In Vitro and in Vivo Metabolism of Lu-AMBA, a GRP-Receptor Binding Compound, and the Synthesis and Characterization of Its Metabolites

The metabolism of 177Lu-AMBA (AMBA = DO3A-CH2CO-G-(4-aminobenzoyl)-QWAVGHLM-NH2), a radiotherapeutic compound in clinical development that binds to GRP and NMB receptors, was studied in vitro (mouse, rat and human plasma, mouse kidney homogenate) and in vivo (by analysis of mouse and rat plasma and urine following IV injection of 177Lu-AMBA). The primary metabolites were Lu-DO3A-CH2CO-G-Abz4-R, where R = -Q-OH (A), -QW-OH (B), and -QWAVGH-OH (C). Minor amounts of (D) where R = -QWAVGHLM-OH and (E) -QWAVGHL-OH were also observed. Clearance of 177Lu-AMBA and of radioactivity from mouse and rat blood was rapid in vivo. In mouse and rat urine, only metabolites Lu-A and Lu-B were found—no parent drug was excreted. Unmetalated ligands and natLu and 177Lu complexes for Lu-AMBA metabolites A−E were synthesized, characterized by HPLC and MS, and used to perform in vitro competition and direct binding studies on GRP receptor-positive PC-3 (human prostate) cancer cells. Biodistribution studies with 177Lu-labeled met...