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

Radiolabeled apoptosis imaging agents for early detection of response to therapy

Since apoptosis plays an important role in maintaining homeostasis and is associated with responses to therapy, molecular imaging of apoptotic cells could be useful for early detection of therapeutic effects, particularly in oncology. Radiolabeled annexin V compounds are the hallmark in apoptosis imaging in vivo. These compounds are reviewed from the genesis of apoptosis (cell death) imaging agents up to recent years. They have some disadvantages, including slow clearance and immunogenicity, because they are protein-based imaging agents. For this reason, several studies have been conducted in recent years to develop low molecule apoptosis imaging agents. In this review, radiolabeled phosphatidylserine targeted peptides, radiolabeled bis(zinc(II)-dipicolylamine) complex, radiolabeled 5-fluoropentyl-2-methyl-malonic acid (ML-10), caspase-3 activity imaging agents, radiolabeled duramycin, and radiolabeled phosphonium cation are reviewed as promising low-molecular-weight apoptosis imaging agents.

18F-labeling using click cycloadditions

Due to expanding applications of positron emission tomography (PET) there is a demand for developing new techniques to introduce fluorine-18 (t_1/2 = 109.8 min). Considering that most novel PET tracers are sensitive biomolecules and that direct introduction of fluorine-18 often needs harsh conditions, the insertion of ¹⁸F in those molecules poses an exceeding challenge. Two major challenges during ¹⁸F-labeling are a regioselective introduction and a fast and high yielding way under mild conditions. Furthermore, attention has to be paid to functionalities, which are usually present in complex structures of the target molecule. The Cu-catalyzed azide-alkyne cycloaddition (CuAAC) and several copper-free click reactions represent such methods for radiolabeling of sensitive molecules under the above-mentioned criteria. This minireview will provide a quick overview about the development of novel ¹⁸F-labeled prosthetic groups for click cycloadditions and will summarize recent trends in copper-catalyzed and copper-free click ¹⁸F-cycloadditions.

Preclinical assessment of carboplatin treatment efficacy in lung cancer by 18F-ICMT-11-positron emission tomography

Tumour response to therapy is assessed primarily in the clinic by monitoring reductions in tumour size. However, this approach lacks sensitivity since in many cases several weeks may elapse before there is evidence of tumour shrinkage. There is therefore a need to develop non-invasive imaging techniques for monitoring tumour treatment response in the clinic. Here, we assessed the pre-clinical utility of (18)F-ICMT-11 positron emission tomography--a method for detecting caspase 3/7 activation--in non-small cell lung cancer (NSCLC). (18)F-ICMT-11 uptake was compared to molecular biochemical measures of cell death in PC9 and A549 NSCLC cells following treatment with carboplatin in vitro and in vivo. Carboplatin-induced apoptosis in the ERCC1 low/mutant EGFR PC9 cells was characterised by time and dose-related increased caspase-3/7 activation, poly-ADP-ribose polymerase cleavage and Annexin V staining. 18F-ICMT-11 uptake was consequently increased up to 14-fold at 200 µM carboplatin compared to vehicle treated cells (P<0.01). In contrast, necrosis was the predominant death mechanism in ERCC1 high/wt EGFR A549 cells and no change in (18)F-ICMT-11 uptake was detected. In vivo, histological analysis of PC9 tumour xenografts indicated high pre-therapy necrosis. A 4.6-fold increase in cleaved caspase-3/7 was measured in non-necrotic regions of PC9 tumours at 48 h post carboplatin therapy. Average PET-derived tumour (18)F-ICMT-11 uptake was insensitive to changes in apoptosis in the presence of substantial pre-existing necrosis. PET-based voxel intensity sorting however, identified intra-tumoural regions of high (18)F-ICMT-11 uptake, enabling accurate assessment of apoptosis and therefore therapy response. In A549 tumours that lacked high pre-therapy necrosis, carboplatin induced growth inhibition that was only minimally associated with apoptosis and thus not detectable by (18)F-ICMT-11 PET.

Clinical translation of molecular imaging agents used in PET studies of cancer

Over recent years, there has been a rapid expansion in our knowledge of the factors that regulate tumor growth; this has resulted in the identification of new therapeutic targets and improvements in the long-term survival of cancer patients. New noninvasive biomarkers of drug targets and pathway modulation in vivo are needed to guide therapy selection and detect drug resistance early so that alternative, more effective treatments can be offered. The translation of new therapeutics into the clinic is disappointingly slow, expensive, and subject to high rates of attrition often occurring at late stages (phase 3) of development. In an attempt to mitigate these delays and failures, there has been resurgence in the development of new molecular imaging probes for studies with positron emission tomography (PET) to characterize tumor biology. In the assessment of therapeutic effects, PET allows imaging of entire tumor burden in a noninvasive repeatable manner. This chapter focuses on the clinical translation of PET imaging agents from bench to bedside. New probes are being used to study a diverse range of processes such as angiogenesis, apoptosis, fatty acid metabolism, and growth factor receptor expression. In the future, validation of these novel imaging probes could allow more innovative therapies to be adapted earlier in the clinic leading to improved patient outcomes.

