Radiolabeled Positron Emission Tomography Research Articles

An Exposition of 11C and 18F Radiotracers Synthesis for PET Imaging.

The development of new radiolabeled Positron emission tomography tracers has been extensively utilized to access the increasing diversity in the research process and to facilitate the development in research methodology, clinical usage of drug discovery and patient care. Recent advances in radiochemistry, as well as the latest techniques in automated radio-synthesizer, have encouraged and challenged the radiochemists to produce the routinely developed radiotracers. Various radionuclides like 18F, 11C, 15O, 13N 99mTc, 131I, 124I and 64Cu are used for incorporating into different chemical scaffolds; among them, 18F and 11C tagged radiotracers are mostly explored such as 11C-Methionine, 11C-Choline, 18F-FDG, 18F-FLT, and 18F-FES. This review is focused on the development of radiochemistry routes to synthesize different radiotracers of 11C and 18F for clinical studies.

Dose Formulation, Biodistribution and PET Imaging Studies of a First-in-Class Fluorine-18 Organophosphorus Cholinesterase Inhibitor Tracer in Rat

Background:: To investigate dynamic live tissue organophosphorus nerve agent uptake and distribution fates resulting in acetylcholinesterase inhibition, we recently reported the first-in-class fluorine-18 [18F] radiolabeled Positron Emission Tomography (PET) imaging tracer known as [18F]O-(2-fluoroethyl)-O-(p-nitrophenyl)methylphosphonate. This tracer has been initially studied in live rats with PET imaging. Objective.: We sought to evaluate the PET tracer in vivo using a new dose formulation of saline, ethanol and L-ascorbic acid, and compare the influence of this formulation on in vivo tracer performance to previous data collected using a CH3CN:PBS formulation. Methods:: A high molar activity [18F]tracer radiosynthesis was used. Doses were formulated as saline, ethanol (≤ 1%) and L-ascorbic acid (0.1%), pH 4.0-4.5. Stability was evaluated to 6 h. Dose injection (i.v.) into male rats was followed by either ex vivo biodistribution profiling at 5, 30, 90 min, or dynamic 90 min PET imaging. Rat biodistribution and PET imaging data were compared. Results and Discussion:: An optimized radiosynthesis (8 ± 2 % RCY) resulted in stable doses for 6 h (>99%). Arterial blood included a tracer and a single metabolite. The ex vivo biodistribution and live tissue PET imaging data revealed rapid radioactivity uptake and distributed tissue levels: heart and lung, highest; liver, moderate; and brain, lowest. Conclusions:: Imaging and biodistribution data were highly correlated with expected radioactivity tissue uptake and distribution in target organs. Lower brain radioactivity levels by PET imaging were found for the new formulation (saline, 1% L-ascorbic acid, < 1% ethanol) as compared to the established CH3CN:PBS formulation. Overall, we found that the i.v. dose formulation changed the in vivo profile of an organophosphorus PET tracer that is considered an important finding for future organophosphorus PET tracer studies.

Tau PET Imaging for Staging of Alzheimer's Disease in Down Syndrome.

Alzheimer's disease (AD) pathology and early-onset dementia develop almost universally in Down syndrome (DS). AD is defined neuropathologically by the presence of extracellular plaques of aggregated amyloid β protein and intracellular neurofibrillary tangles (NFTs) of aggregated hyperphosphorylated tau protein. The development of radiolabeled positron emission tomography (PET) ligands for amyloid plaques and tau tangles enables the longitudinal assessment of the spatial pattern of their accumulation in relation to symptomatology. Recent work indicates that amyloid pathology develops 15-20 years before neurodegeneration and symptom onset in the sporadic and autosomal dominant forms of AD, while tau pathology correlates more closely with symptomatic stages evidenced by cognitive decline and dementia. Recent work on AD biomarkers in DS illustrates similarities between DS and sporadic AD. It may soon be possible to apply recently developed staging classifications to DS to obtain a more nuanced understanding of the development AD in DS and to provide more accurate diagnosis and prognosis in the clinic.

Molecular Imaging of Amyloidosis: Will the Heart Be the Next Target After the Brain?

Amyloidosis is a heterogeneous group of diseases with a common feature of extracellular deposition and infiltration of different types of amyloid fibrils in various organs. For example, Alzheimer's disease is characterized by deposition of amyloid β in the brain. Radiolabeled positron emission tomography (PET) tracers, mainly derivatives of thioflavin-T, were recently introduced for identification of amyloid β plaques in Alzheimer's patients. Such advances of amyloid β plaque imaging of the brain may shed light into imaging of other organs in amyloidosis patients, such as the heart. Cardiac infiltration of amyloid confers poor clinical outcomes, which renders early diagnosis for appropriate clinical management. At present, nuclear imaging of cardiac amyloidosis is predominantly accomplished with bone-seeking radiotracers, such as 99m-technetium-labeled pyrophosphate ((99m)Tc-PYP), 99m-technetium-methylene diphosphonate ((99m)Tc-MDP), and 99m-technetium-3,3,-diphosphono-1,2-propanodicarboxylic acid ((99m)Tc-DPD), with conflicting results in terms of diagnostic performance, with the exception for (99m)Tc-DPD, which may differentiate light-chain amyloidosis from transthyretin-related cardiac amyloidosis. Although other non-bone-seeking radiotracers such as iodine-123-labeled amyloid P component ((123)I-SAP), 123-iodine-Meta-iodobenzylguanidine ((123)I-mIBG), 99m-technetium-labeled protease inhibitor, and indium-111-labeled amyloid antibodies have also shown some success in identifying cardiac amyloidosis, the future, however, may lie in labeling derivatives of thioflavin-T. With the recent success of visualizing deposition of amyloid β in the brain, the US Food and Drug Administration-approved PET imaging agent (18)F-florbetapir may be used to target cardiac amyloidosis next.

PET in Anti-Cancer Drug Development and Therapy

Anti-cancer drug development is a major area of research. Monitoring of response to newer anti-cancer drugs has undergone an evolution from structural imaging modalities to targeting functional metabolic activity at cellular level to better define responsive and non-responsive cancerous tissue. This review article highlights the contribution of Positron Emission Tomography (PET) in this field. PET holds a promising role in the future by providing us information pertaining to the drugs effectiveness early in the course of therapy, so that side effects and expenses can be reduced substantially. PET has been used to measure changes in drug induced metabolism, cellular proliferation and tissue perfusion. Also changes induced by immuno-modulating drugs such as apoptosis, telomere activity, growth factor levels and many more can be studied using specific radiolabelled PET tracers whereas conventional imaging modalities which detect changes in tumor size and residual tissue histopathology may not prove useful in such scenario. In future, most PET scanners will be replaced by Hybrid PET-CT scanners, which provide functional and structural information in the same setting. In addition, PET-CT improves characterization of equivocal lesions and decreases interobserver variability. The most important recent patents concerning role of PET in drug development have been presented.