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.
Highlights
The type I insulin like growth factor receptor (IGF1R) is a heterotetrameric complex consisting of two disulfide-linked achains that bind IGF-1 and IGF-2 and two b-chains that include a transmembrane and a tyrosine kinase domain [1]
The ganitumab and L2-(A–C) epitopes were localized to the L2 domain, the F1-(A–C) epitope was localized to the FnIII-1 domain, aIR3 and Mab 391 bound within the CR domain, and 1H7 and 24–57 bound within the FnIII-1 domain (Table 1; Fig. S1)
Competition studies using microbeads loaded with hIGF1R(ECD)-mFc and challenged with FITC-labeled aIR3, 1H7, and ganitumab demonstrated that ganitumab does not bind to the aIR3 CR epitope or the 1H7 FnIII-1 epitope
Summary
The type I insulin like growth factor receptor (IGF1R) is a heterotetrameric complex consisting of two disulfide-linked achains that bind IGF-1 and IGF-2 and two b-chains that include a transmembrane and a tyrosine kinase domain [1]. Four protein structural motifs in the IGF1R ECD have been shown to be involved in ligand binding and selectivity: L1, L2, CR, and FnIII-1 [1]. IGF1R is closely related to the insulin receptor (INSR), there being 35% to 70% identity between their ECDs, depending on the structural motif [1]. Integration of IGF1R and INSR signaling can occur through hybrid receptors, which are preferentially activated by IGF-1 or IGF-2 [7]. Therapeutic 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
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