Discovery of a series of nonpeptide small molecules that inhibit the binding of insulin-like growth factor (IGF) to IGF-binding proteins

Insulin-like growth factors (IGF-I and II) play an important role in metabolic and mitogenic activities through stimulation of the IGF-I receptor on the cell surface. Although the concentration of IGF in blood and cerebrospinal fluid is quite high (> 100 nM), this large pool of IGF is biologically inactive because of its association with six distinct binding proteins, which form high-affinity complexes with IGF. Thus, inhibitors of IGF-binding proteins (IGFBPs), especially IGFBP-3, could potentially alter the distribution between the "free" and "bound" forms of IGF and thereby elevate biologically active IGF-I to exert a beneficial effect on those patients with diseases that respond to the application of exogenous IGF-I. Whereas IGF-I peptide variants, which bind to IGFBPs but not the IGF-I receptor, have been shown to be potent IGF/IGFBP inhibitors, small molecule nonpeptide IGF/IGFBP inhibitors have the potential advantages of oral bioavailability and flexible dosing regimen. Here we report the discovery of several isoquinoline analogues, exemplified by I and 2, which bind IGFBP-3 as well as other IGFBPs at low nanomolar concentrations. More importantly, both compounds were shown to be able to release biologically active IGF-I from the IGF-I/IGFBP-3 complex. These results point to the feasibility of developing orally active therapeutics to treat IGF-responsive diseases by optimization of the lead molecules 1 and 2.