Platelet-Derived Growth Factor Receptor α Contributes to Human Hepatic Stellate Cell Proliferation and Migration

Platelet-derived growth factor receptor α (PDGFRα), a tyrosine kinase receptor, is up-regulated in hepatic stellate cells (HSCs) during chronic liver injury. HSCs mediate hepatic fibrosis through their activation from a quiescent state partially in response to profibrotic growth factors. HSC activation entails enhanced expression of profibrotic genes, increase in proliferation, and increase in motility, which facilitates migration within the hepatic lobule. We show colocalization of PDGFRα in murine carbon tetrachloride, bile duct ligation, and 0.1% 3,5-diethoxycarbonyl-1,4-dihydrocollidine models of chronic liver injury, and investigate the role of PDGFRα on proliferation, profibrotic gene expression, and migration in primary human HSCs (HHSteCs) using the PDGFRα-specific inhibitory monoclonal antibody olaratumab. Although lacking any effects on HHSteC transdifferentiation assessed by gene expression of ACTA2, TGFB1, COL1A1, SYP1, and FN1, olaratumab specifically reduced HHSteC proliferation (AlamarBlue assay) and cell migration (transwell migration assays). Using phospho-specific antibodies, we show that olaratumab attenuates PDGFRα activation in response to PDGF-BB, and reduced phosphorylation of extracellular signal-regulated kinase 1 and 2, Elk-1, p38, Akt, focal adhesion kinase, mechanistic target of rapamycin, C10 regulator of kinase II, and C10 regulator of kinase-like, suggesting that PDGFRα contributes to mitogenesis and actin reorganization through diverse downstream effectors. Our findings support a distinct contribution of PDGFRα signaling to HSC proliferation and migration and provide evidence that inhibition of PDGFRα signaling could alter the pathogenesis of hepatic fibrosis.

Phase 1 study of narnatumab, an anti-RON receptor monoclonal antibody, in patients with advanced solid tumors

Purpose Macrophage-stimulating 1-receptor (RON) is expressed on macrophages, epithelial cells, and a variety of tumors. Narnatumab (IMC-RON8; LY3012219) is a neutralizing monoclonal antibody that blocks RON binding to its ligand, macrophage-stimulating protein (MSP). This study assessed safety, maximum tolerated dose (MTD), pharmacokinetics, pharmacodynamics, and efficacy of narnatumab in patients with advanced solid tumors. Methods Narnatumab was administered intravenously weekly at 5, 10, 15, or 20 mg/kg or every 2 weeks at 15, 20, 30, or 40 mg/kg in 4-week cycles. Results Thirty-nine patients were treated, and 1 dose-limiting toxicity (DLT) (grade 3 hyponatremia, 5 mg/kg) was reported. The most common narnatumab-related adverse events (AEs) were fatigue (20.5%) and decreased appetite, diarrhea, nausea, and vomiting (10.3% each). Except for 2 treatment-related grade 3 AEs (hyponatremia, hypokalemia), all treatment-related AEs were grade 1 or 2. Narnatumab had a short half-life (<7 days). After Cycle 2, no patients had concentrations above 140 μg/mL (concentration that demonstrated antitumor activity in animal models), except for 1 patient receiving 30 mg/kg biweekly. Eleven patients had a best response of stable disease, ranging from 6 weeks to 11 months. Despite only 1 DLT, due to suboptimal drug exposure, the dose was not escalated beyond 40 mg/kg biweekly. This decision was based on published data reporting that mRNA splice variants of RON are highly prevalent in tumors, accumulate in cytoplasm, and are not accessible by large-molecule monoclonal antibodies. Conclusions Narnatumab was well tolerated and showed limited antitumor activity with this dosing regimen.

Inhibition of fibroblast growth factor receptor 3-dependent lung adenocarcinoma with a human monoclonal antibody

Activating mutations in fibroblast growth factor receptor 3 (FGFR3) have been identified in multiple types of human cancer and in congenital birth defects. In human lung cancer, fibroblast growth factor 9 (FGF9), a high-affinity ligand for FGFR3, is overexpressed in 10% of primary resected non-small cell lung cancer (NSCLC) specimens. Furthermore, in a mouse model where FGF9 can be induced in lung epithelial cells, epithelial proliferation and ensuing tumorigenesis is dependent on FGFR3. To develop new customized therapies for cancers that are dependent on FGFR3 activation, we have used this mouse model to evaluate a human monoclonal antibody (D11) with specificity for the extracellular ligand-binding domain of FGFR3, that recognizes both human and mouse forms of the receptor. Here, we show that D11 effectively inhibits signaling through FGFR3 in vitro, inhibits the growth of FGFR3-dependent FGF9-induced lung adenocarcinoma in mice, and reduces tumor-associated morbidity. Given the potency of FGF9 in this mouse model and the absolute requirement for signaling through FGFR3, this study validates the D11 antibody as a potentially useful and effective reagent for treating human cancers or other pathologies that are dependent on activation of FGFR3.

Summary: This study validates the FGF9 lung adenocarcinoma mouse model as a tool to screen and evaluate potential therapeutics that are designed to inhibit FGF9 or its target receptor, FGFR3.

