Synergistic Effect of Lapatinib and the Class 1 HDAC Inhibitor SNDX-275 in Breast Cancer.

Abstract Background: Inflammatory breast cancer (IBC) is a rare but aggressive form of primary breast cancer with high metastasis rates and poor survival outcomes in patients. Currently, no specific targeted therapy is available to improve patient outcomes, although agents (i.e. trastuzumab and lapatinib) targeting the human epidermal growth factor 2 (HER2) have shown promise in clinical trials. Histone deactylases (HDACs) represent another family of proteins for which inhibitors have been clinically validated and shown to inhibit proliferation of breast cancer cells in vitro and in vivo. In these studies we determined the single agent activity of the class 1 selective HDAC inhibitor entinostat (SNDX-275) in IBC cell models and whether SNDX-275 was synergistic with the HER2 targeted agent lapatinib.Methods: SNDX-275 activity was evaluated in SUM190, SUM149 and KPL-4 IBC cell lines using standard proliferation assays and compared to the non-IBC cell lines MDA-MB-231, SKBr3 and MCF-7. Apoptotic activity and cell cycle analysis were analyzed. SNDX-275 combination with lapatinib was initially determined in vivo in a HER2+ breast cancer model and subsequently in the SUM190, SUM149, KPL-4 IBC cells. For xenograft studies, athymic nude mice bearing human breast (BT474) tumor xenografts were treated with SNDX-275 at 15 or 30 mg/kg/day and lapatinib at 30 mg/kg/ 2xday or 75 mg/kg/ 2xday.Results: Significant anti-proliferative activity of SNDX-275 was observed in IBC (IC50, 250–500 nM) when compared with the non-IBC breast cancer cell lines MDA-MB-231, SKBr3, and MCF-7 (IC50 2–5 mM). Cell cycle analysis showed the onset of apoptosis in IBC cell lines (10%-17%); in the non-IBC cell lines, very little apoptosis occurred (0.8%–3.1%), although G1 stage arrest was seen in the non-IBC cell lines MDA-231 and MCF-7. The SNDX-275–induced apoptosis in IBC cell lines was dependent on caspase 9 rather than Caspase 8 cleavage indicating that the intrinsic apoptotic pathway is activated. The experiments with lapatinib demonstrated a significant benefit of the SNDX-275/lapatinib combination in both the BT474 xenograft study as well as the IBC cell lines tested. In the animal group that was treated with 15 mg/kg SNDX-275 plus 75 mg/kg lapatinib, synergistic effects were observed with tumor regression that was continued at least for 4 weeks after treatment was stopped. Similarly, synergistic anti-proliferative activity was found in almost all (4 of the 5) cell lines tested (SUM190, SUM149, KPL-4, and BT474). Investigation into the mechanism of SNDX-275–mediated apoptosis and the combined effects of lapatinib and SNDX-275 in IBC are under way. Our data demonstrate that HDACi as single agents and particularly in combination with HER2 targeted agents represent a promising new approach for clinical development in IBC breast cancer and patients with HER2-overexpressing breast cancer. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 3135.