The Old, the New, and the Future
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Cancer Biol Med 2018. doi: 10.20892/j.issn.2095-3941.2018.0030 REVIEW G2/M inhibitors as pharmacotherapeutic opportunities for glioblastoma: the old, the new, and the future Angel Mauricio Castro-Gamero1, Julia Alejandra Pezuk2, María Sol Brassesco3, Luiz Gonzaga Tone4,5 1Human Genetics Laboratory, Institute of Natural Sciences, Federal University of Alfenas (UNIFAL-MG), Alfenas 37130-001, Brazil; 2Biotechnology and Innovation in Health Program and Pharmacy Program, Anhanguera University São Paulo (UNIAN- SP), São Paulo 05145-200, Brazil; 3Department of Biology, Faculty of Philosophy, Sciences and Letters at Ribeirão Preto, University of São Paulo, Ribeirão Preto 14040-901, Brazil; 4Department of Pediatrics; 5Department of Genetics, Ribeirão Preto School of Medicine, University of São Paulo, Ribeirão Preto 14049-900, Brazil ABSTRACT Glioblastoma (GBM) is one of the deadliest tumors and has a median survival of 3 months if left untreated. Despite advances in rationally targeted pharmacological approaches, the clinical care of GBM remains palliative in intent. Since the majority of altered signaling cascades involved in cancer establishment and progression eventually affect cell cycle progression, an alternative approach for cancer therapy is to develop innovative compounds that block the activity of crucial molecules needed by tumor cells to complete cell division. In this context, we review promising ongoing and future strategies for GBM therapeutics aimed towards G2/M inhibition such as anti-microtubule agents and targeted therapy against G2/M regulators like cyclin-dependent kinases, Aurora inhibitors, PLK1, BUB, 1, and BUBR1, and survivin. Moreover, we also include investigational agents in the preclinical and early clinical settings. Although several drugs were shown to be gliotoxic, most of them have not yet entered therapeutic trials. The use of either single exposure or a combination with novel compounds may lead to treatment alternatives for GBM patients in the near future. KEYWORDS Chemotherapy; PLK1; AURK; survivin; BUB; BUR1 Introduction cells appear2. Accordingly, a grade IV astrocytoma (also called "glioblastoma", "glioblastoma multiforme", "grade IV Glioblastoma glioblastoma", and "GBM") represents the most common and most aggressive primary malignant brain tumor, with 3 per For many years, tumors of the central nervous system (CNS) 100,000 people diagnosed every year3. GBM is were primarily categorized according to histopathological histopathologically characterized by brisk mitotic activity, criteria determined under microscopic observation, where cellular and nuclear atypia, vascular thrombosis, and similarities and phenotypic differences depended on cellular microvascular hyperproliferation and necrosis, with 80% origin and their presumed levels of differentiation. In being primary or de novo occurring though malignant particular, gliomas are diffusely infiltrating glial cell tumors transformation from lower-grade gliomas (sometimes 4 that are responsible for 80% of malignant tumors initiated in referred to as secondary GBMs) . GBM is one of the most deadly types of tumors5. If left the brain and have been classified by the World Health untreated, this dismal tumor has a median survival of 3 Organization (WHO) depending on the type of the affected months6. In addition to maximal safe surgical resection and glial cell, thus integrating the nomenclature with grading radiotherapy (RT), the standard chemotherapeutic agent for system1. Histological diagnosis is correlated with tumor grade its treatment since 2005 is the alkylant prodrug temozolomide on a scale of I to IV based on how normal or abnormal the (TMZ), which was first approved by the Food and Drug Administration (FDA) for use in recurrent GBM based on Correspondence to: Angel Mauricio Castro-Gamero the phase II trial by Yung and colleagues7. Posteriorly, in the E-mail: [email protected] pivotal phase III study, Stupp and colleagues randomized Received February 19, 2018; accepted June 26, 2018. Available at www.cancerbiomed.org ~600 patients diagnosed with GBM from various treatment Copyright © 2018 by Cancer Biology & Medicine centers. Their investigation consisted of radiation alone or Cancer Biol Med Vol 15, No 4 November 2018 355 radiotherapy with continuous daily TMZ, which wildtype gliomas18,19. demonstrated an improved 14.6-month median survival in This current classification represents a conceptual and the second group, versus 12.1 months in control patients. practical advance over its 2007 predecessor, reinforcing the Two-year survival was also increased by 26.5% compared to need for molecular/genomic diagnosis, new molecular 10.4% for those treated with radiotherapy alone. Nowadays, approaches, as