Abstract 110: Preclinical evaluation of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[18F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide: A novel pyridazinoindole ligand for PET imaging of TSPO in cancer

There exists a continued need for the development of improved positron emission tomography (PET) biomarkers for glioma imaging to aid in tumor diagnosis, inform clinical outcome, and quantify response to therapeutic intervention. Translocator protein (TSPO) expression is elevated in cancer and has been linked with disease progression and diminished survival and tends to be a hallmark of aggressive tumors. Our laboratory has pursued development of TSPO PET ligands as candidates for molecular imaging of glioma, as TSPO ligand binding correlates with tumor grade in this setting.

We report the first exploration of the structure-activity relationship (SAR) around the pyridazinoindole ring and subsequent radiofluorination of a potent and novel pyridazinoindole-based TSPO-selective ligand for cancer imaging. Library and SAR development around the N3 position of the pyridazinoindole scaffold led to the discovery of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (VUIIS8310), a novel TSPO ligand exhibiting a binding affinity comparable to SSR180575, yet bearing a fluorine atom for subsequent radiolabeling with fluorine-18 (18F) to give 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[18F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (18F-VUIIS8310).

Initial production feasibility and radiochemical development were evaluated with microfluidics and subsequently translated to traditional box-based methods. Subsequently, quantitative, preclinical elevation of 18F-VUIIS8310 was conducted in a rat glioma model, given our previous experience in this setting. Glioma-bearing rats were imaged in a microPET system, with dynamic PET acquisitions acquired simultaneously upon tracer injection. PET ligand reversibility and specificity were evaluated by ligand displacement studies that utilized VUIIS8310. 18F-VUIIS8310 exhibited elevated uptake and specific, displaceable binding in tumor, in contrast with normal brain, which exhibited very low uptake. 18F-VUIIS8310 uptake in the tumor, relative to normal brain, reached a tumor-to-normal brain ratio of greater than 10:1. Ex vivo histological analysis of resected tissue correlated well with PET imaging data.

These preclinical studies illuminate 18F-VUIIS8310 as a promising, novel TSPO PET ligand for imaging glioma and potentially other solid tumors. Importantly, the in vivo stability and high signal-to-noise achieved between tumor and surrounding normal brain suggest the potential of this PET ligand for early tumor detection within this setting. Future head-to-head comparisons between this tracer and emerging, potent TSPO ligands (J. Med. Chem. 2013, 56, 3429) are underway.

Citation Format: Yiu-Yin Cheung, Jason R. Buck, Michael L. Nickels, Dewei Tang, H. Charles Manning. Preclinical evaluation of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[18F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide: A novel pyridazinoindole ligand for PET imaging of TSPO in cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 110. doi:10.1158/1538-7445.AM2014-110

3'-Deoxy-3'-[18F]-Fluorothymidine PET imaging reflects PI3K-mTOR-mediated pro-survival response to targeted therapy in colorectal cancer

Biomarkers that predict response to targeted therapy in oncology are an essential component of personalized medicine. In preclinical treatment response studies that featured models of wild-type KRAS or mutant BRAF colorectal cancer treated with either cetuximab or vemurafenib, respectively, we illustrate that [¹⁸F]-FLT PET, a non-invasive molecular imaging readout of thymidine salvage, closely reflects pro-survival responses to targeted therapy that are mediated by PI3K-mTOR activity. Activation of pro-survival mechanisms forms the basis of numerous modes of resistance. Therefore, we conclude that [¹⁸F]-FLT PET may serve a novel and potentially critical role to predict tumors that exhibit molecular features that tend to reflect recalcitrance to MAPK-targeted therapy. Though these studies focused on colorectal cancer, we envision that the results may be applicable to other solid tumors as well.

