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Background: The receptor tyrosine kinase, HER3 and its ligand, heregulin (HRG), have been implicated in the initiation and progression of multiple cancer types including: breast, lung, and head & neck cancers. Seribantumab is a fully human, monoclonal IgG2 antibody that binds to the ligand-binding domain of HER3 and inhibits HRG-mediated signaling. Previously, seribantumab was tested in combination with exemestane in a placebo-controlled, Phase 2 study in post-menopausal women with ER/PR+, HER2 negative metastatic breast cancer (mBC). Although the trial failed to meet its primary efficacy objective of a 50% reduction in hazard ratio in the seribantumab/exemestane treatment vs. the placebo/exemestane control group, a positive trend in PFS and a statistically significant improvement in median OS was observed in patients in the seribantumab/exemestane treatment group. Seribantumab has also been tested in three randomized Phase 2 studies adding to standard of care (SOC) in non-small cell lung, ER/PR+ mBC, and platinum resistant/refractory ovarian cancer. These studies were retrospectively analyzed to determine correlation between HRG mRNA levels in tumor tissue and PFS. In each of these studies, the presence of tumor cell HRG mRNA was prognostic for shortened PFS with SOC treatment. Further, the addition of seribantumab to SOC therapy improved PFS for patients with HRG+ tumors. These data support the hypothesis that HRG expression may define a drug tolerant cancer cell phenotype characterized by poor response to multiple classes of cytotoxic and targeted therapies, including aromatase inhibitors and SERDs. Additionally, blockade of HRG-induced HER3 signaling by seribantumab may counter such protective effects of HRG on cancer cells, with the potential for improved outcomes in HRG+ patients. It is estimated that ˜45% of hormone-receptor positive, HER2 negative advanced breast cancers are HRG+ and that HRG expression may contribute to accelerated clinical progression observed in this subset of patients. Trial design: In the upcoming randomized, double-blinded, multi-center, Phase 2 study, ER/PR receptor-positive, HER2 negative mBC patients with HRG+ tumors will be prospectively selected using a HRG RNA in situ hybridization assay. Approximately 200 women will be screened to enroll 80 HRG+ subjects. Eligible subjects will be randomized in a 1:1 ratio to receive seribantumab/fulvestrant or placebo/fulvestrant until investigator-assessed disease progression or unacceptable toxicity, whichever comes first. Subjects will have progressed on one or two prior hormonal therapies, one of which must have been a CDKi-containing regimen. The goal of this study is to determine if the combination of seribantumab + fulvestrant is more effective than placebo + fulvestrant based on PFS (primary end point) in HRG positive subjects. Secondary endpoints include OS, objective response rate, and time to progression. Safety will also be assessed. Enrollment is expected to begin in 2017 at approximately 80 sites globally. Citation Format: Kaufman PA, Pipas M, Finn GJ, Mathews SE, Zhang H, Richards J, Kudla AJ, Bloom T, Zalutskaya AA, Llorin-Sangalang J, Pinto AC, Ettl J. SHERBOC: A double-blind, placebo-controlled, phase 2 trial of seribantumab (MM-121) plus fulvestrant in postmenopausal women with hormone receptor-positive, heregulin positive, HER2 negative metastatic breast cancer whose disease progressed after prior systemic therapy [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr OT3-06-01.

ErbB3 is a member of the human epidermal growth factor receptor (ErbB or HER) family which is comprised of four receptors (ErbB1-4). A defining feature of the ErbB network is that two members of the family, ErbB2 and ErbB3, are non-autonomous. ErbB2 lacks the capacity to interact with a growth-factor ligand, whereas the kinase activity of ErbB3 is defective. Heregulin (HRG), the ErbB3 ligand, has been identified as a potent driver of proliferation and enhanced survival. HRG expression leads to a distinct tumor cell phenotype characterized by an inability to respond to the effects of numerous Standard of Care (SOC) therapies, including chemotherapies, anti-hormonal agents and other targeted therapeutics. In surveys of HRG expression, we have shown the presence of HRG+ cells in approximately 50% of the cases of most solid tumor types. We hypothesize that these HRG+ cells are protected from the effects of SOC therapy and continue to proliferate even in the presence of SOC, resulting in limited clinical benefit. In this model, if HRG activity is blocked, HRG+ cells become susceptible to SOC, resulting in enhanced clinical benefit. Seribantumab is a fully human anti-ErbB3 monoclonal antibody designed to block HRG activity by inhibiting the binding of HRG to ErbB3. In the presence of seribantumab, HRG+ tumor cells are predicted to be able to respond to co-administered SOC therapy. For hormone receptor positive (HR+) breast cancer, hormone deprivation strategies have proven clinical benefit in the adjuvant and metastatic settings. Unfortunately, clinical benefit from these therapies can be short-lived in some patients. Optimal clinical management of these patients requires a comprehensive molecular understanding of the drivers of rapid clinical progression. We and others have found that HRG mRNA expression measured in tumor samples defines a subgroup of patients who derive only limited clinical benefit from SOC when compared to patients whose tumors do not express HRG. This was observed in a previously published Phase 2 clinical study with exemestane, and preclinically with multiple classes of anti-hormonal agents, including letrozole and fulvestrant -- treatments that currently represent the mainstay of treatment options for HR+, HER2 negative (HER2-) advanced breast cancer. Here we will present data supporting the hypothesis that phenotypically distinct HRG+ cells in breast cancer models persist despite treatment with SOC and various novel classes of therapy. We will also show that the addition of the anti-ErbB3 antibody seribantumab to these other therapies promotes sustained treatment responses. Continued expansion of HRG+ cells could be the key to rapid clinical progression in breast cancer patients treated with SOC therapy. These findings support the development of seribantumab in combination with anti-hormonal agents in a planned Phase 3 clinical trial in HR+, HER2- advanced breast cancer. Citation Format: Finn GJ, Zhang H, Blois A, Mathews SE, Kudla AJ, Baum JS, Demars NA, Cieslewicz MJ, Czibere A. Phenotypically distinct HRG positive cancer cells impact standard of care therapies in metastatic breast cancer models [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 P6-15-02.

