Activity of sirolimus in patients with myelodysplastic syndrome - results of a pilot study

Over expression of mTOR gene predicts overall survival in myelodysplastic syndromes

BACKGROUND

Myelodysplastic syndromes (MDS) is defined as heterogenous disease, it contains heterogenous leukemic stem cells with various degree of cell differentiation. The perturbation of genes involved in myeloid progenitor cell growth, differentiation and proliferation lead to morphologic dysplasia, maturation arrest, ineffective hematopoiesis hence the cytopenias and propensity to develop into acute myeloid leukemia (AML). Heterogeneous subsets of MDS patients have been defined by their clinical and biologic abnormalities. These different features lead to the development of different prognostic system; however, these approaches are limited in predicting clinical course, and management of patients remains challenging given the uncertainty of the time course of disease progression. It is of importance to identify transcriptomic marker causing maturational and differentiation arrest which could help in understanding the pathogenesis of disease.

METHODS AND RESULTS

We have studied differential gene expression profiles (GEPs) in CD34 + marrow cells from myelodysplastic syndrome (MDS) patients (n = 14) and control CD34 + cells using Affymetrix Human Clariom S microarray with 20,000 well annotated genes. We found 4165 genes significantly (p < 0.05) differentially expressed in MDS. Using stringent bioinformatics analyses, we were able to identify few genes (MAPK8, JUNB, mTOR) which were differentially upregulated i.e. 5.39, 73.61 and 2.7 fold change observed in MDS than control and also validated (n = 60) these genes by RT - qPCR. Kaplan - Meier survival analysis indicated that MAPK8 and JUNB could be poor prognostic marker as patients with increased expression showed poor survival, whereas surprisingly mTOR increased expression proved to be good prognostic marker. The correlation analysis showed that the level of gene (MAPK8, JUNB, mTOR) expression was significantly (p ≤ 0.05) associated with frequency of genetic lesions. Interestingly the increased expression of MAPK8 was significantly accompanied with ASXL1 gene mutation.

CONCLUSION

Our study showed an elevation of TNF and AMPK signalling pathways in MDS. TNF signalling might be mediating the proliferative advantage to myeloid clonal cells (mutation carrying cells) over normal cells, whereas, AMPK signalling could be acting as protector against it (favouring normal cells). Hence it would be interesting to explore the functions and pathways associated with mTOR, AMPK, MAPK8 and JUNB in myelopoiesis related diseases like MDS.

Haematological Drugs Affecting Lipid Metabolism and Vascular Health

Many drugs affect lipid metabolism and have side effects which promote atherosclerosis. The prevalence of cancer-therapy-related cardiovascular (CV) disease is increasing due to development of new drugs and improved survival of patients: cardio-oncology is a new field of interest and research. Moreover, drugs used in transplanted patients frequently have metabolic implications. Increasingly, internists, lipidologists, and angiologists are being consulted by haematologists for side effects on metabolism (especially lipid metabolism) and arterial circulation caused by drugs used in haematology. The purpose of this article is to review the main drugs used in haematology with side effects on lipid metabolism and atherosclerosis, detailing their mechanisms of action and suggesting the most effective therapies.

The Role of mTOR Inhibitors in Hematologic Disease: From Bench to Bedside

The mTOR pathway plays a central role in many cellular processes, such as cellular growth, protein synthesis, glucose, and lipid metabolism. Aberrant regulation of mTOR is a hallmark of many cancers, including hematological malignancies. mTOR inhibitors, such as Rapamycin and Rapamycin analogs (Rapalogs), have become a promising class of agents to treat malignant blood diseases-either alone or in combination with other treatment regimens. This review highlights experimental evidence underlying the molecular mechanisms of mTOR inhibitors and summarizes their evolving role in the treatment of hematologic disease, including leukemia, lymphoma, myeloma, immune hemocytopenia, and graft-versus-host disease (GVHD). Based on data presented in this review, we believe that mTOR inhibitors are becoming a trusted therapeutic in the clinical hematologist's toolbelt and should be considered more routinely in combination therapy for the management of hematologic disease.

