10 Combinations of Ionizing Radiation and Other Sensitizing Agents
Minesh P. Mehta
M. P. Mehta, MD
Department of Human Oncology, University of Wisconsin Hospital Medical School, 600 Highland Avenue, K4 312-3684, Madison, WI 53792, USA
10.1
Introduction
Several chapters in this book cover broad classes of radiosensitizing agents. Two specific agents, recently used as “radiation enhancers,” are addressed in the present chapter. They include the redox modulator motexafin-gadolinium (MGd), and the alkylating agent temozolomide. Each of these agents came to clinical testing through the recognition of unique preclinical radiosensitizing mechanisms and data that suggest enhancement of radiation cytotoxic- ity; they share the common theme of having been
clinically tested primarily in tumors of the central nervous system (CNS), in a series of recent clinical trials, primarily because although radiation therapy (RT) has a central role in managing the majority of primary and secondary CNS neoplasms, outcomes for most patients afflicted by these tumors remain poor, and furthermore, these neoplasms, in many ways, present an ideal opportunity for enhancing the efficacy of RT in combination with radiosensitiz- ers (Patel et al. 2004).
The major reasons for considering these neo- plasms as candidates for radiosensitization are as follows:
1. The majority of CNS neoplasms cause death due to local progression, thereby underscoring the need for local control (Wallner et al. 1989).
2. Evidence exists that improving local con- trol improves survival (Patchell et al. 1990;
Noordijk et al. 1994; Mintz et al. 1996; Andrews et al. 2004).
3. A dose-response relationship for RT has been established (Walker et al. 1979).
4. These neoplasms refl ect a rapidly growing popu- lation with high cell turnover in a milieu of slowly proliferating cells, thereby affording an opportu- nity for tumor-selective localization of several radiosensitizers (McGinn et al. 1996).
5. Linear (dose escalation linked to lengthening of treatment duration) radiation dose-escalation strategies have been limited by late normal tissue toxicity (Lee et al. 1999). Historically, several classes of radiation sensitizers, including S-phase halogenated pyrimidines (Phillips et al. 1995;
Prados et al. 1999), oxygen mimetics (Evans et al. 1990), and others (Mehta et al. 2001a), have been tested without clear evidence of clinical ben- efi t. Recently, agents with very different mecha- nisms of action have gained attention and are in clinical trials (Abraham et al. 1992; Rodrigus 2003; Mehta and Suh 2004). These agents, MGd, a redox modulator, and temozolomide, a DNA alkylator, are the focus of this chapter and each is addressed in sequence.
CONTENTS
10.1 Introduction 139
10.2 Motexafin–Gadolinium 140 10.2.1 Structure and Chemistry 140 10.2.2 Imaging Properties 140 10.2.3 Phase-I Results 140 10.2.4 Phase-II Results 141 10.2.5 Phase-III Results 141 10.2.6 Conclusion 143 10.3 Temozolomide 143
10.3.1 Structure and Chemistry 143 10.3.2 Mechanism of Action 143 10.3.3 Brain Metastases 144 10.3.4 Glioblastoma Multiforme 145 10.3.5 Conclusion 146
References 147
10.2
Motexafin–Gadolinium
10.2.1
Structure and Chemistry
Motexafin-gadolinium (MGd) is an expanded metal- loporphyrin, generally included in the family of compounds referred to as the texaphyrins (Fig. 10.1), and was previously referred to as gadolinium-texa- phyrin. Motexafin-gadolinium, a redox active drug, specifically targets tumor cells and enhances the radiation response in several preclinical models (Xu et al. 2001; Magda et al. 2001; Biaglow and Miller 2005). As an avid electron acceptor, it catalyzes the oxidation of key intracellular reducing metabolites.
These compounds, including glutathione, ascorbate, dihydrolipoate, protein thiols, and others, are neces- sary to maintain intracellular energy balance and play a key role in repairing cellular radiation injury.
