Abstract P6-11-04: Profound prevention of experimental brain metastases of breast cancer by temozolomide in a MGMT-dependent manner

Purpose: Brain metastases of breast cancer cause neurocognitive damage and are incurable. We evaluated in experimental brain metastasis model a role of temozolomide, an oral brain permeable alkylating agent characterized by significant uptake in the central nervous system, in the prevention of brain metastases of breast cancer. Material and methods: To assess preventive role of temozolomide, mice were inoculated with 175,000 triple-negative 231-BR-EGFP cells in 0.1 mL PBS in the left ventricle of the heart. Three days after tumor cell inoculation, mice were randomized to temozolomide at the dose of 50 mg/kg delivered by oral gavage in saline, 5 days a week for 4 weeks, or vehicle. Subsequent experiments used temozolomide doses of 25, 10, 5, 1 and 0.5 mg/kg. To evaluate the efficacy of temozolomide in treating established BM, mice received temozolomide (50 mg/kg) beginning on either day 18 or day 24 post-injection of 231-BR-EGFR cells, 5 days a week for two and one week, respectively. To investigate the impact of temozolomide on survival, mice injected with 231-BR-EGFP cells were randomized to vehicle, temozolomide on days 3-14, or temozolomide on days 17-28 post-injection, per the schedule described above. To determine the functional contribution of MGMT expression in the BM preventive model, similar experiments were performed using 231-BR-EGFP cells with induced MGMT expression, and MGMT-positive Jimt-1 cells. Metastases were counted in step sections of one hemisphere of each brain. Additionally, the percentage of MGMT-positive tumor cells in 62 patient-matched sets of breast cancer primary tumors and resected brain metastases was determined immunohistochemically. Results: Temozolomide, when dosed at 50, 25, 10 or 5 mg/kg, 5 days/week, beginning 3 days after inoculation, completely prevented the formation of experimental brain metastases from MGMT-negative 231-BR-EGFP cell line. At a 1 mg/kg dose, temozolomide prevented 68% of large brain metastases, and was ineffective at a dose of 0.5 mg/kg. When the 50 mg/kg dose was administered beginning on days 18 or 24, temozolomide efficacy was reduced or absent. Both schedules of temozolomide (days 3-14 and days 17-28) significantly increased survival (P = .0003 by long-rank test). Earlier administration of temozolomide resulted in long term survival of 6 and 2 out of 10 mice, respectively; a significant difference compared to vehicle (P < .0001 and .0003, respectively).Temozolomide was ineffective at preventing brain metastases in the MGMT-positive 231-BR-EGFP and Jimt-BR3 sublines. In 62 patient-matched sets of primary breast tumors and resected brain metastases 43.5% of the specimens had concordant low MGMT expression, while in another 14.5% sets high MGMT staining in the primary tumor corresponded with low staining in the brain metastasis. Conclusions: Temozolomide profoundly prevents the outgrowth of experimental brain metastases of breast cancer in a MGMT-dependent manner. The majority of patients had low MGMT expressing brain metastases. These data provide a compelling rationale for investigating preventive efficacy of temozolomide in high-risk advanced breast cancer patients. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P6-11-04.