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Journal of Clinical Oncology, Vol 26, No 25 (September 1), 2008: pp. 4189-4199 © 2008 American Society of Clinical Oncology. DOI: 10.1200/JCO.2007.11.5964
Correlation of O6-Methylguanine Methyltransferase (MGMT) Promoter Methylation With Clinical Outcomes in Glioblastoma and Clinical Strategies to Modulate MGMT Activity
From the Laboratory of Tumor Biology and Genetics, Department of Neurosurgery, Centre Hospitalier Universitaire Vaudois and University of Lausanne; National Center of Competence in Research Molecular Oncology, Lausanne; Department of Neurology, University Hospital Zurich, Zurich, Switzerland; Division of Hematology/Oncology, Case Western Reserve University, Cleveland, OH; Department of Oncology–Cancer Biology, Johns Hopkins University School of Medicine, Baltimore, MD; Department of Neuro-Oncology, UniversitätsKlinikum Heidelberg, Heidelberg, Germany; Department of Human Oncology, University of Wisconsin Medical School, Madison, WI; and the Department of Neuro-Oncology, The University of Texas M. D. Anderson Cancer Center, Houston, TX Corresponding author: Monika E. Hegi, PhD, Laboratory of Tumor Biology and Genetics, Department of Neurosurgery BH-19-110, Centre Hospitalier Universitaire Vaudois and University of Lausanne, CH-1011 Lausanne, Switzerland; e-mail: monika.hegi{at}chuv.ch
Resistance to alkylating agents via direct DNA repair by O6-methylguanine methyltransferase (MGMT) remains a significant barrier to the successful treatment of patients with malignant glioma. The relative expression of MGMT in the tumor may determine response to alkylating agents, and epigenetic silencing of the MGMT gene by promoter methylation plays an important role in regulating MGMT expression in gliomas. MGMT promoter methylation is correlated with improved progression-free and overall survival in patients treated with alkylating agents. Strategies to overcome MGMT-mediated chemoresistance are being actively investigated. These include treatment with nontoxic pseudosubstrate inhibitors of MGMT, such as O6-benzylguanine, or RNA interference-mediated gene silencing of MGMT. However, systemic application of MGMT inhibitors is limited by an increase in hematologic toxicity. Another strategy is to deplete MGMT activity in tumor tissue using a dose-dense temozolomide schedule. These alternative schedules are well tolerated; however, it remains unclear whether they are more effective than the standard dosing regimen or whether they effectively deplete MGMT activity in tumor tissue. Of note, not all patients with glioblastoma having MGMT promoter methylation respond to alkylating agents, and even those who respond will inevitably experience relapse. Herein we review the data supporting MGMT as a major mechanism of chemotherapy resistance in malignant gliomas and describe ongoing studies that are testing resistance-modulating strategies.
Alkylating agents, including the chloroethylnitrosoureas (carmustine [BCNU], lomustine, and fotemustine), procarbazine, and temozolomide, are commonly used to treat malignant brain tumors. These agents cause DNA damage by adding alkyl groups to DNA, which triggers DNA repair, thereby inducing apoptosis. Temozolomide is a methylating agent that modifies the DNA at several sites, most commonly N7-methylguanine and N3-methyladenine, which constitute nearly 90% of the total methylation events.1 However, these adducts are efficiently repaired by the base excision repair pathway and have a low cytotoxic potential. Only 5% to 10% of the methylation events mediated by temozolomide yield O6-methylguanine, but if the methyl group is not removed before cell division, these adducts trigger the DNA mismatch repair (MMR) pathway and are highly cytotoxic.2 O6-methylguanine methyltransferase (MGMT) is a cellular DNA repair protein that rapidly reverses alkylation (including methylation) at the O6 position of guanine,3 thereby neutralizing the cytotoxic effects of alkylating agents such as temozolomide. The protective effect of MGMT activity against the cytotoxic effects of alkylating agents has been demonstrated in human cell lines, xenograft models, and MGMT transgenic mice.4-9 High levels of MGMT activity in tumor tissue are associated with resistance to alkylating agents.7 In contrast, epigenetic silencing of the MGMT gene by promoter methylation results in decreased MGMT expression in tumor cells.10,11 Methylation of the MGMT promoter has been observed in a variety of tumor types,12 which is consistent with the observation that epigenetic silencing of genes is a common mechanism for inactivating tumor suppressor genes during malignant progression.13,14 Herein, we review the role of MGMT in conferring resistance to alkylating agents and strategies to modulate MGMT activity in malignant gliomas.
O6-methylguanine methyltransferase is ubiquitously expressed in normal human tissues.15 Most of our knowledge about MGMT as a DNA repair protein is based on observations after exposure to alkylating agents.5 In several ways, the MGMT-mediated repair process (Fig 1) is unique and differs from other DNA repair pathways; MGMT is not part of a repair complex but acts alone. It specifically removes the methyl group from the O6 position of guanine, thereby restoring the nucleotide to its native form without causing any DNA strand breaks, and it is a so-called suicide enzyme. On transfer of the alkyl group to an internal cysteine residue in the active site of MGMT, the enzyme is irreversibly inactivated, thus requiring de novo protein synthesis to maintain enzyme activity. In fact, the process is saturable; an excess of O6-methylguanine in the DNA can deplete MGMT. Although the O6 position of guanine is not the most common target of alkylating agents, the resulting promutagenic lesions act as an important trigger for cytotoxicity and apoptosis.16 Left unrepaired, this modified guanine preferentially pairs with thymine during DNA replication, which activates the MMR pathway. However, MMR only targets and corrects the newly synthesized daughter strand, leaving behind the O6-methylguanine in the template strand. It has been hypothesized that the MMR pathway, in an attempt to repair this mismatch, undergoes reiterative cycles of resynthesis and attempted repair (ie, futile cycling). This results in DNA double-strand breaks, thereby activating apoptotic pathways, leading to cell death.17,18 The essential role of the MMR pathway is illustrated by studies showing that cells deficient in both MGMT and the MMR pathway are 100 times more resistant to methylating agents.19 It has also recently been shown that treatment of glioblastoma (GBM) with temozolomide seems to select for inactivation of MMR as a result of mutation or loss of expression of the MMR-associated protein MSH6.20 Consequently, the highest therapeutic activity of alkylating agents is expected in tumor cells with low levels of MGMT and an intact MMR system.5 Given the central role of MGMT in resistance to alkylating agents and its unique properties, MGMT is an ideal potential target for biochemical modulation of drug resistance.21-23
Analytic Determination of MGMT Several methods for measuring MGMT levels within tumors have been described. The MGMT protein can be detected in tissue samples by immunohistochemistry,24 enzyme activity can be measured by high-performance liquid chromatography (HPLC),25,26 and epigenetic silencing of the MGMT gene by promoter methylation can be assessed using a methylation-specific polymerase chain reaction assay (MSP).27,28 Determination of MGMT activity by HPLC requires fresh tissue and is most suitable for quantitative studies in pure populations of cells.25,26 Immunohistochemical analysis has revealed heterogeneous MGMT expression patterns within tumors, often with regions of intense staining bounded by adjacent cells that lack expression.24,29,30 Tumor-infiltrating lymphocytes, microglia, and blood vessels may also express high levels of MGMT protein (Fig 2). Both HPLC and immunohistochemistry have been used to study the correlation between MGMT activity and drug resistance in cell lines and xenografts derived from a variety of human tumors.32 These studies have shown that MGMT expression varies widely in different tumor types and cell lines.