Abstract C129: Antitumor efficacy and non-clinical safety of the high affinity anti-FGFR4 antibody H4: Implications for targeting the FGF19-FGFR4 axis in oncology

While FGFR4 and its ligand FGF19 represent a promising target for cancers in which FGF19 is overexpressed or gene amplified, inhibition of this pathway carries a potential toxicity risk due to the role of FGFR4 in bile acid homeostasis. Treatment with an anti-FGF19 antibody caused significant toxicity in cynomolgus monkeys that was attributed to perturbations in the enterohepatic circulation of bile acids. Given that FGF19 signals through other FGFRs besides FGFR4, we hypothesized that the toxicity profile of an anti-FGFR4 antibody might be different than that of an anti-FGF19 antibody. To test this we set out to identify an anti-FGFR4 antibody that inhibits FGF19 binding and displays anti-tumor activity in models overexpressing FGF19, and then determine its toxicity profile.

A phage screening effort yielded several anti-FGFR4 antibodies including H4, a high affinity fully human IgG1. H4 binds to the extracellular domain of FGFR4 (Kd = 12 pM) but does not cross react with FGFR1, FGFR2, or FGFR3. Furthermore, H4 efficiently blocks FGF19 binding to FGFR4 (IC50 = 589 pM) and inhibits FGF19-mediated downstream signaling in Hep3B2 cells. In human liver cancer cells that overexpress FGF19, H4 inhibits proliferation (IC50 250-1,100 nM). Anti-tumor activity was evaluated using cell line-derived xenograft tumors established from the FGF19 overexpressing and gene amplified cell lines HuH-7 and Hep3B2. H4 inhibited tumor growth in these models (T/C of 50% and 8%, respectively), and the antitumor effect was accompanied by alterations in several biomarkers associated with FGFR4 pathway inhibition including increased expression of CYP7A, the gene encoding the rate-limiting enzyme of bile acid synthesis.

Finally, to assess the potential toxicity of H4 treatment, single dose intravenous toxicology studies were conducted in rats (20, 60, and 200 mg/kg) and cynomolgus monkeys (5, 20, and 100 mg/kg). Although H4 binds with similar affinity to monkey and rat FGFR4, the toxicology profiles were distinct. H4 was well tolerated in rats with no significant findings up to 200 mg/kg. Effects in monkeys included sporadic malformed feces, reduced food intake, elevated serum ALT (up to 44 fold over baseline) and AST activities, and elevated fecal bile acid concentrations at all doses. Hyperplasia of the gall bladder epithelium occurred at 100 mg/kg. There were no microscopic findings in the liver. Qualitatively this toxicity profile is similar to that of the anti-FGF19 antibody, although the maximum tolerated dose (MTD) of H4 in the monkey after a single dose was considered to be &gt;100 mg/kg. These data are relevant for drug development in Oncology in light of the efforts aimed at developing anti-FGFR4 antibodies and small molecule inhibitors specific for FGFR4.

Citation Format: Timothy R. Mack, Colleen Burns Burns, Xuemei Guo, William John Feaver, Marie Prewett, Jennifer Gruber, Amelie Forest, Armando R. Irizarry, Ruslan Novosiadly, Nick Loizos, Andrew Dropsey. Antitumor efficacy and non-clinical safety of the high affinity anti-FGFR4 antibody H4: Implications for targeting the FGF19-FGFR4 axis in oncology. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C129.

A human monoclonal antibody targeting the stem cell factor receptor (c-Kit) blocks tumor cell signaling and inhibits tumor growth

Stem cell factor receptor (c-Kit) exerts multiple biological effects on target cells upon binding its ligand stem cell factor (SCF). Aberrant activation of c-Kit results in dysregulated signaling and is implicated in the pathogenesis of numerous cancers. The development of more specific and effective c-Kit therapies is warranted given its essential role in tumorigenesis. In this study, we describe the biological properties of CK6, a fully human IgG1 monoclonal antibody against the extracellular region of human c-Kit. CK6 specifically binds c-Kit receptor with high affinity (EC 50 = 0.06 nM) and strongly blocks its interaction with SCF (IC 50 = 0.41 nM) in solid phase assays. Flow cytometry shows CK6 binding to c-Kit on the cell surface of human small cell lung carcinoma (SCLC), melanoma, and leukemia tumor cell lines. Furthermore, exposure to CK6 inhibits SCF stimulation of c-Kit tyrosine kinase activity and downstream signaling pathways such as mitogen-activated protein kinase (MAPK) and protein kinase B (AKT), in addition to reducing tumor cell line growth in vitro. CK6 treatment significantly decreases human xenograft tumor growth in NCI-H526 SCLC (T/C% = 57) and Malme-3M melanoma (T/C% = 58) models in vivo. The combination of CK6 with standard of care chemotherapy agents, cisplatin and etoposide for SCLC or dacarbazine for melanoma, more potently reduces tumor growth (SCLC T/C% = 24, melanoma T/C% = 38) compared with CK6 or chemotherapy alone. In summary, our results demonstrate that CK6 is a c-Kit antagonist antibody with tumor growth neutralizing properties and are highly suggestive of potential therapeutic application in treating human malignancies harboring c-Kit receptor.

