Abstract LB-119: MGMT immunohistochemistry (IHC) improves patients’ selection for temozolomide (TMZ) treatment in advanced chemorefractory MGMT-methylated colorectal cancer

Abstract

Abstract Background: We showed that standard dose TMZ (150 mg/mq/day day1-5q28) is tolerable and active in 32 heavily pre-treated patients with advanced CRC and MGMT promoter methylation (Pietrantonio et al., Ann Oncol 2014). In a subsequent study, we included 32 pts treated with dose-dense TMZ (75 mg/mq/day day 1-21q28, for up to 6 cycles or until progression/unacceptable toxicity). Additional predictive markers are needed for improved selection of patients for TMZ therapy in metastatic CRC. Retained MGMT expression by IHC was proposed as marker for negative selection of pts with brain tumors. We conducted a retrospective analysis to assess MGMT expression in our 2 studies and to correlate this with the outcomes. Methods: All 64 patients included in the 2 studies were defined as MGMT methylation-positive according to methylation-specific PCR. A total of 40 patients had tissue available for IHC. Expression of nuclear MGMT protein was defined scored semiquantitatively according to extension and intensity. Using ROC analysis, IHC score <4 was considered as MGMT-low expression vs. IHC score 4-12 as MGMT-high. Extended RAS-BRAF mutations were assessed by Sanger sequencing. Results: Herein, we report for the first time the results of the dose-dense TMZ study. We obtained 4 confirmed partial responses and 2 stable disease, accounting for a response rate of 12% and a clinical benefit of 18%. Median PFS was 1.8 months. OS data are not mature. Thus, the results are comparable to those obtained in the previous study. MGMT-low expression was found in 15 (38%) samples and MGMT-high in 25 (62%). RAS-BRAF mutations were found in 28 (70%) pts and were not correlated with MGMT expression (p = 1). Response rate was significantly higher in patients with MGMT-low (53%) vs. those with MGMT-high (p<0.0001). Progression-free survival was significantly longer in MGMT-low vs. MGMT-high(5 vs. 2.3 months; p = 0.001). Further analyses are ongoing to validate MGMT IHC as a biomarker and to correlate it with quantitative gene methylation analysis. Conclusions: In this translational study on MGMT methylated CRC, absence of MGMT expression was significantly correlated with tumor response and benefit from TMZ treatment. Citation Format: Filippo De Braud, Filippo Pietrantonio, Maria Di Bartolomeo, Katia Fiorella Dotti, Claudia Maggi, Roberto Iacovelli, Marta Caporale, Rosa Berenato, Federica Perrone, Elena Tamborini, Alessio Pellegrinelli, Stefano Federici, Fabrizio Festinese, Giuseppe Pelosi, Ilaria Bossi, Paola Valentina Consonni, Susanna Maggi, Massimo Milione. MGMT immunohistochemistry (IHC) improves patients’ selection for temozolomide (TMZ) treatment in advanced chemorefractory MGMT-methylated colorectal cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-119. doi:10.1158/1538-7445.AM2015-LB-119