In this study, we have evaluated the in vitro
and in vivo
efficacy of a new type of targeted therapy, an anti-ErbB3 antibody, MM-121. ErbB3 has emerged as a critical partner in EGFR, HER2, and MET oncogene addicted cancers. In particular, its many tyrosine phosphorylation sites serve as potent modules to activate intracellular signaling, especially the PI3K pathway (21
). In many EGFR and HER2 driven cancers, ErbB3 tyrosine phosphorylation is necessary for transmittal of these downstream signaling events, and treatment with the appropriate TKI leads to loss of ErbB3 phosphorylation and cessation of downstream signaling. Recent studies have revealed that reactivation of ErbB3/PI3K signaling is a major mechanism of acquired resistance to EGFR and HER2 inhibitors (5
). In particular heregulin-induced activation of ErbB3 may cause resistance to gefitinib (10
). For these reasons, there has been great enthusiasm for targeting ErbB3 directly as a therapy. However, unlike other ErbB family members, ErbB3 is kinase dead (11
). Thus, it seems that antibody therapies directed against the extracellular domain of ErbB3 appear to be the most effective method to disrupt its function.
The in vivo
activity of MM-121, an anti-ErbB3 antibody, was assessed in this study. This antibody was chosen for clinical development because it potently blocks ligand-dependent activation of ErbB3 (, and Figs. S4, S5
). In this study, we also provide evidence that MM-121 blocks ligand binding and leads to receptor internalization and degradation. Interestingly, although the antibody blocks ligand-dependent activation of ErbB3 phosphorylation in all cell lines examined, antibody mediated receptor internalization and downregulation appears to be cell line dependent. Our results also imply, that in the presence of a strong autocrine ligand activation of ErbB3 as in the ACHN or DU145 cells, MM-121 blocks ligand activation of ErbB3 without downregulating ErbB3 expression ( and ). Yet, in other cell lines, such as the NCI-N87 cells, receptor internalization and degradation may contribute to MM-121 activity (). However, we currently do not know why MM-121 downregulates ErbB3 expression in some cells, but not others. Of note, for MM-121 to be effective in a ligand-independent manner (i.e. only by downregulation of ErbB3), treatment would need to result in elimination of most of the ErbB3 protein. It would appear that in cancers with ligand-independent activation of ErbB3 (e.g. HER2
amplified cancers) MM-121 does not sufficiently downregulate ErbB3 to low enough levels, and thus has minimal effect on cell viability.
MM-121 exerted substantial anti-tumor activity in three xenograft tumor cell lines (DU145, OVCAR8, and ACHN) as a single-agent therapy. However, it was ineffective in many other cancer models. Of particular note, this antibody did not block ErbB3 phosphorylation and was ineffective in the HER2
amplified cancer cell lines (Table S1, Figs. S5 and S6
). We hypothesize that the high concentration of HER2 receptors on the membrane likely obviates the need for ligand-dependent activation of ErbB3. On the other hand, it is not surprising that cancers with high ligand expression in non-HER2
amplified cancers were the most responsive to single-agent therapy (). Although these biomarkers are useful towards understanding the biology behind responsiveness to this antibody, it is hard to predict how applicable they will be in the clinical development of this antibody, and more work needs to be done to elucidate possible biomarker-response relationships. Indeed, predicting response may require a panel of biomarkers that will likely include heregulin. Based on these findings, we hypothesize that MM-121 will be most effective when treating ligand-dependent tumors. Of note, since ovarian cancers, pancreatic cancers, papillary thyroid cancers and medulloblastomas express heregulin in more than 70% of analyzed primary tumors (24
), therapy using MM-121 may demonstrate activity in these malignancies.
There are likely some cancers that express phosphorylated ErbB3, yet ErbB3 is not critical for the growth and survival. In these cancers, MM-121 may downregulate ErbB3 phosphorylation and/or total ErbB3 without therapeutic impact as a single-agent. In such cancers, PI3K-AKT signaling may not be solely dependent on ErbB3, and/or downregulation of PI3K-AKT may not be sufficient to induce growth arrest and/or apoptosis. In these cancers, there may be a benefit for combining MM-121 with other targeted therapies.
Recent studies have highlighted the central role of re-activation of ErbB3 signaling as a mechanism of acquired resistance to EGFR and HER2 based therapies. Therefore, an effective ErbB3 antibody may retard the development of resistance to these inhibitors. In this study, we observed that MM-121 potently prevented resistance to anti-EGFR based therapies in vitro
and in vivo
(, , and ). Importantly, we assessed the activity of MM-121 in the genetically engineered mouse model of lung cancers driven by EGFR T790M-L858R
. The “gatekeeper” T790M mutation in EGFR
is observed in 50% of lung cancers that become resistant to the EGFR TKIs, gefitinib and erlotinib (19
). This mouse model faithfully recapitulates human lung cancers that harbor this mutation in that tumors derived from these mice are resistant to gefitinib yet sensitive to irreversible EGFR inhibitors (18
). These cancers demonstrated a transient response to cetuximab (). However, resistance to cetuximab was associated with an activation of ErbB3 phosphorylation and increased expression of heregulin (). This suggests that activation of ErbB3 in a ligand-dependent manner caused resistance. Importantly, addition of MM-121 to cetuximab blocked the reactivation of ErbB3 and led to a greater and more durable response. Taken in its entirety, these data provide the clinical basis for the evaluation of MM-121 in combination with anti-EGFR and HER2 based therapies. This may lead to more impressive responses and improved times to progression for patients with EGFR or HER2 driven cancers.