替莫唑胺在侵袭性垂体瘤及垂体腺癌中的治疗进展

替莫唑胺(TMZ)是一种口服二代咪唑并四嗪类具有抗肿瘤活性的烷化剂,通过对DNA鸟嘌呤的甲基化来干扰基因转录而诱导DNA损伤,目前是治疗恶性胶质瘤的一线化疗药物.最近有关于TMZ治疗侵袭性垂体腺瘤及垂体癌的报道,有效率分别为60％和69％,且未发生明显并发症.O6-甲基鸟嘌呤-DNA转移酶(MGMT)的表达水平与其疗效呈负相关,从而可能干扰其疗效,由于目前缺乏对垂体癌及侵袭性垂体瘤的有效治疗手段,所以不应否认TMZ对此类患者的治疗有效性.TMZ对侵袭性垂体瘤及垂体癌的治疗机制及有效性仍需进一步探索.     

NF-κB信号通路在脑胶质瘤耐药机制中的研究进展

脑胶质瘤是中枢神经系统最为常见的原发性肿瘤,其中胶质母细胞瘤(glioblastoma multiforme,GBM)发病率较高,其治疗效果主要依赖于手术切除程度及术后化疗效果。由于颅内血脑屏障的作用和耐药基因O~6-甲基鸟嘌呤-DNA甲基转移酶的产生,使得化疗药物在脑胶质瘤患者中的作用大大受限,难以达到预期的临床效果。因此,对脑胶质瘤耐药机制的深入研究意义重大。随着近年来对核转录因子κB(nuclear factor kappa B,NF-κB)通路的深入研究,证明该通路的激活可促进肿瘤细胞扩散和诱导耐药,若拮抗NF-κB通路上下游信号分子可显著改善耐药现象。本文就近年来NF-κB的研究进展以及NF-κB如何参与胶质瘤耐药作一综述。     

DNA损伤修复机制——解读2015年诺贝尔化学奖

Tomas Lindahl, Paul Modrich和Aziz Sancar三位科学家因发现“DNA损伤修复机制”获得了2015年诺贝尔化学奖．Lindahl首次发现Escherichia Coli中参与碱基切除修复的第一个蛋白质--尿嘧啶 DNA糖基化酶（UNG）; Modrich重建了错配修复的体外系统,从大肠杆菌到哺乳动物深入探究了错配修复的机制; Sancar利用纯化的UvrA、UvrB、UvrC重建了核苷酸切除修复的关键步骤,阐述了核苷酸切除修复的分子机制．DNA损伤是由生物所处体外环境和体内因素共同导致的,面对不同种类的损伤,机体启动多种不同的修复机制修复损伤,保护基因组稳定性．这些修复机制包括：光修复(light repairing);核苷酸切除修复（nucleotide excision repair, NER）;碱基切除修复（base excision repair, BER）;错配修复（mismatch repair, MMR）;以及DNA双链断裂修复（DNA double strand breaks repair, DSBR）．其中DNA双链断裂修复又分同源重组（homologous recombination, HR）和非同源末端连接（non homologous end joining, NHEJ）两种方式．本文将对上述几种修复的机制进行总结与讨论．     

多形性脑胶质母细胞瘤化学及基因治疗的研究进展

多形性脑胶质母细胞瘤是最常见的颅内肿瘤之一且预后差。即使积极采用手术,放疗及化疗的联合治疗方法,中位生存期小于1年。传统的恶性胶质瘤的化学治疗主要是应用细胞周期性或非周期性细胞毒性药物,直接杀死肿瘤细胞。随着人们对胶质瘤细胞生物学行为的深入研究,近年来新的细胞毒性药物(如替莫唑胺)不断开发出来;替莫唑胺及亚硝基脲类药物耐药性得到了部分解决以及非细胞毒性药物如酪氨酸激酶抑制剂及细胞间信号传导通路抑制剂在临床中的应用。此外,在导入自杀基因、修复或导入抑癌基因、肿瘤免疫基因治疗、血管生成抑制基因治疗、对流增强传送运输系统等基因治疗策略方面亦展开了广泛的临床研究。使得化学治疗及基因治疗成为GBM手术及放疗的有益补充。     

