A first synthesis of 18F-radiolabeled lapatinib: a potential tracer for positron emission tomographic imaging of ErbB1/ErbB2 tyrosine kinase activity

Novel Receptor Tyrosine Kinase Pathway Inhibitors for Targeted Radionuclide Therapy of Glioblastoma

Glioblastoma (GB) remains the most fatal brain tumor characterized by a high infiltration rate and treatment resistance. Overexpression and/or mutation of receptor tyrosine kinases is common in GB, which subsequently leads to the activation of many downstream pathways that have a critical impact on tumor progression and therapy resistance. Therefore, receptor tyrosine kinase inhibitors (RTKIs) have been investigated to improve the dismal prognosis of GB in an effort to evolve into a personalized targeted therapy strategy with a better treatment outcome. Numerous RTKIs have been approved in the clinic and several radiopharmaceuticals are part of (pre)clinical trials as a non-invasive method to identify patients who could benefit from RTKI. The latter opens up the scope for theranostic applications. In this review, the present status of RTKIs for the treatment, nuclear imaging and targeted radionuclide therapy of GB is presented. The focus will be on seven tyrosine kinase receptors, based on their central role in GB: EGFR, VEGFR, MET, PDGFR, FGFR, Eph receptor and IGF1R. Finally, by way of analyzing structural and physiological characteristics of the TKIs with promising clinical trial results, four small molecule RTKIs were selected based on their potential to become new therapeutic GB radiopharmaceuticals.

Hypervalent aryliodine compounds as precursors for radiofluorination

Over the last 2 decades or so, hypervalent iodine compounds, such as diaryliodonium salts and aryliodonium ylides, have emerged as useful precursors for labeling homoarenes and heteroarenes with no-carrier-added cyclotron-produced [¹⁸ F]fluoride ion (t_1/2  = 109.8 min). They permit rapid and effective radiofluorination at electron-rich as well as electron-deficient aryl rings, and often with unrestricted choice of ring position. Consequently, hypervalent aryliodine compounds have found special utility as precursors to various small-molecule ¹⁸ F-labeling synthons and to many radiotracers for biomedical imaging with positron emission tomography. This review summarizes this advance in radiofluorination chemistry, with emphasis on precursor synthesis, radiofluorination mechanism, method scope, and method application.

Fluorine-18 labelled building blocks for PET tracer synthesis

This review presents a comprehensive overview of the synthesis and application of fluorine-18 labelled building blocks since 2010.

Recent Advances in the Development and Application of Radiolabeled Kinase Inhibitors for PET Imaging

Over the last 20 years, intensive investigation and multiple clinical successes targeting protein kinases, mostly for cancer treatment, have identified small molecule kinase inhibitors as a prominent therapeutic class. In the course of those investigations, radiolabeled kinase inhibitors for positron emission tomography (PET) imaging have been synthesized and evaluated as diagnostic imaging probes for cancer characterization. Given that inhibitor coverage of the kinome is continuously expanding, in vivo PET imaging will likely find increasing applications for therapy monitoring and receptor density studies both in- and outside of oncological conditions. Early investigated radiolabeled inhibitors, which are mostly based on clinically approved tyrosine kinase inhibitor (TKI) isotopologues, have now entered clinical trials. Novel radioligands for cancer and PET neuroimaging originating from novel but relevant target kinases are currently being explored in preclinical studies. This article reviews the literature involving radiotracer design, radiochemistry approaches, biological tracer evaluation and nuclear imaging results of radiolabeled kinase inhibitors for PET reported between 2010 and mid-2015. Aspects regarding the usefulness of pursuing selective vs. promiscuous inhibitor scaffolds and the inherent challenges associated with intracellular enzyme imaging will be discussed.

PET/Computed Tomography Using New Radiopharmaceuticals in Targeted Therapy

Targeted therapy is gaining prominence in the management of different cancers. Given different mechanism of action compared with traditional chemoradiotherapy, selection of patients for targeted therapy and monitoring response to these agents is difficult with conventional imaging. Various new PET radiopharmaceuticals have been evaluated for molecular imaging of these targets to achieve specific patient selection and response monitoring. These PET/computed tomography (CT) agents target the cell surface receptors, hormone receptors, receptor tyrosine kinases, or angiogenesis components. This article reviews the established and potential role of PET/CT with new radiopharmaceuticals for guiding targeted therapy.

