[18F]Atorvastatin Pharmacokinetics and Biodistribution in Healthy Female and Male Rats.

Statins are 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors that are widely used to prevent cardiovascular diseases. However, a series of pleiotropic mechanisms have been associated with statins, particularly with atorvastatin. Therefore, the assessment of [18F]atorvastatin kinetics with positron emission tomography (PET) may elucidate the mechanism of action of statins and the impact of sexual dimorphism, which is one of the most debated interindividual variations influencing the therapeutic efficacy. [18F]Atorvastatin was synthesized via a previously optimized 18F-deoxyfluorination strategy, used for preclinical PET studies in female and male Wistar rats (n = 7 for both groups), and for subsequent ex vivo biodistribution assessment. PET data were fitted to several pharmacokinetic models, which allowed for estimating relevant kinetic parameters. Both PET imaging and biodistribution studies showed negligible uptake of [18F]atorvastatin in all tissues compared with the primary target organ (liver), excretory pathways (kidneys and small intestine), and stomach. Uptake of [18F]atorvastatin was 38 ± 3% higher in the female liver than in the male liver. The irreversible 2-tissue compartment model showed the best fit to describe [18F]atorvastatin kinetics in the liver. A strong correlation (R2 > 0.93) between quantitative Ki (the radiotracer's unidirectional net rate of influx between compartments) and semi-quantitative liver's SUV (standard uptake value), measured between 40 to 90 min, showed potential to use the latter parameter, which circumvents the need for blood sampling as a surrogate of Ki for monitoring [18F]atorvastatin uptake. Preclinical assays showed faster uptake and clearance for female rats compared to males, seemingly related to a higher efficiency for exchanges between the arterial input and the hepatic tissue. Due to the slow [18F]atorvastatin kinetics, equilibrium between the liver and plasma concentration was not reached during the time frame studied, making it difficult to obtain sufficient and accurate kinetic information to quantitatively characterize the radiotracer pharmacokinetics over time. Nevertheless, the reported results suggest that the SUV can potentially be used as a simplified measure, provided all scans are performed at the same time point. Preclinical PET-studies with [18F]atorvastatin showed faster uptake and clearance in female compared to male rats, apparently related to higher efficiency for exchange between arterial blood and hepatic tissue.

MCR Scaffolds Get Hotter with 18F-Labeling.

Imaging techniques, such as positron emission tomography (PET), represent great progress in the clinical development of drugs and diagnostics. However, the efficient and timely synthesis of appropriately labeled compounds is a largely unsolved problem. Numerous small drug-like molecules with high structural diversity can be synthesized via convergent multicomponent reactions (MCRs). The combination of PET labeling with MCR synthesis of biologically active compounds can greatly simplify radioanalytical and imaging-based analysis. In a proof-of-concept study, we optimized robust on-site radiolabeling conditions that were subsequently applied to several structurally different drug-like MCR scaffolds (e.g., arenes, ß-lactam, tetrazole, and oxazole). These labeled scaffolds were synthesized via pinacol-derived aryl boronic esters (arylBPin) by copper-mediated oxidative 18F-fluorination with radiochemical conversions (RCCs) from 15% to 76%.

o-Boronato- and o-Trifluoroborato-Phosphonium Salts Supported by L-α-Amino Acid Side Chain.

