Prediction of response to temozolomide (TMZ)-based therapy by loss of MGMT expression in patients with advanced neuroendocrine tumors (NET)

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Background: TMZ, an oral alkylating agent, has recently been shown to be active in a subset of patients with NETs. In other tumor types, methyl-guanine methyl transferase (MGMT) expression is associated with TMZ resistance through its ability to remove methyl/alkyl groups from the O6 -position of guanine, thereby preventing TMZ-induced DNA damage. Methods: To better define NET pts who benefit from TMZ, we performed a retrospective analysis of 76 NET pts treated with TMZ-based regimens at 3 institutions, identified using IRB-approved observational and clinical trial databases. Immunohistochemical MGMT expression was measured in a subset of pts with available paraffin-embedded archival tissue specimens in a blinded fashion, utilizing mouse monoclonal antibodies to MGMT. Nuclear MGMT expression was scored as either present or absent in tumor cells and correlated with tumor type and treatment outcome. Results: Of 76 pts, 30 received TMZ/thalidomide and 46 received TMZ/bevacizumab. Both regimens incorporate oral TMZ, administered at a dose of 150 mg/m2 qd for 7d, every other week. 63 pts were enrolled on phase II studies, and 13 pts were treated outside a formal study protocol. Pt characteristics were: M:F=42:34; median age 57.5 (20–75); tumor type carcinoid (38), pancreatic NET (35), paraganglioma/pheochromocytoma (3). PR or CR (as measured by RECIST) was observed in 11/35 (31%) pancreatic NETs and 0/38 carcinoid pts (P<0.001), as well as 1/3 pts with paraganglioma/pheo. In 21 pts with available specimens, complete absence of MGMT expression was observed in 5/8 pancreatic NET and 0/13 carcinoid tumors (P<0.01). In this pt subset, absence of MGMT expression was associated with response to TMZ ( Table 1 ). Conclusions: Treatment with TMZ is associated with radiologic response in approximately one third of pts with pancreatic NET, but not in pts with carcinoid tumors. Absence of MGMT expression appears to define pts with pancreatic NET who achieve significant benefit from TMZ.

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No significant financial relationships to disclose.