Novel synthesis and initial preclinical evaluation of (18)F-[FDG] labeled rhodamine: a potential PET myocardial perfusion imaging agent

Development of new copper-64 labeled rhodamine: a potential PET myocardial perfusion imaging agent

Background

Myocardial perfusion imaging (MPI) is one of the most commonly performed investigations in nuclear medicine procedures. Due to the longer half-life of the emerging positron emitter copper-64 and its availability from low energy cyclotron, together with its well-known coordination chemistry, we have synthesized ⁶⁴Cu-labeled NOTA- and ⁶⁴Cu-NOTAM-rhodamine conjugates as potential cardiac imaging agents using PET.

Results

⁶⁴Cu-NOTA- and ⁶⁴Cu-NOTAM-rhodamine conjugates were synthesized using a traightforward and one-step simple reaction. Radiochemical yields were greater than 97% (decay corrected), with a total synthesis time of less than 25 min. Radiochemical purities were always greater than 98% as assessed by TLC and HPLC. These synthetic approaches hold considerable promise as a simple method for ⁶⁴Cu-rhodamine conjugates synthesis, with high radiochemical yield and purity. Biodistribution studies in normal Fischer rats at 60 min post-injection, demonstrated significant heart uptake and a good biodistribution profile for both the radioconjugates. However, the ⁶⁴Cu-NOTAM-rhodamine conjugate has shown more heart uptake (~ 10% ID/g) over the ⁶⁴Cu-NOTA-rhodamine conjugate (5.6% ID/g).

Conclusions

These results demonstrate that these radioconjugates may be useful probes for the PET evaluation of MPI.

Sweetening Pharmaceutical Radiochemistry by 18F-Fluoroglycosylation: Recent Progress and Future Prospects

In the field of ¹⁸F-chemistry for the development of radiopharmaceuticals for positron emission tomography (PET), various labeling strategies by the use of prosthetic groups have been implemented, including chemoselective ¹⁸F-labeling of biomolecules. Among those, chemoselective ¹⁸F-fluoroglycosylation methods focus on the sweetening of pharmaceutical radiochemistry by offering a highly valuable tool for the synthesis of ¹⁸F-glycoconjugates with suitable in vivo properties for PET imaging studies. A previous review covered the various ¹⁸F-fluoroglycosylation methods that were developed and applied as of 2014 (Maschauer and Prante, BioMed. Res. Int. 2014, 214748). This paper is an updated review, providing the recent progress in ¹⁸F-fluoroglycosylation reactions and the preclinical application of ¹⁸F-glycoconjugates, including small molecules, peptides, and high-molecular-weight proteins.

Fluorine-18-Labeled Fluorescent Dyes for Dual-Mode Molecular Imaging

Recent progress realized in the development of optical imaging (OPI) probes and devices has made this technique more and more affordable for imaging studies and fluorescence-guided surgery procedures. However, this imaging modality still suffers from a low depth of penetration, thus limiting its use to shallow tissues or endoscopy-based procedures. In contrast, positron emission tomography (PET) presents a high depth of penetration and the resulting signal is less attenuated, allowing for imaging in-depth tissues. Thus, association of these imaging techniques has the potential to push back the limits of each single modality. Recently, several research groups have been involved in the development of radiolabeled fluorophores with the aim of affording dual-mode PET/OPI probes used in preclinical imaging studies of diverse pathological conditions such as cancer, Alzheimer's disease, or cardiovascular diseases. Among all the available PET-active radionuclides, 18F stands out as the most widely used for clinical imaging thanks to its advantageous characteristics (t1/2 = 109.77 min; 97% β+ emitter). This review focuses on the recent efforts in the synthesis and radiofluorination of fluorescent scaffolds such as 4,4-difluoro-4-bora-diazaindacenes (BODIPYs), cyanines, and xanthene derivatives and their use in preclinical imaging studies using both PET and OPI technologies.

Efficient cartridge purification for producing high molar activity [18F]fluoro-glycoconjugates via oxime formation

INTRODUCTION

¹⁸F-fluoroglycosylation via oxime formation is a chemoselective and mild radiolabeling method for sensitive molecules. Glycosylation can also improve the bioavailability, in vivo kinetics, and stability of the compound in blood, as well as accelerate clearance of biomolecules. A typical synthesis procedure for ¹⁸F-fluoroglycosylation with [¹⁸F]FDG (2-deoxy-2-[¹⁸F]fluoro-d-glucose) and [¹⁸F]FDR (5-deoxy-5-[¹⁸F]fluoro-d-ribose) involves two HPLC (high performance liquid chromatography) purifications: one after ¹⁸F-fluorination of the carbohydrate to remove its labeling precursor, and a second one after the oxime formation step to remove the aminooxy precursor. The two HPLC purifications can be time consuming and complicate the adaptation of the synthetic strategy in nuclear medicine applications and automated synthesis. We have developed a procedure in which SPE (solid phase extraction) and resin purification methods replace both of the needed HPLC purification steps.

METHODS

We used [¹⁸F]FDR and [¹⁸F]FDG as prosthetic groups to radiolabel two aminooxy-modified model molecules, a tetrazine and a PSMA (prostate specific membrane antigen) inhibitor. After fluorination, the excess carbohydrate precursor was removed by derivatizing it with 4,4'-dimethoxytrityl chloride (DMT-Cl). The DMT moiety increases the hydrophobicity of the unreacted precursor making the separation from the fluorinated precursor possible with simple C18 Sep-Pak cartridge. For removal of the aminooxy precursor, we used a commercially available aldehyde resin (AminoLink, Thermo Fisher Scientific). C18 Sep-Pak SPE cartridge was used to separate [¹⁸F]FDR and [¹⁸F]FDG from the ¹⁸F-fluoroglycoconjugate end product.

