microPET and Autoradiographic Imaging of GRP Receptor Expression with ⁶⁴Cu-DOTA-[Lys³]Bombesin in Human Prostate Adenocarcinoma Xenografts

Inhibition of ¹²⁵I-[Tyr³]BBN binding to GRPR on human prostate cancer cell line PC-3 by DOTA-[Lys³]BBN conjugate (IC₅₀ = 2.2 ± 0.5 nmol/L). Results expressed as percentage of binding are mean ± SD of 2 experiments in triplicate.

Time-dependent internalization of ⁶⁴Cu-DOTA-[Lys³]BBN by PC-3 prostate cancer cells incubated at 37°C during 2 h. Cells were preincubated for 2 h at 4°C. Data are mean ± SD of percentage of acid-resistant (internalized) radioactivity in cells (2 experiments in triplicate).

Biodistribution of ⁶⁴Cu-DOTA-[Lys³]BBN in male athymic nude mice bearing subcutaneous PC-3 (AI) and CWR22 (AD) tumors. Mice were intravenously injected with 370 kBq (10 μCi) of radioligand and killed at 15 min, 30 min, 1 h, 2 h, and 4 h. Data are presented as mean %ID/g ± SD (n = 4). Sm int = small intestine.

Receptor-blocking study: biodistribution of ⁶⁴Cu-DOTA-[Lys³]BBN at 1-h time point without (white bars) and with (black bars) coinjection of 200 μg of BBN to determine GRPR-specific binding. Significant inhibition of activity accumulation in PC-3 tumor (P < 0.001) and pancreas (P < 0.001) was observed. Data are presented as mean %ID/g ± SD in reference to total injected dose of radiotracer (n = 4 for each group).

(Left) Coronal microPET image of tumor-bearing mouse (PC-3 on left shoulder and CWR22 on right shoulder) 1 h after administration of ⁶⁴Cu-DOTA-[Lys³]BBN. (Right) Digital autoradiograph of section containing tumors.

Kidney, liver, PC-3 tumor, and CWR22 tumor uptake comparison as obtained from traditional biodistribution (n = 4), microPET (n = 3), and autoradiographic quantification (n = 3). Quantification of microPET and autoradiography revealed similar activity accumulation in both PC-3 tumor and pancreas, which were both lower than those obtained from biodistribution study.