PET Radiotracers of the Cardiovascular System

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.

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

Development of ⁶⁸Ga labeled fatty acids is of immense interest due to the availability of ⁶⁸Ga through a generator and its superiority over SPECT based tracers in carrying out dynamic imaging on a PET scanner. Our present work explores the influence of different chelators on the cardiac uptake and pharmacokinetics of the ⁶⁸Ga-labeled fatty acids. Two new ⁶⁸Ga labeled fatty acids were synthesized by conjugation of 11-aminoundecanoic acid with the bifunctional chelators (BFCs) viz. p-SCN-Bn-DTPA (S-2-(4-isothiocyanatobenzyl)-diethylenetriaminepentaacetic acid) and p-SCN-Bn-NODAGA (S-2-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1-glutaric acid-4,7-acetic acid) and their comparison was carried out with the previously reported ⁶⁸Ga-NOTA-undecanoic acid. Both the conjugates were radiolabeled with ⁶⁸Ga in high yields and purities (>95%). Their formation was established by preparation and characterization of their inactive analogs with ^natGa at macroscopic levels. Biodistribution studies of the complexes in Swiss mice showed lower initial myocardial uptake for ⁶⁸Ga-NODAGA-undecanoic acid (3.8±0.6%ID/g) and ⁶⁸Ga-DTPA-undecanoic acid (1.3±0.5%ID/g) complexes in comparison to previously reported ⁶⁸Ga-NOTA-undecanoic acid complex (7.4±2.8%ID/g) at 2min p.i. However, significant retention of the tracer in the myocardium was observed in the case of ⁶⁸Ga-NODAGA-undecanoic complex, which led to improved heart/non-target ratios of the complex over time in comparison to the other ⁶⁸Ga complexes. Similarly, the DTPA complex exhibited increased washout from the liver in comparison to other ⁶⁸Ga derivatives. The β oxidation mechanism in myocytes was investigated by isolating the myocardial extract post intravenous injection of the respective ⁶⁸Ga complexes and analyzing them by radio-HPLC, which showed metabolic transformation of the parent fatty acid complex peak in all the three complexes. This study has provided an insight into the design characteristics of ⁶⁸Ga labeled fatty acids to achieve the desired myocardial imaging characteristics.

Utility of positron emission tomography for drug development for heart failure

Only about 1 in 5,000 investigational agents in a preclinical stage acquires Food and Drug Administration approval. Among many reasons for this includes an inefficient transition from preclinical to clinical phases, which exponentially increase the cost and the delays the process of drug development. Positron emission tomography (PET) is a nuclear imaging technique that has been used for the diagnosis, risk stratification, and guidance of therapy. However, lately with the advance of radiochemistry and of molecular imaging technology, it became evident that PET could help novel drug development process. By using a PET radioligand to report on receptor occupancy during novel agent therapy, it may help assess the effectiveness, efficacy, and safety of such a new medication in an early preclinical stage and help design successful clinical trials even at a later phase. In this article, we explore the potential implications of PET in the development of new heart failure therapies and review PET's application in the respective pathophysiologic pathways such as myocardial perfusion, metabolism, innervation, inflammation, apoptosis, and cardiac remodeling.

Imaging radiation-induced normal tissue injury

Technological developments in radiation therapy and other cancer therapies have led to a progressive increase in five-year survival rates over the last few decades. Although acute effects have been largely minimized by both technical advances and medical interventions, late effects remain a concern. Indeed, the need to identify those individuals who will develop radiation-induced late effects, and to develop interventions to prevent or ameliorate these late effects is a critical area of radiobiology research. In the last two decades, preclinical studies have clearly established that late radiation injury can be prevented/ameliorated by pharmacological therapies aimed at modulating the cascade of events leading to the clinical expression of radiation-induced late effects. These insights have been accompanied by significant technological advances in imaging that are moving radiation oncology and normal tissue radiobiology from disciplines driven by anatomy and macrostructure to ones in which important quantitative functional, microstructural, and metabolic data can be noninvasively and serially determined. In the current article, we review use of positron emission tomography (PET), single photon emission tomography (SPECT), magnetic resonance (MR) imaging and MR spectroscopy to generate pathophysiological and functional data in the central nervous system, lung, and heart that offer the promise of, (1) identifying individuals who are at risk of developing radiation-induced late effects, and (2) monitoring the efficacy of interventions to prevent/ameliorate them.

Synthesis and evaluation of 15-(4-(2-[¹⁸F]Fluoroethoxy)phenyl)pentadecanoic acid: a potential PET tracer for studying myocardial fatty acid metabolism

15-(4-(2-[¹⁸F]fluoroethoxy)phenyl)pentadecanoic acid ([¹⁸F]7) was synthesized as a PET probe for assessing myocardial fatty acid metabolism. The radiosynthesis of [¹⁸F]7 was accomplished using a two-step reaction, starting with the corresponding tosylate ester, methyl 15-(4-(2-(tosyloxy)ethoxy)phenyl)pentadecanoate (5), and gave the radiolabeled fatty acid, [¹⁸F]7 in a radiolabeling yield of 55-60% and a specific activity of >2000 Ci/mmol (decay corrected to EOB). The biological evaluation of [¹⁸F]7 in rats displayed high uptake in heart (1.94%ID/g at 5 min), which was higher than the uptake (%ID/g) in blood, lung, muscle, pancreas, and brain. MicroPET studies of [¹⁸F]7 in Sprague-Dawley rats demonstrated excellent images of the myocardium when compared with [¹¹C]palmitate images in the same animal. Moreover, the tracer kinetics of [¹⁸F]7 paralleled those seen with [¹¹C]palmitate, with an early peak followed by biphasic washout. When compared to [¹¹C]palmitate, [¹⁸F]7 exhibited a slower early clearance (0.17 ± 0.01 vs 0.30 ± 0.02, P < 0.0001) and a significantly higher late clearance (0.0030 ± 0.0005 vs 0.0006 ± 0.00013, P < 0.01). These initial studies suggest that [¹⁸F]7 could be a potentially useful clinical PET tracer to assess abnormal myocardial fatty acid metabolism.