HER2 is an important biomarker for breast and gastric cancer prognosis and patient treatment decisions. HER2 positivity, as defined by IHC or FISH testing, remains an imprecise predictor of patient response to HER2-targeted therapies. Challenges to accurate HER2 assessment and patient stratification may include intratumoral heterogeneity, lack of assays that are quantitative and/or objective, and differences between measuring HER2 at the protein vs. DNA level. We have developed a novel immunofluorescence method for absolute quantitation of HER2 protein expression at the single cell level on formalin-fixed, paraffin-embedded (FFPE) patient samples. Our assay utilizes automated image analysis software to identify cells, classify them as tumor or non-tumor cells, and quantitate the level of HER2 staining for each tumor cell. The HER2 staining level is then converted to absolute HER2 receptor numbers per cell using an array composed of cell lines that span a range of HER2 levels. This cell pellet array standard is stained in parallel with each tissue sample. With this approach we quantitate HER2 expression at the single cell level and describe the heterogeneity of HER2 expression within breast cancer tissue samples. Our assay provides additional insight into the absolute level of HER2 expression and heterogeneity of HER2 expression within tumors, and allows for direct comparison with the currently existing HER2 assessments by standard IHC and/or FISH. This new assay could be used to increase understanding of the relationship between HER2 expression and patient response to HER2-targeted therapies. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P1-07-03.

Heregulin (HRG) is the cognate ligand of the ErbB3 receptor and has been identified as a potent driver of a distinct tumor cell phenotype characterized by enhanced survival that appears to be inherently more resistant to standard of care therapies. Seribantumab (MM-121) is a fully human monoclonal antibody designed to block HRG from binding to ErbB3 and to prevent the establishment of HRG-driven cancer cell survival in response to cytotoxic therapies. Clinical evidence from randomized studies suggests that patients who receive standard therapy have shorter progression-free survival (PFS) when their tumors are positive for HRG RNA expression compared to patients receiving standard therapy with HRG-negative tumors. The addition of seribantumab was able to enhance the anti-tumor effect of these respective standard therapies and prolong PFS in patients with HRG-positive tumors. Profiling data indicates that positive HRG expression can be found in more than a third of most solid tumors identifying a high unmet medical need. In order to identify HRG-driven tumors with a more difficult-to-treat phenotype, an RNA in situ hybridization (ISH) diagnostic assay has been developed. Details will be presented illustrating the benefits of RNA ISH over alternate detection methods for HRG. This includes the visualization of tumor cell HRG expression with high sensitivity and specificity in small tissue specimens such as fine needle aspirates and core needle biopsies. This assay is currently being used to select patients for inclusion into diagnostic-driven clinical studies. Citation Format: Sara Mathews, Gregory Finn, Arthur J. Kudla, Victoria Rimkunas, Peter Laivins, Gavin MacBeath, Akos Czibere, Jason Baum. Identification of Heregulin (HRG) expression as a driver of a difficult-to-treat cancer phenotype and development of a companion diagnostic for the HRG-ErbB3 targeting drug seribantumab. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Targeting the Vulnerabilities of Cancer; May 16-19, 2016; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(1_Suppl):Abstract nr A19.

Ligand-stimulated activation of FGF receptors (FGFRs) in skeletal muscle cells represses terminal myogenic differentiation. Skeletal muscle cell lines and subsets of primary cells are dependent on FGFs to repress myogenesis and maintain growth. To understand the intracellular events that transduce these signals, MM14 skeletal muscle cells were transfected with expression vectors encoding chimeric receptors. The chimeras are comprised of the PDGF β receptor (PDGFβR) extracellular domain, the FGFR-1 intracellular domain, and either the PDGFβR or FGFR-1 transmembrane domain. The chimeric receptors were autophosphorylated upon PDGF-BB stimulation and are capable of stimulating mitogen-activated protein kinase activity. Activation of the tyrosine kinase domain of either chimera repressed myogenesis, suggesting intracellular responses regulating skeletal muscle differentiation are transduced by activation of the FGFR-1 tyrosine kinase. Unexpectedly, we found that activation of either chimeric receptor failed to stimulate cellular proliferation. Thus, it appears that regulation of skeletal muscle differentiation by FGFs requires only activation of the FGFR tyrosine kinase. In contrast, stimulation of proliferation may require additional, as yet unidentified, signals involving the receptor ectodomain, the FGF ligand, and heparan sulfate either alone, or in combination.