Irreversible oxidation of these reducing metabo- lites diminishes the cellular capacity for radiation- induced DNA damage. Furthermore, MGd inhibits thioredoxin reductase, a key enzyme in restoring the intracellular pools of these reducing agents. The oxi- dation of these compounds by MGd not only causes intracellular bioenergetic disruption, but also gen- erates reactive oxygen species, a process known as futile redox cycling (Miller et al. 1999). As a con- sequence, cells irradiated in the presence of MGd are unable to adequately repair radiation damage, and cell death ensues (Donnelly et al. 1986).
10.2.2
Imaging Properties
It has been demonstrated that substitution of Gd by other lanthanides abrogates the radiation-sensitiz- ing properties of MGd (Rockwell et al. 2002). The presence of trace amounts of Gd in this molecule [in significantly lower concentrations than the amount of Gd in magnetic resonance imaging (MRI) contrast agents] permits visualization of the drug on MRI, thus providing tumor-specific kinetic information (Fig. 10.2). Several preclinical (Woodburn 2001; De Stasio 2001) and clinical (Kesslering et al. 1998;
Ford et al. 2003; Mehta et al. 2004) data have now confirmed this tumor-specific intracellular locali- zation and visualization phenomenon. Ford et al.
(2003) have effectively demonstrated the impact of dose and cumulative dose in this context through a phase-I trial in which intra-tumor Gd was estimated by co-imaging control vials containing known con- centrations of Gd and verifying these data by obtain- ing tissue specimens for actual measurements.
10.2.3
Phase-I Results
An initial single-dose phase-I trial established the maximum tolerated dose (MTD) to be 22.3 mg/kg with reversible acute renal failure as the dose-limit- ing toxicity (DLT) at 29.6 mg/kg (Rosenthal et al.
2000). A subsequent multicenter phase-IB/II trial tested ten daily MGd injections with cranial RT in patients with brain metastases. This study estab- lished the MTD to be 6.3 mg/kg day -1 for 10 days with reversible hepatic transaminitis as the DLT at 8.4 mg/kg day -1 for 10 days (Carde et al. 2001). Sub- sequent testing of this regimen identified 5.5 mg/
kg day -1 as the dose level associated with the least clinical toxicity and thus was established as the stan- dard for further testing in the phase-II and phase- III setting using the 10-day regimen (Mehta et al.
2002).
A biodistribution study of C14-labeled MGd in SMTF tumor-bearing mice injected with 10 mmol/
kg of drug demonstrated significant tumor uptake within 1 h, at which time the blood:tumor ratio is almost unity (Rodrigus 2003). Rapid and sub- stantial blood/plasma washout follows, but MGd is retained in tumors at high levels t5 h, thereby sub- stantially improving the tumor:blood ratio; there- fore, the 2- to 5-h window following MGd admin- istration is generally recommended for RT delivery.
Fig. 10.1. Structure of motexafi n–gadolinium (MGd)
N N
N N
N O
O
O O
G d
O O OC H
3
O O OCH3
c A A Oc O
More prolonged schedules have also been tested. A 22-dose (in 6 weeks) regimen in glioblastoma multi- forme (GBM) patients also receiving concurrent RT established the MTD at 5 mg/kg per day using a five- times-per-week schedule in weeks 1 and 2 and three times per week in weeks 3–6 (110 mg/kg cumulative dose; Mehta et al. 1999). The Childrens’ Oncol- ogy Group completed a phase-I trial in children with intrinsic pontine glioma testing up to 9.2 mg/
kg day -1 for a total of 30 fractions over 6 weeks with concurrent RT with the maximum safe dose for phase-II testing being 4.4 mg/kg day -1u30 (Mehta et al. 2001b). The dose-limiting toxicities at higher doses were transaminitis, hypertension, and rash.
The estimated median survival in the 44 enrolled patients is 313 days, with 95% confidence interval of 248–389 days. Redox modulation can also affect the effectiveness of chemotherapy and MGd synergy with several cytotoxic agents including bleomycin, doxorubicin, temozolomide, taxanes, platinoids, etc., as has been demonstrated in preclinical models (Miller et al. 2001). Clinical trials to determine the MTDs of these combinations are underway.