The MSP assay has been developed for determining the MGMT methylation status of CpG islands in the gene promoter (Fig 3).28,33 The MSP assay detects CpG island methylation with high sensitivity and specificity. This assay requires only small quantities of DNA and can be performed on DNA extracted from paraffin-embedded tissue samples; however, fixation may cause deterioration of the DNA. Therefore, best results are obtained with cryopreserved tumor specimens.
For diagnostic purposes in patients with glioma, one advantage of the MSP assay lies in the fact that detection of the methylated MGMT allele can be attributed solely to neoplastic cells.34 Therefore, nontumor tissue contamination of the surgical specimen does not interfere with the result. Furthermore, MGMT gene expression may be induced in tumor cells in response to DNA damage from alkylating agents or radiotherapy (RT) and also by corticosteroids,6,35 although this remains to be demonstrated in primary gliomas. Aberrant methylation of CpGs is a heritable change in the DNA that may be less susceptible to treatment-induced alterations. Recent reports have reviewed and discussed the different techniques for assessing MGMT and whether they can predict clinical outcome in patients with glioma.36-38 The correlation with outcome was variable throughout the studies analyzed, regardless of the techniques used; however, the overall interpretation of these results is limited as a result of much of the data coming from small series or cohorts of mixed glioma subtypes, use of different cut-offs (eg, for immunohistochemistry), and heterogeneity of treatment or complete lack of treatment information. The latter is highly relevant, because MGMT status is thought to predict benefit primarily from treatment with alkylating agents.33,39 Clearly, the methods useful for diagnostic purposes will have to be standardized and validated in prospective studies.40,41
MGMT Expression/Methylation and Clinical Outcome in Malignant Glioma
More recently, MGMT promoter methylation was analyzed in tissue samples from patients with newly diagnosed GBM treated within an international, randomized phase III trial (European Organisation for Research and Treatment of Cancer [EORTC] trial 26981-22981 and National Cancer Institute of Canada [NCIC] trial CE.3) comparing RT alone with RT plus concomitant and adjuvant temozolomide.33 Paraffin-embedded tissue samples were available for 307 patients (54%). High-quality DNA, allowing for analysis by MSP assay, could be extracted from two thirds of the available tissue, and 45% of this group were shown to have a methylated MGMT promoter. The MSP assay consisted of a two-stage polymerase chain reaction using nested primers.49 The results showed that MGMT promoter methylation was associated with improved overall survival in patients treated with RT plus temozolomide but not in patients initially treated with RT alone (Fig 5).33 Among patients whose tumors contained a methylated MGMT promoter, patients treated with RT plus temozolomide had a median survival of 21.7 months, compared with 15.3 months for patients assigned to RT alone. In patients treated with initial RT alone, MGMT promoter methylation was also associated with a slight improvement in survival among patients surviving beyond 9 months. This was expected because more than 70% of the patients in the RT arm received chemotherapy (most likely with an alkylating agent) at progression, and 60% of these patients received temozolomide at progression. When progression-free survival (PFS) was analyzed, thereby eliminating second-line therapy as a confounding factor, PFS was only prolonged in patients with a methylated MGMT promoter who were treated with RT plus temozolomide (Fig 6).33 In patients with a methylated MGMT promoter, median PFS was 10.3 months for patients treated with RT plus temozolomide compared with 5.9 months in the RT-only group (P = .001).
Given the important role of MGMT in tumor resistance to alkylating agents, a variety of MGMT inhibitors have been investigated with the goal of modulating chemoresistance (reviewed by McMurry50). O6-benzylguanine (O6-BG) is a potent MGMT-inactivating agent that has been studied in combination with alkylating agents. It inactivates MGMT stoichiometrically.51 However, O6-BG lowers MGMT levels in normal cells as well, increasing toxicity from chemotherapy. A dose-escalation study was conducted to determine the dose of O6-BG that would effectively suppress MGMT activity in anaplastic gliomas.52 Escalating doses of O6-BG were administered 6 or 18 hours before tumor resection, and MGMT activity was measured in snap-frozen tumor tissue. A dose of 120 mg/m2 given 6 hours before surgery inhibited MGMT activity in more than 90% of patients, whereas MGMT activity was inhibited in only 45% of patients who received O6-BG 18 hours before surgery (Table 2). Although this analysis did not take into account that MGMT activity may have been constitutively low as a result of promoter methylation in 30% to 50% of patients, these data suggest either that O6-BG is only able to transiently inhibit the enzyme or that resynthesis of MGMT can occur rapidly in tumor tissue.52 No toxic side effects were observed in this study. Importantly, this study illustrated how transient the reduction in MGMT activity may be with such a treatment approach.