Facile synthesis of SSR180575 and discovery of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[(18)F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide, a potent pyridazinoindole ligand for PET imaging of TSPO in cancer

A novel synthesis of the translocator protein (TSPO) ligand 7-chloro-N,N,5-trimethyl-4-oxo-3-phenyl-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (SSR180575, 3) was achieved in four steps from commercially available starting materials. Focused structure-activity relationship development about the pyridazinoindole ring at the N3 position led to the discovery of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (14), a novel ligand of comparable affinity. Radiolabeling with fluorine-18 ((18)F) yielded 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[(18)F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide ([(18)F]-14) in high radiochemical yield and specific activity. In vivo studies of [(18)F]-14 revealed this agent as a promising probe for molecular imaging of glioma.

Inducible loss of one Apc allele in Lrig1-expressing progenitor cells results in multiple distal colonic tumors with features of familial adenomatous polyposis

Individuals with familial adenomatous polyposis (FAP) harbor a germline mutation in adenomatous polyposis coli (APC). The major clinical manifestation is development of multiple colonic tumors at a young age due to stochastic loss of the remaining APC allele. Extracolonic features, including periampullary tumors, gastric abnormalities, and congenital hypertrophy of the retinal pigment epithelium, may occur. The objective of this study was to develop a mouse model that simulates these features of FAP. We combined our Lrig1-CreERT2/+ mice with Apcfl/+ mice, eliminated one copy of Apc in leucine-rich repeats and immunoglobulin-like domains protein 1 (Lrig1)-positive (Lrig1(+)) progenitor cells with tamoxifen injection, and monitored tumor formation in the colon by colonoscopy and PET. Initial loss of one Apc allele in Lrig1(+) cells results in a predictable pattern of preneoplastic changes, culminating in multiple distal colonic tumors within 50 days of induction, as well as the extracolonic manifestations of FAP mentioned above. We show that tumor formation can be monitored by noninvasive PET imaging. This inducible stem cell-driven model recapitulates features of FAP and offers a tractable platform on which therapeutic interventions can be monitored over time by colonoscopy and noninvasive imaging.

Optical metabolic imaging identifies glycolytic levels, subtypes, and early-treatment response in breast cancer

Abnormal cellular metabolism is a hallmark of cancer, yet there is an absence of quantitative methods to dynamically image this powerful cellular function. Optical metabolic imaging (OMI) is a noninvasive, high-resolution, quantitative tool for monitoring cellular metabolism. OMI probes the fluorescence intensities and lifetimes of the autofluorescent metabolic coenzymes reduced NADH and flavin adenine dinucleotide. We confirm that OMI correlates with cellular glycolytic levels across a panel of human breast cell lines using standard assays of cellular rates of glucose uptake and lactate secretion (P < 0.05, r = 0.89). In addition, OMI resolves differences in the basal metabolic activity of untransformed from malignant breast cells (P < 0.05) and between breast cancer subtypes (P < 0.05), defined by estrogen receptor and/or HER2 expression or absence. In vivo OMI is sensitive to metabolic changes induced by inhibition of HER2 with the antibody trastuzumab (herceptin) in HER2-overexpressing human breast cancer xenografts in mice. This response was confirmed with tumor growth curves and stains for Ki67 and cleaved caspase-3. OMI resolved trastuzumab-induced changes in cellular metabolism in vivo as early as 48 hours posttreatment (P < 0.05), whereas fluorodeoxyglucose-positron emission tomography did not resolve any changes with trastuzumab up to 12 days posttreatment (P > 0.05). In addition, OMI resolved cellular subpopulations of differing response in vivo that are critical for investigating drug resistance mechanisms. Importantly, OMI endpoints remained unchanged with trastuzumab treatment in trastuzumab-resistant xenografts (P > 0.05). OMI has significant implications for rapid cellular-level assessment of metabolic response to molecular expression and drug action, which would greatly accelerate drug development studies.

A peptide-based positron emission tomography probe for in vivo detection of caspase activity in apoptotic cells

PURPOSE

Apoptosis, or programmed cell death, can be leveraged as a surrogate measure of response to therapeutic interventions in medicine. Cysteine aspartic acid-specific proteases, or caspases, are essential determinants of apoptosis signaling cascades and represent promising targets for molecular imaging. Here, we report development and in vivo validation of [(18)F]4-fluorobenzylcarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone ([(18)F]FB-VAD-FMK), a novel peptide-based molecular probe suitable for quantification of caspase activity in vivo using positron emission tomography (PET).

