Imatinib analogs as potential agents for PET imaging of Bcr-Abl and c-KIT expression at a kinase level

Synthesis and Evaluation of a 18F-Labeled Ligand for PET Imaging of Colony-Stimulating Factor 1 Receptor

Neuroinflammation involves activation of glial cells in the brain, and activated microglia play a particularly important role in neurodegenerative diseases such as Alzheimer’s disease (AD). In this study, we developed 5-cyano-N-(4-(4-(2-[¹⁸F]fluoroethyl)piperazin-1-yl)-2-(piperidin-1-yl)phenyl)furan-2-carboxamide ([¹⁸F]1) for PET imaging of colony-stimulating factor 1 receptor (CSF1R), an emerging target for neuroinflammation imaging. Non-radioactive ligand 1 exhibited binding affinity comparable to that of a known CSF1R inhibitor, 5-cyano-N-(4-(4-methylpiperazin-1-yl)-2-(piperidin-1-yl)phenyl)furan-2-carboxamide (CPPC). Therefore, we synthesized radioligand [¹⁸F]1 by radiofluorination of chlorine-substituted precursor 7 in 13–15% decay-corrected radiochemical yield. Dynamic PET/CT images showed higher uptake in the lipopolysaccharide (LPS)-treated mouse brain than in control mouse brain. Ex vivo biodistribution study conducted at 45 min after radioligand injection showed that the brain uptake in LPS mice increased by 78% compared to that of control mice and was inhibited by 22% in LPS mice pretreated with CPPC, indicating specificity of [¹⁸F]1 for CSF1R. A metabolism study demonstrated that the radioligand underwent little metabolism in the mouse brain. Taken together, these results suggest that [¹⁸F]1 may hold promise as a radioligand for CSF1R imaging.

A perspective on the radiopharmaceutical requirements for imaging and therapy of glioblastoma

Despite numerous clinical trials and pre-clinical developments, the treatment of glioblastoma (GB) remains a challenge. The current survival rate of GB averages one year, even with an optimal standard of care. However, the future promises efficient patient-tailored treatments, including targeted radionuclide therapy (TRT). Advances in radiopharmaceutical development have unlocked the possibility to assess disease at the molecular level allowing individual diagnosis. This leads to the possibility of choosing a tailored, targeted approach for therapeutic modalities. Therapeutic modalities based on radiopharmaceuticals are an exciting development with great potential to promote a personalised approach to medicine. However, an effective targeted radionuclide therapy (TRT) for the treatment of GB entails caveats and requisites. This review provides an overview of existing nuclear imaging and TRT strategies for GB. A critical discussion of the optimal characteristics for new GB targeting therapeutic radiopharmaceuticals and clinical indications are provided. Considerations for target selection are discussed, i.e. specific presence of the target, expression level and pharmacological access to the target, with particular attention to blood-brain barrier crossing. An overview of the most promising radionuclides is given along with a validation of the relevant radiopharmaceuticals and theranostic agents (based on small molecules, peptides and monoclonal antibodies). Moreover, toxicity issues and safety pharmacology aspects will be presented, both in general and for the brain in particular.

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.

Applying Quality by Design Principles in the Development and Preparation of a New Radiopharmaceutical: Technetium-99m-Imatinib Mesylate

