Central nervous system tumors comprising the primary cancers and brain metastases remain the most lethal neoplasms and challenging to treat. Substantial evidence points to a paramount role for inflammation in the pathology leading to gliomagenesis, malignant progression and tumor aggressiveness in the central nervous system (CNS) microenvironment. This review summarizes the salient contributions of oxidative stress, interleukins, tumor necrosis factor-α(TNF-α), cyclooxygenases, and transcription factors such as signal transducer and activator of transcription 3 (STAT3) and nuclear factor kappa-light-chain-enhancer of activated B-cells (NF-κB) and the associated cross-talks to the inflammatory signaling in CNS cancers. The roles of reactive astrocytes, tumor associated microglia and macrophages, metabolic alterations, microsatellite instability, O6-methylguanine DNA methyltransferase (MGMT) DNA repair and epigenetic alterations mediated by the isocitrate dehydrogenase 1 (IDH1) mutations have been discussed. The inflammatory pathways with relevance to the brain cancer treatments have been highlighted.

Isocitrate dehydrogenases 1 and 2 (IDH1,2), the key Krebs cycle enzymes that generate NADPH reducing equivalents, undergo heterozygous mutations in >70% of low- to mid-grade gliomas and ~20% of acute myeloid leukemias (AMLs) and gain an unusual new activity of reducing the α-ketoglutarate (α-KG) to D-2 hydroxyglutarate (D-2HG) in a NADPH-consuming reaction. The oncometabolite D-2HG, which accumulates >35 mM, is widely accepted to drive a progressive oncogenesis besides exacerbating the already increased oxidative stress in these cancers. More importantly, D-2HG competes with α-KG and inhibits a large number of α-KG-dependent dioxygenases such as TET (Ten-eleven translocation), JmjC domain-containing KDMs (histone lysine demethylases), and the ALKBH DNA repair proteins that ultimately lead to hypermethylation of the CpG islands in the genome. The resulting CpG Island Methylator Phenotype (CIMP) accounts for major gene expression changes including the silencing of the MGMT (O6-methylguanine DNA methyltransferase) repair protein in gliomas. Glioma patients with IDH1 mutations also show better therapeutic responses and longer survival, the reasons for which are yet unclear. There has been a great surge in drug discovery for curtailing the mutant IDH activities, and arresting tumor proliferation; however, given the unique and chronic metabolic effects of D-2HG, the promise of these compounds for glioma treatment is uncertain. This comprehensive review discusses the biology, current drug design and opportunities for improved therapies through exploitable synthetic lethality pathways, and an intriguing oncometabolite-inspired strategy for primary glioblastoma.

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Whether the antimutagenic DNA repair protein MGMT works solo in human cells and if it has other cellular functions is not known. Here, we show that human MGMT associates with PCNA and in turn, with the cell cycle inhibitor, p21cip1 in glioblastoma and other cancer cell lines. MGMT protein was shown to harbor a nearly perfect PCNA-Interacting Protein (PIP box) motif. Isogenic p53-null H1299 cells were engineered to express the p21 protein by two different procedures. Reciprocal immunoprecipitation/western blotting, Far-western blotting, and confocal microscopy confirmed the specific association of MGMT with PCNA and the ability of p21 to strongly disrupt the MGMT-PCNA complexes in tumor cells. Alkylation DNA damage resulted in a greater colocalization of MGMT and PCNA proteins, particularly in HCT116 cells deficient in p21 expression. p21 expression in isogenic cell lines directly correlated with markedly higher levels of MGMT mRNA, protein, activity and greater resistance to alkylating agents. In other experiments, four glioblastoma cell lines synchronized at the G1/S phase using either double thymidine or thymidine-mimosine blocks and subsequent cycling consistently showed a loss of MGMT protein at mid- to late S-phase, irrespective of the cell line, suggesting such a downregulation is fundamental to cell cycle control. MGMT protein was also specifically degraded in extracts from S-phase cells and evidence strongly suggested the involvement of PCNA-dependent CRL4Cdt2 ubiquitin-ligase in the reaction. Overall, these data provide the first evidence for non-repair functions of MGMT in cell cycle and highlight the involvement of PCNA in MGMT downregulation, with p21 attenuating the process.

