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

Abstract

The design of dual mode fluorescent-PET peptidic tracers that can be labeled with [(18)F]fluoride at high specific activity and high yield has been challenged by the short half-life of (18)F 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 [(18)F]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 [(18)F]fluoride via isotope exchange within 20 min in a single step at high specific activity (>3 Ci/μmol) and in good yield to provide 275 mCi and high radiochemical purity. Neither drying of the [(18)F]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/μmol, 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/μmol, 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.

Trimeric Radiofluorinated Sulfonamide Derivatives to Achieve In Vivo Selectivity for Carbonic Anhydrase IX–Targeted PET Imaging

“…The AmBF 3 -conjugated CA-IX-targeting tracers reported here were both obtained in good radiochemical yields (16.3%-36.8%) and specific activity (40-207 GBq/mmol) within 30 min of synthesis time. These radiosynthesis data (radiochemical yield and specific activity) are comparable with those previously reported for other AmBF 3 -conjugates (31,36). For in vivo evaluations, imaging and biodistribution studies were conducted with immunodeficient mice inoculated with HT-29 human colorectal cancer cells.…”

“…Features of this radiolabeling strategy include 1-step synthesis without azeotropic drying, good radiochemical yields, high purity, specific activity and in vitro/vivo stability, and relative ease of purification without high-performance liquid chromatography (5). This approach for 18 F labeling has been successfully applied to RGD and somatostatin analogs for cancer imaging (31,36). The AmBF 3 -conjugated CA-IX-targeting tracers reported here were both obtained in good radiochemical yields (16.3%-36.8%) and specific activity (40-207 GBq/mmol) within 30 min of synthesis time.…”

“…Previously, our group reported the synthesis of a dual-mode imaging agent by coupling rhodamine and cycloRGD (arginineglycine-aspartic acid) to an AmBF 3 -conjugated alkyne 4 amenable to 18 F labeling (31). Using the modular adaptability of this synthon, we coupled 3 AEBS/ABS moieties via copper-catalyzed azide alkyne cycloaddition to target CA-IX expression.…”

Dual PET and Near-Infrared Fluorescence Imaging Probes as Tools for Imaging in Oncology

“…Many such nontargeted agents include nanoparticles, which are untargeted and accumulate in a tumor owing to enhanced permeation and retention or target new or irregular vasculature with 18 F, 64 Cu, and 124 I isotopes that are linked to protoporphyrins, cypates, or Bodipy fluorophores [28]. Cyclic Arg-Gly-Asp (cRGD) is a popular target that is PET/Fl labeled as 18 F-rhodamine [42]. This agent incorporates multiple cRGD integrin-specific ligands with increased affinity for integrin receptors.…”

A fast and effective determination of the biodistribution and subcellular localization of fluorescent immunoliposomes in freshly excised animal organs

“…Such researches often rely on fluorescent agents that can be attached to drugs, tracers and other non-fluorescent contrast agents in order to non-invasively monitor their properties and biodistribution by fluorescence detection [5]. For instance fluorescence imaging is exploited to characterize contrast agents aimed for applications in positron emission tomography (PET) [7] or magnetic resonance imaging (MRI) [8]. Due to the limited penetration depth of light, fluorescence imaging is more feasible in diagnostic imaging of superficial diseases such as rheumatoid arthritis [9], skin, head and neck and breast cancers and for endoscopic imaging of colon cancers as well as intraoperative setups where the surgeon directly visualizes diseased tissues in real time [10].…”