Other studies have attempted to determine the maximum-tolerated dose of chemotherapy that can be administered in combination with O6-BG. One study showed that 120 mg/m2 of O6-BG inhibited MGMT activity to undetectable levels for up to 18 hours in the tumor, and O6-BG either alone or in combination with 13 mg/m2 of BCNU resulted in no serious side effects.26 However, in a subsequent phase I study, 100 mg/m2 of O6-BG increased hematologic toxicity when used in conjunction with escalating doses of BCNU (13.5, 27, 40, and 55 mg/m2).53 At the highest dose of BCNU (55 mg/m2), grade 3 to 4 thrombocytopenia and neutropenia became dose limiting. The maximum-tolerated dose of BCNU in combination with 100 mg/m2 of O6-BG was 40 mg/m2 administered at 6-week intervals. In the following phase II trial, 18 patients with recurrent or progressive nitrosourea-resistant malignant gliomas were treated with O6-BG and BCNU.54 No objective tumor responses were observed, but five patients had stable disease for 6 to 18 weeks. Again, 66% of the patients experienced grade 3 to 4 thrombocytopenia and neutropenia. These studies did not stratify patients based on the MGMT methylation status of their tumors. O6-(4-bromothenyl)guanine (lomeguatrib, PaTrin-2; KuDOS Pharmaceuticals, Cambridge, United Kingdom) is similar to O6-BG. Preclinical data demonstrated that lomeguatrib sensitized an ovarian tumor cell line to temozolomide,55 and the combination of lomeguatrib and temozolomide significantly improved inhibition of tumor growth in human melanoma xenografts.56 Lomeguatrib is also an oral agent and could be conveniently combined with temozolomide. No data on inhibition of MGMT in the tumor or any safety data on the combination of lomeguatrib and temozolomide are yet available. Furthermore, 2-amino-O4-benzylpteridine derivatives are in preclinical studies and have greater tumor specificity,57,58 which could counter the problem of hematologic toxicity associated with systemic application of other MGMT inhibitors. The cytotoxicity of BCNU in combination with O4-benzylfolates seems to be a function of the R-folate receptor, which is overexpressed in many tumor types.58 The compound O4-benzylfolic acid, which has the added advantage of greater water solubility, was shown to be 30 times more effective than O6-BG and was effective against the mutant P140K-MGMT that is resistant to O6-BG. Thus this class of compounds may have greater clinical potential.
Temozolomide and MGMT Depletion: Alternative Dosing Schedules Subsequent clinical trials with temozolomide have explored a variety of dosing schedules (Table 3) with the intention of maximizing MGMT depletion in tumor cells. The first alternative regimen tested was the extended daily regimen,59 which was subsequently used in combination with RT in the trials conducted by the EORTC. These regimens increase exposure to temozolomide over a 28-day cycle by approximately 1.5- to two-fold compared with the standard 5-day regimen, and the hope is that these alternative regimens will increase antitumor activity compared with the 5-day regimen without dramatically increasing hematologic toxicity. However, to date, there is no empiric evidence that this can be achieved in clinical practice.
This concept of enhanced MGMT depletion with alternative temozolomide dosing regimens has been rigorously tested by Tolcher et al.67 Patients received temozolomide at various doses and on two alternative dosing schedules: either 7 consecutive days of every 14 days (7 days on/7 days off) or 21 consecutive days of every 28 days (21/28-day schedule). Serial blood samples were taken to assess MGMT enzyme activity in PBMCs. This study demonstrated a time- and dose-dependent decrease in MGMT activity with both regimens. Continuous dosing for 7 days at a dose of 75 to 175 mg/m2 produced a rapid reduction from mean baseline MGMT activity (72% on day 8), which seemed to be fairly dose dependent. However, during the 7-day rest period, there was recovery of MGMT activity to approximately 55% of baseline (Fig 7).67 The 21-day continuous schedule at a lower daily dose (85 to 125 mg/m2) resulted in a similar reduction (approximately 70%) in mean MGMT activity by day 15, which was sustained through day 21.
This study established a benchmark against which to evaluate alternative dosing regimens and illustrated several important concepts with regard to MGMT depletion with temozolomide. The depletion of MGMT activity was a function of both the total cumulative dose and the area under the concentration-time curve.67 The daily and cumulative doses administered using a variety of dose-dense regimens are listed in Table 3. Higher doses of temozolomide seem to deplete MGMT levels in PBMCs more rapidly than lower doses. Administration of temozolomide for 21 consecutive days at a daily dose that resulted in a comparable cumulative dose per cycle as the 7-days-on/7-days-off regimen resulted in comparable depletion of MGMT activity at lower daily doses. However, this regimen seems to achieve more protracted MGMT depletion at least in PBMCs. On the other hand, the 7-days-on/7-days-off regimen allows for better recovery of MGMT in PBMCs, which may result in less hematologic toxicity. Currently, it is not known which of these schedules is more effective at depleting MGMT in tumor cells or which will strike a better balance between antitumor activity and hematologic toxicity. Although the Tolcher study provides the best rationale for protracted temozolomide schedules, there remain a number of important unanswered questions. In particular, it remains unknown how these schedules affect MGMT activity in brain tumor tissue. Studies reported by Spiro et al26,62 suggest that depletion of MGMT activity in PBMCs occurs at lower doses of O6-BG or temozolomide and is not a reliable predictor of MGMT depletion in tumor tissue.
