Cancer Letters 354 (2014) 21–27

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Cancer Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c a n l e t

Mini-review

Molecular mechanisms of endometrial stromal sarcoma and undifferentiated endometrial sarcoma as premises for new therapeutic strategies Andelko Hrzenjak a,b,1,*, Martina Dieber-Rotheneder c,d,1, Farid Moinfar c,2, Edgar Petru d, Kurt Zatloukal c a

Department of Internal Medicine, Division of Pulmonology, Medical University of Graz, Graz, Austria Ludwig Boltzmann Institute for Lung Vascular Research, Medical University of Graz, Graz, Austria c Institute of Pathology, Medical University of Graz, Graz, Austria d Department of Obstetrics and Gynecology, Medical University of Graz, Graz, Austria b

A R T I C L E

I N F O

Article history: Received 25 June 2014 Received in revised form 11 August 2014 Accepted 11 August 2014 Keywords: Mesenchymal uterine tumors Endometrial stromal sarcomas Chemotherapy Histone deacetylase inhibitors Vorinostat

A B S T R A C T

Endometrial stromal sarcoma (ESS) and undifferentiated endometrial sarcoma (UES) are very rare gynecologic malignancies. Due to the rarity and heterogeneity of these tumors, little is known about their epidemiology, pathogenesis, and molecular pathology. Our previous studies have described deregulation of histone deacetylases expression in ESS/UES samples. Some of these enzymes can be inhibited by substances which are already approved for treatment of cutaneous T-cell lymphoma. On the basis of published data, they may also provide a therapeutic option for ESS/UES patients. Our review focuses on molecular mechanisms of ESS/UES. It describes various aspects with special emphasis on alteration of histone deacetylation and its possible relevance for novel therapies. © 2014 Elsevier Ireland Ltd. All rights reserved.

Pathophysiology Endometrial stromal tumors belong to the rarest uterine neoplasms. Uterine mesenchymal tumors comprise less than 5% of all primary uterine malignancies with endometrial stromal tumors accounting for less than 10% thereof [1]. According to the 2003 WHO classification, endometrial stromal tumors are divided into (i) non-invasive endometrial stromal nodule (ESN), a well-circumscribed benign mesenchymal tumor consisting of uniform cells closely resembling the uterine stromal cells of normal proliferative-phase endometrium; (ii) low-grade endometrial stromal sarcoma (ESS), an infiltrative tumor with stromal cells cytologically almost identical to those observed in the ESN but associated with low aggressive malignant behavior, and (iii) undifferentiated endometrial sarcomas (UES) [2]. In this classification the differentiation between low-grade and undifferentiated tumors is not made on mitotic count, but on the basis of nuclear pleomorphism and necrosis [3]. Strictly defined microscopic criteria (as nuclear atypia) support the WHO 2003 classification of ESS and UES [4] and are helpful in predicting recurrence [5].

* Corresponding author. Tel.: +43(0)31638580049; fax: +43(0)31638572058 E-mail address: [email protected] (A. Hrzenjak). 1 These authors contributed equally to this work. 2 Present address: Department of Pathology, Hospital of the Sisters of Charity, Linz. http://dx.doi.org/10.1016/j.canlet.2014.08.013 0304-3835/© 2014 Elsevier Ireland Ltd. All rights reserved.

Prognostic factors in uterine sarcomas have recently also been summarized by Gadducci [6]. These changes in definition and diagnostic criteria of ESS and UES make the interpretation of studies on biologic and prognostic features in low-grade ESS and UES particularly hazardous and require careful verification. The heterogeneous group of undifferentiated sarcomas often lacks specific differentiation and usually bears no morphological resemblance to endometrial sarcoma. They can be subdivided into groups with either uniform or pleomorphic nuclei [7]. The overall 5-year survival rate of patients with low-grade ESS ranges from 68% to 100%, whereas the 5-year survival rate of UES patients is markedly lower. Due to the rarity and heterogeneity of endometrial stromal neoplasms, little is known about their epidemiology, pathogenesis, and molecular pathology. These circumstances make investigations of their various aspects difficult, which is also reflected by the lack of efficient therapy. Recent therapeutic options for ESS/UES Therapeutic options for ESS/UES have been summarized indepth in previous reviews [8–10]. Current primary therapy for endometrial stromal sarcoma is surgery, mainly abdominal hysterectomy. The role of bilateral salpingo-oophorectomy and ovary preservation remain controversial [11–14]. Lymphadenectomy does not have an effect on survival [12]. Surgical removal of the primary