EXPERIMENTAL DESIGN

Supported by molecular modeling studies and subsequent in vitro assays suggesting probe feasibility, the labeled pan-caspase inhibitory peptide, [(18)F]FB-VAD-FMK, was produced in high radiochemical yield and purity using a simple two-step, radiofluorination. The biodistribution of [(18)F]FB-VAD-FMK in normal tissue and its efficacy to predict response to molecularly targeted therapy in tumors was evaluated using microPET imaging of mouse models of human colorectal cancer.

RESULTS

Accumulation of [(18)F]FB-VAD-FMK was found to agree with elevated caspase-3 activity in response to Aurora B kinase inhibition as well as a multidrug regimen that combined an inhibitor of mutant BRAF and a dual PI3K/mTOR inhibitor in (V600E)BRAF colon cancer. In the latter setting, [(18)F]FB-VAD-FMK PET was also elevated in the tumors of cohorts that exhibited reduction in size.

CONCLUSIONS

These studies illuminate [(18)F]FB-VAD-FMK as a promising PET imaging probe to detect apoptosis in tumors and as a novel, potentially translatable biomarker for predicting response to personalized medicine.

Microwave-assisted, one-pot reaction of 7-azaindoles and aldehydes: a facile route to novel di-7-azaindolylmethanes

A novel and highly efficient synthetic method leveraging microwave-assisted organic synthesis (MAOS) to yield di-7-azaindolylmethanes (DAIMs) is reported. Under MAOS conditions, reaction of 7-azaindole with aldehydes resulted predominantly in DAIMs, as opposed to the expected 7-azaindole addition products that form at ambient temperature. Based upon studies of different indoles and azaindoles with various aromatic and aliphatic aldehydes, we herein propose a mechanism where rapid and efficient microwave heating promotes nucleophilicity of 7-azaindoles towards the corresponding alkylidene-azaindolene intermediate to form the DAIM. This sequence provides a versatile approach to efficiently synthesize novel DAIMs that may be useful pharmaceuticals.

Aurora kinase A promotes inflammation and tumorigenesis in mice and human gastric neoplasia

BACKGROUND & AIMS

Chronic inflammation contributes to the pathogenesis of gastric tumorigenesis. The aurora kinase A (AURKA) gene is frequently amplified and overexpressed in gastrointestinal cancers. We investigated the roles of AURKA in inflammation and gastric tumorigenesis.

METHODS

We used quantitative real-time reverse transcription polymerase chain reaction, immunofluorescence, immunohistochemistry, luciferase reporter, immunoblot, co-immunoprecipitation, and in vitro kinase assays to analyze AGS and MKN28 gastric cancer cells. We also analyzed Tff1(-/-) mice, growth of tumor xenografts, and human tissues.

RESULTS

We correlated increased expression of AURKA with increased levels of tumor necrosis factor-α and inflammation in the gastric mucosa of Tff1(-/-) mice (r = 0.62; P = .0001). MLN8237, an investigational small-molecule selective inhibitor of AURKA, reduced nuclear staining of nuclear factor-κB (NF-κB) p65 in human gastric cancer samples and mouse epithelial cells, suppressed NF-κB reporter activity, and reduced expression of NF-κB target genes that regulate inflammation and cell survival. Inhibition of AURKA also reduced growth of xenograft tumors from human gastric cancer cells in mice and reversed the development of gastric tumors in Tff1(-/-) mice. AURKA was found to regulate NF-κB activity by binding directly and phosphorylating IκBα in cells. Premalignant and malignant lesions from the gastric mucosa of patients had increased levels of AURKA protein and nuclear NF-κB, compared with healthy gastric tissue.