well as further studies to gain a better RT combined with concomitant and adjuvant TMZ after understanding of the role of these mutational profiles in the surgical resection, namely STUPP treatment, is widely used survival of patients and their prognostic values. for newly diagnosed GBM patients8,9. Nonetheless, only Accordingly, many analyses of the genomic landscape of 15%–20% of patients survive 5 years after diagnosis, and no GBM were published by the Cancer Genome Atlas Research other therapies have demonstrated a robust survival benefit Network in 200820-22 and revealed specific genomic, in recurrent disease6,10. epigenomic, transcriptomic, and proteomic alterations in TMZ is an imidazotetrazine derivative of the alkylating core pathways that define novel specific tumor subgroups. agent dacarbazine that delivers a methyl group to the purine Thus, some studies have predicted that genomic diagnoses bases of DNA (O6-guanine, N7-guanine, and N3-adenine). will overrule and dictate the diagnosis in the future23. Although O6-methylguanine (O6-MeG) is the primary However, these subdivisions still play no role in current cytotoxic lesion, it can be reversed by the action of the repair diagnostics and treatment decisions, but do help to overcome enzyme methylguanine methyltransferase (MGMT), thereby some of the molecular heterogeneity in GBM and improve neutralizing the cytotoxic effects of TMZ11. Accordingly, high treatment. expression of MGMT in glioma cells is the predominant The concept of exploiting cell division as a therapeutic mechanism underlying tumor resistance to alkylating target has been in practice since the advent of chemotherapy. agents12. Moreover, patients with methylated-MGMT treated Briefly, antitumor treatments can affect the cell cycle through with TMZ showed a 21.7-month median overall survival three modes of action: blocking DNA synthesis, causing DNA (OS) compared with 12.7 months in those with damage, or perturbing mitotic processes. As with many solid unmethylated promoters13, proving a direct association tumors, GBM is defined as a highly heterogeneous cancer between MGMT expression and tumor response to TMZ with different cell populations coexisting within the tumor therapy14. mass, each one with a distinct proliferative status directly Moreover, results from the European Organization for connected to key molecules regulating cell cycle progression Research and Treatment of Cancer and National Cancer and mitosis. In this regard, the diagnostic and prognostic Institute of Canada trial recognized methylated-MGMT as relevance of cell cycle biomarkers strongly reinforce the need the strongest predictor of outcome and benefit from TMZ to characterize signaling pathways and highlight their treatment8,15. Similarly, the recent meta-analysis by Zhao and potential for novel targeted therapies for GBM. Hence, in the colleagues16 involving 7,886 patients, highlighted the present review, we feature classic compounds used in anti- universal predictive value of MGMT methylation in newly GBM therapy, and review recent advances on new diagnosed GBM patients, elderly GBM patients, and therapeutic approaches that are based on the inhibition of recurrent GBM patients16. cell cycle molecules. Over the last two decades, many researchers have highlighted the importance of GBM molecular subtyping, but Anti-microtubule agents only recently was the WHO Classification for CNS Tumors able to integrate phenotypic and genotypic parameters, and Mitosis disruption is widely used in clinics, and even with the subdivided GBM in three categories based on the status of rapid expansion in the number of classes of compounds with the isocitrate dehydrogenase (IDH) gene17. Consequently, antineoplastic activity, anti-microtubule agents are still GBM are currently classified as IDH-wildtype (approximately among the most strategic and have played a pivotal role in 90% of cases that correspond most frequently to the the curative and palliative treatment of cancer over the last clinically defined primary GBM), GBM IDH-mutant 50 years. (approximately 10% of cases that closely correspond to the In general, anti-microtubule compounds can be divided so-called secondary GBM), and GBM not otherwise specified into tubulin destabilizing and stabilizing agents. In the first (NOS), a diagnosis that is reserved for tumors without full group, we can include vinblastine and vincristine, alkaloids IDH evaluation17. Importantly, some studies already have isolated from the periwinkle plant Cantharanthus rosea, and showed that OS of IDH-mutants are greater than IDH- colchicine, isolated from the meadow saffron Colchicum 356 Castro-Gamero et al. G2/M inhibitors for glioblastoma treatment autumnale, all of which have been shown to inhibit strategies, including conjugates of paclitaxel