Synthesis and characterization of 111In-DTPA-N-TIMP-2: a radiopharmaceutical for imaging matrix metalloproteinase expression.

The matrix metalloproteinases (MMPs) are enzymes involved in the turnover of the extracellular matrix. Their overexpression in tumors is implicated in the metastatic process and may provide a target for diagnostic tumor imaging by using a radiolabeled inhibitor. MMPs are inhibited by endogenous tissue inhibitors of metalloproteinases (TIMPs). Thus, TIMPs are potential targeting molecules which could be used as vehicles for selective radionuclide delivery by virtue of their binding to MMPs. The aim of this work was to produce a radiopharmaceutical with which to evaluate this potential. The 127 amino acid N-terminal domain of recombinant human TIMP-2 (N-TIMP-2) was conjugated with the bifunctional chelator diethylenetriamine pentaacetic acid (DTPA). Singly modified DTPA-N-TIMP-2 conjugate (identified by electrospray ionization mass spectrometry) was isolated by anion-exchange chromatography. The primary site of DTPA modification on N-TIMP-2 was mapped to lysine-116, which is distant from the site of MMP interaction. The conjugate was radiolabeled with indium-111 to give 111In-DTPA-N-TIMP-2 with a specific activity of at least 4 MBq/microg and a radiochemical yield and purity of >95%, by incubation with 111InCl3, without need for postlabeling purification. The product was sterile, pyrogen-free, and stable in serum over 48 h and retained full inhibitory activity in a fluorimetric binding assay. With these attributes, 111In-DTPA-N-TIMP-2 is a suitable radiopharmaceutical for in vivo biological and clinical investigation of the potential benefits of imaging MMP expression.