Semiautomated radiosynthesis and biological evaluation of [18F]FEAU: a novel PET imaging agent for HSV1-tk/sr39tk reporter gene expression

3D-printed PEEK reactors and development of a complete continuous flow system for chemical synthesis

This paper presents the development of milli- and microfluidic reactors made of polyether ether ketone (PEEK) and 3D-printed equipment for a complete continuous flow system.

Cre/lox-assisted non-invasive in vivo tracking of specific cell populations by positron emission tomography

Many pathophysiological processes are associated with proliferation, migration or death of distinct cell populations. Monitoring specific cell types and their progeny in a non-invasive, longitudinal and quantitative manner is still challenging. Here we show a novel cell-tracking system that combines Cre/lox-assisted cell fate mapping with a thymidine kinase (sr39tk) reporter gene for cell detection by positron emission tomography (PET). We generate Rosa26-mT/sr39tk PET reporter mice and induce sr39tk expression in platelets, T lymphocytes or cardiomyocytes. As proof of concept, we demonstrate that our mouse model permits longitudinal PET imaging and quantification of T-cell homing during inflammation and cardiomyocyte viability after myocardial infarction. Moreover, Rosa26-mT/sr39tk mice are useful for whole-body characterization of transgenic Cre mice and to detect previously unknown Cre activity. We anticipate that the Cre-switchable PET reporter mice will be broadly applicable for non-invasive long-term tracking of selected cell populations in vivo.Non-invasive cell tracking is a powerful method to visualize cells in vivo under physiological and pathophysiological conditions. Here Thunemann et al. generate a mouse model for in vivo tracking and quantification of specific cell types by combining a PET reporter gene with Cre-dependent activation that can be exploited for any cell population for which a Cre mouse line is available.

Radiosynthesis of 6'-Deoxy-6'[18F]Fluorosucrose via Automated Synthesis and Its Utility to Study In Vivo Sucrose Transport in Maize (Zea mays) Leaves

Sugars produced from photosynthesis in leaves are transported through the phloem tissues within veins and delivered to non-photosynthetic organs, such as roots, stems, flowers, and seeds, to support their growth and/or storage of carbohydrates. However, because the phloem is located internally within the veins, it is difficult to access and to study the dynamics of sugar transport. Radioactive tracers have been extensively used to study vascular transport in plants and have provided great insights into transport dynamics. To better study sucrose partitioning in vivo, a novel radioactive analog of sucrose was synthesized through a completely chemical synthesis route by substituting fluorine-18 (half-life 110 min) at the 6' position to generate 6'-deoxy-6'[(18)F]fluorosucrose ((18)FS). This radiotracer was then used to compare sucrose transport between wild-type maize plants and mutant plants lacking the Sucrose transporter1 (Sut1) gene, which has been shown to function in sucrose phloem loading. Our results demonstrate that (18)FS is transported in vivo, with the wild-type plants showing a greater rate of transport down the leaf blade than the sut1 mutant plants. A similar transport pattern was also observed for universally labeled [U-(14)C]sucrose ([U-(14)C]suc). Our findings support the proposed sucrose phloem loading function of the Sut1 gene in maize, and additionally demonstrate that the (18)FS analog is a valuable, new tool that offers imaging advantages over [U-(14)C]suc for studying phloem transport in plants.

Design of a functional cyclic HSV1-TK reporter and its application to PET imaging of apoptosis

Positron emission tomography (PET) is a sensitive and noninvasive imaging method that is widely used to explore molecular events in living subjects. PET can precisely and quantitatively evaluate cellular apoptosis, which has a crucial role in various physiological and pathological processes. In this protocol, we describe the design and use of an engineered cyclic herpes simplex virus 1-thymidine kinase (HSV1-TK) PET reporter whose kinase activity is specifically switched on by apoptosis. The expression of cyclic TK (cTK) in healthy cells leads to inactive product, whereas the activation of apoptosis through the caspase-3 pathway cleaves cTK, thus restoring its activity and enabling PET imaging. In addition to detailing the design and construction of the cTK plasmid in this protocol, we include assays for evaluating the function and specificity of the cTK reporter in apoptotic cells, such as assays for measuring the cell uptake of PET tracer in apoptotic cells, correlating doxorubicin (Dox)-induced cell apoptosis to cTK function recovery, and in vivo PET imaging of cancer cell apoptosis, and we also include corresponding data acquisition methods. The time to build the entire cTK reporter is ∼2-3 weeks. The selection of a stable cancer cell line takes ∼4-6 weeks. The time to implement assays regarding cTK function in apoptotic cells and the in vivo imaging varies depending on the experiment. The cyclization strategy described in this protocol can also be adapted to create other reporter systems for broad biomedical applications.