The muscle LIM protein (MLP) is a muscle-specific LIM-only factor that exhibits a dual subcellular localization, being present in both the nucleus and in the cytoplasm. Overexpression of MLP in C2C12 myoblasts enhances skeletal myogenesis, whereas inhibition of MLP activity blocks terminal differentiation. Thus, MLP functions as a positive developmental regulator, although the mechanism through which MLP promotes terminal differentiation events remains unknown. While examining the distinct roles associated with the nuclear and cytoplasmic forms of MLP, we found that nuclear MLP functions through a physical interaction with the muscle basic helix-loop-helix (bHLH) transcription factors MyoD, MRF4, and myogenin. This interaction is highly specific since MLP does not associate with nonmuscle bHLH proteins E12 or E47 or with the myocyte enhancer factor-2 (MEF2) protein, which acts cooperatively with the myogenic bHLH proteins to promote myogenesis. The first LIM motif in MLP and the highly conserved bHLH region of MyoD are responsible for mediating the association between these muscle-specific factors. MLP also interacts with MyoD-E47 heterodimers, leading to an increase in the DNA-binding activity associated with this active bHLH complex. Although MLP lacks a functional transcription activation domain, we propose that it serves as a cofactor for the myogenic bHLH proteins by increasing their interaction with specific DNA regulatory elements. Thus, the functional complex of MLP-MyoD-E protein reveals a novel mechanism for both initiating and maintaining the myogenic program and suggests a global strategy for how LIM-only proteins may control a variety of developmental pathways.

Human epidermal growth factor receptor 2 (HER2) is an important biomarker for breast and gastric cancer prognosis and patient treatment decisions. HER2 positivity, as defined by IHC or fluorescent in situ hybridization testing, remains an imprecise predictor of patient response to HER2-targeted therapies. Challenges to correct HER2 assessment and patient stratification include intratumoral heterogeneity, lack of quantitative and/or objective assays, and differences between measuring HER2 amplification at the protein versus gene level. We developed a novel immunofluorescence method for quantitation of HER2 protein expression at the single-cell level on FFPE patient samples. Our assay uses automated image analysis to identify and classify tumor versus non-tumor cells, as well as quantitate the HER2 staining for each tumor cell. The HER2 staining level is converted to HER2 protein expression using a standard cell pellet array stained in parallel with the tissue sample. This approach allows assessment of HER2 expression and heterogeneity within a tissue section at the single-cell level. By using this assay, we identified distinct subgroups of HER2 heterogeneity within traditional definitions of HER2 positivity in both breast and gastric cancers. Quantitative assessment of intratumoral HER2 heterogeneity may offer an opportunity to improve the identification of patients likely to respond to HER2-targeted therapies. The broad applicability of the assay was demonstrated by measuring HER2 expression profiles on multiple tumor types, and on normal and diseased heart tissues.

We have previously cloned and sequenced a newt keratinocyte growth factor receptor (KGFR) cDNA which exhibited a unique spatial and temporal expression pattern in the regenerating newt limb. In this report, we further characterize the biochemical and functional properties of this newt KGFR. A stable Chinese hamster ovary transfectant overexpressing the newt KGFR was capable of binding both 125I-fibroblast growth factor-1 (FGF-1) and 125I-FGF-7 but not 125I-FGF-2, indistinguishable from the human KGFR. Scatchard analysis and cross-linking studies further support the conclusion that FGF-1 and FGF-7 are the ligands for the newt KGFR. In addition to their ability to bind to FGFs, both the human and the newt KGFR are also capable of repressing differentiation in mouse MM14 myoblasts. MM14 cells express FGFR1 and are repressed from differentiation by FGF-1, FGF-2, and FGF-4 but not FGF-7. Co-transfection of MM14 cells with either a human or newt KGFR expression construct conferred a response to FGF-7 as determined by a human alpha-cardiac actin/luciferase reporter construct. The response to FGF-7 was similar to the endogenous FGF response as FGF-7 prevented MM14 myoblasts from undergoing terminal differentiation. Thus, both the human and the newt KGFRs transduce signals similar to those transduced via the endogenous mouse FGFR1. Together these data indicate that this newly isolated newt KGFR is a functional receptor as it binds two FGF family members with high affinity and mediates signaling in skeletal muscle myoblasts. Because the binding pattern of the newt KGFR is similar to the pattern observed for its mammalian counterpart, it emphasizes the strict conservation that this ligand/receptor system has undergone through evolution.

