The Novel HSP90 inhibitor, IPI-493, is highly effective in human gastrostrointestinal stromal tumor xenografts carrying heterogeneous KIT mutations

Patient-derived xenograft models of gastrointestinal stromal tumors provide a ready-to-use platform for translational research

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal malignancy of the gastrointestinal tract. Most GISTs harbor mutations in oncogenes, such as KIT, and are treated with tyrosine kinase inhibitors (TKIs), such as imatinib. Most tumors develop secondary mutations, inducing drug resistance against the available TKIs, requiring novel therapies. We established a GIST patient-derived xenograft (PDX) platform of GIST that can be used for preclinical drug testing. Tumor tissue from consenting GIST patients was transplanted subcutaneously to NMRI nu/nu mice. Once tumor growth was observed, the tumor was re-transplanted to a next generation of mice. Tumors were characterized histopathologically and molecularly at every re-transplantation and compared with the original patient tumor. We transplanted 112 tumor samples from 99 GIST patients, resulting in 12 established and well-characterized GIST models with different mutations and TKI sensitivity. Three models harbor secondary KIT mutations. One model is characterized by a primary, imatinib-resistant PDGFRA exon 18 p.D842V mutation. Our established platform of well-characterized GIST PDX models, covering the most relevant driver mutations, serves as an excellent tool for preclinical drug testing and tumor biology studies.

Targeting protein-protein interactions in drug discovery: Modulators approved or in clinical trials for cancer treatment

Protein-protein interactions (PPIs) form complex cellular networks fundamental to many key biological processes, including signal transduction, cell proliferation and DNA repair. In consequence, their perturbation is often associated with many human diseases. Targeting PPIs offers a promising approach in drug discovery and ongoing advancements in this field hold the potential to provide highly specific therapies for a wide range of complex diseases. Despite the development of PPI modulators is challenging, advances in the genetic, proteomic and computational level have facilitated their discovery and optimization. Focusing on anticancer drugs, in the last years several PPI modulators have entered clinical trials and venetoclax, which targets Bcl-2 family proteins, has been approved for treating different types of leukemia. This review discusses the clinical development status of drugs modulating several PPIs, such as MDM2-4/p53, Hsp90/Hsp90, Hsp90/CDC37, c-Myc/Max, KRAS/SOS1, CCR5/CCL5, CCR2/CCL2 or Smac/XIAP, in cancer drug discovery.

KIT mutations and expression: current knowledge and new insights for overcoming IM resistance in GIST

Gastrointestinal stromal tumor (GIST) is the most common sarcoma located in gastrointestinal tract and derived from the interstitial cell of Cajal (ICC) lineage. Both ICC and GIST cells highly rely on KIT signal pathway. Clinically, about 80-90% of treatment-naive GIST patients harbor primary KIT mutations, and special KIT-targeted TKI, imatinib (IM) showing dramatic efficacy but resistance invariably occur, 90% of them was due to the second resistance mutations emerging within the KIT gene. Although there are multiple variants of KIT mutant which did not show complete uniform biologic characteristics, most of them have high KIT expression level. Notably, the high expression level of KIT gene is not correlated to its gene amplification. Recently, accumulating evidences strongly indicated that the gene coding, epigenetic regulation, and pre- or post- protein translation of KIT mutants in GIST were quite different from that of wild type (WT) KIT. In this review, we elucidate the biologic mechanism of KIT variants and update the underlying mechanism of the expression of KIT gene, which are exclusively regulated in GIST, providing a promising yet evidence-based therapeutic landscape and possible target for the conquer of IM resistance. Video Abstract.

Heat shock protein 90: biological functions, diseases, and therapeutic targets

Heat shock protein 90 (Hsp90) is a predominant member among Heat shock proteins (HSPs), playing a central role in cellular protection and maintenance by aiding in the folding, stabilization, and modification of diverse protein substrates. It collaborates with various co-chaperones to manage ATPase-driven conformational changes in its dimer during client protein processing. Hsp90 is critical in cellular function, supporting the proper operation of numerous proteins, many of which are linked to diseases such as cancer, Alzheimer's, neurodegenerative conditions, and infectious diseases. Recognizing the significance of these client proteins across diverse diseases, there is a growing interest in targeting Hsp90 and its co-chaperones for potential therapeutic strategies. This review described biological background of HSPs and the structural characteristics of HSP90. Additionally, it discusses the regulatory role of heat shock factor-1 (HSF-1) in modulating HSP90 and sheds light on the dynamic chaperone cycle of HSP90. Furthermore, the review discusses the specific contributions of HSP90 in various disease contexts, especially in cancer. It also summarizes HSP90 inhibitors for cancer treatment, offering a thoughtful analysis of their strengths and limitations. These advancements in research expand our understanding of HSP90 and open up new avenues for considering HSP90 as a promising target for therapeutic intervention in a range of diseases.

Advances in immunology and immunotherapy for mesenchymal gastrointestinal cancers

Mesenchymal gastrointestinal cancers are represented by the gastrointestinal stromal tumors (GISTs) which occur throughout the whole gastrointestinal tract, and affect human health and economy globally. Curative surgical resections and tyrosine kinase inhibitors (TKIs) are the main managements for localized GISTs and recurrent/metastatic GISTs, respectively. Despite multi-lines of TKIs treatments prolonged the survival time of recurrent/metastatic GISTs by delaying the relapse and metastasis of the tumor, drug resistance developed quickly and inevitably, and became the huge obstacle for stopping disease progression. Immunotherapy, which is typically represented by immune checkpoint inhibitors (ICIs), has achieved great success in several solid tumors by reactivating the host immune system, and been proposed as an alternative choice for GIST treatment. Substantial efforts have been devoted to the research of immunology and immunotherapy for GIST, and great achievements have been made. Generally, the intratumoral immune cell level and the immune-related gene expressions are influenced by metastasis status, anatomical locations, driver gene mutations of the tumor, and modulated by imatinib therapy. Systemic inflammatory biomarkers are regarded as prognostic indicators of GIST and closely associated with its clinicopathological features. The efficacy of immunotherapy strategies for GIST has been widely explored in pre-clinical cell and mouse models and clinical experiments in human, and some patients did benefit from ICIs. This review comprehensively summarizes the up-to-date advancements of immunology, immunotherapy and research models for GIST, and provides new insights and perspectives for future studies.

