Evaluation of radiolabeled type IV collagen fragments as potential tumor imaging agents

The objective of this study was to examine radiopharmaceuticals that target the alpha3 beta1 integrin to determine if these agents target tumors for diagnostic imaging and/or targeted radiotherapy of cancer. Prior studies had shown that residues 531-542 from the alpha1 chain of type IV collagen bind a variety of tumor cell alpha3 beta1 integrins. A peptide mimic of this sequence containing all D-amino acids (designated D-Hep-III) was synthesized by solid-phase methods. The tetraazamacrocyclic chelator, TETA, was conjugated to the peptide while it was resin-bound. TETA-D-Hep-III and D-Hep-III were radiolabeled with Cu-64 and I-125, respectively, in high specific activity and radiochemical purity. Heterologous competitive binding assays between D-Hep-III and either I-125-D-Hep-III or Cu-64-TETA-D-Hep-III indicated low micromolar affinity of D-Hep-III. The biodistribution of each radiolabeled analogue of D-Hep-III was carried out in rats and tumor-bearing mice. Both analogues were rapidly cleared from the blood in normal rats, with the kidneys receiving the highest accumulation of each. SKOV3 human ovarian tumor cells, known to strongly express alpha3 beta1, were xenografted in SCID mice. Localization of I-125-D-Hep III and Cu-64-TETA-D-Hep III in the xenografts were low (<2% ID/g), and in the case of I-125-D-Hep III, not inhibited by a competitive dose of D-Hep III. The low tumor accumulation is likely not due to receptor down-regulation, but rather due to the weak affinity of the radioligands for the alpha3 beta1 integrin.