Molecular imaging of c-Met tyrosine kinase activity

Pharmacodynamics of Akt drugs revealed by a kinase-modulated bioluminescent indicator

Measuring pharmacodynamics (PD)-the biochemical effects of drug dosing-and correlating them with therapeutic efficacy in animal models is crucial for the development of effective drugs but traditional PD studies are labor and resource intensive. Here we developed a kinase-modulated bioluminescent indicator (KiMBI) for rapid, noninvasive PD assessment of Akt-targeted drugs, minimizing drug and animal use. Using KiMBI, we performed a structure-PD relationship analysis on the brain-active Akt inhibitor ipatasertib by generating and characterizing two novel analogs. One analog, ML-B01, successfully inhibited Akt in both the brain and the body. Interestingly, capivasertib, ipatasertib and ML-B01 all exhibited PD durations beyond their pharmacokinetic profiles. Furthermore, KiMBI revealed that the PD effects of an Akt-targeted proteolysis-targeting chimera degrader endured for over 3 days. Thus, bioluminescence imaging with Akt KiMBI provides a noninvasive and efficient method for in vivo visualization of the PD of Akt inhibitors and degraders.

Kinase-Modulated Bioluminescent Indicators Enable Noninvasive Imaging of Drug Activity in the Brain

Aberrant kinase activity contributes to the pathogenesis of brain cancers, neurodegeneration, and neuropsychiatric diseases, but identifying kinase inhibitors that function in the brain is challenging. Drug levels in blood do not predict efficacy in the brain because the blood-brain barrier prevents entry of most compounds. Rather, assessing kinase inhibition in the brain requires tissue dissection and biochemical analysis, a time-consuming and resource-intensive process. Here, we report kinase-modulated bioluminescent indicators (KiMBIs) for noninvasive longitudinal imaging of drug activity in the brain based on a recently optimized luciferase-luciferin system. We develop an ERK KiMBI to report inhibitors of the Ras-Raf-MEK-ERK pathway, for which no bioluminescent indicators previously existed. ERK KiMBI discriminates between brain-penetrant and nonpenetrant MEK inhibitors, reveals blood-tumor barrier leakiness in xenograft models, and reports MEK inhibitor pharmacodynamics in native brain tissues and intracranial xenografts. Finally, we use ERK KiMBI to screen ERK inhibitors for brain efficacy, identifying temuterkib as a promising brain-active ERK inhibitor, a result not predicted from chemical characteristics alone. Thus, KiMBIs enable the rapid identification and pharmacodynamic characterization of kinase inhibitors suitable for treating brain diseases.

Visualization of Diagnostic and Therapeutic Targets in Glioma With Molecular Imaging

Gliomas, particularly high-grade gliomas including glioblastoma (GBM), represent the most common and malignant types of primary brain cancer in adults, and carry a poor prognosis. GBM has been classified into distinct subgroups over the years based on cellular morphology, clinical characteristics, biomarkers, and neuroimaging findings. Based on these classifications, differences in therapeutic response and patient outcomes have been established. Recently, the identification of complex molecular signatures of GBM has led to the development of diverse targeted therapeutic regimens and translation into multiple clinical trials. Chemical-, peptide-, antibody-, and nanoparticle-based probes have been designed to target specific molecules in gliomas and then be visualized with multimodality molecular imaging (MI) techniques including positron emission tomography (PET), single-photon emission computed tomography (SPECT), near-infrared fluorescence (NIRF), bioluminescence imaging (BLI), and magnetic resonance imaging (MRI). Thus, multiple molecules of interest can now be noninvasively imaged to guide targeted therapies with a potential survival benefit. Here, we review developments in molecular-targeted diagnosis and therapy in glioma, MI of these targets, and MI monitoring of treatment response, with a focus on the biological mechanisms of these advanced molecular probes. MI probes have the potential to noninvasively demonstrate the pathophysiologic features of glioma for diagnostic, treatment, and response assessment considerations for various targeted therapies, including immunotherapy. However, most MI tracers are in preclinical development, with only integrin αVβ3 and isocitrate dehydrogenase (IDH)-mutant MI tracers having been translated to patients. Expanded international collaborations would accelerate translational research in the field of glioma MI.