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tumor is frequently combined with subsequent adjuvant treatment involving radio- and/or chemotherapy. However, in most cases chemotherapy is moderately active and has palliative character only. Standardized systemic therapy in ESS/UES is not established yet. The expression of molecular targets for tyrosine kinase inhibitors (TKI) in ESS and UES was reported [15–17]. However, only a few cases of responses to imatinib in patients with uterine sarcomas expressing at least one target of TKI have been described. A single case of UES with EGFR expression temporarily responded to imatinib [18]. Furthermore, a complete metabolic response to imatinib mesylate in a patient with a low-grade ESS has been reported [19]. In contrast, a retrospective immunohistochemical and molecular analysis of potential targets of TKI in 52 ESS and 13 UES highly question the use of TKI in endometrial stromal tumors [20]. In some studies, a therapeutic use of progestins and aromatase inhibitors in the treatment of low-grade ESS has been shown. Aromatase inhibitors block the enzyme aromatase, which turns androgen into estrogen, thus reducing estrogen production in postmenopausal women. As a consequence, the amount of body estrogen for stimulation of estrogen-receptor positive tumor cells is decreased. Progestins bind to progesterone receptors and downregulate gene transcription. This is especially true for estrogen receptors, leading to the reduction of circulating estrogens and decrease in endometrial gland and stromal proliferation [3]. In their review, Thanopolou et al. summarized, among other results, data of 18 patients with recurrent/metastatic ESS treated with aromatase inhibitors. Five complete responses and 11 partial responses were seen. In addition, a negative impact of hormonal replacement therapy in ESS was demonstrated [10]. A high percentage of ESS cases express hormone receptors, especially estrogen (40–100%) and progesterone (60–100%) receptors [10,21,22]. On the other hand, contradictory data were published regarding the expression of androgen receptors in uterine sarcomas [23,24]. Hormonal therapies seem to have differential efficacy in ESS and UES. ESS tumors frequently express estrogen and progesterone receptors and usually show better response to hormonal therapies. Although hormonal therapy has been shown to be able to stabilize disease or to induce a remission, it must be stressed that this effect depends on the receptor status [8]. On the other side, UES tumors usually do not express hormone receptors and, therefore, are not susceptible to hormonal therapy. Aromatase inhibitors, including letrozole, seem to be promising agents which can be used either as adjuvant or as first-line treatment [10]. Because of tumor rarity, one can hardly expect that novel molecularly targeted therapies will be specifically developed against endometrial stromal sarcoma. Therefore, therapies used for other solid tumors, might be investigated with regard to their efficacy for treatment of endometrial stromal sarcomas. However, to establish the basis for such studies, molecular pathophysiology of ESS/UES has to be elucidated in more detail. Genetic alterations in ESS/UES Chromosomal and cytogenetic studies have shown some heterogeneous genetic aberrations in ESS and UES. One of the most frequent genetic aberrations found is the t(7;17) (p15;q21) chromosomal translocation, first described by Hennig et al. [25]. This non-random chromosomal change is mainly present in endometrial stromal sarcomas [26,27]. At the sites of the 7p15 and 17q21 breakpoints Koontz et al. found fusion of two zinc-finger proteins, the so-called JAZF1/JJAZ1 gene fusion [28]. The JAZF1/JJAZ1 gene fusion seems to be quite distinctive for ESS. We have previously found the JAZF1/JJAZ1 fusion in 80% of 18 classic ESS, and in none of the two UES [29]. This is in line with previous data from others, showing that UES were mostly negative for this translocation, whereas the percentage of positive ESS cases and non-malignant endometrial