A number of studies in patients with high-grade glioma have investigated alternative temozolomide dosing schedules and have begun to look at important clinical questions regarding the best treatment strategy in both the first-line and recurrent settings (Table 4).68-74
One particularly relevant question is whether concomitant RT plus temozolomide is sufficient to confer a survival benefit in patients with newly diagnosed GBM. This question is the object of an ongoing international Intergroup trial designed to establish the optimal sequence and relative contribution of the concomitant and adjuvant chemotherapy compared with RT alone in patients with anaplastic astrocytoma. More than 700 patients will be randomly assigned in the Concurrent and Adjuvant Temozolomide in Non-deleted Anaplastic Astrocytoma trial. One study in Greece has examined whether dose intensification of the maintenance regimen affects clinical outcome. In a randomized multicenter phase II trial, patients with newly diagnosed GBM received the standard regimen of concomitant RT plus temozolomide followed by temozolomide at a dose of 150 mg/m2 x 5 days every other week compared with RT alone.68 The results suggest that this dose-intensified maintenance regimen may improve PFS in the absence of significant hematologic toxicity.68 Patients treated with RT plus temozolomide had a median PFS of nearly 11 months, and 37% of patients had not yet experienced disease progression at 1 year. This was substantially better than the PFS achieved with RT plus temozolomide in the EORTC/NCIC trial (median, 7 months; 1-year PFS, 27%).75 However, this improvement in PFS did not seem to translate into a substantial improvement in overall survival; median overall survival was 13.4 months with RT plus temozolomide (compared with 14.6 months in the EORTC/NCIC trial75) and only 7.7 months in the RT-only arm.68 Although patient selection cannot be ruled out, the poor survival outcome, particularly in the control arm, may have been due to the small number of patients who received chemotherapy at recurrence. Nevertheless, this trial provides evidence of the safety and feasibility of a dose-intensified temozolomide maintenance regimen. A French phase II study also examined the effect of the 7-days-on/7-days-off regimen (150 mg/m2/d) as both neoadjuvant therapy before RT for up to four cycles (8 weeks) and adjuvant (ie, maintenance) therapy after RT until progression (up to eight cycles) in patients with inoperable tumors.76 This study provided information about antitumor activity in the neoadjuvant setting, although the median number of cycles was only three (range, one to eight cycles). Consistent with the previous attempts at neoadjuvant therapy for GBM, 25% of patients had a partial response, and 31% had stable disease. Median PFS in this group of patients with poor prognosis was 3.8 months, and median overall survival was 6 months. Hematologic toxicity requires careful monitoring; 24% of patients developed grade 3 or 4 thrombocytopenia, 14% had grade 4 granulocytopenia, and 14% had grade 4 lymphopenia. In addition, five patients developed interstitial pneumopathy, and six patients required dose reductions. This study also provided evidence that MGMT expression is correlated with response to temozolomide; despite dose intensification, patients with high levels of MGMT expression in their tumor tissue were unlikely to achieve response to treatment and were more likely to experience disease progression early. To date, the largest clinical experience with an alternative temozolomide regimen in the first-line setting is from the joint German-Swiss randomized phase III trial of the Neuro-oncology Working Group within the German Cancer Society (NOA-08).72 This trial is currently investigating the benefit of temozolomide (100 mg/m2 7 days on/7 days off) versus RT alone as a primary therapy for high-grade glioma until treatment failure and will enroll 340 patients older than 65 years. In the recurrent setting, several studies have investigated dose-dense regimens. Italian investigators studying temozolomide at 75 mg/m2 on a 21/28-day schedule reported grade 3 lymphocytopenia in one fourth of patients and an increased incidence of infections.73,76a,77 In our previous experience with temozolomide administered at 75 mg/m2 for 6 to 7 weeks concurrent with RT, up to 80% of patients developed profound lymphocytopenia, further promoted by the frequent administration of corticosteroids at diagnosis.65 A small Belgian phase II trial has also examined the 21/28-day schedule at a dose of 100 mg/m2 in 19 patients with recurrent anaplastic astrocytoma and anaplastic oligoastrocytoma.76a A report on the safety profile of this regimen indicated a high incidence of grade 3 and 4 lymphopenia in 10 and 9 patients, respectively. In addition, there were two suspected opportunistic infections: one herpes zoster reactivation during lymphopenia and one case of P carinii pneumonia after the patient had discontinued study treatment. Therefore, it seems that regular lymphocyte counts and prophylaxis against opportunistic infections may be required when using this regimen, particularly in the recurrent setting. Similar observations have been made in patients with melanoma who were treated with the daily schedule (75 mg/m2 x 6 to 7 weeks).65 The high frequency of lymphopenia in patients receiving prolonged daily schedules of temozolomide raises the question of whether more intermittent dosing might be better tolerated. The most compelling data come from a German study in 39 patients with recurrent GBM who were treated with 150 mg/m2 on the 7-days-on/7-days-off regimen, which was associated with a relatively low incidence of lymphopenia.70,71 This regimen produced an overall response rate of 9.5%, a 6-month PFS rate of 48%, and a median PFS of 21 weeks (approximately 5 months),70 which is superior to the data reported on the standard 5-day dosing regimen.78 These data have since been confirmed and extended.74 Importantly, in the more recent trial involving 90 patients, there was no significant difference in median PFS between patients with a methylated or unmethylated MGMT promoter at initial diagnosis.74 This suggests either that patients with a methylated MGMT promoter at diagnosis acquired resistance to temozolomide during progression20,45 or that this regimen may indeed overcome MGMT-mediated resistance, thereby extending time to progression in patients with an unmethylated promoter.
A variety of molecular markers may have prognostic value in patients with malignant glioma. These markers include high expression of MGMT, overexpression of the epidermal growth factor receptor, presence of the epidermal growth factor receptor vIII mutation, expression of the YKL-40 gene, tenascin expression, loss or mutation of the PTEN gene, loss of chromosome 10, and mutation or loss of the p53 gene.79-82 Although MGMT expression seems to have a strong influence on response to alkylating agents and clinical outcome in patients with GBM, to date none of these markers, including MGMT, has been definitively confirmed. An international group of investigators from the United States, Canada, and Europe is collaborating on a successor clinical trial to the EORTC/NCIC study. This trial, RTOG 0525, led by the Radiation Therapy Oncology Group Intergroup (RTOG) along with the EORTC and the North Central Cancer Treatment Group, will test whether increasing the dose-intensity of adjuvant temozolomide will improve survival compared with the standard regimen established by the EORTC/NCIC study.83,84 The study design is shown in Figure 8.83,84 The underlying hypothesis for this phase III trial is based on the laboratory finding that prolonged exposure of tumor cells to temozolomide results in reduced MGMT activity.85 The trial accrued 1,153 patients, and both clinical and molecular markers are being used to assign patients to the two treatment arms. Clinical factors included the validated subgroups identified by the recursive partitioning analysis of the RTOG, and patients in classes III, IV, and V were eligible for study entry. In addition, immediate tumor tissue sample submission was mandatory to prospectively assess MGMT promoter methylation status (using a quantitative MSP assay41), and patients were then stratified by MGMT promoter methylation status before randomization. This ensures a balanced number of patients with MGMT-methylated tumors assigned to the control and experimental arms, which will be critical for validation of MGMT promoter methylation as a molecular marker of response and/or prognosis. Results are expected in about 2 years.