The distinct effects of cytokines on cellular growth and differentiation suggest that specific signaling pathways mediate these diverse biological activities. Fibroblast growth factors (FGFs) are well-established inhibitors of skeletal muscle differentiation and may operate via activation of specific signaling pathways distinct from recently identified mitogen signaling pathways. We examined whether platelet-derived growth factor (PDGF)-activated signaling pathways are sufficient to mediate FGF-dependent repression of myogenesis by introducing the PDGF beta receptor into a mouse skeletal muscle cell line. Addition of PDGF-BB to cells expressing the PDGF beta receptor activated the PDGF beta receptor tyrosine kinase, stimulated mitogen-activated protein (MAP) kinase, and increased the steady-state levels of junB and c-fos mRNAs. Despite the activation of these intracellular signaling molecules, PDGF beta receptor activation elicited no detectable effect on cell proliferation or differentiation. In contrast to PDGF-BB, addition of FGF-2 to myoblasts activated signaling pathways that resulted in DNA synthesis and repression of differentiation. Because of the low number of endogenous FGF receptors expressed, FGF-stimulated signaling events, including tyrosine phosphorylation and activation of MAP kinase, could be detected only in cells expressing higher levels of a transfected FGF receptor cDNA. As the PDGF beta receptor- and FGF receptor-stimulated signaling pathways yield different biological responses in these skeletal muscle cells, we hypothesize that FGF-mediated repression of skeletal muscle differentiation activates signaling pathways distinct from those activated by the PDGF beta receptor. Activation of PDGF beta receptor tyrosine kinase activity, stimulation of MAP kinase, and upregulation of immediate-early gene expression are not sufficient to repress skeletal muscle differentiation.

Fibroblast growth factors (FGFs) are a family of nine proteins that bind to three distinct types of cell surface molecules: (i) FGF receptor tyrosine kinases (FGFR-1 through FGFR-4); (ii) a cysteine-rich FGF receptor (CFR); and (iii) heparan sulfate proteoglycans (HSPGs). Signaling by FGFs requires participation of at least two of these receptors: the FGFRs and HSPGs form a signaling complex. The length and sulfation pattern of the heparan sulfate chain determines both the activity of the signaling complex and, in part, the ligand specificity for FGFR-1. Thus, the heparan sulfate proteoglycans are likely to play an essential role in signaling. We have recently identified a role for FGF in limb bud development in vivo. In the chick limb bud, ectopic expression of the 18 kDa form of FGF-2 or FGF-2 fused to an artificial signal peptide at its amino terminus causes skeletal duplications. These data, and the observations that FGF-2 is localized to the subjacent mesoderm and the apical ectodermal ridge in the early developing limb, suggest that FGF-2 plays an important role in limb outgrowth. We propose that FGF-2 is an apical ectodermal ridge-derived factor that participates in limb outgrowth and patterning.

The signaling network downstream of the ErbB family of receptors has been extensively targeted by cancer therapeutics; however, understanding the relative importance of the different components of the ErbB network is nontrivial. To explore the optimal way to therapeutically inhibit combinatorial, ligand-induced activation of the ErbB-phosphatidylinositol 3-kinase (PI3K) axis, we built a computational model of the ErbB signaling network that describes the most effective ErbB ligands, as well as known and previously unidentified ErbB inhibitors. Sensitivity analysis identified ErbB3 as the key node in response to ligands that can bind either ErbB3 or EGFR (epidermal growth factor receptor). We describe MM-121, a human monoclonal antibody that halts the growth of tumor xenografts in mice and, consistent with model-simulated inhibitor data, potently inhibits ErbB3 phosphorylation in a manner distinct from that of other ErbB-targeted therapies. MM-121, a previously unidentified anticancer therapeutic designed using a systems approach, promises to benefit patients with combinatorial, ligand-induced activation of the ErbB signaling network that are not effectively treated by current therapies targeting overexpressed or mutated oncogenes.

TPS4148 Background: HER2 overexpression occurs in ≤ 20% of GE cancers, and front-line fluoropyrimidine/platinum with TRAS increases survival in HER2+ GE junction/gastric cancer patients. Weekly PTX...