Recent developments in the identification and biosynthesis of antitumor drugs derived from microorganisms

Secondary metabolites in microorganisms represent a resource for drug discovery and development. In particular, microbial-derived antitumor agents are in clinical use worldwide. Herein, we provide an overview of the development of classical antitumor drugs derived from microorganisms. Currently used drugs and drug candidates are comprehensively described in terms of pharmacological activities, mechanisms of action, microbial sources, and biosynthesis. We further discuss recent studies that have demonstrated the utility of gene-editing technologies and synthetic biology tools for the identification of new gene clusters, expansion of natural products, and elucidation of biosynthetic pathways. This review summarizes recent progress in the discovery and development of microbial-derived anticancer compounds with emphasis on biosynthesis.

Heterocyclic Compounds as Hsp90 Inhibitors: A Perspective on Anticancer Applications

Heat shock proteins (Hsps) have garnered special attention in cancer therapy as molecular chaperones with regulatory/mediatory effects on folding, maintenance/stability, maturation, and conformation of proteins as well as their effects on prevention of protein aggregation. Hsp90 ensures the stability of various client proteins needed for the growth of cells or the survival of tumor cells; therefore, they are overexpressed in tumor cells and play key roles in carcinogenesis. Accordingly, Hsp90 inhibitors are recognized as attractive therapeutic agents for investigations pertaining to tumor suppression. Natural Hsp90 inhibitors comprising geldanamycin (GM), reclaimed analogs of GM including 17-AAG and DMAG, and radicicol, a natural macrocyclic antifungal, are among the first potent Hsp90 inhibitors. Herein, recently synthesized heterocyclic compounds recognized as potent Hsp90 inhibitors are reviewed along with the anticancer effects of heterocyclic compounds, comprising purine, pyrazole, triazine, quinolines, coumarin, and isoxazoles molecules.

The heat shock response and small molecule regulators

The heat shock response (HSR) is a highly conserved cellular pathway that is responsible for stress relief and the refolding of denatured proteins [1]. When a host cell is exposed to conditions such as heat shock, ischemia, or toxic substances, heat shock factor-1 (HSF-1), a transcription factor, activates the genes that encode for the heat shock proteins (Hsps), which are a family of proteins that work alongside other chaperones to relieve stress and refold proteins that have been denatured (Burdon, 1986) [2]. Along with the refolding of denatured proteins, Hsps facilitate the removal of misfolded proteins by escorting them to degradation pathways, thereby preventing the accumulation of misfolded proteins [3]. Research has indicated that many pathological conditions, such as diabetes, cancer, neuropathy, cardiovascular disease, and aging have a negative impact on HSR function and are commonly associated with misfolded protein aggregation [4,5]. Studies indicate an interplay between mitochondrial homeostasis and HSF-1 levels can impact stress resistance, proteostasis, and malignant cell growth, which further support the role of Hsps in pathological and metabolic functions [6]. On the other hand, Hsp activation by specific small molecules can induce the heat shock response, which can afford neuroprotection and other benefits [7]. This review will focus on the modulation of Hsps and the HSR as therapeutic options to treat these conditions.

Pimitespib is effective on cecal GIST in a mouse model of familial GISTs with KIT-Asp820Tyr mutation through KIT signaling inhibition

Three families with multiple gastrointestinal stromal tumors (GISTs) caused by a germline Asp820Tyr mutation at exon 17 of the c-kit gene (KIT-Asp820Tyr) have been reported. We previously generated a knock-in mouse model of the family, and the mice with KIT-Asp818Tyr corresponding to human KIT-Asp820Tyr showed a cecal tumor equivalent to human GIST. In the model mice, we reported that tyrosine kinase inhibitor, imatinib, could stabilize but not decrease the cecal tumor volume. In this report, we examined whether a heat shock protein 90 inhibitor, pimitespib (TAS-116), has an inhibitory effect on phosphorylation of KIT-Asp818Tyr and can decrease the cecal tumor volume in the model mice. First, we showed that pimitespib inhibited KIT phosphorylation both dose- and time-dependently in KIT-Asp818Tyr transfected murine Ba/F3 cells. Then, four 1-week courses of pimitespib were orally administered to heterozygous (KIT-Asp818Tyr/+) model mice. Each course consisted of once-daily administration for consecutive 5 days followed by 2 days-off. Cecal tumors were dissected, and tumor volume was histologically analyzed, Ki-67 labeling index was immunohistochemically examined, and apoptotic figures were counted. Compared to the vehicle treated mice, pimitespib administered mice showed statistically significantly smaller cecal tumor volume, lower Ki-67 labeling index, and higher number of apoptotic figures in 10 high power fields (P = 0.0344, P = 0.0019 and P = 0.0269, respectively). Western blotting revealed that activation of KIT signaling molecules was strongly inhibited in the tumor tissues of pimitespib-administered mice compared to control mice. Thus, pimitespib seemed to inhibit in vivo tumor progression effectively in the model mice. These results suggest that the progression of multiple GISTs in patients with germline KIT-Asp820Tyr might be controllable by pimitespib.

Targeting heat shock protein 90 for anti-cancer drug development

INTRODUCTION

Heat shock proteins (HSPs) constitute a large family of proteins involved in protein folding and maturation. HSP expression is induced by heat shock or other stressors including cellular damage and hypoxia. The major groups, which are classified based on their molecular weight, include HSP27, HSP40, HSP60, HSP70, HSP90, and large HSP (HSP110 and glucose-regulated protein 170). HSPs play a significant role in cellular proliferation, differentiation, survival, apoptosis, and carcinogenesis. The human HSP90 family consists of five members and has a strong association with cancer.

OBJECTIVES

The primary objective is to review the important functions of heat shock protein 90 in cancer, especially as an anti-cancer drug target.

RESULTS

The HSP90 proteins not only play important roles in cancer development, progression, and metastasis, but also have potential clinical use as biomarkers for cancer diagnosis or assessing disease progression, and as therapeutic targets for cancer therapy. In this chapter, we discuss the roles of HSP90 in cancer biology and pharmacology, focusing on HSP90 as an anti-cancer drug target. An understanding of the functions and molecular mechanisms of HSP90 is critical for enhancing the accuracy of cancer diagnosis as well as for developing more effective and less toxic chemotherapeutic agents.