Quantitative and Dynamic Imaging of ATM Kinase Activity by Bioluminescence Imaging

Ataxia telangiectasia mutated (ATM) is a serine/threonine kinase critical to the cellular DNA damage response, including DNA double strand breaks (DSBs). ATM activation results in the initiation of a complex cascade of events facilitating DNA damage repair, cell cycle checkpoint control, and survival. Traditionally, protein kinases have been analyzed in vitro using biochemical methods (kinase assays using purified proteins or immunological assays) requiring a large number of cells and cell lysis. Genetically encoded biosensors based on optical molecular imaging such as fluorescence or bioluminescence have been developed to enable interrogation of kinase activities in live cells with a high signal to background. We have genetically engineered a hybrid protein whose bioluminescent activity is dependent on the ATM-mediated phosphorylation of a substrate. The engineered protein consists of the split luciferase-based protein complementation pair with a CHK2 (a substrate for ATM kinase activity) target sequence and a phospho-serine/threonine-binding domain, FHA2, derived from yeast Rad53. Phosphorylation of the serine residue within the target sequence by ATM would lead to its interaction with the phospho-serine-binding domain, thereby preventing complementation of the split luciferase pair and loss of reporter activity. Bioluminescence imaging of reporter-expressing cells in cultured plates or as mouse xenografts provides a quantitative surrogate for ATM kinase activity and therefore the cellular DNA damage response in a noninvasive, dynamic fashion.

Analysis of progress and challenges for various patterns of c-MET-targeted molecular imaging: a systematic review

BACKGROUND

Mesenchymal-epithelial transition factor also named c-MET is a receptor tyrosine kinase for the hepatocyte growth factor that plays a pivotal role in tumorigenesis. c-MET-targeted therapies have been tested in preclinical models and patients, with significant benefits for cancer treatment. In recent years, many studies have shown that the expression level and activation status of c-MET are closely correlated to c-MET-targeted therapy response and clinical prognosis, thus highlighting the importance of evaluating the c-MET status during and prior to targeted therapy. Molecular imaging allows the monitoring of abnormal alterations of c-MET in real time and in vivo.

RESULTS

In this review, we initially summarize the recent advances in c-MET-targeted molecular imaging, with a special focus on the development of imaging agents ranging in size from monoclonal antibody to small molecule. The aim of this review is to report the preclinical results and clinical application of all molecular imaging studies completed until now for in vivo detection of c-MET in cancer, in order to be beneficial to development of molecular probe and the combination of molecular imaging technologies for in vivo evaluation of c-MET. Various molecular probe targeted to c-MET possesses distinctive advantages and disadvantages. For example, antibody-based probes have high binding affinity but with long metabolic cycle as well as remarkable immunogenicity.

CONCLUSIONS

Although studies for c-MET-targeted molecular imaging have made many important advances, most of imaging agents specifically target to extracellular area of c-MET receptor; however, it is difficult to reflect entirely activation of c-MET. Therefore, small molecule probes based on tyrosine kinase inhibitors, which could target to intracellular area of c-MET without any immunogenicity, should be paid more attention.

Design, synthesis and molecular modeling studies of new series of antitumor 1,2,4-triazines with potential c-Met kinase inhibitory activity

