Abstract 1879: Pyrazolopyrimidines as novel selective allosteric MALT1 inhibitors with in vivo activity in ABC-DLBCL

Diffuse large B cell lymphoma (DLBCL) is the most common histological subtype of Non-Hodgkin's lymphoma, comprising 30% to 40% of all newly diagnosed cases. DLBCL is a biologically and clinically diverse disease with more than a dozen subtypes classified by the World Health Organization (WHO). Gene expression profiling groups DLBCL into three molecular subtypes, named according to their cell of origin, and which include germinal center B-cell-like (GCB) DLBCL, activated B-cell-like (ABC) DLBCL and primary mediastinal B cell lymphoma (PMBL). The current standard of care (SoC) is R-CHOP chemo-immunotherapy. Although the SoC is curative in a substantial proportion of patients, 40% of patients, especially in the ABC-DLBCL subtype, do not achieve durable remissions and suffer from progressive disease. In ABC-DLBCL, recurrent mutations in the B-cell receptor (BCR) and Toll-like receptor (TLR) pathways lead to constitutive NF-κB signaling. Mutations in the BCR pathway include gain-of-function mutations of CD79A/B (~20%) and CARD11 (~10%), and loss-of-function mutations of A20 (~25%). In the TLR pathway, MYD88 is commonly mutated (~37%). In recent years, knowledge of these aberrantly regulated pathways and their underlying mutations guided the development and investigation of newer molecular targeted agents, e.g. BTK, SYK, PI3K, PKC and MALT1 inhibitors. Among these strategies, inhibition of MALT1 provides the advantage that it is downstream of most of the reported BCR pathway mutations, including CARD11 mutations, and most of the targeted kinases with respect to potentially emerging resistance. However, no MALT1 inhibitor is in the clinic until now.

Here, we report the identification of pyrazolopyrimidines as a new class of allosteric MALT1 inhibitors and their optimization for in vivo. The lead compound shows nanomolar potency in biochemical and cellular MALT1 protease reporter assays. In addition, pyrazolopyrimidines are highly selective for MALT1 and demonstrate differential cell killing of CARD11 mutant ABC-DLBCL cells vs control cells in proliferation assays. Furthermore, we demonstrate in vivo activity of pyrazolopyrimidines in CD79 and CARD11 mutant ABC-DLBCL xenograft models, with the lead compound causing regression in a CARD11 mutant xenograft model.

Citation Format: Andreas Weiss, Thomas Radimerski, Daniel Wyss, Rita Andraos, Alexandra Buhles, Dario Sterker, Jean Quancard, Carole Pissot, Oliver Simic, Marc Bigaud, Frederic Bornancin, Achim Schlapbach, Catherine Regnier, Paul McSheehy, Elisabeth Buchdunger, Markus Wartmann, Martin Renatus, Ralf Endres, William Sellers, Francesco Hofmann. Pyrazolopyrimidines as novel selective allosteric MALT1 inhibitors with in vivo activity in ABC-DLBCL [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1879.

CHZ868, a Type II JAK2 Inhibitor, Reverses Type I JAK Inhibitor Persistence and Demonstrates Efficacy in Myeloproliferative Neoplasms

Although clinically tested JAK inhibitors reduce splenomegaly and systemic symptoms, molecular responses are not observed in most myeloproliferative neoplasm (MPN) patients. We previously demonstrated that MPN cells become persistent to type I JAK inhibitors that bind the active conformation of JAK2. We investigated whether CHZ868, a type II JAK inhibitor, would demonstrate activity in JAK inhibitor persistent cells, murine MPN models, and MPN patient samples. JAK2 and MPL mutant cell lines were sensitive to CHZ868, including type I JAK inhibitor persistent cells. CHZ868 showed significant activity in murine MPN models and induced reductions in mutant allele burden not observed with type I JAK inhibitors. These data demonstrate that type II JAK inhibition is a viable therapeutic approach for MPN patients.

Activity of the Type II JAK2 Inhibitor CHZ868 in B Cell Acute Lymphoblastic Leukemia

A variety of cancers depend on JAK2 signaling, including the high-risk subset of B cell acute lymphoblastic leukemias (B-ALLs) with CRLF2 rearrangements. Type I JAK2 inhibitors induce paradoxical JAK2 hyperphosphorylation in these leukemias and have limited activity. To improve the efficacy of JAK2 inhibition in B-ALL, we developed the type II inhibitor CHZ868, which stabilizes JAK2 in an inactive conformation. CHZ868 potently suppressed the growth of CRLF2-rearranged human B-ALL cells, abrogated JAK2 signaling, and improved survival in mice with human or murine B-ALL. CHZ868 and dexamethasone synergistically induced apoptosis in JAK2-dependent B-ALLs and further improved in vivo survival compared to CHZ868 alone. These data support the testing of type II JAK2 inhibition in patients with JAK2-dependent leukemias and other disorders.