CONCLUSION

We have provided an overview of the complex relationship between cancer and HSP90. HSP90 proteins play an important role in tumorigenesis and may be used as potential clinical biomarkers for the diagnosis and predicting prognostic outcome of patients with cancer. HSP90 proteins may be used as therapeutic targets for cancer therapy, prompting discovery and development of novel chemotherapeutic agents.

Inhibitors of HSP90 in melanoma

HSP90 (heat shock protein 90) is an ATP-dependent molecular chaperone involved in a proper folding and maturation of hundreds of proteins. HSP90 is abundantly expressed in cancer, including melanoma. HSP90 client proteins are the key oncoproteins of several signaling pathways controlling melanoma development, progression and response to therapy. A number of natural and synthetic compounds of different chemical structures and binding sites within HSP90 have been identified as selective HSP90 inhibitors. The majority of HSP90-targeting agents affect N-terminal ATPase activity of HSP90. In contrast to N-terminal inhibitors, agents interacting with the middle and C-terminal domains of HSP90 do not induce HSP70-dependent cytoprotective response. Several inhibitors of HSP90 were tested against melanoma in pre-clinical studies and clinical trials, providing evidence that these agents can be considered either as single or complementary therapeutic strategy. This review summarizes current knowledge on the role of HSP90 protein in cancer with focus on melanoma, and provides an overview of structurally different HSP90 inhibitors that are considered as potential therapeutics for melanoma treatment.

Old and New Approaches to Target the Hsp90 Chaperone

The 90-kDa heat shock protein (Hsp90) is a molecular chaperone that ensures cellular proteostasis by maintaining the folding, stabilization, activation, and degradation of over 400 client proteins. Hsp90 is not only critical for routine protein maintenance in healthy cells, but also during states of cellular stress, such as cancer and neurodegenerative diseases. Due to its ability to affect phosphorylation of numerous client proteins, inhibition of Hsp90 has been an attractive anticancer approach since the early 1990's, when researchers identified a druggable target on the amino terminus of Hsp90 for a variety of cancers. Since then, 17 Hsp90 inhibitors that target the chaperone's Nterminal domain, have entered clinical trials. None, however, have been approved thus far by the FDA as a cancer monotherapy. In these trials, a major limitation observed with Hsp90 inhibition at the N-terminal domain was dose-limiting toxicities and relatively poor pharmacokinetic profiles. Despite this, preclinical and clinical research continues to show that Hsp90 inhibitors effectively target cancer cell death and decrease tumor progression supporting the rationale for the development of novel Hsp90 inhibitors. Here, we present an in-depth overview of the Hsp90 inhibitors used in clinical trials. Finally, we present current shifts in the field related to targeting the carboxy-terminal domain of Hsp90 as well as to the development of isoform-selective inhibitors as a means to bypass the pitfalls of current Hsp90 inhibitors and improve clinical trial outcomes.

17-Aminogeldanamycin selectively diminishes IRE1α-XBP1s pathway activity and cooperatively induces apoptosis with MEK1/2 and BRAFV600E inhibitors in melanoma cells of different genetic subtypes

Outcomes of melanoma patient treatment remain unsatisfactory despite accessibility of oncoprotein-targeting drugs and immunotherapy. Here, we reported that 17-aminogeldanamycin more potently activated caspase-3/7 in BRAF^V600E melanoma cells than geldanamycin, another inhibitor of heat shock protein 90 (HSP90). 17-aminogeldanamycin alleviated self-triggered compensatory increase in HSP70 mRNA level and induced endoplasmic reticulum (ER) stress, which was followed by selective diminution of cytoprotective IRE1α-XBP1s pathway activity of unfolded protein response (UPR), inhibition of ERK1/2 activity and induction of apoptosis. Concomitantly, ATF6/p50 level and expression of PERK-dependent genes, CHOP and BIM, remained unaltered. This might result from an inframe deletion in EIF2AK3 leading to a PERK^L21del variant revealed by whole-exome sequencing in melanoma cell lines. 17-aminogeldanamycin exhibited similar activity in NRAS^Q61R melanoma cells that harbored a heterozygous inactivating variant of NAD(P)H:quinone oxidoreductase 1 (NQO1^P187S). In addition, 17-aminogeldanamycin acted cooperatively with trametinib (an inhibitor of MEK1/2) and vemurafenib (an inhibitor of BRAF^V600E) in induction of apoptosis in melanoma cell lines as evidenced by in-cell caspase-3/7 activation and PARP cleavage that occurred earlier compared with either drug used alone. As trametinib and vemurafenib did not significantly affect HSP70 and GRP78 transcript levels, cooperation of MEK/BRAF^V600E inhibitors and 17-aminogeldanamycin might result from a concurrent inhibition of the RAS/RAF/MEK/ERK cascade and IRE1α-dependent signaling, and cell-intrinsic ER homeostasis can determine the extent of the drug cooperation. Our study indicates that 17-aminogeldanamycin takes several advantages compared with other HSP90-targeting compounds, and can complement activity of BRAF/MEK inhibitors in melanoma cells of different genetic subtypes.

TAS-116 inhibits oncogenic KIT signalling on the Golgi in both imatinib-naïve and imatinib-resistant gastrointestinal stromal tumours

BACKGROUND

Despite the effectiveness of imatinib mesylate (IM), most gastrointestinal stromal tumours (GISTs) develop IM resistance, mainly due to the additional kinase-domain mutations accompanied by concomitant reactivation of KIT tyrosine kinase. Heat-shock protein 90 (HSP90) is one of the chaperone molecules required for appropriate folding of proteins such as KIT.

METHODS

We used a novel HSP90 inhibitor, TAS-116, which showed specific binding to HSP90α/β with low toxicity in animal models. The efficacy and mechanism of TAS-116 against IM-resistant GIST were evaluated by using IM-naïve and IM-resistant GIST cell lines. We also evaluated the effects of TAS-116 on the other HSP90 client protein, EGFR, by using lung cell lines.

RESULTS

TAS-116 inhibited growth and induced apoptosis in both IM-naïve and IM-resistant GIST cell lines with KIT activation. We found KIT was activated mainly in intracellular compartments, such as trans-Golgi cisternae, and TAS-116 reduced autophosphorylated KIT in the Golgi apparatus. In IM-resistant GISTs in xenograft mouse models, TAS-116 caused tumour growth inhibition. We found that TAS-116 decreased phosphorylated EGFR levels and inhibited the growth of EGFR-mutated lung cancer cell lines.