CONCLUSIONS

In analyses of gastric cancer cell lines, human tissue samples, and mouse models, we found AURKA to be up-regulated during chronic inflammation to promote activation of NF-κB and tumorigenesis. AURKA inhibitors might be developed as therapeutic agents for gastric cancer.

Translocator protein 18 kDa (TSPO) is regulated in white and brown adipose tissue by obesity

Translocator protein 18 kDa (TSPO) is an outer-mitochondrial membrane transporter which has many functions including participation in the mitochondrial permeability transition pore, regulation of reactive oxygen species (ROS), production of cellular energy, and is the rate-limiting step in the uptake of cholesterol. TSPO expression is dysregulated during disease pathologies involving changes in tissue energy demands such as cancer, and is up-regulated in activated macrophages during the inflammatory response. Obesity is associated with decreased energy expenditure, mitochondrial dysfunction, and chronic low-grade inflammation which collectively contribute to the development of the Metabolic Syndrome. Therefore, we hypothesized that dysregulation of TSPO in adipose tissue may be a feature of disease pathology in obesity. Radioligand binding studies revealed a significant reduction in TSPO ligand binding sites in mitochondrial extracts from both white (WAT) and brown adipose tissue (BAT) in mouse models of obesity (diet-induced and genetic) compared to control animals. We also confirmed a reduction in TSPO gene expression in whole tissue extracts from WAT and BAT. Immunohistochemistry in WAT confirmed TSPO expression in adipocytes but also revealed high-levels of TSPO expression in WAT macrophages in obese animals. No changes in TSPO expression were observed in WAT or BAT after a 17 hour fast or 4 hour cold exposure. Treatment of mice with the TSPO ligand PK11195 resulted in regulation of metabolic genes in WAT. Together, these results suggest a potential role for TSPO in mediating adipose tissue homeostasis.

A novel reporter system for molecular imaging and high-throughput screening of anticancer drugs

Apoptosis is irreversible programmed cell death, characterized by a cellular cascade activation of caspase 3, which subsequently degrades proteins and other components of cells with a motif sequence. Here we report a novel reporter system to detect apoptosis, growth arrest, and cell death based on controlled and self-amplified protein degradation. The key element of the reporter system is an apoptotic sensor chimerical protein which consists of three components: procaspase 3, ubiquitin (Ub), and a strong consensus sequence of N-degron. Between each of these units is a DEVD (Asp-Glu-Val-Asp) sequence, which acts as the cleavage target of caspase 3. This non-conventional signal loss approach is much more sensitive than other native methods that are based on signal gain. The superior sensitivity is demonstrated by its effective application in 386-well high-throughput screening (HTS) with low drug concentrations and a short incubation time. The HTS selection process using this reporter system is very simple and economic. The simplicity eliminates potential errors introduced by multiple steps; there is no need for any substrate. Furthermore, the cells in the assay need not be disrupted, and the morphology of the cells can provide additional information on mechanisms. After HTS, the intact cells can also be used for other analytic analysis. This system thus has a potentially important role in the discovery and development of new anticancer drugs. It also appears to be very versatile, can be used both in vitro and in vivo with different linked reporter genes, and can be used for a variety of imaging applications.

Knockdown of aberrantly upregulated aryl hydrocarbon receptor reduces tumor growth and metastasis of MDA-MB-231 human breast cancer cell line

The aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor that belongs to the basic-helix-loop-helix (bHLH)-Per-ARNT-Sim (PAS) superfamily of transcription factors, mediates toxic response induced by environmental chemicals such as polycyclic aromatic hydrocarbons (PAH). AhR is expressed at high levels in several human breast carcinoma cell lines in direct correlation with the degree of their malignancy. Recent studies suggest a possible role for AhR in cancer independent of PAH. Therefore, we established stable AhR knockdown cells of the human breast cancer cell line MDA-MB-231 and analyzed their tumorigenic properties in in vitro and in vivo model systems. In addition we analyzed their response to radiation and chemotherapeutic treatment. AhR knockdown attenuated these cells tumorigenic properties in vitro including proliferation, anchorage independent growth, migration and apoptosis and reduced orthotopic xenograft tumor growth and lung metastasis in vivo. Notably, we observed that AhR knockdown enhanced radiation-induced apoptosis as well as significantly decreased cell clonogenic survival. Furthermore, AhR knockdown in MDA-MB-231 cells sensitized them to paclitaxel treatment, evident by a decrease in the required cytotoxic dose. Subsequent analysis revealed AhR knockdown significantly reduced phosphorylation of AKT, which impacts cell proliferation and survival. Apoptosis-focused gene expression analyses revealed an altered expression of genes regulating apoptosis in MDA-MB-231 cells. Collectively, our data identify AhR as a potential novel therapeutic target in the treatment of metastatic breast cancer.

