Introduction: Bombesin, a neuropeptide with potential for breast and prostate tumor targeting, is rapidly metabolized in vivo, and as a result, uptake in tumor xenografts in mice is poor. An improvement can be expected from the introduction of nonnatural amino acids and spacers. Leu13 was replaced by cyclohexylalanine and Met14 by norleucine. Two spacers, -betaAla-betaAla- and 3,6-dioxa-8-aminooctanoic acid, were inserted between the receptor-binding amino acid sequence (7-14) of bombesin (BBS) and the retroN(alpha)-carboxymethyl histidine chelator used for labeling with the [99mTc](CO)3 core and the rhenium (Re) congener.
Methods: The biological characterization of the new compounds was performed both in vitro on prostate carcinoma PC-3 cells (binding affinity, internalization/externalization) and in vivo (biodistribution in nude mice with tumor xenografts). The stability was also investigated in human plasma. The Re analogues were prepared for chemical characterization.
Results: The nonnatural amino acids led to markedly slower degradation in human plasma and PC-3 cell cultures. The receptor affinity of the new technetium 99m ([99mTc])-labeled BBS analogues was similar to the unmodified compound with Kd<1 nM. Uptake in the pancreas and in PC-3 tumor xenografts in nude mice was blocked by unlabeled BBS. The best target-to-nontarget uptake ratio was clearly due to the presence of the more polar spacer, -betaAla-betaAla-.
Conclusions: The different spacers did not have a significant effect on stability or receptor affinity but had a clear influence on the uptake in healthy organs and tumors. Uptake in the kidneys was lower than in the liver, which is likely to be due to the lipophilicity of the compounds. A specific, high uptake was also observed in the gastrin-releasing peptide receptor-rich pancreas. Thus, with the introduction of spacers the in vivo properties of the compounds can be improved while leaving the affinity unaffected.