Dovas, Athanassios

ABSTRACT Glioblastoma (GBM) is universally lethal despite decades of research to find effective treatments. This highlights the need to identify druggable targets essential for sustaining the malignant phenotype but dispensable for normal tissue. We propose that the enzyme α-tubulin acetyl transferase ( ATAT1 ) meets these criteria. ATAT1 acetylates α-tubulin at lysine 40, which increases microtubule stability and promotes microtubule-based transport. While ATAT1 knockout mice have only a very mild phenotype, ATAT1 suppression in GBM has multiple therapeutic effects by reducing tumor invasion, proliferation, and therapeutic resistance. These translate not only into improved survival with ATAT1 targeting by itself, but also into synergy when ATAT1 deletion is combined with FDA approved therapies. This study strongly supports our conclusion that ATAT1 is a promising therapeutic target in GBM.

Abstract The prognosis for patients afflicted with GBM has remained dismal in spite of decades of investigations, which speaks to the need to identify targets that drive essential functions in GBM, using well-tolerated, brain permeant inhibitors. Here we report our studies of MT-125, a small molecule inhibitor of non-muscle myosin IIA and IIB. MT-125 has high brain penetrance and an excellent safety profile; blocks GBM invasion and cytokinesis; and prolongs survival in murine GBM models. By impairing mitochondrial fission, MT-125 increases redox stress and induces DNA damage and ferroptosis; and these effects explain why this drug synergizes with radiotherapy in prolonging survival in murine GBM models. MT-125 also induces oncogene addiction to receptor tyrosine kinase (RTK) signaling through a mechanism that is also driven by redox stress, and it also synergizes with FDA-approved RTK and MAP Kinase pathway inhibitors in vitro. Furthermore, MT-125 also synergizes with an FDA approved, RTK inhibitor in vivo, with the combination producing long-term, tumor free survival in an aggressive, preclinical murine model of GBM. MT-125’s long-term administration is well-tolerated at doses well above what is predicted to be therapeutic in multiple mammalian species. These data provide a powerful rationale for the clinical development of NMII targeting therapeutics to treat cancer and demonstrates that MT-125 has strong clinical potential for the treatment of GBM.

Abstract We have shown that members of the non-muscle myosin II (NMII) family of molecular motors are non-redundant drivers of both invasion and proliferation in glioblastoma (GBM), consistent with the canonical roles NMII isoforms play in cell motility and cytokinesis. However, we have also found that NMII also has a non-canonical role by regulating oncogenic signaling pathways relevant to GBM pathogenesis. This unexpected finding has led us to identify and characterize MT-125, a highly specific small molecule inhibitor of the two predominant NMII isoforms in GBM (NMIIA and NMIIB), and we have studied its effects in pre-clinical GBM models. MT-125 has high brain penetrance and retention; an excellent safety profile in rodents and canines with no changes in metabolic or hematologic indices after prolonged administration; is well-tolerated with daily dosing for at least 85 consecutive days; blocks GBM invasion and cytokinesis, consistent with the canonical roles of NMII; and prolongs median survival by 35% as a single agent in murine GBM models. MT-125 increases signaling along both the PDGFR- and MAPK-driven pathways through a mechanism that involves the upregulation of reactive oxygen species, leading to oncogene addiction to these pathways. As a consequence, MT-125 is synthetically lethal when combined with FDA-approved, CNS-permeant inhibitors of PDGFR and mTOR in vitro. Combining MT-125 with sunitinib, a PDGFR inhibitor, or paxalisib, a combined PI3 Kinase/mTOR inhibitor, in vivo doubles median survival in a highly aggressive murine orthotopic GBM model. Furthermore, combining MT-125 with sunitinib produces tumor free remissions of >100 days--2.5-fold greater than median survival of either drug alone—in approximately 40% of mice. Our results provide a powerful rationale for developing NMII targeting strategies to treat cancer and demonstrate that MT-125 has strong clinical potential for the treatment of GBM.

