Chemical design of a radiolabeled gelatinase inhibitor peptide for the imaging of gelatinase activity in tumors

Since elevated levels of gelatinases [matrix metalloproteinase (MMP)-2 and MMP-9] are associated with a poor prognosis in cancer patients, these enzymes are potential targets for tumor imaging. In the present study, a cyclic decapeptide, cCTTHWGFTLC (CTT), was selected as a mother compound because of its selective inhibitory activity toward gelatinases. For imaging gelatinase activity in tumors, we designed a CTT-based radiopharmaceutical taking into consideration that (1) the HWGF motif of the peptide is important for the activity, (2) hydrophilic radiolabeled peptides show low-level accumulation in the liver and (3) an increase in the negative charge of radiolabeled peptides is effective in reducing renal accumulation. Thus, a highly hydrophilic and negatively charged radiolabel, indiun-111-diethylenetriaminepentaacetic acid (111In-DTPA), was attached to an N-terminal residue distant from the HWGF motif (111In-DTPA-CTT). In MMP-2 inhibition assays, In-DTPA-CTT significantly inhibited the proteolytic activity in a concentration-dependent fashion. When injected into normal mice, 111In-DTPA-CTT showed low levels of radioactivity in the liver and kidney. A comparison of the pharmacokinetic characteristics of 111In-DTPA-CTT with those of other CTT derivatives having different physicochemical properties revealed that the increase in hydrophilicity and negative charge caused by the conjugation of 111In-DTPA reduced levels of radioactivity in the liver and kidney. In tumor-bearing mice, a significant correlation was observed between the accumulation in the tumor as well as tumor-to-blood ratio of 111In-DTPA-CTT and gelatinase activity. These findings support the validity of the chemical design of 111In-DTPA-CTT for reducing accumulation in nontarget tissues and maintaining the inhibitory activity of the mother compound. Furthermore, 111In-DTPA-CTT derivatives would be potential radiopharmaceuticals for the imaging of gelatinase activity in metastatic tumors in vivo.