Fully automated production of diverse 18F-labeled PET tracers on the ELIXYS multireactor radiosynthesizer without hardware modification

Fully automated radiosynthesizers are continuing to be developed to meet the growing need for the reliable production of PET tracers made under current good manufacturing practice guidelines. There is a current trend toward supporting kitlike disposable cassettes that come preconfigured for particular tracers, thus eliminating the need for cleaning protocols between syntheses and enabling quick transitions to synthesizing other tracers. Though ideal for production, these systems are often limited for the development of novel tracers because of pressure, temperature, and chemical compatibility considerations. This study demonstrated the versatile use of the ELIXYS fully automated radiosynthesizer to adapt and produce 8 different (18)F-labeled PET tracers of varying complexity.

METHODS

Three-reactor syntheses of 2-deoxy-2-(18)F-fluoro-β-d-arabinofuranosylcytosine (d-(18)F-FAC), 2-deoxy-2-(18)F-fluoro-5-methyl-β-l-arabinofuranosyluracil (l-(18)F-FMAU), and 2-deoxy-2-(18)F-fluoro-5-ethyl-β-d-arabinofuranosyluracil (d-(18)F-FEAU) along with the 1-reactor syntheses of d-(18)F-FEAU, (18)F-FDG, 3-deoxy-3-(18)F-fluoro-l-thymidine ((18)F-FLT), (18)F-fallypride, 9-(4-(18)F-fluoro-3-hydroxymethylbutyl)-guanine ((18)F-FHBG), and N-succinimidyl-4-(18)F-fluorobenzoate ((18)F-SFB), were all produced using ELIXYS without the need for any hardware modifications or reconfiguration. Synthesis protocols were adapted and slightly modified from those in the literature but were not fully optimized. Furthermore, (18)F-FLT, (18)F-FDG, and (18)F-fallypride were produced sequentially on the same day and used for preclinical imaging of A431 tumor-bearing severe combined immunodeficient mice and wild-type BALB/c mice. To assess future translation to the clinical setting, several batches of tracers were subjected to a full set of quality control tests.

RESULTS

All tracers were produced with radiochemical yields comparable to those in the literature. (18)F-FLT, (18)F-FDG, and (18)F-fallypride were successfully used to image the mice, with results consistent with those reported in the literature. All tracers that were subjected to clinical quality control tests passed.

CONCLUSION

The ELIXYS radiosynthesizer facilitates rapid tracer development and is capable of producing multiple (18)F-labeled PET tracers suitable for clinical applications using the same hardware setup.

Antiviral drug ganciclovir is a potent inhibitor of microglial proliferation and neuroinflammation

Aberrant microglial responses contribute to neuroinflammation in many neurodegenerative diseases, but no current therapies target pathogenic microglia. We discovered unexpectedly that the antiviral drug ganciclovir (GCV) inhibits the proliferation of microglia in experimental autoimmune encephalomyelitis (EAE), a mouse model for multiple sclerosis (MS), as well as in kainic acid-induced excitotoxicity. In EAE, GCV largely prevented infiltration of T lymphocytes into the central nervous system (CNS) and drastically reduced disease incidence and severity when delivered before the onset of disease. In contrast, GCV treatment had minimal effects on peripheral leukocyte distribution in EAE and did not inhibit generation of antibodies after immunization with ovalbumin. Additionally, a radiolabeled analogue of penciclovir, [(18)F]FHBG, which is similar in structure to GCV, was retained in areas of CNS inflammation in EAE, but not in naive control mice, consistent with the observed therapeutic effects. Our experiments suggest GCV may have beneficial effects in the CNS beyond its antiviral properties.

ELIXYS - a fully automated, three-reactor high-pressure radiosynthesizer for development and routine production of diverse PET tracers

BACKGROUND

Automated radiosynthesizers are vital for routine production of positron-emission tomography tracers to minimize radiation exposure to operators and to ensure reproducible synthesis yields. The recent trend in the synthesizer industry towards the use of disposable kits aims to simplify setup and operation for the user, but often introduces several limitations related to temperature and chemical compatibility, thus requiring reoptimization of protocols developed on non-cassette-based systems. Radiochemists would benefit from a single hybrid system that provides tremendous flexibility for development and optimization of reaction conditions while also providing a pathway to simple, cassette-based production of diverse tracers.

METHODS

We have designed, built, and tested an automated three-reactor radiosynthesizer (ELIXYS) to provide a flexible radiosynthesis platform suitable for both tracer development and routine production. The synthesizer is capable of performing high-pressure and high-temperature reactions by eliminating permanent tubing and valve connections to the reaction vessel. Each of the three movable reactors can seal against different locations on disposable cassettes to carry out different functions such as sealed reactions, evaporations, and reagent addition. A reagent and gas handling robot moves sealed reagent vials from storage locations in the cassette to addition positions and also dynamically provides vacuum and inert gas to ports on the cassette. The software integrates these automated features into chemistry unit operations (e.g., React, Evaporate, Add) to intuitively create synthesis protocols. 2-Deoxy-2-[18F]fluoro-5-methyl-β-l-arabinofuranosyluracil (l-[18F]FMAU) and 2-deoxy-2-[18F]fluoro-β-d-arabinofuranosylcytosine (d-[18F]FAC) were synthesized to validate the system.

