68Ga labeled fatty acids for cardiac metabolic imaging: Influence of different bifunctional chelators

Modern Developments in Bifunctional Chelator Design for Gallium Radiopharmaceuticals

The positron-emitting radionuclide gallium-68 has become increasingly utilised in both preclinical and clinical settings with positron emission tomography (PET). The synthesis of radiochemically pure gallium-68 radiopharmaceuticals relies on careful consideration of the coordination chemistry. The short half-life of 68 min necessitates rapid quantitative radiolabelling (≤10 min). Desirable radiolabelling conditions include near-neutral pH, ambient temperatures, and low chelator concentrations to achieve the desired apparent molar activity. This review presents a broad overview of the requirements of an efficient bifunctional chelator in relation to the aqueous coordination chemistry of gallium. Developments in bifunctional chelator design and application are then presented and grouped according to eight categories of bifunctional chelator: the macrocyclic chelators DOTA and TACN; the acyclic HBED, pyridinecarboxylates, siderophores, tris(hydroxypyridinones), and DTPA; and the mesocyclic diazepines.

Synthesis and evaluation of radiogallium-labeled long-chain fatty acid derivatives as myocardial metabolic imaging agents

Since long-chain fatty acids work as the primary energy source for the myocardium, radiolabeled long-chain fatty acids play an important role as imaging agents to diagnose metabolic heart dysfunction and heart diseases. With the aim of developing radiogallium-labeled fatty acids, herein four fatty acid-based tracers, [67Ga]Ga-HBED-CC-PDA, [67Ga]Ga-HBED-CC-MHDA, [67Ga]Ga-DOTA-PDA, and [67Ga]Ga-DOTA-MHDA, which are [67Ga]Ga-HBED-CC and [67Ga]Ga-DOTA conjugated with pentadecanoic acid (PDA) and 3-methylhexadecanoic acid (MHDA), were synthesized, and their potential for myocardial metabolic imaging was evaluated. Those tracers were found to be chemically stable in 0.1 M phosphate buffered saline. Initial [67Ga]Ga-HBED-CC-PDA, [67Ga]Ga-HBED-CC-MHDA, [67Ga]Ga-DOTA-PDA, and [67Ga]Ga-DOTA-MHDA uptakes in the heart at 0.5 min postinjection were 5.01 ± 0.30%ID/g, 5.74 ± 1.02%ID/g, 5.67 ± 0.22%ID/g, and 5.29 ± 0.10%ID/g, respectively. These values were significantly lower than that of [123I]BMIPP (21.36 ± 2.73%ID/g). For their clinical application as myocardial metabolic imaging agents, further structural modifications are required to increase their uptake in the heart.

Integrated approaches to unravel the impact of protein lipoxidation on macromolecular interactions

Protein modification by lipid derived reactive species, or lipoxidation, is increased during oxidative stress, a common feature observed in many pathological conditions. Biochemical and functional consequences of lipoxidation include changes in the conformation and assembly of the target proteins, altered recognition of ligands and/or cofactors, changes in the interactions with DNA or in protein-protein interactions, modifications in membrane partitioning and binding and/or subcellular localization. These changes may impact, directly or indirectly, signaling pathways involved in the activation of cell defense mechanisms, but when these are overwhelmed they may lead to pathological outcomes. Mass spectrometry provides state of the art approaches for the identification and characterization of lipoxidized proteins/residues and the modifying species. Nevertheless, understanding the complexity of the functional effects of protein lipoxidation requires the use of additional methodologies. Herein, biochemical and biophysical methods used to detect and measure functional effects of protein lipoxidation at different levels of complexity, from in vitro and reconstituted cell-like systems to cells, are reviewed, focusing especially on macromolecular interactions. Knowledge generated through innovative and complementary technologies will contribute to comprehend the role of lipoxidation in pathophysiology and, ultimately, its potential as target for therapeutic intervention.