The synthesis of o-boronato- and o-trifluoroborato-phosphonium salts supported by the L-amino acid side chain is described. The synthesis of these new class of amino acid derivatives was achieved by stereoselective quaternization of o-(pinacolato)boronatophenylphosphine with ß- or γ-iodo amino acid derivatives which are prepared from L-serine or L-aspartic acid, respectively. The quaternization of the phosphine was performed using either iodo amino ester or carboxylic acid derivatives. In addition, free carboxylic acid and amine derivatives were obtained by saponification or HCl acidolysis of o-boronato-phosphonium amino esters, respectively. The usefulness of these compounds in peptide coupling was demonstrated by coupling an o-boronato-phosphonium amino ester with an aspartic acid moiety. When the o-boronato-phosphonium amino acid or dipeptide derivatives were mixed with fluoride, the corresponding o-trifluoroborated products were cleanly and rapidly obtained in high isolated yields. The hydrolysis of these compounds at room temperature using a phosphate buffer pH 7/CD3CN mixture has shown only traces of free fluoride F(-) after several days. Finally, a preliminary radiolabeling essay has proven the facile [(18)F]-fluoride incorporation and high stability of the radiolabeled product in aqueous conditions. Indeed, this new class of boron-phosphonium amino acid derivatives shows promising properties for their applications in synthesis and labeling of peptides.

Recent advances in chelator design and labelling methodology for (68) Ga radiopharmaceuticals.

Gallium-68 has the potential to become the technetium-99m of positron emission tomography with ideal decay characteristics and a long-lived parent isotope for generator production. The work in the area of (68) Ga is focused on two key areas: (1) synthesis of a library of bifunctional chelators, which can be quickly radiolabelled to form kinetically inert complexes under mild conditions compatible with biomolecules and (2) development of radiosynthetic methodologies for clinical use and to facilitate radiolabelling of a wide range of chelators under mild conditions. Recent advances in these areas, with particular focus on the past 3 years, are covered herein.

Mapping Arginase Expression with 18F-Fluorinated Late-Generation Arginase Inhibitors Derived from Quaternary α-Amino Acids.

Arginase hydrolyzes L-arginine and influences levels of polyamines and nitric oxide. Arginase overexpression is associated with inflammation and tumorigenesis. Thus, radiolabeled arginase inhibitors may be suitable PET tracers for staging arginase-related pathophysiologies. We report the synthesis and evaluation of 2 radiolabeled arginase inhibitors, 18F-FMARS and 18F-FBMARS, developed from α-substituted-2-amino-6-boronohexanoic acid derivatives. Methods: Arylboronic ester-derived precursors were radiolabeled via copper-mediated fluorodeboronation. Binding assays using arginase-expressing PC3 and LNCaP cells were performed. Autoradiography of lung sections from a guinea pig model of asthma overexpressing arginase and dynamic small-animal PET imaging with PC3-xenografted mice evaluated the radiotracers' specific binding and pharmacokinetics. Results:18F-fluorinated compounds were obtained with radiochemical yields of up to 5% (decay-corrected) and an average molar activity of 53 GBq⋅µmol-1 Cell and lung section experiments indicated specific binding that was blocked up to 75% after pretreatment with arginase inhibitors. Small-animal PET studies indicated fast clearance of the radiotracers (7.3 ± 0.6 min), arginase-mediated uptake, and a selective tumor accumulation (SUV, 3.0 ± 0.7). Conclusion: The new 18F-fluorinated arginase inhibitors have the potential to map increased arginase expression related to inflammatory and tumorigenic processes. 18F-FBMARS showed the highest arginase-mediated uptake in PET imaging and a significant difference between uptake in control and arginase-inhibited PC3 xenografted mice. These results encourage further research to examine the suitability of 18F-FBMARS for selecting patients for treatments with arginase inhibitors.

[18F]Atorvastatin: synthesis of a potential molecular imaging tool for the assessment of statin-related mechanisms of action.