48 Background: HER2 is overexpressed in gastric and gastroesophageal junction (GEJ) tumors. Though treatment of these tumors with the HER2-targeted agent trastuzumab offers clinical benefit, patients ultimately progress. Signaling by HER2/HER3 heterodimers following activation by HER3’s ligand heregulin has been shown to be a resistance mechanism to trastuzumab. The objective of this study was to quantitate HER2, HER3 and heregulin in gastric and GEJ tumor samples to determine whether these tumors express components for signaling through this complex and thus have potential for developing trastuzumab resistance. Methods: A novel immunofluorescent assay was used to quantitate HER2 and cytokeratin in 236 gastric and GEJ tumors. Using image analysis, each cell within a section was classified as tumor or non-tumor based on cytokeratin level. HER2 fluorescence was also quantitated in each cell and converted to number of HER2 receptors per cell using a standard curve from an array of cell lines with defined HER2. The assay’s single cell output enables an assessment of HER2 heterogeneity in any tissue. In those tumors overexpressing HER2, HER3 and heregulin were quantitated using immunofluorescence as for HER2 and by RT-PCR. Results: In concordance with published data, HER2 was overexpressed in ~10% of gastric and ~20% of GEJ tumors. HER2-overexpressing tumors could be divided into two groups with levels of HER2 consistent with IHC scores of 2+ and 3+ as assessed by clinical assays. All HER2-overexpressing samples also expressed HER3 and heregulin. Heregulin expression was detected in both tumor cells and/or in adjacent stroma. Conclusions: HER3 and heregulin are present in HER2-overexpressing gastric and GEJ tumors. To evaluate the importance of HER3 and heregulin in these patients, a randomized open label second line Phase 2 trial has been initiated with MM-111, a HER2/HER3 bispecific antibody that inhibits HER3 activity in HER2-overexpressing cells by blocking heregulin binding. HER2, HER3 and heregulin expression will be retrospectively analyzed in tumor samples from patients in this trial, providing further insight into the role of this complex in gastric and GEJ cancer.

Oncogenic Ras (H-Ras G12V) inhibits skeletal myogenesis through multiple signaling pathways. Previously, we demonstrated that the major downstream effectors of Ras (i.e., MEK/MAPK, RalGDS and Rac/Rho) play a minor, if any, role in the differentiation-defective phenotype of Ras myoblasts. Recently, NFkappaB, another Ras signaling target, has been shown to inhibit myogenesis presumably by stimulating cyclin D1 accumulation and cell cycle progression. In this study, we address the involvement of NFkappaB activation in the Ras-induced inhibition of myogenesis. Using H-Ras G12V and three G12V effector-loop variants, we detect high levels of NFkappaB transcriptional activity in C3H10T1/2-MyoD cells treated with differentiation medium. Myogenesis is blocked by all Ras proteins tested, yet only in the case of H-Ras G12V are cyclin D1 levels increased and cell cycle progression maintained. Expression of IkappaBalpha SR, an inhibitor of NFkappaB, does not reverse the differentiation-defective phenotype of Ras expressing cultures, but does induce differentiation in cultures treated with tumor necrosis factor (TNFalpha) or in cultures expressing the RelA/p65 subunit of NFkappaB. These data confirm that NFkappaB is a target of Ras and suggest that the cellular actions of NFkappaB require additional signals that are discriminated by the Ras effector-loop variants. Results with IkappaBalpha SR convincingly demonstrate that H-Ras G12V does not rely on NFkappaB activity to block myogenesis, an observation that continues to implicate another unidentified signaling pathway(s) in the inhibition of skeletal myogenesis by Ras.

Increasing evidence suggests amplification or overexpression of human epidermal growth factor receptor-2 (HER2), and HER3 levels are correlated to decreased survival in gastric cancers. Our previous studies established that MM-111, a novel bispecific antibody that specifically targets the HER2/HER3 heterodimer, blocks heregulin (HRG) binding to HER3 and corresponding downstream signaling pathways. In this study we used computational and experimental biology to assess the activity of MM-111 combination therapies in treating gastric cancer. First, we utilized a multi-scale computational network model of HER2-positive tumors that relates HER family signal transduction with cell growth to predict that MM-111 plus trastuzumab would synergistically inhibit tumor growth. The mechanism underlying this prediction is that the combinatorial blockade of HER2 and HER3 suppresses signal propagation through both the AKT and ERK cascades and this leads to synergistic cell growth arrest. Cell viability and signaling experiments in a gastric cancer model (NCI-N87) confirmed these predictions. In particular, MM-111 plus trastuzumab caused 25% greater cell growth inhibition than the additive effects of the individual treatments (compared to Bliss independence; p< 0.05). Next, we assessed the effect of exogenous HRG on this combination therapy. HRG reduced the activity of trastuzumab but did not affect MM-111’s activity, and it enhanced the synergy of this combination (50% greater than Bliss independence; p < 10−8). We then assessed the effect of adding paclitaxel to the trastuzumab plus MM-111 combination. Our key finding was that activation of HER3 by HRG greatly reduced paclitaxel's activity. MM-111, but not trastuzumab, restored cell sensitivity to paclitaxel in the presence of HRG. A synergistic effect (35% greater than Bliss independence; p < 10−8) was observed for the combination of MM-111, trastuzumab and paclitaxel. Similar combination effects were also achieved using a three-dimensional NCI-N87 spheroid assay: Inhibition of HER2, HER3, and the AKT and ERK pathways correlated with spheroid growth inhibition. Furthermore, HRG-overexpressing NCI-N87 (NCI-N87-HRG) xenografts were less sensitive to paclitaxel than the wild type xenografts. MM-111 restored the sensitivity to paclitaxel in these xenografts, even when given as a second line treatment regimen after tumor progression on the front-line regimen (trastuzumab plus 5-fluorouracil). Trastuzumab alone did not overcome the HRG-induced paclitaxel resistance, but its combination with MM-111 showed significantly greater activity than either drug alone. Taken together, our data suggest that combination of MM-111, trastuzumab and paclitaxel is worthy of investigation as a potential therapeutic strategy for the treatment of HER2-positive gastric cancer. Citation Format: Bo Zhang, Johanna Lahdenranta, Jinyan Du, Daniel Kirouac, Stephanie Nguyen, Ryan Overland, Violette Paragas, Arthur Kudla, Ulrik Nielsen, Charlotte McDonagh, Matthew Onsum. MM-111, a bispecific HER2 and HER3 antibody, synergistically combines with trastuzumab and paclitaxel in preclinical models of gastric cancer. [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 4633. doi:10.1158/1538-7445.AM2013-4633