Synthesis and structure-activity relationships of 5,6,7-substituted pyrazolopyrimidines: discovery of a novel TSPO PET ligand for cancer imaging

Focused library synthesis and structure-activity relationship development of 5,6,7-substituted pyrazolopyrimidines led to the discovery of 2-(5,7-diethyl-2-(4-(2-fluoroethoxy)phenyl)pyrazolo[1,5-a]pyrimidin-3-yl)-N,N-diethylacetamide (6b), a novel translocator protein (TSPO) ligand exhibiting a 36-fold enhancement in affinity compared to another pyrazolopyrimidine-based TSPO ligand, 6a (DPA-714). Radiolabeling with fluorine-18 ((18)F) facilitated production of 2-(5,7-diethyl-2-(4-(2-[(18)F]fluoroethoxy)phenyl)pyrazolo[1,5-a]pyrimidin-3-yl)-N,N-diethylacetamide ((18)F-6b) in high radiochemical yield and specific activity. In vivo studies of (18)F-6b were performed which illuminated this agent as an improved probe for molecular imaging of TSPO-expressing cancers.

Limits of [18F]-FLT PET as a biomarker of proliferation in oncology

Background

Non-invasive imaging biomarkers of cellular proliferation hold great promise for quantifying response to personalized medicine in oncology. An emerging approach to assess tumor proliferation utilizes the positron emission tomography (PET) tracer 3’-deoxy-3’[¹⁸F]-fluorothymidine, [¹⁸F]-FLT. Though several studies have associated serial changes in [¹⁸F]-FLT-PET with elements of therapeutic response, the degree to which [¹⁸F]-FLT-PET quantitatively reflects proliferative index has been continuously debated for more that a decade. The goal of this study was to elucidate quantitative relationships between [¹⁸F]-FLT-PET and cellular metrics of proliferation in treatment naïve human cell line xenografts commonly employed in cancer research.

Methods and Findings

[¹⁸F]-FLT-PET was conducted in human cancer xenograft-bearing mice. Quantitative relationships between PET, thymidine kinase 1 (TK1) protein levels and immunostaining for proliferation markers (Ki67, TK1, PCNA) were evaluated using imaging-matched tumor specimens. Overall, we determined that [¹⁸F]-FLT-PET reflects TK1 protein levels, yet the cell cycle specificity of TK1 expression and the extent to which tumors utilize thymidine salvage for DNA synthesis decouple [¹⁸F]-FLT-PET data from standard estimates of proliferative index.

Conclusions

Our findings illustrate that [¹⁸F]-FLT-PET reflects tumor proliferation as a function of thymidine salvage pathway utilization. Unlike more general proliferation markers, such as Ki67, [¹⁸F]-FLT PET reflects proliferative indices to variable and potentially unreliable extents. [¹⁸F]-FLT-PET cannot discriminate moderately proliferative, thymidine salvage-driven tumors from those of high proliferative index that rely primarily upon de novo thymidine synthesis. Accordingly, the magnitude of [¹⁸F]-FLT uptake should not be considered a surrogate of proliferative index. These data rationalize the diversity of [¹⁸F]-FLT-PET correlative results previously reported and suggest future best-practices when [¹⁸F]-FLT-PET is employed in oncology.

