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

• Published in: Bioconjugate chemistry 2009-06, Vol.20 (6), p.1171-1178
• Main Authors: Linder, Karen E, Metcalfe, Edmund, Arunachalam, Thangavel, Chen, Jianqing, Eaton, Stephen M, Feng, Weiwei, Fan, Hong, Raju, Natarajan, Cagnolini, Aldo, Lantry, Laura E, Nunn, Adrian D, Swenson, Rolf E
• Format: Article
• Language: English
• Subjects: Animals; Cell Line, Tumor; Cells; Humans; Kidney - metabolism; Ligands; Liver - metabolism; Male; Metabolism; Metabolites; Mice; Oligopeptides - blood; Oligopeptides - chemical synthesis; Oligopeptides - metabolism; Oligopeptides - pharmacokinetics; Pancreas; Rats; Receptors, Bombesin - metabolism; Rodents; Tissue Distribution; Tumors
• ISSN: 1043-1802; 1520-4812
• DOI: 10.1021/bc9000189

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 metabolites A−E were performed in PC-3 tumor-bearing mice and the results compared with intact 177Lu-AMBA. IC50 values for unmetalated metabolite ligands A−E were >400 nM in PC-3 cells in competition binding studies against 177Lu-AMBA. No direct binding to PC-3 cells was observed with 177Lu-labeled A−C, confirming IC50 results. 177Lu-labeled metabolites A−E showed no uptake in GRP-receptor positive tumor or pancreas in PC-3 tumor bearing mice. All metabolites were rapidly excreted via the renal route (∼78−87%) within 1 h. These results demonstrate that the tumor uptake observed with 177Lu-AMBA is due to parent drug and not due to any of its identified metabolites.