There is a desperate need for novel and efficacious chemotherapeutic strategies for human brain cancers. There are abundant molecular alterations along the p53 and MDM2 pathways in human glioma, which play critical roles in drug resistance. The present study was designed to evaluate the in vitro and in vivo antitumor activity of a novel brain-penetrating small molecule MDM2 degrader, termed SP-141. In a panel of nine human glioblastoma and medulloblastoma cell lines, SP-141, as a single agent, potently killed the brain tumor-derived cell lines with IC50 values ranging from 35.8 to 688.8 nM. Treatment with SP-141 resulted in diminished MDM2 and increased p53 and p21cip1 levels, G2/M cell cycle arrest, and marked apoptosis. In intracranial xenograft models of U87MG glioblastoma (wt p53) and DAOY medulloblastoma (mutant p53) expressing luciferase, treatment with SP-141 caused a significant 4- to 9-fold decrease in tumor growth in the absence of discernible toxicity. Further, combination treatment with a low dose of SP-141 (IC20) and temozolomide, a standard anti-glioma drug, led to synergistic cell killing (1.3- to 31-fold) in glioma cell lines, suggesting a novel means for overcoming temozolomide resistance. Considering that SP-141 can be taken up by the brain without the need for any special delivery, our results suggest that SP-141 should be further explored for the treatment of tumors of the central nervous system, regardless of the p53 status of the tumor.

The Near-infrared Fluorescence (NIRF) molecular imaging of cancer is known to be superior in sensitivity, deeper penetration, and low phototoxicity compared to other imaging modalities. In view of an increased need for efficient and targeted imaging agents, we synthesized a NAD(P)H quinone oxidoreductase 1 (NQO1)-activatable NIR fluorescent probe (NIR-ASM) by conjugating dicyanoisophorone (ASM) fluorophore with the NQO1 substrate quinone propionic acid (QPA). The probe remained non-fluorescent until activation by NQO1, whose expression is largely limited to malignant tissues. With a large Stokes shift (186 nm) and a prominent near-infrared emission (646 nm) in response to NQO1, NIR-ASM was capable of monitoring NQO1 activity in vitro and in vivo with high specificity and selectivity. We successfully employed the NIR-ASM to differentiate cancer cells from normal cells based on NQO1 activity using fluorescence microscopy and flow cytometry. Chemical and genetic approaches involving the use of ES936, a specific inhibitor of NQO1 and siRNA and gene transfection procedures unambiguously demonstrated NQO1 to be the sole target activating the NIR-ASM in cell cultures. NIR-ASM was successfully used to detect and image the endogenous NQO1 in three live tumor-bearing mouse models (A549 lung cancer, Lewis lung carcinoma, and MDMAMB 231 xenografts) with a high signal-to-low noise ratiometric NIR fluorescence response. When the NQO1-proficient A549 tumors and NQO1-deficient MDA-MB-231 tumors were developed in the same animal, only the A549 malignancies activated the NIR-ASM probe with a strong signal. Because of its high sensitivity, rapid activation, tumor selectivity, and nontoxic properties, the NIR-ASM appears to be a promising agent with clinical applications.

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There is great interest in repurposing disulfiram (DSF), a rapidly metabolizing nontoxic drug, for brain cancers and other cancers. To overcome the instability and low therapeutic efficacy, we engineered passively-targeted DSF-nanoparticles (DSFNPs) using biodegradable monomethoxy (polyethylene glycol) d,l-lactic-co-glycolic acid (mPEG-PLGA) matrix. The physicochemical properties, cellular uptake and the blood brain-barrier permeability of DSFNPs were investigated. The DSFNPs were highly stable with a size of ∼70 nm with a >90% entrapment. Injection of the nanoparticles labeled with HITC, a near-infrared dye into normal mice and tumor-bearing nude mice followed by in vivo imaging showed a selective accumulation of the formulation within the brain and subcutaneous tumors for >24 h, indicating an increased plasma half-life and entry of DSF into desired sites. The DSFNPs induced a potent and preferential killing of many brain tumor cell lines in cytotoxicity assays. Confocal microscopy showed a quick internalization of the nanoparticles in tumor cells followed by initial accumulation in lysosomes and subsequently in mitochondria. DSFNPs induced high levels of ROS and led to a marked loss of mitochondrial membrane potential. Activation of the MAP-kinase pathway leading to a nuclear translocation of apoptosis-inducing factor and altered expression of apoptotic and anti-apoptotic proteins were also observed. DSFNPs induced a powerful and significant regression of intracranial medulloblastoma xenografts compared to the marginal efficacy of unencapsulated DSF. Together, we show that passively targeted DSFNPs can affect multiple targets, trigger potent anticancer effects, and can offer a sustained drug supply for brain cancer treatment through an enhanced permeability retention (EPR).