One in nine Australian women are diagnosed with breast cancer and nearly 3,000 will die of metastatic diseases annually, most commonly to bone, liver, lung and brain. Once brain metastases are established, prognosis for these patients is extremely poor, with a median survival of ~ 1-2 months if untreated. The limited efficacy of current systemic and targeted therapies against triple negative (ER-/PR-/HER2-) metastatic breast tumour is contributing to the increase in incidence of breast to brain metastases. We have developed a unique mouse model of triple negative breast cancer metastasis to brain (4T1Br4) that is suited for testing the efficacy of anti-metastatic drugs. Recent in vitro and animal studies have shown that brain permeable histone deacetylases (HDAC) inhibitors such as vorinostat enhance the radiosensitivity of brain metastatic breast cancer and reduce brain metastasis in experimental models.
Here, we demonstrated the superior inhibitory and radiosensitising properties of novel HDAC inhibitors (SB939 and 1179.4b) against 4T1Br4 brain metastatic cells.
Specifically, we report that SB939 and 1179.4b (IC50 = 600 and 90 nM, respectively) are significantly more potent than vorinostat (IC50 = 1.5 μM) at inhibiting 4T1Br4 cell proliferation. Importantly, inhibition by 1179.4b was 4.4-fold more selective for 4T1Br4 tumour cells compared to normal fibroblasts (IC50 ~ 400 nM). In a colony forming assay, vorinostat (1 μM) did not inhibit 4T1Br4 colony formation whereas at the same concentration, SB939 partially, and 1179.4b completely blocked colony formation. Similarly, SB939 or 1179.4b, even at 2.5 or 17 times lower concentrations than vorinostat, enhanced the effect of irradiation and the activation of γ-H2AX, a marker of DNA double strand breaks. Taken together, our results demonstrate the superior potency of SB939 and 1179.4b against brain metastatic cells and warrant further investigation in vivo.