A phase I study of olaratumab, an anti-platelet-derived growth factor receptor alpha (PDGFRα) monoclonal antibody, in patients with advanced solid tumors

PURPOSE

The platelet-derived growth factor receptor (PDGFR) has an important role in tumorigenesis and tumor progression. Olaratumab (IMC-3G3) is a fully human monoclonal antibody that selectively binds human PDGFRα and blocks ligand binding. This phase I study assessed the safety, maximum tolerated dose (MTD), recommended phase II dose (RP2D), pharmacokinetics, and preliminary antitumor activity of olaratumab in patients with advanced solid tumors.

METHODS

Patients were enrolled into five dose-escalating cohorts of 3-6 patients each. Olaratumab was administered intravenously weekly at 4, 8, or 16 mg/kg (cohorts 1-3) or once every other week at 15 or 20 mg/kg (cohorts 4-5), with 4 weeks/cycle.

RESULTS

Nineteen patients were treated in five cohorts. There were no dose-limiting toxicities; the MTD was not identified with the doses studied. The most common olaratumab-related adverse events (AE) were fatigue and infusion reactions (10.5 % each). With the exception of 1 patient (20 mg/kg) experiencing two grade 3 drug-related AEs after the dose-limiting toxicity assessment period, all drug-related AEs were grade 1 or 2. The trough concentrations (C min) for 16 mg/kg weekly and 20 mg/kg biweekly were higher than 155 μg/mL, and the concentration found to be efficacious in preclinical xenograft models. Twelve patients (63.2 %) had a best response of stable disease [median duration of 3.9 months (95 % CI 2.3-8.7)].

CONCLUSIONS

Olaratumab was well tolerated and showed preliminary antitumor activity. RP2Ds are 16 mg/kg weekly and 20 mg/kg biweekly. Phase II studies of olaratumab as monotherapy and in combination are ongoing in several tumor types.

Abstract C159: A human monoclonal antibody targeting the stem cell factor receptor (c-Kit) blocks tumor cell signaling and inhibits tumor growth

Stem cell factor receptor also known as c-Kit is a receptor tyrosine kinase that mediates cell growth, survival, and differentiation signals in response to its ligand stem cell factor (SCF). Aberrant c-Kit expression and/or activation through mutations or autocrine/paracrine signaling mechanisms occur in various malignancies and promote tumor development. Specific therapeutic targeting of c-Kit in cancer is warranted given its cancer role. In this study, we characterize the biological properties of CK6, a fully human IgG1 monoclonal antibody against the extracellular region of human c-Kit. CK6 specifically binds human c-Kit receptor with high affinity (EC50= 0.06nM) and strongly blocks its interaction with SCF (IC50= 0.41nM) in solid phase assays. Flow cytometry shows CK6 binding to the cell surface of small cell lung carcinoma (SCLC), melanoma, leukemia, and other human c-Kit expressing tumor cell lines. Furthermore, exposure to CK6 inhibits SCF stimulation of c-Kit tyrosine kinase activity and downstream signaling pathways in these tumor cell lines. Reduced levels of phosphorylated c-Kit, mitogen-activated protein kinase (MAPK) and protein kinase B/Akt were observed. Given these findings, we evaluated the antitumor growth efficacy of CK6 in several human xenograft tumor models in vivo. CK6 monotherapy treatment significantly suppressed tumor growth of NCI-H526 SCLC (T/C%= 50) and Malme-3M Melanoma (T/C%= 58) xenograft models. The combination of CK6 with standard of care (SOC) chemotherapy agents, cisplatin and etoposide for SCLC or dacarbazine for melanoma, led to enhanced tumor growth inhibition (SCLC T/C%= 12; melanoma T/C%= 38) compared to CK6 monotherapy or SOC alone. In summary, our results demonstrate that CK6 is a c-Kit antagonist antibody with tumor growth neutralizing properties and are highly suggestive of potential therapeutic application in treating human cancers harboring c-Kit receptor.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C159.

Citation Format: Maria Lebron, Laura Brennan, Chris Damoci, Marie Prewett, Marina Starodubtseva, Michael Amatulli, Yiwei Zhang, Douglas Burtrum, Paul Balderes, Kris Persaud, David Surguladze, Nick Loizos, Keren Paz, Helen Kotanides. A human monoclonal antibody targeting the stem cell factor receptor (c-Kit) blocks tumor cell signaling and inhibits tumor growth. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C159.

Platelet-derived growth factor receptor alpha (PDGFRα) targeting and relevant biomarkers in ovarian carcinoma

OBJECTIVE

Platelet-derived growth factor receptor alpha (PDGFRα) is believed to be associated with cell survival. We examined (i) whether PDGFRα blockade enhances the antitumor activity of taxanes in ovarian carcinoma and (ii) potential biomarkers of response to anti-PDGFRα therapy.

METHODS

PDGFRα expression in 176 ovarian carcinomas was evaluated with tissue microarray and correlated to survival outcome. Human-specific monoclonal antibody to PDGFRα (IMC-3G3) was used for in vitro and in vivo experiments with or without docetaxel. Gene microarrays and reverse-phase protein arrays with pathway analyses were performed to identify potential predictive biomarkers.

