FK 506: an immunosuppressant for the 1990s?

A Novel Derivative of (-)mycousnine Produced by the Endophytic Fungus Mycosphaerella nawae, Exhibits High and Selective Immunosuppressive Activity on T Cells

An endophytic fungus, Mycosphaerella nawae ZJLQ129, was isolated from the leaves of the traditional Chinese medicine Smilax china. From the fermentation broth and mycelium, a dibenzofurane compound (-)mycousnine (1) was isolated. Chemical modification of it to the amide derivative (-)mycousnine enamine (2), which is new to science, was found to have high and selective immunosuppressive activity: similar to cyclosporin A, (-)mycousnine enamine (2) selectively inhibited T cell proliferation, suppressed the expression of the surface activation antigens CD25 and CD69 and the formation and expression of the cytokines interleukin-2 as well as interferon γ in activated T cells, but did not show any effect on the proliferation of B cells and cancer cells (PANC-1 and A549) and the activation of macrophages. Furthermore, the cytotoxicity of (-)mycousnine enamine was lower than that of cyclosporin A, and its therapeutic index (TC50/EC50) was 4,463.5, which is five-fold higher than that of cyclosporin A. We conclude that (-)mycousnine enamine (2), the semi-synthestic product prepared from the native product (-)mycousnine (1) of the endophyte M. nawae is a novel effective immunosuppressant showing low toxicity and high selectivity.

Feeding an army: The metabolism of T cells in activation, anergy, and exhaustion

Through the direct control of infection or by providing cytokine signals to other cellular players, T cells play a central role in the orchestration of the immune response. However, in many disease states, T cells are rendered dysfunctional, unable to carry out their effector functions. As T cell activation is bioenergetically demanding, some T cell dysfunction can have metabolic underpinnings. In this review, we will discuss how T cells are programmed to fuel their effector response, and how programmed or pathologic changes can disrupt their ability to generate the energy needed to proliferate and carry out their critical functions.

Action of tacrolimus in arginine induced experimental acute pancreatitis

OBJECTIVE

To determine whether tacrolimus administered to rats, in the presence of pancreatitis induced by L-Arginine, interferes with the serum levels of amylase and glucose and the histological pattern of the pancreatic parenchyma.

METHODS

Forty Wistar rats were divided into four groups with 10 rats each: control group (C), tacrolimus group (T), pancreatitis group (P) and pancreatitis-tacrolimus group (PT). We evaluated serum levels of amylase, glucose, and tacrolimus and made histological assessments of the pancreas. Induction of pancreatitis was made by inoculation of L-Arginine at a dose of 500 mg/100g body weight intraperitoneally, and tacrolimus treatment at a dose of 1ìg/kg subcutaneously for four days.

RESULTS

Serum amylase was higher (p = 0.0000) in groups PT, P and T than in the control group. The PT group mean was higher (p = 0.0009) than in the T group, but did not differ (p = 0.6802) from the average of the P group. There was no difference between groups P and T (p = 0.2568). Neither in mean blood glucose between the groups (p = 0.4920); serum levels of tacrolimus were similar in PT and T groups (p = 0.7112). There were no histological changes in groups T and C and no hemorrhage in the pancreas of rats in groups P and PT. In group P, there was no edema in 30%, mild edema in 20% and in 50%, moderate; as for inflammatory infiltration, it was moderate in 80% and absent in 20%, and atrophy of the parenchyma was moderate in 60% and severe in 40%. In the PT group, there was edema, inflammatory infiltration or atrophy in the pancreas in all rats.

CONCLUSION

Treatment with Tacrolimus induced an increase in serum amylase in normal mice, but did not affect blood glucose or the histological pattern of the pancreatic parenchyma. In the presence of pancreatitis induced by L-Arginine tacrolimus induced edema, inflammatory infiltration and more severe atrophy in the pancreatic parenchyma.

