Tracer level radiochemistry to clinical dose preparation of 177Lu-labeled cyclic RGD peptide dimer

Aim: Integrin alpha(v)beta(3) plays a significant role in angiogenesis during tumor growth and metastasis, and is a receptor for the extracellular matrix proteins with the exposed arginine(R)-glycine(G)-aspartic acid(D) tripeptide sequence. The over-expression of integrin alpha(v)beta(3) during tumor growth and metastasis presents an interesting molecular target for both early detection and treatment of rapidly growing solid tumors. Considering the advantages of Lu-177 for targeted radiotherapy and enhanced tumor targeting capability of cyclic RGD peptide dimer, an attempt has been made to optimize the protocol for the preparation of clinical dose of Lu-177 labeled DOTA-E[c(RGDfK)](2) (E = Glutamic acid, f = phenyl alanine, K = lysine) as a potential agent for targeted tumor therapy. Methods: Lu-177 was produced by thermal neutron bombardment on enriched Lu2O3 (82% in Lu-176) target at a flux of 1 x 10(14) n/cm(2).s for 21 d. Therapeutic dose of Lu-177-DOTA-E[c(RGDfK)](2) (7.4 GBq) was prepared by adding the aqueous solution of the ligand and (LuCl3)-Lu-177 to 0.1 M NH(4)OAC buffer containing gentisic acid and incubating the reaction mixture at 90 degrees C for 30 mm. The yield and radiochemical purity of the complex was determined by HPLC technique. Parameters, such as, ligand-to-metal ratio, pH of the reaction mixture, incubation time and temperature were varied using tracer quantity of Lu-177 (37 MBq) in order to arrive at the optimized protocol for the preparation of clinical dose. Biological behavior of the radiotracer prepared was studied in C57/BL6 mice bearing melanoma tumors. Results: Lu-177 was produced with a specific activity of 950 +/- 50 GBq/mg (25.7 +/- 1.4 Ci/mg) and radionuclidic purity of 99.98%. A careful optimization of several parameters showed that Lu-177-DOTA-E[c(RGDfK)](2) could be prepared with adequately high radiochemical purity using a ligand-to-metal ratio similar to 2. Based on these studies therapeutic dose of the agent with 7.4 GBq of Lu-177 was formulated in similar to 63 GBg/mu M specific activity with high yield (98.2 +/- 0.7%), radiochemical purity and in vitro stability. Biodistribution studies carried out in C57/BL6 mice bearing melanoma tumors revealed specific accumulation of the radiolabeled conjugate in tumor (3.80 +/- 0.55% ID/g at 30 mm p.i.) with high tumor to blood and tumor to muscle ratios. However, the uptake of the radiotracer in the tumor was found to be reduced to 1.51 +/- 0.32 %ID/g at 72 h p.i. Conclusions: The present work successfully demonstrates the formulation of an optimized protocol for the preparation of Lu-177 labeled DOTA-E[c(RGDfK)](2) for PRRT applications using Lu-177 produced by direct neutron activation in a medium flux research reactor. (C) 2013 Elsevier Inc. All rights reserved.