3'-Deoxy-3'-18F-fluorothymidine PET predicts response to (V600E)BRAF-targeted therapy in preclinical models of colorectal cancer

Selective inhibition of oncogenic targets and associated signaling pathways forms the basis of personalized cancer medicine. The clinical success of (V600E)BRAF inhibition in melanoma, coupled with the emergence of acquired resistance, underscores the importance of rigorously validating quantitative biomarkers of treatment response in this and similar settings. Because constitutive activation of BRAF leads to proliferation in tumors, we explored 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) PET to noninvasively quantify changes in tumor proliferation that are associated with pharmacologic inhibition of (V600E)BRAF downstream effectors and that precede changes in tumor volume.

METHODS

Human colorectal cancer (CRC) cell lines expressing (V600E)BRAF were used to explore relationships between upregulation of p27 and phosphorylation of BRAF downstream effectors on small-molecule (V600E)BRAF inhibitor exposure. Athymic nude mice bearing (V600E)BRAF-expressing human CRC cell line xenografts were treated with a small-molecule (V600E)BRAF inhibitor (or vehicle) daily for 10 d. Predictive (18)F-FLT PET was conducted before changes in tumor volume occurred. Correlations were evaluated among PET, inhibition of phosphorylated MEK (p-MEK) and phosphorylated-ERK (p-ERK) by Western blot, tumor proliferation by histology, and small-molecule exposure by matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry (IMS).

RESULTS

Treatment of CRC cell lines with PLX4720 reduced proliferation associated with target inhibition and upregulation of p27. In vivo, PLX4720 treatment reduced (18)F-FLT uptake, but not (18)F-FDG uptake, in Lim2405 xenografts before quantifiable differences in xenograft volume. Reduced (18)F-FLT PET reflected a modest, yet significant, reduction of Ki67 immunoreactivity, inhibition of p-MEK and p-ERK, and elevated tumor cell p27 protein levels. Both (18)F-FLT PET and (18)F-FDG PET accurately reflected a lack of response in HT-29 xenografts, which MALDI imaging mass spectrometry suggested may have stemmed from limited PLX4720 exposure.

CONCLUSION

We used preclinical models of CRC to demonstrate (18)F-FLT PET as a sensitive predictor of response to (V600E)BRAF inhibitors. Because (18)F-FLT PET predicted reduced proliferation associated with attenuation of BRAF downstream effectors, yet (18)F-FDG PET did not, these data suggest that (18)F-FLT PET may represent an alternative to (18)F-FDG PET for quantifying clinical responses to BRAF inhibitors.

Rapid, Microwave-Assisted Organic Synthesis of Selective (V600E)BRAF Inhibitors for Preclinical Cancer Research

We report a dramatically improved total synthesis of two highly selective (V600E)BRAF inhibitors, PLX4720 and PLX4032, that leverages microwave-assisted organic synthesis (MAOS). Compared with previously reported approaches, our novel MAOS method significantly reduces overall reaction time without compromising yield. In addition to providing a gram-scale route to these compounds for preclinical oncology research, we anticipate this approach could accelerate the synthesis of azaindoles in high-throughput, library-based formats.

Evaluation of CD44 and CD133 as cancer stem cell markers for colorectal cancer

Colorectal cancer (CRC) is a major cause of cancer-related mortality in the world. Recently, a number of studies have demonstrated that cancer stem cells (CSCs) present in colorectal cancer tissues, are responsible for resistance to conventional therapies. Therefore, effective recognition of CSCs is of great importance. In the present study, to explore the potential characterizations of CSCs by the expression of specific cell surface markers such as CD133 and CD44, we screened six CRC cell lines using western blotting, immunofluorescence and flow cytometry. SW620, one of the six cell lines analyzed, was sorted into four subpopulations by fluorescence activated cell sorting (FACS). The capability of colony formation, proliferation rate, apoptosis, drug resistance, as well as their migratory and invasion potential were detected. The results revealed that the combination of CD44 and CD133 correlates with the features of CSCs in SW620 cells. CD44 positive cells showed more robust colony formation, higher proliferation, less spontaneous apoptosis, a higher resistance to drug-induced cell death, and were enriched after drug treatment. Among CD44 positive SW620 cells, the CD133 negative subpopulation was more migratory and invasive, which means that CD44⁺CD133⁻ correlates with most of features proposed for CSCs. Overall, the data presented herein showed that CRCs have a wide range of expression for CD44 and CD133; it is unlikely the CSCs can be characterized by any single marker or the same set of markers for all colon cancer cells. For SW620 cells, the CSCs are likely represented by the CD44⁺CD133⁻ surface marker. This finding of CSC markers represented by one positive and one negative is in line with CSCs in other tumors, such as CD34⁺CD38⁻ for acute myeloid leukemia; CD44⁺CD24⁻ for breast and pancreatic tumors. The absence of surface molecule(s) on CSCs will make it even more difficult to track and target this group of minority cells.