stromal nodules (ENS) was quite high [28,30,31]. Overall, data suggest that JAZF1/JJAZ1 gene fusion is present only in a subset of primary ESS tumors. One recent study showed that 32% of ESS (n = 27) and none of UES cases (n = 17) were positive for this gene fusion [7]. These variations in prevalence might be based on (i) methods used for tissue collection and/or preservation, (ii) detection methods, or (iii) differences between patient populations. They also indicate the heterogeneity of ESS/UES, an issue which can only be solved by analyzing larger number of samples. Recently, new chromosomal translocation t(10;17) (q22;p13) was reported in a distinct group of ESS, fusing two genes: YWHAE, encoding a member of the 14-3-3 family, and either FAM22A or FAM22B, respectively [32,33]. Tumors with YWHE-FAM rearrangements are associated with high-grade morphology and aggressive clinical behavior which is important for prognostic and therapeutic purposes [34,35]. Although detection of these genetic alterations is undoubtedly an improvement in diagnostics for differentiation between ESS and UES, clinical utility and potential benefit for therapy needs to be established. Wnt pathway deregulation in ESS/UES The role of Wnt pathway in embryogenesis, in adult tissue homeostasis and in tumor development is quite well described. The canonical Wnt signaling pathway, involving ß-catenin, is deregulated in ESS/UES [36]. With a genome-wide cDNA library, more than 300 genes deregulated in ESS could be detected. Among the most strongly deregulated genes, there were secreted frizzled-related protein 4 (SFRP4) and SFRP1, putative modulators of the Wntsignaling pathway [37,38]. SFRP4 was down-regulated in ESS and UES as compared with non-malignant proliferative endometrium, as shown by QRT-PCR and in situ hybridization (Fig. 1a and 1b). SFRP4 expression in UES was even lower than in ESS, but this finding was validated in a relatively low number of UES cases only and requires further proof. Interestingly, recent methylation studies have not shown hypermethylation of the SFRP4 promoter sequence which would have explained such down-regulation [39]. Thus, other mechanisms must be responsible for deregulation of SFRP4 in ESS and UES. Different SFRP-family members can bind to Wnt molecules and prevent their binding on frizzled receptors located in the cell membrane and subsequent activation of the Wnt pathway. This results in activation of disheveled protein and inhibition of glycogen synthase kinase-3ß, an enzyme responsible for ß-catenin phosphorylation. Subsequently, non-phosphorylated ß-catenin accumulates and is translocated into the cell nucleus forming complexes with T-cell factor/lymphoid enhancing factor (TCF/ LEF). The latter is a transcription factor that stimulates TCF/LEF mediated gene expression and further activates the expression of numerous genes stimulating cell proliferation. Indeed, ß-catenin is increased in ESS and UES in comparison with non-malignant endometrium, indicating activation of the Wnt signaling pathway in tumor tissue (Fig. 1b). In addition, an increased translocation of ß-catenin from cytoplasm into the nucleus and a positive correlation with proliferation marker Ki-67, especially in more aggressive UES cases has been shown [36]. Overall, SFRP4 seems to act as a tumor suppressor gene regulating the cytosolic ß-catenin pool in the cell. Interestingly, Feng et al. found that proliferation markers, such as Ki-67, are also predictive for a high recurrence of ESS [40]. By using immunostaining, Ng et al. detected nuclear ß-catenin staining in 40% of ESS and suggested this method to be potentially useful for diagnosis, especially for distinguishing ESS from leiomyosarcoma, which are negative for nuclear ß-catenin [41]. Kildal et al. found strong nuclear ß-catenin staining in 61% of the 82 ESS cases [42]. However, they also found nuclear ß-catenin staining in 31% of normal endometrial stroma samples. Thus, the diagnostic and

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a Fold change (SFRP4 vs. GAPDH)

