Receptor binding assay, biodistribution and micro-SPECT/CT imaging of ¹¹¹In-AMBA in human prostate tumor-bearing mice

Objectives Gastrin-releasing peptide receptors (GRPRs) are overexpressed on a variety of human tumors like prostate, breast, and lung cancer. Bombesin (BN) is a 14 amino acid peptide with high affinity for these GRPRs. We synthesized DO3A-CH₂-CO-G-4-aminobenzoyl-Q-W-A-V-G-H-L-M-NH₂-(AMBA), a peptide chelated with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), and radiolabeled this BN analogue with ¹¹¹InCl₃. Biological activity of ¹¹¹In-AMBA was evaluated in PC-3 prostate tumor-bearing SCID mice.

Methods A solid phase approach was used to synthesize AMBA. The affinity of AMBA to BBN type 2 receptor was determined by a competitive displacement cell-binding assay using ¹²⁵I-Tyr₄-BN. The PC-3 tumor-bearing SCID mice were imaged by microSPECT/CT and sacrificed for biodistribution at 1, 4, 8, 24 and 48 hr after iv injection of ¹¹¹In-AMBA.

Results The purity of synthesized AMBA was greater than 95%. The IC₅₀ and Ki of AMBA in the human bombesin 2 receptor were 0.33 ± 0.09 nM and 0.26 ± 0.07 nM, respectively. The radiolabeling efficiency of ¹¹¹In-AMBA was 95.43 ± 1.37 %. In biodistribution, the higher uptake of tumor and tumor/muscle ratio was 3.87 ± 0.65 %ID/g and 11.7 at 8 hours after injection, respectively. The microSPECT/CT imaging studies with ¹¹¹In-AMBA suggested the higher uptake of tumor was maintained between 8 and 48 hours post-injection.

Conclusions Our result revealed AMBA has high affinity with BBN type 2 receptor. The results demonstrated a good uptake in the GRPR-over expression PC-3 tumor-bearing SCID mice. ¹¹¹In-AMBA is a potential agent for imaging GRPR-positive tumors in humans