Although differences in MGMT promoter methylation may determine the clinical course in patients with GBM treated with temozolomide, it is presently not recommended to use the MGMT promoter methylation assay to determine who should receive temozolomide and who should not.38 First, an independent confirmation of the retrospective analysis from the EORTC/NCIC trial is necessary. Second, there is reason to believe that alternative dose-intensified schedules may overcome resistance in patients with an unmethylated MGMT promoter.86 Third, allocating patients with GBM to specific treatments on the basis of MGMT promoter methylation status will only assume clinical relevance when effective alternative treatments become available. At present, the only established alternative is nitrosourea-based chemotherapy, which is also subject to resistance mediated by MGMT. A promising strategy to overcome resistance mediated by MGMT seems to be depletion of MGMT by prolonged exposure to low doses of alkylating agents. For these agents, the feasibility of intensified dosing schedules has been demonstrated. Optimizing this approach in conjunction with modulation of dosing schedules is of paramount importance to maximize the clinical effectiveness of temozolomide. However, myelosuppression continues to be dose limiting when MGMT depletion is maximized by dose intensification or the addition of MGMT inhibitors such as O6-BG. The development of tumor-specific MGMT inhibitors may overcome this limitation. This is an area of intense ongoing research, which will hopefully result in further improvements in clinical outcomes.
Although all authors completed the disclosure declaration, the following author(s) indicated a financial or other interest that is relevant to the subject matter under consideration in this article. Certain relationships marked with a "U" are those for which no compensation was received; those relationships marked with a "C" were compensated. For a detailed description of the disclosure categories, or for more information about ASCO's conflict of interest policy, please refer to the Author Disclosure Declaration and the Disclosures of Potential Conflicts of Interest section in Information for Contributors. Employment: N/A Leadership: N/A Consultant: Monika E. Hegi, Merck-Serono, Oncomethylone Sciences; James G. Herman, Oncomethylome Sciences; Roger Stupp, Schering Plough; Wolfgang Wick, Schering Plough; Michael Weller, Schering Plough Stock: N/A Honoraria: Monika E. Hegi, Schering Plough, Oncomethylone Sciences; Roger Stupp, Schering Plough; Michael Weller, Schering Plough; Mark R. Gilbert, Schering Plough, Genentech Research Funds: Monika E. Hegi, Funds, Schering Plough, Oncomethylone Sciences; Mark R. Gilbert, Funds, Schering Plough Testimony: N/A Other: N/A
Conception and design: Monika E. Hegi, Wolfgang Wick, Michael Weller, Minesh P. Mehta, Mark R. Gilbert Provision of study materials or patients: Monika E. Hegi, Wolfgang Wick, Minesh P. Mehta Collection and assembly of data: Wolfgang Wick, Mark R. Gilbert Data analysis and interpretation: Roger Stupp, Wolfgang Wick, Michael Weller, Mark R. Gilbert Manuscript writing: Monika E. Hegi, Lili Liu, James G. Herman, Roger Stupp, Wolfgang Wick, Michael Weller, Mark R. Gilbert Final approval of manuscript: Monika E. Hegi, James G. Herman, Roger Stupp, Wolfgang Wick, Michael Weller, Minesh P. Mehta, Mark R. Gilbert
We thank Jeff Riegel, PhD, ProEd Communications, Inc, for his medical editorial assistance with this manuscript.
Supported in part by Schering-Plough International (funding for medical editorial assistance). Authors’ disclosures of potential conflicts of interest and author contributions are found at the end of this article.
1. Trivedi RN, Almeida KH, Fornsaglio JL, et al: The role of base excision repair in the sensitivity and resistance to temozolomide-mediated cell death. Cancer Res 65:6394-6400, 2005 2. Liu L, Taverna P, Whitacre CM, et al: Pharmacologic disruption of base excision repair sensitizes mismatch repair-deficient and -proficient colon cancer cells to methylating agents. Clin Cancer Res 5:2908-2917, 1999 3. Pegg AE, Dolan ME, Moschel RC: Structure, function, and inhibition of O6-alkylguanine-DNA alkyltransferase. Prog Nucleic Acid Res Mol Biol 51:167-223, 1995[Medline] 4. Dumenco LL, Allay E, Norton K, et al: The prevention of thymic lymphomas in transgenic mice by human O6-alkylguanine-DNA alkyltransferase. Science 259:219-222, 1993 5. Gerson SL: Clinical relevance of MGMT in the treatment of cancer. J Clin Oncol 20:2388-2399, 2002 6. Kaina B, Fritz G, Mitra S, et al: Transfection and expression of human O6-methylguanine-DNA methyltransferase (MGMT) cDNA in Chinese hamster cells: The role of MGMT in protection against the genotoxic effects of alkylating agents. Carcinogenesis 12:1857-1867, 1991 7. Baer JC, Freeman AA, Newlands ES, et al: Depletion of O6-alkylguanine-DNA alkyltransferase correlates with potentiation of temozolomide and CCNU toxicity in human tumour cells. Br J Cancer 67:1299-1302, 1993[Medline] 8. Redmond SM, Joncourt F, Buser K, et al: Assessment of P-glycoprotein, glutathione-based detoxifying enzymes and O6-alkylguanine-DNA alkyltransferase as potential indicators of constitutive