The receptor tyrosine kinase c-Met is an attractive target for therapeutic treatment of cancers nowadays. Herein we describe the design and synthesis of a novel series of 1,2,4-triazine derivatives based on our lead NCI 748494/1, possessing different N-linkers to aromatic and heterocyclic rings. In addition, a molecular hybrid series combining the 1,2,4-triazine scaffold to the well-known anticancer drug 6-mercaptopurine (6-MP) was synthesized in order to explore its "double-drug" antitumor effect. The synthesized compounds were evaluated for their in vitro antitumor activity against three c-Met addicted cancer cell lines (A549, HT-29 and MKN-45). Most compounds showed moderate to excellent antitumor activity. Compound 3d showed potent inhibitory activity more than reference Foretinib, BMS-777607 and NCI 748494/1 with IC₅₀ values in the range 0.01-0.31 µM against the cancer cell lines. The calculated IC₅₀ of 3d against c-Met kinase was found to be 2.71 µM, which is more potent than NCI 748494/1 (IC₅₀ = 31.70 µM). Docking studies were performed to identify the binding mode of 3d with c-Met kinase domain in comparison to moderate and weak derivatives. The present study clearly demonstrates that 1,2,4-triazine ring exhibits promising antitumor activity and the double-drug optimization strategy led to identifying 3d as a potent c-Met kinase inhibitor suitable for further development.

Quantitative and Dynamic Imaging of ATM Kinase Activity

Ataxia telangiectasia mutated (ATM) is a serine/threonine kinase critical to the cellular DNA-damage response, including DNA double-strand breaks (DSBs). ATM activation results in the initiation of a complex cascade of events facilitating DNA damage repair, cell cycle checkpoint control, and survival. Traditionally, protein kinases have been analyzed in vitro using biochemical methods (kinase assays using purified proteins or immunological assays) requiring a large number of cells and cell lysis. Genetically encoded biosensors based on optical molecular imaging such as fluorescence or bioluminescence have been developed to enable interrogation of kinase activities in live cells with a high signal to background. We have genetically engineered a hybrid protein whose bioluminescent activity is dependent on the ATM-mediated phosphorylation of a substrate. The engineered protein consists of the split luciferase-based protein complementation pair with a CHK2 (a substrate for ATM kinase activity) target sequence and a phospho-serine/threonine-binding domain, FHA2, derived from yeast Rad53. Phosphorylation of the serine residue within the target sequence by ATM would lead to its interaction with the phospho-serine-binding domain, thereby preventing complementation of the split luciferase pair and loss of reporter activity. Bioluminescence imaging of reporter expressing cells in cultured plates or as mouse xenografts provides a quantitative surrogate for ATM kinase activity and therefore the cellular DNA damage response in a noninvasive, dynamic fashion.

Novel Bioluminescent Activatable Reporter for Src Tyrosine Kinase Activity in Living Mice

Aberrant activation of the Src kinase is implicated in the development of a variety of human malignancies. However, it is almost impossible to monitor Src activity in an in vivo setting with current biochemical techniques. To facilitate the noninvasive investigation of the activity of Src kinase both in vitro and in vivo, we developed a genetically engineered, activatable bioluminescent reporter using split-luciferase complementation. The bioluminescence of this reporter can be used as a surrogate for Src activity in real time. This hybrid luciferase reporter was constructed by sandwiching a Src-dependent conformationally responsive unit (SH2 domain-Srcpep) between the split luciferase fragments. The complementation bioluminescence of this reporter was dependent on the Src activity status. In our study, Src kinase activity in cultured cells and tumor xenografts was monitored quantitatively and dynamically in response to clinical small-molecular kinase inhibitors, dasatinib and saracatinib. This system was also applied for high-throughput screening of Src inhibitors against a kinase inhibitor library in living cells. These results provide unique insights into drug development and pharmacokinetics/phoarmocodynamics of therapeutic drugs targeting Src signaling pathway enabling the optimization of drug administration schedules for maximum benefit. Using both Firefly and Renilla luciferase imaging, we have successfully monitored Src tyrosine kinase activity and Akt serine/threonine kinase activity concurrently in one tumor xenograft. This dual luciferase reporter imaging system will be helpful in exploring the complex signaling networks in vivo. The strategies reported here can also be extended to study and image other important kinases and the cross-talks among them.