CONCLUSION

TAS-116 may be a novel promising drug to overcome tyrosine kinase inhibitor-resistance in both GIST and EGFR-mutated lung cancer.

Assessment of the platelet-derived growth factor receptor alpha antibody olaratumab in a panel of patient-derived soft tissue sarcoma xenografts

Background

Soft tissue sarcoma (STS) comprises a family of rare, heterogeneous tumors of mesenchymal origin. Single-agent doxorubicin remains the first-line standard-of-care treatment for advanced and inoperable STS, but response rates are only around 15%. In 2016, phase Ib/II clinical trial results reported an overall survival benefit of 11.8 months when combining doxorubicin and the platelet-derived growth factor receptor alpha (PDGFRA)-directed antibody olaratumab compared to doxorubicin alone, without providing a scientific rationale for such unprecedented therapeutic effect. We decided to evaluate the efficacy of olaratumab in a panel of STS patient-derived xenografts (PDX).

Methods

NMRI nu/nu mice were bilaterally transplanted with tumor tissue of patient-derived xenograft models expressing PDGFRA, including models of leiomyosarcoma (UZLX-STS22), malignant peripheral nerve sheath tumor (UZLX-STS39), myxofibrosarcoma (UZLX-STS59) and undifferentiated pleomorphic sarcoma (UZLX-STS84). Mice were randomly divided into four different treatment groups: (1) control, (2) doxorubicin (3 mg/kg once weekly), (3) anti-PDGFRA [olaratumab (60 mg/kg twice weekly) + mouse anti-PDGFRA antibody 1E10 (20 mg/kg twice weekly)] and (4) the combination of doxorubicin and anti-PDGFRA (same dose/schedule as in the single treatment arms). Tumor volume, histopathology and Western blotting were used to assess treatment efficacy.

Results

Anti-PDGFRA treatment as a single agent did not reduce tumor growth and did not result in significant anti-proliferative or pro-apoptotic activity. Combining doxorubicin and anti-PDGFRA did not reduce tumor burden, though a mild inhibition of proliferation was observed in UZLX-STS39 and -STS59. A pro-apoptotic effect was observed in all models except UZLX-STS22. Antitumor effects on histology were not significantly different comparing doxorubicin and the combination treatment. Moreover, anti-PDGFRA treatment, both as a single agent as well as combined with doxorubicin, did not result in inhibition of the downstream MAPK and PI3K/AKT signaling pathways.

Conclusions

We were not able to demonstrate significant antitumor effects of anti-PDGFRA treatment in selected STS PDX models, neither alone nor in combination with doxorubicin. This is in line with the very recent results of the phase III clinical trial NCT02451943 ANNOUNCE, which did not confirm the clinical benefit of olaratumab in combination with doxorubicin over single agent doxorubicin.

Electronic supplementary material

The online version of this article (10.1186/s12885-019-5872-1) contains supplementary material, which is available to authorized users.

Heat Shock Protein 90 as a Prognostic Marker and Therapeutic Target for Adrenocortical Carcinoma

Background: Adrenocortical carcinoma (ACC) is a rare tumor entity with restricted therapeutic opportunities. HSP90 (Heat Shock Protein 90) chaperone activity is fundamental for cell survival and contributes to different oncogenic signaling pathways. Indeed, agents targeting HSP90 function have shown therapeutic efficacy in several cancer types. We have examined the expression of HSP90 in different adrenal tumors and evaluated the use of HSP90 inhibitors in vitro as possible therapy for ACC. Methods: Immunohistochemical expression of HSP90 isoforms was investigated in different adrenocortical tumors and associated with clinical features. Additionally, a panel of N-terminal (17-allylamino-17-demethoxygeldanamycin (17-AAG), luminespib, and ganetespib) and C-terminal (novobiocin and silibinin) HSP90 inhibitors were tested on various ACC cell lines. Results: Within adrenocortical tumors, ACC samples exhibited the highest expression of HSP90β. Within a cohort of ACC patients, HSP90β expression levels were inversely correlated with recurrence-free and overall survival. In functional assays, among five different compounds tested luminespib and ganetespib induced a significant decrease in cell viability in single as well as in combined treatments with compounds of the clinically used EDP-M scheme (etoposide, doxorubicin, cisplatin, mitotane). Inhibition of cell viability correlated furthermore with a decrease in proliferation, in cell migration and an increase in apoptosis. Moreover, analysis of cancer pathways indicated a modulation of the ERK1/2-and AKT-pathways by luminespib and ganetespib treatment. Conclusions: Our findings emphasize HSP90 as a marker with prognostic impact and promising target with N-terminal HSP90 inhibitors as drugs with potential therapeutic efficacy toward ACC.

Novel Insights into the Treatment of Imatinib-Resistant Gastrointestinal Stromal Tumors

Gastrointestinal stromal tumors (GIST) have emerged as a compelling clinical and biological model for the rational development of therapeutic strategies targeting critical oncogenic events over the past two decades. Oncogenic activation of KIT or PDGFRA receptor tyrosine kinases is the crucial driver for GIST tumor initiation, transformation, and cancer cell proliferation. Three tyrosine kinase inhibitors (TKIs) with KIT inhibitory activity - imatinib, sunitinib, and regorafenib - are approved to treat advanced GIST and have successfully exploited this addiction to KIT oncogenic signaling, demonstrating remarkable activity in a disease that historically had no successful systemic therapy options. However, GIST refractory to approved TKIs remain an unmet clinical need, as virtually all patients with metastatic GIST eventually progress on any given therapy. The main and best-established mechanism of resistance is the polyclonal expansion of multiple subpopulations harboring different secondary KIT mutations. The present review aims at summarizing current and forthcoming treatment directions in advanced imatinib-resistant GIST supported by a strong biological rationale.