drug resistance in human colorectal tumors. Cancer Res 51:2092-2097, 1991 9. Hermisson M, Klumpp A, Wick W, et al: O6-methylguanine DNA methyltransferase and p53 status predict temozolomide sensitivity in human malignant glioma cells. J Neurochem 96:766-776, 2006[CrossRef][Medline] 10. Qian XC, Brent TP: Methylation hot spots in the 5' flanking region denote silencing of the O6-methylguanine-DNA methyltransferase gene. Cancer Res 57:3672-3677, 1997 11. Watts GS, Pieper RO, Costello JF, et al: Methylation of discrete regions of the O6-methylguanine DNA methyltransferase (MGMT) CpG island is associated with heterochromatinization of the MGMT transcription start site and silencing of the gene. Mol Cell Biol 17:5612-5619, 1997[Abstract] 12. Esteller M, Hamilton SR, Burger PC, et al: Inactivation of the DNA repair gene O6-methylguanine-DNA methyltransferase by promoter hypermethylation is a common event in primary human neoplasia. Cancer Res 59:793-797, 1999 13. Esteller M, Corn PG, Baylin SB, et al: A gene hypermethylation profile of human cancer. Cancer Res 61:3225-3229, 2001 14. Ting AH, Jair KW, Schuebel KE, et al: Differential requirement for DNA methyltransferase 1 in maintaining human cancer cell gene promoter hypermethylation. Cancer Res 66:729-735, 2006 15. Gerson SL, Trey JE, Miller K, et al: Comparison of O6-alkylguanine-DNA alkyltransferase activity based on cellular DNA content in human, rat and mouse tissues. Carcinogenesis 7:745-749, 1986 16. Ochs K, Kaina B: Apoptosis induced by DNA damage O6-methylguanine is Bcl-2 and caspase-9/3 regulated and Fas/caspase-8 independent. Cancer Res 60:5815-5824, 2000 17. Karran P: Mechanisms of tolerance to DNA damaging therapeutic drugs. Carcinogenesis 22:1931-1937, 2001 18. Roos WP, Batista LF, Naumann SC, et al: Apoptosis in malignant glioma cells triggered by the temozolomide-induced DNA lesion O6-methylguanine. Oncogene 26:186-197, 2007[CrossRef][Medline] 19. Stojic L, Brun R, Jiricny J: Mismatch repair and DNA damage signalling. DNA Repair (Amst) 3:1091-1101, 2004[CrossRef][Medline] 20. Cahill DP, Levine KK, Betensky RA, et al: Loss of the mismatch repair protein MSH6 in human glioblastomas is associated with tumor progression during temozolomide treatment. Clin Cancer Res 13:2038-2045, 2007 21. Bignami M, O'Driscoll M, Aquilina G, et al: Unmasking a killer: DNA O(6)-methylguanine and the cytotoxicity of methylating agents. Mutat Res 462:71-82, 2000[CrossRef][Medline] 22. Gerson SL: MGMT: Its role in cancer aetiology and cancer therapeutics. Nat Rev Cancer 4:296-307, 2004[CrossRef][Medline] 23. Pegg AE: Repair of O(6)-alkylguanine by alkyltransferases. Mutat Res 462:83-100, 2000[CrossRef][Medline] 24. Zaidi NH, Liu L, Gerson SL: Quantitative immunohistochemical estimates of O6-alkylguanine-DNA alkyltransferase expression in normal and malignant human colon. Clin Cancer Res 2:577-584, 1996[Abstract] 25. Dolan ME, Larkin GL, English HF, et al: Depletion of O6-alkylguanine-DNA alkyltransferase activity in mammalian tissues and human tumor xenografts in nude mice by treatment with O6-methylguanine. Cancer Chemother Pharmacol 25:103-108, 1989[CrossRef][Medline] 26. Spiro TP, Gerson SL, Liu L, et al: O6-benzylguanine: A clinical trial establishing the biochemical modulatory dose in tumor tissue for alkyltransferase-directed DNA repair. Cancer Res 59:2402-2410, 1999 27. Galm O, Herman JG: Methylation-specific polymerase chain reaction. Methods Mol Med 113:279-291, 2005[Medline] 28. Herman JG, Graff JR, Myohanen S, et al: Methylation-specific PCR: A novel PCR assay for methylation status of CpG islands. Proc Natl Acad Sci U S A 93:9821-9826, 1996 29. Möllemann M, Wolter M, Felsberg J, et al: Frequent promoter hypermethylation and low expression of the MGMT gene in oligodendroglial tumors. Int J Cancer 113:379-385, 2005[CrossRef][Medline] 30. Silber JR, Blank A, Bobola MS, et al: Lack of the DNA repair protein O6-methylguanine-DNA methyltransferase in histologically normal brain adjacent to primary human brain tumors. Proc Natl Acad Sci U S A 93:6941-6946, 1996 31. Godard S, Getz G, Delorenzi M, et al: Classification of human astrocytic gliomas on the basis of gene expression: A correlated group of genes with angiogenic activity emerges as a strong predictor of subtypes. Cancer Res 63:6613-6625, 2003 32. Liu L, Spiro TP, Qin X, et al: Differential degradation rates of inactivated alkyltransferase in blood mononuclear cells and tumors of patients after treatment with O6-benzylguanine. Clin Cancer Res 7:2318-2324, 2001 33. Hegi ME, Diserens AC, Gorlia T, et al: MGMT gene silencing and benefit from temozolomide in glioblastoma. N Engl J Med 352:997-1003, 2005 34. Shen L, Kondo Y, Rosner GL, et al: MGMT promoter methylation and field defect in sporadic colorectal cancer. J Natl Cancer Inst 97:1330-1338, 2005 35. Grombacher T, Mitra S, Kaina B: Induction of the alkyltransferase (MGMT) gene by DNA damaging agents and the glucocorticoid dexamethasone and comparison with the response of base excision repair genes. Carcinogenesis 17:2329-2336, 1996 36. Brell M, Tortosa A, Verger E, et al: Prognostic significance of O6-methylguanine-DNA methyltransferase determined by promoter hypermethylation and immunohistochemical expression