过表达TNFAIP1促进替莫唑胺对人脑胶质瘤细胞U251迁移和侵袭的抑制作用

为了深入研究人脑胶质瘤治疗时耐药的发生机制,并寻求潜在的治疗手段,以人脑胶质瘤细胞U251为研究对象,研究肿瘤坏死因子α诱导蛋白1(TNFAIP1)对其迁移和侵袭的影响.半定量PCR和蛋白质免疫印迹(Western blot)试验发现,替莫唑胺(TMZ)会上调U251细胞内TNFAIP1的表达.在U251细胞中过表达T...     

多形胶质母细胞瘤细胞系端粒酶活性的测定

端粒缩短见于星形细胞瘤发展过程中,但其长度在胶质母细胞瘤／细胞系相对稳定,提示胶质瘤细胞内存在端粒修复机制的可能性．为证实此点,利用端粒重复片段扩增技术（ＴＲＡＰ）,对８株人／大鼠多形胶质母细胞系的蛋白提取液中端粒酶活性加以测定．结果显示：８例胶质瘤样本的反应液均可见端粒ＰＣＲ扩增片段;用无ＤＮａｓｅ的ＲＮａｓｅ事先处理蛋白提取液,可明显降低或消除ＰＣＲ产物的出现,说明ＴＲＡＰ反应中的ＰＣＲ扩增是在端粒酶的介导下进行而非ＤＮＡ污染或其它端粒修复因子所致．从而不但建立起检测人癌细胞内端粒酶活性的可靠方法,也为针对端粒酶的胶质母细胞瘤生物／药物治疗提供了实验依据．     

MGMT 甲基化影响替莫唑胺对胶质母细胞瘤疗效的研究

目的:探讨MGMT甲基化如何影响替莫唑胺对胶质母细胞瘤的治疗效果。方法:选取41个胶质母细胞瘤患者(根据相同替莫唑胺化疗方案治疗下临床结局的不同分为两组)的肿瘤组织,采用甲基化特异性聚合酶链反应分析胶质瘤组织中MGMT基因启动子区过甲基化状态,同时采用免疫组织化学法分析胶质瘤组织中MGMT蛋白表达情况。结果:临床结局不佳组中,以MGMT蛋白表达阳性的肿瘤为主(72.2%),而在结局相对良好组中,MGMT蛋白表达的阳性率仅为39.1%;在MGMT蛋白表达阳性的22例胶质母细胞瘤组织中,7例MGMT启动子甲基化,阳性率为31.8%,在MGMGT蛋白表达阴性的19例中,14例MGMT启动子甲基化,阳性率为73.7%(P<0.05)。结论:MGMT基因启动子区的甲基化状态与MGMT蛋白的表达相关。MGMT基因启动子过甲基化,MGMT蛋白表达较低;MGMT基因启动子去甲基化,MGMT蛋白表达较高。MGMT启动子过甲基化通过抑制MGMT基因的表达而增加替莫唑胺的疗效。     

维生素C联合替莫唑胺对胶质瘤细胞的毒性作用及其机制*

目的:探讨维生素C(VC)联合替莫唑胺(TMZ)对胶质瘤细胞活力的毒性作用及其机制。方法:在体外条件下培养人胶质瘤细胞BMG-1和SHG44细胞,设对照组(不施加VC与TMZ)、TMZ组(0.2 mmol/L)、VC(0.5 mmol/L)+TMZ(0.2 mmol/L)组,TMZ(0.2 mmol/L TMZ)+U0126(10 μmol/L)组,每组实验重复3次。采用MTT实验检测细胞生存率;流式细胞术和Annexin V-FITC/PI染色检测细胞凋亡情况; ROS检测试剂盒检测活性氧簇(ROS)水平, Western blot检测与凋亡、自噬及ERK通路相关蛋白的表达。结果:与对照组比较,TMZ组胶质瘤细胞的存活率显著下降(P＜0.05)。与TMZ组比较,VC+TMZ组胶质细胞瘤细胞的存活率显著下降(P＜0.01),VC+TMZ组中细胞凋亡率显著升高,且Bax、Cleaved caspase-3及Cleaved PARP蛋白表达显著增加,Bcl-2表达显著降低,而ROS水平及细胞自噬率显著降低,LC3-II/LC3-1表达显著降低,p62表达显著增加(P均＜0.05)。同时,联用可降低BMG-1和SHG44细胞中的p-ERK1/2相关蛋白的表达水平,且提高细胞凋亡率(P均＜0.05)。结论:VC联合TMZ能够增强对胶质瘤细胞的毒性,而这一作用是通过ERK信号通路来促进细胞凋亡并抑制替莫唑胺所介导的自噬作用。     

