Effects of radiolabeling on the optical properties of a dual-labeled peptide

Objectives Multimodality imaging provides complementary functional and anatomical information for diagnosis, treatment planning, and therapeutic monitoring. Significant effort has been placed on using NIRF dyes that can be derivatized and subjected to radiolabeling schemes with reduced risk of physical degradation or loss of fluorescence. In this study we labeled a DOTA-MMP-targeting peptide (M1) with IRDye800CW and a radioisotope and examined the effects of labeling conditions on the properties of the dual-agent.

Methods M1 was labeled with IRDye800CW-NHS in phosphate buffer to form M2. M2 underwent 64Cu labeling in 0.1 M sodium acetate with heating at 50oC for 1 h. Stability studies were performed in PBS, DTPA and serum to evaluate peptide, fluorescent, and radiotracer stability. Generator-produced 68Ga was also used to label M2 by fractionated elution, adjusting the eluate to pH 4 with sodium acetate, and heating at 95oC for 5 mins. Stability studies were performed as described above. Cold Cu and Ga were used to label M2 for evaluation of optical properties in response to the different labeling conditions.

Results M2 was prepared with >90% purity and proved stable in PBS and water for up to 2 weeks as no additional peaks were observed on the UV and fluorescence channels of the HPLC. Radiolabeling with 64Cu and 68Ga resulted in rapid labeling, within 5 mins.; radiochemical purity (RCP) was >95% in both cases and did not require further purification. 64Cu-M2 showed excellent stability in PBS and DTPA, but decreased RCP was observed in serum studies at 24 h. 68Ga-M2 was stable in all conditions. Experiments with cold Cu and Ga labeling showed no major loss of optical properties or fluorescent signal.

Conclusions An efficient dual-labeling strategy was developed for NIR/PET imaging using two different radiometals. Retention of fluorescent properties indicates the feasibility and utility of this approach for the development of novel multimodality imaging agents