in anaplastic gliomas. Clin Cancer Res 11:5167-5174, 2005 37. Maxwell JA, Johnson SP, Quinn JA, et al: Quantitative analysis of O6-alkylguanine-DNA alkyltransferase in malignant glioma. Mol Cancer Ther 5:2531-2539, 2006 38. Stupp R, Hegi ME: Methylguanine methyltransferase testing in glioblastoma: When and how? J Clin Oncol 25:1459-1460, 2007 39. Criniere E, Kaloshi G, Laigle-Donadey F, et al: MGMT prognostic impact on glioblastoma is dependent on therapeutic modalities. J Neurooncol 83:173-179, 2007[CrossRef][Medline] 40. Preusser M, Janzer R, Felsberg J, et al: Anti-MGMT immunohistochemistry in glioblastoma multiforme: Observer variability and lack of association with patient survival impede its use as clinical biomarker. Brain Pathol [epub ahead of print on April 8, 2008] 41. Vlassenbroeck I, Califice S, Diserens A-C, et al: Validation of real-time MSP to determine MGMT promoter methylation in glioma. J Mol Diagn 10:332-337, 2008 42. Chen ZP, Yarosh D, Garcia Y, et al: Relationship between O6-methylguanine-DNA methyltransferase levels and clinical response induced by chloroethylnitrosourea therapy in glioma patients. Can J Neurol Sci 26:104-109, 1999[Medline] 43. Esteller M, Garcia-Foncillas J, Andion E, et al: Inactivation of the DNA-repair gene MGMT and the clinical response of gliomas to alkylating agents. N Engl J Med 343:1350-1354, 2000 44. Jaeckle KA, Eyre HJ, Townsend JJ, et al: Correlation of tumor O6 methylguanine-DNA methyltransferase levels with survival of malignant astrocytoma patients treated with bis-chloroethylnitrosourea: A Southwest Oncology Group study. J Clin Oncol 16:3310-3315, 1998[Abstract] 45. Paz MF, Yaya-Tur R, Rojas-Marcos I, et al: CpG island hypermethylation of the DNA repair enzyme methyltransferase predicts response to temozolomide in primary gliomas. Clin Cancer Res 10:4933-4938, 2004 46. Belanich M, Pastor M, Randall T, et al: Retrospective study of the correlation between the DNA repair protein alkyltransferase and survival of brain tumor patients treated with carmustine. Cancer Res 56:783-788, 1996 47. Friedman HS, McLendon RE, Kerby T, et al: DNA mismatch repair and O6-alkylguanine-DNA alkyltransferase analysis and response to Temodal in newly diagnosed malignant glioma. J Clin Oncol 16:3851-3857, 1998 48. Hegi ME, Diserens A-C, Godard S, et al: Clinical trial substantiates the predictive value of O-6-methylguanine-DNA methyltransferase promoter methylation in glioblastoma patients treated with temozolomide. Clin Cancer Res 10:1871-1874, 2004 49. Palmisano WA, Divine KK, Saccomanno G, et al: Predicting lung cancer by detecting aberrant promoter methylation in sputum. Cancer Res 60:5954-5958, 2000 50. McMurry TB: MGMT inhibitors: The Trinity College-Paterson Institute experience, a chemist's perception. DNA Repair (Amst) 6:1161-1169, 2007[CrossRef][Medline] 51. Pegg AE: Mammalian O6-alkylguanine-DNA alkyltransferase: Regulation and importance in response to alkylating carcinogenic and therapeutic agents. Cancer Res 50:6119-6129, 1990 52. Schold SC Jr, Kokkinakis DM, Chang SM, et al: O6-benzylguanine suppression of O6-alkylguanine-DNA alkyltransferase in anaplastic gliomas. Neuro-oncol 6:28-32, 2004[Abstract] 53. Friedman HS, Pluda J, Quinn JA, et al: Phase I trial of carmustine plus O6-benzylguanine for patients with recurrent or progressive malignant glioma. J Clin Oncol 18:3522-3528, 2000 54. Quinn JA, Pluda J, Dolan ME, et al: Phase II trial of carmustine plus O6-benzylguanine for patients with nitrosourea-resistant recurrent or progressive malignant glioma. J Clin Oncol 20:2277-2283, 2002 55. Barvaux VA, Lorigan P, Ranson M, et al: Sensitization of a human ovarian cancer cell line to temozolomide by simultaneous attenuation of the Bcl-2 antiapoptotic protein and DNA repair by O6-alkylguanine-DNA alkyltransferase. Mol Cancer Ther 3:1215-1220, 2004 56. Middleton MR, Kelly J, Thatcher N, et al: O6-(4-bromothenyl)guanine improves the therapeutic index of temozolomide against A375M melanoma xenografts. Int J Cancer 85:248-252, 2000[Medline] 57. Nelson ME, Loktionova NA, Pegg AE, et al: 2-amino-O4-benzylpteridine derivatives: Potent inactivators of O6-alkylguanine-DNA alkyltransferase. J Med Chem 47:3887-3891, 2004[CrossRef][Medline] 58. Ladino CA, Chari RV, Bourret LA, et al: Folate-maytansinoids: Target-selective drugs of low molecular weight. Int J Cancer 73:859-864, 1997[CrossRef][Medline] 59. Brock CS, Newlands ES, Wedge SR, et al: Phase I trial of temozolomide using an extended continuous oral schedule. Cancer Res 58:4363-4367, 1998 60. Newlands ES, Blackledge GR, Slack JA, et al: Phase I trial of temozolomide (CCRG 81045: M&B 39831: NSC 362856). Br J Cancer 65:287-291, 1992[Medline] 61. Newlands ES, Stevens MF, Wedge SR, et al: Temozolomide: A review of its discovery, chemical properties, pre-clinical development and clinical trials. Cancer Treat Rev 23:35-61, 1997[CrossRef][Medline] 62. Spiro TP, Liu L, Majka S, et al: Temozolomide: The effect of once- and twice-a-day dosing on tumor tissue levels of the DNA repair protein O(6)-alkylguanine-DNA-alkyltransferase. Clin Cancer Res 7:2309-2317, 2001 63. Middleton MR, Lee SM, Arance A, et al: O6-methylguanine formation, repair protein depletion and clinical outcome