RESULTS

When compared to low or no PDGFRα expression, increased PDGFRα expression was associated with significantly poorer overall survival of patients with ovarian cancer (P=0.014). Although treatment with IMC-3G3 alone did not affect cell viability or increase apoptosis, concurrent use of IMC-3G3 with docetaxel significantly enhanced sensitization to docetaxel and apoptosis. In an orthotopic mouse model, IMC-3G3 monotherapy had no significant antitumor effects in SKOV3-ip1 (low PDGFRα expression), but showed significant antitumor effects in HeyA8-MDR (high PDGFRα expression). Concurrent use of IMC-3G3 with docetaxel, compared with use of docetaxel alone, significantly reduced tumor weight in all tested cell lines. In protein ontology, the EGFR and AKT pathways were downregulated by IMC-3G3 therapy. MAPK and CCNB1 were downregulated only in the HeyA8-MDR model.

CONCLUSION

These data identify IMC-3G3 as an attractive therapeutic strategy and identify potential predictive markers for further development.

Stromal platelet-derived growth factor receptor α (PDGFRα) provides a therapeutic target independent of tumor cell PDGFRα expression in lung cancer xenografts

In lung cancer, platelet-derived growth factor receptor α (PDGFRα) is expressed frequently by tumor-associated stromal cells and by cancer cells in a subset of tumors. We sought to determine the effect of targeting stromal PDGFRα in preclinical lung tumor xenograft models (human tumor, mouse stroma). Effects of anti-human (IMC-3G3) and anti-mouse (1E10) PDGFRα monoclonal antibodies (mAb) on proliferation and PDGFRα signaling were evaluated in lung cancer cell lines and mouse fibroblasts. Therapy studies were conducted using established PDGFRα-positive H1703 cells and PDGFRα-negative Calu-6, H1993, and A549 subcutaneous tumors in immunocompromised mice treated with vehicle, anti-PDGFRα mAbs, chemotherapy, or combination therapy. Tumors were analyzed for growth and levels of growth factors. IMC-3G3 inhibited PDGFRα activation and the growth of H1703 cells in vitro and tumor growth in vivo, but had no effect on PDGFRα-negative cell lines or mouse fibroblasts. 1E10 inhibited growth and PDGFRα activation of mouse fibroblasts, but had no effect on human cancer cell lines in vitro. In vivo, 1E10-targeted inhibition of murine PDGFRα reduced tumor growth as single-agent therapy in Calu-6 cells and enhanced the effect of chemotherapy in xenografts derived from A549 cells. We also identified that low expression cancer cell expression of VEGF-A and elevated expression of PDGF-AA were associated with response to stromal PDGFRα targeting. We conclude that stromal PDGFRα inhibition represents a means for enhancing control of lung cancer growth in some cases, independent of tumor cell PDGFRα expression.

A phase Ib/II study evaluating the efficacy of doxorubicin (D) with or without a human anti-PDGFRα monoclonal antibody olaratumab (IMC-3G3) in the treatment of advanced soft tissue sarcoma (STS)

TPS10099

Background: PDGFRα is a receptor commonly overexpressed in STS. Olaratumab is a fully human IgG1 MAb which specifically binds to human PDGFRα and blocks PDGFR-mediated signaling pathways. Olaratumab demonstrated significant tumor inhibition as a monotherapy and in combination with standard chemotherapy in preclinical human sarcoma xenograft models. A Phase 1b/2 clinical trial for patients with advanced or metastatic STS is currently enrolling patients in 9 sites across the United States; approximately 45 patients have been randomized. Methods: In Phase 1b, patients received olaratumab (15 mg/kg) via intravenous infusion on Days 1 and 8 of each 21-day cycle and D (75 mg/m²) 1 hour after olaratumab on Day 1. Phase 1b was completed without encountering dose-limiting toxicities; enrollment into Phase 2 has begun. The primary endpoint of the Phase 2 portion is to compare progression-free survival in patients with advanced STS when treated with D plus olaratumab versus D alone. Planned enrollment goal for Phase 2 is 130 patients. Patients are required to be ≥ 18 years old with ECOG 0-2, have appropriate organ function and histologically confirmed, measurable, and advanced STS. There is no restriction on the number of prior therapies. Patients are randomized 1:1 to either D plus olaratumab (Arm A) at same dose/schedule as in Phase 1b or D alone (Arm B). All patients can receive dexrazoxane. Upon completion of 8 cycles, patients in Arm A continue with olaratumab monotherapy. Patients in Arm B who develop disease progression during or after treatment can subsequently receive olaratumab monotherapy. Patients are stratified to treatment arms according to PDGFRα expression (positive vs. negative), number of previous lines of treatment, sarcoma subtype, and ECOG performance status. Response assessment occurs every 6 weeks. Exploratory analyses include biomarkers of olaratumab pharmacodynamic activity including PDGF ligands, PDGFR downstream molecules, VEGF, and changes in vascularity of tumor specimens.

