Ex vivo characterization of neuroinflammatory and neuroreceptor changes during epileptogenesis using candidate positron emission tomography biomarkers

OBJECTIVE

Identification of patients at risk of developing epilepsy before the first spontaneous seizure may promote the development of preventive treatment providing opportunity to stop or slow down the disease.

METHODS

As development of novel radiotracers and on-site setup of existing radiotracers is highly time-consuming and expensive, we used dual-centre in vitro autoradiography as an approach to characterize the potential of innovative radiotracers in the context of epilepsy development. Using brain slices from the same group of rats, we aimed to characterise the evolution of neuroinflammation and expression of inhibitory and excitatory neuroreceptors during epileptogenesis using translational positron emission tomography (PET) tracers; ¹⁸ F-flumazenil (¹⁸ F-FMZ; GABA_A receptor), ¹⁸ F-FPEB (metabotropic glutamate receptor 5; mGluR5), ¹⁸ F-flutriciclamide (translocator protein; TSPO, microglia activation) and ¹⁸ F-deprenyl (monoamine oxidase B, astroglia activation). Autoradiography images from selected time points after pilocarpine-induced status epilepticus (SE; baseline, 24 and 48 hours, 5, 10 and 15 days and 6 and 12-14 weeks after SE) were normalized to a calibration curve, co-registered to an MRI-based 2D region-of-interest atlas, and activity concentration (Bq/mm² ) was calculated.

RESULTS

In epileptogenesis-associated brain regions, ¹⁸ F-FMZ and ¹⁸ F-FPEB showed an early decrease after SE. ¹⁸ F-FMZ decrease was maintained in the latent phase and further reduced in the chronic epileptic animals, while ¹⁸ F-FPEB signal recovered from day 10, reaching baseline levels in chronic epilepsy. ¹⁸ F-flutriciclamide showed an increase of activated microglia at 24 hours after SE, peaking at 5-15 days and decreasing during the chronic phase. On the other hand, ¹⁸ F-deprenyl autoradiography showed late astrogliosis, peaking in the chronic phase.

SIGNIFICANCE

Autoradiography revealed different evolution of the selected targets during epileptogenesis. Our results suggest an advantage of combined imaging of inter-related targets like glutamate and GABA_A receptors, or microglia and astrocyte activation, in order to identify important interactions, especially when using PET imaging for the evaluation of novel treatments.

Hyperpolarised 13C MRI: a new horizon for non-invasive diagnosis of aggressive breast cancer

Hyperpolarised 13C MRI (HP-MRI) is a novel imaging technique that allows real-time analysis of metabolic pathways in vivo.1 The technology to conduct HP-MRI in humans has recently become available and is starting to be clinically applied. As knowledge of molecular biology advances, it is increasingly apparent that cancer cell metabolism is related to disease outcomes, with lactate attracting specific attention. 2 Recent reviews of breast cancer screening programs have raised concerns and increased public awareness of over treatment. The scientific community needs to shift focus from improving cancer detection alone to pursuing novel methods of distinguishing aggressive breast cancers from those which will remain indolent. HP-MRI offers the opportunity to identify aggressive tumour phenotypes and help monitor/predict therapeutic response. Here we report one of the first cases of breast cancer imaged using HP-MRI alongside correlative conventional imaging, including breast MRI.

Preferential targeting of disseminated liver tumors using a recombinant adeno-associated viral vector

A novel selectively targeting gene delivery approach has been developed for advanced hepatocellular carcinoma (HCC), a leading cause of cancer mortality whose prognosis remains poor. We combine the strong liver tropism of serotype-8 capsid-pseudotyped adeno-associated viral vectors (AAV8) with a liver-specific promoter (HLP) and microRNA-122a (miR-122a)-mediated posttranscriptional regulation. Systemic administration of our AAV8 construct resulted in preferential transduction of the liver and encouragingly of HCC at heterotopic sites, a finding that could be exploited to target disseminated disease. Tumor selectivity was enhanced by inclusion of miR-122a-binding sequences (ssAAV8-HLP-TK-122aT4) in the expression cassette, resulting in abrogation of transgene expression in normal murine liver but not in HCC. Systemic administration of our tumor-selective vector encoding herpes simplex virus-thymidine kinase (TK) suicide gene resulted in a sevenfold reduction in HCC growth in a syngeneic murine model without toxicity. In summary, we have developed a systemically deliverable gene transfer approach that enables high-level expression of therapeutic genes in HCC but not normal tissues, thus improving the prospects of safe and effective treatment for advanced HCC.

