Kiel D. Neumann

Dynamic nuclear polarization of biocompatible 13C-enriched carbonates for in vivo pH imaging

A hyperpolarization technique using carbonate precursors of biocompatible molecules was found to yield high concentrations of hyperpolarized (13)C bicarbonate in solution. This approach enabled large signal gains for low-toxicity hyperpolarized (13)C pH imaging in a phantom and in vivo in a murine model of prostate cancer.

[11C]Para-Aminobenzoic Acid: A Positron Emission Tomography Tracer Targeting Bacteria-Specific Metabolism

Imaging studies are frequently used to support the clinical diagnosis of infection. These techniques include computed tomography (CT) and magnetic resonance imaging (MRI) for structural information and single photon emission computed tomography (SPECT) or positron emission tomography (PET) for metabolic data. However, frequently, there is significant overlap in the imaging appearance of infectious and noninfectious entities using these tools. To address this concern, recent approaches have targeted bacteria-specific metabolic pathways. For example, radiolabeled sugars derived from sorbitol and maltose have been investigated as PET radiotracers, since these are efficiently incorporated into bacteria but are poor substrates for mammalian cells. We have previously shown that para-aminobenzoic acid (PABA) is an excellent candidate for development as a bacteria-specific imaging tracer as it is rapidly accumulated by a wide range of pathogenic bacteria, including metabolically quiescent bacteria and clinical strains, but not by mammalian cells. Therefore, in this study, we developed an efficient radiosynthesis for [11C]PABA, investigated its accumulation into Escherichia coli and Staphylococcus aureus laboratory strains in vitro, and showed that it can distinguish between infection and sterile inflammation in a murine model of acute bacterial infection.

Imaging Active Infection in vivo Using D-Amino Acid Derived PET Radiotracers

Occult bacterial infections represent a worldwide health problem. Differentiating active bacterial infection from sterile inflammation can be difficult using current imaging tools. Present clinically viable methodologies either detect morphologic changes (CT/ MR), recruitment of immune cells (111In-WBC SPECT), or enhanced glycolytic flux seen in inflammatory cells (18F-FDG PET). However, these strategies are often inadequate to detect bacterial infection and are not specific for living bacteria. Recent approaches have taken advantage of key metabolic differences between prokaryotic and eukaryotic organisms, allowing easier distinction between bacteria and their host. In this report, we exploited one key difference, bacterial cell wall biosynthesis, to detect living bacteria using a positron-labeled D-amino acid. After screening several 14C D-amino acids for their incorporation into E. coli in culture, we identified D-methionine as a probe with outstanding radiopharmaceutical potential. Based on an analogous procedure to that used for L-[methyl-11C]methionine ([11C] L-Met), we developed an enhanced asymmetric synthesis of D-[methyl-11C]methionine ([11C] D-Met), and showed that it can rapidly and selectively differentiate both E. coli and S. aureus infections from sterile inflammation in vivo. We believe that the ease of [11C] D-Met radiosynthesis, coupled with its rapid and specific in vivo bacterial accumulation, make it an attractive radiotracer for infection imaging in clinical practice.

Quantification of 89Zr-Iron oxide nanoparticle biodistribution using PET-MR and ultrashort TE sequences: Letter to the Editor

One developing application of magnetic nanoparticles is to use magnetically linked drugs in intra-arterial chemotherapy (IAC) procedures. These magnetic drugs can then be selectively removed by deploying an endovascular magnetic device downstream of the targeted organ, thus limiting off-target drug toxicities. In vitro studies used radiolabeled iron oxide nanoparticles (IONP) to quantify the number of particles captured on the device using a gamma counter. To demonstrate efficacy in vivo, accurate quantification of the drug’s distribution on the device, within the targeted organ, and systemically is necessary. The purpose of this study was to validate positron emission tomography / magnetic resonance imaging (PET/MRI) image-based quantification, using Zr-PET signal, transverse relaxation rate (R 2), and quantitative magnetic susceptibility (v) of ferromagnetic Zr-IONP biodistribution in vitro and in vivo.

Detection of Bacteria-Specific Metabolism Using Hyperpolarized [2-13C]Pyruvate

The differentiation of bacterial infection from other causes of inflammation is difficult in clinical practice and is critical where patient outcomes rely heavily on early interventions. In addition to physical exam and laboratory markers, several imaging modalities are frequently employed, but these techniques generally target the host immune response, rather than the living microorganisms themselves. Here, we describe a method to detect bacteria-specific metabolism using hyperpolarized (HP) 13C magnetic resonance spectroscopy. This technology allows visualization of the real-time conversion of enriched 13C substrates to their metabolic products, identified by their distinct chemical shifts. We have identified the rapid metabolism of HP [2-13C]pyruvate to [1-13C]acetate as a metabolic signature of common bacterial pathogens. We demonstrate this conversion in representative Gram-negative and Gram-positive bacteria, namely, Escherichia coli and Staphylococcus aureus, and its absence in key mammalian cell types. Furthermore, this conversion was successfully modulated in three mutant strains, corresponding to deletions of relevant enzymes.