Abstract C54: In vitro and in vivo anti-tumor activity of the antimacrophage stimulating 1-receptor antibody IMC-RON8 in breast and bladder cancer models

Macrophage stimulating 1-receptor (RON) is a member of the c-Met receptor tyrosine kinase family. RON is normally expressed on macrophages and epithelial cells. However, it is overexpressed and activated in a large number of human tumors. The fully human anti-RON antibody, IMC-RON8, blocks the RON ligand, macrophage-stimulating protein (MSP), from binding to RON and has been demonstrated to have antitumor activity against human colon, lung, and pancreatic xenografts in mice. Overexpression of RON correlates with a worse clinical outcome for patients in at least two human cancer indications, namely breast and bladder. Given this correlation, we investigated the effect of IMC-RON8 in several in vitro and in vivo systems with the RON-positive breast and bladder cancer cell lines JIMT-1 and BFTC-905, respectively. Both of these were found from a screen of breast and bladder cancer cell lines designed to identify those that are RON positive and responsive to MSP through activation of the MAP Kinase signaling pathway. IMC-RON8 inhibited the phosphorylation of MAP Kinase in response to 5nM MSP stimulation for JIMT-1 and BFTC-905. IMC-RON8 also inhibited the MSP-induced cellular migration of JIMT-1 in a wound healing assay, completely preventing progression towards closing of the wound. As measured by flow cytometry, IMC-RON8 at 33.3nM induced the internalization of 38% and 31% cell-surface RON expressed on JIMT-1 and BFTC-905 cells, respectively, following 24 hours of treatment at 37°C. To investigate IMC-RON8-induced RON downmodulation, clonal Hela cells stably expressing a RON-GFP protein were generated. These cells were treated with MSP, a RON agonist antibody (RON2), or IMC-RON8 and the level of RON-GFP was then measured by confocal microscopy in a live-cell time lapse experiment. IMC-RON8 treatment actively induced receptor interalization and caused RON-GFP degradation (reduction of the GFP signal) relative to IgG controls, although to a lesser extent than MSP and RON2. In an in vivo JIMT-1 xenograft model, IMC-RON8 significantly inhibited tumor growth with a %T/C value of 59 when administered at 60 mg/kg twice a week. Combination of IMC-RON8 with the chemotherapeutic agent, docetaxel at 12 mg/kg once per week, significantly improved the anti-tumor effects compared to either monotherapy with a %T/C value of 20. In an in vivo BFTC-905 xenograft model, IMC-RON8 significantly inhibited tumor growth with a %T/C value of 58 when administered at 60 mg/kg twice a week. BFTC-905 tumors removed after the last IMC-RON8 dose (6 total doses over a 3 week study period), showed a decrease in the total level of RON receptor present in tumors relative to controls (p-value of &lt;0.0001). These preclinical results therefore demonstrate that IMC-RON8 possesses antitumor activity in vitro and in vivo and may be effective as a single agent or in combination with a conventional cytotoxic therapy for the treatment of cancer. Taken together, these data suggest the mechanism of action for IMC-RON8 antitumor activity can be both ligand-blocking activity and receptor downmodulation; both leading to a decrease in ligand-induced tumor cell proliferation. IMC-RON8 is presently in Phase I studies to treat patients with advanced solid tumors.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr C54.

Rationale for the development of IMC-3G3, a fully human immunoglobulin G subclass 1 monoclonal antibody targeting the platelet-derived growth factor receptor alpha

A large body of evidence suggests that the platelet-derived growth factor (PDGF) family and associated receptors are potential targets in oncology therapeutic development because of their critical roles in the proliferation and survival of various cancers and in the regulation and growth of the tumor stroma and blood vessels. Several small molecules that nonspecifically target the PDGF signaling axis are in current use or development as anticancer therapies. However, for the majority of these agents, PDGF and its receptors are neither the primary targets nor the principal mediators of anticancer activity. IMC-3G3, a fully human monoclonal antibody of the immunoglobulin G subclass 1, specifically binds to the human PDGF receptor alpha (PDGFRalpha) with high affinity and blocks PDGF ligand binding and PDGFRalpha activation. The results of preclinical studies and the frequent expression of PDGFRalpha in many types of cancer and in cancer-associated stroma support a rationale for the clinical development of IMC-3G3. Currently, IMC-3G3 is being evaluated in early clinical development for patients with several types of solid malignancies.

Abstract 1793: Platelet-derived growth factor receptor alpha blockade significantly enhances sensitization to docetaxel in ovarian carcinoma

Objective: Platelet-derived growth factor receptor alpha (PDGFRα) is a member of class III receptor tyrosine kinase and is associated with cell survival. Here, we examined whether PDGFRα blockade enhances the anti-tumor activity of taxanes in ovarian carcinoma.

Methods: PDGFRα expression was evaluated with RT-PCR and Western blot in multiple ovarian cancer cell lines. Human-specific monoclonal antibody to PDGFRα (IMC-3G3, ImClone Systems, NJ, USA) was used for in-vitro (cell viability assay, apoptosis assay) and in-vivo experiments with or without docetaxel.