Use of immunosuppressive therapy for management of myelodysplastic syndromes: a systematic review and meta-analysis

Immunosuppressive therapy (IST) is one therapy option for treatment of patients with lower-risk myelodysplastic syndromes (MDS). However, the use of several different immunosuppressive regimens, the lack of high-quality studies, and the absence of validated predictive biomarkers pose important challenges. We conducted a systematic review and meta-analysis according to the Meta-Analysis of Observational Studies in Epidemiology (MOOSE) guidelines and searched MEDLINE via PubMed, Ovid EMBASE, COCHRANE registry of clinical trials (CENTRAL), and the Web of Science without language restriction from inception through September 2018, as well as relevant conference proceedings and abstracts, for prospective cohort studies or clinical trials investigating IST in MDS. Fixed and Random-effects models were used to pool response rates. We identified nine prospective cohort studies and 13 clinical trials with a total of 570 patients. Overall response rate was 42.5% [95% confidence interval (CI): 36.1-49.2%] including a complete remission rate of 12.5% (95%CI: 9.3-16.6%) and red blood cell transfusion independence rate of 33.4% (95% CI: 25.1-42.9%). The most commonly used forms of IST were anti-thymocyte globulin alone or in combination with cyclosporin A with a trend towards higher response rates with combination therapy. Progression rate to acute myeloid leukemia was 8.6% per patient year (95%CI: 3.3-13.9%). Overall survival and adverse events were only inconsistently reported. We were unable to validate any biomarkers predictive of a therapeutic response to IST. IST for treatment of lower-risk MDS patients can be successful to alleviate transfusion burden and associated sequelae.

Immunosuppressive therapy in myelodysplastic syndromes: a borrowed therapy in search of the right place

INTRODUCTION

Myelodysplastic syndromes (MDS) encompass a heterogenous collection of clonal hematopoietic stem cell disorders defined by dysregulated hematopoiesis, peripheral cytopenias, and a risk of leukemic progression. Increasing data support the role of innate and adaptive immune pathways in the pathogenesis and disease course of MDS. The role of immunosuppressive therapy has an established role in the treatment of other hematologic diseases, such as aplastic anemia whose pathogenesis is postulated to reflect that of MDS with regards to many aspects of immune activation. Areas covered: This paper discusses the current understanding of immune dysregulation as it pertains to MDS, the clinical experience with immunosuppressive therapy in the management of MDS, as well as future prospects which will likely improve therapeutic options and outcomes for patients with MDS. Expert commentary: Though limited by paucity of high quality data, immunomodulatory and immunosuppressive therapies for the treatment of MDS have shown meaningful clinical activity in selected patients. Continued clarification of the immune pathways that are dysregulated in MDS and establishing predictors for clinical benefit of immunosuppressive therapy are vital to improve the use and outcomes with these therapies.

Impaired Mitophagy of Nucleated Erythroid Cells Leads to Anemia in Patients with Myelodysplastic Syndromes

Myelodysplastic syndromes (MDS) are a heterogeneous group of clonal stem cell disorders characterized by cytopenia and dysplasia. Anemia is the most common symptom in patients with MDS. Mitophagy and mitochondrial dysfunction might be involved in the development of MDS. In this study, we investigated the change of mitophagy in erythroid precursors in MDS patients. We found that NIX-mediated mitophagy was impaired in bone marrow nucleated red blood cells (NRBC) of MDS patients, associated with an increased amount of damaged mitochondria and increased ROS level which might lead to apoptosis and ineffective erythropoiesis. The results showed that the amount of mitochondria in GlycoA⁺ NRBC positively correlated with the count of ring sideroblasts in bone marrow samples. Meanwhile, the level of autophagy-associated marker LC3B in GlycoA⁺ NRBC had a positive correlation with hemoglobin (Hb) levels, and the amount of mitochondria in GlycoA⁺ NRBC had a negative correlation with Hb levels in high-risk MDS patients. Our results indicated that mitophagy might involve the pathogenesis of anemia associated with MDS. Autophagy might be a novel target in treatments of MDS patients.