60 UES 50 40 30

ESS proliferative endometria

20

** 10

*

0 Tissue type Proliferative endometrium

ESS

UES

ß-catenin Immunoblotting

SFRP4 In situ hybridization

b

Fig. 1. Deregulation of the Wnt pathway in endometrial stromal sarcoma. (a) As determined by qRT-PCR, the expression level of SFRP4 in both ESS and UES was much lower compared to proliferative endometrium. Number of cases: ESS, n = 10; UES, n = 4. The presented data represent means ± SD (*p < 0.05; **p < 0.01). (b) In situ hybridization (upper panel) using antisense SFRP4-specific probes revealed high amount of SFRP4 mRNA in the stroma of proliferative endometrium. There was no staining of endometrial glands (arrow-head). SFRP4 mRNA in the ESS and UES sample was greatly decreased. Immunoblotting for ß-catenin (lower panel) was performed on paraffinembedded tissue slices. In ESS tumor cells granular cytoplasmic staining was observed, whereas in UES tumor cells staining was more prominent in nuclei, suggesting nuclear translocation of ß-catenin. All sections were counterstained with hematoxylin. Original magnifications = 600×. For details see [36] (data originally published by Wiley).

prognostic value of this staining remains uncertain. Kurihara et al. found coincident overexpression of ß-catenin and cyclin D1, a direct target gene of ß-catenin, in UES with nuclear uniformity [39]. In contrast, this overexpression was very rare among endometrial stromal nodules and low-grade ESS. Recently, cyclin D1 was reported as diagnostic immunomarker for YWHAE-FAM22 ESS [32]. Histone de/acetylation in ESS/UES Many cancer types, including gynecological malignancies, are characterized by epigenetic alterations [43,44]. Acetylation and deacetylation of histone proteins are chemical modifications controlled by two groups of enzymes – histone acetylases (HATs) and histone deacetylases (HDACs). These enzymes regulate the acetylation level of histone proteins, thereby influencing the chromatin condensation and chromatin susceptibility for different transcription factors. Moreover, not only histone proteins, but also many other cellular proteins, are prone to those epigenetic modifications. Thus, the potential influence of de/acetylation on different cellular events goes far beyond our present knowledge. So far, 18 different isoforms of HDACs are known. The majority of them are zinc-dependent metalloproteases grouped into class I, II and IV, respectively. Class III HDACs are not zinc-dependent, but NAD+-dependent enzymes [45,46]. Because of their localization both in cell nucleus and in

cytoplasm, class II HDACs (HDAC 4, 5, 6, 7, 9 and 10) have a very broad range of substrates making their inhibition of special interest. HDAC might be of particular relevance for ESS and UES since HDAC2 was found to be expressed in all ESS samples with strong immunohistological positivity indicating overexpression in approximately 80% of samples (Fig. 2) [47]. Despite > 80% homology on the protein level between HDAC1 and HDAC2, which are both constitutively expressed class I HDACs, HDAC1 was not overexpressed in analyzed tumor samples when compared to non-neoplastic endometrial tissue [47]. Inhibition of HDAC2 expression by sodium valproate resulted in increased inhibition of cell-growth which has been accompanied by G1 arrest and by reduction of the number of cells in the S-phase of the cell cycle, as determined by fluorescence activated cell sorting (FACS). Interestingly, in the endometrial stromal sarcoma cell line ESS-1 [48], increased growth inhibition was primarily based on arrested cell proliferation and not on apoptosis. The effects of valproate on ESS-1 cells were attributed to the inhibition of HDAC2 which has been verified by comparative studies using cells transfected with HDAC2-specific siRNA. Further clues on the role of HDAC in ESS and UES-related cell lines have been obtained in studies using Vorinostat (suberoylanilide hydroxamic acid, SAHA, Zollinza™) which is a potent inhibitor of both

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ESS

UES

HDAC2

Proliferative endometrium

Fig. 2. HDAC2 deregulation in endometrial stromal sarcoma. Immunohistochemical staining by using specific anti HDAC2 antibody showed very weak or negative reaction in proliferative endometrium. Only endometrial gland cells showed intensive positive nuclear reaction for HDAC2. In contrast, both ESS and UES samples were strongly positive. Original magnifications = 400×. For details see [47] (data originally published by AACR).

