Insulin-like growth factor-I (IGF-I) has been shown to play a role in the progression of atherosclerosis in experimental animal models. IGF binding protein-4 (IGFBP-4) binds to IGF-I and prevents its association with receptors. Overexpression of a protease resistant form of IGFBP-4 has been shown to inhibit the ability of IGF-I to stimulate normal smooth muscle cell growth in mice. Based on these observations, we prepared a protease resistant form of IGFBP-4 and infused it into hypercholesterolemic pigs. Infusion of the protease resistant mutant inhibited lesion development by 53.3±6.1% (n=6, p<0.01). Control vessels that received an equimolar concentration of IGF-I and the protease resistant IGFBP-4 showed no reduction in lesion size compared to control lesions that were infused with vehicle. Infusion of nonmutated form of IGFBP-4 did not significantly inhibit lesion development. PCNA analysis showed that the mutant IGFBP-4 appeared to inhibit cell proliferation. The area occupied by extracellular matrix was also reduced proportionally compared to total lesion area. Immunoblotting revealed that the mutant IGFBP-4 remained intact whereas the wild type IGFBP-4 that was infused was proteolytically cleaved. Further analysis of the lesions revealed that a marker protein, IGFBP-5, whose synthesis is stimulated by IGF-I, was decreased in the lesions that received the protease resistant, IGFBP-4 mutant whereas, there was no change in lesions that received wild type IGFBP-4 or the mutant protein plus IGF-I. These findings clearly illustrate that infusion of protease resistant IGFBP-4 into the perilesion environment results in inhibition of cell proliferation and attenuation of the development of neointima. The findings support the hypothesis that inhibiting IGFBP-4 proteolysis in the lesion microenvironment could be an effective means for regulating neointimal expansion.