RESULTS

l-[18F]FMAU and d-[18F]FAC were successfully synthesized in 165 and 170 min, respectively, with decay-corrected radiochemical yields of 46% ± 1% (n = 6) and 31% ± 5% (n = 6), respectively. The yield, repeatability, and synthesis time are comparable to, or better than, other reports. d-[18F]FAC produced by ELIXYS and another manually operated apparatus exhibited similar biodistribution in wild-type mice.

CONCLUSION

The ELIXYS automated radiosynthesizer is capable of performing radiosyntheses requiring demanding conditions: up to three reaction vessels, high temperatures, high pressures, and sensitive reagents. Such flexibility facilitates tracer development and the ability to synthesize multiple tracers on the same system without customization or replumbing. The disposable cassette approach simplifies the transition from development to production.

High-pressure, compact, modular radiosynthesizer for production of positron emitting biomarkers

A robust, modular, semi-automated synthesis unit useful for conducting radiochemical reactions under pressurized conditions (up to ∼200psi [1.4MPa]) for the production of PET biomarkers has been developed. This compact unit (7.6cm×33.0cm×58.4cm) is capable of performing any single step reaction that is generally encountered in radiochemical syntheses, and multiple units can be combined for more complex syntheses. The versatility of a 3-unit system is exemplified by reliably conducting the multi-step syntheses of 2'-deoxy-2'-[(18)F]fluoro-1-β-arabinofuranosyl-uracil and -cytosine derivatives, which involve corrosive and moisture sensitive reagents under pressurized conditions.

Plug-and-play modules for flexible radiosynthesis

We present a plug-and-play radiosynthesis platform and accompanying computer software based on modular subunits that can easily and flexibly be configured to implement a diverse range of radiosynthesis protocols. Modules were developed that perform: (i) reagent storage and delivery, (ii) evaporations and sealed reactions, and (iii) cartridge-based purifications. The reaction module incorporates a simple robotic mechanism that removes tubing from the vessel and replaces it with a stopper prior to sealed reactions, enabling the system to withstand high pressures and thus provide tremendous flexibility in choice of solvents and temperatures. Any number of modules can rapidly be connected together using only a few fluidic connections to implement a particular synthesis, and the resulting system is controlled in a semi-automated fashion by a single software interface. Radiosyntheses of 2-[(18)F]fluoro-2-deoxy-d-glucose ([(18)F]FDG), 1-[(18)F]fluoro-4-nitrobenzene ([(18)F]FNB), and 2'-deoxy-2'-[(18)F]fluoro-1-β-d-arabinofuranosyl cytosine (d-[(18)F]FAC) were performed to validate the system and demonstrate its versatility.

Nucleoside-based probes for imaging tumor proliferation using positron emission tomography

Cancer is one of the leading causes of human death, and early detection can be beneficial for its timely therapy and management. For the early detection of cancer, positron emission tomography (PET) is more accurate and sensitive than other imaging modalities, such as computed tomography and magnetic resonance imaging. [(18) F]-Labeled fluorodeoxyglucose is the most useful PET probe in early detection of cancer; however, its nonspecific accumulation and consequent false-positive findings warrant the identification of other PET probes. Thymidine (TdR) and its analogs have been radiolabeled for PET imaging of cellular proliferation and DNA synthesis. Because of its in vivo instability, radiolabeled TdR has not been successful in PET imaging. However, some of its radiolabeled analogs have been developed for PET imaging of cellular proliferation and DNA synthesis. In this review, the radiochemistry and production of (11) C-TdR and (11) C/(18) F-labeled TdR analogs published to date are presented.

An improved strategy for the synthesis of [¹⁸F]-labeled arabinofuranosyl nucleosides

The expression of the herpes simplex virus type-1 thymidine kinase (HSV1-tk) gene can be imaged efficaciously using a variety of 2'-[(18)F]fluoro-2'-deoxy-1-b-D-arabinofuranosyl-uracil derivatives [[(18)F]-FXAU, X=I(iodo), E(ethyl), and M(methyl)]. However, the application of these derivatives in clinical and translational studies has been impeded by their complicated and long syntheses (3-5h). To remedy these issues, in the study at hand we have investigated whether microwave or combined catalysts could facilitate the coupling reaction between sugar and nucleobase and, further, have probed the feasibility of establishing a novel approach for [(18)F]-FXAU synthesis. We have demonstrated that the rate of the trimethylsilyl trifluoromethanesulfonate (TMSOTf)-catalyzed coupling reaction between the 2-deoxy-sugar and uracil derivatives at 90 °C can be significantly accelerated by microwave-driven heating or by the addition of Lewis acid catalyst (SnCl(4)). Further, we have observed that the stability of the α- and β-anomers of [(18)F]-FXAU derivatives differs during the hydrolysis step. Using the microwave-driven heating approach, overall decay-corrected radiochemical yields of 19%-27% were achieved for [(18)F]-FXAU in 120min at a specific activity of >22MBq/nmol (595Ci/mmol). Ultimately, we believe that these high yielding syntheses of [(18)F]-FIAU, [(18)F]-FMAU and [(18)F]-FEAU will facilitate routine production for clinical applications.

