Paradoxical effects of the phage display-derived peptide antagonist IGF-F1-1 on insulin-like growth factor-1 receptor signaling

Acquired Resistance to Drugs Targeting Tyrosine Kinases

Resistance to chemotherapeutic drugs exemplifies the greatest hindrance to effective treatment of cancer patients. The molecular mechanisms responsible have been investigated for over 50 years and have revealed the lack of a single cause, but instead, multiple mechanisms including induced expression of membrane transporters that pump drugs out of cells (multidrug resistance (MDR) phenotype), changes in the glutathione system, and altered metabolism. Treatment of cancer patients/cancer cells with chemotherapeutic agents and/or molecularly targeted drugs is accompanied by acquisition of resistance to the treatment administered. Chemotherapeutic agent resistance was initially assumed to be due to induction of mutations leading to a resistant phenotype. While this has occurred for molecularly targeted drugs, it is clear that drugs selectively targeting tyrosine kinases (TKs) cause the acquisition of mutational changes and resistance to inhibition. The first TK to be targeted, Bcr-Abl, led to the generation of several drugs including imatinib, dasatinib, and sunitinib that provided a rich understanding of this phenomenon. It became clear that mutations alone were not the only cause of resistance. Additional mechanisms were involved, including alternative splicing, alternative/compensatory signaling pathways, and epigenetic changes. This review will focus on resistance to tyrosine kinase inhibitors (TKIs), receptor TK (RTK)-directed antibodies, and antibodies that inactivate specific RTK ligands. New approaches and concepts aimed at avoiding the generation of drug resistance will be examined. Many RTKs, including the IGF-1R, are dependence receptors that induce ligand-independent apoptosis. How this signaling paradigm has implications on therapeutic strategies will also be considered.

Acquired resistance to drugs targeting receptor tyrosine kinases

Development of resistance to chemotherapeutic drugs represents a significant hindrance to the effective treatment of cancer patients. The molecular mechanisms responsible have been investigated for over half a century and have revealed the lack of a single cause. Rather, a multitude of mechanisms have been delineated ranging from induction and expression of membrane transporters that pump drugs out of cells (multidrug resistance (MDR) phenotype), changes in the glutathione system and altered metabolism to name a few. Treatment of cancer patients/cancer cells with chemotherapeutic agents and/or molecularly targeted drugs is accompanied by acquisition of resistance to the treatment administered. Chemotherapeutic agent resistance was initially assumed to be due to induction of mutations leading to a resistant phenotype. This has also been true for molecularly targeted drugs. Considerable experience has been gained from the study of agents targeting the Bcr-Abl tyrosine kinase including imatinib, dasatinib and sunitinib. It is clear that mutations alone are not responsible for the many resistance mechanisms in play. Rather, additional mechanisms are involved, ranging from epigenetic changes, alternative splicing and the induction of alternative/compensatory signaling pathways. In this review, resistance to receptor tyrosine kinase inhibitors (RTKIs), RTK-directed antibodies and antibodies that inactivate ligands for RTKs are discussed. New approaches and concepts aimed at avoiding the generation of drug resistance will be examined. The recent observation that many RTKs, including the IGF-1R, are dependence receptors that induce apoptosis in a ligand-independent manner will be discussed and the implications this signaling paradigm has on therapeutic strategies will be considered.

Defining the pathway to insulin-like growth factor system targeting in cancer

The insulin-like growth factors (IGFs; IGF-1 and IGF-2) play central roles in cell growth, differentiation, survival, transformation and metastasis. The biologic effects of the IGFs are mediated by the IGF-1 receptor (IGF-1R), a receptor tyrosine kinase with homology to the insulin receptor (IR). Dysregulation of the IGF system is well recognized as a key contributor to the progression of multiple cancers, with IGF-1R activation increasing the tumorigenic potential of breast, prostate, lung, colon and head and neck squamous cell carcinoma (HNSCC). Despite this relationship, targeting the IGF-1R has only recently undergone development as a molecular cancer therapeutic. As it has taken hold, we are witnessing a robust increase and interest in targeting the inhibition of IGF-1R signaling. This is accentuated by the list of over 30 drugs, including monoclonal antibodies (mAbs) and tyrosine kinase inhibitors (TKIs) that are under evaluation as single agents or in combination therapies. The IGF-binding proteins (IGFBPs) represent the third component of the IGF system consisting of a class of six soluble secretory proteins. They represent a unique class of naturally occurring IGF-antagonists that bind to and sequester IGF-1 and IGF-2, inhibiting their access to the IGF-1R. Due to their dual targeting of the IGFs without affecting insulin action, the IGFBPs are an untapped "third" class of IGF-1R inhibitors. In this commentary, we highlight some of the significant aspects of and prospects for targeting the IGF-1R and describe what the future may hold.

Insulin-like growth factor-1 receptor and ligand targeting in head and neck squamous cell carcinoma

IGF-1 receptor (IGF-1R) signaling is associated with increased tumorigenesis of epithelial cancers. In light of recent epidemiological studies correlating high circulating levels of IGF-1 with increased risk of second primary tumors (SPTs) of the head and neck, we examined IGF system and epidermal growth factor receptor (EGFR) expression in human head and neck squamous cell carcinoma (HNSCC) matched pairs and a cross-section of HNSCC cell lines. Employing the latter, we demonstrated that IGF-1 stimulated S-phase transition in a PI 3-K/Akt and Erk-dependent manner in 5 of 8 cell lines, with Erk activation being dependent upon EGFR kinase activity. IGF-1 stimulated thymidine incorporation was inhibited by treatment with IGFBP-3, the IGF-1R tyrosine kinase inhibitor NVP-AEW541, or the EGFR tyrosine kinase inhibitor AG1478. Combining IGFBP-3 with NVP-AEW541 or AG1478 abrogated IGF-1 responses at 10-fold lower doses than either compound alone. These results demonstrate the potential for co-targeting the IGF system and EGFR in HNSCC.