New Hsp90 Inhibitor-based Therapies for Ewing Sarcoma

Ewing sarcoma (EwS) is one of the most common malignant bone and soft tissue tumours in children and adolescents. Despite advances in EwS treatment, the prognosis is dismal and survival rates for patients with metastatic or recurrent disease are unacceptably low. Therefore, there is an urgent need to develop novel EwS therapies. The project New Hsp90 Inhibitor-based Therapies for Ewing Sarcoma (HSP90IES) aims to expand the target space of EwS drugs through the rational design of novel Hsp90 C-terminal domain (CTD) inhibitors (Aim 1) and progress beyond the state-of-the-art, by designing innovative Hsp90-mediated targeting chimeras (HEMTACs) for the degradation of EwS-specific oncogenic proteins (Aim 2), and small molecule drug conjugates (SMDCs) for Hsp90 inhibitor (Hsp90i)-mediated targeted delivery of inhibitors of anti-apoptotic proteins to tumours to improve their selective toxicity (Aim 3). Most cancers are characterised by dysregulated protein homeostasis and show increased dependence on heat shock proteins. Indeed, the Hsp90 chaperone is overexpressed in many cancers, including EwS, and represents an important target for anticancer drug development. Because Hsp90 mediates the folding of some 400 proteins, its inhibition can disrupt multiple signalling pathways simultaneously, leading to potent anticancer effects resembling combination therapy. Importantly, Hsp90i have specificity towards tumour cells, making them a promising new class of anticancer agents. Unfortunately, all Hsp90i that have been clinically studied bind to the N-terminal domain (NTD) and, at the same concentration at which they inhibit Hsp90, also induce the heat shock response. This generally leads to cytostatic activity and resistance. (i) Unlike Hsp90 NTD inhibitors, Hsp90 CTD inhibitors, which include our novel molecules, can separate these activities, providing opportunities for the development of therapeutically useful anticancer agents. We have previously shown high efficacy of dual MCL-1/BCL-XL inhibition against EwS cells, which however was hampered by dose-limiting toxicity in mouse EwS xenografts. To overcome this toxicity, we make use of the accumulation of Hsp90i in cancer cells. (ii) The SMDCs we will develop consist of the Hsp90i, which primarily serves as a delivery vehicle to cancer cells, a cleavable linker, and BCL-XL or an MCL-1 inhibitor as an anticancer payload. With SMDCs, we aim to avoid the induction of apoptosis in healthy tissue of pediatric patients. In a complementing approach we will leverage the advantages of degraders. (iii) HEMTACs induce the formation of the ternary complex between the protein of interest (POI), HEMTAC and the Hsp90/E3 ligase, which initiates the degradation of POI by the ubiquitin-proteasome system. With our innovative HEMTACs consisting of Hsp90i, a non-cleavable linker and an EWS::FLI1 inhibitor, we aim to degrade the transcription factor EWS::FLI1, which is the major oncogene in >85% of EwS cases. The HSP90IES project covers the full cycle of early-stage lead discovery, including computational design, medicinal chemistry, in vitro and in vivo profiling of cancer activity, and is organised into three interrelated scientific and two communication and management work packages (WP). The overall goal of the project will be achieved through several specific tasks in each WP, carried out by the group of Dr Tomašič at the University of Ljubljana, and four partners: the groups of Dr Distel and Prof. Kovar and at the Children's Cancer Research Institute, Austria, the group of Dr Zubriené at Vilnius University, Lithuania, and the group of Dr Grdadolnik at the National Institute of Chemistry, Slovenia. The expertise of the project partners is complementary and synergistic, and they have the experience and the necessary equipment for the successful implementation of the project overarching goal: Development of innovative Hsp90i-based molecular tools and Hsp90i as preclinical leads for the treatment of EwS.