Tacrolimus: review of pharmacokinetics, pharmacodynamics, and pharmacogenetics to facilitate practitioners' understanding and offer strategies for educating patients and promoting adherence

Kidney transplantation requires lifelong immunosuppression with agents that prevent allograft rejection. Immunosuppressive regimens typically include a steroid, an immune modulator (eg, azathioprine, mycophenolate mofetil, or mycophenolate sodium), and a calcineurin inhibitor, either cyclosporine or tacrolimus. Tacrolimus is metabolized by cytochrome P450 3A4 in both the liver and small intestine. Drugs that are substrates of cytochrome P450 3A4, as well as inhibitors and inducers of cytochrome P450 3A4, can cause significant interactions with tacrolimus. A review of the pharmacodynamics and pharmacokinetics of tacrolimus is important to enhance practitioners' understanding when using tacrolimus after kidney transplantation. It is also important to educate patients and their families about tacrolimus. Patients' adherence to this medication regimen is pivotal for allograft survival. A consistent and comprehensive approach to education and discharge teaching is a key component of adherence and the attainment of therapeutic drug levels. At Shands Jacksonville Transplant Center, discharge education and teaching tools aid the transplant professionals and facilitate patients' adherence. This in turn supports the goals of maintaining therapeutic serum levels of tacrolimus and improving renal allograft survival.

Tacrolimus: Review of Pharmacokinetics, Pharmacodynamics, and Pharmacogenetics to Facilitate Practitioners' Understanding and Offer Strategies for Educating Patients and Promoting Adherence

Kidney transplantation requires lifelong immunosuppression with agents that prevent allograft rejection. Immunosuppressive regimens typically include a steroid, an immune modulator (eg, azathioprine, mycophenolate mofetil, or mycophenolate sodium), and a calcineurin inhibitor, either cyclosporine or tacrolimus. Tacrolimus is metabolized by cytochrome P450 3A4 in both the liver and small intestine. Drugs that are substrates of cytochrome P450 3A4, as well as inhibitors and inducers of cytochrome P450 3A4, can cause significant interactions with tacrolimus. A review of the pharmacodynamics and pharmacokinetics of tacrolimus is important to enhance practitioners' understanding when using tacrolimus after kidney transplantation. It is also important to educate patients and their families about tacrolimus. Patients' adherence to this medication regimen is pivotal for allograft survival. A consistent and comprehensive approach to education and discharge teaching is a key component of adherence and the attainment of therapeutic drug levels. At Shands Jacksonville Transplant Center, discharge education and teaching tools aid the transplant professionals and facilitate patients' adherence. This in turn supports the goals of maintaining therapeutic serum levels of tacrolimus and improving renal allograft survival.

Calcium channel blockers as the treatment of choice for hypertension in renal transplant recipients: fact or fiction

Posttransplantation hypertension has been identified as an independent risk factor for chronic allograft dysfunction and loss. Based on available morbidity and mortality data, posttransplantation hypertension must be identified and managed appropriately. During the past decade, calcium channel blockers have been recommended by some as the antihypertensive agents of choice in this population, because it was theorized that their vasodilatory effects would counteract the vasoconstrictive effects of the calcineurin inhibitors. With increasing data becoming available, reexamining the use of traditional antihypertensive agents, including diuretics and beta-blockers, or the newer agents, angiotensin-converting enzyme (ACE) inhibitors and angiotensin II receptor blockers, may be beneficial. Transplant clinicians must choose antihypertensive agents that will provide their patients with maximum benefit, from both a renal and a cardiovascular perspective. Beta-blockers, diuretics, and ACE inhibitors have all demonstrated significant benefit on morbidity and mortality in patients with cardiovascular disease. Calcium channel blockers have been shown to possess the ability to counteract cyclosporine-induced nephrotoxicity. When compared with beta-blockers, diuretics, and ACE inhibitors, however, the relative risk of cardiovascular events is increased with calcium channel blockers. With the long-term benefits of calcium channel blockers on the kidney unknown and a negative cardiovascular profile, these agents are best reserved as adjunctive therapy to beta-blockers, diuretics, and ACE inhibitors.