[Study on autophagy in nucleated red blood cells in patients with myelodysplastic syndromes]

Objective: To investigate the change of autophagy level of bone marrow nucleated red blood cell (RBC) in patients with myelodysplastic syndromes (MDS) . Methods: Fifty-four MDS patients and thirty-three controls were enrolled in this study. The mitophagy were observed by transmission electron microscopy (TEM) . The level of autophagy-associated protein LC3B in GlycoA(+) nucleated RBC was measured by flow cytometry. The expressions of ULK1 and mTOR mRNA in GlycoA(+) nucleated RBC were measured by real-time PCR. The expression of the mitochondrial outer membrane protein TOM20 in GlycoA(+) nucleated RBC was detected by Western blot. Results: Autophagosomes or autolysosomes were scarcely observed by TEM in MDS patients. The expression of LC3B in GlycoA(+) nucleated RBC in high-risk MDS patients (0.22±0.12) was significantly lower than that in normal controls (0.43±0.22, P<0.001) , and lower than that in low-risk MDS patients (0.40±0.16, P=0.001) . The expression of AMPK [0.26 (0.60) ] in GlycoA(+) nucleated RBC in high-risk MDS patients was significantly lower than that in controls [1.00 (2.07) , P<0.017) . The expression of ULK1 mRNA in GlycoA(+) nucleated RBC in high-risk MDS patients [0.27 (3.31) ] was significantly lower than that in controls [1.07 (4.41) , P<0.017]. The level of mTOR mRNA in GlycoA(+) nucleated RBC in high-risk MDS patients [1.82 (3.74) ] was significantly higher than that in controls [1.26 (1.38) , P<0.017]. The level of LC3B in GlycoA(+) nucleated RBC was negatively correlated with the HGB (r=0.529, P=0.009) in high-risk MDS patients. The expression of mitochondrial outer membrane protein TOM20 in high-risk MDS patients was 9.42±4.42. Conclusion: Autophagy is impaired in nucleated RBC of MDS patients.

Mammalian-target of rapamycin inhibition with temsirolimus in myelodysplastic syndromes (MDS) patients is associated with considerable toxicity: results of the temsirolimus pilot trial by the German MDS Study Group (D-MDS)

The mammalian-target of rapamycin (also termed mechanistic target of rapamycin, mTOR) pathway integrates various pro-proliferative and anti-apoptotic stimuli and is involved in regulatory T-cell (TREG) development. As these processes contribute to the pathogenesis of myelodysplastic syndromes (MDS), we hypothesized that mTOR modulation with temsirolimus (TEM) might show activity in MDS. This prospective multicentre trial enrolled lower and higher risk MDS patients, provided that they were transfusion-dependent/neutropenic or relapsed/refractory to 5-azacitidine, respectively. All patients received TEM at a weekly dose of 25 mg. Of the 9 lower- and 11 higher-risk patients included, only 4 (20%) reached the response assessment after 4 months of treatment and showed stable disease without haematological improvement. The remaining patients discontinued TEM prematurely due to adverse events. Median overall survival (OS) was not reached in the lower-risk group and 296 days in the higher-risk group. We observed a significant decline of bone marrow (BM) vascularisation (P = 0·006) but were unable to demonstrate a significant impact of TEM on the balance between TREG and pro-inflammatory T-helper-cell subsets within the peripheral blood or BM. We conclude that mTOR-modulation with TEM at a dose of 25 mg per week is accompanied by considerable toxicity and has no beneficial effects in elderly MDS patients.

The immune landscape of myelodysplastic syndromes

Even though the pathogenesis of myelodysplastic syndromes (MDS) is dominated by specific molecular defects involving hematopoietic precursors, also immune mechanisms seem to play a fundamental functional role. In this review we will first describe the clinical and laboratory autoimmune manifestations often detectable in MDS patients. We will then focus on studies addressing the possible influence of different immune cell subpopulations on the disease onset and evolution. We will finally consider therapeutic approaches based on immunomodulation, ranging from immunosuppressants to vaccination and transplantation strategies.

