We hypothesized that complete inhibition of HER3 is required for the optimal action of HER2 or PI3K/Akt inhibitors against HER2+ tumors. The HER3 monoclonal antibody LJM716 (LJM) inhibits ligand-induced and ligand-independent dimerization of HER3. Treatment with LJM inhibited P-HER3 and P-Akt in a panel of HER2+ breast cancer cell lines. In BT474 and SKBR3 cells, LJM inhibited HER2-HER3 dimers as assessed by coprecipitation of HER2 with HER3 antibodies. Using a DUAL antibody proximity base assay, formalin-fixed sections of HER2+ xenografts treated with LJM showed a reduction of HER2-HER3 dimers and P-HER3 compared to untreated controls. Treatment with LJM alone also induced regression of established BT474 xenografts. Mice with BT474 xenografts were treated with lapatinib and trastuzumab (Lap/T), LJM and trastuzumab (LJM/T) or lapatinib, trastuzumab and LJM716 (Lap/T/LJM). After 3 weeks of treatment, mice from the 3 groups exhibited nearly complete tumor regression (volume <25 mm3). At 28 weeks of follow-up with no treatment, 8/12 mice in the Lap/T group and 5/7 mice in the LJM/T exhibited tumor regrowth (≥200 mm3), whereas only 2 of 15 mice in the Lap/T/LJM group did so (p=0.0046, Log-rank test).

We expanded our studies to examine the combination of LJM716 with the p110α (PI3Kα) specific inhibitor BYL719 (BYL). LJM in combination with BYL inhibited growth in monolayer and 3-D Matrigel and S473 P-Akt in HER2+ breast cancer cells with mutations in the PI3K pathway better than either single agent. MDA-MB-453 xenografts regressed completely after 3 weeks of therapy with BYL/LJM whereas either single agent inhibited xenograft growth only partially. All mice bearing BT474 xenografts treated with T/LJM, T/BYL, LJM/BYL or T/LJM/BYL exhibited tumor regression after 24 days of treatment. Twenty weeks after treatment discontinuation, only 1/8 mice in the T/LJM/BYL exhibited tumor re-growth whereas >80% of mice in all other groups did so (p=0.0009, Log-rank test). Finally, we hypothesized that LJM would be at least equivalent to the HER2 antibody pertuzumab (P) in combination with T against HER2+ tumors. Inhibition of growth in vitro and P-Akt were superior with LJM/T compared to P/T in vitro. Mice bearing BT474 xenografts treated with either LJM/T or P/T had a similar nearly complete tumor regression after 4 weeks of treatment. We are currently monitoring potential tumor regrowth in these mice.

In summary, treatment with LJM inhibited HER2-HER3 dimers, P-HER3 and P-Akt in HER2+ breast cancer cells with PI3K pathway mutations. As a single agent, LJM markedly inhibited HER2+ xenograft growth. Treatment with LJM in combination with Lap/T improved survival of mice with HER2+ xenografts compared to Lap/T. Further, LJM sensitized breast cancer cells and xenografts to a p110α-specific inhibitor. Finally, the HER3 antibody in combination with T inhibited PI3K/Akt and xenograft growth as well as the combination of P/T.

Citation Format: Joan T. Garrett, Cammie R. Sutton, Carl Uli Bialucha, Seth A. Ettenberg, Jerry Wallweber, Lisa DeFazio-Eli, Carlos L. Arteaga. A HER3 antibody that blocks ligand-independent HER2-HER3 dimerization sensitizes to HER2 and PI3K inhibitors . [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5461. doi:10.1158/1538-7445.AM2013-5461