Medical therapy for inflammatory bowel disease

CD and UC represent a spectrum of chronic IBD that present in protean ways and are accompanied by a variety of systemic sequelae. Sulfasalazine and the newer 5-aminosalicylates are important in the management of mild-to-moderate disease, whereas corticosteroids remain the primary therapy for most patients with moderate-to-severe disease (Tables 2-5). The toxicities associated with long-term steroid therapy, combined with their ineffectiveness as maintenance medications, have led to increased use of immunomodulators, such as azathioprine and 6-MP, for the treatment of steroid-dependent and steroid-resistant IBD. Infliximab is a novel therapeutic adjunct for chronically active and fistulizing CD that will herald a new era of biologic therapy for IBD. Meanwhile, CSA remains an alternative to urgent colectomy in severe UC unresponsive to corticosteroids and also for CD patients with severe disease or refractory fistulas. Finally, continued insights into the etiopathogenic pathways in IBD will provide evolving and innovative approaches until the eventual causes and cures are elucidated. In the meantime, clinicians should remain optimistic regarding current ability to reduce the morbidity and maintain the quality of life for patients suffering with these frustrating diseases.

3-(13-Hydroxytridecyl)-1-[13-(3-pyridyl)tridecyl]pyridinium chloride (YM-53792), a novel inhibitor of NF-AT activation

A compound, YM-53792, was identified as an inhibitor of interleukin-2 (IL-2) gene promoter activity, using a Jurkat cell-based reporter system in which the luciferase gene is regulated by the IL-2 gene promoter. Production of IL-2, interleukin-4 (IL-4) and interleukin-5 (IL-5) from human peripheral blood mononuclear cells was suppressed by YM-53792 in a dose-dependent fashion. Since expression of these cytokine genes is known to be regulated by NF-AT, we examined whether the promoter activity created by multimerization of NF-AT elements was inhibited with YM-53792. YM-53792 inhibited this promoter activity, but not AP-1- and NF-kappaB-driven promoter activities nor SV40 enhancer/promoter activity. In addition, electrophoretic mobility shift assays did not detect NF-AT/DNA complexes when nuclear extract prepared from YM-53792-treated, PMA/A23187-stimulated Jurkat cells was used, whereas AP-1/DNA complexes were observed. These results suggest that YM-53792 specifically inhibits the activation of NF-AT.

Synergism between sirolimus and 1,25-dihydroxyvitamin D3 in vitro and in vivo

The active form of vitamin D, 1 alpha, 25-(OH)2D3, displays immunomodulatory effects in vitro and in vivo at pharmacological levels. We evaluated the dose-effect relationship of 1,25(OH)2D3 and sirolimus (rapamycin, RAP) in vitro, on the inhibition of PHA-stimulated PBMC proliferation, by using the median effect analysis. Pharmacological concentrations of 1,25(OH)2D3 (between 10(-9) and 3 x 10(-6) M) interacted synergistically with RAP (combination index value of 0.01 for 50% suppression of PBMC proliferation). In vivo, the effect of 1,25(OH)2D3 and RAP combinations on the evolution of experimental allergic encephalomyelitis in SJL mice was analyzed. 1,25(OH)2D3, given i.p., in monotherapy, at a dose of 2 micrograms/kg every two days, from day -3 until day +19 after disease induction, or RAP, injected daily at a dose of 0.3 mg/kg for the same period, decreased EAE incidence (paralysis in 70 and 55% of the animals, respectively, versus 98% in the placebo treated group, p < 0.001). The combination treatment using the two drugs in these subtherapeutical doses provided near-total clinical (8% paralysis, p < 0.001 compared to monotherapy with 1,25(OH)2D3 or RAP) and histological protection, comparable to that obtained with RAP in monotherapy at a threefold higher dose (1 mg/kg/d). When the two drugs were given using an alternate day administration schedule (RAP at 0.6 mg/kg and 1,25(OH)2D3 at 2 micrograms/kg, each given on alternate days from day -3 to 19), near total protection was again obtained (13% paralysis, p < 0.001 versus control). These in vitro and in vivo data support the existence of synergistic interactions between 1,25(OH)2D3 and RAP. Considering the narrow therapeutic windows of both RAP and vitamin D-related compounds in autoimmune disease models, combinations of these drugs could find clinical application in reducing their individual therapeutically efficient doses to non-toxic levels.

