Dual Mode Fluorescent 18F-PET Tracers: Efficient Modular Synthesis of Rhodamine-[cRGD]2-[18F]-Organotrifluoroborate, Rapid, and High Yielding One-Step 18F-Labeling at High Specific Activity, and Correlated in Vivo PET Imaging and ex Vivo Fluorescence

The design of dual mode fluorescent-PET peptidic tracers that can be labeled with [F-18]fluoride at high specific activity and high yield has been challenged by the short half-life of F-18 and its aqueous indolence toward nucleophilic displacement, that often necessitates multistep reactions that start with punctiliously dry conditions. Here we present a modular approach to constructing a fluorescent dimeric peptide with a pendant radioprosthesis that is labeled in water with [F-18]fluoride ion in a single, user-friendly step. The modular approach starts with grafting a new zwitterionic organotrifluoroborate radioprosthesis onto a pentaerythritol core with three pendent alkynes that enable successive grafting of a bright fluorophore (rhodamine) followed by two peptides (cylcoRGD). The construct is labeled with [F-18]fluoride via isotope exchange within 20 min in a single step at high specific activity (>3 Ci/mu mol) and in good yield to provide 275 mCi and high radiochemical purity. Neither drying of the [F-18]fluoride ion solution nor HPLC purification of the labeled tracer is required. Facile chemical synthesis of this dual mode tracer along with a user-friendly one-step radiolabeling method affords very high specific activity. In vivo PET images of the dual mode tracer are acquired at both high and low specific activities. At very high specific activity, i.e., 3.5 Ci/mu mol, tumor uptake is relatively high (5.5%ID/g), yet the associated mass is below the limits of fluorescent detection. At low specific activity, i.e., 0.01 Ci/mu mol, tumor uptake in the PET image is reduced by approximately 50% (2.9%ID/g), but the greater associated mass enables fluorescence detection in the tumor. These data highlight a facile production of a dual mode fluorescent-PET tracer which is validated with in vivo and ex vivo images. These data also define critical limitations for the use of dual mode tracers in small animals.