伊立替康联合替莫唑胺对儿童难治性实体瘤近期疗效的研究

目的：研究伊立替康（Irinotecan,CPT-11）联合替莫唑胺（Temozolomide,TMZ）对儿童难治性实体瘤的近期疗效;方法：选取了11名难治性儿童实体瘤患者并争取了家属同意,分别给予了伊立替康：60mg／m2／d,ivgtt,连用5天;替莫唑胺：75mg／m2／d,连用5天,在伊立替康前一小时口服;用药期间同时给予思密达或者头孢菌素类抗生素预防药物相关性腹泻,并定期复查血象以及肝肾功;28天一疗程,每疗程行复查CT或MRI进行疗效评定和药物安全性评价。结果：11例患者经过2-6个疗程的治疗：疾病控制率（DCR）达到54％;明显延长了患者的无疾病进展生存期,中位无疾病进展生存期3．3个月。用药期间不良事件主要包括0-Ⅲ级的药物相关性腹泻、呕吐、血小板减少、贫血等;结论：伊立替康联合替莫唑胺方案能明显延长儿童难治实体瘤患者未进展存活时间,且耐受性好、安全性高,可以作为儿童晚期实体瘤患者的首选治疗方案。     

YKL-40在脑胶质母细胞瘤微环境中的作用

胶质母细胞瘤是大脑及其他中枢神经系统最常见的恶性肿瘤,其复杂的肿瘤微环境是胶质母细胞瘤临床治疗的主要挑战,也是胶质母细胞瘤患者复发率高、生存率低的主要原因。YKL-40,这一分泌性蛋白质与多种类型的癌症预后不良相关,且在高级别胶质瘤尤其是胶质母细胞瘤患者中血清水平与肿瘤组织表达水平显著升高,而在低级别胶质瘤中并未发现这一特征。这提示,YKL-40与胶质瘤分级及胶质母细胞瘤恶性发展过程密切相关。针对YKL-40的抗体治疗也被证明能够与电离辐射协同抑制胶质母细胞瘤血管生成及恶性发展。基于YKL-40的临床价值,本文将从肿瘤微环境的角度,归纳总结YKL-40在恶性肿瘤中的相关研究成果,并讨论其在胶质母细胞瘤发生发展中的相关作用及临床应用前景。     

Disentangling the therapeutic tactics in GBM: From bench to bedside and beyond

Glioblastoma multiforme (GBM) is one of the most common and malignant form of adult brain tumor with a high mortality rate and dismal prognosis. The present standard treatment comprising surgical resection followed by radiation and chemotherapy using temozolomide can broaden patient's survival to some extent. However, the advantages are not palliative due to the development of resistance to the drug and tumor recurrence following the multimodal treatment approaches due to both intra- and intertumoral heterogeneity of GBM. One of the major contributors to temozolomide resistance is O⁶-methylguanine-DNA methyltransferase. Furthermore, deficiency of mismatch repair, base excision repair, and cytoprotective autophagy adds to temozolomide obstruction. Rising proof additionally showed that a small population of cells displaying certain stem cell markers, known as glioma stem cells, adds on to the resistance and tumor progression. Collectively, these findings necessitate the discovery of novel therapeutic avenues for treating glioblastoma. As of late, after understanding the pathophysiology and biology of GBM, some novel therapeutic discoveries, such as drug repurposing, targeted molecules, immunotherapies, antimitotic therapies, and microRNAs, have been developed as new potential treatments for glioblastoma. To help illustrate, “what are the mechanisms of resistance to temozolomide” and “what kind of alternative therapeutics can be suggested” with this fatal disease, a detailed history of these has been discussed in this review article, all with a hope to develop an effective treatment strategy for GBM.     