Molecular imaging of targeted therapies with positron emission tomography: the visualization of personalized cancer care

INTRODUCTION

Molecular imaging has been defined as the visualization, characterization and measurement of biological processes at the molecular and cellular level in humans and other living systems. In oncology it enables to visualize (part of) the functional behaviour of tumour cells, in contrast to anatomical imaging that focuses on the size and location of malignant lesions. Available molecular imaging techniques include single photon emission computed tomography (SPECT), positron emission tomography (PET) and optical imaging. In PET, a radiotracer consisting of a positron emitting radionuclide attached to the biologically active molecule of interest is administrated to the patient. Several approaches have been undertaken to use PET for the improvement of personalized cancer care. For example, a variety of radiolabelled ligands have been investigated for intratumoural target identification and radiolabelled drugs have been developed for direct visualization of the biodistibution in vivo, including intratumoural therapy uptake. First indications of the clinical value of PET for target identification and response prediction in oncology have been reported. This new imaging approach is rapidly developing, but uniformity of scanning processes, standardized methods for outcome evaluation and implementation in daily clinical practice are still in progress. In this review we discuss the available literature on molecular imaging with PET for personalized targeted treatment strategies.

CONCLUSION

Molecular imaging with radiolabelled targeted anticancer drugs has great potential for the improvement of personalized cancer care. The non-invasive quantification of drug accumulation in tumours and normal tissues provides understanding of the biodistribution in relation to therapeutic and toxic effects.

Spirocyclic hypervalent iodine(III)-mediated radiofluorination of non-activated and hindered aromatics

Fluorine-18 (t½=109.7 min) is the most commonly used isotope to prepare radiopharmaceuticals for molecular imaging by positron emission tomography (PET). Nucleophilic aromatic substitution reactions of suitably activated (electron-deficient) aromatic substrates with no-carrier-added [(18)F]fluoride ion are routinely carried out in the synthesis of radiotracers in high specific activities. Despite extensive efforts to develop a general (18)F-labelling technique for non-activated arenes there is an urgent and unmet need to achieve this goal. Here we describe an effective solution that relies on the chemistry of spirocyclic hypervalent iodine(III) complexes, which serve as precursors for rapid, one-step regioselective radiofluorination with [(18)F]fluoride. This methodology proves to be efficient for radiolabelling a diverse range of non-activated functionalized arenes and heteroarenes, including arene substrates bearing electron-donating groups, bulky ortho functionalities, benzylic substituents and meta-substituted electron-withdrawing groups. Polyfunctional molecules and a range of previously elusive (18)F-labelled building blocks, compounds and radiopharmaceuticals are synthesized.

Radiofluorination of diaryliodonium tosylates under aqueous-organic and cryptand-free conditions

Positron emission tomography (PET) has growing importance as a molecular imaging technique in clinical research and drug development. Methods for producing PET radiotracers utilizing cyclotron-produced [(18)F]fluoride ion (t1/2 = 109.7 min) without the need for complete removal of irradiated target [(18)O]water and addition of cryptand are keenly sought for practical convenience and efficiency. Several structurally diverse diaryliodonium tosylates, XArI(+)Ar'Y TsO(-) (X = H or p-MeO), were investigated in a microfluidic apparatus for their reactivity towards radiofluorination with high specific activity (no-carrier-added) [(18)F]fluoride ion in mixtures of DMF and irradiated target [(18)O]water in the absence of cryptand. Salts bearing a para or ortho electron-withdrawing group Y (e.g., Y = p-CN) reacted rapidly (∼3 min) to give the expected major [(18)F]fluoroarene product, [(18)F]FArY, in useful moderate radiochemical yields even when the solvent had an [(18)O]water content up to 28%. Salts bearing electron-withdrawing groups in meta position (e.g., Y = m-NO2), or an electron-donating substituent (Y = p-OMe), gave low radiochemical yields under the same conditions.

[¹¹C]Sorafenib: radiosynthesis and preclinical evaluation in tumor-bearing mice of a new TKI-PET tracer

INTRODUCTION

Tyrosine kinase inhibitors (TKIs) like sorafenib are important anticancer therapeutics with thus far limited treatment response rates in cancer patients. Positron emission tomography (PET) could provide the means for selection of patients who might benefit from TKI treatment, if suitable PET tracers would be available. The aim of this study was to radiolabel sorafenib (1) with carbon-11 and to evaluate its potential as TKI-PET tracer in vivo.