PET Imaging of Hepatocellular Carcinomas: 18F-Fluoropropionic Acid as a Complementary Radiotracer for 18F-Fluorodeoxyglucose

Objective:

To evaluate the preclinical value of ¹⁸F-fluoropropionic acid (¹⁸F-FPA) and ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography (PET) for imaging HCCs.

Methods:

The ¹⁸F-FPA and ¹⁸F-FDG uptake patterns in 3 HCC cell lines (Hep3B, HepG2, and SK-Hep1) were assessed in vitro and in vivo. The ¹⁸F-FPA uptake mechanism was investigated using inhibition experiments with orlistat and 5-tetradecyloxy-2-furoic acid. The ¹⁸F-FPA PET imaging was performed in different tumor animal models and compared with ¹⁸F-FDG. We also evaluated the expressions of glucose transporter-1 (GLUT1), fatty acid synthase (FASN), and matrix metalloproteinase-2 (MMP2) in these cell lines.

Results:

In vitro experiments showed that the radiotracer uptake patterns were complementary in the HCC cell lines. Orlistat and 5-tetradecyloxy-2-furoic acid decreased the uptake of ¹⁸F-FPA. The tumor-to-liver ratio of ¹⁸F-FPA was superior to that of ¹⁸F-FDG in the SK-Hep1 and HepG2 tumors (P < .05). However, in the Hep3B tumors, the tumor-to-liver normalized uptake of ¹⁸F-FDG was higher than ¹⁸F-FPA (P < .01). FASN was highly expressed in cell lines with high ¹⁸F-FPA uptake, whereas GLUT1 was highly expressed in cell lines with high ¹⁸F-FDG uptake. The ¹⁸F-FPA uptake correlated with FASN (r = 0.89, P = .014) and MMP2 (r = 0.77, P = .002) expressions.

Conclusions:

PET imaging with ¹⁸F-FPA combined with ¹⁸F-FDG can be an alternative for detecting HCC.

Emerging Tracers for Nuclear Cardiac PET Imaging

Myocardial perfusion imaging using positron emission tomography (PET) has several advantages over single photon emission computed tomography (SPECT). The recent advances in SPECT technology have shown promise, but there is still a large need for PET in the clinical management of coronary artery disease (CAD). Especially, absolute quantification of myocardial blood flow (MBF) using PET is extremely important. In spite of considerable advances in the diagnosis of CAD, novel PET radiopharmaceuticals remain necessary for the diagnosis of CAD because clinical use of current cardiac radiotracers is limited by their physical characteristics, such as decay mode, emission energy, and half-life. Thus, the use of a radioisotope that has proper characteristics and a proper half-life to develop myocardial perfusion agents could overcome these limitations. In this review, the current state of cardiac PET and a general overview of novel 18F or 68Ga-labeled radiotracers, including their radiosynthesis, in vivo characterization, and evaluation, are provided. The future perspectives are discussed in terms of their potential usefulness based on new image analysis methods and hybrid imaging.

Synthesis and evaluation of 68Ga labeled palmitic acid for cardiac metabolic imaging

This work evaluates the potential of a ⁶⁸Ga labeled long chain 16C fatty acid for cardiac metabolic imaging. For radiolabeling with ⁶⁸Ga, hexadecanedioic acid was coupled with the chelator p-NH₂-Bn-NOTA. Under the optimized conditions, NOTA-hexadecanoic acid could be radiolabeled with ⁶⁸Ga in ≥95% yields. In biodistribution studies carried out in Swiss mice, ⁶⁸Ga-NOTA-hexadecanoic acid showed low myocardial uptake at 2 min p.i. (3.7 ± 1.3%ID/g). While ⁶⁸Ga-NOTA-hexadecanoic acid cleared rapidly from non-target organs such as blood, lungs, intestine and kidney, wash out from liver was slow. Radio-HPLC analyses of myocardial extracts of rats injected with ⁶⁸Ga-NOTA-hexadecanoic acid confirmed its metabolic transformation in the myocardium.