-based affinity chromatography.

RESULTS

Myriocin derivatives were synthesized and evaluated using CTLL-2 proliferation and SPT activity assays. Guided by these results, affinity chromatography matrices were prepared and two specific myriocin-binding proteins were isolated from CTLL-2 lysates. Analyses of these polypeptides establish conclusively that they are murine LCB1 and LCB2, mammalian homologs of two yeast proteins that have been genetically linked to sphingolipid biosynthesis.

CONCLUSION

The ability of myriocin-containing matrices to bind factors that have SPT activity and the exclusive isolation of LCB1 and LCB2 as myriocin-binding proteins demonstrates that the two proteins are directly responsible for SPT activity and that myriocin acts directly upon these polypeptides.

Differential Activation of c-Jun NH2-Terminal Kinase and p38 Pathways During FTY720-Induced Apoptosis of T Lymphocytes That Is Suppressed by the Extracellular Signal-Regulated Kinase Pathway

FTY720 is a novel immunosuppressive drug derived from a metabolite from Isaria sinclairii that is known to induce apoptosis of rat splenic T cells. In this study, we examined the intracellular signaling pathway triggered by FTY720. Treatment of human Jurkat T lymphocytes with FTY720-induced apoptosis characterized by DNA fragmentation. The same treatment induced activation of protein kinases such as c-Jun NH2-terminal kinase (JNK), p38/CSBP (CSAID-binding protein), and a novel 36-kDa myelin basic protein (MBP) kinase, but not extracellular signal-regulated kinase (ERK). Pretreatment of Jurkat cells with DEVD-CHO blocked FTY720-induced DNA fragmentation as well as the activation of p38/CSBP. However, DEVD-CHO treatment failed to inhibit FTY720-induced activation of JNK and the 36-kDa MBP kinase. We have also demonstrated that activation of the ERK signaling pathway completely suppressed the FTY720-induced apoptotic process including activation of caspase 3 and activation of JNK and the 36-kDa MBP kinase. Furthermore, transient expression of constitutively active mitogen-activated protein kinase/ERK kinase (MEK) protected the cells from FTY720-induced cell death. The effect of MEK was canceled by coexpression of a mitogen-activated protein kinase phosphatase, CL100. These results indicate that JNK and p38 pathways are differentially regulated during FTY720-induced apoptosis and that activation of ERK pathway alone is sufficient to cancel the FTY720-induced death signal.

Prophylaxis of acute renal allograft rejection using FTY720 in combination with subtherapeutic doses of cyclosporine

BACKGROUND

In rodent transplant models, FTY720 exerts a synergistic affect with cyclosporine (CsA) to prolong allograft survival. The present experiments sought to test this combination in subhuman primates.

METHODS

Cynomolgus monkeys were transplanted with kidney allografts that were incompatible in mixed lymphocyte culture reactions. The animals were treated with daily intramuscular injections of CsA using doses selected to maintain whole blood trough concentrations at therapeutic values between 40 and 200 ng/ml. The 4 experimental groups included CsA without or with 0.1, 0.3, or 1 mg/kg/day FTY720 delivered daily by intravenous bolus injection. Therapeutic effects were suggested both by the graft histology of biopsy within the first 10 posttransplant days and by the length of host survival.

RESULTS

Whereas recipients treated with CsA alone rejected kidney allografts at a median survival time of 8.5 days (n=4), those treated with either 0.1 or 0.3 mg/kg/day FTY720 in addition to CsA showed significant prolongation of kidney allograft survival to 71 days (n=3; P<0.04) or 63 days (n=5; P<0.05), respectively. The hosts in the 1.0 mg/kg/day FTY720 group survived 48 days, with 2 of 5 recipients succumbing at 9 or 17 days postgraft, suggesting possible complications caused by overimmunosuppression. Biopsies of the 0.1 mg/kg/day FTY720 group on posttransplant day 7 documented mild to moderate rejection (grade I), indicated by multiple focal areas of tubular destruction. The histology results of transplants in the 0.3 or 1 mg/kg/day FTY720 group showed only minimal interstitial inflammatory infiltrates (borderline grade), with no evidence of tubular or arterial damage. Serum creatinine values among the animals in the 0.1 mg/kg/day FTY720 group showed increases in 2 of 3 recipients by day 20 and in the third by day 41 postgraft. Among the 0.3 mg/kg/day FTY720 group, 3 of 5 recipients maintained baseline creatinine values to 45 days postgraft; 1 recipient had stable kidney function for 120 days postgraft.