Targeted nitric oxide delivery preferentially induces glioma cell chemosensitivity via altered p53 and O6‐Methylguanine‐DNA Methyltransferase activity

Glioblastoma multiforme is the most common malignant central nervous system tumor, and also among the most difficult to treat due to a lack of response to chemotherapeutics. New methods of countering the mechanisms that confer chemoresistance to malignant gliomas could lead to significant advances in the quest to identify novel drug combinations or targeted drug delivery systems for cancer therapy. In this study, we investigate the use of a targeted nitric oxide (NO) donor as a pretreatment to sensitize glioma cells to chemotherapy. The protein chlorotoxin (CTX) has been shown to preferentially target glioma cells, and we have developed CTX–NO, a glioma‐specific, NO‐donating CTX derivative. Pretreatment of cells with CTX–NO followed by 48‐h exposure to either carmustine (BCNU) or temozolomide (TMZ), both common chemotherapeutics used in glioma treatment, resulted in increased efficacy of both therapeutics. After CTX–NO exposure, both T98G and U‐87MG human malignant glioma cells show increased sensitivity to BCNU and TMZ. Further investigation revealed that the consequences of this combination therapy was a reduction in active levels of the cytoprotective enzyme MGMT and altered p53 activity, both of which are essential in DNA repair and tumor cell resistance to chemotherapy. The combination of CTX–NO and chemotherapeutics also led to decreased cell invasion. These studies indicate that this targeted NO donor could be an invaluable tool in the development of novel approaches to treat cancer. Biotechnol. Bioeng. 2013; 110: 1211–1220. © 2012 Wiley Periodicals, Inc.     

High expression of leptin receptor leads to temozolomide resistance with exhibiting stem/progenitor cell features in gliobalastoma

Glioblastoma is a highly aggressive malignant disease with notable resistance to chemotherapy. In this study, we found that leptin receptor (ObR)-positive glioblastoma cells were resistant to temozolomide (TMZ), and TMZ-resistant cells exhibited high expression of ObR. ObR can serve as a marker to enrich glioblastoma cells with some stem/progenitor cell traits, which explained the reason for TMZ resistance of ObR+ cells. STAT3-mediated SOX2/OCT4 signaling axis maintained the stem/progenitor cell properties of ObR+ cells, which indirectly regulated glioblastoma TMZ resistance. These findings gain insight into the molecular link between obesity and glioblastoma, and better understanding of this drug-resistant population may lead to the development of more effective therapeutic interventions for glioblastoma.     

Dynamic inhibition of ATM kinase provides a strategy for glioblastoma multiforme radiosensitization and growth control

Glioblastoma multiforme (GBM) is notoriously resistant to treatment. Therefore, new treatment strategies are urgently needed. ATM elicits the DNA damage response (DDR), which confers cellular radioresistance; thus, targeting the DDR with an ATM inhibitior (ATMi) is very attractive. Herein, we show that dynamic ATM kinase inhibition in the nanomolar range results in potent radiosensitization of human glioma cells, inhibits growth and does not conflict with temozolomide (TMZ) treatment. The second generation ATMi analog KU-60019 provided quick, reversible and complete inhibition of the DDR at sub-micromolar concentrations in human glioblastoma cells. KU-60019 inhibited the phosphorylation of the major DNA damage effectors p53, H2AX and KAP1 as well as AKT. Colony-forming radiosurvival showed that continuous exposure to nanomolar concentrations of KU-60019 effectively radiosensitized glioblastoma cell lines. When cells were co-treated with KU-60019 and TMZ, a slight increase in radiation-induced cell killing was noted, although TMZ alone was unable to radiosensitize these cells. In addition, without radiation, KU-60019 with or without TMZ reduced glioma cell growth but had no significant effect on the survival of human embryonic stem cell (hESC)-derived astrocytes. Altogether, transient inhibition of the ATM kinase provides a promising strategy for radiosensitizing GBM in combination with standard treatment. In addition, without radiation, KU-60019 limits growth of glioma cells in co-culture with human astrocytes that seem unaffected by the same treatment. Thus, inter-fraction growth inhibition could perhaps be achieved in vivo with minor adverse effects to the brain.Key words: AKT, DNA repair, KU-60019, temozolomide     

