Bindra, Ranjit S.

Abstract Glioblastoma is the most common type of malignant primary brain tumor and a major cause of morbidity and mortality. In 2021, the World Health Organization updated the classification of Central Nervous System (CNS) tumors to restrict glioblastomas to isocitrate dehydrogenase-wildtype (IDHwt) tumors, improving understanding of the prognosis and optimal therapy for these tumors. This revision also enables more homogeneous populations of patients to be enrolled in clinical trials, facilitating the evaluation of novel therapies. In this updated consensus review from the Society for Neuro-Oncology (SNO) and the European Association of Neuro-Oncology (EANO), the current management of patients with glioblastoma is discussed. In addition, novel therapies such as immunotherapies, viral therapies, targeted molecular therapies, theranostics, and antibody–drug conjugates will be reviewed, as well as the current challenges and future directions for research.

Abstract Background Acquired resistance to temozolomide (TMZ) chemotherapy due to DNA mismatch repair (MMR) enzyme deficiency is a barrier to improving outcomes for isocitrate dehydrogenase (IDH) wild-type glioblastoma (GBM) patients. KL-50 is a new imidazotetrazine-based therapeutic designed to induce DNA interstrand cross-links, and subsequent double-stranded breaks, in an MMR-independent manner in cells with O-6-methylguanine-DNA methyltransferase (MGMT) deficiency. Previous research showed its efficacy against LN229 glioma cells with MMR and MGMT knockdown. Its activity against patient-derived GBM that model post-TMZ recurrent tumors is unclear. Methods We created MMR-deficient GBM patient-derived xenografts through exposure to TMZ, followed by treatment with additional TMZ or KL-50. We also generated isogenic, MSH6 knockout (KO) patient-derived GBM and tested them for sensitivity to TMZ and KL-50. Results KL-50 extended the median survival of mice intracranially engrafted with either patient-derived TMZ-naïve GBM6 or TMZ-naïve GBM12 by 1.75-fold and 2.15-fold, respectively (P < 0.0001). A low dose (4 Gy) of fractionated RT further extended the survival of KL-50-treated GBM12 mice (median survival = 80 days for RT + KL-50 vs. 71 days KL-50 alone, P = 0.018). KL-50 also extended the median survival of mice engrafted with post-TMZ, MMR-deficient GBM6R-m185 (140 days for KL-50 vs. 37 days for vehicle, P < 0.0001). MSH6 KO increased TMZ IC50 for GBM6 and GBM12 cultures by >5-fold and >12-fold for cell death and live cell count outputs, respectively. In contrast, MSH6-KO actually decreased KL-50 IC50 by 10–80%. Conclusion KL-50-based compounds are a promising new strategy for the treatment of MGMT-deficient, MMR-deficient GBM that recurs after frontline TMZ.

Abstract DNA damage response (DDR) mechanisms are critical to maintenance of overall genomic stability, and their dysfunction can contribute to oncogenesis. Significant advances in our understanding of DDR pathways have raised the possibility of developing therapies that exploit these processes. In this expert-driven consensus review, we examine mechanisms of response to DNA damage, progress in development of DDR inhibitors in IDH-wild-type glioblastoma and IDH-mutant gliomas, and other important considerations such as biomarker development, preclinical models, combination therapies, mechanisms of resistance and clinical trial design considerations.

Abstract BACKGROUND Preclinical studies have demonstrated that IDH1-mutant (IDHmt) gliomas harbor a BRCAness phenotype with a defect in homologous recombination that confers PARP inhibitor sensitivity. METHODS The phase II component of ABTC 1801 examined the efficacy of the combination of pamiparib (BeiGene BGB-290) with low dose metronomic temozolomide in recurrent IDHmt grade 2 and 3 gliomas. In this two-stage design, Arm A enrolled patients who had failed two lines and Arm B a single line of alkylator therapy. Pamiparib and temozolomide doses based on phase I results was 60 mg BID and 20 mg daily. Primary endpoint was objective response rate (ORR). RESULTS 39 patients (Arm A 15, Arm B 24) enrolled in the phase II. Median age was 45 (range: 25-69), 56% male, and median KPS 90. 72% of tumors were MGMT methylated. There were no confirmed responses in Arm A and one in Arm B; neither arm met the threshold for additional accrual. One-third of subjects discontinued treatment for reasons other than progressive disease. Estimated median PFS was 5.8 months in Arm A and 11.3 months in Arm B. Seven subjects (Arm A 3, Arm B 4) developed DLT during treatment. Grade 3+ toxicity was principally hematologic; 9 patients had grade 3 anemia, 7 grade 3 and 3 grade 4 neutropenia, and 3 grade 3 thrombocytopenia. In enhancing and non-enhancing tumors from 8 surgical arm patients, median unbound drug tumor/plasma ratios were 0.98 and 0.92, respectively; and median unbound pamiparib tumor concentrations were 209 and 188 nmol/L (or nmol/kg), respectively, which were >20-fold the in vitro IC50 for PARP inhibition. CONCLUSIONS While pamiparib appeared to achieve sufficient pharmacologically active concentrations in both enhancing and non-enhancing tumors, combination with temozolomide did not produce a meaningful ORR in IDHmt recurrent gliomas. Cumulative hematologic toxicity was substantial.