Biodistribution and radiation dosimetry of 18F-CP-18, a potential apoptosis imaging agent, as determined from PET/CT scans in healthy volunteers

(18)F-CP-18, or (18S,21S,24S,27S,30S)-27-(2-carboxyethyl)-21-(carboxymethyl)-30-((2S,3R,4R,5R,6S)-6-((2-(4-(3-F18-fluoropropyl)-1H-1,2,3-triazol-1-yl)acetamido)methyl)-3,4,5-trihydroxytetrahydro-2H-pyran-2-carboxamido)-24-isopropyl-18-methyl-17,20,23,26,29-pentaoxo-4,7,10,13-tetraoxa-16,19,22,25,28-pentaazadotriacontane-1,32-dioic acid, is being evaluated as a tissue apoptosis marker for PET imaging. The purpose of this study was to determine the biodistribution and estimate the normal-organ radiation-absorbed doses and effective dose from (18)F-CP-18.

METHODS

Successive whole-body PET/CT scans were obtained at approximately 7, 45, 90, 130, and 170 min after intravenous injection of (18)F-CP-18 in 7 healthy human volunteers. Blood samples and urine were collected between the PET/CT scans, and the biostability of (18)F-CP-18 was assessed using high-performance liquid chromatography. The PET scans were analyzed to determine the radiotracer uptake in different organs. OLINDA/EXM software was used to calculate human radiation doses based on the biodistribution of the tracer.

RESULTS

(18)F-CP-18 was 54% intact in human blood at 135 min after injection. The tracer cleared rapidly from the blood pool with a half-life of approximately 30 min. Relatively high (18)F-CP-18 uptake was observed in the kidneys and bladder, with diffuse uptake in the liver and heart. The mean standardized uptake values (SUVs) in the bladder, kidneys, heart, and liver at around 50 min after injection were approximately 65, 6, 1.5, and 1.5, respectively. The calculated effective dose was 38 ± 4 μSv/MBq, with the urinary bladder wall having the highest absorbed dose at 536 ± 61 μGy/MBq using a 4.8-h bladder-voiding interval for the male phantom. For a 1-h voiding interval, these doses were reduced to 15 ± 2 μSv/MBq and 142 ± 15 μGy/MBq, respectively. For a typical injected activity of 555 MBq, the effective dose would be 21.1 ± 2.2 mSv for the 4.8-h interval, reduced to 8.3 ± 1.1 mSv for the 1-h interval.

CONCLUSION

(18)F-CP-18 cleared rapidly through the renal system. The urinary bladder wall received the highest radiation dose and was deemed the critical organ. Both the effective dose and the bladder dose can be reduced by frequent voiding. From the radiation dosimetry perspective, the apoptosis imaging agent (18)F-CP-18 is suitable for human use.

Recent trends in bioorthogonal click-radiolabeling reactions using fluorine-18

The increasing application of positron emission tomography (PET) in nuclear medicine has stimulated the extensive development of a multitude of novel and versatile bioorthogonal conjugation techniques especially for the radiolabeling of biologically active high molecular weight compounds like peptides, proteins or antibodies. Taking into consideration that the introduction of fluorine-18 (t(1/2) = 109.8 min) proceeds under harsh conditions, radiolabeling of these biologically active molecules represents an outstanding challenge and is of enormous interest. Special attention has to be paid to the method of 18F-introduction. It should proceed in a regioselective manner under mild physiological conditions, in an acceptable time span, with high yields and high specific activities. For these reasons and due to the high number of functional groups found in these compounds, a specific labeling procedure has to be developed for every bioactive macromolecule. Bioorthogonal strategies including the Cu-assisted Huisgen cycloaddition and its copper-free click variant, both Staudinger Ligations or the tetrazine-click reaction have been successfully applied and represent valuable alternatives for the selective introduction of fluorine-18 to overcome the afore mentioned obstacles. This comprehensive review deals with the progress and illustrates the latest developments in the field of bioorthogonal labeling with the focus on the preparation of radiofluorinated building blocks and tracers for molecular imaging.

Managing lymphoma with non-FDG radiotracers: current clinical and preclinical applications

Nuclear medicine imaging modalities such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) have played a prominent role in lymphoma management. PET with [(18)F]Fluoro-2-deoxy-D-glucose (FDG) is the most commonly used tool for lymphoma imaging. However, FDG-PET has several limitations that give the false positive or false negative diagnosis of lymphoma. Therefore, development of new radiotracers with higher sensitivity, specificity, and different uptake mechanism is in great demand in the management of lymphoma. This paper reviews non-FDG radiopharmaceuticals that have been applied for PET and SPECT imaging in patients with different types of lymphoma, with attention to diagnosis, staging, therapy response assessment, and surveillance for disease relapse. In addition, we introduce three radiolabeled anti-CD20 antibodies for radioimmunotherapy, which is another important arm for lymphoma treatment and management. Finally, the relatively promising radiotracers that are currently under preclinical development are also discussed in this paper.

Temporal and spatial evolution of therapy-induced tumor apoptosis detected by caspase-3-selective molecular imaging

PURPOSE

Induction of apoptosis in tumors is considered a desired goal of anticancer therapy. We investigated whether the dynamic temporal and spatial evolution of apoptosis in response to cytotoxic and mechanism-based therapeutics could be detected noninvasively by the caspase-3 radiotracer [(18)F]ICMT-11 and positron emission tomography (PET).

EXPERIMENTAL DESIGN

The effects of a single dose of the alkylating agent cyclophosphamide (CPA or 4-hydroperoxycyclophosphamide), or the mechanism-based small molecule SMAC mimetic birinapant on caspase-3 activation was assessed in vitro and by [(18)F]ICMT-11-PET in mice bearing 38C13 B-cell lymphoma, HCT116 colon carcinoma, or MDA-MB-231 breast adenocarcinoma tumors. Ex vivo analysis of caspase-3 was compared to the in vivo PET imaging data.

