Improving the tumor uptake of 99mTc-labeled neuropeptides using stabilized peptide analogues

Two neuropeptides, bombesin (BBS) and neurotensin (NT) and their radiolabeled analogues, have great potential for tumour targeting, either for diagnosis (e.g., with Tc-99m) or therapy (e.g., with Y-90 or Re-188). In this study, we investigated NT(8-13) and BBS(7-14) analogues with Nalpha-histidinyl acetate linked to the N-terminus of the peptide. This His-derivative forms a stable and inert tridentate complex with the Tc-99m(CO)(3) and the Re-188(CO)(3) moieties. The stability of Tc-99m-labeled neurotensin and bombesin analogues was tested in human plasma samples and in tumour cell cultures in the presence and absence of specific enzyme inhibitors. The inhibitor of ACE (angiotensin converting enzyme) was the most effective in inhibiting the peptide cleavage of both NT(8-13) and BBS(7-14). In agreement with this finding, the replacement of Ile(12) by tert-leucine (NT) and Leu(13) by cyclohexylalanin (BBS) brought about a better stability. With NT(8-13) analogues, higher tumour to nontarget (t/nt) ratios and the same affinity to the receptor was observed, but with BBS(7-14) derivatives the affinity was lower and the t/nt ratio was not significantly improved. Toxicity tests showed no effect in mice of up to a five-hundred-fold higher dose than planned for patient application, which started successfully with NT(8-13) analogues.