CONCLUSIONS

Addition of FTY720 therapy to a subtherapeutic CsA immunosuppressive regimen delays the rejection of renal allografts in subhuman primates.

A novel immunosuppressant, FTY720, increases the efficiency of a superantigen-induced peripheral T-cell deletion whilst inhibiting negative selection in the thymus

A novel immunosuppressant, FTY720, was generated by chemical modification of ISP-I, an immunosuppressive compound purified from culture filtrates of Isaria sinclairii. FTY720 directly induces apoptotic cell death in lymphocytes, which is believed to be the mechanism by which this drug exerts its immunosuppressive effect. We examined the effect of FTY720 treatment on antigen-induced apoptotic cell death in peripheral T cells and thymocytes. A superantigen, staphylococcus enterotoxin B (SEB), induces T-cell antigen receptor (TCR) Vbeta-specific apoptotic cell death in mature T cells in vivo. In this well-documented experimental system, FTY720 administration significantly enhanced the efficiency of superantigen-induced T-cell deletion. We also determined that apoptotic cell death with DNA fragmentation induced in T-hybridoma cells after stimulation in vitro with anti-TCR antibodies was enhanced in the presence of non-cytolytic doses of FTY720. In sharp contrast, negative selection of T cells in the thymus, another example of antigen-induced apoptosis, was found to be inhibited by FTY720 treatment. A rescue effect was observed on clonal deletion in the H-Y-specific TCRalpha beta transgenic male thymus. In a chicken egg albumin (OVA)-specific TCRalphabeta transgenic system, OVA-induced apoptotic cell death of CD4+CD8+ thymocytes was also inhibited by FTY720 injection. Thus, FTY720 increased the susceptibility of mature T cells to TCR-mediated apoptosis but decreased that of immature thymocytes. The results in this report suggest that the potent immunosuppressive effect of FTY720 is, in part, a result of the augmentation of effects on antigen-induced apoptosis in mature T cells, and that two distinct apoptotic cell death pathways are operating in mature and immature T cells.

Preclinical evaluation of a new immunosuppressive agent, FTY720

FTY720 is a synthetic analog of a fungal metabolite that shows potent immunosuppressive activity in vitro and in vivo with little apparent toxicity. The drug displays marked synergistic effects in vivo with CsA and/or rapamycin. Therefore, this drug may improve the therapeutic window of agents that target cytokine synthesis or signal transduction. Because of these promising findings, the agent is likely to be tested in humans as an adjunct to clinical immunosuppressive regimens.

FTY720, a Novel Immunosuppressant, Induces Sequestration of Circulating Mature Lymphocytes by Acceleration of Lymphocyte Homing in Rats. I. FTY720 Selectively Decreases the Number of Circulating Mature Lymphocytes by Acceleration of Lymphocyte Homing

FTY720, given i.v. or orally at 0.03 mg/kg or more, significantly prolonged skin allograft survival in a dose-dependent manner and showed more potent immunosuppressive activity than cyclosporin A (CsA) or tacrolimus (FK506) in MHC-incompatible rat strains of WKAH donors and F344 recipients. However, unlike CsA or FK506, FTY720 up to 1000 nM did not affect IL-2 production in allogeneic MLC. Within 3 to 24 h after a single oral administration of FTY720 at 0.1 to 1 mg/kg, the number of lymphocytes in the rats was markedly decreased in the peripheral blood and thoracic duct lymph and partially in spleen. By contrast, the number of lymphocytes in peripheral lymph nodes (PLN), mesenteric lymph nodes (MLN), and Peyer’s patches (PP) was significantly increased at the same time. Intravenous transfusion of calcein-labeled rat lymphocytes into rats revealed that FTY720 significantly accelerated lymphocyte homing to PLN, MLN, and PP, dose dependently. Since FTY720-induced lymphocyte homing was completely blocked by simultaneous treatment of the calcein-labeled lymphocytes with mAbs against CD62L, CD49d, and CD11a before the transfusion, the acceleration of lymphocyte homing by FTY720 appears to be mediated by lymphocyte-homing receptors. These findings indicate that FTY720 sequesters circulating mature lymphocytes into PLN, MLN, and PP by acceleration of lymphocyte homing and thereby decreases the number of lymphocytes in peripheral blood, thoracic duct lymph, and spleen. Based on these observations, sequestration of circulating mature-lymphocytes is presumed to be a main mechanism of the immunosuppressive activity of FTY720.