RESULTS

Drug treatment increased the mean [(18)F]ICMT-11 tumor uptake with a peak at 24 hours for CPA (40 mg/kg; AUC40-60: 8.04 ± 1.33 and 16.05 ± 3.35 %ID/mL × min at baseline and 24 hours, respectively) and 6 hours for birinapant (15 mg/kg; AUC40-60: 20.29 ± 0.82 and 31.07 ± 5.66 %ID/mL × min, at baseline and 6 hours, respectively). Voxel-based spatiotemporal analysis of tumor-intrinsic heterogeneity suggested that discrete pockets of caspase-3 activation could be detected by [(18)F]ICMT-11. Increased tumor [(18)F]ICMT-11 uptake was associated with caspase-3 activation measured ex vivo, and early radiotracer uptake predicted apoptosis, distinct from the glucose metabolism with [(18)F]fluorodeoxyglucose-PET, which depicted continuous loss of cell viability.

CONCLUSION

The proapoptotic effects of CPA and birinapant resulted in a time-dependent increase in [(18)F]ICMT-11 uptake detected by PET. [(18)F]ICMT-11-PET holds promise as a noninvasive pharmacodynamic biomarker of caspase-3-associated apoptosis in tumors.

Synthesis, 18F-radiolabeling, and in vivo biodistribution studies of N-fluorohydroxybutyl isatin sulfonamides using positron emission tomography

The effector caspases-3 and -7 play a central role in programmed type I cell death (apoptosis). Molecular imaging using positron emission tomography (PET) by tracking the activity of executing caspases might allow the detection of the early onset as well as therapy monitoring of various diseases induced by dysregulated apoptosis. Herein, four new fluorinated diastereo- and enantiopure isatin sulfonamide-based potent and selective caspase-3 and -7 inhibitors were prepared by cyclic sulfate ring-opening with fluoride. All fluorohydrins exhibited excellent in vitro affinities (up to IC50 = 11.8 and 0.951 nM for caspase-3 and -7, respectively), which makes them appropriate PET radiotracer candidates. Therefore, N-(4-[(18)F]fluoro-3(R)-hydroxybutyl)- and N-(3(S)-[(18)F]fluoro-4-hydroxybutyl)-5-[1-(2(S)-(methoxymethyl)pyrrolidinyl)sulfonyl]isatin were synthesized in 140 min with 24% and 10% overall radiochemical yields and specific activities of 10-127 GBq/μmol using [(18)F]fluoride in the presence of Kryptofix and subsequent acidic hydrolysis. In vivo biodistribution studies in wild-type mice using PET/computed tomography imaging proved fast clearance of the tracer after tail vein injection.

Chelator-accelerated one-pot 'click' labeling of small molecule tracers with 2-[¹⁸F]fluoroethyl azide

2-[¹⁸F]Fluoroethyl azide ([¹⁸F]FEA) can readily be obtained by nucleophilic substitution of 2-azidoethyl-4-toluenesulfonate with [¹⁸F]fluoride (half-life 110 min), and has become widely used as a reagent for ‘click’ labeling of PET tracers. However, distillation of [¹⁸F]FEA is typically required, which is time-consuming and unpractical for routine applications. In addition, copper(I)-catalyzed cycloaddition of [¹⁸F]FEA with non-activated alkynes, and with substrates containing labile functional groups, can be challenging. Herein, we report a highly efficient and practical ligand-accelerated one-pot/two-step method for ‘click’ labeling of small molecule tracers with [¹⁸F]FEA. The method exploits the ability of the copper(I) ligand bathophenanthrolinedisulfonate to accelerate the rate of the cycloaddition reaction. As a result, alkynes can be added directly to the crude reaction mixture containing [¹⁸F]FEA, and as cyclisation occurs almost immediately at room temperature, the reaction is tolerant to labile functional groups. The method was demonstrated by reacting [¹⁸F]FEA with a series of alkyne-functionalized 6-halopurines to give the corresponding triazoles in 55–76% analytical radiochemical yield.

New frontiers in the design and synthesis of imaging probes for PET oncology: current challenges and future directions

Despite being developed over 30 years ago, 2-deoxy-2-[(18)F]fluoro-D-glucose remains the most frequently used radiotracer in PET oncology. In the last decade, interest in new and more specific radiotracers for imaging biological processes of oncologic interest has increased exponentially. This review summarizes the strategies underlying the development of those probes together with their validation and status of clinical translation; a brief summary of new radiochemistry strategies applicable to PET imaging is also included. The article finishes with a consideration of the challenges imaging scientists must overcome to bring about increased adoption of PET as a diagnostic or pharmacologic tool.

Synthesis, microwave spectrum, and conformational properties of 2-fluoroethyl azide (FCH2CH2N3)

A novel synthesis producing neat 2-fluoroethyl azide (FCH2CH2N3) is described. A conformational analysis using microwave spectroscopy augmented by quantum chemical calculations at the CCSD(T)/cc-pVTZ, B3LYP/aug-cc-pVTZ, and B3LYP/cc-pVTZ levels of theory has been performed for this compound. The spectra of the ground vibrational state and two vibrationally excited states of one rotameric form were assigned. A large number of transitions was assigned, and very accurate values were obtained for the rotational and quartic centrifugal distortion constants. The identified conformer has synclinal orientations for the F-C-C-N and C-C-N-N chains of atoms bringing the fluorine atom and the azido group into close proximity. It is concluded from consideration of absolute intensities that this conformer is indeed the preferred form of the molecule in accord with the theoretical calculations. The experimental and CCSD(T) rotational constants are in very good agreement, whereas much larger discrepancies were seen for the experimental and B3LYP quartic centrifugal distortion constants.