Thrombocytopenia in MDS: epidemiology, mechanisms, clinical consequences and novel therapeutic strategies

Thrombocytopenia is commonly seen in myelodysplastic syndrome (MDS) patients, and bleeding complications are a major cause of morbidity and mortality. Thrombocytopenia is an independent factor for decreased survival and has been incorporated in newer prognostic scoring systems. The mechanisms of thrombocytopenia are multifactorial and involve a differentiation block of megakaryocytic progenitor cells, leading to dysplastic, hypolobated and microscopic appearing megakaryocytes or increased apoptosis of megakaryocytes and their precursors. Dysregulated thrombopoietin (TPO) signaling and increased platelet destruction through immune or nonimmune mechanisms are frequently observed in MDS. The clinical management of patients with low platelet counts remains challenging and approved chemotherapeutic agents such as lenalidomide and azacytidine can also lead to a transient worsening of thrombocytopenia. Platelet transfusion is the only supportive treatment option currently available for clinically significant thrombocytopenia. The TPO receptor agonists romiplostim and eltrombopag have shown clinical activity in clinical trials in MDS. In addition to thrombopoietic effects, eltrombopag can inhibit leukemic cell proliferation via TPO receptor-independent effects. Other approaches such as treatment with cytokines, immunomodulating drugs and signal transduction inhibitors have shown limited activity in selected groups of MDS patients. Combination trials of approved agents with TPO agonists are ongoing and hold promise for this important clinical problem.

Mito-protective autophagy is impaired in erythroid cells of aged mtDNA-mutator mice

Somatic mitochondrial DNA (mtDNA) mutations contribute to the pathogenesis of age-related disorders, including myelodysplastic syndromes (MDS). The accumulation of mitochondria harboring mtDNA mutations in patients with these disorders suggests a failure of normal mitochondrial quality-control systems. The mtDNA-mutator mice acquire somatic mtDNA mutations via a targeted defect in the proofreading function of the mtDNA polymerase, PolgA, and develop macrocytic anemia similar to that of patients with MDS. We observed an unexpected defect in clearance of dysfunctional mitochondria at specific stages during erythroid maturation in hematopoietic cells from aged mtDNA-mutator mice. Mechanistically, aberrant activation of mechanistic target of rapamycin signaling and phosphorylation of uncoordinated 51-like kinase (ULK) 1 in mtDNA-mutator mice resulted in proteasome-mediated degradation of ULK1 and inhibition of autophagy in erythroid cells. To directly evaluate the consequence of inhibiting autophagy on mitochondrial function in erythroid cells harboring mtDNA mutations in vivo, we deleted Atg7 from erythroid progenitors of wild-type and mtDNA-mutator mice. Genetic disruption of autophagy did not cause anemia in wild-type mice but accelerated the decline in mitochondrial respiration and development of macrocytic anemia in mtDNA-mutator mice. These findings highlight a pathological feedback loop that explains how dysfunctional mitochondria can escape autophagy-mediated degradation and propagate in cells predisposed to somatic mtDNA mutations, leading to disease.

Sirolimus, tacrolimus and low-dose methotrexate based graft-versus-host disease prophylaxis after non-ablative or reduced intensity conditioning in related and unrelated donor allogeneic hematopoietic cell transplant