Results: All eleven tested ovarian cancer cell lines had variable expression of PDGFRα. OVCA433 cell line had the highest level of expression of PDGFRα. IMC-3G3 blocked PDGFRα phosphorylation in response to PDGF-AA stimulation. IMC-3G3 alone had no affect on cell viability in all tested cell lines. However, IMC-3G3 significantly enhanced sensitization to docetaxel: IC50 reduction rate, SKOV3-ip1, 25.5%; HeyA8-parental, 31.7%; OVCA433, 35.3%; and HeyA8-MDR, 44.2% (all, p&lt;0.05). While treatment with IMC-3G3 alone did not increase apoptosis, concurrent use with docetaxel significantly enhanced docetaxel-mediated apoptosis (mean percentage of total apoptotic cells in SKOV3-ip1, untreated vs IMC-3G3 alone vs docetaxel alone vs docetaxel with IMC-3G3, 7.3% vs 7.1% vs 15.4% vs 22.2%, p&lt;0.001). In the in-vivo experiment with SKOV3-ip1 bearing mice, administration of IMC-3G3 monotherapy had no significant anti-tumor effects compared to the control group (mean tumor weight, 2.5 ± 0.32 vs 2.9 ± 0.51 grams, p&gt;0.05). Concurrent use of IMC-3G3 with docetaxel significantly reduced tumor weight compared to docetaxel alone (mean tumor weight, 0.22 ± 0.07 vs 0.63 ± 0.14 grams, tumor reduction rate 65.1%, p&lt;0.05).

Conclusion: IMC-3G3 significantly enhances the anti-tumor effects of docetaxel. Therefore, targeting PDGFRα may be an attractive approach for increasing the cytotoxic effects of chemotherapy.

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 1793.

Lack of detection of agonist activity by antibodies to platelet-derived growth factor receptor alpha in a subset of normal and systemic sclerosis patient sera

OBJECTIVE

To investigate whether agonist anti-platelet-derived growth factor receptor alpha (anti-PDGFRalpha) antibodies are present in the serum of patients with systemic sclerosis (SSc; scleroderma).

METHODS

Sera were obtained from healthy subjects and scleroderma patients. An electrochemiluminescence binding assay was performed for detection of serum autoantibodies to PDGFRalpha, PDGFRbeta, epidermal growth factor receptor (EGFR), and colony-stimulating factor receptor 1 (CSFR1). Serum immunoglobulin was purified by protein A/G chromatography. To assess Ig agonist activity, PDGFRalpha-expressing cells were incubated with pure Ig and the level of receptor phosphorylation determined in an enzyme-linked immunoassay, as well as by Western blotting. Ig agonist activity was also assessed in a mitogenic assay and by MAP kinase activation in a PDGFRalpha-expressing cell line.

RESULTS

Sera from 34.3% of the healthy subjects and 32.7% of the SSc patients contained detectable autoantibodies to PDGFRalpha and PDGFRbeta, but not EGFR or CSFR1. Purified Ig from these sera was shown to retain PDGFR binding activity and, at 200-1,000 microg/ml, exhibited no agonist activity in a cell-based PDGFRalpha phosphorylation assay and did not stimulate a mitogenic response or MAP kinase activation in a PDGFRalpha-expressing cell line. Two purified Ig samples that were unable to bind PDGFRalpha did exhibit binding activity to a nonglycosylated form of PDGFRalpha.

CONCLUSION

Although approximately one-third of sera from scleroderma patients contained detectable autoantibodies to PDGFR, these antibodies were not specific to scleroderma, since they were also detected in a similar percentage of samples from normal subjects. PDGFRalpha agonist activity was not demonstrated when purified Ig from these sera was tested in cell-based assays.

Identifying protein interactions by hydroxyl-radical protein footprinting

Hydroxyl-radical protein footprinting is a straightforward and direct method to map protein sites involved in macromolecular interactions. The first step is to radioactively end-label the protein. Using hydroxyl radicals as a peptide backbone cleavage reagent, the protein is then cleaved in the absence and presence of ligand. Cleavage products are separated by high resolution gel electrophoresis. The digital image of the footprinting gel can be subjected to quantitative analysis to identify changes in the sensitivity of the protein to hydroxyl-radical cleavage. Molecular weight markers are electrophoresed on the same gel and hydroxyl-radical cleavage sites assigned by interpolation between the known cleavage sites of the markers. The results are presented in the form of a difference plot that shows regions of the protein that change their susceptibility to cleavage while bound to a ligand.