Mathematical Model of the Firefly Luciferase Complementation Assay Reveals a Non-Linear Relationship between the Detected Luminescence and the Affinity of the Protein Pair Being Analyzed

The firefly luciferase complementation assay is widely used as a bioluminescent reporter technology to detect protein-protein interactions in vitro, in cellulo, and in vivo. Upon the interaction of a protein pair, complemented firefly luciferase emits light through the adenylation and oxidation of its substrate, luciferin. Although it has been suggested that kinetics of light production in the firefly luciferase complementation assay is different from that in full length luciferase, the mechanism behind this is still not understood. To quantitatively understand the different kinetics and how changes in affinity of a protein pair affect the light emission in the assay, a mathematical model of the in vitro firefly luciferase complementation assay was constructed. Analysis of the model finds that the change in kinetics is caused by rapid dissociation of the protein pair, low adenylation rate of luciferin, and increased affinity of adenylated luciferin to the enzyme. The model suggests that the affinity of the protein pair has an exponential relationship with the light detected in the assay. This relationship causes the change of affinity in a protein pair to be underestimated. This study underlines the importance of understanding the molecular mechanism of the firefly luciferase complementation assay in order to analyze protein pair affinities quantitatively.

Proliferation of colorectal cancer is promoted by two signaling transduction expression patterns: ErbB2/ErbB3/AKT and MET/ErbB3/MAPK

One of the recent breakthroughs in cancer research is the identification of activating mutations in various receptor tyrosine kinase(RTK) pathways in many cancers including colorectal cancer(CRC). We hypothesize that, alternative to mutations, overexpression of various oncogenic RTKs may also underpin CRC pathogenesis, and different RTK may couple with distinct downstream signaling pathways in different subtypes of human CRC. By immunohistochemistry, we show here that RTK members ErbB2, ErbB3 and c-Met were in deed differentially overexpressed in colorectal cancer patient samples leading to constitutive activation of RTK signaling pathways. Using ErbB2 specific inhibitor Lapatinib and c-Met specific inhibitor PHA-665752, we further demonstrated that this constitutive activation of RTK signaling is necessary for the survival of colorectal cancer cells. Furthermore, we show that RTK overexpression pattern dictates the use of downstream AKT and/or MAPK pathways. Our data are important additions to current oncogenic mutation models, and further explain the clinical variation in therapeutic responses of colorectal cancer. Our findings advocate for more personalized therapy tailored to individual patients based on their type of RTK expression in addition to their mutation status.

Interrogating tumor metabolism and tumor microenvironments using molecular positron emission tomography imaging. Theranostic approaches to improve therapeutics

Positron emission tomography (PET) is a noninvasive molecular imaging technology that is becoming increasingly important for the measurement of physiologic, biochemical, and pharmacological functions at cellular and molecular levels in patients with cancer. Formation, development, and aggressiveness of tumor involve a number of molecular pathways, including intrinsic tumor cell mutations and extrinsic interaction between tumor cells and the microenvironment. Currently, evaluation of these processes is mainly through biopsy, which is invasive and limited to the site of biopsy. Ongoing research on specific target molecules of the tumor and its microenvironment for PET imaging is showing great potential. To date, the use of PET for diagnosing local recurrence and metastatic sites of various cancers and evaluation of treatment response is mainly based on [(18)F]fluorodeoxyglucose ([(18)F]FDG), which measures glucose metabolism. However, [(18)F]FDG is not a target-specific PET tracer and does not give enough insight into tumor biology and/or its vulnerability to potential treatments. Hence, there is an increasing need for the development of selective biologic radiotracers that will yield specific biochemical information and allow for noninvasive molecular imaging. The possibility of cancer-associated targets for imaging will provide the opportunity to use PET for diagnosis and therapy response monitoring (theranostics) and thus personalized medicine. This article will focus on the review of non-[(18)F]FDG PET tracers for specific tumor biology processes and their preclinical and clinical applications.