Encouraging results have been reported with sirolimus, tacrolimus and low-dose methotrexate after non-myeloablative allogeneic hematopoietic cell transplant. We conducted a retrospective analysis of 71 patients with lymphoid malignancies treated with this prophylaxis regimen after non-myeloablative or reduced intensity allogeneic hematopoietic cell transplant. Grafts were human leukocyte antigen (HLA)-matched related in 29 (41%), matched unrelated in 36 (51%) and 9/10 HLA-matched unrelated in six (8%) patients. The regimen was well tolerated and over 90% of patients completed the planned treatment. The cumulative incidences of 1-year grade B-D and C-D acute graft-versus-host disease (GVHD) were 0.28 (95% confidence interval [CI], 0.18-0.39) and 0.07 (95% CI, 0.03-0.15), respectively, and of 1- and 2-year chronic GVHD (National Institutes of Health criteria) in 70 evaluable patients were 0.15 (95% CI, 0.08-0.24) and 0.33 (95% CI, 0.22-0.44), respectively. The median day of onset of acute GVHD was 123 days (range, 17-268 days). Peri-transplant rituximab or anti-thymocyte globulin did not affect GVHD. The cumulative incidence of 1-year non-relapse mortality and relapse were 4% and 20%, respectively. With a median follow-up of 3.5 (range: 0.18-5.1) years, overall survival and progression-free survival at 2 years were 82% and 66%, respectively. This GVHD regimen results in a low incidence and severity of acute and chronic GVHD after reduced intensity and non-myeloablative allogeneic hematopoietic cell transplant for lymphoid malignancies. The study also highlights the incidence of late onset acute GVHD in non-myeloablative/reduced intensity conditioning, and the contribution of the new GVHD staging system that more accurately reflects clinical outcomes.

Current therapy of myelodysplastic syndromes

After being a neglected and poorly-understood disorder for many years, there has been a recent explosion of data regarding the complex pathogenesis of myelodysplastic syndromes (MDS). On the therapeutic front, the approval of azacitidine, decitabine, and lenalidomide in the last decade was a major breakthrough. Nonetheless, the responses to these agents are limited and most patients progress within 2 years. Allogeneic stem cell transplantation remains the only potentially curative therapy, but it is associated with significant toxicity and limited efficacy. Lack or loss of response after standard therapies is associated with dismal outcomes. Many unanswered questions remain regarding the optimal use of current therapies including patient selection, response prediction, therapy sequencing and combinations, and management of resistance. It is hoped that the improved understanding of the underpinnings of the complex mechanisms of pathogenesis will be translated into novel therapeutic approaches and better prognostic/predictive tools that would facilitate accurate risk-adaptive therapy.

Immunomodulatory treatment of myelodysplastic syndromes: antithymocyte globulin, cyclosporine, and alemtuzumab

It is now well accepted that a subgroup of patients with myelodysplastic syndromes (MDS) can recover from pancytopenia following immunosuppressive treatment (IST). For many years immunosuppression with antilymphocyte antibodies has been a standard treatment approach for patients with severe aplastic anemia (SAA). The initial concept of using immunosuppression to treat pancytopenic patients with MDS was based on the premise that MDS might share with SAA an autoimmune basis for the bone marrow failure common to both conditions. The idea was supported by reports of favorable outcomes in occasional cases of MDS treated with antithymocyte globulin (ATG). Today, various forms of IST have been successfully used to restore hematopoiesis in MDS in many centers worldwide. In this review we outline the rationale for use of IST in MDS, and describe studies which help to define the patients with MDS likely to respond to IST. We summarize 18 published clinical trials using IST for MDS and discuss how these studies have helped to define the MDS subgroups likely to respond to treatment, the nature and durability of the response, the impact of IST on long-term outcome, and the best treatment approach.