In vivo imaging of glucose uptake and metabolism in tumors

Tumors have a greater reliance on anaerobic glycolysis for energy production than normal tissues. We developed a noninvasive method for imaging glucose uptake in vivo that is based on magnetic resonance imaging and allows the uptake of unlabeled glucose to be measured through the chemical exchange of protons between hydroxyl groups and water. This method differs from existing molecular imaging methods because it permits detection of the delivery and uptake of a metabolically active compound in physiological quantities. We show that our technique, named glucose chemical exchange saturation transfer (glucoCEST), is sensitive to tumor glucose accumulation in colorectal tumor models and can distinguish tumor types with differing metabolic characteristics and pathophysiologies. The results of this study suggest that glucoCEST has potential as a useful and cost-effective method for characterizing disease and assessing response to therapy in the clinic.

Radiosynthesis of the D2/3 agonist [3-11C]-(+)-PHNO using [11C]iodomethane

We report here a radiosynthesis for the D(2/3) agonist (+)-4-([3-(11)C]propyl)-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol (3-[(11)C]-(+)-PHNO) labelled at the terminal carbon of the N-propyl chain. The protocol is based on (11)C-methylation of an N-acetyl precursor. This initial step is followed by a reduction with LiAlH(4) to give ([3-(11)C]-(+)-PHNO). We first applied the method for the synthesis of a model compound, N-3-([(11)C]propyl)-1,2,3,4-tetrahydroisoquinoline, which we obtained in 77-97% analytical radiochemical yield (n=6) in 20 min. Similarly, we prepared ([3-(11)C]-(+)-PHNO) in 55-60% analytical radiochemical yield (n=5) using a one-pot procedure. We have also been able to implement the complete process on a semi-automated module. This platform delivered purified and formulated [3-(11)C]PHNO with an average radiochemical yield of 9% (n=13, range 2-30%, non-decay corrected), a radiochemical purity >95%, and a specific radioactivity of 26.8-81.1 GBq/μmol in a total time of 63-65 min.

[18F]fluoromethyl-[1,2-2H4]-choline: a novel radiotracer for imaging choline metabolism in tumors by positron emission tomography

Current radiotracers for positron emission tomography imaging of choline metabolism have poor systemic metabolic stability in vivo. We describe a novel radiotracer, [(18)F]fluoromethyl-[1,2-(2)H(4)]-choline (D4-FCH), that employs deuterium isotope effect to improve metabolic stability. D4-FCH proved more resistant to oxidation than its nondeuterated analogue, [(18)F]fluoromethylcholine, in plasma, kidneys, liver, and tumor, while retaining phosphorylation potential. Tumor radiotracer levels, a determinant of sensitivity in imaging studies, were improved by deuterium substitution; tumor uptake values expressed as percent injected dose per voxel at 60 min were 7.43 +/- 0.47 and 5.50 +/- 0.49 for D4-FCH and [(18)F]fluoromethylcholine, respectively (P = 0.04). D4-FCH was also found to be a useful response biomarker. Treatment with the mitogenic extracellular kinase inhibitor PD0325901 resulted in a reduction in tumor radiotracer uptake that occurred in parallel with reductions in choline kinase A expression. In conclusion, D4-FCH is a very promising metabolically stable radiotracer for imaging choline metabolism in tumors.