RESULTS

[¹⁸F]FDR and [¹⁸F]FDG were efficiently purified from their precursors, free fluorine-18, and other impurities. The aldehyde resin quantitatively removed the unreacted aminooxy precursors after the oxime formation. The fluorine-18 labeled oxime end products were obtained with high radiochemical purity (>99%) and molar activity (>600 GBq μmol^-1).

CONCLUSIONS

We have developed an efficient cartridge purification method for producing high molar activity ¹⁸F-glycoconjugates synthesized via oxime formation.

Radiosynthesis and preclinical evaluation of [18F] 4-(2-fluoroethoxy)-2H-chromen-2-one as a novel myocardial perfusion imaging agent

Recently we developed [18F] 4-(2-fluoroethoxy)-2H-chromen-2-one as a novel 18F myocardial perfusion imaging radiotracer. It was synthesized in good radiochemical yield (>90%). The total time from radiosynthesis to its purification was less than 40 min, with excellent radiochemical purity (≥99%). It showed good stability over a period of 5 h at room temperature. The partition coefficient (log P) of radiotracer was found to be 2.70, suggesting the lipophilic nature of radiotracer. Ex vivo biodistribution study of radiotracer in normal Wistar rats for 30 min post-injection, demonstrated a good heart uptake (>1.3% ID/g) and favorable pharmacokinetics. Additionally, the radiotracer showed significant excretion (>11% ID) by liver, which is indicative of its rapid clearance. Further, in vivo biodistribution study of radiotracer in New Zealand White rabbit provided the clear PET/CT images of cardiomyocytes and myocardial perfusion. All these experimental findings suggest that [18F] 4-(2-fluoroethoxy)-2H-chromen-2-one could be used as a potential hit for myocardial perfusion imaging.

New chemical and radiochemical routes to [18F]Rho6G-DEG-F, a delocalized lipophilic cation for myocardial perfusion imaging with PET

New chemical and radiochemical syntheses are described for the preparation of [¹⁸F]Rho6G-DEG-F, an ¹⁸F-labeled analogue of the fluoresecent dye rhodamine 6G, which has shown promise as myocardidal perfusion imaging agent. Tosylated precursors of [¹⁸F]Rho6G-DEG-F amenable to ¹⁸F-labeling were obtained either through a two-step synthesis from rhodamine 6G lactone (33% yield), or in one step from rhodamine 575 (64% yield), then purified by preparative C₁₈ chromatography. Manual synthesis of [¹⁸F]Rho6G-DEG-F was achieved in a single radiochemical step from either the tosylate salt or the tosylate/formate double salt in DMSO under standard nucleophillic aliphatic ¹⁸F-fluorination conditions (K[¹⁸F]F/K₂CO₃/Kryptofix 2.2.2.). Incorporation of the [¹⁸F]F^- was found to be satisfactory (≥34% by TLC), despite the protic character of the precursor molecules. [¹⁸F]Rho6G-DEG-F was manually synthesized in final decay-corrected radiochemical yields of 11-26% (tosylate salt) and 9-21% (tosylate/formate double salt). The protocol was transferred to an automated synthesis unit, where the product was obtained in 3-9% radiochemical yield (n=3) decay corrected to start-of-synthesis, >99% radiochemical purity, and a molar activity of 122-267 GBq/μmol (3.3-7.2 Ci/μmol).

Cardiac metabolic imaging: current imaging modalities and future perspectives

In this review, current imaging techniques and their future perspectives in the field of cardiac metabolic imaging in humans are discussed. This includes a range of noninvasive imaging techniques, allowing a detailed investigation of cardiac metabolism in health and disease. The main imaging modalities discussed are magnetic resonance spectroscopy techniques for determination of metabolite content (triglycerides, glucose, ATP, phosphocreatine, and so on), MRI for myocardial perfusion, and single-photon emission computed tomography and positron emission tomography for quantitation of perfusion and substrate uptake.

Research Progress on 18F-Labeled Agents for Imaging of Myocardial Perfusion with Positron Emission Tomography

Coronary artery disease (CAD) is the leading cause of death in the world. Myocardial perfusion imaging (MPI) plays a significant role in non-invasive diagnosis and prognosis of CAD. However, neither single-photon emission computed tomography nor positron emission tomography clinical MPI agents can absolutely satisfy the demands of clinical practice. In the past decades, tremendous developments happened in the field of ¹⁸F-labeled MPI tracers. This review summarizes the current state of ¹⁸F-labeled MPI tracers, basic research data of those tracers, and the future direction of MPI tracer research.

Fluorine-18 labelled building blocks for PET tracer synthesis

This review presents a comprehensive overview of the synthesis and application of fluorine-18 labelled building blocks since 2010.

Synthesis and evaluation of novel F-18-labeled pyrimidine derivatives: potential FAK inhibitors and PET imaging agents for cancer detection

Compound [¹⁸F]-8a exhibited good in vivo biodistribution data in mice bearing S180 tumor. And the microPET imaging study of [¹⁸F]-8a in S180 tumor-bearing mice was also preformed, which illustrated that the uptake in S180 tumor at 60 min post-injection of [¹⁸F]-8a was obvious.