Platelet apoptosis and agonist-mediated activation in myelodysplastic syndromes

Patients with myelodysplastic syndromes (MDS) have a defect in the differentiation of bone marrow multipotent progenitor cells. Thrombocytopenia in MDS patients may be due to premature megakaryocyte death, but platelet apoptotic mechanisms may also occur. This study aimed to study function and apoptotic state of platelets from MDS patients with different platelet count. Reticulated platelets, platelet activation, activated caspases and annexin-V binding were evaluated by flow cytometry. Pro-apoptotic Bax and Bak proteins were determined by western blots and plasma thrombopoietin by ELISA. Microparticle-associated procoagulant activity and thrombin generation capacity of plasma were determined by an activity kit and calibrated automated thrombography, respectively. High plasma thrombopoietin levels and low immature circulating platelet count showed a pattern of hypoplastic thrombocytopenia in MDS patients. Platelets from MDS patients showed reduced activation capacity and more apoptosis signs than controls. Patients with the lowest platelet count showed less platelet activation and the highest extent of platelet apoptosis. On this basis, patients with thrombocytopenia should suffer more haemorrhagic episodes than is actually observed. Consequently, we tested whether there were some compensatory mechanisms to counteract their expected bleeding tendency. Microparticle-associated procoagulant activity was enhanced in MDS patients with thrombocytopenia, whereas their plasma thrombin generation capacity was similar to control group. This research shows a hypoplastic thrombocytopenia that platelets from MDS patients possess an impaired ability to be stimulated and more apoptosis markers than those from healthy controls, indicating that MDS is a stem cell disorder, and then, both number and function of progeny cells, might be affected.

Defining the role of sirolimus in the management of graft-versus-host disease: from prophylaxis to treatment

Graft-versus-host disease (GVHD) remains a major cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation (HSCT). Measures developed that have significantly reduced GVHD were also frequently associated with an increased risk of relapse. GVHD and graft-versus-tumor (GVT) effects are tightly linked, and balance between both reactions is difficult to achieve. To have an impact on the outcome and quality of life after HSCT, improvements in current strategies to prevent and treat GVHD while preserving the GVT effect are clearly needed. Sirolimus (rapamycin) is a lipophilic macrocytic lactone with immunosuppressive, antitumor, and antiviral properties. Because of its multiple modes of activities, it is being increasingly used in the management of GVHD. This review aims to summarize its mechanisms of action and potential advantages over other immunosuppressors and to analyze the most relevant studies investigating its role in both prevention and treatment of GVHD.

The effect of decitabine on megakaryocyte maturation and platelet release

Thrombocytopenia is a common feature of myelodysplastic syndromes (MDS). 5-aza-2'-deoxycytidine (decitabine) has been used to treat MDS with an approximately 20% response rate in thrombocytopenia. However, the mechanism of how decitabine increases platelet count is not clear. In this study, we investigated the effect of decitabine on megakaryocyte maturation and platelet release in the mouse. The effect of decitabine on megakaryocyte maturation was studied in an in vitro megakaryocyte differentiation model utilising mouse bone marrow cells and mouse megakaryoblastic cell line L8057. Decitabine (2.5 μM) is able to induce L8057 cells to differentiate into a megakaryocyte-like polyploidy cells with positive markers of acetylcholinesterase and αIIb integrin (CD41). Higher expression of αIIb integrin was also found in primary mouse bone marrow cells and human cord blood CD34+ cells cultured with both thrombopoietin and decitabine as compared to thrombopoietin alone. In addition, we noted a 30% platelet count increase in Balb/c mice 12 hours after the injection of decitabine at a clinically relevant dose (15 mg/m2), suggesting a rapid platelet release from the spleen or bone marrow. Our data suggest that decitabine increases platelet counts by enhancing platelet release and megakaryocyte maturation.

Prognostic molecular markers in myelodysplastic syndromes

Cytogenetic findings in myelodysplastic syndromes play an important role in diagnosis, prognostication and clinical decision making. Therefore, they became an important aspect in scoring systems such as the International Prognostic Scoring System (IPSS) and the WHO-adapted Prognostic Scoring System (WPSS). Ongoing efforts to refine the categorization of karyotypes with regard to prognosis and therapeutic options will change scoring systems in the near future. In order to learn more about the pathophysiology of myelodysplastic syndromes, various molecular genetic aberrations are identified and their impact on prognosis discussed. New screening methods such as gene expression or single nucleotide polymorphism analysis are good candidates to find entrance in clinical practice in the future as they are useful tools in further elucidation of the underlying defects in myelodysplastic syndromes and the development of more specific classifications of the disease concerning risk assessment.