class I and II HDACs [49–51]. Vorinostat binds directly to the active centre of histone deacetylases, thereby acting as chelator for zinc ion and blocking the catalytic site of those enzymes [52]. In cell culture, vorinostat efficiently inhibits cell growth of various tumor cell types in a micromolar range. These inhibitory effects are mainly based on the activation of apoptosis, inhibition of cell growth or the activation of autophagic mechanisms and seem to be, at least to some degree, cell-type specific. Vorinostat efficiently inhibited growth of uterine sarcoma cells (MES-SA cell line) [53] by influencing different HDAC members (HDAC 2, 3 and 7) [54]. In MES-SA cells the growth inhibition was based on increased activation of apoptosis. Efficacy of vorinostat on endometrial stromal sarcoma and uterine sarcoma cells (ESS-1 and MES-SA) are summarized in Fig. 3. The cell lines differed in expression of CD10, Cyclin D1, P53 and PTEN-mutation. In contrast to MESSA cells, ESS-1 cells are positive for CD10, cyclinD1and PTEN mutation but negative for P53. Other in vitro studies in ESS-1 with vorinostat showed similar results, indicating inhibition of G1/S transition based on down-regulation of HDAC7 expression. These inhibitory effects were not apoptosis-based, but relied on vorinostat-based activation of autophagy [55]. Interestingly, a combination of vorinostat with a tumor necrosis factor-related apoptosis-inducing ligand (TRAIL/Apo-2L) remarkably enhanced the cytotoxic effect in the sarcoma cell lines ESS-1 and MES-SA. In this study the induction of apoptosis or autophagy by vorinostat could be related to the expression of TRAIL receptor 1/DR 4 and caspase-8, respectively [56]. In nude mice xenografts of MES-SA cells 50 mg/kg/day vorinostat administered over 3 weeks resulted in a more than 50% reduction of tumor size, as compared with placebo [54]. This reduction of

ESS-1

Vorinostat concentration (µM)

tumor size was rather based on vorinostat-based activation of apoptosis than on diminished tumor-cell proliferation. Based on these results, the main mechanism of cell growth inhibition by vorinostat seems to depend on the cell-type. On the other hand, these differences might also be dependent on duration of vorinostat treatment and/or the time-interval between treatment and final cell proliferation analysis. HDAC inhibitors (HDACIs) as possible targeted therapy for ESS/UES Histone deacetylase inhibitors belong to a novel, powerful class of drugs which are promising for treatment of different hematologic and solid malignancies [57]. So far, two different HDAC inhibitors have been approved by the US Food and Drug Administration (FDA): Vorinostat (SAHA, Zolinza™) [58] and romidepsin (FK228, Istodax™) [59]. Both of them are approved as therapies for cutaneous T-cell lymphoma. Romidepsin (depsipeptide, FK228, Istodax™), a prodrug whose disulphide bond must be reduced to yield the active form, is also approved for peripheral T-cell lymphoma (PTCL) [60]. Further hydroxamic acid-based HDACIs, including panobinostat (LBH589) and belinostat (PXD101), are active in a wide range of hematologic malignancies and different solid tumors, as recently reviewed by Grassadonia et al. [61] and Anne et al. [62]. Comprehensive summaries of clinical trials and their outcomes with different HDAC inhibitors, either as monotherapy or as combination therapy, have been published recently [63,64]. Although up to now vorinostat and romidepsin have been approved for the treatment of T-cell lymphoma, numerous in vitro and in vivo studies also have shown its antineoplastic effect in many other

MES-SA

Vorinostat concentration (µM)

Fig. 3. Vorinostat inhibits proliferation of endometrial stromal sarcoma (ESS) and uterine sarcoma (MES-SA) cells. Cells were treated with indicated vorinostat concentrations and after 24, 48 or 72 hours the cell number was determined. Cell proliferation was measured by using [3H]thymidine uptake assay. Results are expressed as percentage inhibition of [3H]thymidine incorporation upon vorinostat treatment and normalized to untreated cells. For details see [54] and [55] (data originally published by BioMed Central and Wiley, respectively).