Glioblastoma (GBM) is a lethal brain cancer with a five-year survival rate of <5%. Approximately half of GBM tumors lack the DNA repair protein O6-methylguanine DNA methyltransferase (MGMT), which reverses O6-alkylguanine (O6G) lesions. Patients presenting MGMT– GBM are treated with surgery followed by radiation therapy and temozolomide (TMZ), an imidazotetrazine prodrug that produces O6-methylguanine (O6MeG) lesions. However, ~50% of these patients will develop TMZ resistance by silencing of the DNA mismatch repair (MMR) pathway. We recently reported that the novel N3-(2-fluoroethyl)imidazotetrazine “KL-50” is efficacious and well-tolerated in murine models of TMZ-resistant GBM (Lin et al. Science 2022, 377, 502). Herein, we rigorously establish that KL-50 generates DNA interstrand crosslinks (ICLs) by DNA alkylation to generate O6-(2-fluoroethyl)guanine (O6FEtG), displacement of fluoride to form an N1,O6-ethanoguanine (N1,O6EtG) intermediate, and ring-opening by the adjacent cytidine. 2-Chloroethylating agents, such as lomustine and mitozolomide (MTZ), generate the same ICL by an analogous mechanism. However, DNA ICLs form >10-fold more slowly from O6FEtG than O6ClEtG, and this slower rate of cross-linking allows MGMT to reverse the initial O6FEtG in healthy tissue while also reducing MGMT–DNA cross-links arising from addition of MGMT to the N1,O6EtG intermediate. KL- 50 is efficacious in an intracranial patient-derived murine xenograft of TMZ-resistant, MGMT–/MMR– GBM (mOS = 205, 28, and 26 d for KL-50, TMZ, and vehicle-treated control, respectively) and in murine models of newly-diagnosed MGMT–/MMR+ GBM, suggesting its use in recurrent and up-front settings, respectively. These studies underscore the significance of considering the rates of chemical DNA modification and biochemical DNA repair in the design of systemic DNA alkylation agents.

Abstract Isocitrate dehydrogenase (IDH) mutant gliomas are the most common adult, malignant primary brain tumors diagnosed in patients younger than 50, constituting an important cause of morbidity and mortality. In recent years, there has been significant progress in understanding the molecular pathogenesis and biology of these tumors, sparking multiple efforts to improve their diagnosis and treatment. In this consensus review from the Society for Neuro-Oncology (SNO), the current diagnosis and management of IDH-mutant gliomas will be discussed. In addition, novel therapies, such as targeted molecular therapies and immunotherapies, will be reviewed. Current challenges and future directions for research will be discussed.

Abstract Glioblastomas are the most common form of malignant primary brain tumor and an important cause of morbidity and mortality. In recent years there have been important advances in understanding the molecular pathogenesis and biology of these tumors, but this has not translated into significantly improved outcomes for patients. In this consensus review from the Society for Neuro-Oncology (SNO) and the European Association of Neuro-Oncology (EANO), the current management of isocitrate dehydrogenase wildtype (IDHwt) glioblastomas will be discussed. In addition, novel therapies such as targeted molecular therapies, agents targeting DNA damage response and metabolism, immunotherapies, and viral therapies will be reviewed, as well as the current challenges and future directions for research.

Abstract O6-methylguanine-DNA methyltransferase (MGMT) is an enzyme that removes alkyl groups at the O6-position of guanine in DNA. MGMT expression is reduced or absent in many tumor types derived from a diverse range of tissues, most notably in glioma. Low MGMT expression confers significant sensitivity to DNA alkylating agents such as temozolomide, providing a natural therapeutic index over normal tissue. In this study, we sought to identify novel approaches that could maximally exploit the therapeutic index between tumor cells and normal tissues based on MGMT expression, as a means to enhance selective tumor cell killing. Temozolomide, unlike other alkylators, activated the ataxia telangiectasia and Rad3–related (ATR)–checkpoint kinase 1 (Chk1) axis in a manner that was highly dependent on MGMT status. Temozolomide induced growth delay, DNA double-strand breaks, and G2–M cell-cycle arrest, which led to ATR-dependent phosphorylation of Chk1; this effect was dependent on reduced MGMT expression. Treatment of MGMT-deficient cells with temozolomide increased sensitivity to ATR inhibitors both in vitro and in vivo across numerous tumor cell types. Taken together, this study reveals a novel approach for selectively targeting MGMT-deficient cells with ATR inhibitors and temozolomide. As ATR inhibitors are currently being tested in clinical trials, and temozolomide is a commonly used chemotherapeutic, this approach is clinically actionable. Furthermore, this interaction potently exploits a DNA-repair defect found in many cancers. Significance: Monofunctional alkylating agents sensitize MGMT-deficient tumor cells to ATR inhibitors.

AbstractHomologous recombination (HR) DNA repair‐deficient (HRD) breast cancers have been shown to be sensitive to DNA repair targeted therapies. Burgeoning evidence suggests that sporadic breast cancers, lacking germline BRCA1/BRCA2 mutations, may also be HRD. We developed a functional ex vivo RAD51‐based test to identify HRD primary breast cancers. An integrated approach examining methylation, gene expression, and whole‐exome sequencing was employed to ascertain the aetiology of HRD. Functional HRD breast cancers displayed genomic features of lack of competent HR, including large‐scale state transitions and specific mutational signatures. Somatic and/or germline genetic alterations resulting in bi‐allelic loss‐of‐function of HR genes underpinned functional HRD in 89% of cases, and were observed in only one of the 15 HR‐proficient samples tested. These findings indicate the importance of a comprehensive genetic assessment of bi‐allelic alterations in the HR pathway to deliver a precision medicine‐based approach to select patients for therapies targeting tumour‐specific DNA repair defects. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.