Phase I/II study of the mammalian target of rapamycin inhibitor everolimus (RAD001) in patients with relapsed or refractory hematologic malignancies

PURPOSE

Everolimus (RAD001, Novartis), an oral derivative of rapamycin, inhibits the mammalian target of rapamycin (mTOR), which regulates many aspects of cell growth and division. A phase I/II study was done to determine safety and efficacy of everolimus in patients with relapsed or refractory hematologic malignancies.

EXPERIMENTAL DESIGN

Two dose levels (5 and 10 mg orally once daily continuously) were evaluated in the phase I portion of this study to determine the maximum tolerated dose of everolimus to be used in the phase II study.

RESULTS

Twenty-seven patients (9 acute myelogenous leukemia, 5 myelodysplastic syndrome, 6 B-chronic lymphocytic leukemia, 4 mantle cell lymphoma, 1 myelofibrosis, 1 natural killer cell/T-cell leukemia, and 1 T-cell prolymphocytic leukemia) received everolimus. No dose-limiting toxicities were observed. Grade 3 potentially drug-related toxicities included hyperglycemia (22%), hypophosphatemia (7%), fatigue (7%), anorexia (4%), and diarrhea (4%). One patient developed a cutaneous leukocytoclastic vasculitis requiring a skin graft. One patient with refractory anemia with excess blasts achieved a major platelet response of over 3-month duration. A second patient with refractory anemia with excess blasts showed a minor platelet response of 25-day duration. Phosphorylation of downstream targets of mTOR, eukaryotic initiation factor 4E-binding protein 1, and/or, p70 S6 kinase, was inhibited in six of nine patient samples, including those from the patient with a major platelet response.

CONCLUSIONS

Everolimus is well tolerated at a daily dose of 10 mg daily and may have activity in patients with myelodysplastic syndrome. Studies of everolimus in combination with therapeutic agents directed against other components of the phosphatidylinositol 3-kinase/Akt/mTOR pathway are warranted.

Rapamycin: something old, something new, sometimes borrowed and now renewed

The molecular target of rapamycin (mTOR) is central to a complex intracellular signaling pathway and is involved in diverse processes including cell growth and proliferation, angiogenesis, autophagy, and metabolism. Although sirolimus (rapamycin), the oldest inhibitor of mTOR, was discovered more than 30 years ago, renewed interest in this pathway is evident by the numerous rapalogs recently developed. These newer agents borrow from the structure of sirolimus and, although there are some pharmacokinetic differences, they appear to differ little in terms of pharmacodynamic effects and overall tolerability. Given the multitude of potential applications for this class of agents and the decrease in cost that can be expected upon the expiration of sirolimus patents, renewed focus on this agent is warranted.

The incidence and impact of thrombocytopenia in myelodysplastic syndromes

Thrombocytopenia and platelet dysfunction contribute to hemorrhagic complications in the myelodysplastic syndromes (MDS). Reliable data regarding the frequency and consequences of thrombocytopenia in MDS are lacking. An extensive literature review indicated that the prevalence of thrombocytopenia (platelets<100x10(9)/L) in MDS ranged from 40% to 65%; the median frequency of thrombocytopenia prior to any MDS therapy was 65% (range, 23-93%). A retrospective review of patients who were referred to the University of Texas M. D. Anderson Cancer Center (MDACC) identified 1605 of 2410 patients (67%) with thrombocytopenia at referral. Of these, 1756 patients were classified using the International Prognostic Scoring System (IPSS), and 896 patients (51%) had intermediate-2 or high-risk disease. Treatment-related thrombocytopenia was observed in studies that involved azacitidine, tipifarnib, decitabine, lenalidomide, sirolimus, and combination chemotherapy with idarubicin, cytarabine, and topotecan. The reported incidence of hemorrhagic complications in the literature ranged from 3% to 53%, and the frequency of hemorrhagic deaths ranged from 14% to 24%. At MDACC, 460 patients had a coded cause of death: hemorrhage as a contributory cause of death, 20%; hemorrhage as the only cause of death, 10%. Thrombocytopenia was common in MDS, and there was an increased prevalence in higher risk IPSS categories. Many approved and investigational MDS therapies caused or exacerbated preexisting thrombocytopenia. The incidence of severe bleeding in MDS was greater than reported in current guidelines.