Prospective study of FK506 side effects: anxiety or akathisia?

FK506 is a macrolide immunosuppressant agent used in solid organ and bone marrow transplantation and for autoimmune disorders. FK506 is reported to have a number of neuropsychiatric side effects, including anxiety and tremor. Because FK506 was implicated in causing akathisia in a case report, we did a prospective, cross-sectional study of 25 renal transplant recipients to determine whether akathisia occurred and/or had a relationship to FK506 plasma levels. The Symptom Checklist-90-R, Hamilton Anxiety (HAM-A), and Akathisia Rating (ARS) scales were administered. Higher FK506 plasma levels correlated with higher HAM-A scores. ARS scores did not correlate with FK506 plasma levels; however, when FK506 plasma levels were divided into "high" (> or = 0.9 ng/mL) and "low" (< 0.9 ng/mL) groups, total ARS and HAM-A scores were significantly higher in the "high" group. We discuss implications of these findings as well as management.

Three-times-daily monotherapy with tacrolimus (FK 506) in kidney transplantation

BACKGROUND

Tacrolimus (FK 506) was introduced into organ transplantation as a powerful immunosuppressive but with several adverse effects. In fact, an appropriate protocol for using this agent has not yet been established. On the basis of pharmacokinetic studies and the reports of its administration as a continuous intravenous infusion, we designed the regimen of every eight hours in order to reduce the daily dosage.

METHODS

Tacrolimus was given to nine patients in the Japanese FK506 study. Although it was discontinued in two patients within two months because of adverse effects and acute rejection, seven other patients tolerated the agent for a long period and received the drug three times a day. Concomitant prednisolone was gradually tapered and finally withdrawn in these patients.

RESULTS

The smaller dosage of tacrolimus led to a higher trough concentration of 14.1 +/- 1.6 ng/mL in the three-times-a-day regimen compared with 12.7 +/- 1.9 ng/ml in twice-a-day therapy (P < 0.01). The daily dosage of tacrolimus proved to be reducible even further. Our target trough concentration was decreased finally to < 5 ng/mL. Concomitant prednisolone was withdrawn in all of the seven patients. The course of these patients has been uneventful for a year after withdrawal of prednisolone.

CONCLUSION

Our regimen of three-times-a-day oral administration of tacrolimus is a likely protocol for reduction of its daily dosage, which lowers its adverse effects while maintaining its immunosuppressive action at a lower trough concentration without concomitant prednisolone.

Polysaccharide (ANK-102) from Polianthes tuberosa cells deteriorates the resistance of mice to Listeria monocytogenes infection

Modulatory effect on the murine self defense system by a newly discovered acidic polysaccharide (ANK-102) produced by P. tuberosa cells in liquid culture was examined. Pretreatment with ANK-102 deteriorated the murine survival against lethal infection of Listeria monocytogenes, an intracellular gram-positive bacterium eliminated mainly by macrophages through T-cell mediated immune response. Pretreatment with ANK-102 resulted in the accumulation of Mac 1 and Mac 2 positive cells in the peritoneal cavity of the infected animals and the reduction of Thy1.2 expression on the surface of the thymocytes. A new type of immunosuppressive polysaccharide ANK-102 was introduced.

FK506 enhances fibrogenesis in in vitro and in vivo models of liver fibrosis in rats

BACKGROUND/AIMS

The immunosuppressant FK506 is undergoing clinical trials in transplantation and autoimmune diseases. FK506 modulates several cytokines and exerts hepatoprotection in acute models. This study was initiated to determine whether FK506 would be beneficial as an antifibrogenic agent.

METHODS

Fibrosis was induced by carbon tetrachloride administration to rats. Half of those animals were treated with FK506. The rats were killed after 8 weeks of carbon tetrachloride treatment. The livers were evaluated by histology, Northern hybridizations, and collagen quantitation. Additionally, rat fibroblasts were incubated with and without FK506 and used for Northern hybridization analysis.