Positron emission tomography reporter genes and reporter probes: gene and cell therapy applications

Positron emission tomography (PET) imaging reporter genes (IRGs) and PET reporter probes (PRPs) are amongst the most valuable tools for gene and cell therapy. PET IRGs/PRPs can be used to non-invasively monitor all aspects of the kinetics of therapeutic transgenes and cells in all types of living mammals. This technology is generalizable and can allow long-term kinetics monitoring. In gene therapy, PET IRGs/PRPs can be used for whole-body imaging of therapeutic transgene expression, monitoring variations in the magnitude of transgene expression over time. In cell or cellular gene therapy, PET IRGs/PRPs can be used for whole-body monitoring of therapeutic cell locations, quantity at all locations, survival and proliferation over time and also possibly changes in characteristics or function over time. In this review, we have classified PET IRGs/PRPs into two groups based on the source from which they were derived: human or non-human. This classification addresses the important concern of potential immunogenicity in humans, which is important for expansion of PET IRG imaging in clinical trials. We have then discussed the application of this technology in gene/cell therapy and described its use in these fields, including a summary of using PET IRGs/PRPs in gene and cell therapy clinical trials. This review concludes with a discussion of the future direction of PET IRGs/PRPs and recommends cell and gene therapists collaborate with molecular imaging experts early in their investigations to choose a PET IRG/PRP system suitable for progression into clinical trials.

Study of [18F]FLT and [123I]IaraU for cellular imaging in HSV1 tk-transfected murine fibrosarcoma cells: evaluation of the tracer uptake using 5-fluoro, 5-iodo and 5-iodovinyl arabinosyl uridines as competitive probes

As one of the most intensively studied probes for imaging of the cellular proliferation, [(18)F]FLT was investigated whether the targeting specificity of thymidine kinase 1 (TK1) dependency could be enhanced through a synergistic effect mediated by herpes simplex type 1 virus (HSV1) tk gene in terms of the TK1 or TK2 expression. 5-[(123)I]Iodo arabinosyl uridine ([(123)I]IaraU) was prepared in a radiochemical yield of 8% and specific activity of 21 GBq/μmol, respectively. Inhibition of the cellular uptake of these two tracers was compared by using the arabinosyl uridine analogs such as 5-iodo, 5-fluoro and 5-(E)-iodovinyl arabinosyl uridine along with 2'-fluoro-5-iodo arabinosyl uridine (FIAU). Due to potential instability of the iodo group, accumulation index of 1.6 for [(123)I]IaraU by HSV1-TK vs. control cells could virtually be achieved at 1.5 h, but dropped to 0.2 compared to 2.0 for [(18)F]FLT at 5 h. The results from competitive inhibition by these nucleosides against the accumulation of [(18)F]FLT implied that FLT exerted a mixed TK1- and TK2-dependent inhibition with HSV1-tk gene transfection because of the shifting of thymidine kinase status. Taken together, the combination of [(18)F]FLT and HSV1-TK provides a synergistic imaging potency.

Structure-guided engineering of human thymidine kinase 2 as a positron emission tomography reporter gene for enhanced phosphorylation of non-natural thymidine analog reporter probe

Positron emission tomography (PET) reporter gene imaging can be used to non-invasively monitor cell-based therapies. Therapeutic cells engineered to express a PET reporter gene (PRG) specifically accumulate a PET reporter probe (PRP) and can be detected by PET imaging. Expanding the utility of this technology requires the development of new non-immunogenic PRGs. Here we describe a new PRG-PRP system that employs, as the PRG, a mutated form of human thymidine kinase 2 (TK2) and 2'-deoxy-2'-18F-5-methyl-1-β-L-arabinofuranosyluracil (L-18F-FMAU) as the PRP. We identified L-18F-FMAU as a candidate PRP and determined its biodistribution in mice and humans. Using structure-guided enzyme engineering, we generated a TK2 double mutant (TK2-N93D/L109F) that efficiently phosphorylates L-18F-FMAU. The N93D/L109F TK2 mutant has lower activity for the endogenous nucleosides thymidine and deoxycytidine than wild type TK2, and its ectopic expression in therapeutic cells is not expected to alter nucleotide metabolism. Imaging studies in mice indicate that the sensitivity of the new human TK2-N93D/L109F PRG is comparable with that of a widely used PRG based on the herpes simplex virus 1 thymidine kinase. These findings suggest that the TK2-N93D/L109F/L-18F-FMAU PRG-PRP system warrants further evaluation in preclinical and clinical applications of cell-based therapies.

