Comparative study on DOTA-derivatized bombesin analog labeled with ⁹⁰Y and ¹⁷⁷Lu: in vitro and in vivo evaluation

Abstract 

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 ⁹⁰Y and ¹⁷⁷Lu, 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 BN1.1 (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 ⁹⁰Y and with ¹⁷⁷Lu [specific activity (SA), 15 Ci/mg Lu] were investigated and optimized to provide ⁹⁰Y-DOTA-BN[2–14]NH₂ and ¹⁷⁷Lu-DOTA-BN[2–14]NH₂ of high SA. The stability of the radiolabeled 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

⁹⁰Y-DOTA-BN[2–14]NH₂ and ¹⁷⁷Lu-DOTA-BN[2–14]NH₂ were obtained with radiochemical yield >98% and high SA (67.3 GBq ⁹⁰Y/μmol and 33.6 GBq ¹⁷⁷Lu/μmol, 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 ^natY- and ^natLu-labeled analogs to GRP receptors were high (IC₅₀=1.78, 1.99, and 1.34 nM, respectively), especially for the ^natLu-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 ¹⁷⁷Lu-labeled peptide (84.87%) than for the ⁹⁰Y-labeled one (80.79%), while the efflux rate was slower for ¹⁷⁷Lu-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 ¹⁷⁷Lu-labeled peptide than that for the ⁹⁰Y-labeled (81% vs. 42%, respectively).

Conclusions

Our studies demonstrated that DOTA-BN[2–14]NH₂ can be labeled with ⁹⁰Y (NCA) and ¹⁷⁷Lu (CA) with high radiochemical yields. The in vitro and in vivo comparison between ⁹⁰Y-DOTA-BN[2–14]NH₂ and ¹⁷⁷Lu-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 ¹⁷⁷Lu-labeled derivative.

Keywords: Lutetium-177, Yttrium-90, β-Emitters, Bombesin analog, GRP analogs