Abstract P2-09-08: Therapeutic potential of the dual HER1/2 inhibitor AZD8931 in circumventing endocrine resistance
Authors: Morrison GD , Fu X , Shea M , Mitchell T , Giuliano M , Osborne CK , Klinowska T , Rimawi MF , Schiff R 
Published: August 2014

Background: Crosstalk between ER and HER-family signaling pathways has been suggested to play a role in the development of endocrine resistance. Preclinical studies have shown that AZD8931, a dual tyrosine kinase inhibitor (TKI) of HER1 (EGFR) and HER2, elicits an equipotent inhibition of HER1, HER2, and HER3 signaling, and is consequently more effective in blocking ligand-dependent HER signaling than the dual HER 1/2 TKI lapatinib. Using in vitro and in vivo models we investigated AZD8931's therapeutic potential in enhancing endocrine therapy and in overcoming the growth of tumor cells resistant to tamoxifen (Tam).

Materials and Methods: The effect of different TKIs (AZD8931 and lapatinib) on endocrine therapy [Tam and Fulvestrant (Ful)] was tested in ER+ MCF7 and T47D parental cells and their Tam-resistant derivatives (TamRes). In vitro growth, proliferation, and apoptosis were assessed using an in situ cell cytometer, EDU incorporation, and Annexin V-FITC/c-PARP, respectively. HER ligands in the parental and TamRes cells were profiled using RNASeq analysis of these lines. Western blot analysis was used to analyze the effect of AZD8931, lapatinib, and the HER1 TKI gefitinib on HER 1/2 pathway activation upon ligand stimulation. Nude mice with transplantable MCF7/TamRes xenografts at 200 mm3 were randomized to continued Tam, Tam+AZD8931, Fulvestrant (Ful), or Ful+AZD8931 treatments.

Results: We found that neither lapatinib nor AZD8931 significantly enhanced endocrine sensitivity in parental MCF7 breast cancer cells, although AZD8931 did enhance endocrine sensitivity in parental T47D cells. Furthermore, AZD8931 combined with either Tam or Ful inhibited cell growth more profoundly than lapatinib in the T47D TamRes cell model, and was significantly, though modestly, more potent than lapatinib in the MCF7 TamRes model when combined with Tam. In both TamRes models, AZD8931 significantly inhibited cell proliferation and induced apoptosis, with the highest effects seen in combination with Ful. Interestingly, multiple HER ligands are upregulated in both MCF7 and T47D TamRes models, which could explain the superiority of AZD8931 over lapatinib in these models. Indeed, in EGF and heregulin (HRG) stimulated conditions, AZD8931 more potently inhibited HER signaling (i.e., phosphorylated (p) levels of HER1/2, MAPK, and AKT) than lapatinib or gefitinib. Finally, AZD8931 significantly delayed the growth of MCF7 TamRes xenografts in the presence of continued Tam or Ful. These tumors were also inhibited by Ful alone, but the strongest inhibition was achieved by Ful and AZD8931 in combination. However, despite the marked tumor growth delay, no tumor regression was found in any of these treatments.

Conclusion: This study provides evidence that AZD8931 has greater inhibitory efficacy than lapatinib in endocrine resistant models that are dependent on ligand activation of the HER pathway. Although the AZD8931 combination with Ful robustly slowed growth of TamRes tumors in vivo, the absence of tumor regression suggests that additional escape pathways are also involved and should also be targeted to fully circumvent tamoxifen resistance.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P2-09-08.