10.2.4
Phase-II Results
The most extensive phase-II data are available in brain-metastases patients receiving 10 doses of MGd at 5.5 mg/kg day -1 with cranial RT (3 Gyu10=30 Gy) (Carde et al. 1998). Sixty-one patients with brain metastases from various histologic types of primary
tumors were enrolled to this phase-Ib/II trial. In the evaluable patients, the overall response rate was 77%. The median survival was compared with a 528-patient database from Germany and a case- matched comparison was performed with the Radia- tion Therapy Oncology Group (RTOG) database. In both instances, a favorable survival trend for MGd- treated patients was noticed, and these data were utilized to estimate a potential clinical benefit in order to appropriately power a phase-III trial. The other phase-II data come from an analysis of this agent in GBM. Two trials, one a study from the Uni- versity of California at Los Angeles (UCLA), and the other, a multi-institutional trial, have now been conducted in this context (Suh et al. 2002; Manon et al. 2004). The most mature data are available from the UCLA trial, and in this 33-patient case-matched comparison study, a potential median survival gain of almost 6 months was identified.
10.2.5
Phase-III Results
Based on the results of the phase-II trial, a multi- center phase-III trial of cranial RT (30 Gy/10 frac- tions) for brain metastases with or without daily MGd (5.5 mg/kg day -1) was recently completed. Four hundred one patients, 251 with non-small cell lung cancer (NSCLC) and 75 with breast cancer, were enrolled. The study was conducted with two primary co end points, overall survival, and time to neuro- logic progression. The latter end point, measured
Fig. 10.2. The tumor specifi city and MR imaging capabilities of MGd in a patient with brain metas- tases. Both scans are non-contrast MR images. The image on the left is obtained prior to admin- istration of MGd and the image on the right is post-MGd
both by the investigator and by a blinded events- review committee (ERC), reflects one of the most unique aspects of this trial. In addition, prospective and ongoing assessment of neurocognitive function was carried out by a blinded consultant using a val- idated clinical battery designed to assess various neurocognitive functional domains. Although no significant differences were noted in overall sur- vival, both the investigator and the ERC assessment demonstrated significantly larger neurologic pro- gression free survival for the MGd-treated patients with NSCLC (Fig. 10.3; Mehta et al. 2003). This trial also provided a large database to analyze the safety spectrum of MGd. Overall, the drug was extremely well tolerated and permitted the delivery of 96%
of all intended RT fractions. The vast majority of toxicities were grade 1 or 2 in severity, and the most common was skin discoloration, consistent with the color of the drug. The most common grade-3 or greater toxicity was transient hypertension (in
<6% of patients, although an additional approxi- mately 27% of patients also experienced grade-1 or grade-2 hypertension). This transient hypertension occurred during or within 1 h of MGd administra- tion, and was in almost all cases self-limiting. In the majority of instances, no intervention was required.
It did not preclude drug or radiation administration in the majority of patients.
Although the first phase-III trial demonstrated improved neurologic progression-free survival in
NSCLC patients, this was not a pre-planned analy- sis and therefore was insufficient for regulatory approval. Consequently, a follow-up phase-III trial enrolling only NSCLC patients was recently com- pleted, enrolling just over 550 patients. The results of this trial were just becoming available at the time of this writing, and as per the formal corporate announcement, the primary end point of improve- ment in time to neurologic deterioration was not reached. The median time to neurologic progres- sion, as assessed by the ERC, was 10 months with whole-brain RT and 15.4 months with MGd added to whole-brain RT (p=0.122). Although this difference appears to be clinically important, it did not reach statistical significance, as the number of patients alive and evaluable for neurologic progression was small by 10 months; furthermore, as per the cor- porate announcement, when patients from France were excluded, for the remaining 348 patients, the median time to neurologic progression was 8.8 vs 24.2 months (p=0.004). The precise explanation for this geographic variance has not yet been provided.
Whereas the bulk of the clinical results available to date are from patients with brain metastases, MGd is being evaluated in several other diseases including GBM, primary NSCLC, cancer of the pancreas, head and neck malignancies, pediatric pontine glioma, etc. At the Annual Meeting of the American Society of Clinical Oncology (ASCO) in 2003, Ford et al. (2003) presented survival data from
Fig. 10.3. The KaplanMeier neurologic progression-free survival curve from the pre-specifi ed stratum of 251 patients with brain metastases from non-small cell lung cancer treated with MGd (solid upper line) or control (dotted lower line). D days, HR hazard ratio, M months, MGd motexafi n gadolinium, NR not reached, WBRT whole-brain radiation therapy. (From Mehta et al. 2003)
their phase-I/II GBM trial and compared these with a case-matched cohort from the RTOG database.
