Does MGMT (O6-methylguanine–DNA methyltransferase) have a role in metastatic Ewing sarcoma (ES) patients (pts) undergoing temozolomide (TMZ) and irinotecan (IRI)?

11030 Background: TMZ+IRI has significant activity in metastatic ES. Epigenetic silencing of the MGMT DNA gene by promoter methylation has been associated with response to TMZ in glioblastoma. Our aim was to assess if MGMT methylation 1) has a role in ES progression and 2) is predictive of response to TMZ. Methods: 1) In 10 ES cell lines presence of MGMT gene (Real-time PCR), methylation of its promoter (methylation-specific PCR) and protein expression (western blot) were assessed. MGMT protein (IHC) and methylation of its promoter was searched in 97 ES pts samples (74 localized; 23 metastatic). 2) In metastatic ES pts treated with TMZ+IRI, with pre-treatment FFPE tissue and measurable disease, the relation between RECIST response, PFS and MGMT expression (IHC) was assessed. Results: 1) The expression of MGMT gene and its protein was detected and concordant (p = 0.02) in all ES cell lines evaluated; methylation was a rare event. In ES tissue samples the methylation of the MGMT gene was found at a low intensity as compared with the unmethylated gene, but the protein expression was relatively low: 36% in localized, 65% in metastatic pts (p 0.03). 2) 24 pts (median age 19 years, range 3-50 years; F/M: 7/17) treated with TMZ + IRI from 2010 to 2015 were identified. Line of treatment: 8 patients were in 1st line; 16 ≥ 2nd line. Median n of cycles was 6 (range: 2-31). Pattern of metastases: 16 multiple sites, 4 lungs, 3 multiple sites + bone marrow, 1 bone. MGMT was positive in 63% of cases. ORR: 16.5% (1 CR , 3 PR); SD: 50% (13 pts); PD: 33.5% (7 pts). According to MGMT expression the ORR was 11% in negative and 20% in MGMT positive patients (p = 0.8). 6-mos PFS rate was 59% (38-80 %IC), no differece according to MGMT expression (pos 61% vs neg 56%, p 0.7). Conclusions: Whereas in cell lines the MGMT gene and its protein expression is a generalized event, in tissue samples MGMT protein is present in a minority of localized pts, and might be associate with tumor progression. Methylation of MGMT gene does not seem responsible for its regulation in ES, and post-transcriptional mechanisms are more likely to be involved. The presence of MGMT protein does not predict the response to TMZ + IRI in this small series.