RESULTS

Surprisingly, FK506 exacerbated hepatic inflammation and fibrosis. Increased hepatic collagen and higher messenger RNA levels of transforming growth factor beta 1 and collagens I, III, and IV were found in the FK506-treated group. Rat fibroblasts treated with FK506 expressed higher levels of collagens I and III, fibronectin, macrophage-colony stimulating factor, tissue inhibitor of metalloprotease, and transforming growth factor beta 1 messenger RNAs.

CONCLUSIONS

These findings suggest that FK506 increases the expression of extracellular matrix genes and enhances fibrosis in the rat model. While further studies are needed to elucidate these profibrogenic mechanisms, caution is indicated for the unrestricted use of FK506 in patients subject to recurrent fibrogenic stimulation.

Acute effects of different immunosuppressive drugs on pancreatic, islet, renal, and arterial hepatic blood flow in anesthetized rats

The effects of four different immunosuppressive drugs on organ blood flow were investigated. Sprague-Dawley rats were injected intravenously with 0.2 ml of either 15-deoxyspergualin (DSG; 5 mg/kg body weight), RS 61443 (80 mg/kg body weight), FK 506 (0.5 mg/kg body weight), cyclosporin A (9.5 mg/kg body weight), or the vehicles used. At 15 or 60 min after injection of the drugs, the blood perfusion of the whole pancreas, the pancreatic islets, and the kidneys, as well as the arterial blood flow to the liver, were measured in anesthetized animals using a microsphere technique. Fifteen minutes after administration, both FK 506 and DSG decreased the fraction of whole pancreatic blood flow diverted through the islets. FK 506 and cyclosporin A reduced renal blood flow, but only 60 min after injection of the drug. None of the drugs influenced hepatic blood flow. RS 61443 did not affect the blood flow of the organ systems investigated. These differences in the effects of the drugs tested on blood flow might have some important implications on their efficacy and side effects. Thus, in view of its lack of influence on organ blood flow, RS 61443 seems to be preferable, at least when compared with cyclosporin A and FK 506 in the context of organ transplantation.

Treatment of inflammatory bowel disease from now to the millennium

After decades of therapeutic stasis, treatment advances are occurring in inflammatory bowel disease. Recognition that mesalazine was the active moiety of sulphasalazine has led to a number of new methods of delivering mesalazine without sulphapyridine, with improved toxicity ratios. Current attempts to deliver topical steroids directly to the large bowel have yet to be established as therapeutically effective. Immunosuppressive treatment has been used for many years but recent evidence has firmly established its value and cyclosporin has recently been added to the therapeutic armamentarium. Increasing understanding of the basic processes of inflammation has yielded targets for anti-inflammatory treatments aimed both at the processes of immune activation and of attraction by chemotaxis of neutrophils from the circulation to the lamina propria. Some of these novel treatments, which will be assessed in forthcoming years, involve large molecular weight bioengineered peptides and antibodies that are likely to be expensive and difficult to administer. Other treatment, e.g. 5-lipoxygenase or thromboxane synthesis inhibitors or platelet-activating factor antagonists, are conventional lower molecular weight compounds that are easier to produce and are orally active. It is predicted that 5-lipoxygenase inhibitors will be the next therapeutic advance in inflammatory bowel disease. Such a prediction may founder if blanket suppression of multiple inflammatory mechanisms, rather than targeted actions, is required in inflammation.

FK506 and cyclosporin a regulate proliferation and proto-oncogene expression in HTLV-1-associated myelopathy/tropical-spastic-paraparesis-derived T cells

Human T-cell-leukemia-virus-type-1 (HTLV-1) infection is associated with adult T-cell leukemia/lymphoma (ATL) and HTLV-1-associated myelopathy (HAM)/tropical spastic paraparesis (TSP). The T-cell-targeting immunosuppressants, FK506 and cyclosporin A (CsA), suppressed proliferation of the HAM/TSP-derived T-cell lines, H89-59, H89-79 and H109. FK506 and CsA also reduced expression of the proto-oncogenes, c-myc and c-fos, but not c-jun and interleukin-2-receptor-alpha (IL-2R alpha) gene in H109 cells. The growth-inhibitory effects of FK506 and CsA were not abrogated by interleukin 2 (IL-2). These results suggest that the inhibitory effects of FK506 and CsA are independent of IL-2, and are associated with the reduction of c-myc and c-fos gene expression.