A fluoride-derived electrophilic late-stage fluorination reagent for PET imaging

The unnatural isotope fluorine-18 ((18)F) is used as a positron emitter in molecular imaging. Currently, many potentially useful (18)F-labeled probe molecules are inaccessible for imaging because no fluorination chemistry is available to make them. The 110-minute half-life of (18)F requires rapid syntheses for which [(18)F]fluoride is the preferred source of fluorine because of its practical access and suitable isotope enrichment. However, conventional [(18)F]fluoride chemistry has been limited to nucleophilic fluorination reactions. We report the development of a palladium-based electrophilic fluorination reagent derived from fluoride and its application to the synthesis of aromatic (18)F-labeled molecules via late-stage fluorination. Late-stage fluorination enables the synthesis of conventionally unavailable positron emission tomography (PET) tracers for anticipated applications in pharmaceutical development as well as preclinical and clinical PET imaging.

Highlighting the Versatility of the Tracerlab Synthesis Modules. Part 1: Fully Automated Production of [F]Labelled Radiopharmaceuticals using a Tracerlab FX(FN)

The field of radiochemistry is moving towards exclusive use of automated synthesis modules for production of clinical radiopharmaceutical doses. Such a move comes with many advantages, but also presents radiochemists with the challenge of re-configuring synthesis modules for production of radiopharmaceuticals that require non-conventional radiochemistry whilst maintaining full automation. This review showcases the versatility of the Tracerlab FX(FN) synthesis module by presenting simple, fully automated methods for producing [(18)F]FLT, [(18)F]FAZA, [(18)F]MPPF, [(18)F]FEOBV, [(18)F]sodium fluoride, [(18)F]fluorocholine and [(18)F]SFB.

The improved syntheses of 5-substituted 2'-[18F]fluoro-2'-deoxy-arabinofuranosyluracil derivatives ([18F]FAU, [18F]FEAU, [18F]FFAU, [18F]FCAU, [18F]FBAU and [18F]FIAU) using a multistep one-pot strategy

INTRODUCTION

We and others have previously reported a four-step radiosynthesis of a series of 2'-deoxy-2'-[(18)F]fluoro-5-substituted-1-β-D-arabinofuranosyluracil derivatives including [(18)F]FAU, [(18)F]FEAU, [(18)F]FFAU, [(18)F]FCAU, [(18)F]FBAU and [(18)F]FIAU as thymidine derivatives for tumor proliferation and/or reporter gene expression imaging with positron emission tomography (PET). Although the radiosynthesis has been proven to be reproducible and efficient, this complicated multistep reaction is difficult to incorporate into an automated cGMP-compliant radiosynthesis module for routine production. Recently, we have developed a simple and efficient one-pot method for routine production of [(18)F]FMAU. In this study, we studied the feasibility of radiosynthesizing [(18)F]FAU, [(18)F]FEAU, [(18)F]FFAU, [(18)F]FCAU, [(18)F]FBAU and [(18)F]FIAU using this newly developed method.

METHODS

Similar to the radiosynthesis of [(18)F]FMAU, 5-substituted 2'-[(18)F]fluoro-2'-deoxy-arabinofuranosyluracil derivatives ([(18)F]FAU, [(18)F]FEAU, [(18)F]FFAU, [(18)F]FCAU, [(18)F]FBAU and [(18)F]FIAU) were synthesized in one-pot radiosynthesis module in the presence of Friedel-Crafts catalyst TMSOTf and HMDS.

RESULTS

This one-pot radiosynthesis method could be used to produce [(18)F]FAU, [(18)F]FEAU, [(18)F]FFAU, [(18)F]FCAU, [(18)F]FBAU and [(18)F]FIAU. The overall radiochemical yields of these tracers varied from 4.1%±0.8% to 10.1%±1.9% (decay-corrected, n=4). The overall reaction time was reduced from 210 min to 150 min from the end of bombardment, and the radiochemical purity was >99%.

CONCLUSIONS

The improved radiosyntheses of [(18)F]FAU, [(18)F]FEAU, [(18)F]FFAU, [(18)F]FCAU, [(18)F]FBAU and [(18)F]FIAU have been achieved with reasonable yields and high purity using a multistep one-pot method. The synthetic time has been reduced, and the reaction procedures have been significantly simplified. The success of this approach may make PET tracers [(18)F]FAU, [(18)F]FEAU, [(18)F]FFAU, [(18)F]FCAU, [(18)F]FBAU and [(18)F]FIAU more accessible for preclinical and clinical research.