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Table 1 A survey of completed clinical trials with HDAC inhibitor monotherapies in solid tumors. Drug/ Tumor type (A) vorinostat Gastrointestinal cancer Advanced solid tumors Advanced prostate cancer Solid tumors Glioblastoma multiforme GBM Advanced cancer Relapsed NSCLC Thyroid cancer Recurrent/metastatic head and neck cancer Metastatic breast cancer Epithelial ovarian or peritoneal carcinoma Relapsed or refractory breast, colorectal or NSCLC Mesothelioma Advanced cancer Advanced cancer (solid/hematological: 50/23) (B) romidepsin Thyroid carcinoma Head and neck cancer Recurrent glioma Prostate cancer SCLC Colorectal cancer Lung cancer Renal cell cancer (C) belinostat Unresectable hepatocellular carcinoma Recurrent or refractory thymic epithelial tumors Epithelial ovariancancer Micropapillary ovarian tumors Advanced malignant pleural mesothelioma Advanced refractory solid tumors (D) panabinostat Refractory metastatic renal cell carcinoma (E) entinostat Pretreated metastatic melanoma

Trial phase

Number of patients

Response

First author, year (reviewed in)

I I II I II I II II II II II II I I I

16 57 27 18 66 25 16 19 12 12 27 16 13 23 73

7 SD (>8 weeks) 12 SD, 1 PR 2 SD (84 and 135 d) 9 SD, 9 PD PFS 6 m 9/52. no prolongation of QTc-interval 8 SD (TTP: median 2.3 m) 9 SD (TTP: median 24 w) 3 SD, 1 PR (unconfirmed) 4 SD (4-14 m) 1 PR, 9 SD (2 PFS > 6 m) 8 SD (TTP: median 33.5 d) 2 PR, 4 SD PR, 1/14, SD 2/14 1 CR, 5 PR (2 unconfirmed) 16 SD

Doi et al., 2012 (63) Ramalingam et al., 2010 (57) Bradley et al., 2009 (57) Fujiwara et al., 2009 (57) Galanis et al., 2009 (64) Munster et al., 2009 (63) Traynor et al., 2009 (64) Woyach et al., 2009 (64) Blumenschein et al., 2008 (64) Luu et al., 2008 (64) Modesitt et al., 2008 (64) Vansteenkiste et al., 2008 (64) Krug et al., 2006 (63) Rubin et al., 2006 (63) Kelly et al., 2005 (64)

II II I/II II II II II II

20 14 8/35 25 16 25 18 25

13 SD 2 SD PFS median 8 w, 1 PFS of 6 m 2 PR ≥6 m 3 SD, PFS median 1.8 m 4 SD 9 SD 1 CR

Sherman et al., 2013 (64) Haigentz et al., 2012 (64) Iwamoto et al., 2011 (64) Molife et al., 2010 (64) Otterson et al., 2010 (64) Whitehead et al., 2009 (64) Schrump et al., 2008 (64) Stadler et al., 2006 (64)

I/II II II

54 41 32

Yeo et al., 2012 (64) Giaccone et al., 2011 (64) Mackay et al., 2010 (64)

II I

13 46

PFS (median: 2.64 m), SD:45.2% PFS at 6 m: 46% PFS (median: 2.3 m), SD 9/15 PFS (median: 13.4 m), SD 2/12 2 SD, PFS (median: 1 m) 18 SD

II

20

none

Hainsworth et al., 2011 (64)

II

28

7 SD(4/3), (TTP median: 55/51 d)

Hauschield et al., 2008 (64)

Ramalingam et al., 2009 (64) Steele et al., 2008 (64)

Abbreviations: FL, follicular lymphoma; MZL, marginal zone lymphoma; MCL, mantle cell lymphoma; NSCLC, non-small cell lung cancer; CR, complete response; PR, partial response; SD, stable disease; ORR, overall response rate; PFS, progression free survival; TTP, time to progression; d, days; w, weeks; m, months.