Fluorine-18 labeling by click chemistry: multiple probes in one pot

Click chemistry has been widely applied in drug development including radiopharmaceuticals and has shown great advantages. Here we reported a novel strategy for rapid preparation of multiple (18)F labeled PET probes in one pot using the 'Click Reaction' of Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition of terminal alkynes and organic azides (CuAAC). Preliminary results showed its high efficiency and potential for speeding up the preclinical screening of PET probes.

Synthesis of new fluorinated, 2-substituted 5-pyrrolidinylsulfonyl isatin derivatives as caspase-3 and caspase-7 inhibitors: nonradioactive counterparts of putative PET-compatible apoptosis imaging agents

Downstream caspases-3 and -7 are essential to execute the programmed type I cell death (apoptosis). In order to better understand their role, specific inhibitors of these enzymes are required, which after radiolabeling can be applied to non-invasively visualize and monitor apoptotic pathways in vivo using Positron Emission Tomography (PET). Therefore, 2-methoxyethyl-, 2-methoxypropyl-, 2-ethoxymethyl-, 2-(2-fluoroethoxymethyl)-, and 2-(2,2,2-trifluoroethoxymethyl)pyrrolidinyl analogues of (S)-5-[1-(2-methoxymethylpyrrolidinyl)sulfonyl]isatin (2) were prepared and their in vitro binding affinities towards caspases-1, -3, -6 and -7 were evaluated and compared to that of the lead structure 2. While the inhibition potencies against caspases-1 and -6 were in the micromolar range, all synthesized compounds exhibited excellent and selective inhibition of caspases-3 and -7 in the nanomolar range up to IC50=4.79 nM and 7.47 nM, respectively. These highly potent 2-substituted analogues of 2 might be developed as anti-apoptosis agents and some selected fluorinated inhibitors might be useful as potential PET radiotracers for apoptosis imaging after (18)F-labeling.

Evaluation of a 125I-labelled benzazepinone derived voltage-gated sodium channel blocker for imaging with SPECT

Voltage-gated sodium channels (VGSCs) are a family of transmembrane proteins that mediate fast neurotransmission, and are integral to sustain physiological conditions and higher cognitive functions. Imaging of VGSCs in vivo holds promise as a tool to elucidate operational functions in the brain and to aid the treatment of a wide range of neurological diseases. To assess the suitability of 1-benzazepin-2-one derived VGSC blockers for imaging, we have prepared a (125)I-labelled analogue of BNZA and evaluated the tracer in vivo. In an automated patch-clamp assay, a diastereomeric mixture of the non-radioactive compound blocked the Na(v)1.2 and Na(v)1.7 VGSC isoforms with IC(50) values of 4.1 ± 1.5 μM and 0.25 ± 0.07 μM, respectively. [(3)H]BTX displacement studies revealed a three-fold difference in affinity between the two diastereomers. Iodo-destannylation of a tin precursor with iodine-125 afforded the two diastereomerically pure tracers, which were used to assess binding to VGSCs in vivo by comparing their tissue distributions in mice. Whilst the results point to a lack of VGSC binding in vivo, SPECT imaging revealed highly localized uptake in the interscapular region, an area typically associated with brown adipose tissue, which in addition to high metabolic stability of the iodinated tracer, demonstrate the potential of 1-benzazepin-2-ones for in vivo imaging.

Recent advances in the molecular imaging of programmed cell death: part I--pathophysiology and radiotracers

In humans, apoptosis (programmed cell death) is the most common form of cell death after necrosis. Apoptosis is a series of genetically preprogrammed biochemical and morphologic energy-requiring events that, after a specific external or internal stimulus, results in the physiologic disappearance of a cell via its self-disintegration and packaging of its contents into membrane vesicles called apoptotic bodies. Apoptotic bodies can readily be ingested, with their nutrients and even organelles recycled by neighboring cells or phagocytes without local inflammation. In contrast, necrosis is characterized by the primary loss of plasma membrane integrity and the uncontrolled release of a cell's contents, often causing local inflammation, tissue damage, and scarring. Alternate forms of cell death also exist, associated with specific molecular mechanisms involving enzymes, organelles, genes, external stimuli, or blockade of normal cell proliferation. In this review we will briefly outline the molecular mechanisms of apoptosis that can be imaged with radiotracers now under development.

Biomarkers and molecular probes for cell death imaging and targeted therapeutics

Cell death is a critically important biological process. Disruption of homeostasis, either by excessive or deficient cell death, is a hallmark of many pathological conditions. Recent research advances have greatly increased our molecular understanding of cell death and its role in a range of diseases and therapeutic treatments. Central to these ongoing research and clinical efforts is the need for imaging technologies that can locate and identify cell death in a wide array of in vitro and in vivo biomedical samples with varied spatiotemporal requirements. This review article summarizes community efforts over the past five years to identify useful biomarkers for dead and dying cells, and to develop molecular probes that target these biomarkers for optical, radionuclear, or magnetic resonance imaging. Apoptosis biomarkers are classified as either intracellular (caspase enzymes, mitochondrial membrane potential, cytosolic proteins) or extracellular (plasma membrane phospholipids, membrane potential, surface exposed histones). Necrosis, autophagy, and senescence biomarkers are described, as well as unexplored cell death biomarkers. The article discusses possible chemotherapeutic and theranostic strategies, and concludes with a summary of current challenges and expected eventual rewards of clinical cell death imaging.