Glutathione depletion sensitizes cisplatin- and temozolomide-resistant glioma cells in vitro and in vivo

Malignant glioma is a severe type of brain tumor with a poor prognosis and few options for therapy. The main chemotherapy protocol for this type of tumor is based on temozolomide (TMZ), albeit with limited success. Cisplatin is widely used to treat several types of tumor and, in association with TMZ, is also used to treat recurrent glioma. However, several mechanisms of cellular resistance to cisplatin restrict therapy efficiency. In that sense, enhanced DNA repair, high glutathione levels and functional p53 have a critical role on cisplatin resistance. In this work, we explored several mechanisms of cisplatin resistance in human glioma. We showed that cellular survival was independent of the p53 status of those cells. In addition, in a host-cell reactivation assay using cisplatin-treated plasmid, we did not detect any difference in DNA repair capacity. We demonstrated that cisplatin-treated U138MG cells suffered fewer DNA double-strand breaks and DNA platination. Interestingly, the resistant cells carried higher levels of intracellular glutathione. Thus, preincubation with the glutathione inhibitor buthionine sulfoximine (BSO) induced massive cell death, whereas N-acetyl cysteine, a precursor of glutathione synthesis, improved the resistance to cisplatin treatment. In addition, BSO sensitized glioma cells to TMZ alone or in combination with cisplatin. Furthermore, using an in vivo model the combination of BSO, cisplatin and TMZ activated the caspase 3–7 apoptotic pathway. Remarkably, the combined treatment did not lead to severe side effects, while causing a huge impact on tumor progression. In fact, we noted a remarkable threefold increase in survival rate compared with other treatment regimens. Thus, the intracellular glutathione concentration is a potential molecular marker for cisplatin resistance in glioma, and the use of glutathione inhibitors, such as BSO, in association with cisplatin and TMZ seems a promising approach for the therapy of such devastating tumors.Malignant gliomas are the most common and aggressive type of primary brain tumor in adults. Current therapy includes surgery for tumor resection, followed by radiotherapy and/or concomitant adjuvant chemotherapy with temozolomide (TMZ) or chloroethylating nitrosoureas (CNUs). However, these protocols have limited success, and patients diagnosed with glioma have a dismal prognosis, with a median survival of 15 months and a 5-year survival rate of ~2%.¹ Several molecular mechanisms for cell resistance to these agents have been described. Because both are alkylating agents, the repair enzyme O₆-methylguanine-DNA methyltransferase (MGMT) is certainly a first barrier that is associated with increased tumor resistance.^{2, 3} The p53 status has also been proposed to act in an opposite manner in glioma cell resistance to TMZ or CNUs. Although p53 mutation is shown to be more resistant to TMZ treatment, owing to the induction of cell death,⁴ the p53 protein protects glioma cells after CNU treatment, most likely by improving other DNA repair systems.⁵Cisplatin is one of the most effective anticancer drugs and is used as a first-line treatment for a wide spectrum of solid tumors, such as ovarian, lung and testicular cancer,⁶ and it is used for adjuvant therapy in gliomas.⁷ Cisplatin is a molecule formed by one platinum ion that is surrounded by four ligands at the cis position: two chloride atoms and two amine molecules. The mechanism of action of cisplatin is mainly based on DNA damage. Once inside the cell, cisplatin becomes activated by the substitution of one or two chloride atoms by water, a process known as aquation. Owing to this process, the drug becomes positively charged and interacts with the DNA molecule, inducing the formation of DNA adducts. Activated cisplatin preferentially binds to purine bases in the nucleophilic N7 sites, where the majority of adducts occur between two guanines on the same strand, whereas ~3–5% of cisplatin adducts react with purines at the opposite strands, forming interstrand crosslinks (ICLs). The DNA lesions, in turn, trigger a series of signal-transduction pathways, leading to cell-cycle arrest, DNA repair and apoptosis.⁸Although relatively efficient, resistance to cisplatin, either intrinsic or acquired, during cycles of therapy is common, and overcoming tumor resistance remains the major challenge for cisplatin anticancer therapy. Cellular cisplatin resistance is a multifactorial phenomenon that may include decreased drug uptake, enhanced DNA repair capacity and higher glutathione (GSH) concentration.⁹GSH is a highly abundant, low-molecular-weight peptide in the cell, and it is well known for its critical importance in maintaining the cellular oxidative balance as a free radical scavenger. Additionally, GSH has a protective role against xenobiotic agents once its highly reactive thiol group binds and inactivates those agents. In fact, the GSH content and glutathione S-transferase (GST) have long been associated with cisplatin resistance in numerous cell lines and tumor tissues.^{9, 10, 11}Considering these possible pathways, it is not clear, however, which one determinates cisplatin resistance in glioma cells. Aiming to better understand the molecular mechanisms of resistance to this drug, four human glioma cell lines with different p53 status were investigated. We showed that cellular resistance was found to be independent of p53 as well as of the DNA repair capacity of the cells. On the other hand, the GSH levels within the cell were shown to act as a decisive resistance barrier to cisplatin, reducing the induction of DNA damage in the treated cells. Also, both in an in vitro and in vivo model depletion of GSH by an inhibitor (buthionine sulfoximine, BSO) sensitized the glioma cell lines to cisplatin. Interestingly, BSO also potentiated TMZ cytotoxicity. Thus, combination with BSO, cisplatin and TMZ turned out to be an extremely powerful approach to improve cytotoxicity in glioma, thus providing an exciting alternative for glioma treatment.     