New targets and therapies for gastrointestinal stromal tumors

INTRODUCTION

The majority of gastrointestinal stromal tumors (GIST) are driven by an abnormal receptor tyrosine kinase (RTK) signaling, occurring mainly due to somatic mutations in KIT or platelet derived growth factor receptor alpha (PDGFRA). Although the introduction of tyrosine kinase inhibitors (TKIs) has revolutionized therapy for GIST patients, with time the vast majority of them develop TKI resistance. Advances in understanding the molecular background of GIST resistance allows for the identification of new targets and the development of novel strategies to overcome or delay its occurrence. Areas covered: The focus of this review is on novel, promising therapeutic approaches to overcome heterogeneous resistance to registered TKIs. These approaches involve new TKIs, including drugs specific for a mutated form of KIT/PDGFRA, drugs with inhibitory effect against multiple RTKs, compounds targeting dysregulated downstream signaling pathways, drugs affecting KIT expression and degradation, inhibitors of cell cycle, and immunotherapeutics. Expert commentary: As the resistance to standard TKI treatment can be heterogeneous, a combinational approach for refractory GIST could be beneficial. Moreover, the understanding of the molecular background of resistant disease would allow development of a more personalized approach for these patients and their response to targeted therapy could be monitored closely using 'liquid biopsy'.

Hsp90 inhibitor geldanamycin attenuates the cytotoxicity of sunitinib in cardiomyocytes via inhibition of the autophagy pathway

Sunitinib malate (sunitinib) is an orally available, multitargeted tyrosine kinase inhibitor with antitumor and antiangiogenic activities. Although sunitinib is effective for the treatment of patients with gastrointestinal stromal tumor, advanced renal cell carcinoma, or pancreatic neuroendocrine tumor, adverse cardiac events associated with sunitinib administration have been reported. Here, we examined the effect of geldanamycin, an inhibitor of heat shock protein (Hsp) 90, on sunitinib-induced cytotoxicity in cardiomyocytes. First, we found that treatment with geldanamycin or other Hsp90 inhibitors (tanespimycin, ganetespib, or BIIB021) significantly attenuated sunitinib-induced cytotoxicity in rat H9c2 cardiomyocytes, suggesting a drug-class effect of Hsp90 inhibitors. We then examined the mechanisms underlying sunitinib-induced cytotoxicity and found that sunitinib induced autophagy in H9c2 cells and that pretreatment with geldanamycin inhibited the induction of autophagy by promoting degradation of the autophagy-related proteins Atg7, Beclin-1, and ULK1. Pharmacological assessment with autophagy inhibitors confirmed that geldanamycin attenuated the cytotoxicity of sunitinib by interfering with autophagy. In addition, we found that the molecular chaperone Hsp70, which is induced by geldanamycin, was not involved in the attenuation of sunitinib-induced cytotoxicity. Finally, to provide more clinically relevant data, we confirmed that geldanamycin attenuated sunitinib-induced cytotoxicity in human induced pluripotent stem cell-derived cardiomyocytes. Together, these data suggest that geldanamycin attenuates sunitinib-induced cytotoxicity in cardiomyocytes by inhibiting the autophagy pathway. Thus, the further investigation of combination or sequential treatment with an Hsp90 inhibitor and sunitinib is warranted as a potential strategy of attenuating the cardiotoxicity associated with sunitinib administration in the clinical setting.

Heat Shock Proteins and Cancer

Heat shock proteins (HSPs) constitute a large family of proteins involved in protein folding and maturation whose expression is induced by heat shock or other stressors. The major groups are classified based on their molecular weights and include HSP27, HSP40, HSP60, HSP70, HSP90, and large HSPs. HSPs play a significant role in cellular proliferation, differentiation, and carcinogenesis. In this article we comprehensively review the roles of major HSPs in cancer biology and pharmacology. HSPs are thought to play significant roles in the molecular mechanisms leading to cancer development and metastasis. HSPs may also have potential clinical uses as biomarkers for cancer diagnosis, for assessing disease progression, or as therapeutic targets for cancer therapy.

A Hyperactive Signalosome in Acute Myeloid Leukemia Drives Addiction to a Tumor-Specific Hsp90 Species

Acute myeloid leukemia (AML) is a heterogeneous and fatal disease with an urgent need for improved therapeutic regimens given that most patients die from relapsed disease. Irrespective of mutation status, the development of aggressive leukemias is enabled by increasing dependence on signaling networks. We demonstrate that a hyperactive signalosome drives addiction of AML cells to a tumor-specific Hsp90 species (teHsp90). Through genetic, environmental, and pharmacologic perturbations, we demonstrate a direct and quantitative link between hyperactivated signaling pathways and apoptotic sensitivity of AML to teHsp90 inhibition. Specifically, we find that hyperactive JAK-STAT and PI3K-AKT signaling networks are maintained by teHsp90 and, in fact, gradual activation of these networks drives tumors increasingly dependent on teHsp90. Thus, although clinically aggressive AML survives via signalosome activation, this addiction creates a vulnerability that can be exploited with Hsp90-directed therapy.

GIST treatment options after tyrosine kinase inhibitors

OPINION STATEMENT

The management of advanced gastrointestinal stromal tumor (GIST) has been dramatically altered by the development of tyrosine kinase inhibitors. The disease, which had a median overall survival of 12 months for patients with unresectable disease, now has a median survival approaching 5 or more years. The challenge faced clinically is how to care for patients when they have progressed on all approved therapies. Clinical trials evaluating the role of novel combination therapies with investigational agents that target AKT/PI3K pathways are of interest especially given the preclinical rationale available. The addition of an mTOR inhibitor can be tried as these are available, but requires care and monitoring for additional toxicities. With improved understanding of this disease, which we thought of as one biology, personalized therapies are being studied and tested and is particularly relevant for GIST that are less responsive to the standard kinase inhibitors, such as platelet-derived growth factor alpha (PDGFRA) D842V and wild-type/succinate dehydrogenase (SDH)-deficient GIST. IGF1R inhibitors as a class are not being developed because of the lack of significant efficacy in many clinical trials and the efficacy in WT GIST has been limited; to date drugs targeting VEGFR, such as sunitinib and regorafenib, appear to be the best agents available for this group of patients. The exciting findings seen with CTLA4 and PD-1/PD-L1 antibodies in melanoma and other solid tumors is exciting, especially because there is a growing body of evidence that such approaches have biologic rationale; clinical trials evaluating these agents are awaited with interest. Last, recent work has shed light on older agents that may have a role in GIST. Moving forward to test these agents alone or in combination with TKIs offers potentially new strategies for treating advanced disease.