Abstract LB-36: JAK2/STAT5 inhibition circumvents resistance to PI3K/mTOR blockade: A rationale for co-targeting these pathways in metastatic breast cancer

Hyperactive PI3K/mTOR signaling is prevalent in the majority of human malignancies (1) and its inhibition exhibits potent antitumor activity in a wide spectrum of solid cancers. Unfortunately, single-agent targeted cancer therapy is usually short-lived and thwarted by different resistance mechanisms (2). Here, we report the discovery of a JAK2/STAT5-evoked positive feedback loop that causes adaptive resistance to dual PI3K/mTOR inhibition. Mechanistically, PI3K/mTOR inhibition increased IRS1-dependent activation of JAK2/STAT5 and secretion of IL-8 in several cell lines and primary triple-negative breast tumors. Genetic or pharmacological inhibition of JAK2 abrogated this vicious feedback loop. Combined PI3K/mTOR and JAK2 inhibition synergistically reduced cancer cell viability in vitro as well as tumor growth in vivo, and decreased tumor seeding and metastasis due to its impact on the IL-8 receptor CXCR1+ tumor-initiating subpopulation of cells. We further found that combined PI3K/mTOR and JAK2 inhibition increased event-free as a well as overall survival of tumor bearing animals. This study reveals a new link between growth factor signaling, JAK/STAT activation, cytokine secretion and metastasis. Our results provide a rationale for combined targeting of the PI3K/mTOR and JAK2/STAT5 pathways in triple-negative breast cancer, a particularly aggressive and currently incurable disease.

Citation Format: Adrian Britschgi, Rita Andraos, Heike Brinkhaus, Inna Klebba, Masato Murakami, Vincent Romanet, Urs Müller, Thomas Radimerski, Mohamed Bentires-Alj. JAK2/STAT5 inhibition circumvents resistance to PI3K/mTOR blockade: A rationale for co-targeting these pathways in metastatic breast cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr LB-36. doi:10.1158/1538-7445.AM2013-LB-36

JAK2/STAT5 inhibition circumvents resistance to PI3K/mTOR blockade: a rationale for cotargeting these pathways in metastatic breast cancer

Hyperactive PI3K/mTOR signaling is prevalent in human malignancies and its inhibition has potent antitumor consequences. Unfortunately, single-agent targeted cancer therapy is usually short-lived. We have discovered a JAK2/STAT5-evoked positive feedback loop that dampens the efficacy of PI3K/mTOR inhibition. Mechanistically, PI3K/mTOR inhibition increased IRS1-dependent activation of JAK2/STAT5 and secretion of IL-8 in several cell lines and primary breast tumors. Genetic or pharmacological inhibition of JAK2 abrogated this feedback loop and combined PI3K/mTOR and JAK2 inhibition synergistically reduced cancer cell number and tumor growth, decreased tumor seeding and metastasis, and also increased overall survival of the animals. Our results provide a rationale for combined targeting of the PI3K/mTOR and JAK2/STAT5 pathways in triple-negative breast cancer, a particularly aggressive and currently incurable disease.

Modulation of activation-loop phosphorylation by JAK inhibitors is binding mode dependent

Janus kinase (JAK) inhibitors are being developed for the treatment of rheumatoid arthritis, psoriasis, myeloproliferative neoplasms, and leukemias. Most of these drugs target the ATP-binding pocket and stabilize the active conformation of the JAK kinases. This type I binding mode can lead to an increase in JAK activation loop phosphorylation, despite blockade of kinase function. Here we report that stabilizing the inactive state via type II inhibition acts in the opposite manner, leading to a loss of activation loop phosphorylation. We used X-ray crystallography to corroborate the binding mode and report for the first time the crystal structure of the JAK2 kinase domain in an inactive conformation. Importantly, JAK inhibitor-induced activation loop phosphorylation requires receptor interaction, as well as intact kinase and pseudokinase domains. Hence, depending on the respective conformation stabilized by a JAK inhibitor, hyperphosphorylation of the activation loop may or may not be elicited.

Bim and Mcl-1 exert key roles in regulating JAK2V617F cell survival

Background

The JAK2^V617F mutation plays a major role in the pathogenesis of myeloproliferative neoplasms and is found in the vast majority of patients suffering from polycythemia vera and in roughly every second patient suffering from essential thrombocythemia or from primary myelofibrosis. The V617F mutation is thought to provide hematopoietic stem cells and myeloid progenitors with a survival and proliferation advantage. It has previously been shown that activated JAK2 promotes cell survival by upregulating the anti-apoptotic STAT5 target gene Bcl-xL. In this study, we have investigated the role of additional apoptotic players, the pro-apoptotic protein Bim as well as the anti-apoptotic protein Mcl-1.

Methods

Pharmacological inhibition of JAK2/STAT5 signaling in JAK2^V617F mutant SET-2 and MB-02 cells was used to study effects on signaling, cell proliferation and apoptosis by Western blot analysis, WST-1 proliferation assays and flow cytometry. Cells were transfected with siRNA oligos to deplete candidate pro- and anti-apoptotic proteins. Co-immunoprecipitation assays were performed to assess the impact of JAK2 inhibition on complexes of pro- and anti-apoptotic proteins.

Results

Treatment of JAK2^V617F mutant cell lines with a JAK2 inhibitor was found to trigger Bim activation. Furthermore, Bim depletion by RNAi suppressed JAK2 inhibitor-induced cell death. Bim activation following JAK2 inhibition led to enhanced sequestration of Mcl-1, besides Bcl-xL. Importantly, Mcl-1 depletion by RNAi was sufficient to compromise JAK2^V617F mutant cell viability and sensitized the cells to JAK2 inhibition.

Conclusions

We conclude that Bim and Mcl-1 have key opposing roles in regulating JAK2^V617F cell survival and propose that inactivation of aberrant JAK2 signaling leads to changes in Bim complexes that trigger cell death. Thus, further preclinical evaluation of combinations of JAK2 inhibitors with Bcl-2 family antagonists that also tackle Mcl-1, besides Bcl-xL, is warranted to assess the therapeutic potential for the treatment of chronic myeloproliferative neoplasms.