with a 4 hr schedule of temozolomide in the treatment of advanced melanoma: Results of a phase II study. Int J Cancer 88:469-473, 2000[CrossRef][Medline] 64. Stupp R, Dietrich P-Y, Ostermann Kraljevic S, et al: Promising survival for patients with newly diagnosed glioblastoma multiforme treated with concomitant radiation plus temozolomide followed by adjuvant temozolomide. J Clin Oncol 20:1375-1382, 2002 65. Su YB, Sohn S, Krown SE, et al: Selective CD4+ lymphopenia in melanoma patients treated with temozolomide: A toxicity with therapeutic implications. J Clin Oncol 22:610-616, 2004 66. Ganière V, Christen G, Bally F, et al: Listeria brain abscess, Pneumocystis pneumonia and Kaposi's sarcoma after temozolomide. Nat Clin Pract Oncol 3:339-343, 2006; quiz following 343[CrossRef][Medline] 67. Tolcher AW, Gerson SL, Denis L, et al: Marked inactivation of O6-alkylguanine-DNA alkyltransferase activity with protracted temozolomide schedules. Br J Cancer 88:1004-1011, 2003[CrossRef][Medline] 68. Athanassiou H, Synodinou M, Maragoudakis E, et al: Randomized phase II study of temozolomide and radiotherapy compared with radiotherapy alone in newly diagnosed glioblastoma multiforme. J Clin Oncol 23:2372-2377, 2005 69. Chinot O, Barrié M, Cournède A, et al: Phase II study of temozolomide (TMZ) administered on a 7 days on-7 days off regimen as primary treatment before radiotherapy (RT) in inoperable newly diagnosed glioblastoma multiforme (GBM). J Clin Oncol 23:119s, 2005 (suppl; abstr 1523) 70. Wick W, Steinbach JP, Kuker WM, et al: One week on/one week off: A novel active regimen of temozolomide for recurrent glioblastoma. Neurology 62:2113-2115, 2004 71. Wick W, Weller M: How lymphotoxic is dose-intensified temozolomide? The glioblastoma experience. J Clin Oncol 23:4235-4236, 2005; author reply 4236 72. Hertie-Institut fur klinische Hirnforschung: Clinical trials: NOA-08—Temozolomide (one week on/one week off) versus radiotherapy for first-line therapy of anaplastic astrocytoma and glioblastoma in the elderly: A randomized phase III study (Methvsalem). Available at: http://www.hih-tuebingen.de/en/an/research0/neuro-onkologie/kls0/ 73. Tosoni A, Cavallo G, Ermani M, et al: Is protracted low-dose temozolomide feasible in glioma patients? Neurology 66:427-429, 2006 74. Wick A, Felsberg J, Steinbach JP, et al: Efficacy and tolerability of temozolomide in an alternating weekly regimen in patients with recurrent glioma. J Clin Oncol 25:3357-3361, 2007 75. Stupp R, Mason WP, van den Bent MJ, et al: Radiotherapy plus concomitant and adjuvant temozolomide for glioblastoma. N Engl J Med 352:987-996, 2005 76. Chinot OL, Barrie M, Fuentes S, et al: Correlation between O6-methylguanine-DNA methyltransferase and survival in inoperable newly diagnosed glioblastoma patients treated with neoadjuvant temozolomide. J Clin Oncol 25:1470-1475, 2007 76a. Neyns B, Chaskis C, Joosens E, et al: A multicenter cohort study of dose-dense temozolomide (21 of 28 days) for the treatment of recurrent anaplastic astrocytoma or oligoastrocytoma. Cancer Invest 26:269-277, 2008[CrossRef][Medline] 77. Brandes AA, Tosoni A, Cavallo G, et al: Temozolomide 3 weeks on and 1 week off as first-line therapy for recurrent glioblastoma: Phase II study from Gruppo Italiano Cooperativo di Neuro-Oncologia (GICNO). Br J Cancer 95:1155-1160, 2006[CrossRef][Medline] 78. Yung WK, Albright RE, Olson J, et al: A phase II study of temozolomide vs. procarbazine in patients with glioblastoma multiforme at first relapse. Br J Cancer 83:588-593, 2000[CrossRef][Medline] 79. Hill C, Hunter SB, Brat DJ: Genetic markers in glioblastoma: Prognostic significance and future therapeutic implications. Adv Anat Pathol 10:212-217, 2003[CrossRef][Medline] 80. Sansal I, Sellers WR: The biology and clinical relevance of the PTEN tumor suppressor pathway. J Clin Oncol 22:2954-2963, 2004 81. Smith JS, Tachibana I, Passe SM, et al: PTEN mutation, EGFR amplification, and outcome in patients with anaplastic astrocytoma and glioblastoma multiforme. J Natl Cancer Inst 93:1246-1256, 2001 82. Tanwar MK, Gilbert MR, Holland EC: Gene expression microarray analysis reveals YKL-40 to be a potential serum marker for malignant character in human glioma. Cancer Res 62:4364-4368, 2002 83. National Cancer Institute: Radiation therapy and temozolomide in treating patients with newly diagnosed glioblastoma or gliosarcoma. http://www.clinicaltrials.gov/ct/gui/show/NCT00304031?order=1 84. European Organisation for Research and Treatment of Cancer: Brain tumour group: Ongoing trials. http://groups.eortc.be/brain/html.trials.html 85. Gander M, Leyvraz S, Decosterd L, et al: Sequential administration of temozolomide and fotemustine: Depletion of O6-alkyl guanine-DNA transferase in blood lymphocytes and in tumours. Ann Oncol 10:831-838, 1999 86. Weller M, Steinbach JP, Wick W: Temozolomide: A milestone in the pharmacotherapy of brain tumors. Future Oncol 1:747-754, 2005[CrossRef][Medline] Submitted March 7, 2007; accepted March 4, 2008.
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Copyright © 2008 by the American Society of Clinical Oncology, Online ISSN: 1527-7755. Print ISSN: 0732-183X
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ABSTRACT
INTRODUCTION