Peptidomimetics derived from natural products

Although much has been written in recent years about rational drug design, no drug has been designed de novo, that is, without using a natural substrate or inhibitor or screening lead as a starting point. Instead, as we have seen, medicinal chemists continue to depend upon serendipitous discovery of novel biological activities and novel chemical entities for structures on which to begin work. What rational drug design really means at present is rational drug discovery and rational optimization. These result from the application of modern structural and mechanistic biochemistry, and good synthetic chemistry, to obtain structures with the desired spectrum of biological activities. Traditionally, lead compounds were discovered in plant and animal extracts, and more recently in microorganisms and chemical libraries. These traditional approaches continue, but are augmented by advances in molecular biology, which now provide pure proteins in quantity for screening and structure determination, as well as for characterization by modern biophysical methods. Remarkably, x-ray and NMR methods can now provide the most important information needed to design new drugs, that is, the conformations of ligands bound to target proteins. Approaches to identifying possible ligands based only on the knowledge of the enzyme active site are being developed. Some of these, such as CAVEAT, have been recently reviewed. In spite of these impressive gains, de novo design of new drugs will not be achieved until we learn how to logically build specific inhibitors of a target enzyme knowing only the protein sequence of the enzyme or the amino acid sequence of the messenger substances. We have a long way to go, because by this very rigorous definition, even the successful design of a new nonpeptide drug beginning with enzyme-ligand NMR or x-ray structure constitutes rational optimization. However, as this article has illustrated, we have made great progress. Some of the current and futuristic approaches to drug design are shown in Fig. 8. Development of useful enzyme inhibitors, designed by knowing the enzyme catalytic mechanism or discovered by screening for natural inhibitors, is a very successful rational method. Discovery of receptor antagonists by screening protocols is also productive.(ABSTRACT TRUNCATED AT 400 WORDS)

Prevention of cyclophosphamide-induced diabetes in the NOD/WEHI mouse with deoxyspergualin

Ten out of 20 (50%) 17-week-old female NOD/WEHI mice developed an acute form of autoimmune diabetes when injected with two large doses of cyclophosphamide (CY), given at 14-day intervals. If these mice were treated under a prophylactic regimen with 2.5 mg/kg body weight per day of the novel immunosuppressant deoxyspergualin (DSP) the onset of diabetes was completely prevented. Moreover, DSP-treated animals showed reduced signs of pancreatic insulitis, had lower percentages of splenic lymphoid cells (SLC) expressing IL-2 receptors and Ly-6C antigens on their surfaces, and these cells released lower amounts of interferon-gamma (IFN) when stimulated in vitro. These data, providing evidence for the capacity of DSP to protect NOD/WEHI mice from experimental autoimmune diabetes and to modulate histo-immunological pathogenic pathways, indicate DSP as a drug of potential interest in the treatment of human insulin-dependent diabetes mellitus.

New approaches to the therapy of autoimmune diseases: rheumatoid arthritis as a paradigm

Several therapeutic agents currently are used to treat rheumatoid arthritis (RA). However, there is no compelling evidence that any of these agents substantially alters the long-term destructive course of RA. Advances in biotechnology have led to a better understanding of mechanisms that underlie autoimmune diseases such as RA. Although the etiology of RA remains unknown, there now is considerable insight regarding the immune and inflammatory pathways that ultimately lead to cartilage and bone destruction. Therapies with monoclonal antibodies directed against cell surface constituents, fusion toxins against cell activation markers, and cytokine inhibitors all have been shown to be safe and possibly efficacious in early open trials in RA. They now are being more rigorously tested in double-blind, placebo-controlled trials. Early experience with these biologic agents in humans, as well as data obtained from the use of these agents in animal models of autoimmune disease, are reviewed. In addition, experimental studies with "blocking peptides" and immunization with autoreactive T cell receptor peptides will be reviewed, and implications for therapy in RA will be discussed.