Highly efficient click labeling using 2-[¹⁸F]fluoroethyl azide and synthesis of an ¹⁸FN-hydroxysuccinimide ester as conjugation agent

INTRODUCTION

Click labeling using 2-[¹⁸F]fluoroethyl azide has been proven to be promising methods of radiolabeling small molecules and peptides, some of which are undergoing clinical evaluations. However, the previously reported method afforded low yield, poor purities and under desirable reproducibility.

METHODS

A vacuum distillation method was used to isolate 2-[¹⁸F]fluoroethyl azide, and the solvent effect of acetonitrile and dimethylformamide (DMF) on the click labeling using Cu(I) from copper sulfate/sodium ascorbate was studied. The labeling conditions were optimized to radiosynthesize a hydroxysuccinimide ester (N-hydroxysuccinimide, or NHS).

RESULTS

2-[¹⁸F]fluoroethyl azide was isolated by the vacuum distillation method with >80% yield within 10min in a "pure" and click-ready form. It was found that the amount of DMF was critical for maintaining high levels of Cu(I) from copper sulfate/sodium ascorbate in order to rapidly complete the click labeling reaction. The addition of bathophenanthrolinedisulfonic acid disodium salt to the mixture of copper sulfate/sodium ascorbate also greatly improved the click labeling efficiency. Through exploiting these optimizations, a base-labile NHS ester was rapidly radiosynthesized in 90% isolated yield with good chemical and radiochemical purities.

CONCLUSIONS

We have developed a general method to click-label small molecules efficiently using [¹⁸F]2 for research and clinical use. This NHS ester can be used for conjugation chemistry to label antibodies, peptides and small molecules as positron emission tomography tracers.

The impact of click chemistry in medicinal chemistry

INTRODUCTION

The copper(I)-catalyzed 1,3-dipolar cycloaddition of alkynes and azides to form 1,2,3-triazoles is the most popular reaction in click chemistry. This reaction is also near-perfect, in terms of its robustness, due to the high degree of reliability and complete specificity. Furthermore, this reaction has been used increasingly in drug discovery, because the formed 1,2,3-triazole can act as both a bioisostere and a linker.

AREAS COVERED

This review provides an overview of a most important click reaction, 1,3-dipolar cycloadditions of alkynes and azides, in the drug discovery.

EXPERT OPINION

Click chemistry is a very powerful tool, in the drug discovery, because it is very efficient in the creation of compound libraries through combinatorial methodology. However, the 1,2,3-triazole ring itself is not a commonly used pharmacophore and has rarely been found in marketed drugs, demonstrating that there are still some limitations during the use of 1,2,3-triazole in the molecules of drug candidates. Hopefully, in the next decade, we will witness the emergence of 1,2,3-triazole-bearing drugs on the market as this click reaction is used more and more widely in the drug discovery.

Automated GMP synthesis of [(18)F]ICMT-11 for in vivo imaging of caspase-3 activity

INTRODUCTION

Isatin-5-sulfonamide ([(18)F]ICMT-11) is a sub-nanomolar inhibitor of caspase-3 previously evaluated as an apoptosis imaging agent. Herein, an alternative radiosynthesis of [(18)F]ICMT-11 with increased purity and specific activity is presented. Finally, a GMP-applicable automated radiosynthesis of [(18)F]ICMT-11 is described.

METHODS

The preparation of [(18)F]ICMT-11 was evaluated under a variety of reaction conditions, including reaction solvent, by employing alternative phase transfer catalysts and under different deprotection conditions. Following initial investigations, the process was transferred onto a fully automated GE FASTlab synthesis platform for further development and optimisation.

RESULTS

The synthesis of [(18)F]ICMT-11 was successfully validated under GMP conditions, resulting in a yield of 4.6 ± 0.4 GBq with a radiochemical purity of >98% at EOS and a specific activity of 685 ± 237 GBq/μmol within 90 min. Quality control was carried out in accordance with the European Pharmacopoeia and demonstrated that [(18)F]ICMT-11 can be consistently manufactured on the FASTlab to meet specifications.

CONCLUSIONS

A simplified methodology for the synthesis of the apoptosis imaging agent, [(18)F]ICMT-11, has been achieved by the S(N)2 displacement of a tosylate leaving group with [(18)F]fluoride ion. This results in an increased purity and specific activity over the original copper catalysed "Click" synthetic stratagem reaction involving 2-[(18)F]fluoroethylazide with an alkyne precursor and is now suitable for routine clinical application.

Synthesis and evaluation of nucleoside radiotracers for imaging proliferation

INTRODUCTION

Uncontrolled proliferation is a fundamental characteristic of cancer, and consequently, imaging of tumor proliferative status finds interest clinically both as a diagnostic tool and for evaluation of response to treatment. Positron emission tomography (PET) radiotracers based on a nucleoside core, such as 3'-[18F]fluoro-3'-deoxythymidine ([18F]FLT), have been extensively studied for this purpose. However, [18F]FLT suffers from poor DNA incorporation leading to occasional poor correlation of [18F]FLT tumor uptake with other proliferation indicators such as Ki-67 immunostaining.