Design, synthesis, and biological characterization of a caspase 3/7 selective isatin labeled with 2-[18F]fluoroethylazide

Imaging of programmed cell death (apoptosis) is important in the assessment of therapeutic response in oncology and for diagnosis in cardiac and neurodegenerative disorders. The executioner caspases 3 and 7 ultimately effect cellular death, thus providing selective molecular targets for in vivo quantification of apoptosis. To realize this potential, we aimed to develop 18F-labeled isatin sulfonamides with high metabolic stability and moderate lipophilicity while retaining selectivity and affinity for caspase 3/7. A small library of isatins modified with fluorinated aromatic groups and heterocycles was synthesized. A lead compound incorporating 2'-fluoroethyl-1,2,3-triazole was identified with subnanomolar affinity for caspase 3. "Click labeling" provided the 18F-labeled tracer in 65 +/- 6% decay-corrected radiochemical yield from 2-[18F]fluoroethylazide. The compound showed high stability in vivo with rapid uptake and elimination in healthy tissues and tumor. The novel 18F-labeled isatin is a candidate radiotracer for further preclinical evaluation for imaging of apoptosis.

Methods for 18F-labeling of RGD peptides: comparison of aminooxy [18F]fluorobenzaldehyde condensation with 'click labeling' using 2-[18F]fluoroethylazide, and S-alkylation with [18F]fluoropropanethiol

Three strategies for chemoselective labeling of RGD peptides with (18)F have been compared. Aminooxy [(18)F]fluorobenzaldehyde conjugation provided 40 +/- 12% decay-corrected radiochemical yield using a fully automated method. An one-pot protocol for 'click labeling' of the RGD scaffold with 2-[(18)F]fluoroethylazide afforded 47 +/- 8% decay-corrected radiochemical yield. Attempted conjugation with 3-[(18)F]fluoropropanethiol led to extensive decomposition and was therefore found unsuitable for labeling of the RGD peptide investigated. The results suggest that 'click labeling' of RGD peptides provides an attractive alternative to aminooxy aldehyde condensation, however, 2-[(18)F]-fluoroethylazide may be too small to allow separation of large (18)F-labeled RGD peptides from their precursors.

A New Method for Radiosynthesis of C-Labeled Carbamate Groups and its Application for a Highly Efficient Synthesis of the Kappa-Opioid Receptor Tracer [C]GR103545

¹¹C-labeled carbamates can be obtained in a three-component coupling reaction of primary or secondary amines with CO₂ and ¹¹C-methylation reagents. [¹¹C]Methyl-triflate mediated methylation of carbamino adducts provides the corresponding ¹¹C-labeled carbamate groups in excellent yields under mild conditions (temperatures ≤ 40°C, 2 min reaction time). The utility of the method has been demonstrated by a highly efficient radiosynthesis of [¹¹C]GR103545.

Relationship between ketamine-induced psychotic symptoms and NMDA receptor occupancy: a [(123)I]CNS-1261 SPET study

RATIONALE

Ketamine induces effects resembling both positive and negative psychotic symptoms of schizophrenia. These are thought to arise through its action as an uncompetitive antagonist of the N-methyl-D-aspartate (NMDA) receptor.

OBJECTIVES

We used [(123)I]CNS-1261 to study ketamine binding to NMDA receptors in healthy human controls in vivo and its relationship to positive and negative psychotic symptom induction.

MATERIALS AND METHODS

Ten healthy controls underwent two single-photon emission tomography scans with [(123)I]CNS-1261. On each occasion, they received a bolus infusion of either ketamine or saline. The Brief Psychiatric Rating Scale (BPRS) was administered at the end of each scan. Predefined regions of interest were used to estimate change in volume of distribution of [(123)I]CNS-1261 following ketamine administration. Two normalised-to-cortex binding indices were also used in order to study effects of ketamine on NMDA receptor availability by region, after correction for global and nonspecific effects.

RESULTS

Ketamine-induced reduction in [(123)I]CNS-1261 volume of distribution in all regions showed the strongest correlation with BPRS negative subscale (p < 0.01). With the normalised-to-cortex measures, NMDA receptor binding in middle inferior frontal cortex showed a significant correlation with BPRS negative subscale (BI1 r = 0.88, BI2 r = 95.9, p < 0.001).