BACKGROUND: Statins are lipid-lowering agents that inhibit cholesterol synthesis and are clinically used in the primary and secondary prevention of cardiovascular diseases. However, a considerable group of patients does not respond to statin treatment, and the reason for this is still not completely understood. [18F]Atorvastatin, the 18F-labeled version of one of the most widely prescribed statins, may be a useful tool for statin-related research. RESULTS: [18F]Atorvastatin was synthesized via an optimized ruthenium-mediated late-stage 18F-deoxyfluorination. The defluoro-hydroxy precursor was produced via Paal-Knorr pyrrole synthesis and was followed by coordination of the phenol to a ruthenium complex, affording the labeling precursor in approximately 10% overall yield. Optimization and automation of the labeling procedure reliably yielded an injectable solution of [18F]atorvastatin in 19% ± 6% (d.c.) with a molar activity of 65 ± 32 GBq·µmol-1. Incubation of [18F]atorvastatin in human serum did not lead to decomposition. Furthermore, we have shown the ability of [18F]atorvastatin to cross the hepatic cell membrane to the cytosolic and microsomal fractions where HMG-CoA reductase is known to be highly expressed. Blocking assays using rat liver sections confirmed the specific binding to HMG-CoA reductase. Autoradiography on rat aorta stimulated to develop atherosclerotic plaques revealed that [18F]atorvastatin significantly accumulates in this tissue when compared to the healthy model. CONCLUSIONS: The improved ruthenium-mediated 18F-deoxyfluorination procedure overcomes previous hurdles such as the addition of salt additives, the drying steps, or the use of different solvent mixtures at different phases of the process, which increases its practical use, and may allow faster translation to clinical settings. Based on tissue uptake evaluations, [18F]atorvastatin showed the potential to be used as a tool for the understanding of the mechanism of action of statins. Further knowledge of the in vivo biodistribution of [18F]atorvastatin may help to better understand the origin of off-target effects and potentially allow to distinguish between statin-resistant and non-resistant patients.

64Cu PET Imaging of the CXCR4 Chemokine Receptor Using a Cross-Bridged Cyclam Bis-Tetraazamacrocyclic Antagonist.

Expression of the chemokine receptor chemokine C-X-C motif receptor 4 (CXCR4) plays an important role in cancer metastasis, in autoimmune diseases, and during stem cell-based repair processes after stroke and myocardial infarction. Previously reported PET imaging agents targeting CXCR4 suffer from either high nonspecific uptake or bind only to the human form of the receptor. The objective of this study was to develop a high-stability 64Cu-labeled small-molecule PET agent for imaging both human and murine CXCR4 chemokine receptors. Methods: Synthesis, radiochemistry, stability and radioligand binding assays were performed for the novel tracer 64Cu-CuCB-bicyclam. In vivo dynamic PET studies were performed on mice bearing U87 (CXCR4 low-expressing) and U87.CXCR4 (human-CXCR4 high-expressing) tumors. Biodistribution and receptor blocking studies were performed on CD1-IGS immunocompetent mice. CXCR4 expression on tumor and liver disaggregates was confirmed using a combination of immunohistochemistry, quantitative polymerase chain reaction, and Western blot. Results:64Cu-CuCB-bicyclam has a high affinity for both the human and the murine variants of the CXCR4 receptor (half-maximal inhibitory concentration, 8 nM [human]/2 nM [murine]) and can be obtained from the parent chelator that has low affinity. In vitro and in vivo studies demonstrate specific uptake in CXCR4-expressing cells that can be blocked by more than 90% using a higher-affinity antagonist, with limited uptake in non-CXCR4-expressing organs and high in vivo stability. The tracer was also able to selectively displace the CXCR4 antagonists AMD3100 and AMD3465 from the liver. Conclusion: The tetraazamacrocyclic small molecule 64Cu-CuCB-bicyclam has been shown to be an imaging agent for the CXCR4 receptor that is likely to be applicable across a range of species. It has high affinity and stability and is suitable for preclinical research in immunocompetent murine models.

Synthesis, characterization and in vivo evaluation of a magnetic cisplatin delivery nanosystem based on PMAA-graft-PEG copolymers.