The clinical impact and accessibility of ^99mTc tracers for cancer diagnosis would be greatly enhanced by the availability of a new, simple, and easy labeling process and radiopharmaceuticals. In this study, Technetium-99m-imatinib mesylate ([^99mTc]TcIMT) was developed and prepared as a new radiopharmaceutical for breast cancer diagnosis. The effect of critical process parameters on the product quality and stability of [^99mTc]TcIMT was investigated using the quality by design concept of the ICH Q8 (Pharmaceutical Development) guideline. [^99mTc]TcIMT was subjected to in vitro cell binding studies to determine healthy and cancer cell affinity using HaCaT and MCF-7 cells, respectively. The optimal radiolabeling procedure with 1 mg of IMT, 500 μg of stannous chloride, 0.1 mg of ascorbic acid, and ^1mCi ^99mTc radioactivity was obtained for [^99mTc]TcIMT. The pH of the reaction mixture was adjusted to 10 and allowed to react for 15 min at room temperature. The radiochemical purity of [^99mTc]TcIMT was found to be higher than 90% at room temperature up to 6 h. Chromatography analysis revealed >85% [^99mTc]TcIMT complex formation with promising stability in saline, cell medium, and serum up to 6 h. The radiolabeled complex showed a higher cell-binding ratio to MCF-7 cells (88.90% ± 3.12) than HaCaT cells (45.64 ± 4.72) when compared to ^99mTc. Our findings show that the developed preparation method for [^99mTc]TcIMT falls well within the proven acceptable ranges. Applying quality by design (QbD) principles is feasible and worthwhile for the preparation of [^99mTc]TcIMT. In conclusion, radiochemical purity, stability, and in vitro cell binding evaluation of the [^99mTc]TcIMT complex indicate that the agent can be utilized for imaging of breast cancer cells.

PET Imaging of Receptor Tyrosine Kinases in Cancer

Overexpression and/or mutations of the receptor tyrosine kinase (RTK) subfamilies, such as epidermal growth factor receptors (EGFR) and vascular endothelial growth factor receptors (VEGFR), are closely associated with tumor cell growth, differentiation, proliferation, apoptosis, and cellular invasiveness. Monoclonal antibodies (mAb) and tyrosine kinase inhibitors (TKI) specifically inhibiting these RTKs have shown remarkable success in improving patient survival in many cancer types. However, poor response and even drug resistance inevitably occur. In this setting, the ability to detect and visualize RTKs with noninvasive diagnostic tools will greatly refine clinical treatment strategies for cancer patients, facilitate precise response prediction, and improve drug development. Positron emission tomography (PET) agents using targeted radioactively labeled antibodies have been developed to visualize tumor RTKs and are changing clinical decisions for certain cancer types. In the present review, we primarily focus on PET imaging of RTKs using radiolabeled antibodies with an emphasis on the clinical applications of these immunoPET probes. Mol Cancer Ther; 17(8); 1625-36. ©2018 AACR.

Novel derivatives of anaplastic lymphoma kinase inhibitors: Synthesis, radiolabeling, and preliminary biological studies of fluoroethyl analogues of crizotinib, alectinib, and ceritinib

Anaplastic lymphoma kinase (ALK), an oncogenic receptor tyrosine kinase, is a therapeutic target in various cancers, including non-small cell lung cancer. Although several ALK inhibitors, including crizotinib, ceritinib, and alectinib, are approved for cancer treatment, their long-term benefit is often limited by the cancer's acquisition of resistance owing to secondary point mutations in ALK. Importantly, some ALK inhibitors cannot cross the blood-brain barrier (BBB) and thus have little or no efficacy against brain metastases. The introduction of a lipophilic moiety, such as a fluoroethyl group may improve the drug's BBB penetration. Herein, we report the synthesis of fluoroethyl analogues of crizotinib 1, alectinib 4, and ceritinib 9, and their radiolabeling with ¹⁸F for pharmacokinetic studies. The fluoroethyl derivatives and their radioactive analogues were obtained in good yields with high purity and good molar activity. A cytotoxicity screen in ALK-expressing H2228 lung cancer cells showed that the analogues had up to nanomolar potency and the addition of the fluorinated moiety had minimal impact overall on the potency of the original drugs. Positron emission tomography in healthy mice showed that the analogues had enhanced BBB penetration, suggesting that they have therapeutic potential against central nervous system metastases.

Synthesis, in vitro and in vivo evaluation of 18F-fluoronorimatinib as radiotracer for Imatinib-sensitive gastrointestinal stromal tumors

INTRODUCTION

Gastrointestinal stromal tumors (GIST) have a wide range of mutations, but can mostly be treated with Imatinib, until eventually resistance towards this tyrosine kinase inhibitor is acquired. Early and non-invasive determination of the sensitivity of the tumor and its metastases towards Imatinib by positron emission tomography (PET) would be beneficial for therapy planning and monitoring.