The database median survival was 12 months, and the observed median survival on the UCLA MGd cohort was 17.6 months, with a hazard ratio of 2.0.
These encouraging results have prompted the RTOG to pursue additional phase-II testing of this agent in GBM, in combination with temozolomide. The com- bination of MGd with other cytocidal agents repre- sents a novel research direction. Phase-I studies are currently underway, and the recent observation of synergy between temozolomide and MGd, both of which cross the blood-brain barrier and therefore could be combined in the treatment of brain tumors, is very exciting.
A "cutting-edge" application of MGd lies in its potential as an agent for gadolinium-neutron cap- ture therapy (GdNCT). Historically, boron-neutron capture therapy (BNCT) has been investigated in CNS tumors, with modest efficacy, limited in large part by the lack of significant tumor specificity of most boronated compounds. The cross-sectional area of interaction for neutrons with Gd is signifi- cantly greater than for boron, and recent work has demonstrated that >90% of the nuclei of GBM cells in culture localize MGd in the nucleus (De Stasio et al. 2005). Because the neutron-capture reaction produces Auger electrons with very limited path lengths, such extensive intranuclear localization is a prerequisite for further evaluation of GdNCT.
10.2.6 Conclusion
Motexafin-gadolinium represents one of the novel radiosensitizers that have undergone recent clinical evaluation. It has shown early, but tentative, promise, and some benefit in subset analysis. This agent is con- tinuing to be evaluated very actively in the clinic, and its precise roles remain to be adequately defined.
10.3
Temozolomide
10.3.1
Structure and Chemistry
Temozolomide, 3,4-dihydro-3-methyl-4-oxoimidazole [5,1-D-as-tetrazine-8-caroxamide] is a second-gen- eration oral alkylating prodrug that is spontaneously
hydrolyzed to its active metabolite, 5-(3-methyltri- azen-1yl)imidazole-4-carboximide (MTIC), under normal physiologic conditions, and has nearly 100% bioavailability, which can be slightly reduced by concomitant meals. It readily crosses the blood- brain barrier, producing cerebrospinal fluid concen- trations that are approximately 30% of plasma con- centrations (Reid et al. 1997; Patel et al. 2003).
10.3.2
Mechanism of Action
The primary cytotoxicity of temozolomide results from its ability to methylate DNA bases. Although several bases can and are methylated, the most ubiq- uitous methylation events involve guanine at the N-7 and O-6 positions and adenine at the N-3 posi- tion. Methylation of the O-6 position of guanine accounts for approximately 5–6% of the total meth- ylation events but is believed to represent the pri- mary mechanism of cytotoxicity (Fig. 10.4; Ludlum 1990; Denny et al. 1994). Under normal circum- stances, O-6 methylation is repaired by the enzyme methyl-guanine methyl transferase (MGMT), which demethylates guanine by accepting the methyl group onto itself, rendering the enzyme inactive (Fig. 10.5; Pegg et al. 1983; Pieper 1997). Repair of further methylation events requires de novo synthe- sis of additional MGMT. Recent data suggest that in several tumors there is epigenetic silencing of MGMT through promoter region methylation, thus resulting in sensitivity to temozolomide (Qian and Brent 1997; Watts et al. 1997; Jaeckle et al. 1998;
Estellar et al. 1999; Silber et al. 1999; Esteller et al. 2000; Hegi et al. 2004; Paz et al. 2004; Esteller
N3adenine
% 9
s e t i s r e h t O
% 6 1
O6 e n i n a u g
*
% 5
N7guanine
% 0 7
)
% ( s t c u d d a l a t o t e s a b / n o i t i s o P
Fig. 10.4. Distribution of DNA-methylation events from temo- zolomide by site
and Herman 2004). Repetitive drug administration would therefore likely result in MGMT exhaustion, and potentially sensitize the tumor to cytotoxicity from temozolomide (Tolcher et al. 2003). Addi- tional mechanisms to enhance the effect of temo- zolomide cytotoxicity have focused on the specific inhibition of MGMT, using the experimental agent O-6 benzylguanine, which appeared promising in preclinical studies, but produced unacceptable clinical toxicity, thereby resulting in drastic dose reduction of temozolomide, and effectively abolish- ing the therapeutic gain (Dolan et al. 1990; Schold et al. 2004; Wedge et al. 1996a,b; Chinnasamy et al. 1997). It is also becoming apparent that MGMT is not the only step in the repair/death process fol- lowing temozolomide exposure, and the mismatch repair system, as well as the p53 mutation status, may be significant (Karran and Marinus 1982;
Liu et al. 1996; Friedman et al. 1998; Tentori et al.