METHODS

N3-((1-(2-[18F]fluoroethyl)-1H-[1,2,3]-triazol-4-yl)methyl)thymidine ([18F]2) and N3-((1-(2-[18F]fluoroethyl)-1H-[1,2,3]-triazol-4-yl)methyl)-4'-thio-β-thymidine ([18F]3) were synthesized by click chemistry from [18F]fluoroethyl azide and by direct nucleophilic substitution of a tosylate precursor. Metabolic stability and phosphorylation potential of the radiotracers were evaluated in vitro and compared to [18F]FLT. Further, metabolic stability and biodistribution analysis of [18F]2 and [18F]3 were evaluated in vivo.

RESULTS

Stable isotope standards and radiochemistry precursors were synthesized by modification of existing literature procedures. [18F]2 and [18F]3 were synthesized in a radiochemical yield of 8%-12% (end of synthesis, non-decay corrected). Both nucleosides were stable to metabolic degradation by thymidine phosphorylase, and in vivo stability analysis showed only one metabolite for [18F]3. No phosphorylation of [18F]2 could be detected in HCT116 cell homogenates, and in the same assay, only minor (∼8%) phosphorylation of [18F]3 was observed. Biodistribution in Balb/c mice indicated rapid clearance for [18F]2 and [18F]3 to a lesser extent.

CONCLUSIONS

The favorable biodistribution and metabolic profile of [18F]3 warrant further investigation as a next-generation PET proliferation marker.

Development of radiotracers for oncology--the interface with pharmacology

There is an increasing role for positron emission tomography (PET) in oncology, particularly as a component of early phase clinical trials. As a non-invasive functional imaging modality, PET can be used to assess both pharmacokinetics and pharmacodynamics of novel therapeutics by utilizing radiolabelled compounds. These studies can provide crucial information early in the drug development process that may influence the further development of novel therapeutics. PET imaging probes can also be used as early biomarkers of clinical response and to predict clinical outcome prior to the administration of therapeutic agents. We discuss the role of PET imaging particularly as applied to phase 0 studies and discuss the regulations involved in the development and synthesis of novel radioligands. The review also discusses currently available tracers and their role in the assessment of pharmacokinetics and pharmacodynamics as applied to oncology.

Imaging apoptosis with positron emission tomography: 'bench to bedside' development of the caspase-3/7 specific radiotracer [(18)F]ICMT-11

The capacity to evade apoptosis has been defined as one of the hallmarks of cancer and, thus, effective anti-cancer therapy often induces apoptosis. A biomarker for imaging apoptosis could assist in monitoring the efficacy of a wide range of current and future therapeutics. Despite the potential, there are limited clinical examples of the use of positron emission tomography for imaging of apoptosis. [(18)F]ICMT-11 is a novel reagent designed to non-invasively image caspase-3 activation and, hence, drug-induced apoptosis. Radiochemistry development of [(18)F]ICMT-11 has been undertaken to improve specific radioactivity, reduce content of stable impurities, reduce synthesis time and enable automation for manufacture of multi-patient dose. Due to the promising mechanistic and safety profile of [(18)F]ICMT-11, the radiotracer is transitioning to clinical development and has been selected as a candidate radiotracer by the QuIC-ConCePT consortium for further evaluation in preclinical models and humans. A successful outcome will allow use of the radiotracer as qualified method for evaluating the pharmaceutical industry's next generation therapeutics.

Copper-free click--a promising tool for pre-targeted PET imaging

The copper-free click (CFC) reaction has been evaluated for its potential application to in vivo pre-targeting for PET imaging. A promising biodistribution profile is demonstrated when employing [(18)F]2-fluoroethylazide ([(18)F]1) and optimisation of the CFC reaction with a series of cyclooctynes shows that reactions proceed efficiently with tantalizing opportunities for application-specific tuning.

Improved radiosynthesis of the apoptosis marker 18F-ICMT11 including biological evaluation

We improved the specific radioactivity of the apoptosis imaging isatin derivative (18)F-ICMT11. We then evaluated (18)F-ICMT11 in EL4 tumor-bearing mice 24h after treatment with etoposide/cyclophosphamide combination therapy. Dynamic PET imaging demonstrated increased uptake in the drug-treated (0.115±0.011 SUV) compared to the vehicle-treated EL4 tumors (0.083±0.008 SUV). This effect correlated to the observed increases in apoptotic index.

Multimodality molecular imaging of apoptosis in oncology

OBJECTIVE

The purposes of this review are to describe the signaling pathways of and the cellular changes that occur with apoptosis and other forms of cell death, summarize tracers and modalities used for imaging of apoptosis, delineate the relation between apoptosis and inhibition of protein translation, and describe spectroscopic technologies that entail high-frequency ultrasound and infrared and midinfrared light in characterizing the intracellular events of apoptosis.

CONCLUSION

Apoptosis is a highly orchestrated set of biochemical and morphologic cellular events. These events present many potential targets for the imaging of apoptosis in vivo. Imaging of apoptosis can facilitate early assessment of anticancer treatment before tumor shrinkage, which may increase the effectiveness of delivery of chemotherapy and radiation therapy and speed drug development.