Molecular imaging of cell-based cancer immunotherapy

Cell-based cancer immunotherapy represents a new and powerful weapon in the arsenal of anticancer treatments. Non-invasive monitoring of the disposition, migration and destination of therapeutic cells will facilitate the development of cell based therapy. The therapeutic cells can be modified intrinsically by a reporter gene or labeled extrinsically by introducing imaging probes into the cells or on the cell surface before transplant. Various advanced non-invasive molecular imaging techniques are playing important roles in optimizing cellular therapy by tracking cells and monitoring the therapeutic effects of transplanted cells in vivo. This review will summarize the application of multiple molecular imaging modalities in cell-based cancer immunotherapy.

Automated synthesis of 2'-deoxy-2'-[18F]fluoro-5-methyl-1-β-D-arabinofuranosyluracil ([18F]-FMAU) using a one reactor radiosynthesis module

2'-Deoxy-2'-[(18)F]fluoro-5-methyl-1-β-D-arabinofuranosyluracil ([(18)F]-FMAU) is an established PET probe used to monitor cellular proliferation. For clinical applications, a fully automated cGMP-compliant radiosynthesis would be preferred. However, the current synthesis of [(18)F]-FMAU requires a multistep procedure, making the development of an automated protocol difficult and complicated. Recently, we have developed a significantly simplified one-pot reaction condition for the synthesis of [(18)F]-FMAU in the presence of Friedel-Crafts catalysts. Here, we report a fully automated synthesis of [(18)F]-FMAU based on a one reactor radiosynthesis module using our newly developed synthetic method. The product was purified on a semi-preparative high-performance liquid chromatography integrated with the synthesis module using 6% EtOH in 10 mM phosphate buffer or 8% MeCN/water. [(18)F]-FMAU was obtained in 12±3% radiochemical yield (decay corrected overall yield based on [(18)F]-F(-), n=4) with 383±33 mCi/μmol specific activity at the time of injection. The α/β anomer ratio was 4:6. The overall reaction time was about 150 min from the end of bombardment and the radiochemical purity was >99%. This automated synthesis should also be suitable for the production of other 5-substituted thymidine analogues.

Improved synthesis of 2'-deoxy-2'-[18F]-fluoro-1-beta-D-arabinofuranosyl-5-iodouracil ([18F]-FIAU)

An improved synthesis of 2'-[(18)F]-fluoro-2'-deoxy-1-beta-D-arabinofuranosyl-5-iodouracil ([(18)F]-FIAU) has been developed. The method utilizes trimethylsilyl trifluoromethanesulfonate (TMSOTf) catalyzed coupling of 2-deoxy-2-[(18)F]-fluoro-1,3,5-tri-O-benzoyl-d-arabinofuranose with 2,4-bis(trimethylsilyloxy)-5-iodouracil to yield the protected dibenzoyl-[(18)F]-FIAU. Dibenzoyl-[(18)F]-FIAU was deprotected with sodium methoxide to yield a mixture of alpha- and beta-anomers in a ratio of 1:1, which were purified by HPLC. The procedure described in this article eliminates the need for HBr activation of the sugar prior to coupling with silylated iodouracil and is suitable for automation. The total reaction time was about 110 min, starting from [(18)F]-fluoride. The average isolated yield of the required beta-anomer was 10+/-6% (decay corrected) with average specific activity of 125 mCi/micromol.

Different strategies for reducing intestinal background radioactivity associated with imaging HSV1-tk expression using established radionucleoside probes

One limitation of HSV1-tk reporter positron emission tomography (PET) with nucleoside analogues is the high background radioactivity in the intestine. We hypothesized that endogenous expression of thymidine kinase in bacterial flora could phosphorylate and trap such radiotracers, contributing to the high radioactivity levels in the bowel, and therefore explored different strategies to increase fecal elimination of radiotracer. Intestinal radioactivity was assessed by in vivo microPET imaging and ex vivo tissue sampling following intravenous injection of 18F-FEAU, 124I-FIAU, or 18F-FHBG in a germ-free mouse strain. We also explored the use of an osmotic laxative agent and/or a 100% enzymatically hydrolyzed liquid diet. No significant differences in intestinal radioactivity were observed between germ-free and normal mice. 18F-FHBG-derived intestinal radioactivity levels were higher than those of 18F-FEAU and 124I-FIAU; the intestine to blood ratio was more than 20-fold higher for 18F-FHBG than for 18F-FEAU and 124I-FIAU. The combination of Peptamen and Nulytely lowered intestinal radioactivity levels and increased (2.2-fold) the HSV1-tk transduced xenograft to intestine ratio for 18F-FEAU. Intestinal bacteria in germ-free mice do not contribute to the high intestinal levels of radioactivity following injection of radionucleoside analogues. The combination of Peptamen and Nulytely increased radiotracer elimination by increasing bowel motility without inducing dehydration.