2001; Srivenugopal et al. 2001; Roos et al. 2004).
In limited preclinical studies, some synergy between temozolomide and radiation has been dem- onstrated, but the precise mechanisms for this have not yet been elucidated (Wedge et al. 1997; Van Rijn et al. 2000). Another study demonstrated that temo- zolomide has the capacity to inhibit the invasiveness of glioma cells in combination with RT. Radiation enhances the alpha-v/beta-3 integrin expression, which intensifies the invasiveness and migration of glioma cells. Temozolomide is able to inhibit these effects by promoting the cleavage of focal adhesion kinase (Wick et al. 2002). The clinical ability to dose temozolomide in a repetitive daily fashion has led to the testing of several combinations of the drug as a
putative “radiation enhancer” and RT, with the bulk of the experience coming from brain tumors. In this chapter, we focus on the data primarily as they relate to the combination of temozolomide with concur- rent RT.
10.3.3
Brain Metastases
In a randomized phase-II trial of 52 patients, Antonadou et al. (2002a) compared WBRT (40 Gy in 20 fractions) alone or with concomitant temo- zolomide (75 mg/m2 day -1) followed by six cycles of temozolomide (200 mg/m2 day -1 for 5 days every 28 days). The primary end points were radiologic response and neurologic functional status. The sec- ondary end points were overall survival, safety, and tolerability. The addition of temozolomide improved the response rate (96 vs 67%; p=0.017). The pro- portion of patients with improved function was greater in the group receiving temozolomide. The drug was well tolerated, with a significant increase in nausea and vomiting, but no difference in the rates of headache or fatigue. No grade-3 or grade-4 myleosuppression occurred. Overall survival with the addition of temozolomide was not significantly improved.
Another phase-II study randomized 82 patients to WBRT (30 Gy in ten fractions) alone or with con- comitant temozolomide (75 mg/m2 day -1) followed by two cycles of temozolomide (200 mg/m2 day -1 for 5 days every 28 days) beginning 4 weeks after WBRT (Verger et al. 2003). Temozolomide was well tolerated with no acute neurologic toxicity. No sig- nificant difference in radiologic response was found between the two arms. Progression-free survival at 90 days was significantly greater in those receiv- ing temozolomide (72 vs 54%; p=0.03). The rate of neurologic death was also significantly lower in the experimental arm (41 vs 69%; p=0.029), but there was no difference in overall survival.
There has been one phase-III study, randomizing 134 patients (82% with lung primaries) to WBRT (30 Gy in ten fractions) alone or with concomitant temozolomide (75 mg/m2 day -1) followed by six cycles of temozolomide (200 mg/m2 day -1 for 5 days every 28 days) beginning 4 weeks after WBRT.
Patients treated with temozolomide had a signifi- cantly greater response rate (53.4 vs 33.3%; p=0.039).
This improvement in response with temozolomide was even more pronounced in patients <60 years of age (76.7 vs 37.0%; p=0.003) as well as those with a
Fig. 10.5. Proposed mechanism of action of temozolomide.
The primary cytotoxic injury results from an inability to remove the methyl adduct from the O-6 position of guanine, persistence of which leads to futile DNA repair through an intact mismatch repair and possible apoptotic death through wild-type p53
KPS t90 (70.6 vs 32.4%; p=0.003). There was no sig- nificant difference in overall survival (Antonadou et al. 2002b).
