Making defect-free macromolecules is a challenging issue in chemical synthesis. This challenge is especially pronounced in dendrimer synthesis where exponential growth quickly leads to steric ...congestion. To overcome this difficulty, proportionate branching in dendrimer growth is proposed. In proportionate branching, both the number and the length of branches increase exponentially but in opposite directions to mimic tree growth. The effectiveness of this strategy is demonstrated through the synthesis of a fluorocarbon dendron containing 243 chemically identical fluorine atoms with a MW of 9082 Da. Monodispersity is confirmed by nuclear magnetic resonance spectroscopy, mass spectrometry, and small-angle X-ray scattering. Growing different parts proportionately, as nature does, could be a general strategy to achieve defect-free synthesis of macromolecules.
Abstract
In vivo
positron emission tomography (PET) imaging is a key modality to evaluate disease status of brain tumors. In recent years, tremendous efforts have been made in developing PET imaging ...methods for pediatric brain tumors. Carbon-11 labelled tryptophan derivatives are feasible as PET imaging probes in brain tumor patients with activation of the kynurenine pathway, but the short half-life of carbon-11 limits its application. Using a transgenic mouse model for the sonic hedgehog (Shh) subgroup of medulloblastoma, here we evaluated the potential of the newly developed 1-(2-
18
Ffluoroethyl)-L-tryptophan (1-L-
18
FFETrp) as a PET imaging probe for this common malignant pediatric brain tumor. 1-L-
18
FFETrp was synthesized on a PETCHEM automatic synthesizer with good chemical and radiochemical purities and enantiomeric excess values. Imaging was performed in tumor-bearing Smo/Smo medulloblastoma mice with constitutive actvation of the Smoothened (Smo) receptor using a PerkinElmer G4 PET-X-Ray scanner. Medulloblastoma showed significant and specific accumulation of 1-L-
18
FFETrp. 1-L-
18
FFETrp also showed significantly higher tumor uptake than its D-enantiomer, 1-D-
18
FFETrp. The uptake of 1-L-
18
FFETrp in the normal brain tissue was low, suggesting that 1-L-
18
FFETrp may prove a valuable PET imaging probe for the Shh subgroup of medulloblastoma and possibly other pediatric and adult brain tumors.
Central nervous system tumors are the most common pediatric solid tumors; they are also the most lethal. Unlike adults, childhood brain tumors are mostly primary in origin and differ in type, ...location and molecular signature. Tumor characteristics (incidence, location, and type) vary with age. Children present with a variety of symptoms, making early accurate diagnosis challenging. Neuroimaging is key in the initial diagnosis and monitoring of pediatric brain tumors. Conventional anatomic imaging approaches (computed tomography (CT) and magnetic resonance imaging (MRI)) are useful for tumor detection but have limited utility differentiating tumor types and grades. Advanced MRI techniques (diffusion-weighed imaging, diffusion tensor imaging, functional MRI, arterial spin labeling perfusion imaging, MR spectroscopy, and MR elastography) provide additional and improved structural and functional information. Combined with positron emission tomography (PET) and single-photon emission CT (SPECT), advanced techniques provide functional information on tumor metabolism and physiology through the use of radiotracer probes. Radiomics and radiogenomics offer promising insight into the prediction of tumor subtype, post-treatment response to treatment, and prognostication. In this paper, a brief review of pediatric brain cancers, by type, is provided with a comprehensive description of advanced imaging techniques including clinical applications that are currently utilized for the assessment and evaluation of pediatric brain tumors.
