Radiolabeled high affinity peptidomimetic antagonist selectively targets αvβ3 receptor-positive tumor in mice

Objectives The aim of this research was to synthesize radiolabeled peptidomimetic integrin α v β 3 antagonists that selectively target integrin α v β 3 receptor and clear rapidly from the whole body. Methods Integrin α v β 3 antagonists, 4-[2-(3,4,5,6-tetrahydropyrimidine-2-ylamino)ethyloxy]benzoyl-2-( S )-aminoethylsulfonyl-amino-β-alanine (IA) and 4-[2-(3,4,5,6-tetrahydro-pyrimidin-2-ylamino)-ethyloxy]benzoyl-2-( S )-[ N -(3-amino-neopenta-1-carbamyl)]-aminoethylsulfonylamino-β-alanine hydrochloride (IAC), a hydrophobic carbamate derivative of IA, were conjugated with 2- p -isothiocyanatobenzyl-DOTA at the amino terminus and labeled with 111 In. The 111 In labeled IA and IAC were subjected to in vitro receptor binding, biodistribution and imaging studies using nude mice bearing the receptor-positive M21 human melanoma xenografts. Results The 111 In-labeled IA (40%) and -IAC (72%) specifically bound in vitro to α v β 3 (0.8 μM) at a molar excess. This receptor binding was completely blocked by a molar excess of cold IA to α v β 3 . The higher receptor-binding affinity of the 111 In-labeled IAC was reflected in higher tumor uptake and retention: 5.6±1.4 and 4.5±0.7 %ID/g vs. 3.8±0.9 and 2.0±0.3 %ID/g for the 111 In-labeled IA at 0.33 and 2 h. The tumor uptakes were inhibited by the co-injection of 200 μg of IA, indicating that the uptake was receptor mediated. These antagonists were excreted primarily via the renal system. The 111 In activity retained in the whole body was quite comparable between the 111 In-labeled IA (24% ID) and the 111 In-labeled IAC (33% ID) at 2 h. The higher peak tumor uptake and longer retention resulted in higher tumor-to-background ratios for the 111 In-labeled IAC at 2 h with 9.7, 2.3, 0.8, 1.9, 7.1, 2.2, 0.9, 3.7 and 9.9 for blood, liver, kidney, lung, heart, stomach, intestine, bone and muscle, respectively. The imaging studies with the 111 In-labeled IAC also clearly visualized the receptor-positive tumor at 4 h. Conclusions The 111 In-labeled IAC showed an improve tumor targeting kinetics with rapid accumulation and prolonged retention in the α v β 3 receptor-positive tumor. This together with the rapid whole-body clearance pharmacokinetics warrants further studies on this IAC analog for molecular imaging of tumor-induced angiogenic vessels and various malignant human tumors expressing the receptor.