48 Background: ErbB2 (HER2) overexpression has been reported in 7-34% of gastric cancers. ErbB3 (HER3) is the preferred dimerization partner of ErbB2, and ErbB2/ErbB3 heterodimer activation is implicated in the progression and metastasis of ErbB2+ tumors. Activation of ErbB3 signaling is a postulated resistance mechanism to current ErbB2-directed therapies and select chemotherapies. In line with this research, ErbB3 levels are associated with poor prognosis in gastric cancers. MM-111 is a bi-specific antibody that docks to ErbB2 and inhibits ErbB3 signaling in cells that overexpress ErbB2. In this study, MM-111 was evaluated in ErbB2+ gastric cancer by testing the activity of MM-111 in ErbB2+ pre-clinical models of gastric cancer, and by assessing the prevalence of potentially predictive biomarkers in a panel of archived gastric and gastroesophageal junction (GEJ) tumors. Methods: MM-111 was tested in ErbB2+ gastric cancer xenografts that were either untreated or after tumors ceased to respond to trastuzumab/5-FU. Xenografts were analyzed at multiple time points for the expression of ErbB-receptor family members and their downstream signaling by Luminex -assays. Preclinical data indicate that ErbB2, ErbB3, and heregulin are predictive biomarkers for MM-111. In order to determine the prevalence of our potentially predictive biomarkers in gastric and GEJ cancers, we obtained commercially archived tumor tissue and assayed the tissue for ErbB2 and ErbB3 expression levels using quantitative IHC, and measured heregulin transcript levels by RT-PCR. Results: MM-111 synergizes with various treatment regimens in the 2nd line treatment setting in ErbB2+ gastric cancer xenografts. In our models, the combination of MM-111, trastuzumab, and paclitaxel is particularly effective after tumors progressed on trastuzumab/5-FU. MM-111 inhibits the activity of the ErbB –signaling axis in these models. In addition, 23% of GEJ tumor samples and 20% of gastric samples were positive for potentially predictive biomarkers. Conclusions: ErbB2+xenograft tumors that stop responding to trastuzumab-based therapies benefit from MM-111–based regimens.

Several FGF family members are expressed in skeletal muscle; however, the roles of these factors in skeletal muscle development are unclear. We examined the RNA expression, protein levels, and biological activities of the FGF family in the MM14 mouse skeletal muscle cell line. Proliferating skeletal muscle cells express FGF-1, FGF-2, FGF-6, and FGF-7 mRNA. Differentiated myofibers express FGF-5, FGF-7, and reduced levels of FGF-6 mRNA. FGF-3, FGF-4, and FGF-8 were not detectable by RT-PCR in either proliferating or differentiated skeletal muscle cells. FGF-I and FGF-2 proteins were present in proliferating skeletal muscle cells, but undetectable after terminal differentiation. We show that transfection of expression constructs encoding FGF-1 or FGF-2 mimics the effects of exogenously applied FGFs, inhibiting skeletal muscle cell differentiation and stimulating DNA synthesis. These effects require activation of an FGF tyrosine kinase receptor as they are blocked by transfection of a dominant negative mutant FGF receptor. Transient transfection of cells with FGF-1 or FGF-2 expression constructs exerted a global effect on myoblast DNA synthesis, as greater than 50% of the nontransfected cells responded by initiating DNA synthesis. The global effect of cultures transfected with FGF-2 expression vectors was blocked by an anti-FGF-2 monoclonal antibody, suggesting that FGF-2 was exported from the transfected cells. Despite the fact that both FGF-l and FGF-2 lack secretory signal sequences, when expressed intracellularly, they regulate skeletal muscle development. Thus, production of FGF-1 and FGF-2 by skeletal muscle cells may act as a paracrine and autocrine regulator of skeletal muscle development in vivo.