CONCLUSIONS

[(123)I]CNS-1261 binding was modulated by ketamine, a drug known to compete for the same site on the NMDA receptor in vitro. Ketamine may induce negative symptoms through direct inhibition of the NMDA receptor, and positive symptoms may arise through a different neurochemical pathway.

Synthesis and reactivity of [18F]-N-fluorobenzenesulfonimide

A novel [18F]NF reagent and two novel radiochemical transformations have been developed: [18F]NFSi has been prepared from sodium dibenzenesulfonimide and reacted in the presence of silyl enol ethers and allylsilanes to deliver labelled fluorinated ketones and allylic fluorides respectively; the radiosynthesis of the fluorinated A ring of vitamin D3 has also been completed with success.

"Click labeling" with 2-[18f]fluoroethylazide for positron emission tomography

As an effort in the development of more flexible (18)F-labeling chemistry, we report herein on the use of the Cu(I)-catalyzed Huisgen cycloaddition, also known as the "click reaction", to form (18)F-labeled 1,2,3-triazoles. Nucleophilic fluorination of 2-azidoethyl-4-toluenesulfonate followed by distillation provided 2-[(18)F]fluoroethylazide in 55% radiochemical yield (decay-corrected). 2-[(18)F]fluoroethylazide was reacted with a small library of terminal alkynes in the presence of excess Cu(2+)/ascorbate or copper powder. The most reactive alkyne, N-benzylpropynamide provided nearly quantitative incorporation of 2-[(18)F]fluoroethylazide after 15 min at ambient temperature, whereas the majority of the alkyne substrates provided excellent yields of the corresponding (18)F-labeled 1,2,3-triazoles following heating to 80 degrees C. Using the method described, a model peptide was obtained in 92.3 +/- 0.3% (n = 3) radiochemical yield (decay-corrected) after purification by semipreparative HPLC.

[123I]TPCNE--a novel SPET tracer for the sigma-1 receptor: first human studies and in vivo haloperidol challenge

[123I]TPCNE (1(trans-[123I]iodopropen-2-yl)-4-[(4-cyanophenoxy)methyl] piperidine; Ki = 0.67 nM; log P = 3.36) is a novel sigma-1 receptor SPET ligand. In this study, we developed an optimized labeling method for [123I]TPCNE and investigated the kinetics, binding characteristics, and whole-body distribution of this tracer for the first time in humans. We also performed a challenge with the sigma-1 receptor antagonist haloperidol against [123I]TPCNE. Seven healthy volunteers were recruited. Dynamic brain SPET scans were performed following i.v. administration of 185 MBq [123I]TPCNE in all seven subjects. Three of the subjects were given oral haloperidol (2.5 mg) approximately 1 h before the scan. The dynamic data were analyzed with both reversible and irreversible compartmental models.[123I]TPCNE showed high uptake in brain and liver. All non-haloperidol-treated subjects showed a high whole-brain uptake (average: 8.7% of injected activity). No significant clearance of the tracer was seen up to 30 h post injection. In the haloperidol-treated subjects, the time-activity curves clearly demonstrated clearance of the tracer from the brain. Regional radioactivity concentrations were reduced by haloperidol from 42% in the cerebellum to 73% in the thalamus.[(123)I]TPCNE demonstrated high brain uptake, with highest binding found in the posterior cingulate. A region in which binding was unaffected by haloperidol pretreatment could not be identified, and the time-activity data were best described by an irreversible model.