METHODS

We developed a synthesis strategy towards the precursor molecule, performed the ¹⁸F-synthesis and in the following evaluated the radioligand in vitro regarding its lipophilicity, stability and biological activity (KIT binding properties) as well as its in vivo properties in GIST tumor-bearing mice.

RESULTS

[¹⁸F]fluoronorimatinib could be obtained in an overall radiochemical yield of 22.2±3.3% within 90min. The radioligand showed high GIST cell uptake and was able to distinguish between Imatinib-sensitive and resistant tumor cell lines (GIST-T1, GIST882, GIST430) in vitro. Further biological evaluations of the ligand towards 9 different GIST-relevant KIT mutations showed comparable binding affinities compared to the structural lead Norimatinib (65nM vs. 53nM for wt-KIT). The in vivo evaluation of the newly developed radioligand showed tumor-to-background-ratios comparable to previously described, similar radiotracers.

CONCLUSIONS

Thus, [¹⁸F]fluoronorimatinib is able to distinguish between Imatinib-resistant and sensitive KIT mutations. Although no improvement of in vivo tumor-to-background ratios could be achieved compared to formerly described radioligands, the hepatic uptake could be considerably reduced, being advantageous for the imaging of GIST. Advances in knowledge and implications for patient care: We were able to show that it is possible to significantly reduce the unfavorably high hepatic uptake of small-molecule radioligands applicable for GIST PET imaging. This work can thus be the basis for further work intending to develop a PET-radioligand for Imatinib-dependent GIST imaging.

Synthesis of a [(18)F]-labeled ceritinib analogue for positron emission tomography of anaplastic lymphoma kinase, a receptor tyrosine kinase, in lung cancer

Anaplastic lymphoma kinase (ALK), an oncogenic receptor tyrosine kinase, has emerged as a therapeutic target in solid and hematologic tumors. Although several ALK inhibitors have gained recent approval for therapy, non-invasive indicators of target engagement or for use as predictive biomarkers in vivo are lacking. Therefore, we designed and synthesized a radiolabeled analogue of the ALK inhibitor ceritinib, [(18)F]fluoroethyl-ceritinib ([(18)F]-FEC), for use with positron emission tomography. We used two methods to synthesize [(18)F]-FEC. First, [(18)F]fluoroethyl-tosylate was prepared, coupled with ceritinib, and the product purified to yield [(18)F]-FEC. Alternatively, a precursor compound was synthesized, directly fluorinated with (18)F-fluoride, and purified to yield [(18)F]-FEC. The first method produced [(18)F]-FEC with an average decay-corrected yield of 24% (n = 4), specific activity of 1200 mCi/µmol, and >99% purity; synthesis time was 115 min from the end of bombardment. The second method produced [(18)F]-FEC with an average yield of 7% (n = 4), specific activity of 1500 mCi/µmol, and >99% purity; synthesis time was 65 min from the end of bombardment. The synthesis of a novel (18)F-labeled analogue of ceritinib has been achieved in acceptable yields, at high purity, and with high specific activity. The compound is a potential positron emission tomography imaging agent for the detection of ALK-overexpressing solid tumors such as lung cancer.

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.

Theragnostic imaging using radiolabeled antibodies and tyrosine kinase inhibitors

During the past decade, the efficacy of new molecular targeted drugs such as tyrosine kinase inhibitors (TKIs) and monoclonal antibodies has been proven worldwide, and molecular targeted therapies have become the mainstream in cancer therapy. However, clinical use of these new drugs presents unexpected adverse effects or poor therapeutic effects. Therefore, we require diagnostic tools to estimate the target molecule status in cancer tissues and predict therapeutic efficacy and adverse effects. Although immunohistochemical, polymerase chain reaction (PCR) and fluorescence in situ hybridization (FISH) analyses of biopsy samples are conventional and popular for this diagnostic purpose, molecular imaging modalities such as positron emission tomography (PET) and single photon emission computed tomography (SPECT) are also useful for noninvasive estimation of gene and protein expression and drug pharmacokinetics. In this review, we introduce new radiolabeled TKIs, antibodies, and their clinical application in molecular targeted therapy and discuss the issues of these imaging probes.