Preclinical molecular imaging of the translocator protein (TSPO) in a metastases model based on breast cancer xenografts propagated in the murine brain

Previous studies have demonstrated the feasibility of translocator protein (TSPO) imaging to visualize and quantify human breast adenocarcinoma (MDA-MB-231) cells in vivo using a TSPO-targeted near-infrared (NIR) probe (NIR-conPK11195). This study aimed to extend the use of the TSPO-targeted probe to a more biologically relevant and clinically important tumor microenvironment as well as to assess our ability to longitudinally detect the presence and progression of breast cancer cells in the brain. The in vivo biodistribution and accumulation of NIR-conPK11195 and free (unconjugated) NIR dye were quantitatively evaluated in intracranial MDA-MB-231-bearing mice and non-tumor-bearing control mice longitudinally once a week from two to five weeks post-inoculation. The in vivo time-activity curves illustrate distinct clearance profiles for NIR-conPK11195 and free NIR dye, resulting in preferential accumulation of the TSPO-targeted probe in the intracranial tumor bearing hemisphere (TBH) with significant tumor contrast over normal muscle tissue (p < 0.005 at five weeks; p < 0.01 at four weeks). In addition, the TSPO-labeled TBHs demonstrated significant contrast over the TBHs of mice injected with free NIR dye (p < 0.001 at four and five weeks) as well as over the TSPO-labeled non-tumor-bearing hemispheres (NTBHs) of control mice (p < 0.005 at four and five weeks). Overall, TSPO-targeted molecular imaging appears useful for visualizing and quantifying breast cancer xenografts propagated in the murine brain and may assist in preclinical detection, diagnosis and monitoring of metastatic disease as well as drug discovery. Furthermore, these results indicate it should be possible to perform TSPO-imaging of breast cancer cells in the brain using radiolabeled TSPO-targeted agents, particularly in light of the fact that [11C]-labeled TSPO probes such as [11C]-PK 11195 have been successfully used to image gliomas in the clinic.

[18F]FLT-PET to predict pharmacodynamic and clinical response to cetuximab therapy in Ménétrier's disease

Molecular imaging biomarkers of proliferation hold great promise for quantifying response to personalized medicine. One such approach utilizes the positron emission tomography (PET) tracer 3'-deoxy-3'[18F]-fluorothymidine ([18F]FLT), an investigational agent whose uptake reflects thymidine salvage-dependent DNA synthesis. The goal of this study was to evaluate [18F]FLT-PET in the setting of Ménétrier's disease (MD), a rare, premalignant hyperproliferative disorder of the stomach treatable with cetuximab therapy. Over 15 months, a patient with confirmed MD underwent cetuximab therapy and was followed with sequential [18F]FLT-PET. For comparison to MD, an [18F]FLT-PET study was conducted in another patient to quantify uptake in a normal stomach. Prior to cetuximab therapy, stomach tissue in MD was easily visualized with [18F]FLT-PET, with pre-treatment uptake levels exceeding normal stomach uptake by approximately fourfold. Diminished [18F]FLT-PET in MD was observed following the initial and subsequent doses of cetuximab and correlated with clinical resolution of the disease. To our knowledge, this study reports the first clinical use of [18F]FLT-PET to assess proliferation in a premalignant disorder. We illustrate that the extent of MD involvement throughout the stomach could be easily visualized using [18F]FLT-PET, and that response to cetuximab could be followed quantitatively and non-invasively in sequential [18F]FLT-PET studies. Thus, [18F]FLT-PET appears to have potential to monitor response to treatment in this and potentially other hyperproliferative disorders.