Abstract Tumor cells require the mitotic spindle to drive chromosome separation in metaphase, which has made the spindle a compelling target for development of anti-mitotics. However, most of these spindle inhibitors target microtubules and can thereby be highly neurotoxic. This has motivated development of targeted anti-mitotics that inhibit non-microtubule components of the spindle, including kinesins and mitotic kinases. These drugs are not neurotoxic and prolong survival in tumor patients, but resistance limits their efficacy. We reported (Kenchappa et al., Cell Reports, 2022) that resistance to one of these—ispinesib, a highly specific inhibitor of the mitotic kinesin Kif11—requires two functions of STAT3: activation of transcription in the nucleus and of oxidative phosphorylation in the mitochondria. Inhibiting both functions reverses ispinesib resistance. Furthermore, combining ispinesib with a CNS-permeant STAT3 inhibitor significantly prolongs survival over ispinesib alone in both immunocompetent and PDX GBM models. We now report that resistance to two other targeted anti-mitotics—alisertib (Aurora Kinase inhibitor) and volasertib (Polo Like Kinase inhibitor)—works similarly and can likewise be reversed with STAT3 inhibitors. For all three of these anti-mitotics, resistance goes hand-in-hand with development of therapy-induced senescence (TIS) in approximately half of GBM cells. TIS cells secrete a variety of soluble factors, referred to as the senescence associated secretory phenotype (SASP), which stimulate stemness, angiogenesis, and immunosuppression. We propose that: 1) TIS cells require STAT3 for survival, and that they in turn maintain a drug resistant state by suppressing mitosis in the remaining, non senescent cells through components of the SASP; and 2) by killing TIS cells, STAT3 inhibitors restore anti-mitotic sensitivity in the remaining, non-senescent tumor cells. We propose that this STAT3/TIS model of resistance may also apply to how resistance develops to other GBM therapies, and may guide similar translational approaches to enhancing therapeutic efficacy.

Abstract While the allure of targeted therapies in oncology has been their high degree of specificity and potency for key tumor drivers, they have been disappointing in glioblastoma (GBM), even for drugs that are blood brain barrier permeable and CNS retained. This point is highlighted by the experience with mitotic spindle inhibitors, drugs which block the G2M transition and induce mitotic catastrophe—a phenotype characterized by cell enlargement and polyploidy that leads to apoptotic cell death. We have shown that one of these, a potent inhibitor of the mitotic kinesin Kif11 (ispinesib), is highly active against GBM tumor initiating cells and prolongs survival in murine models of this disease. However, tumors eventually progress, reflecting the development of drug resistance. Although ispinesib resistant GBM cells develop mitotic catastrophe, they become highly resistant to the apoptosis that typically follows and continue to proliferate. We find that this apoptosis resistance requires phosphorylation of the transcription factor STAT3 at two residues—Y705 and S727. Phosphorylation of Y705, mediated by SRC kinase, translocates STAT3 to the nucleus where it induces transcription of anti-apoptotic proteins. Phosphorylation at S727, mediated by EGFR, translocates STAT3 to the mitochondria where it blocks release of cytochrome c—the penultimate effector in apoptosis. Simultaneously inhibiting both SRC and EGFR with FDA-approved, CNS permeant inhibitors reverses this resistance and significantly prolongs survival in ispinesib-treated GBM-bearing mice. Furthermore, we find that resistance to several other mitotic inhibitors also utilizes this STAT3-driven mechanism and can likewise be reversed with combined EGFR and SRC inhibition. Thus, our work demonstrates how a promising therapeutic approach, which has been disappointing in GBM, can in fact be rendered effective by anticipating and prospectively treating ab initio the mechanism that drives treatment resistance.

Significance Glioblastoma, the most common and lethal of primary brain tumors, can diffusely infiltrate brain. Because the components of the cytoskeleton that drive tumor invasion are nonredundant, they have been attractive targets for blocking glioblastoma dispersion. However, we find that while targeting one of these—the molecular motor myosin IIA—effectively blocks glioblastoma invasion, it enhances tumor proliferation and lethality. In this study, we have found that myosin IIA functions as a tumor suppressor by reducing signaling of two oncogenes—ERK1/2 and NF-κB—in a manner regulated by the mechanics of the tumor and surrounding brain. Our results also argue that effective treatment of glioblastoma will require inhibiting targets that drive both invasion and proliferation.