The effects of linking substituents on the in vivo behavior of site-directed, peptide-based, diagnostic radiopharmaceuticals.

A number of human cancers are known to over-express the gastrin-releasing peptide receptor (GRPr) on cell surfaces. The high specificity and affinity of bombesin (BBN), an amphibian analogue of mammalian gastrin-releasing peptide, for the GRPr makes it an ideal candidate for delivery of diagnostic probes, such as 99mTc radiometal, to tumor sites. An optimized targeting agent possesses high tumor uptake with minimal uptake in normal tissues. In this study, 99mTc-targeting vectors of bombesin using various amino acid/aliphatic pharmacokinetic modifiers or linking groups were evaluated to determine the effect of the spacer on receptor binding affinity, internalization/externalization and biodistribution. Conjugates of the general type [DPR-X-BBN] (X = amino acid/aliphatic pharmacokinetic modifier) were synthesized by solid phase peptide synthesis (SPPS) and metallated with either low-valent, radioactive Tc-99m(I) or non-radioactive Re(I)-tricarbonyl precursors. All of the new non-metallated and metallated conjugates were characterized by electrospray ionization mass spectrometry (ESI-MS). Receptor binding affinity, internalization/externalization and biodistribution studies in normal (CF-1) and tumor (human prostate PC-3-bearing mice) are reported. The effectiveness of targeting xenografted PC-3 tumors in rodents for two of the new 99mTc-BBN conjugates is demonstrated herein using small animal single photon emission computed tomography (SPECT).