10.3.4
Glioblastoma Multiforme
In a phase-II study, Stupp and colleagues (2002) administered temozolomide with concomitant RT, followed by adjuvant single-agent temozolomide, to 64 patients with newly diagnosed GBM. The RT consisted of 2 Gy fractions administered for 5 days each week for 6 weeks, for a total dose of 60 Gy. In combination with RT, temozolomide was given at a dose of 75 mg/m2 day -1. As adjuvant therapy, patients received six cycles of temozolomide 200 mg/m2 day -1 for 5 days every 4 weeks. Toxicities were mild, with grade-4 neutropenia occurring in <5% of patients during the induction (n=62) or adjuvant (n=49) phases. The median overall survival was 16 months, and the 2-year survival rate was 31%, suggesting an improvement over historical survival data. Although the study size was small, these impressive results prompted a phase-III evaluation of temozolomide and concurrent RT.
The randomized phase-III trial was carried out by the European Organization for Research and Treatment of Cancer (EORTC) and the National Cancer Institute of Canada (NCIC) and is described in detail in Chapter 12. An overview is shown in Figure 10.6. The study enrolled 573 patients from 15 countries, with a median age of 56 years. Patient
characteristics were well balanced between the two study arms (Stupp et al. 2005). With median follow- up of 28 months, the hazard ratio (HR) for death was 0.63 in favor of the chemoradiation therapy group, representing a 37% relative reduction in the risk of death (Table 10.1). The 2-year survival rates were 26.5 and 10%, respectively. A subset analysis demonstrated a significant benefit from the addition of temozolomide in GBM patients in most patient subgroups, with the exception of patients who had a performance status of two and those who underwent biopsy only vs resection.
Fig. 10.6. The EORTC/NCIC phase-III trial of radiation therapy with or without temozolomide in glioblastoma multiforme: treatment scheme
Table 10.1. The EORTC/NCIC phase-III trial of radiation therapy with or without temozolomide in glioblastoma: sur- vival results and toxicity
RT+temozolomide RT
Effi cacy n=287 n=286
Median overall survival 14.6 months 12.1 months Hazard ratio 0.63 (p<0.001)
Two-year overall survival 26.5% 10%
Median progression-free survival
6.9 months 5.0 months
Hazard ratio 0.54 (p<0.0001) Grade-3/4 toxicity
Neutropenia 7% 0
Thrombocytopenia 12% 0
Rash 2% 0
Infection 3% 2%
Fatigue/constitutional 8% 6%
RT radiation therapy
T
R 2Gy/daydays1-5for6weeks* e
d i m o l o z o m e
T 75mg/m2/daydays1-7 s
k e e w 6 r o f
: y b r e t a l s k e e w 4 d e w o l l o f
e d i m o l o z o m e
T 150-200mg/m2/day 5
- 1 s y a d
q4weeksx6cycles
y p a r e h t n o i t a i d a r
= T R : s n o i t a i v e r b b A
R A N D O M I Z E
3 7 5
= n
T
R 2Gy/daydays1-5for6weeks
y G 0 6 : e s o d l a t o T
* 4 e d a r G
a m o t y c o r t s a
r o h t i w
t u o h t i w
l a c i g r u s
n o i t c e s e r
Toxicity was higher with temozolomide, as expected, but both regimens were well tolerated (Table 10.1). There were no grade-3/4 hematologic toxicities in the RT-only group. In 284 patients who received temozolomide, the following grade-3/4 hematologic toxicities were observed: neutropenia in 21 patients (7%); leukopenia in 20 patients (7%); and thrombocytopenia in 33 patients (12%). During RT, 6 patients (2%) in the radiation therapy group and 9 patients (3%) in the combination group experienced severe infection. Moderate to severe fatigue was experienced by approximately 30% of the patients in both groups. The addition of temozolomide during RT did not appear to increase the rate of grade-3/4 rash (d1% in each group). During adjuvant therapy, 5 patients (2%; all from the temozolomide group) experienced grade-3/4 rash.