Therapeutic Efficacy Assessment of CK6, a Monoclonal KIT Antibody, in a Panel of Gastrointestinal Stromal Tumor Xenograft Models

We evaluated the efficacy of CK6, a KIT monoclonal antibody, in a panel of human gastrointestinal stromal tumor (GIST) xenograft models. Nude mice were bilaterally transplanted with human GIST xenografts (four patient derived and two cell line derived), treated for 3 weeks, and grouped as follows: control (untreated); CK6 (40 mg/kg, 3 × weekly); imatinib (50 mg/kg, twice daily); sunitinib (40 mg/kg, once daily); imatinib + CK6; sunitinib + CK6 (same doses and schedules as in the single-agent treatments). Tumor volume assessment, Western blot analysis, and histopathology were used for evaluation of efficacy. Statistical analysis was performed using Mann-Whitney U (MWU) and Wilcoxon matched-pairs tests. CK6 as a single agent only reduced tumor growth rate in the UZLX-GIST3 model (P = .053, MWU compared to control), while in none of the other GIST models an effect on tumor growth rate was observed. CK6 did not result in significant anti-proliferative or pro-apoptotic effects in any of the GIST models, and moreover, CK6 did not induce a remarkable inhibition of KIT activation. Furthermore, no synergistic effect of combining CK6 with tyrosine kinase inhibitors (TKIs) was observed. Conversely, in certain GIST xenografts, anti-tumor effects seemed to be inferior under combination treatment compared to single-agent TKI treatment. In the GIST xenografts tested, the anti-tumor efficacy of CK6 was limited. No synergy was observed on combination of CK6 with TKIs in these GIST models. Our findings highlight the importance of using relevant in vivo human tumor xenograft models in the preclinical assessment of drug combination strategies.

Hsp90 inhibitor as a sensitizer of cancer cells to different therapies (review)

Hsp90 is a molecular chaperone that maintains the structural and functional integrity of various client proteins involved in signaling and many other functions of cancer cells. The natural inhibitors, ansamycins influence the Hsp90 chaperone function by preventing its binding to client proteins and resulting in their proteasomal degradation. N- and C-terminal inhibitors of Hsp90 and their analogues are widely tested as potential anticancer agents in vitro, in vivo as well as in clinical trials. It seems that Hsp90 competitive inhibitors target different tumor types at nanomolar concentrations and might have therapeutic benefit. On the contrary, some Hsp90 inhibitors increased toxicity and resistance of cancer cells induced by heat shock response, and through the interaction of survival signals, that occured as side effects of treatments, could be very effectively limited via combination of therapies. The aim of our review was to collect the data from experimental and clinical trials where Hsp90 inhibitor was combined with other therapies in order to prevent resistance as well as to potentiate the cytotoxic and/or antiproliferative effects.

MTOR inhibition enhances NVP-AUY922-induced autophagy-mediated KIT degradation and cytotoxicity in imatinib-resistant gastrointestinal stromal tumors

Our previous study demonstrated NVP-AUY922, a HSP90AA1 inhibitor, could enhance mutant KIT degradation in gastrointestinal stromal tumor (GIST) cells through both proteasome- and autophagy-mediated pathways. Herein, we showed rapamycin, a MTOR inhibitor and autophagy inducer, could reduce total and phospho-KIT expression levels and enhance apoptosis in imatinib-resistant GIST cells. The involvement of autophagy in rapamycin-induced KIT downregulation was further confirmed by co-localization of KIT and autophagosome, and partial recovery of KIT expression level by either siRNA-mediated BECN1 and ATG5 silencing or autophagy inhibitors after rapamycin. Rapamycin and NVP-AUY922 synergistically inhibited GIST cells growth in vitro. The combination of low-dose NVP-AUY922 with rapamycin had comparable effects on reducing KIT expression, increasing MAP1LC3B puncta and tumor necrosis, and inhibiting tumor growth as high-dose NVP-AUY922 did in GIST430 xenograft model. Our results suggest the addition of a MTOR inhibitor may reduce NVP-AUY922 dose requirement and potentially improve its therapeutic index in mutant KIT-expressing GISTs.

Phosphoinositide 3-kinase inhibitors combined with imatinib in patient-derived xenograft models of gastrointestinal stromal tumors: rationale and efficacy

INTRODUCTION

The PI3K signaling pathway drives tumor cell proliferation and survival in gastrointestinal stromal tumor (GIST). We tested the in vivo efficacy of three PI3K inhibitors (PI3Ki) in patient-derived GIST xenograft models.

EXPERIMENTAL DESIGN

One hundred and sixty-eight nude mice were grafted with human GIST carrying diverse KIT genotypes and PTEN genomic status. Animals were dosed orally for two weeks as follows: control group (untreated); imatinib (IMA); PI3Ki (BKM120-buparlisib, BEZ235, or BYL719) or combinations of imatinib with a PI3Ki. Western blotting, histopathology, and tumor volume evolution were used for the assessment of treatment efficacy. Furthermore, tumor regrowth was evaluated for three weeks after treatment cessation.

RESULTS

PI3Ki monotherapy showed a significant antitumor effect, reflected in tumor volume reduction or stabilization, inhibitory effects on mitotic activity, and PI3K signaling inhibition. The IMA+PI3Ki combination remarkably improved the efficacy of either single-agent treatment with more pronounced tumor volume reduction and enhanced proapoptotic effects over either single agent. Response to IMA+PI3Ki was found to depend on the KIT genotype and specific model-related molecular characteristics.

CONCLUSION

IMA+PI3Ki has significant antitumor efficacy in GIST xenografts as compared with single-agent treatment, resulting in more prominent tumor volume reduction and enhanced induction of apoptosis. Categorization of GIST based on KIT genotype and PI3K/PTEN genomic status combined with dose optimization is suggested for patient selection for clinical trials exploring such combinations.

Drug repurposing identifies a synergistic combination therapy with imatinib mesylate for gastrointestinal stromal tumor

Gastrointestinal stromal tumor (GIST) is a rare and therefore often neglected disease. Introduction of the kinase inhibitor imatinib mesylate radically improved the clinical response of patients with GIST; however, its effects are often short-lived, with GISTs demonstrating a median time-to-progression of approximately two years. Although many investigational drugs, approved first for other cancers, have been subsequently evaluated for the management of GIST, few have greatly affected the overall survival of patients with advanced disease. We employed a novel, focused, drug-repurposing effort for GIST, including imatinib mesylate-resistant GIST, evaluating a large library of FDA-approved drugs regardless of current indication. As a result of the drug-repurposing screen, we identified eight FDA-approved drugs, including fludarabine phosphate (F-AMP), that showed synergy with and/or overcame resistance to imatinib mesylate. F-AMP induces DNA damage, Annexin V, and caspase-3/7 activities as the cytotoxic effects on GIST cells, including imatinib mesylate-resistant GIST cells. F-AMP and imatinib mesylate combination treatment showed greater inhibition of GIST cell proliferation when compared with imatinib mesylate and F-AMP alone. Successful in vivo experiments confirmed the combination of imatinib mesylate with F-AMP enhanced the antitumor effects compared with imatinib mesylate alone. Our results identified F-AMP as a promising, repurposed drug therapy for the treatment of GISTs, with potential to be administered in combination with imatinib mesylate or for treatment of imatinib mesylate-refractory tumors.