Real-time SPR characterization of the interactions between multi-epitope proteins and antibodies against classical swine fever virus

The envelope glycoprotein E2 is the major immunodominant protein of the classical swine fever virus and can induce neutralizing antibodies and protective host-immune responses in infected swine. We designed, expressed, and purified multi-epitope protein (GST-BT22) that contains a tandem repeat of the E2 antigenic-determinant residues 693-704, 770-780, and 826-843, each of which is separated by a GGSSGG sequence. In the same manner, we also designed, expressed, and purified a second protein (GST-BT23) that contains a C-terminal sequence consisting of residues 1446-1460 from the classical swine fever virus nonstructural protein NS2-3 separated from the GST-BT22 sequence by a GGSSGG sequence. Western blotting of GST-BT22 and GST-BT23 with serum from a swine that had been experimentally infected with the virus showed that the proteins reacted with anti-serum, whereas GST did not. Surface plasmon resonance was used to quantify the affinities of GST-BT22 and GST-BT23 for serum antibodies (K(a) = 4.31 × 10(8) and 5.01 × 10(8), respectively). GST, used as a control, was reacted an order of magnitude less strongly than did GST-BT22 and GST-BT23. Surface plasmon resonance, therefore, appears to be a sensitive and precise method for epitope evaluation and can be used to characterize the immunogenicity of a recombinant protein.

Multimodality imaging of tumor and bone response in a mouse model of bony metastasis

Cancer drug development generally performs in vivo evaluation of treatment effects that have traditionally relied on detection of morphologic changes. The emergence of new targeted therapies, which may not result in gross morphologic changes, has spurred investigation into more specific imaging methods to quantify response, such as targeted fluorescent probes and bioluminescent cells. The present study investigated tissue response to docetaxel or zoledronic acid (ZA) in a mouse model of bony metastasis. Intratibial implantations of breast cancer cells (MDA-MB-231) were monitored throughout this study using several modalities: molecular resonance imaging (MRI) tumor volume and apparent diffusion coefficient (ADC), micro-computed tomography (µCT) bone volume, bioluminescence imaging (BLI) reporting cancer cell apoptosis, and fluorescence using Osteosense 800 and CatK 680-FAST. Docetaxel treatment resulted in tumor cell kill reflected by ADC and BLI increases and tumor volume reduction, with delayed bone recovery seen in µCT prefaced by increased osteoblastic activity (Osteosense 800). In contrast, the ZA treatment group produced similar values in MRI, BLI, and Osteosense 800 fluorescence imaging readouts when compared to controls. However, µCT bone volume increased significantly by the first week post-treatment and the CatK 680-FAST signal was slightly diminished by 4 weeks following ZA treatment. Multimodality imaging provides a more comprehensive tool for new drug evaluation and efficacy screening through identification of morphology as well as function and apoptotic signaling.

Real-time analysis of the interaction of a multiple-epitope peptide with antibodies against classical swine fever virus using surface plasmon resonance

The E2 envelope glycoprotein is the major immunodominant protein of classical swine fever virus (CSFV), and can induce neutralizing antibodies and protective immune responses in infected swine. We developed a tandem-repeat multiple-epitope recombinant protein that contains two copies of each of the regions of E2 spanned by residues 693-704, 770-780, and 826-843, coupled by two copies of the region spanned by residues 1446-1460 of the CSFV nonstructural protein NS2-3. The chemically synthesized gene was expressed in Escherichia coli as a fusion with glutathione S-8 (GST), named GST-BT21. After it was purified with Glutathione Sepharose 4B, we used Western blotting to characterize the construct and surface plasmon resonance to analyze its affinity and specific interaction with CSFV-positive serum. Purified GST-BT21 protein displayed excellent immunoreactivity with antiserum against CSFV (Tian et al., 2012), and surface plasmon resonance confirmed the specific affinity between BT21, but not GST, and antibodies in serum from animals infected with CSFV. Surface plasmon resonance is a sensitive and precise method for epitope evaluation, and it can be used to characterize the immunogenicity and functions of recombinant proteins.

Theranostic Imaging of the Kinases and Proteases that Modulate Cell Death and Survival

Several signaling cascades are involved in cell death, with a significant amount of crosstalk between them. Despite the complexity of these cascades several key pro-survival and pro-death players have been identified. These include PI3-kinase, AKT and caspase-3. Here we review the approaches used to date to perform molecular imaging of these important targets. We focus in particular on approaches that include the possibility of modulating the activity of these kinases and proteases in a theranostic approach.