Radiosynthesis, ex Vivo Biodistribution, and in Vivo Positron Emission Tomography Imaging Evaluations of [11C]2-Pyridinealdoxime Methiodide ([11C]2-PAM): A First-In-Class Antidote Tracer for Organophosphate Intoxication

2-Pyridinealdoxime methiodide (2-PAM) is a widely used antidote for the treatment of organophosphorus (OP) exposure that reactivates the target protein acetylcholinesterase. Carbon-11 2-PAM was prepared to more fully understand the in vivo mode of action, distribution, and dynamic qualities of this important countermeasure. Alkylation of 2-pyridinealdoxime with [11C]CH3I provided the first-in-class [11C]2-PAM tracer in 3.5% decay corrected radiochemical yield from [11C]CH3I, >99% radiochemical purity, and 4831 Ci/mmol molar activity. [11C]2-PAM tracer distribution was evaluated by ex vivo biodistribution and in vivo dynamic positron emission tomography (PET) imaging in naïve (OP exposure deficient) rats. Tracer alone and tracer coinjected with a body mass-scaled human therapeutic dose of 30 mg/kg nonradioactive 2-PAM demonstrated statistically similar tissue and blood distribution profiles with the greatest uptake in kidney and significantly lower levels in liver, heart, and lung with lesser amounts in blood and brain. The imaging and biodistribution data show that radioactivity uptake in brain and peripheral organs is rapid and characterized by differential tissue radioactivity washout profiles. Analysis of arterial blood samples taken 5 min after injection showed ∼82% parent [11C]2-PAM tracer. The imaging and biodistribution data are now established, enabling future comparisons to outcomes acquired in OP intoxicated rodent models.

Exploring Metabolism In Vivo Using Endogenous 11C Metabolic Tracers

Cancer and other diseases are increasingly understood in terms of their metabolic disturbances. This thinking has revolutionized the field of ex vivo metabolomics and motivated new approaches to detect metabolites in living systems, including proton magnetic resonance spectroscopy (1H-MRS), hyperpolarized 13C MRS, and PET. For PET, imaging abnormal metabolism in vivo is hardly new. Positron-labeled small-molecule metabolites have been used for decades in humans, including 18F-FDG, which is used frequently to detect upregulated glycolysis in tumors. Many current 18F metabolic tracers including 18F-FDG have evolved from their 11C counterparts, chemically identical to endogenous substrates and thus approximating intrinsic biochemical pathways. This mimicry has stimulated the development of new radiochemical methods to incorporate 11C and inspired the synthesis of a large number of 11C endogenous radiotracers. This is in spite of the 20-minute half-life of 11C, which generally limits its use in patients to centers with an on-site cyclotron. Innovation in 11C chemistry has persisted in the face of this limitation, because (1) the radiochemists involved are inspired, (2) the methods of 11C incorporation are diverse, and (3) 11C compounds often show more predictable in vivo behavior, thus representing an important first step in the validation of new tracer concepts. In this mini-review we will discuss some of the general motivations behind PET tracers, rationales for the use of 11C, and some of the special challenges encountered in the synthesis of 11C endogenous compounds. Most importantly, we will try to highlight the exceptional creativity used in early 11C tracer syntheses, which used enzyme-catalyzed and other green methods before these concepts were commonplace.

An improved radiosynthesis of O‐(2‐[18F]fluoroethyl)‐O‐(p‐nitrophenyl)methylphosphonate: A first‐in‐class cholinesterase PET tracer

O-(2-Fluoroethyl)-O-(p-nitrophenyl) methylphosphonate 1 is an organophosphate cholinesterase inhibitor that creates a phosphonyl-serine covalent adduct at the enzyme active site blocking cholinesterase activity in vivo. The corresponding radiolabeled O-(2-[18 F]fluoroethyl)-O-(p-nitrophenyl) methylphosphonate, [18 F]1, has been previously prepared and found to be an excellent positron emission tomography imaging tracer for assessment of cholinesterases in live brain, peripheral tissues, and blood. However, the previously reported [18 F]1 tracer synthesis was slow even with microwave acceleration, required high-performance liquid chromatography separation of the tracer from impurities, and gave less optimal radiochemical yields. In this paper, we report a new synthetic approach to circumvent these shortcomings that is reliant on the facile reactivity of bis-(O,O-p-nitrophenyl) methylphosphonate, 2, with 2-fluoroethanol in the presence of DBU. The cold synthesis was successfully translated to provide a more robust radiosynthesis. Using this new strategy, the desired tracer, [18 F]1, was obtained in a non-decay-corrected radiochemical yield of 8xa0±xa02% (nxa0=xa07) in >99% radiochemical and >95% chemical purity with a specific activity of 3174xa0±xa0345 Ci/mmol (EOS). This new facile radiosynthesis routinely affords highly pure quantities of [18 F]1, which will further enable tracer development of OP cholinesterase inhibitors and their evaluation in vivo.