Inhibition of Casein Kinase II by CX-4945, But Not Yes-associated protein (YAP) by Verteporfin,Enhances the Antitumor Efficacy of Temozolomide in Glioblastoma

Overcoming temozolomide (TMZ) resistance in glioma cancer cells remains a major challenge to the effective treatment of the disease. Increasing TMZ efficacy for patients with glioblastoma (GBM) is urgently needed because TMZ treatment is the standard chemotherapy protocol for adult patients with glioblastoma. O⁶-methylguanine-DNA-methyltransferase (MGMT) overexpression is associated with TMZ resistance, and low MGMT is a positive response marker for TMZ therapy. Here, we used 3 glioma cell lines (SF767, U373, and LN229), which had different levels of TMZ sensitivity. We found TMZ sensitivity is positively correlated with MGMT expression and multidrug-resistance protein ABC subfamily G member 2 (ABCG2) in these cells. CK2-STAT3 signaling and Hippo-YAP signaling are reported to regulate MGMT expression and ABCG2 expression, respectively. We combined CK2 inhibitor CX-4945 and YAP inhibitor verteporfin with TMZ treatment. We found that CX-4945 but not verteporfin can sensitize TMZ-resistant cells SF767 to TMZ and that CX-4945 and TMZ combinational treatment was effective for glioma treatment in mouse models compared with TMZ alone.ImplicationsA combination of CK2 inhibitor with TMZ may improve the therapeutic efficiency of TMZ toward GBM with acquired resistance.     

Tacrine derivatives stimulate human glioma SF295 cell death and alter important proteins related to disease development: An old drug for new targets

Glioblastoma is the most common and aggressive glioma, characterized by brain invasion capability. Being very resistant to the current therapies, since even under treatment, surgery, and chemotherapy with temozolomide (TMZ), patients achieve a median survival of one year. In the search for more effective therapies, new molecules have been designed. For nervous system cancers, molecules able to cross the blood-brain barrier are handled with priority. Accordingly, tacrine was chosen for this study and the inclusion of spiro-heterocyclic rings was done in its structure resulting in new compounds. Cytotoxic activity of tacrine derivatives was assayed using glioblastoma cell line (SF295) as well as analyzing cell death mechanism. Increased caspases activities were observed, confirming apoptosis as cell death type. Some derivatives also increased reactive oxygen species formation and decreased the mitochondrial membrane potential. Moreover, compounds acted on several glioblastoma-related proteins including p53, HLA-DR, beta-catenin, Iba-1, MAP2c, Olig-2, and IDH1. Therefore, tacrine derivatives presented promising results for the development of new glioblastoma therapy, particularly to treat those patients resistant to TMZ.     