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Nine fluorine-containing vesicular acetylcholine transporter (VAChT) inhibitors were synthesized and screened as potential PET tracers for imaging the VAChT. Compound 18a was one of ...the most promising carbonyl-containing benzovesamicol analogs; the minus enantiomer, (−)-18a displayed high potency (VAChT Ki=0.59±0.06nM) and high selectivity for VAChT versus σ receptors (>10,000-fold). The radiosynthesis of (−)-18F18a was accomplished by a two-step procedure with 30–40% radiochemical yield. Preliminary biodistribution studies of (−)-18F18a in adult male Sprague-Dawley rats at 5, 30, 60 and 120min post-injection (p.i.) were promising. The total brain uptake of (−)-18F18a was 0.684%ID/g at 5min p.i. and by 120min p.i. slowly washed out to 0.409 %ID/g; evaluation of regional brain uptake showed stable levels of ∼0.800 %ID/g from 5 to 120min p.i in the VAChT-enriched striatal tissue of rats, indicating the tracer had crossed the blood brain barrier and was retained in the striatum. Subsequent microPET brain imaging studies of (−)-18F18a in nonhuman primates (NHPs) showed high striatal accumulation in the NHP brain; the standardized uptake value (SUV) for striatum reached a maximum value of 5.1 at 15min p.i. The time–activity curve for the target striatal region displayed a slow and gradual decreasing trend 15min after injection, while clearance of the radioactivity from the cerebellar reference region was much more rapid. Pretreatment of NHPs with 0.25mg/kg of the VAChT inhibitor (−)-vesamicol resulted in a ∼90% decrease of striatal uptake compared to baseline studies. HPLC metabolite analysis of NHP plasma revealed that (−)-18F18a had a good in vivo stability. Together, these preliminary results suggest (−)-18F18a is a promising PET tracer candidate for imaging VAChT in the brain of living subjects.
A small fluorocarbon dendron is conjugated to Cys-34 of albumin via a flexible linker to form fluorinated conjugates of albumin.
A small fluorocarbon dendron that contains nine chemically identical ...fluorine atoms was covalently conjugated to albumin via a flexible linker. Two versions were made, which differ by 10% in the linker length. Both versions display split 19F signal and much shorter 19F longitudinal relaxation time than their small molecule counterparts. 10% difference in the flexible linker length has negligible impact on the 19F signal.
Neurofibromatosis type 1 (NF1) is a neurocutaneous disorder. Plexiform neurofibromas (PNFs) are benign tumors commonly formed in patients with NF1. PNFs have a high incidence of developing into ...malignant peripheral nerve sheath tumors (MPNSTs) with a 5-year survival rate of only 30%. Therefore, the accurate diagnosis and differentiation of MPNSTs from benign PNFs are critical to patient management. We studied a fluorine-18 labeled tryptophan positron emission tomography (PET) radiotracer, 1-(2-18Ffluoroethyl)-L-tryptophan (L-18FFETrp), to detect NF1-associated tumors in an animal model. An ex vivo biodistribution study of L-18FFETrp showed a similar tracer distribution and kinetics between the wild-type and triple mutant mice with the highest uptake in the pancreas. Bone uptake was stable. Brain uptake was low during the 90-min uptake period. Static PET imaging at 60 min post-injection showed L-18FFETrp had a comparable tumor uptake with 1⁸Ffluorodeoxyglucose (FDG). However, L-18FFETrp showed a significantly higher tumor-to-brain ratio than FDG (n = 4, p < 0.05). Sixty-minute-long dynamic PET scans using the two radiotracers showed similar kidney, liver, and lung kinetics. A dysregulated tryptophan metabolism in NF1 mice was further confirmed using immunohistostaining. L-18FFETrp is warranted to further investigate differentiating malignant NF1 tumors from benign PNFs. The study may reveal the tryptophan–kynurenine pathway as a therapeutic target for treating NF1.
Sphingosine-1-phosphate receptor (S1PR) activation plays a key role in vascular inflammatory response. Here, we report in vivo validation of 11CTZ3321, a potent S1PR1 radioligand, for imaging ...vascular inflammation in a rat model of carotid injury. The right common carotid artery of male adult Sprague-Dawley rats was injured by balloon overinflation that denuded the endothelium and distended the vessel wall. Animals received a 60-minute micro-positron emission tomography (micro PET) scan with 11CTZ3321 at 72 hours after injury. Ex vivo autoradiography was also conducted. The expression and cellular location of S1PR1 were examined by immunohistological analysis. Three-dimensional (3D) reconstruction of the first 100-second microPET/computed tomography (CT) image indicated the location of bilateral common carotid arteries. 11CTZ3321 displayed significantly higher accumulation (standardized uptake values: 0.93 ± 0.07 vs 0.78 ± 0.09, n = 6, P = .001) in the injured carotid artery than in the contralateral side. Increased tracer uptake in the injured artery was confirmed by autoradiography (photostimulated luminescence measures: 85.5 ± 0.93 vs 71.48 ± 6.22, n = 2). Concordantly, high S1PR1expression was observed in infiltrated inflammatory cells in the injured artery. Our studies demonstrate 11CTZ3321 microPET is able to detect the acute upregulation of S1PR1 expression in inflamed carotid artery. Therefore, 11CTZ3321 has potential to be a PET radiotracer for detecting early inflammatory response and monitoring therapeutic efficacy of vascular inflammation.
