Imaging Tumor Vasculature Noninvasively with Positron Emission Tomography and RGD Peptides Labeled with Copper 64 Using the Bifunctonal Chelates DOTA, Oxo-DO3A. and PCTA

Two novel bifunctional chelates, 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-3,6,9-triacetic acid (PCTA) and 1-oxa4,7,10-triazacyclododecane-4,7,10-triacetic acid (Oxo-DO3A), were found to radiolabel antibodies with copper 64 ( 64 Cu) well for positron emission tomography (PET). In this study, the same chelators were used to radiolabel peptides with 64 Cu for PET imaging of angiogenesis. PCTA, Oxo-DO3A, and 1,4,7,10-tetraazacyclododecane-N,N’,N’’,N’’’-tetraacetic acid (DOTA) were conjugated to cyclic(RGDyK), and their binding affinities were confirmed. Conditions for 64 Cu radiolabeling were optimized for maximum yield and specific activity. The in vitro stability of the radiolabeled compounds was challenged with serum incubation. PET studies were carried out in a non–avb3-expressing tumor model to evaluate the compounds’ specificity for proliferating tumor vasculature and their in vivo pharmacokinetics. The PCTA and Oxo-DO3A bioconjugates were labeled with 64 Cu at higher effective specific activity and radiochemical yield than the DOTA bioconjugate. In the imaging studies, all the 64 Cu bioconjugates could be used to visualize the tumor and the radiotracer uptake was blocked with cyclic-(RGDyK). Target uptake of each bioconjugate was similar, but differences in other tissues were observed. 64 Cu-PCTA-RGD showed the best clearance from nontarget tissue and the highest tumor to nontarget ratios. PCTA was the most promising bifunctional chelate for 64 Cu peptide imaging and warrants further investigation.