Platelet-derived growth factor receptor-alpha: a novel therapeutic target in human hepatocellular cancer

Hepatocellular cancer (HCC) is a disease of poor prognosis. Identifying novel molecular aberrations might present opportunities to identify new therapeutic targets. Due to the similarities between the processes of development and cancer, we used early developing livers to identify genes that might play a primary role in HCC. Platelet-derived growth factor receptor-alpha (PDGFRalpha) was identified from microarray using early developing mouse livers. Expression of PDGFRalpha and its upstream effectors, PDGF-AA and PDGF-CC, were examined in HCC tissues (n = 43) by Western blot, real-time PCR, and immunohistochemistry. Finally, effect of anti-PDGFRalpha antibody (mAb 3G3, ImClone Systems, Inc.) was examined on human hepatoma cells. A high expression of PDGFRalpha was observed during early liver development. HCCs (17 of 21) revealed cytoplasmic PDGFRalpha and activated PDGFRalpha (phospho-Tyr(754)) by immunohistochemistry. Additional HCCs (14 of 22) showed elevated PDGFRalpha levels when compared with the adjacent normal livers by Western blots. Of these 14 patients, 3 showed increased PDGFRalpha gene expression, 3 showed elevated PDGF-AA, and 4 had higher PDGF-CC levels in the tumors compared with adjacent livers. Multiple hepatoma cell lines, when treated with mAb 3G3, showed significant decreases in cell proliferation and survival (P < 0.05). In conclusion, approximately 70% of HCC tissues had elevated PDGFRalpha levels due to diverse mechanisms. PDGFRalpha inhibition in hepatoma cells led to diminution of tumor cell survival and proliferation and thus might be of therapeutic significance.

An antibody directed against PDGF receptor beta enhances the antitumor and the anti-angiogenic activities of an anti-VEGF receptor 2 antibody

Platelet-derived growth factor (PDGF) and its receptors (PDGFR) play important roles in tumorigenesis through stimulating tumor growth and promoting angiogenesis via enhancing pericyte recruitment and vessel maturation. Here we produced a neutralizing antibody, 1B3, directed against mouse PDGFRbeta. 1B3 binds to PDGFRbeta with high affinity (9x10(-11)M) and blocks PDGF-BB from binding to the receptor with an IC(50) of approximately 1.2 nM. The antibody also blocks ligand-stimulated activation of PDGFRbeta and downstream signaling molecules, including Akt and MAPK p42/44, in tumor cells. In animal studies, 1B3 significantly enhanced the antitumor and the anti-angiogenic activities of DC101, an antibody directed against mouse vascular endothelial growth factor receptor 2, in a pancreatic (BxPC-3) and a non-small cell lung (NCI-H460) tumor xenograft models. Treatment with the combination of 1B3 and DC101 in BxPC-3 xenograft-bearing mice resulted in tumor regression in 58% of mice compared to that in 18% of mice treated with DC101 alone. Taken together, these results lend great support to use PDGFRbeta antagonists in combinations with other antitumor and/or anti-angiogenic agents in the treatment of a variety of cancers.

Human bone marrow activates the Akt pathway in metastatic prostate cells through transactivation of the alpha-platelet-derived growth factor receptor

The factors regulating the bone tropism of disseminated prostate cancer cells are still vaguely defined. We report that prostate cancer cells that metastasize to the skeleton respond to human bone marrow with a robust stimulation of the phosphatidylinositol 3-kinase/Akt pathway, whereas prostate cells that lack bone-metastatic potential respond negligibly. The majority of this Akt activation is dependent on alpha-platelet-derived growth factor receptor (alpha-PDGFR) signaling, which was shown using the small-molecule inhibitor of PDGFR signaling AG1296. Low concentrations of PDGF-AA and PDGF-BB found in bone marrow aspirates, which were detected by ELISA, do not account for the high levels of alpha-PDGFR signaling. Additionally, neutralizing PDGF binding using a alpha-PDGFR-specific antibody (IMC-3G3) failed to produce a significant inhibition of bone marrow-induced Akt activation. However, the inhibitory effect of IMC-3G3 rivaled that of AG1296 when incubation was done under conditions that stimulated alpha-PDGFR internalization. We conclude that alpha-PDGFR is activated by multiple soluble factors contained within human bone marrow, in addition to its natural ligands, and this transactivation is dependent on receptor localization to the plasma membrane. Therefore, alpha-PDGFR expression may provide select prostate phenotypes with a growth advantage within the bone microenvironment.

Targeting the platelet-derived growth factor receptor alpha with a neutralizing human monoclonal antibody inhibits the growth of tumor xenografts: implications as a potential therapeutic target

Platelet-derived growth factor receptor alpha (PDGFRalpha) is a type III receptor tyrosine kinase that is expressed on a variety of tumor types. A neutralizing monoclonal antibody to human PDGFRalpha, which did not cross-react with the beta form of the receptor, was generated. The fully human antibody, termed 3G3, has a Kd of 40 pmol/L and blocks both PDGF-AA and PDGF-BB ligands from binding to PDGFRalpha. In addition to blocking ligand-induced cell mitogenesis and receptor autophosphorylation, 3G3 inhibited phosphorylation of the downstream signaling molecules Akt and mitogen-activated protein kinase. This inhibition was seen in both transfected and tumor cell lines expressing PDGFRalpha. The in vivo antitumor activity of 3G3 was tested in human glioblastoma (U118) and leiomyosarcoma (SKLMS-1) xenograft tumor models in athymic nude mice. Antibody 3G3 significantly inhibited the growth of U118 (P=0.0004) and SKLMS-1 (P <0.0001) tumors relative to control. These data suggest that 3G3 may be useful for the treatment of tumors that express PDGFRalpha.

