A 99mTc(I)-Postlabeled High Affinity Bombesin Analogue as a Potential Tumor Imaging Agent

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Abstract
The overexpression of neuropeptide receptors observed in many cancers provides an attractive target for tumor imaging and therapy. Bombesin is a peptide exhibiting a high affinity for the gastrin releasing peptide (GRP) receptor, which is overexpressed by a variety of tumors such as breast or prostate cancer. In the present study, we have evaluated if the bombesin analogue [Nα-histidinyl acetate]bombesin(7−14), radiolabeled with the novel [99mTc(OH2)3(CO)3]+, has the potential to be used as a diagnostic radiopharmaceutical. Receptor saturation studies, carried out on the GRP receptor-expressing PC-3 human prostate cancer cell line, revealed for [99mTc(CO)3−Nα-histidinyl acetate]bombesin(7−14) Kd values in the subnanomolar range. Competitive binding assays, using the cold rhenium(I)-labeled analogue as a surrogate for the 99mTc-conjugate, also showed high affinity binding. Incubation of the radioconjugate with PC-3 cells resulted in a rapid temperature- and time-dependent specific internalization. At 37 °C more than 70% was internalized within the first 15 min and remained constant up to 2 h. Despite the weak proteolytic stability of [99mTc(CO)3−Nα-histidinyl acetate]bombesin(7−14) in vitro, biodistribution studies, performed in PC-3 tumor-bearing mice, showed low uptake in the tumor (0.89 ± 0.27% ID/g 30 min pi) but high uptake into the pancreas (7.11 ± 3.93% ID/g 30 min pi), a GRP receptor-positive organ. Blockade experiment (coinjection of 300 μg bombesin/mouse with the radioligand) showed specificity of the uptake. Despite the low tumor uptake, tumor-to-blood ratios of 2.0 and 2.7 and tumor-to-muscle ratios of 8.9 and 8.0 were obtained at 30 min and 1.5 h postinjection, respectively. The promising results merit the future in vivo investigation of 99mTc/188Re-tricarbonyl-labeled bombesin analogues.

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