Two subsequent trials, one a phase-III study from Greece, and a second phase-II trial from Germany (Athanassiou et al. 2005; Combs et al. 2004), further corroborated these results, and in March 2005, the U.S. Food and Drug Administration granted approval to temozolomide for the first-line treatment of GBM in combination with RT. From a radiation-sensitizing perspective, the phase-II German trial is of particular interest, as it utilized a lower dose of temozolomide (50 mg/m2 day -1) during RT, without any adjuvant drug, and achieved comparable survival outcome, thereby leading to several questions specific to drug dose and scheduling.
Although the findings of EORTC/NCIC trial 22981/26981 showed an increase in survival in patients receiving concomitant and adjuvant temo- zolomide for GBM, a large proportion of these patients only marginally benefited from this regi- men. This observation might be explained by the role of MGMT. The gene that encodes for MGMT is located on chromosome 10q26. The loss of function of this gene is most often due to epigenetic changes, specifically promoter-region methylation, leading to lack of MGMT expression. Epigenetic silencing of the MGMT gene through promoter methylation has been found to lead to increased overall survival and better response to treatment with temozolomide and BCNU in patients with gliomas (Silber et al. 1999;
Esteller et al. 2000). To determine if MGMT gene promoter methylation status had an impact on sur- vival in the patients enrolled in EORTC 22981/26981, Hegi et al. (2005) evaluated the methylation status of the MGMT gene promoter region in patients with available and adequate specimens. Methyla- tion status was assessed for 206 of the 573 original
patients (36%) through methylation-specific poly- merase chain reaction (PCR). The MGMT promoter methylation was found in 45% of assessable patients.
Both treatment arms, RT alone and RT plus temo- zolomide, were equally represented in this sample population. Median overall survival, irrespective of treatment assignment, was increased in patients with methylated MGMT promoter regions compared with those with unmethylated MGMT promoter regions.
When treatment assignment was considered with MGMT promoter methylation status, a survival ben- efit from the addition of temozolomide to RT was seen only in patients with MGMT promoter meth- ylation (Table 10.2).
Table 10.2. Impact of MGMT promoter region methylation status on survival by treatment arm in the EORTC/NCIC phase-III trial
TMZ+RT RT Signifi cance (p)
Overall MS (months) 14.6 12.1 <0.001 MS of methylated GBM 21.7 15.3 0.007 MS of unmethylated GBM 12.7 11.8 0.06 MGMT methyl-guanine methyl transferase, TMZ temozolo- mide, RT radiation therapy, MS median survival, GBM glio- blastoma multiforme
A further research direction with significant implications on radiosensitization comes from the recognition that the cytotoxicity of temozolomide can be enhanced through inhibition of the enzyme poly-ADP-ribose polymerase 1 (PARP-1), and initial preclinical and clinical studies are showing prom- ising results. Furthermore, inhibition of PARP-1 also leads to radiosensitization, thereby creating an opportunity for three-way synergy (Tentori et al. 2002; Curtin et al. 2004; Calabrese et al. 2004;
Lapidus et al. 2005; Plummer et al. 2005).
10.3.5 Conclusion
Temozolomide has demonstrated modest activ- ity in patients with recurrent or newly diagnosed brain metastases from various malignancies. Recent trials of temozolomide with RT suggest a significant increase in response rates, especially for metasta- ses from lung cancer. Histology appears to have a bearing on the effect of temozolomide, as well as when it is used with RT. For example, in a phase-II study of dose-intense alternating weekly regimen
of temozolomide, the response rate was 24% for NSCLC, 19% for breast cancer, and 40% for mela- noma (Siena et al. 2003). This differential activity could putatively be associated with methylation of the promoter region of the MGMT gene which syn- thesizes MGMT, the enzyme responsible for repair- ing the lethal methylation lesion produced by temo- zolomide. It is therefore possible to consider a future treatment strategy with temozolomide, based on the MGMT promoter-region methylation status of the tumor. The frequency of this finding ranges from 36 to 42% in squamous and adenocarcinoma (of the lung), respectively (Furonaka et al. 2005). In GBM, the concurrent administration of temozolomide and RT has improved survival, with a large increase in patients with MGMT promoter-region methylation.
Consequently, its role in other gliomas, both low grade and anaplastic, is actively being explored.
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