Mapping protein-ligand interactions by hydroxyl-radical protein footprinting

Hydroxyl-radical protein footprinting is a direct method to map protein sites involved in macromolecular interactions. The first step is to radioactively end-label the protein. Using hydroxyl radicals as a peptide backbone cleavage reagent, the protein is then cleaved in the absence and presence of ligand. Cleavage products are separated by high-resolution gel electrophoresis. The digital image of the footprinting gel can be subjected to quantitative analysis to identify changes in the sensitivity of the protein to hydroxyl-radical cleavage. Molecular weight markers are electrophoresed on the same gel and hydroxyl-radical cleavage sites assigned by interpolation between the known cleavage sites of the markers. The results are presented in the form of a difference plot that show regions of the protein that change their susceptibility to cleavage while bound to a ligand.

Mapping interactions of Escherichia coli GreB with RNA polymerase and ternary elongation complexes

Escherichia coli GreA and GreB modulate transcription elongation by interacting with the ternary elongation complex (containing RNA polymerase, DNA template, and RNA transcript) to induce hydrolytic cleavage of the transcript and release of the 3'-terminal fragment. Hydroxyl radical protein footprinting and alanine-scanning mutagenesis were used to investigate the interactions of GreB with RNA polymerase alone and in a ternary elongation complex. A major determinant for binding GreB to both RNA polymerase and the ternary elongation complex was identified. In addition, the hydroxyl radical footprinting indicated major conformational changes of GreB, in terms of reorientations of the N- and C-terminal domains with respect to each other, particularly upon interactions with the ternary elongation complex.

Determinants for Escherichia coli RNA polymerase assembly within the beta subunit

We used binding assays and other approaches to identify fragments of the Escherichia coli RNAP beta subunit involved in the obligatory interaction with the alpha subunit to form the stable assembly intermediate alpha2beta as well as in the interaction to recruit the beta' subunit into the alpha2beta sub-assembly. We show that two regions of evolutionarily conserved sequence near the C terminus of beta (conserved regions H and I) are central to the assembly of RNAP and likely make subunit-subunit contacts with both alpha and beta'.

Intron-encoded endonuclease I-TevII binds across the minor groove and induces two distinct conformational changes in its DNA substrate

I-TevII is the homing endonuclease encoded by the sunY intron of bacteriophage T4. The enzyme cleaves an intronless sunY gene near the exon I-exon II junction, thereby initiating intron homing into its cognate intronless allele. Specifically, I-TevII cleaves its DNA target 13 to 15 nucleotides (nt) downstream of the sunY intron insertion site, generating 2-nl 3'-OH extensions. Here, we present evidence that I-TevII makes predominantly minor groove contacts in two regions of its recognition sequence, as does I-TevI, the other homing endonuclease encoded by phage T4. Following cleavage, I-TevII was shown to remain bound to one of its DNA products, suggesting possible additional roles for the endonuclease in the mobility process. Interestingly, two distinct conformational changes were detected by gel analysis in the DNA substrate following binding by I-TevII, one occurring in the absence of Mg2+, the second being dependent on the presence of Mg2+. The Mg(2+)-induced distortion accompanies a nick in one strand, and may serve to bring the cleavage site on the other strand into proximity with the catalytic domain of the protein.

Evolution of mobile group I introns: recognition of intron sequences by an intron-encoded endonuclease

Mobile group I introns are hypothesized to have arisen after invasion by endonuclease-encoding open reading frames (ORFs), which mediate their mobility. Consistent with an endonuclease-ORF invasion event, we report similarity between exon junction sequences (the recognition site for the mobility endonuclease) and intron sequences flanking the endonuclease ORF in the sunY gene of phage T4. Furthermore, we have demonstrated the ability of the intron-encoded endonuclease to recognize and cleave these intron sequences when present in fused form in synthetic constructs. These observations and accompanying splicing data are consistent with models in which the invading endonuclease ORF is provided safe haven within a splicing element. In turn the intron is afforded immunity to the endonuclease product, which imparts mobility to the intron.

The td intron endonuclease I-TevI makes extensive sequence-tolerant contacts across the minor groove of its DNA target

I-TevI, a double-strand DNA endonuclease encoded by the mobile td intron of phage T4, has specificity for the intronless td allele. Genetic and physical studies indicate that the enzyme makes extensive contacts with its DNA substrate over at least three helical turns and around the circumference of the helix. Remarkably, no single nucleotide within a 48 bp region encompassing this interaction domain is essential for cleavage. Although two subdomains (DI and DII) contain preferred sequences, a third domain (DIII), a primary region of contact with the enzyme, displays much lower sequence preference. While DII and DIII suffice for recognition and binding of I-TevI, all three domains are important for formation of a cleavage-competent complex. Mutational, footprinting and interference studies indicate predominant interactions of I-TevI across the minor groove and phosphate backbone of the DNA. Contacts appear not to be at the single nucleotide level; rather, redundant interactions and/or structural recognition are implied. These unusual properties provide a basis for understanding how I-TevI recognizes T-even phage DNA, which is heavily modified in the major groove. These recognition characteristics may increase the range of natural substrates available to the endonuclease, thereby extending the invasive potential of the mobile intron.