malignancies. Clinical trials of HDACIs in solid tumors have been extensively reviewed by Qiu et al. [57] and Slingerland et al. [64]. A survey on completed clinical trials with HDACI monotherapies in solid tumors is shown in Table 1 [57,63,64]. Although in clinical studies with solid tumors complete response to HDAC treatment was never observed, partial responses and especially stabilization of disease was found in many patients [63]. Clinical trials in patients with solid tumors of the breast, colorectum and non-small cell lung cancer did not necessarily indicate significant response rates. Response to therapy with vorinostat or romidepsin in combination with other chemotherapeutics or with radiotherapy was markedly improved. However, these approaches were associated with higher toxicity in some cases [63]. Only a few studies have described positive effects of HDAC inhibitors in gynecological tumors. Dowdy et al. showed that the HDAC inhibitor trichostatin A (TSA) and paclitaxel caused synergistic activation of apoptosis in endometrial cancer cell lines [65]. Several independent studies shed some light on the importance of class I HDAC in various gynecological malignancies. Expression of class I HDACs (HDAC 1, 2 and 3) was shown to be inversely correlated with the prognosis of endometrial and ovarian carcinomas [66]. Interestingly, the inhibition of class I HDACs, especially of HDAC3, suppresses ovarian cancer cell growth in vitro by drug-induced gene silencing [67]. Multiple clinical trials, either with HDACIs alone or in combination with other agents are currently underway. They are used in a wide range of hematologic malignancies and many different solid tumors [63,68,69]. The most frequently used HDACIs in open clinical studies comprise vorinostat (n = 57), panobinostat (n = 41),

romidepsin (n = 23), valproic acid (in cancer: n = 22), entinostat (n = 9) and belinostat (n = 6) (http://clinicaltrials.gov/; March 2014). Conclusion and future perspectives In the last decade, the number of papers on ESS and UES, two very rare gynecological tumor entities, has considerably increased. Despite this, our knowledge about the molecular mechanisms of ESS/ UES and available therapeutic options is still limited and needs to be improved. Detailed understanding of molecular pathophysiology and cell biology of these tumors is an important premise for development of more potent systemic therapies. On the other side, presently available chemotherapeutic drugs established in the treatment of other malignancies should also be tested for their efficacy in ESS/UES. This would not only save financial resources, but would also decrease the time for the development of new drugs. These aspects are of special importance because ESS/UES tumors are very rare and the financial aspect of the development of drugs for orphandiseases is particularly challenging. Investigations of targeted drugs interfering with different pathways may provide new therapeutic strategies for ESS and UES. Several in vitro studies suggest that interfering with some basic mechanisms including the induction of apoptosis and/or inhibition of tumor-cell proliferation may be the way for future experiments and clinical trials. In numerous in vitro and in vivo studies HDAC inhibitors turned out to be potent candidates as mono- or as combined-therapy in various malignancies. They frequently enhance the effects of standard therapies in an additive or synergistic manner. Some of them are already approved by the FDA as therapeutic options for cutaneous T-cell lymphoma.

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Importantly, the concentrations used are usually in low micromolar range and the side effects observed are relatively mild [68,70]. Vorinostat has been tested in various in vitro and in vivo systems including numerous clinical trials, in which its tumor-inhibitory effects have been shown. Previous studies have demonstrated its pronounced inhibitory effects in endometrial sarcoma cells (ESS-1 and MES-SA), both in vitro and in vivo. Based on these promising pre-clinical data, we propose HDAC inhibitors as to be further tested for treatment of gynecological tumors in general and endometrial stromal sarcoma in particular. Conflict of interest statement None declared. Acknowledgement This work was supported by Lore Saldow Research Fund. This paper is dedicated to the memory of Mrs. Lore Saldow. We thank Helmut Denk, Johannes Haybaeck and Leopold Froehlich for stimulating discussion. Figures presented in this review represent own results and were previously published from our group. According to the copyright policies from the original publishers, there is no need for additional copyright permission for these figures. Originally, these figures were part of our papers published by: BioMed Central (for Fig. Mol. Cancer), AACR (Mol. C. Ther.) and Wiley (J. Pathol.). Some figures contain additional data which have not been published before.

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