O6-methylguanine DNA methyltransferase and p53 status predict temozolomide sensitivity in human malignant glioma cells

Temozolomide (TMZ) is a methylating agent which prolongs survival when administered during and after radiotherapy in the first-line treatment of glioblastoma and which also has significant activity in recurrent disease. O6-methylguanine DNA methyltransferase (MGMT) is a DNA repair enzyme attributed a role in cancer cell resistance to O6-alkylating agent-based chemotherapy. Using a panel of 12 human glioma cell lines, we here defined the sensitivity to TMZ in acute cytotoxicity and clonogenic survival assays in relation to MGMT, mismatch repair and p53 status and its modulation by dexamethasone, irradiation and BCL-X(L). We found that the levels of MGMT expression were a major predictor of TMZ sensitivity in human glioma cells. MGMT activity and clonogenic survival after TMZ exposure are highly correlated (p < 0.0001, r2 = 0.92). In contrast, clonogenic survival after TMZ exposure does not correlate with the expression levels of the mismatch repair proteins mutS homologue 2, mutS homologue 6 or post-meiotic segregation increased 2. The MGMT inhibitor O6-benzylguanine sensitizes MGMT-positive glioma cells to TMZ whereas MGMT gene transfer into MGMT-negative cells confers protection. The antiapoptotic BCL-X(L) protein attenuates TMZ cytotoxicity in MGMT-negative LNT-229 but not in MGMT-positive LN-18 cells. Neither ionizing radiation (4 Gy) nor clinically relevant concentrations of dexamethasone modulate MGMT activity or TMZ sensitivity. Abrogation of p53 wild-type function strongly attenuates TMZ cytotoxicity. Conversely, p53 mimetic agents designed to stabilize the wild-type conformation of p53 sensitize glioma cells for TMZ cytotoxicity. Collectively, these results suggest that the determination of MGMT expression and p53 status will help to identify glioma patients who will or will not respond to TMZ.     

A novel approach to overcome temozolomide resistance in glioma and melanoma: Inactivation of MGMT by gene therapy

Malignant glioma is the most common primary brain tumor. Malignant melanoma is the most malignant of skin tumor. The two malignancies are poorly responsive to conventional treatment regimens such as chemotherapy. Temozolomide (TMZ) is a DNA-alkylating agent used for the treatment of glioma, astrocytoma, and melanoma. Resistance to alkylating agents such as TMZ correlates with increased expression of DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT). Several studies in animal models have demonstrated that decreasing MGMT level with gene therapy could overcome TMZ resistance and enhance tumor cell death. In the present review, we provide an overview of recent advances in this field.     

Drug Resistance in Glioblastoma: A Mini Review

Glioblastoma multiforme (GBM) is recognized as the most common and lethal form of central nervous system cancer. Currently used surgical techniques, chemotherapeutic agents, and radiotherapy strategies have done very little in extending the life expectancies of patients diagnosed with GBM. The difficulty in treating this malignant disease lies both in its inherent complexity and numerous mechanisms of drug resistance. In this review, we summarize several of the primary mechanisms of drug resistance. We reviewed available published literature in the English language regarding drug resistance in glioblastoma. The reasons for drug resistance in glioblastoma include drug efflux, hypoxic areas of tumor cells, cancer stem cells, DNA damage repair, and miRNAs. Many potential therapies target these mechanisms, including a series of investigated alternative and plant-derived agents. Future research and clinical trials in glioblastoma patients should pursue combination of therapies to help combat drug resistance. The emerging new data on the potential of plant-derived therapeutics should also be closely considered and further investigated.