METHODS

Synthetic methods were developed in which sorafenib was labeled at two different positions, followed by a metabolite analysis in rats and a PET imaging study in tumor-bearing mice.

RESULTS

[methyl-(11)C]-1 and [urea-(11)C]-1 were synthesized in yields of 59% and 53%, respectively, with a purity of >99%. The identity of the products was confirmed by coinjection on HPLC with reference sorafenib. In an in vivo metabolite analysis [(11)C]sorafenib proved to be stable. The percentage of intact product in blood-plasma after 45 min was 90% for [methyl-(11)C]-1 and 96% for [urea-(11)C]-1, respectively. Due to the more reliable synthesis, further research regarding PET imaging was performed with [methyl-(11)C]-1 in nude mice bearing FaDu (head and neck cancer), MDA-MB-231 (breast cancer) or RXF393 (renal cancer) xenografts. Highest tracer accumulation at a level of 2.52 ± 0.33%ID/g was observed in RXF393, a xenograft line extensively expressing the sorafenib target antigen Raf-1 as assessed by immunohistochemistry.

CONCLUSION

In conclusion, we have synthesized [(11)C]sorafenib as PET tracer, which is stable in vivo and has the capability to be used as PET tracer for imaging in tumor-bearing mice.

PET imaging with small-molecule tyrosine kinase inhibitors: TKI-PET

The discovery and increased understanding of tumor targets has led to the development and approval of 12 small molecule tyrosine kinase inhibitors (TKIs). Despite tremendous efforts in TKI development, treatment efficacies with these therapeutics are still too low and improvements require a personalized medicine approach. Positron emission tomography (PET) with radiolabeled TKIs (TKI-PET) is a tracking, quantification and imaging method, which provides a unique understanding of the behavior of these drugs in vivo and of the interaction with their target(s). In this article we provide an overview of tracer synthesis and development because each TKI requires a tailor made approach. Moreover, we describe current preclinical work and the first proof-of-principle clinical studies on the application of TKI-PET, illustrating the potential of this approach for improving therapy efficacy and personalized cancer treatment.

Regiospecific syntheses of functionalized diaryliodonium tosylates via [hydroxy(tosyloxy)iodo]arenes generated in situ from (diacetoxyiodo)arenes

Ready access to (18)F-labeled aryl synthons is required for preparing novel radiotracers for molecular imaging with positron emission tomography. Diaryliodonium salts react with cyclotron-produced no-carrier-added [(18)F]fluoride ion to produce [(18)F]aryl fluorides. We aimed to prepare functionalized diaryliodonium salts to serve as potential precursors for producing useful (18)F-labeled aryl synthons, such as (18)F-labeled halomethylbenzenes, benzaldehydes, and benzoic acid esters. Such salts were designed to have one functionalized aryl ring, one relatively electron-rich ring, such as 4-methoxyphenyl or 2-thienyl, and a nonfluorine containing weakly nucleophilic anion. Generation of a [hydroxy(tosyloxy)iodo]arene from a functionalized (diacetoxyiodo)arene in situ followed by treatment with an electron-rich arene, such as anisole or thiophene, or with a functionalized arylstannane gave expedient regiospecific access to a wide range of functionally diverse diaryliodonium tosylates in moderate to high yields (44-98%). The described methodology broadens the scope for producing new functionalized diaryliodonium salts for diverse applications.

PET imaging with radiolabeled antibodies and tyrosine kinase inhibitors: immuno-PET and TKI-PET

During the last decade, the discovery of critical tumor targets has boosted the design of targeted therapeutic agents with monoclonal antibodies (mAbs) and tyrosine kinase inhibitors (TKIs) receiving most of the attention. Immuno-positron emission tomography (immuno-PET) and TKI-PET, the in vivo tracking and quantification of mAbs and TKIs biodistribution with PET, are exciting novel options for better understanding of the in vivo behavior and efficacy of these targeted drugs in individual patients and for more efficient drug development. Very recently, current good manufacturing practice compliant procedures for labeling of mAbs with positron emitters have been described, as well as the preparation of some radiolabeled TKIs, while the first proof of principle studies has been performed in patients. In this review, technical developments in immuno-PET and TKI-PET are described, and their clinical potential is discussed. An overview is provided for the most appealing preclinical immuno-PET and TKI-PET studies, as well as the first clinical achievements with these emerging technologies.