Comparative Study On Dota-Derivatized Bombesin Analog Labeled With Y-90 And Lu-177: In Vitro And In Vivo Evaluation

Introduction: The aim of the study was to compare in vitro and in vivo a novel DOTA-chelated bombesin (BN) analog of the amino acid sequence, QRLGNQWAVGHLM-CONH2 (BN[2-14]NH2), labeled with Y-90 and Lu-177, for its potential use in targeted radiotherapy of tumors expressing gastrin releasing peptide (GRP) receptors. The same amino acid sequence, but with different chelator, referred as BNJ.1 (Gly-Gly-Cys-Aca-QRLGNQWAVGHLM-CONH2), has already been studied and reported; however, the DOTA-chelated one, suitable for labeling with M+3 type radiometals, was not yet described.Methods: The conditions for labeling of DOTA-BN[2-14]NH2 with noncarrier added Y-90 and with Lu-177 [specific activity (SA), 15 Ci/mg Lu] were investigated and optimized to provide Y-90-DOTA-BN[2-14]NH2 and Lu-177-DOTA-BN[2-14]NH2 of high SA. The stability of the radjolabeled compounds in human serum was evaluated over a period of 24 h. The human prostate cancer cell line PC-3, known to express GRP receptors, was used for in vitro evaluation of radiolabeled peptide affinity to GRP receptors and for assessment of cytotoxicity of both nonlabeled and radiolabeled peptide. Biodistribution accompanied by receptor blocking was studied in normal Swiss mice.Results: (90)DOTA-B[2-14]NH2 and Lu-177-DOTA-BN[2-14]NH2 were obtained with radiochemical yield >98% and high SA (67.3 GBq Y-90/mu mol and 33.6 GBq Lu-177/mu mol, respectively). They were stable when incubated in human serum for up to 24 h. The binding affinities of DOTA-BN[2-14]NH2 and both Y-nat- and Lu-nat-labeled analogs to GRP receptors were high (IC50=1.78, 1.99, and 1.34 nM, respectively), especially for the Lu-nat-DOTA-BN[2-14]NH2 complex. The cytotoxicity Study of DOTA-BN[2-14]NH2 to PC-3 cells revealed an IC50=6300 nM after 72 h of exposition, while the labeled derivatives showed no significant cytotoxic effect. The internalization rate to PC-3 cells was more rapid for Lu-177-labeled peptide (84.87%) than for the Y-90-labeled one (80.79%), while the efflux rate was slower for Lu-177-DOTA-BN[2-14]NH2 (46.8% vs. 61.74%). The biodistribution study of both derivatives in normal mice revealed a specific binding to GRP receptor-positive tissues, which could be blocked by coinjection of cold peptide. The effect of receptor blockage in vivo was also more pronounced for the Lu-177-labeled peptide than that for the Y-90-labeled (81% vs. 42%, respectively).Conclusions: Our studies demonstrated that DOTA-BN[2-14]NH2 can be labeled with Y-90 (NCA) and Lu-177 (CA) with high radiochemical yields. The in vitro and in vivo comparison between Y-90-DOTA-BN[2-14]NH2, and Lu-177-DOTA-BN[2-14]NH2 indicated that the change of radiometal in the complex from Y to Lu influence the binding affinity to the GRP receptors with preference to the Lu-labeled derivative. (C) 2009 Published by Elsevier Inc.