Synthesis and evaluation of isatin analogs as caspase-3 inhibitors: introduction of a hydrophilic group increases potency in a whole cell assay

A series of isatin analogs containing a hydrophilic group, including a pyridine ring, ethylene glycol group, and a triazole ring, have been synthesized, and their inhibition potency for caspase-3 was measured both in vitro (i.e., recombinant enzyme) and in whole cells (HeLa cells). The analogs having a hydrophilic group, including 12, 13, 16, 38, and 40, have dramatically increased activity in vitro and in HeLa cells compared to the corresponding unsubstituted N-phenyl isatin analogs.

Radiotracers for noninvasive molecular imaging of tumor cell death

The need to monitor cancer therapy-induced cellular and tissue changes using noninvasive imaging techniques continues to stimulate both basic and clinical research. Monitoring changes in cellular proliferative capacity that occur after treatment with radiation and/or chemotherapy has the potential to provide longitudinal information on the cellular dynamics of tumors before, during, and after therapeutic intervention. Cells can lose their reproductive potential through one of several mechanisms, including apoptosis and autophagy (which are forms of programmed cell death), premature senescence, or necrosis. When a tumor responds to therapy, current imaging methods do not provide information about the exact mechanism of cell death executed. We are now beginning to develop the molecular imaging tools that will enable us to noninvasively image cell death mechanisms both in experimental models and in the clinical cancer environment. Studies with these imaging tools will contribute to a better understanding of therapeutic responses and assist in the design and evaluation of more effective treatments. This review examines the state-of-the-art in the use of (radio)tracers for the purpose of imaging mechanisms of tumor cell inactivation (cell death) in animal models and in clinical trials.

"Click"-capture, ring-opening metathesis polymerization (ROMP), release: facile triazolation utilizing ROMP-derived oligomeric phosphates

Soluble, high-load ring-opening metathesis polymerization (ROMP)-derived oligomeric triazole phosphates (OTP) are reported for application as efficient triazolating reagents of nucleophilic species. Utilizing a "Click"-capture, ROMP, release protocol, the efficient and purification-free, direct triazolation of N-, O-, and S-nucleophilic species was successfully achieved. A variety of OTP derivatives were rapidly synthesized as free-flowing solids on a multigram scale from commercially available materials.

The traceless Staudinger ligation for indirect 18F-radiolabelling

The Staudinger ligation of phosphine-substituted thioesters with (18)F-fluoroethylazide has been successfully applied to access (18)F-labelled molecules in radiochemical yields superior to 95%; the first fluorous variant of a Staudinger radio-ligation has been validated.

Imaging the life and death of tumors in living subjects: Preclinical PET imaging of proliferation and apoptosis

Cancer is characterized by deregulation of cell proliferation and altered cell death apoptosis, which constitutes, in almost all instances, the minimal common platform upon which all neoplastic evolution occurs. The most implicit and clinically attractive anticancer strategies, therefore, consist of eliminating tumor cells by preventing their expansion and ultimately inducing cell death apoptosis. In this context, the non-invasive molecular assessment of tumor cell proliferation and apoptosis status using PET imaging constitutes a major strategy in preclinical studies to assess the efficacy of new anticancer therapeutics using small animal PET imaging, and in clinical settings for the monitoring of treatment responses in patients. For this purpose, a variety of PET tracers targeting specific molecular entities allowing the non-invasive measurement of biological processes, including cell proliferation and apoptosis, are under development for use in preclinical studies and clinical trials to non-invasively image in vivo the lifeline of tumors.

The future of imaging: developing the tools for monitoring response to therapy in oncology: the 2009 Sir James MacKenzie Davidson Memorial lecture

Since the days of Sir James MacKenzie Davidson, radiology discoveries have been shaping the way patients are managed. The lecture on which this review is based focused not on anatomical imaging, which already has a great impact on patient management, but on the molecular imaging of cancer targets and pathways. In this post-genomic era, we have several tools at our disposal to enable the discovery of new probes for stratifying patients for therapy and for monitoring response to therapy sooner than is possible using conventional cross-sectional imaging methods. I describe a chemical library approach to discovering new imaging agents, as well as novel methods for improving the metabolic stability of existing probes. Finally, I describe the evaluation of these probes for clinical use in both pre-clinical and clinical validation. The chemical library approach is exemplified by the discovery of isatin sulfonamide probes for imaging apoptosis in tumours. This approach allowed important desirable features of radiopharmaceuticals to be incorporated into the design strategy. A lead compound, [(18)F]ICMT11, is selectively taken up in vitro in cancer cells and in vivo in tumours undergoing apoptosis. Improvement of the metabolic stability of a probe is exemplified by work on [(18)F]fluoro-[1,2-(2)H(2)]choline ("[(18)F]-D4-choline"), a novel probe for imaging choline metabolism. Deuterium substitution significantly reduced the systemic metabolism of this compound relative to that of non-deuteriated analogues, supporting its future clinical use. In order for probes to be useful for monitoring response a number of validation and/or qualification studies need to be performed, including assessments of whether the probe measures the target or pathway of interest in a specific and reproducible manner, whether the probe is stable to metabolism in vivo, what is the best time to assess response with these probes and finally whether changes in radiotracer uptake are associated with clinical outcome. [(18)F]Fluorothymidine, a probe for proliferation imaging has been validated and qualified for use in breast cancer. In summary, the ability to create new molecules that can report on specific targets and pathways provides a strategy for studying response to treatment in cancer earlier than it is currently possible. This could fundamentally change the way medicine is practised in the next 5-10 years.

