Evaluation of 64Cu-labeled dipicolylamine (DPA) as a small-molecule PET probe for in vivo imaging of phosphatidylserine exposure

1502 Objectives Anionic phosphatidylserine (PS) is highly exposed on the surfaces of cancer cells and tumor blood vessels. Synthetic bis-zinc(II)-dipicolylamine (Zn-DPA) derivatives have strong binding affinities to biological membranes enriched with PS. This study was designed to investigate the feasibility of a small-molecule PET probe, 64Cu-labeled DPA, for in vivo imaging of PS exposure. Methods Bis-dipicolylamine was radiolabeled with 64Cu (t1/2 = 12.7 h) to provide 64Cu-DPA which was then subjected to in vitro and in vivo evaluations, including stability, lipophilicity, U87MG cell uptake, PET imaging, and direct tissue sampling biodistribution studies in a U87MG tumor xenografted mouse model. Results The direct 64Cu-radiolabeling of DPA (n = 6) was achieved with radiochemical purity of >98%. The octanol/water partition coefficient (log P) for 64Cu-DPA was determined to be -1.32 ± 0.07, suggesting that 64Cu-DPA is moderately hydrophilic. A stability study in PBS showed that 64Cu-DPA is stable with >92% of 64Cu-DPA remaining intact after 6 h and in vitro studies revealed 64Cu-DPA binds to U87MG glioma cells and has good tumor cell retention. In PET studies, the U87MG tumors were all clearly visible with high contrast to contralateral background at all measured time points after injection of 64Cu-DPA. Much lower tumor uptake and predominant liver uptake were observed for 64CuCl2-PET, indicating that the tracer uptake in U87MG tumor results from 64Cu-DPA rather than free 64Cu. In addition, 64Cu-DPA demonstrated good tumor retention and high tumor-to-muscle ratio (14.99 ± 0.65 at 24 h postinjection). However, high accumulation and retention of 64Cu-DPA in the liver were observed. Finally, biodistribution results were consistent with the quantification analyses of microPET imaging. Conclusions Our results showed the feasibility of direct radiolabeling of DPA with 64Cu and 64Cu-DPA has the potential for in vivo imaging of PS exposure with PET