Characterization and assessment of the sensitivity and resistance of a newly established human gastrointestinal stromal tumour xenograft model to treatment with tyrosine kinase inhibitors

BACKGROUND

Acquired resistance to tyrosine kinase inhibitors (TKIs) in gastrointestinal stromal tumours (GISTs) is most commonly caused by secondary KIT or PDGFRA mutations. In this study we characterize a newly established GIST xenograft model, UZLX-GIST9, and evaluate the in vivo response of the model to standard TKIs (imatinib, sunitinib, and regorafenib).

METHODS

Tumour fragments from a metastatic lesion of a GIST patient clinically progressing after treatment with imatinib, sunitinib and regorafenib were engrafted in a nude, immunodeficient mouse. Upon sequential passaging from mouse to mouse, tumour fragments were collected for histopathological and molecular characterization. The sensitivity of the model to treatment with TKIs was evaluated in 28 mice [passage 2 (n = 8), passage 4 (n = 20), 41 tumours]. Mice were grouped as follows: control (untreated), imatinib (50 mg/kg/BID), imatinib (100 mg/kg/BID), sunitinib (40 mg/kg/QD), and regorafenib (30 mg/kg/QD). After three weeks of oral treatment, tumours were collected for subsequent analysis. The efficacy of treatment was assessed by tumour volume, histopathology and Western immunoblotting.

RESULTS

UZLX-GIST9 maintains the same typical morphological features and immunohistochemical characteristics as the original patient biopsy and expresses CD117 and DOG1. The KIT mutational profile (p.P577del + W557LfsX5+ D820G) remains the same as the original tissue sample originating from an intraspinal metastatic site. Three week treatment with different TKIs showed that the model is resistant to imatinib. Sunitinib induces tumour growth delay and regorafenib reduces the tumour burden by 30% as compared to control animals. While none of the TKIs had a significant effect on cell proliferation or cell survival, a remarkable increase of necrosis and significant reduction of microvessel density was observed under sunitinib and regorafenib. Western immunoblotting showed a mild reduction in KIT and AKT activation only in regorafenib treated tumours.

CONCLUSIONS

We established a novel human GIST xenograft, UZLX-GIST9, harbouring KIT exon 11 and 17 mutations and maintaining the pheno-and genotype of the original tumour. UZLX-GIST9 shows different levels of response to standard TKIs. This model will help to study TKI resistance and to explore novel treatment approaches for patients with TKI-resistant GIST.

Discovery and development of heat shock protein 90 inhibitors as anticancer agents: a review of patented potent geldanamycin derivatives

INTRODUCTION

There has been research on anticancer strategies which focus on disrupting a single malignant protein. One of the strategies is the inhibition of one protein, heat shock protein 90 (Hsp90). There are many reasons why Hsp90 protein is targeted by anticancer agents: maintenance of cellular homeostasis in organisms involves Hsp90 and its client proteins; moreover, Hsp90 complex is involved in regulating several signal transduction pathways and plays an important role in the maturation of lots of tumor-promoting client proteins. Geldanamycin (GM), the first benzoquinone ansamycin, has shown anticancer activity by binding to Hsp90. Currently, several GM derivatives such as 17-AAG, 17-(2-dimethylaminoethyl)amino-17-demethoxygeldanamycin, IPI-493, and IPI-504 are being progressively developed toward clinical application.

AREAS COVERED

Several research groups have studied GM and its derivatives to develop novel and potent Hsp90 inhibitors for the treatment of cancer. The crystal structure of Hsp90 was utilized to undergo structural optimization of GM derivatives. A wide variety of structural modifications were performed and some of the derivatives are now in clinical studies. The aim of this review was to summarize and analyze the structure-activity relationships of GM derivatives and the focus is on patented novel and pharmaceutically efficacious derivatives published from 1971 to 2012.

EXPERT OPINION

Hsp90 inhibitors offer an effective therapeutic approach for treatment of cancer. To date, the clinical results of 17-AAG, IPI-493, and IPI-504 suggest that these GM derivatives could be used either alone or in combination with other marketed medications for the treatment of cancer patients. As there are not any marketed Hsp90 inhibitors, inhibiting Hsp90 chaperone function remains as a promising strategy that still requires further research.

Autophagy is involved in endogenous and NVP-AUY922-induced KIT degradation in gastrointestinal stromal tumors

Gastrointestinal stromal tumor (GIST) is a prototype of mutant KIT oncogene-driven tumor. Prolonged tyrosine kinase inhibitor (TKI) treatment may result in a resistant phenotype through acquired secondary KIT mutation. Heat shock protein 90 (HSP90AA1) is a chaperone protein responsible for protein maturation and stability, and KIT is a known client protein of HSP90AA1. Inhibition of HSP90AA1 has been shown to destabilize KIT protein by enhancing its degradation via the proteasome-dependent pathway. In this study, we demonstrated that NVP-AUY922 (AUY922), a new class of HSP90AA1 inhibitor, is effective in inhibiting the growth of GIST cells expressing mutant KIT protein, the imatinib-sensitive GIST882 and imatinib-resistant GIST48 cells. The growth inhibition was accompanied with a sustained reduction of both total and phosphorylated KIT proteins and the induction of apoptosis in both cell lines. Surprisingly, AUY922-induced KIT reduction could be partially reversed by pharmacological inhibition of either autophagy or proteasome degradation pathway. The blockade of autophagy alone led to the accumulation of the KIT protein, highlighting the role of autophagy in endogenous KIT turnover. The involvement of autophagy in endogenous and AUY922-induced KIT protein turnover was further confirmed by the colocalization of KIT with MAP1LC3B-, acridine orange- or SQSTM1-labeled autophagosome, and by the accumulation of KIT in GIST cells by silencing either BECN1 or ATG5 to disrupt autophagosome activity. Therefore, the results not only highlight the potential application of AUY922 for the treatment of KIT-expressing GISTs, but also provide the first evidence for the involvement of autophagy in endogenous and HSP90AA1 inhibitor-induced KIT degradation.

