Epitope-Specific Mechanisms of IGF1R Inhibition by Ganitumab

Abstract

BackgroundTherapeutic antibodies targeting the IGF1R have shown diverse efficacy and safety signals in oncology clinical trials. The success of these agents as future human therapeutics depends on understanding the specific mechanisms by which these antibodies target IGF1R signaling.Methodology/Principal FindingsA panel of well-characterized assays was used to investigate the mechanisms by which ganitumab, a fully human anti-IGF1R antibody undergoing clinical testing, inhibits IGF1R activity. Epitope mapping using IGF1R subdomains localized the ganitumab binding site to the L2 domain. Binding of ganitumab inhibited the high-affinity interaction of IGF-1 and IGF-2 required to activate IGF1R in cells engineered for IGF1R hypersensitivity and in human cancer cell lines, resulting in complete blockade of ligand-induced cellular proliferation. Inhibition of IGF1R activity by ganitumab did not depend on endosomal sequestration, since efficient ligand blockade was obtained without evidence of receptor internalization and degradation. Clinically relevant concentrations of ganitumab also inhibited the activation of hybrid receptors by IGF-1 and IGF-2. Ganitumab was not an agonist of homodimeric IGF1R or hybrid receptors in MCF-7 and COLO 205 cells, but low-level IGF1R activation was detected in cells engineered for IGF1R hypersensitivity. This activation seems biologically irrelevant since ganitumab completely inhibited ligand-driven proliferation. The in vivo efficacy profile of ganitumab was equivalent or better than CR and FnIII-1 domain-specific antibodies, alone or in combination with irinotecan. CR domain-specific antibodies only blocked IGF-1 binding to IGF1R but were more potent than ganitumab at inducing homodimer and hybrid receptor downregulation in vitro, however this difference was less obvious in vivo. No inhibition of hybrid receptors was observed with the FnIII-1 domain antibodies, which were relatively strong homodimer and hybrid agonists.Conclusions/SignificanceThe safety and efficacy profile of ganitumab and other anti-IGF1R antibodies may be explained by the distinct molecular mechanisms by which they inhibit receptor signaling.

Analysis of the quantitative balance between insulin-like growth factor (IGF)-1 ligand, receptor, and binding protein levels to predict cell sensitivity and therapeutic efficacy

“…Heterodimers of IGF1R/IR are preferentially activated by IGF ligands; therefore, treatment with anti-IGF therapies would also impact IGF1R/IR receptor activity. For example, ganitumab (AMG 479), a monoclonal antibody against IGF1R, has been shown to be effective against inhibiting IGF ligand stimulated activation of IGF1R/IR heterodimers [ 77 ],[ 78 ].…”

The IGF-Trap: Novel Inhibitor of Carcinoma Growth and Metastasis

“…The mouse monoclonal antibody to human IGFIR-MAb 391 (R&D Systems; ref. 29) was used to treat tumor-bearing mice in vivo. For IHC, rabbit polyclonal antibodies to pIGFIR, Ki67, and cleaved caspase-3 (all from Abcam) and a rat monoclonal antibody to CD31 (Clone MEC 13.3) from BD Biosciences were used.…”

Specificity Delivers: Therapeutic Role of Tumor Antigen-Specific Antibodies in Pancreatic Cancer

“…Binding of ligands, IGF-1 or IGF-2, leads to a conformational change and autophosphorylation of the receptor, which activates downstream signaling pathways such as the PI3K/Akt and the Ras/Raf/MEK/ERK pathways. These pathways are shown to inhibit apoptosis, and improve cell survival, cell proliferation and invasion 28, 29 . IGF-1R, IGF-1 and IGF-2 are overexpressed in PDAs and preclinical studies have shown that the IGF-1R signaling cascade plays a pro-oncogenic role 30 .…”