Amplification of human epidermal growth factor receptor 2 (HER2) occurs in approximately 25% of breast cancers and is associated with increased disease recurrence and poor prognosis. Trastuzumab, a monoclonal antibody targeting HER2, has demonstrated clinical benefit in HER2 over-expressing tumors. However, acquired resistance and disease progression is widely observed in patients treated with trastuzumab. In this experiment our objectives were to dissect the dynamic, molecular mechanisms involved in acquired resistance to trastuzumab and to determine whether MM-111, a novel bispecific antibody fusion protein that specifically targets the HER2/HER3 heterodimer and blocks heregulin binding to HER3, has activity in trastuzumab-resistant tumor cells. BT474 cells were cultured in the presence of trastuzumab and cells were periodically tested for response to trastuzumab and MM-111. Samples were also collected for protein and RNA analyses. Resistance to trastuzumab gradually increased in BT474 cells after four months of exposure to trastuzumab, as measured by a cell proliferation assay. Quantitative flow cytometry analysis showed EGFR, HER2, and HER3 levels on the cell surface of resistant cells were similar to the parental cells. Phospho-protein kinase antibody arrays revealed that signaling pathways associated with the ERK cascade were activated during the development of drug resistance. Western blotting further confirmed that phosphorylation of EGFR, ERK, CREB, c-Jun, and AFT-1 was increased in the resistant cells. Real-time polymerase chain reaction also showed transcript levels of HER ligands, including HRG1α, HRG1α, betacellulin, amphiregulin, epigen, TGFβ, and HB-EGF, dramatically increased in tumor cells that acquired resistance to trastuzumab. Compared to the BT474 parental cells, MM-111 showed a greater inhibition in trastuzumab-resistant cells in a spheroid growth assay. Furthermore, trastuzumab-resistant cells became more sensitive to gefitinib and erlotinib, both EGFR inhibitors. The combination of gefitinib or erlotinib with MM-111 showed greater inhibition than either drug alone. In conclusion, our data suggest that one mechanism by which HER2 overexpressing breast cancer cells develop resistance to trastuzumab is to up-regulate ligand-dependent EGFR and HER3 signaling pathways. The use of MM-111 and EGFR inhibitors may provide an effective therapeutic strategy for the treatment of trastuzumab-resistant cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1888. doi:1538-7445.AM2012-1888

Recent advances in understanding of skeletal muscle differentiation implicate fibroblast growth factors (FGFs) as regulators of myogenesis; however, the identity and actions of factors that repress myogenesis in vivo remain to be established. This review will focus on the fibroblast growth factor family and the evidence for its role in regulating myogenesis in culture and in vivo.

The oncogenic receptor ErbB2 (HER2) is frequently overexpressed in a variety of cancer indications, including breast, gastric, bladder, and lung. ErbB3 (HER3), the preferred dimerization partner of ErbB2, is critical for the survival and growth of ErbB2-overexpressing tumors, but is poorly inhibited by current ErbB2-targeted therapies. MM-111 is a novel bispecific antibody designed to specifically inhibit function of the ErbB2/ErbB3 heterodimer in ErbB2-overexpressing tumors. MM-111 binds simultaneously to ErbB2 and ErbB3 and blocks the interaction of ErbB3 with its ligand, heregulin. Inhibiting the ErbB2/ErbB3 heterodimer prevents ligand-induced activation of the phosphotidylinositol 3-kinase (PI3K) pro-survival pathway, altering cell cycle progression and inhibiting tumor growth. Upregulation of both heregulin and ErbB3 has been observed in tumors that have acquired resistance to the ErbB2-targeted therapies trastuzumab and lapatinib. In addition, ErbB2 and ErbB3 are frequently upregulated in estrogen receptor positive breast cancers that have developed resistance to anti-estrogen therapies. These data indicate that the ErbB2/ErbB3 receptor heterodimer potently activates growth of tumors characterized by ErbB2 overexpression and implicate ErbB3 as a mechanism of resistance to current therapies. Because of its ability to inhibit ErbB3 activity, MM-111 could be effective in combination with ErbB2-targeted therapies by overcoming ErbB3-mediated resistance. Previous data demonstrating that MM-111 acts in concert with trastuzumab to inhibit growth of ErbB2-overexpressing human tumor models support this hypothesis. We now show that MM-111 effectively combines with lapatinib to inhibit growth of tumors driven by the ErbB2/ErbB3 heterodimer. MM-111, as a single agent, is more effective than lapatinib at inhibiting heregulin-driven activation of ErbB3 and AKT in ErbB2-overexpressing cancer cell lines. In these same cell lines, the combination of MM-111 and lapatinib results in greater AKT inhibition than either treatment alone. In addition, we have demonstrated in ErbB2-overexpressing 3D spheroid cancer models that treatment with the MM-111/ lapatinib combination increases the number of cells in early phase apoptosis as compared to single-drug treatments. Finally, in an ErbB2-overexpressing cancer xenograft model, the combination of MM-111 and lapatinib results in greater inhibition of tumor growth than either therapy alone. In conclusion, we show that MM-111 and lapatinib positively combine to inhibit the growth of ErbB2-overexpressing tumors. Our results suggest that concurrent treatment with MM-111 and lapatinib may provide a potent therapeutic regimen for cancer patients whose tumors overexpress ErbB2 by deterring ErbB3-mediated resistance to lapatinib. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 654. doi:10.1158/1538-7445.AM2011-654