A potent combination of the novel PI3K Inhibitor, GDC-0941, with imatinib in gastrointestinal stromal tumor xenografts: long-lasting responses after treatment withdrawal

PURPOSE

Oncogenic signaling in gastrointestinal stromal tumors (GIST) is sustained via PI3K/AKT pathway. We used a panel of six GIST xenograft models to assess efficacy of GDC-0941 as single agent or in combination with imatinib (IMA).

EXPERIMENTAL DESIGN

Nude mice (n = 136) were grafted bilaterally with human GIST carrying diverse KIT mutations. Mice were orally dosed over four weeks, grouped as follows: (A) control; (B) GDC-0941; (C) imatinib, and (D) GDC+IMA treatments. Xenografts regrowth after treatment discontinuation was assessed in groups C and D for an additional four weeks. Tumor response was assessed by volume measurements, micro-PET imaging, histopathology, and immunoblotting. Moreover, genomic alterations in PTEN/PI3K/AKT pathway were evaluated.

RESULTS

In all models, GDC-0941 caused tumor growth stabilization, inhibiting tumor cell proliferation, but did not induce apoptosis. Under GDC+IMA, profound tumor regression, superior to either treatment alone, was observed. This effect was associated with the best histologic response, a nearly complete proliferation arrest and increased apoptosis. Tumor regrowth assays confirmed superior activity of GDC+IMA over imatinib; in three of six models, tumor volume remained reduced and stable even after treatment discontinuation. A positive correlation between response to GDC+IMA and PTEN loss, both on gene and protein levels, was found.

CONCLUSION

GDC+IMA has significant antitumor efficacy in GIST xenografts, inducing more substantial tumor regression, apoptosis, and durable effects than imatinib. Notably, after treatment withdrawal, tumor regression was sustained in tumors exposed to GDC+IMA, which was not observed under imatinib. Assessment of PTEN status may represent a useful predictive biomarker for patient selection.

Heat shock protein 90 inhibition: rationale and clinical potential

Heat shock protein 90 (HSP90) is a molecular chaperone protein essential for cellular survival. Functionally, HSPs promote proper protein folding, prevent misfolding, and restore three-dimensional protein structure which is critical following toxic cellular stresses. Recently, targeting HSP90 pharmacologically has gained traction in cancer therapy. Oncogenic cells depend on their ability to withstand endogenous (anoxia, nutrient deprivation, pH changes, and deranged signaling pathways) and exogenous (chemotherapy and radiation therapy) stressors for survival. Pharmacological inhibition of HSP90 destabilizes proteins and leads to degradation through the proteasome. This article will review the utility of HSP90 inhibition, as well as the current adoption in clinical trials and practice.

The HSP90 inhibitor, AT13387, is effective against imatinib-sensitive and -resistant gastrointestinal stromal tumor models

The majority of gastrointestinal stromal tumors (GIST) are characterized by activating mutations of KIT, an HSP90 client protein. Further secondary resistance mutations within KIT limit clinical responses to tyrosine kinase inhibitors, such as imatinib. The dependence of KIT and its mutated forms on HSP90 suggests that HSP90 inhibition might be a valuable treatment option for GIST, which would be equally effective on imatinib-sensitive and -resistant clones. We investigated the activity of AT13387, a potent HSP90 inhibitor currently being evaluated in clinical trials, in both in vitro and in vivo GIST models. AT13387 inhibited the proliferation of imatinib-sensitive (GIST882, GIST-T1) and -resistant (GIST430, GIST48) cell lines, including those resistant to the geldanamycin analogue HSP90 inhibitor, 17-AAG. Treatment with AT13387 resulted in depletion of HSP90 client proteins, KIT and AKT, along with their phospho-forms in imatinib-sensitive and -resistant cell lines, irrespective of KIT mutation. KIT signaling was ablated, whereas HSP70, a marker of HSP90 inhibition, was induced. In vivo, antitumor activity of AT13387 was showed in both the imatinib-sensitive, GIST-PSW, xenograft model and a newly characterized imatinib-resistant, GIST430, xenograft model. Induction of HSP70, depletion of phospho-KIT and inhibition of KIT signaling were seen in tumors from both models after treatment with AT13387. A combination of imatinib and AT13387 treatment in the imatinib-resistant GIST430 model significantly enhanced tumor growth inhibition over either of the monotherapies. Importantly, the combination of AT13387 and imatinib was well tolerated. These results suggest AT13387 is an excellent candidate for clinical testing in GIST in combination with imatinib.

Mechanisms of Resistance to Hsp90 Inhibitor Drugs: A Complex Mosaic Emerges

The molecular chaperone Hsp90 holds great promise as a cancer drug target, despite some of the initial clinical trials of Hsp90 inhibitor drugs having not lived up to expectation. Effective use of these drugs will benefit greatly from a much more detailed understanding of the factors that contribute to resistance, whether intrinsic or acquired. We review how cell culture studies have revealed a number of different mechanisms whereby cells can be rendered less susceptible to the effects of Hsp90 inhibitor treatment. A major influence is Hsp90 inhibition causing strong induction of the heat shock response, a stress response that increases cellular levels of prosurvival chaperones such as Hsp27 and Hsp70. Another problem seems to be that these inhibitors do not always access the Hsp90 proteins of the mitochondrion, forms of Hsp90 that—in cancer cells—are operating to suppress apoptosis. It should be possible to overcome these drawbacks through the appropriate drug redesign or with the combinatorial use of an Hsp90 inhibitor with a drug that targets either heat shock factor or the chaperone Hsp70. Still though, cells will often differ in the key antiapoptotic versus proapoptotic activities that are dependent on Hsp90, in the key steps in their apoptotic pathways responsive to Hsp90 inhibition or Hsp70 level, as well as the extents to which their survival is dependent on oncogenic tyrosine kinases that are clients of Hsp90. A systems approach will therefore often be required in order to establish the most prominent effects of Hsp90 inhibition in each type of cancer cell.