MGMT is frequently inactivated in pancreatic NET-G2 and is associated with the therapeutic activity of STZ-based regimens

O6-methylguanine-DNA methyltransferase (MGMT) has been linked with alkylating agent resistance and tumor growth suppression. However, its role remains undetermined in pancreatic neuroendocrine tumors (Pan-NET). The MGMT expression was examined by immunohistochemistry in 142 patients to evaluate MGMT immunoreactivity and clinicopathological factors. We analyzed the relationship between MGMT expression and treatment efficacy in 19 patients who received STZ-based regimens. In 142 Pan-NET, 97 cases (68.3%) were judged as MGMT-positive and 45 cases (31.6%) as negative. MGMT negativity was significantly more common in NET-G2 (62.5%) than in NET-G1 (11.2%, p < 0.001). MGMT-negative cases were associated significantly with larger tumor size (p < 0.01), higher Ki-67 index (p < 0.01), higher mitotic index (p < 0.05), and more frequent liver metastasis (p < 0.05). Of the 19 cases treated with STZ, 6 cases were determined as SD and 4 cases as PD in MGMT-positive patients (N = 10), while 5 cases were determined as PR and 4 cases as SD in MGMT-negative patients (N = 9). Progression-free survival in MGMT-negative cases was significantly better than in MGMT-positive cases (p < 0.05). MGMT expression was lower in NET-G2 than in NET-G1, and STZ-based regimens improved the therapeutic outcomes of MGMT-negative Pan-NET. These findings indicate that NET-G2 may represent a better therapeutic target for STZ treatment.