Glioblastoma (GBM) is the most common primary malignant brain tumor in adults and has an exceedingly low median overall survival of only 15 months. Current standard-of-care for GBM consists of gross total surgical resection followed by radiation with concurrent and adjuvant chemotherapy. Temozolomide (TMZ) is the firstchoice chemotherapeutic agent in GBM; however, the development of resistance to TMZ often becomes the limiting factor in effective treatment. While O6-methylguanine-DNA methyltransferase repair activity and uniquely resistant populations of glioma stem cells are the most well-known contributors to TMZ resistance, many other molecular mechanisms have come to light in recent years. Key emerging mechanisms include the involvement of other DNA repair systems, aberrant signaling pathways, autophagy, epigenetic modifications, microRNAs, and extracellular vesicle production. This review aims to provide a comprehensive overview of the clinically relevant molecular mechanisms and their extensive interconnections to better inform efforts to combat TMZ resistance.

A novel mechanism is described by which neurons reenter the cell cycle, which leads to their apoptosis: β-amyloid peptide activates the MEK-ERK pathway aberrantly, resulting in expression of cyclin D1 and cell death. Cyclin D1, in turn, prevents the activation of neuronal CDK5, which is important for neuronal survival.

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Purpose: Patients develop resistance to anti-angiogenic drugs, secondary to changes in the tumor microenvironment, including hypoxia. PIM kinases are pro-survival kinases and their expression increases in hypoxia. The goal of this study was to determine whether targeting hypoxia-induced PIM kinase expression is effective in combination with VEGF-targeting agents. The rationale for this therapeutic approach is based on the fact that anti-angiogenic drugs can make tumors hypoxic, and thus more sensitive to PIM inhibitors. Experimental Design: Xenograft and orthotopic models of prostate and colon cancer were used to assess the effect of PIM activation on the efficacy of VEGF-targeting agents. Immunohistochemistry and in vivo imaging were used to analyze angiogenesis, apoptosis, proliferation, and metastasis. Biochemical studies were performed to characterize the novel signaling pathway linking PIM and HIF-1. Results: PIM was upregulated following treatment with anti-VEGF therapies, and PIM1 overexpression reduced the ability of these drugs to disrupt vasculature and block tumor growth. PIM inhibitors reduced HIF-1 activity, opposing the shift to a pro-angiogenic gene signature associated with hypoxia. Combined inhibition of PIM and VEGF produced a synergistic anti­tumor response characterized by decreased proliferation, reduced tumor vasculature, and decreased metastasis. Conclusions: This study describes PIM kinase expression as a novel mechanism of resistance to anti-angiogenic agents. Our data provide justification for combining PIM and VEGF inhibitors to treat solid tumors. The unique ability of PIM inhibitors to concomitantly target HIF-1 and selectively kill hypoxic tumor cells addresses two major components of tumor progression and therapeutic resistance.

New approaches are needed for the treatment of patients with T-cell acute lymphoblastic leukemia (T-ALL) who fail to achieve remission with chemotherapy. Analysis of the effects of pan-PIM protein kinase inhibitors on human T-ALL cell lines demonstrated that the sensitive cell lines expressed higher PIM1 protein kinase levels, whereas T-ALL cell lines with NOTCH mutations tended to have lower levels of PIM1 kinase and were insensitive to these inhibitors. NOTCH-mutant cells selected for resistance to gamma secretase inhibitors developed elevated PIM1 kinase levels and increased sensitivity to PIM inhibitors. Gene profiling using a publically available T-ALL dataset demonstrated overexpression of PIM1 in the majority of early T-cell precursor (ETP)-ALLs and a small subset of non-ETP ALL. While the PIM inhibitors blocked growth, they also stimulated ERK and STAT5 phosphorylation, demonstrating that activation of additional signaling pathways occurs with PIM inhibitor treatment. To block these pathways, Ponatinib, a broadly active tyrosine kinase inhibitor (TKI) used to treat chronic myelogenous leukemia, was added to this PIM-inhibitor regimen. The combination of Ponatinib with a PIM inhibitor resulted in synergistic T-ALL growth inhibition and marked apoptotic cell death. Treatment of mice engrafted with human T-ALL with these two agents significantly decreased the tumor burden and improved the survival of treated mice. This dual therapy has the potential to be developed as a novel approach to treat T-ALL with high PIM expression.