The development of anticancer drug delivery systems which retain or enhance the cytotoxic properties of the drug to tumorous tissues, while reducing toxicity to other organs is of key importance. We investigated different poly(methacrylic acid)-g-poly(ethyleneglycol methacrylate) polymers as in situ coating agents for magnetite nanocrystallites. The obtained magnetic nano-assemblies were in turn thoroughly characterized for their structural, colloidal and physicochemical properties (drug loading capacity/release, magnetic field triggered drug release, cell uptake and localization) in order to select the best performing system. With the focus on in vivo validation of such magnetic drug delivery systems for first time, we selected cisplatin as the drug, since it is a potent anticancer agent which exhibits serious side effects due to lack of selectivity. In addition, cisplatin would offer facile determination of the metal content in the animal tissues for biodistribution studies. Alongside post-mortem Pt determination in the tissues, the biodistribution of the drug nanocarriers was also monitored in real time with PET-CT (positron emission tomography/computed tomography) with and without the presence of magnetic field gradients; using a novel chelator-free method, the nanoparticles were radiolabeled with 68Ga without having to alter their structure with chemical modifications for conjugation of radiochelators. The ability to be radiolabeled in such a straightforward but very robust way, along with their measured high MRI response, renders them attractive for dual imaging, which is an important functionality for translational investigations. Their anticancer properties were evaluated in vitro and in vivo, in a cisplatin resistant HT-29 human colon adenocarcinoma model, with and without the presence of magnetic field gradients. Enhanced anticancer efficacy and reduced toxicity was recorded for the cisplatin-loaded nanocarriers in comparison to the free cisplatin, particularly when a magnetic field gradient was applied at the tumor site. Post mortem and real-time tissue distribution studies did not reveal increased cisplatin concentration in the tumor site, suggesting that the enhanced anticancer efficacy of the cisplatin-loaded nanocarriers is driven by mechanisms other than increased cisplatin accumulation in the tumors.

Boron-(18) F containing positron emission tomography radiotracers: advances and opportunities.

Standard [(18) F]fluorination methods to form carbon-fluorine bonds can have some limitations such as low yield and the requirement for harsh reaction conditions. Inorganic approaches include the formation of boron-[(18) F]fluorine bonds and have the potential to give high specific activities at room temperature forming a bond that is stable in vivo. There is considerable potential in future applications, particularly in relation to multimodal imaging and the provision of rapid efficient labelling protocols.

Chelator free gallium-68 radiolabelling of silica coated iron oxide nanorods via surface interactions.

The commercial availability of combined magnetic resonance imaging (MRI)/positron emission tomography (PET) scanners for clinical use has increased demand for easily prepared agents which offer signal or contrast in both modalities. Herein we describe a new class of silica coated iron-oxide nanorods (NRs) coated with polyethylene glycol (PEG) and/or a tetraazamacrocyclic chelator (DO3A). Studies of the coated NRs validate their composition and confirm their properties as in vivo T2 MRI contrast agents. Radiolabelling studies with the positron emitting radioisotope gallium-68 (t1/2 = 68 min) demonstrate that, in the presence of the silica coating, the macrocyclic chelator was not required for preparation of highly stable radiometal-NR constructs. In vivo PET-CT and MR imaging studies show the expected high liver uptake of gallium-68 radiolabelled nanorods with no significant release of gallium-68 metal ions, validating our innovation to provide a novel simple method for labelling of iron oxide NRs with a radiometal in the absence of a chelating unit that can be used for high sensitivity liver imaging.

Final step gallium-68 radiolabelling of silica-coated iron oxide nanorods as potential PET/MR multimodal imaging agents.

The investigation of iron oxide-based positron emission tomography/magnetic resonance (PET/MR) multimodal imaging agents is an expanding field in which a variety of nanoparticle sizes, shapes, surface coatings and radioisotopes are open for exploration. This study develops iron oxide nanorods which are coated with various mixtures of poly(ethylene glycol) (PEG) and macrocyclic ligand (DO3A) via the formation of a silica layer on the surface. Gallium-68 radiolabelling of the nanorods was carried out in high radiochemical yields (RCY) and their stability in human serum was demonstrated for all constructs, even in the absence of the macrocyclic chelating unit. Further studies were carried out in an attempt to determine the appropriate amount of PEG coating to give optimal properties for future in vivo studies.