Abstract 1242: Combination of p21-activated kinase 1 (PAK1) inhibitor FRAX1036 and Aurora-A inhibitor alisertib is effective in hormone receptor-positive breast cancer

Estrogen receptor alpha (ERα) signaling plays a critical role in the development and progression of hormone receptor-positive (HR+) breast cancer. One mechanism of tumor progression of HR+ breast cancer is phosphorylation of ERα by non-receptor protein kinases, leading to ligand-independent activation of ERα. Mitotic kinases Aurora A (AURKA) and p21-activated kinase 1 (PAK1) are frequently overexpressed in breast cancer and can phosphorylate ERα on serine 305, increasing the ability of ERα to induce transcription of target genes implicated in tumorigenesis. In addition, PAK1 phosphorylates and activates AURKA and its functional partners, suggesting possible synergistic activity in tumors overexpressing both kinases. We tested the hypothesis that combined inhibition of AURKA and PAK1 would synergistically suppress tumor growth of HR+ breast cancer cells in vitro and in vivo. Treatment of T47D HR+ breast cancer cell line with the PAK1 inhibitor FRAX1036, combined with AURKA inhibitor alisertib, improved cell killing in comparison to single agents. FRAX1036 and alisertib were then evaluated in an orthotopic xenograft model using BT474 HR+ breast cancer cell line. Ten million BT474 cells were implanted into the mammary fat pads of 6-week old NSG mice simultaneously with implantation of estrogen pellets to facilitate growth of xenografts. Mice received 21 days of treatment with FRAX1036 20mg/kg daily and alisertib 10 or 15 mg/kg BID, alone or in combination. We observed exceptional inhibition of tumor growth in the combination treatment groups. Combination of FRAX1036 20mg/kg daily with alisertib 15mg/kg BID was the most effective, with partial or near-complete responses that continued through 21 days of treatment. This combination resulted in significantly better inhibition of tumor growth than alisertib 15mg/kg BID alone or FRAX1036 20mg/kg daily alone (p = 0.003 and p = 1.29e-6, respectively). Similarly, combination treatment of FRAX1036 20mg/kg daily with alisertib 10mg/kg BID was significantly better then alisertib alone or FRAX1036 alone in the corresponding doses (p = 0.002 and p = 3.52e-7, respectively). Analysis of H&E stained sections and Ki-67 proliferation index of BT474 xenograft tumors indicated a highly significant decrease in viable tumor cells and replacement by stroma after combination treatment compared to treatment with single agents or with control. Caspase-3 IHC analysis showed significantly greater proportion of apoptotic cells per tumor