Use of 2-[(18)F]fluoroethylazide for the Staudinger ligation - Preparation and characterisation of GABA(A) receptor binding 4-quinolones

The labelling reagent 2-[(18)F]fluoroethylazide was used in a traceless Staudinger ligation. This reaction was employed to obtain the GABA(A) receptor binding 6-benzyl-4-oxo-1,4-dihydro-quinoline-3-carboxylic acid (2-[(18)F]fluoroethyl) amide. The radiotracer was prepared with a non-decay corrected radiochemical yield of 7%, a radiochemical purity >95% and a specific radioactivity of 0.9 GBq/micromol. The compound showed low brain penetration in normal rats. A series of fluoroalkyl 4-quinolone analogues with nanomolar to sub-nanomolar affinity for the GABA(A) receptor has been prepared as well.

Synthesis of carbon-11-labeled 4-aryl-4H-chromens as new PET agents for imaging of apoptosis in cancer

Carbon-11-labeled 4-aryl-4H-chromenes, 2-amino-7-(dimethylamino)-4-(3-[(11)C]methoxy-5-methoxyphenyl)-4H-chromene-3-carbonitrile ([(11)C]6a), 2-amino-4-(3-bromo-4-[(11)C]methoxy-5-methoxyphenyl)-7-(dimethylamino)-4H-chromene-3-carbonitrile ([(11)C]6c), 2-amino-4-(3-[(11)C]methoxy-5-methoxyphenyl)-4,7-dihydropyrano[2,3-e]indole-3-carbonitrile ([(11)C]6d), 2-amino-4-(3-bromo-4-[(11)C]methoxy-5-methoxyphenyl)-4,7-dihydropyrano[2,3-e]indole-3-carbonitrile ([(11)C]6f), 2-amino-4-(3-[(11)C]methoxy-5-methoxyphenyl)-4,9-dihydropyrano[3,2-g]indole-3-carbonitrile ([(11)C]6g), 2-amino-4-(3-bromo-4-[(11)C]methoxy-5-methoxyphenyl)-4,9-dihydropyrano[3,2-g]indole-3-carbonitrile ([(11)C]6i), 2-amino-4-(3-[(11)C]methoxy-5-methoxyphenyl)-7-methyl-4,7-dihydropyrano[2,3-e]indole-3-carbonitrile ([(11)C]6j) and 2-amino-4-(3-bromo-4-[(11)C]methoxy-5-methoxyphenyl)-7-methyl-4,7-dihydropyrano[2,3-e]indole-3-carbonitrile ([(11)C]6l), were prepared by O-[(11)C]methylation of their corresponding precursors using [(11)C]CH(3)OTf under basic conditions and isolated by a simplified solid-phase extraction (SPE) method in 30-50% radiochemical yields based on [(11)C]CO(2) and decay corrected to end of bombardment (EOB). The overall synthesis time from EOB was 15-20min, the radiochemical purity was >99%, and the specific activity at end of synthesis (EOS) was 111-185GBq/micromol.

Positron emission tomography imaging of drug-induced tumor apoptosis with a caspase-3/7 specific [18F]-labeled isatin sulfonamide

Of the molecular biochemical alterations that occur during apoptosis, activation of caspases, notably caspase-3, is probably the most attractive for developing specific in vivo molecular imaging probes. We recently designed a library of isatin-5 sulfonamides and selected [18F]ICMT-11 for further evaluation on the basis of subnanomolar affinity for activated capsase-3, high metabolic stability, and facile radiolabeling. In this present study, we have demonstrated that [18F]ICMT-11 binds to a range of drug-induced apoptotic cancer cells in vitro and to 38C13 murine lymphoma xenografts in vivo by up to 2-fold at 24 h posttreatment compared to vehicle treatment. We further demonstrated that the increased signal intensity in tumors after drug treatment, detected by whole body in vivo microPET imaging, was associated with increased apoptosis. In summary, we have characterized [18F]ICMT-11 as a caspase-3/7 specific PET imaging radiotracer for the assessment of tumor apoptosis that could find utility in anticancer drug development and the monitoring of early responses to therapy.

Fluorinated isatin derivatives. Part 3. New side-chain fluoro-functionalized pyrrolidinyl sulfonyl isatins as potent caspase-3 and -7 inhibitors

Background: Dysregulation of type I programmed cell death (apoptosis) leads to a variety of diseases, among which cancer, cardiovascular and neurodegenerative disorders are the most prominent and widespread. Effector caspases such as caspases-3 and -7 get activated during the apoptotic signaling cascade and hence represent a biological target for the diagnosis and therapy of apoptosis-associated diseases. Methods: Synthesis of potent fluorinated analogs of the lead compound (S)-(+)-5-[1-(2-methoxymethylpyrrolidinyl)sulfonyl]isatin facilitates the aim-oriented identification of precursor candidates for 18F-radiofluorination resulting in radiolabeled compounds that could be employed as tracers for the specific imaging of apoptosis in vivo, using positron-emission tomography. Conclusion: Within a series of new mono-, di- and trifluoromethylated pyrrolidine ring analogs of the lead compound, high inhibition potencies were found for caspases-3 and -7 with IC50 values up to 30 and 37 nM, respectively. A new oxidative desulfurization–fluorination protocol was shown to be a versatile technique for fluorine incorporation.

Rapid synthesis of the indole-indolone scaffold via [3+2] annulation of arynes by methyl indole-2-carboxylates

The reaction of methyl indole-2-carboxylates and arynes affords a very efficient, high yielding synthesis of a novel indole-indolone ring system, which tolerates considerable functionality, is broad in scope and proceeds under mild reaction conditions.