Approximately 75% of breast cancers are estrogen receptor (ER) positive. Although endocrine therapies such as tamoxifen, fulvestrant, and letrozole have demonstrated significant efficacy in treating ER+ breast cancer patients, intrinsic or acquired resistance has limited their success. Recent studies suggest that crosstalk between ErbB receptor signaling and ER signaling may contribute to resistance to endocrine therapy. Overexpression of human epidermal growth factor receptor 2 (HER2, synonymous with ErbB2) and upregulation of the ErbB3 ligand heregulin are associated with poor prognosis and reduced overall survival. MM-111 is a novel bispecific antibody fusion protein that specifically targets the ErbB2/ErbB3 heterodimer and blocks heregulin binding to ErbB3. MM-111 inhibits ligand-induced ErbB3 phosphorylation, tumor cell cycle progression, and tumor growth when ErbB2 is overexpressed. We hypothesized that combination of endocrine therapies with MM-111 may improve anti-tumor efficacy. In an estrogen-stimulated BT474-M3 ER-positive breast cancer cell three-dimensional spheroid assay, MM-111, when used as a single agent, showed growth inhibitory effects similar to the anti-estrogen drugs tamoxifen and fulvestrant. Combination of MM-111 with anti-estrogen therapy showed superior activity to either drug alone. In the presence of heregulin, MM-111 maintained its growth inhibitory activity, whereas the inhibitory effect of tamoxifen and fulvestrant was diminished. This suggests that activation of ErbB3 confers tumor cell resistance to anti-estrogen therapies. When both estrogen and heregulin were present, the combination of MM-111 and the anti-estrogen drugs demonstrated a significantly greater inhibitory effect than either drug alone. Western blot analysis showed that treatment of BT-474-M3 cells with the combination of MM-111 and the anti-estrogen drugs significantly increased apoptosis markers such as cytochrome C and BAX. Furthermore, an in vivo BT474-M3 xenograft model showed resistance to tamoxifen treatment (5 mg/pellet, 60-day release). In this xenograft model MM-111 sensitized tumor response to tamoxifen and the combination treatment dramatically inhibited tumor growth. In conclusion, the combination of MM-111 and endocrine therapies may provide a potent regimen that overcomes acquired resistance to endocrine therapies in ER+, ErbB2-overexpressing breast cancer patients. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 655. doi:10.1158/1538-7445.AM2011-655

MM-111 is a novel bispecific antibody fusion protein which targets the ErbB2/ErbB3 oncogenic unit, blocking activation of the phosphatidylinositol 3-kinase (PI3K) pro-survival pathway. The anti-ErbB2 arm of MM-111 binds with high affinity to the ErbB2 receptor, which localizes the bispecific molecule to ErbB2 over-expressing tumor cells and promotes binding of the anti-ErbB3 arm to the ErbB3 receptor. MM-111 binding to ErbB3 results in inhibition of ErbB3 signaling by blocking the binding of the ErbB3 physiological ligand heregulin. MM-111 treatment of ErbB2 overexpressing cancer cells inhibits activation of the PI3K pathway with sub-nanomolar potency, blocks cell cycle progression and attenuates tumor cell growth in multiple xenograft models. ErbB2 over-expressing tumor cells are addicted to growth signals provided by the ligand-activated ErbB2/ErbB3 heterodimer. Activation of downstream PI3K pathway signaling also occurs through ligand-independent ErbB2/ErbB2 homodimers and ErbB2/ErbB3 heterodimers. Recently the ErbB2-targeted therapeutic antibody trastuzumab was shown to inhibit basal ErbB3 signaling in the absence of ligand stimulation, purportedly by interrupting ligand-independent ErbB2/ErbB3 heterodimers formed through overexpression of ErbB2. However, in these studies trastuzumab did not effectively block ligand-induced activation of the ErbB2/ErbB3 oncogenic unit. Indeed, there is emerging evidence that ligand-induced ErbB3 activation may have an important role in resistance to trastuzumab. As MM-111 and trastuzumab have distinct and potentially complimentary effects on signaling in cells overexpressing ErbB2, we hypothesized that their combination may synergistically effect inhibition of tumor cell growth driven by the ErbB2/3 signaling network. Our data demonstrate that MM-111 and trastuzumab positively combine to inhibit breast cancer growth in multiple in vitro breast cancer models expressing ErbB2 and ErbB3. Further, in the BT-474 breast cancer xenograft model the combination of MM-111 and trastuzumab results in greater inhibition of tumor growth and an increased number of completely regressed tumors compared to the monotherapy treatment groups. Pharmacodynamic analysis of samples from these studies show that repeated administration of the combined therapeutic agents results in strong inhibition of the ErbB3/PI3K pathway. In conclusion, we show that MM-111 and trastuzumab inhibit the growth of ErbB2 over-expressing breast tumors with distinct mechanisms that act synergistically in combination. Concurrent treatment with MM-111 and trastuzumab may provide a potent therapeutic regimen for ErbB2-overexpressing breast cancer patients and potentially deter acquired resistance to trastuzumab through ErbB3 activation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3485.