Novel 64Cu Labeled RGD2-BBN Heterotrimers for PET Imaging of Prostate Cancer

Bombesin receptor 2 (BB2) and integrin a(v)beta(3) receptor are privileged targets for molecular imaging of cancer because of their overexpression in a number of tumor tissues. The most recent developments in heterodimer-based radiopharmaceuticals concern BB2 and integrin a(v)beta(3)-targeting compounds, consisting of bombesin (BBN) and cyclic arginineglycine-aspartic acid peptides (RGD), connected through short length linkers. Molecular imaging probes based on RGD-BBN heterodimer design exhibit improved tumor targeting efficacy compared to the single-receptor targeting peptide monomers. However, their application in clinical study is restricted because of inefficient synthesis or unfavorable in vivo properties, which could depend on the short linker nature. Thus, the aim of the present study was to develop a RGD(2)-BBN heterotrimer, composed of (7-14)BBN-NH2 peptide (BBN) linked to the E[c(RGDyK)](2) dimer peptide (RGD(2)), bearing the new linker type [Pro-Gly](12). The heterodimer E[c(RGDyK)]ZPEG(3)-Glu-(Pro-Gly)(12)-BBN(7-14)-NH2, (RGD(2)-PG(12)-BBN) was prepared through conventional solid phase synthesis, then conjugated with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) or 1,4,7-triazacyclononane-l-glutaric acid-4,7-diacetic acid (NODA-GA). In 64Cu labeling, the NODA-GA chelator showed superior radiochemical characteristics compared to DOTA (70% vs 40% yield, respectively). Both conjugates displayed dual targeting ability, showing good a(v)beta(3) affinities and high BB2 receptor affinities which, in the case of the NODA-GA conjugate, were in the same range as the best RGD-BBN heterodimer ligands reported to date (K-i = 24 nM). Cu-64-DOTA and (64)CuNODA-GA probes were also found to be stable after 1 h incubation in mouse serum (>90%). In a microPET study in prostate cancer PC-3 xenograft mice, both probes showed low tumor uptake, probably due to poor pharmacokinetic properties in vivo. Overall, our study demonstrates that novel RGD-BBN heterodimer with long linker can be prepared and they preserve high binding affinities to BB2 and integrin a(v)beta(3) receptor binding ability. The present study represents a step forward in the design of effective heterodimer or heterotrimer probes for dual targeting.