Abstract P6-12-09: Pan-HER, an antibody mixture with antitumor activity against drug-resistant HER2-overexpressing breast cancers with high ERBB ligand expression

Background: Amplification/overexpression of ERBB receptors and/or ligands has been associated with resistance to anti-HER2 therapies. Pan-HER is a mixture of six antibodies targeting each of the ERBB receptors, EGFR, HER2 and HER3, with synergistic pairs of antibodies. Each pair of antibodies simultaneously blocks ligand binding and/or induces target degradation, thus preventing compensatory mechanisms to anti-ERBB therapies. We examined the antitumor activity of Pan-HER against drug-sensitive and -resistant HER2+ breast cancer cells and xenografts. Results: Pan-HER exhibited potent growth inhibitory activity against a panel of HER2+ breast cancer cells (BT474, MDA-453, MDA-361, SUM190, HCC1954, UACC893 and SKBR3). Growth inhibition was associated with internalization and degradation of EGFR, HER2 and HER3. Pan-HER was superior to the combination of trastuzumab/pertuzumab (TP) against HER2+/PIK3CA mutant MDA-361, HCC1954, UACC893 and MDA-453 cells. We next compared the effect of Pan-HER against BT474, HCC1954 and MDA-361 xenografts established in nude mice to that of trastuzumab/lapatinib (TL), TP and T-DM1. All treatments were effective across the panel of xenografts. In mice with MDA-361 tumors, Pan-HER and TP were superior to TL. Immunoblot analysis showed significant downregulation of EGFR, HER2 and HER3 only in tumors treated with Pan-HER. After a complete response, treatment was discontinued. Among mice with BT474 xenografts treated with TP, TL and T-DM1, 25-50% of mice exhibited a tumor recurrence within 50 weeks of follow-up, while no recurrences were registered in mice treated with Pan-HER. Tumors recurring after TP and T-DM1 expressed significantly higher HER3 and P-HER3 protein levels and NRG1 mRNA levels. HCC1954 xenografts recurring after T-DM1 also overexpressed NRG1 mRNA compared to tumors before therapy. We next examined the effect of Pan-HER against trastuzumab-resistant HR6 (BT474) cells (Ritter et al. CCR 2007) and HCC1954 and UACC893 cells with acquired resistance to T-DM1 (TDR; IC50 >5-, >6- and 600-fold in HR6, UACC893-TDR and HCC1954-TDR cells, respectively, vs. parental cells). All T-DM1-resistant cells expressed significantly higher HER3 and P-HER3 protein levels and NRG1 mRNA and protein levels. Treatment with the HER3 neutralizing antibody LJM716 resensitized HR6 and HCC1954-TDR cells to T-DM1, suggesting a causal association between the NRG1-HER3 axis and drug resistance. Mice with HR6 tumors were treated with Pan-HER, TL, TP and T-DM1. Only Pan-HER arrested HR6 tumor growth and downregulated EGFR, HER2, HER3, P-HER3 and P-AKT. Finally, HCC1954-TDR tumors rapidly grew in vivo despite treatment with T-DM1. Administration of Pan-HER to mice bearing HCC1954-TDR xenografts growing in the presence of T-DM1, induced rapid tumor regressions. Conclusions: These data suggest that multitarget therapeutic interventions, such as Pan-HER, which simultaneously remove and/or block all ERBB receptors and ligands, are a feasible and effective approach against HER2-overexpressing cancers both sensitive and resistant to anti-HER2 therapies. Citation Format: Schwarz LJ, Hutchinson KE, Estrada MV, Sanders ME, Dugger TC, Formisano L, Guerrero AL, Red-Brewer M, Young CD, Lantto J, Pedersen MW, Kragh M, Horak ID, Arteaga CL. Pan-HER, an antibody mixture with antitumor activity against drug-resistant HER2-overexpressing breast cancers with high ERBB ligand expression [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P6-12-09.