The Akt/Mammalian Target of Rapamycin Signal Transduction Pathway Is Activated in High-Risk Myelodysplastic Syndromes and Influences Cell Survival and Proliferation

The Akt/mammalian target of rapamycin (mTOR) signaling pathway is important for both cell growth and survival. In particular, an impaired regulation of the Akt/mTOR axis has been strongly implicated in mechanisms related to neoplastic transformation, through enhancement of cell proliferation and survival. Myelodysplastic syndromes (MDS) are a group of heterogeneous hematopoietic stem cell disorders characterized by ineffective hematopoiesis and by a high risk of evolution into acute myelogenous leukemia (AML). The pathogenesis of the MDS evolution into AML is still unclear, although some recent studies indicate that aberrant activation of survival signaling pathways could be involved. In this investigation, done by means of immunofluorescent staining, we report an activation of the Akt/mTOR pathway in high-risk MDS patients. Interestingly, not only mTOR was activated but also its downstream targets, 4E-binding protein 1 and p70 ribosomal S6 kinase. Treatment with the selective mTOR inhibitor, rapamycin, significantly increased apoptotic cell death of CD33(+) (but not CD33(-)) cells from high-risk MDS patients. Rapamycin was ineffective in cells from healthy donors or low-risk MDS. Moreover, incubation of high-risk MDS patient CD34(+) cells with rapamycin decreased the in vitro clonogenic capability of these cells. In contrast, the phosphoinositide 3-kinase inhibitor, LY294002, did not significantly affect the clonogenic activity of high-risk MDS cells. Taken together, our results indicate that the Akt/mTOR pathway is critical for cell survival and proliferation in high-risk MDS patients. Therefore, this signaling network could become an interesting therapeutic target for treating more advanced MDS cases.

Possible molecular target therapy with rapamycin in MDS

The authors previously reported the mRNA expression of Glutathione S-transferases theta (GSTT)-1, wild type (623 bp) and mutant (500 bp) in MDS patients. The deletion of 123 bp creates a sequence that is homologues to mammalian target of rapamycin (mTOR). To analyse the function of mutant GSTT-1 gene, stable transformants for the mutant and wild-type GSTT-1 gene, respectively, were established. In this study, the expression of wild and mutant type GSTT-1 gene of those stable transformants and bone marrow cells from MDS patients by RT-PCR was observed in the presence or absence of rapamycin. In result, exposure of rapamycin led to the disappearance of just the mutant gene band. This phenomenon possibly indicates that rapamycin only attacked the mutant GSTT-1 expressing clone.

Mammalian target of rapamycin (mTOR) regulates both proliferation of megakaryocyte progenitors and late stages of megakaryocyte differentiation

A major determinant in platelet production is the megakaryocyte (MK) size that is regulated both by ploidization and the increase in cytoplasmic volume at the end of maturation. Here we investigated the involvement of the mammalian target of rapamycin (mTOR) pathway in the regulation of megakaryopoiesis. We show that phosphorylation of mTOR, p70S6K1, and 4E-BP1 was diminished in thrombopoietin-cultured human MKs after rapamycin treatment. Rapamycin induced an inhibition in the G1/S transition and a decrease in the mean MK ploidy via a diminution of p21 and cyclin D3 occurring at a transcriptional level. Both cycling (2N/4N) and polyploid (8N/16N) MKs were reduced in size, with a size reduction slightly more pronounced in mature polyploid MKs than in immature ones. Rapamycin also induced a delay in the expression of MK markers and prevented the generation of proplatelet MKs. Additional experiments performed in vitro with MKs from mutant mice showed that the decrease in mean ploidy level and the delay in MK differentiation in the presence of rapamycin were less pronounced in CdknIa (p21)–/– MKs than in CdknIa (p21)+/+ MKs. These findings indicate that the mTOR pathway plays an important role during megakaryopoiesis by regulating ploidy, cell size, and maturation, in part by regulating p21 and cyclin D3.