Radiolabeled bombesin analogs for prostate cancer diagnosis: preclinical studies

Introduction: Radionuclide imaging can be a useful tool for the diagnosis of prostate cancer. Bombesin (BBN) is a molecule with high affinity for gastrin releasing peptide (GRP) receptors which are over-expressed in that tumor. This report compares ^99mTc-HYNIC-βAla-BBN(7-14)NH2 [^99mTc-HYNIC-BBN] and ^99mTc≡N(PNP6)-Cys-βAla-BBN(7-14)NH2 [^99mTcN(PNP6)-Cys-BBN] with regard to labeling procedures as well as in vitro and in vivo evaluation (biodistribution and scintigraphic imaging). Methods: Peptide synthesis was performed in an automated peptide synthesizer. HYNIC-BBN was radiolabeled with pertechnetate using tricine and ethylenediamine diacetic acid (EDDA) as coligands. Cys- BBN was radiolabeled in a two-step procedure with the preparation of the precursor ^99mTc-Nitrido first and then introducing diphosphine (PNP6). Radiochemical evaluation of conjugates, as well as studies of stability, transchelation toward cysteine, and partition coefficient were done. Biological studies included internalization, biodistribution in healthy animals and in animals bearing PC3 cancer cells with acquisition of images from the tumor-bearing animals. Results: Both complexes showed a high radiochemical yield along with good stability. Biodistribution studies pointed out strong renal excretion for the former complex due to its hydrophilic profile and marked hepatobiliary excretion for the latter, corresponding to observed lipophilicity. Tumor uptake was higher for ^99mTc-HYNIC-BBN and the same occurred with internalization findings, which exceeded those of ^99mTcN(PNP6)-BBN. Blocking studies in mice bearing PC-3 tumor cells revealed significantly reduced pancreas and tumor uptake, demonstrating receptor specificity of the conjugates. Conclusion: The best radiotracer was ^99mTc-HYNIC-BBN on the basis of high radiochemical yield, fast radiolabeling procedure without need for a purification step, and more consistent tumor uptake