5-[18F]Fluoroalkyl pyrimidine nucleosides: probes for positron emission tomography imaging of herpes simplex virus type 1 thymidine kinase gene expression

INTRODUCTION

The preliminary in vivo evaluation of novel 5-[(18)F]fluoroalkyl-2'-deoxyuridines ([(18)F]FPrDU, [(18)F]FBuDU, [(18)F]FPeDU; [(18)F]1a-c, respectively) and 2'-fluoro-2'-deoxy-5-[(18)F]fluoroalkyl-1-beta-d-arabinofuranosyl uracils ([(18)F]FFPrAU, [(18)F]FFBuAU, [(18)F]FFPeAU; [(18)F]1d-f, respectively) as probes for imaging herpes simplex virus type 1 thymidine kinase (HSV1-tk) gene expression is described.

METHODS

[(18)F]1a-f were successfully synthesized by a rapid and efficient two-step one-pot nucleophilic fluorination reaction using 5-O-mesylate precursors and [(18)F]F(-). For in vivo studies, tumor xenografts were grown in nude mice by implanting RG2 cells stably expressing HSV1-tk (RG2TK+) and wild-type cells (RG2).

RESULTS

Biodistribution studies at 2 h pi revealed that the uptake of [(18)F]1a-b and [(18)F]1d-e in RG2TK+ tumors was not significantly different from control tumors. However, [(18)F]1c and [(18)F]1f had an average 1.6- and 1.7-fold higher uptake in RG2TK+ tumors than control RG2 tumors. Blood activity curves for [(18)F]1c and [(18)F]1f highlight rapid clearance of radioactivity in the blood. Dynamic small animal PET (A-PET) imaging studies of tumor-bearing mice with [(18)F]1c and [(18)F]1f showed higher initial uptake (3.5- and 1.4-fold, respectively) in RG2TK+ tumors than in control tumors, with continued washout of activity from both tumors over time.

CONCLUSIONS

Biological evaluations suggest that [(18)F]1c and [(18)F]1f may have limited potential for imaging HSV1-tk gene expression due to fast washout of activity from the blood, thus significantly decreasing sensitivity and specificity of tracer accumulation in HSV1-tk-expressing tumors.

Engineered antibody fragments with infinite affinity as reporter genes for PET imaging

Reporter gene imaging has great potential for many clinical applications including the tracking of transplanted cells and monitoring of gene therapy. However, currently available reporter gene-reporter probe combinations have significant limitations with the biodistribution of the reporter probe and the specificity and immunogenicity of the reporter gene. The objective of the present study was to evaluate a new approach for reporter gene imaging based on cell surface expression of antibody fragments that can irreversibly bind to radiometal chelates.

METHODS

We developed a new reporter gene, designated 1,4,7,10-tetraazacyclodocecane-N,N',N'',N'''-tetraacetic acid (DOTA) antibody reporter 1 (DAbR1), which consists of the single-chain Fv (scFv) fragment of the anti-Y-DOTA antibody 2D12.5/G54C fused to the human T cell CD4 transmembrane domain. The corresponding reporter probe is yttrium-(S)-2-(4-acrylamidobenzyl)-DOTA (*Y-AABD), a DOTA complex that binds irreversibly to a cysteine residue in the 2D12.5/G54C antibody. U-87 glioma cells were stably transfected with a DAbR1 expression vector. Binding of *Y-AABD to transfected and wild-type cells was studied in vitro and in vivo.

RESULTS

Flow cytometry revealed high expression of the DAbR1 protein on the cell surface of tumor cells. Uptake of 90Y-AABD in DAbR1-expressing human U-87 glioma xenografts was 6.2 (+/-1.3) percentage injected dose per gram (%ID/g) at 1 h and 4.9 (+/-0.62) %ID/g at 24 h after injection. The corresponding tumor-to-plasma ratios were 45:1 and 428:1, respectively. Uptake by U-87 tumors without the DAbR1 gene was 0.16 (+/-0.02) %ID/g at 1 h and 0.05 (+/-0.03) %ID/g at 24 h. PET images in mice with 86Y-AABD demonstrated intense uptake in DAbR1-positive tumors and low background activity in the liver.

CONCLUSION

These findings indicate that cell surface expression of radiometal chelate binding antibodies such as 2D12.5/G54C is a promising strategy for reporter gene imaging.

Synthesis and in vitro evaluation of 5-[(18)f]fluoroalkyl pyrimidine nucleosides for molecular imaging of herpes simplex virus type 1 thymidine kinase reporter gene expression