Immunosuppressive effects of FTY720 alone or in combination with cyclosporine and/or sirolimus

BACKGROUND

We examined the ability of FTY720, a novel immunosuppressant that prolongs the survival of allografts in experimental animal models, to potentiate the immunosuppressive effects of cyclosporine (CsA) and/or sirolimus (SRL) in vitro and in vivo.

METHODS

FTY720 alone (10-5000 ng/ml) or in combination with other drugs was added to human peripheral blood lymphocytes (PBLs) undergoing stimulation in vitro with phytohemagglutinin (PHA) or OKT3 monoclonal antibody. The combination index (CI) values were calculated to evaluate the nature of the interactions between FTY720 and CsA and/or SRL: CI values <1 reflect synergistic, CI=1, additive, and CI>1, antagonistic interactions. In addition, Wistar Furth (RT1u) rat recipients of Buffalo (RT1b) heart allografts were treated with FTY720 alone or in combination with other agents. FTY720 alone was also tested to block small bowel or liver allograft rejection in rats.

RESULTS

FTY720 alone produced only modest inhibition of the proliferation of human PBL stimulated with PHA or OKT3 monoclonal antibody. In combination with CsA or SRL, however, FTY720 produced synergistic effects, namely, CI values of 0.58 and 0.36, respectively. A 14-day course of FTY720 (0.05-8.0 mg/kg/day) by oral gavage prolonged heart allograft survival in dose-dependent fashion. Although a 14-day oral course of CsA (1.0 mg/kg/day) alone was ineffective (mean survival time=7.0+/-0.7 vs. 6.4+/-0.6 days in treated vs. untreated hosts), treatment with a combination of 1.0 mg/kg/day CsA and 0.1 mg/kg/day FTY720 extended allograft survival to 62.4+/-15.6 days (P<0.001; CI=0.15). Similarly, a 14-day oral course of 0.08 mg(kg/day SRL alone was ineffective (6.8+/-0.6 days; NS), but the combination of SRL with 0.5 mg/kg/day FTY720 extended the mean survival time to 34.4+/-8.8 days (CI=0.28). The CsA/SRL (0.5/0.08 mg/kg/day) combination acted synergistically with FTY720 (0.1 mg/kg/day) to prolong heart survivals to >60 days (CI=0.18).

CONCLUSIONS

FTY720 potentiates the immunosuppressive effects of CsA and/or SRL both in vitro (by inhibiting of T-cell proliferative response) and in vivo (by inhibiting allograft rejection).

Novel screen methodologies for identification of new microbial metabolites with pharmacological activity

Micro-organisms continue to provide an important source of chemical diversity for the discovery of compounds with new biological activities. Microbial metabolites discovered recently using assays to detect compounds with potential pharmacological utility are surveyed and found to represent an extensive range of structural types produced by a wide variety of organisms. Assays used for screening samples produced by microbial processes must be robust, sensitive and specific and able to operate above a background of potential interferences from a number of sources. Discovery assays currently in use fall into three main categories cell-based, receptor-ligand interaction and enzyme inhibition assays. Trends in the use of these assays and new developments in assay technology applicable to the screening of microbial samples are examined with particular reference to the high throughput screening environment. For microbial screening to be a competitive route to new drug leads, the disciplines involved must be engineered into a seamlessly integrated process to deliver novel compounds with the required biological properties rapidly.