Closing in on the AMPA receptor: Synthesis and evaluation of 2-acetyl-1-(4′-chlorophenyl)-6-methoxy-7-[11C]methoxy-1,2,3,4-tetrahydroisoquinoline as a potential PET tracer

2-Acetyl-1-(4'-chlorophenyl)-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline, one of the most potent non-competitive AMPA antagonists described to date, has been labelled with carbon-11 and tritium and evaluated as a potential ligand for in vivo imaging of AMPA receptors using PET. The carbon-11 labelled compound showed good initial brain uptake in rats, but with rapid clearance and relatively homogenous distribution. In saturation binding studies, the tritiated racemic ligand was found to be highly potent with a Kd of 14.8+/-1.8 nM. We conclude that the low receptor density labelled with this compound, its rapid clearance from the CNS and low specific binding makes it unsuitable as an in vivo PET imaging agent for AMPA receptors.

Towards NR2B receptor selective imaging agents for PET-synthesis and evaluation of N-[11C]-(2-methoxy)benzyl (E)-styrene-, 2-naphthyl- and 4-trifluoromethoxyphenylamidine

Three potent and selective 11C-labelled NR2B antagonists have been synthesized and evaluated as PET ligands. The brain uptake of the compounds in mice varied substantially and was dominated by metabolism. One compound was found to have favourable uptake and retention in the brain, as well as a binding pattern consistent with the expression of the target receptor as measured by in vitro autoradiography. However, the metabolism of the compounds tested was too rapid to allow for in vivo imaging.

Ketamine displaces the novel NMDA receptor SPET probe [123I]CNS-1261 in humans in vivo

[(123)I]CNS-1261 [N-(1-naphthyl)-N'-(3-iodophenyl)-N-methylguanidine] is a high-affinity SPET ligand with selectivity for the intra-channel PCP/ketamine/MK-801 site of the N-methyl-d-aspartate (NMDA) receptor. This study evaluated the effects of ketamine (a specific competitor for the intra-channel PCP/ketamine/MK-801 site) on [(123)I]CNS-1261 binding to NMDA receptors in vivo. Ten healthy volunteers underwent 2 bolus-plus-infusion [(123)I]CNS-1261 scans, one during placebo and the other during a ketamine challenge. Ketamine administration led to a significant decrease in [(123)I]CNS-1261 V(T) in most of the brain regions examined (P<.05). [(123)I]CNS-1261 appears to be a specific ligand in vivo for the intra-channel PCP/ketamine/MK-801 NMDA binding site.

Studies on the synthesis and biological properties of non-carrier-added [(125)I and (131)I]-labeled arylalkylidenebisphosphonates: potent bone-seekers for diagnosis and therapy of malignant osseous lesions

Arylalkylidenebisphosphonates labeled with nca [(125)I or (131)I] have been synthesized and their biological function investigated. The label was attached to the aromatic group in high yield and under mild conditions by means of iododesilylation. The bone affinities of the radioactive compounds were investigated in normal Balb/C mice. The compound 1-hydroxy(m-iodo[(125,131)I]-phenylethylidene)-1,1-bisphosphonate was found to possess superior bone affinity compared to others, and its in vivo deiodination was insignificant. The uptake in femur 24h after injection was 850 +/- 265% and 986 +/- 118% of injected dose per gram tissue times gram body weight in mice and rats, respectively. The therapeutic potential of the compound was investigated in two tumor models in athymic (nude) rats, one model for mixed lytic/sclerotic metastatic bone-lesions originating from breast cancer and the other model simulating osseous osteosarcoma. The effects in these models compare favorably to those observed for established treatment modalities. The experiments demonstrate that radioiodinated bisphosphonates may have a potential for diagnosis and therapy of malignant osseous lesions.

ROMPgel-supported triphenylphosphine with potential application in parallel synthesis

[reaction: see text] ROMPgel-supported triphenylphosphine was synthesized in three steps (67%) from norbornadiene, 4-bromoiodobenzene, and chlorodiphenylphosphine. The supported reagent has a high loading (2.5 mmol/g) and favorable swelling properties in organic solvents. It has been utilized for the conversion of alcohols to halides, the reduction of ozonides, and the isomerization of alpha,beta-acetylenic esters and in the Staudinger reaction. In general, filtration of the resin from the reaction mixtures and evaporation gave the corresponding products in high yield and purity.