Two novel series of 5-fluoroalkyl-2'-deoxyuridines (FPrDU, FBuDU, FPeDU) and 2'-fluoro-2'-deoxy-5-fluoroalkylarabinouridines (FFPrAU, FFBuAU, FFPeAU) that have three, four, or five methylene units (propyl, butyl, or pentyl) at C-5 were prepared and tested as reporter probes for imaging herpes simplex virus type 1 thymidine kinase (HSV1- tk) gene expression. The Negishi coupling methodology was employed in efficiently synthesizing the radiolabeling precursors. All six 5-[(18)F]fluoroalkyl pyrimidines were readily prepared from 3-N-benzoyl-3',5'-di-O-benzoyl-protected 5-O-mesylate precursors in 17-35% radiochemical yield (decay-corrected). In vitro studies highlighted that all six [(18)F]-labeled nucleosides selectively accumulated in cells expressing the HSV1-TK protein and there was negligible uptake in control cells. [(18)F]FPrDU, [(18)F]FBuDU, [(18)F]FPeDU, and [(18)F]FFBuAU had the best uptake profiles. Despite their selective accumulation in HSV1- tk-expressing cells, all 5-fluoroalkyl pyrimidine nucleosides had low-to-negligible cytotoxic activity (CC50 > 1000-1209 microM). Ultimately, the results demonstrated that 5-[(18)F]fluoropropyl, [(18)F]fluorobutyl, and [(18)F]fluoropentyl pyrimidine nucleosides have the potential to be in vivo HSV1-TK PET reporter probes over a dynamic range of reporter gene expression levels.

Invincible, but Not Invisible: Imaging Approaches Toward In Vivo Detection of Cancer Stem Cells

With evidence emerging in support of a cancer stem-cell model of carcinogenesis, it is of paramount importance to identify and image these elusive cells in their natural environment. The cancer stem-cell hypothesis has the potential to explain unresolved questions of tumorigenesis, tumor heterogeneity, chemotherapeutic and radiation resistance, and even the metastatic phenotype. Intravital imaging of cancer stem cells could be of great value for determining prognosis, as well as monitoring therapeutic efficacy and influencing therapeutic protocols. Cancer stem cells represent a rare population of cells, as low as 0.1% of cells within a human tumor, and the phenotype of isolated cancer stem cells is easily altered when placed under in vitro conditions. This represents a challenge in studying cancer stem cells without manipulation or extraction from their natural environment. Advanced imaging techniques allow for the in vivo observation of physiological events at cellular resolution. Cancer stem-cell studies must take advantage of such technology to promote a better understanding of the cancer stem-cell model in relation to tumor growth and metastasis, as well as to potentially improve on the principles by which cancers are treated. This review examines the opportunities for in vivo imaging of putative cancer stem cells with regard to currently accepted cancer stem-cell characteristics and advanced imaging technologies.

Construction and validation of improved triple fusion reporter gene vectors for molecular imaging of living subjects

Multimodality imaging using several reporter genes and imaging technologies has become an increasingly important tool in determining the location(s), magnitude, and time variation of reporter gene expression in small animals. We have reported construction and validation of several triple fusion genes composed of a bioluminescent, a fluorescent, and a positron emission tomography (PET) reporter gene in cell culture and in living subjects. However, the bioluminescent and fluorescent components of fusion reporter proteins encoded by these vectors possess lesser activities when compared with the bioluminescent and fluorescent components of the nonfusions. In this study, we first created a mutant (mtfl) of a thermostable firefly luciferase (tfl) bearing the peroxisome localization signal to have greater cytoplasmic localization and improved access for its substrate, d-luciferin. Comparison between the three luciferases [mtfl, tfl, and firefly luciferase (fl)] both in cell culture and in living mice revealed that mtfl possessed 6- to 10-fold (in vitro) and 2-fold (in vivo) higher activity than fl. The improved version of the triple fusion vector carrying mtfl as the bioluminescent reporter component showed significantly (P < 0.05) higher bioluminescence than the previous triple fusion vectors. Of the three different red fluorescent reporter genes (jred, hcred, and mrfp1, isolated from jellyfish chromophore, coral Heteractis crispa, and coral Discosoma, respectively) evaluated, mrfp1 was able to preserve highest expression as a component of the triple fusion reporter gene for in vivo fluorescence imaging. A truncated version of wild-type herpes simplex virus 1 (HSV1) thymidine kinase gene (wttk) retained a higher expression level than the truncated mutant HSV1-sr39 TK (ttk) as the third reporter component of this improved triple fusion vector. Multimodality imaging of tumor-bearing mice using bioluminescence and microPET showed higher luciferase activity [(2.7 +/- 0.1 versus 1.9 +/- 0.1) x (10(6) p/s/cm(2)/sr)] but similar level of fluorine-18-labeled 2'-fluoro-2'-deoxyarabinofuranosyl-5-ethyluracil (18F-FEAU) uptake (1.37 +/- 0.15 versus 1.37 +/- 0.2) percentage injected dose per gram] by mtfl-mrfp1-wttk-expressing tumors compared with the fl-mrfp1-wttk-expressing tumors. Both tumors showed 4- to 5-fold higher accumulation (P < 0.05) of 18F-FEAU than fluorine-18-labeled 9-(4-fluoro-3-hydroxymethylbutyl)guanine. This improved triple fusion reporter vector will enable high sensitivity detection of lower numbers of cells from living animals using the combined bioluminescence, fluorescence, and microPET imaging techniques.