Immunomodulators. Future prospects

The future role of the immunomodulators in medical practice is yet to be defined. The key question is whether these new substances will bring remarkable progress in transplantation or in the treatment of such conditions as cancer, AIDS, and autoimmune diseases, or whether they will be of only minor adjunctive importance. As background to the discussion of immunomodulating agents, the immune system is explained, with emphasis on the roles of T and B lymphocytes, macrophages, phagocytes, human leukocyte antigen and the complement system. Special attention is given to the cytokines, particularly the lymphokines. The immunomodulators can be divided into three main groups: immunosuppressive agents, such as FK 506 and rapamycin; immunostimulating agents, of which BCG vaccine is most important; and the remaining immunomodulators, which include the biological response modifiers. The last group, which encompasses the colony-stimulating factors (GM-CSF, G-CSF, and M-CSF), the interleukins, the interferons, and the tumour necrosis factors, is described in detail. Innovative research and medical applications of these cytokines, including indications, contraindications, and adverse reactions, are discussed. The role of monoclonal antibodies against endotoxins is also described.

Aplastic anaemia: pathogenetic mechanisms and treatment with special reference to immunomodulation

Aplastic anaemia (AA) has been defined as a syndrome in which the presence of pancytopenia is accompanied by marrow hypocellularity. Ample laboratory data and clinical observations continue to make immune mediation of bone-marrow failure an attractive hypothesis. Recent progress in the practice of bone-marrow transplantation has led to a survival rate of approximately 80% in the best cases, but such a treatment is only amenable in young patients (less than 45-50 years) with HLA-identical bone-marrow donors. Anti-lymphocyte and thymocyte globulin treatment has been surprisingly effective for AA, resulting in transfusion independence in 40-80% of patients. The mechanism of action is unknown, although effects on immunosuppression appear to be the most likely candidates. Long-term results for patients receiving cyclosporin A treatment will soon be available, and preliminary data show an effect similar to that of antithymocyte globulin. In contrast to successful bone-marrow transplantation, improvement following immunomodulation leaves quantitative abnormalities in all haematopoietic cell lines, and patients are prone to develop clonal (malignant) disease.

Calcineurin phosphatase activity in T lymphocytes is inhibited by FK 506 and cyclosporin A

The immunosuppressive agents cyclosporin A (CsA) and FK 506 bind to distinct families of intracellular proteins (immunophilins) termed cyclophilins and FK 506-binding proteins (FKBPs). Recently, it has been shown that, in vitro, the complexes of CsA-cyclophilin and FK 506-FKBP-12 bind to and inhibit the activity of calcineurin, a calcium-dependent serine/threonine phosphatase. We have investigated the effects of drug treatment on phosphatase activity in T lymphocytes. Calcineurin is expressed in T cells, and its activity can be measured in cell lysates. Both CsA and FK 506 specifically inhibit cellular calcineurin at drug concentrations that inhibit interleukin 2 production in activated T cells. Rapamycin, which binds to FKBPs but exhibits different biological activities than FK 506, has no effect on calcineurin activity. Furthermore, excess concentrations of rapamycin prevent the effects of FK 506, apparently by displacing FK 506 from FKBPs. These results show that calcineurin is a target of drug-immunophilin complexes in vivo and establish a physiological role for calcineurin in T-cell activation.

Cyclosporine, FK-506 and other drugs in organ transplantation

As experience of the most effective way to use cyclosporine for immunosuppression in organ transplantation grows, new drugs are emerging, which may improve the potency of future immunosuppressive protocols or at least provide alternative drugs in selected situations. These newer agents include FK506, which is undergoing extensive clinical trials, rapamycin, RS-61443 and deoxyspergualin. The increasing understanding of the mechanism of action of some of these drugs on signal transduction pathways in the T cell should allow the development of drugs with perhaps more specific actions.