Sphingosine 1-phosphate receptor 1 (S1P1) plays a pivotal signaling role in inflammatory response; because S1P1 modulation has been identified as a therapeutic target for various diseases, a PET ...tracer for S1P1 would be a useful tool. Fourteen fluorine-containing analogues of S1P ligands were synthesized and their in vitro binding potency measured; four had high potency and selectivity for S1P1 (S1P1 IC50 < 10 nM, >100-fold selectivity for S1P1 over S1P2 and S1P3). The most potent ligand, 28c (IC50 = 2.63 nM for S1P1) was 18F-labeled and evaluated in a mouse model of LPS-induced acute liver injury to determine its S1P1-binding specificity. The results from biodistribution, autoradiography, and microPET imaging showed higher 18F28c accumulation in the liver of LPS-treated mice than controls. Increased expression of S1P1 in the LPS model was confirmed by immunohistochemical analysis (IHC). These data suggest that 18F28c is a S1P1 PET tracer with high potential for imaging S1P1 in vivo.
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Twelve optically pure enantiomers were obtained using either crystallization or chiral high performance liquid chromatography (HPLC) separation methodologies to resolve six racemic ...sigma-1 (σ1) receptor ligands. The in vitro binding affinities of each enantiomer for σ1, σ2 receptors and vesicular acetylcholine transporter (VAChT) were determined. Out of the 12 optically pure enantiomers, five displayed very high affinities for σ1 (Ki<2nM) and high selectivity for σ1 versus σ2 and VAChT (>100-fold). The minus enantiomer, (−)-14a ((−)-TZ3108) (Ki-σ1=1.8±0.4nM, Ki-σ2=6960±810nM, Ki-VAChT=980±87nM), was chosen for radiolabeling and further in vivo evaluation in rodents and nonhuman primates (NHPs). A biodistribution study in Sprague Dawley rats showed brain uptake (%ID/gram) of (−)-18FTZ3108 reached 1.285±0.062 at 5min and 0.802±0.129 at 120min. NHP microPET imaging studies revealed higher brain uptake of (−)-18FTZ3108 and more favorable pharmacokinetics compared to its racemic counterpart. Pretreatment of the animal using two structurally different σ1 ligands significantly decreased accumulation of (−)-18FTZ3108 in the brain. Together, our in vivo evaluation results suggest that (−)-18FTZ3108 is a promising positron emission tomography (PET) tracer for quantifying σ1 receptor in the brain.
HIV-associated neuroinflammation is believed to be a major contributing factor in the development of HIV-associated neurocognitive disorders (HAND). In this study, we used micropositron emission ...tomography (PET) imaging to quantify neuroinflammation in HIV-1 transgenic rat (Tg), a small animal model of HIV, known to develop neurological and behavioral problems.
Dynamic (18)FDPA-714 PET imaging was performed in Tg and age-matched wild-type (WT) rats in three age groups: 3-, 9-, and 16-month-old animals. As a positive control for neuroinflammation, we performed unilateral intrastriatal injection of quinolinic acid (QA) in a separate group of WT rats. To confirm our findings, we performed multiplex immunofluorescent staining for Iba1 and we measured cytokine/chemokine levels in brain lysates of Tg and WT rats at different ages.
(18)FDPA-714 uptake in HIV-1 Tg rat brains was generally higher than in age-matched WT rats but this was not statistically significant in any age group. (18)FDPA-714 uptake in the QA-lesioned rats was significantly higher ipsilateral to the lesion compared to contralateral side indicating neuroinflammatory changes. Iba1 immunofluorescence showed no significant differences in microglial activation between the Tg and WT rats, while the QA-lesioned rats showed significant activation. Finally, cytokine/chemokine levels in brain lysates of the Tg rats and WT rats were not significantly different.
Microglial activation might not be the primary mechanism for neuropathology in the HIV-1 Tg rats. Although (18)FDPA-714 is a good biomarker of neuroinflammation, it cannot be reliably used as an in vivo biomarker of neurodegeneration in the HIV-1 Tg rat.
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