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Effects of Insulinlike Growth Factor Binding Proteins on Insulinlike Growth Factor-I Biodistribution in Tumor-Bearing Nude Mice

Department of Nuclear Medicine, Warren G. Magnuson Clinical Center, National Institutes of Health, Bethesda, Maryland

Correspondence: For correspondence or reprints contact: Jorge A. Carrasquillo, MD, Bldg. 10, Rm. 1C496, National Institutes of Health, 10 Center Dr., MSC 1180, Bethesda, MD 20892-1180.

ABSTRACT

This study evaluated the biodistribution and tumor targeting ability of radiolabeled insulinlike growth factor (IGF)-I. Because IGF binding proteins (IGFBPs) play a critical role in modulating IGF activity, the binding properties of ¹²⁵I-labeled IGF-I to IGFBPs were investigated in vitro and in vivo. Because a large amount of the IGF-I was catabolized in vivo, we also studied the catabolism of IGF-I by tumor cells in vitro. Methods: ¹²⁵I-labeled-IGF-I was prepared using the chloramine T method. The biodistribution of ¹²⁵I-labeled-IGF-I in tumor-bearing nude mice was compared between groups injected with ¹²⁵I-labeled IGF-I alone or coinjected with unlabeled peptide. In vitro and in vivo chromatography studies were performed to evaluate the binding profile to IGFBPs and the degree of catabolites in serum as well as urine. Results: Data indicated that the binding of radiolabeled IGF-I to IGFBPs in vitro was dose dependent. However, there was a difference in complex formation between the serum and the heparinized plasma. In heparinized plasma, the radioactivity shifted from a 30- to 50-kDa complex to a 150-kDa complex and to a free ligand, because the binding of heparin with IGFBPs decreased its affinity for IGF-I. In plasma prepared with acid citrate dextrose a binding pattern identical to that of serum was observed. Moreover, there was a binding difference between mouse and rat. The ¹²⁵I-labeled IGF-I catabolized very quickly when incubated at 37°C but not at all at 4°C. In tumor-bearing nude mice, the uptake of radioactivity in normal tissues decreased quickly, particularly in the kidneys. In mice coinjected with unlabeled carrier, the radioactivity in most normal tissues was lower and the tumor uptake higher than in the mice without carrier. Conclusion: These data confirm that ¹²⁵I-labeled IGF-I is avidly bound to IGFBPs, both in vitro and in vivo. By partially saturating this binding with unlabeled peptides, a favorable biodistribution was achieved, including faster clearance from normal tissue and higher tumor uptake, which resulted in better tumor-to-nontumor ratios. Nevertheless, the rapid catabolism and release of the radiolabel from tumor tissue result in a suboptimal targeting agent.

Key Words: insulinlike growth factor-I • insulinlike growth factor receptor • insulinlike growth factor binding proteins • endocytosis

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