Synthesis and biological evaluation of 2,2-disubstituted 2-aminoethanols: analogues of FTY720

Desymmerization of symmetric FTY720 by substitution of different alkyl groups for one of the prochiral hydroxymethyl groups was performed. The size of the alkyl groups and the absolute configuration at quaternary carbon were important on immunosuppressive activity.

An immunosuppressive agent, FTY720, increases intracellular concentration of calcium ion and induces apoptosis in HL-60

We previously reported that FTY720 is an efficient inducer of apoptosis in lymphocytes and cultured cell lines. In the present study, HL-60 human promyerocytoma cells also induced apoptosis through in vitro treatment with the drug, demonstrating extensive DNA fragmentation 6 hr after incubation. The major target of FTY720 was the common signalling pathway of apoptosis, since a rapid (< 1 min) increase in the intracellular Ca2+ concentration ([Ca2+]i) was found in the cells treated with the drug. Calcium chelation in the culture medium with EGTA did not affect the [Ca2+]i mobilization. A phospholipase C inhibitor, U73122, inhibited the increase in [Ca2+]i as well as the fragmentation of the nuclear DNA, whereas U73343, a non-effective analogue of U73122, had little effect. These results suggest that FTY720-induced apoptosis is mediated through an activation of phospholipase C and the subsequent release of Ca2+ from intracellular calcium pools. In addition, the treatment of HL-60 with pertussis toxin (PTX) did not inhibit Ca2+ mobilization or apoptosis, suggesting that the activation of phospholipase C is independent of PTX-sensitive G-proteins.

A new immunosuppressant, FTY720, induces bcl-2-associated apoptotic cell death in human lymphocytes

FTY720 is a unique immunosuppressive drug produced by modification of a metabolite from Isaria sinclairii. In vitro treatment of human mononuclear cells with FTY720 resulted in a dose-dependent reduction of cell viability. These treated cells demonstrated characteristic DNA ladder formation on agarose gel electrophoresis. Jurkat cells transfected with human bcl-2 gene were resistant to FTY720; their neo type was susceptible to the drug. A rapid acceleration of cell death in human mononuclear cells was seen as early as 2 hr after incubation with FTY720. The intracellular Bax protein increased remarkably 1 hr after the culture; it markedly decreased in the surviving cells at 2 and 3 hr. Coincidental to the Bax decrease. Bcl-2 progressively decreased beginning 2 hr after the culture. Thus, the ratio of Bcl-2 to Bax was decreased by the enhanced expression of Bax immediately after FTY720-treatment, resulting in rapid cell death acceleration. The surviving cells (FTY720-resistant cells) at 2 and 3 hr after culture showed a similar ratio of Bcl-2 to Bax as was observed in the control cells. These results suggest that FTY720 displays bcl-2-associated apoptotic cell death in human mononuclear cells.

A novel immunosuppressant, FTY720, with a unique mechanism of action, induces long-term graft acceptance in rat and dog allotransplantation

A new compound with an immunosuppressive property was purified from culture filtrates of Isaria sinclairii and was chemically modified to FTY720. Rat spleen cells incubated with FTY720 demonstrated features characteristic of apoptosis--such as the absence of surface microvilli, chromatin condensation, and the formation of apoptotic bodies--by electron microscopy, and genemic DNA fragmentation by agarose gel electrophoresis. When FTY720 was administered in liver-allografted rats at a dose of 0.5 mg/kg from day 1 to day 14 after transplantation, the recipients survived significantly longer than the control group. Pretransplant treatment with 5 mg/kg of FTY720 one day before and on the day of grafting induced a remarkable prolongation of recipient survival, and three of 10 recipients survived for longer than 50 days. Furthermore, administration of FTY720 at 5 mg/kg on days 3 and day 4 after grafting also prolonged survival. In canine kidney allografting, a pretransplant 2-day course of FTY720 at 5 mg/kg prolonged graft survival. Daily administration of FTY720 in combination with CsA resulted in a significant prolongation of graft survival in a synergistic manner. In addition, FTY720 appeared to be nontoxic in canine recipients. These results demonstrated that FTY720, having a unique mechanism of action, induces long-term graft acceptance in rat and dog allotransplantation.