Preparation and evaluation of bombesin peptide derivatives as potential tumor imaging agents: effects of structure and composition of amino acid sequence on in vitro and in vivo characteristics

ObjectivesAmong the many clinically relevant peptide receptor systems, bombesin (BN) receptors have attracted enormous attraction due to their overexpression in various frequently occurring human tumors including breast and prostate, thus making such receptors promising targets with radiolabeled BN analogs. The present study describes the preparation and evaluation of a series of new BN derivatives as potential tumor imaging agents.MethodsSeveral new BN derivatives with the common structure MAG3-X-BN(1–14 or 6–14), where X=Asp or Asp-Asp, were synthesized by solid-phase peptide synthesis. S-benzoylmercaptoacetic acid was incorporated at the end of synthesis via manual conjugation to yield MAG3-BN conjugates. Radiolabeling with 99mTc was accomplished by ligand exchange method. The receptor-binding affinity assays were performed in MDA-MB-231, MCF-7, T47-D and PC-3 cancer cell lines. In vivo biodistribution and clearance kinetics were assessed in Balb/c mice, and tumor targeting efficacy was determined in nude mice bearing breast tumor xenografts.ResultsThe peptides were prepared conveniently and radiolabeled efficiently with 99mTc (up to 95% labeling efficiency). In vitro cell binding assays demonstrated high affinity (values in the nanomolar range) of 99mTc peptides towards breast and prostate cancer cell lines. In addition, the radioconjugates displayed significant internalization (values ranged between 19% and 35%) in tumor cells. In vivo biodistribution and biokinetics are characterized by efficient clearance from the blood and variable degrees of excretion through the renal pathway. In vivo tumor targeting studies displayed variable uptake capacity of different BN derivatives, underlining the influence of specific amino acid sequence on tumor targeting profiles. Tumor uptake was always higher than the radioactivity in the blood and muscle, with good tumor retention and good tumor-to-blood and tumor-to-muscle ratios, indicating the potential of these agents for targeting tumors in vivo.ConclusionsThe combination of favorable in vitro and in vivo properties may render these BN peptides as potential candidates for targeting BN/GRP receptor-positive tumors. They deserve further evaluation to determine their real strength. The present data indeed provide useful information regarding peptide structure–pharmacologic activity relationship, which might be useful in designing and developing new BN-like peptides for efficient targeting of tumors in vivo.