Comparative in vitro and in vivo evaluation of two 64Cu-labeled bombesin analogs in a mouse model of human prostate adenocarcinoma

Bombesin (BBN), an analog of human gastrin-releasing peptide (GRP), binds to the GRP receptor (GRPR) with high affinity and specificity. Overexpression of GRPR has been discovered in mostly androgen-independent human prostate tissues and, thus, provides a potential target for prostate cancer diagnosis and therapy. We have previously demonstrated the feasibility of the positron emission tomography (PET) imaging using Cu-64-1,4,7,10-tetraazadodecane-N,N',N,N'-tetraacetic acid (DOTA)-[Lys(3)]BBN to detect GRPRpositive prostate cancer. In this study, we compared the receptor affinity, metabolic stability, tumor-targeting efficacy, and pharmacokinetics of a truncated BBN analog Cu-64-DOTA-Aca-BBN(7-14) with Cu-64-DOTA-[Lys(3)]BBN. Binding of each DOTA conjugate to GRPR on PC-3 and 22Rv1 prostate cancer cells was evaluated with competitive binding assay using I-125-[Tyr(4)]BBN as radioligand. In vivo pharmacokinetics was determined on male nude mice subcutaneously implanted with PC-3 cells. Dynamic microPET imaging was performed to evaluate the systemic distribution of the tracers. Metabolic stability of the tracers in blood, urine, tumor, liver and kidney was studied using high-performance liquid chromatography. The results showed that I-125-[Tyr4]BBN has a K-d of 14.8 +/- 0.4 nM against PC-3 cells, and the receptor concentration on PC-3 cell surface is approximately 2.7 +/- 0.1 X 10(6) receptors per cell. The 50% inhibitory concentration value for DOTA-Aca-BBN(7-14) is 18.4 +/- 0.2 nM, and that for DOTA-[Lys(3)]BBN is 2.2 +/- 0.5 nM. DOTA-[Lys(3)]BBN shows a better tumor contrast and absolute tumor activity accumulation compared to DOTA-Aca-BBN(7-14). Studies on metabolic stability for both tracers on organ homogenates showed that Cu-64-DOTA-[Lys(3)]BBN is relatively stable. This study dernonstrated that both tracers are Suitable for targeted PET imaging to detect the expression of GRPR in prostate cancer, while Cu-64-DOTA-[Lys(3)]BBN may have a better potential for clinical translation. (c) 2006 Elsevier Inc. All rights reserved.