Preclinical evaluation of a new, stabilized neurotensin(8--13) pseudopeptide radiolabeled with (99m)tc.

The rapid degradation of neurotensin (NT) limits its clinical use in cancer imaging and therapy. Thus, a new NT(8--13) pseudopeptide, NT-VIII, was synthesized. Some changes were introduced in the sequence of NT(8--13) to stabilize the molecule against enzymatic degradation: Arg(8) was N-methylated, and Lys and Tle replaced Arg(9) and Ile(12), respectively. Finally, (NalphaHis)Ac was coupled to the N-terminus for (99m)Tc(CO)(3) labeling. This peptide was characterized both in vitro and in vivo. METHODS: The new analog was labeled with (99m)Tc(CO)(3). Its metabolic stability was analyzed both in human plasma and in HT-29 cells. Binding properties, receptor downregulation, and internalization were tested with HT-29 cells. Biodistribution was evaluated in nude mice with HT-29 xenografts. RESULTS: (99m)Tc(CO)(3)NT-VIII showed a high stability in plasma, where most of the peptide remained intact after 24 h of incubation at 37 degreesC. However, the degradation in HT-29 cells was more rapid (46% of intact (99m)Tc(CO)(3)NT-VIII after 24 h at 37 degreesC). Binding to NT1 receptors (NTR1) was saturable and specific. Scatchard analysis showed a high affinity for (99m)Tc(CO)(3)NT-VIII, with a dissociation constant similar to (125)I-NT (1.8 vs. 1.6 nmol/L). After interacting with NTR1, (99m)Tc(CO)(3)NT-VIII was rapidly internalized, with more than 90% internalized after 30 min. It also distributed and cleared rapidly in nude mice bearing HT-29 xenografts. The highest rates of accumulation were found in kidney and tumor at all time points tested. Tumor uptake was highly specific because it could be blocked by coinjection with a high dose of (NalphaHis)Ac-NT(8--13). Tumors were clearly visualized in scintigraphy images. CONCLUSION: The changes that were introduced stabilized the molecule against enzymatic degradation without affecting binding properties. Moreover, the increase in stability enhanced tumor uptake, making this derivative a promising candidate for clinical use.