Simultaneous PET-MRI in oncology: a solution looking for a problem?

With the recent development of integrated positron emission tomography-magnetic resonance imaging (PET-MRI) scanners, new possibilities for quantitative molecular imaging of cancer are realized. However, the practical advantages and potential clinical benefits of the ability to record PET and MRI data simultaneously must be balanced against the substantial costs and other requirements of such devices. In this review, we highlight several of the key areas where integrated PET-MRI measurements, obtained simultaneously, are anticipated to have a significant impact on clinical and/or research studies. These areas include the use of MR-based motion corrections and/or a priori anatomical information for improved reconstruction of PET data, improved arterial input function characterization for PET kinetic modeling, the use of dual-modality contrast agents, and patient comfort and practical convenience. For widespread acceptance, a compelling case could be made if the combination of quantitative MRI and specific PET biomarkers significantly improves our ability to assess tumor status and response to therapy, and some likely candidates are now emerging. We consider the relative advantages and disadvantages afforded by PET-MRI and summarize current opinions and evidence as to the likely value of PET-MRI in the management of cancer.

Quantitative preclinical imaging of TSPO expression in glioma using N,N-diethyl-2-(2-(4-(2-18F-fluoroethoxy)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide

There is a critical need to develop and rigorously validate molecular imaging biomarkers to aid diagnosis and characterization of primary brain tumors. Elevated expression of translocator protein (TSPO) has been shown to predict disease progression and aggressive, invasive behavior in a variety of solid tumors. Thus, noninvasive molecular imaging of TSPO expression could form the basis of a novel, predictive cancer imaging biomarker. In quantitative preclinical PET studies, we evaluated a high-affinity pyrazolopyrimidinyl-based TSPO imaging ligand, N,N-diethyl-2-(2-(4-(2-(18)F-fluoroethoxy)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide ((18)F-DPA-714), as a translational probe for quantification of TSPO levels in glioma.

METHODS

Glioma-bearing rats were imaged with (18)F-DPA-714 in a small-animal PET system. Dynamic images were acquired simultaneously on injection of (18)F-DPA-714 (130-200 MBq/0.2 mL). Blood was collected to derive the arterial input function (AIF), with high-performance liquid chromatography radiometabolite analysis performed on selected samples for AIF correction. Compartmental modeling was performed using the corrected AIF. Specific tumor cell binding of DPA-714 was evaluated by radioligand displacement of (3)H-PK 11195 with DPA-714 in vitro and displacement of (18)F-DPA-714 with an excess of DPA-714 in vivo. Immediately after imaging, tumor and healthy brain tissues were harvested for validation by Western blotting and immunohistochemistry.

RESULTS

(18)F-DPA-714 was found to preferentially accumulate in tumors, with modest uptake in the contralateral brain. Infusion with DPA-714 (10 mg/kg) displaced (18)F-DPA-714 binding by greater than 60% on average. Tumor uptake of (18)F-DPA-714 was similar to another high-affinity TSPO imaging ligand, (18)F-N-fluoroacetyl-N-(2,5-dimethoxybenzyl)-2-phenoxyaniline, and agreed with ex vivo assay of TSPO levels in tumor and healthy brain.

CONCLUSION

These studies illustrate the feasibility of using (18)F-DPA-714 for visualization of TSPO-expressing brain tumors. Importantly, (18)F-DPA-714 appears suitable for quantitative assay of tumor TSPO levels in vivo. Given the relationship between elevated TSPO levels and poor outcome in oncology, these studies suggest the potential of (18)F-DPA-714 PET to serve as a novel predictive cancer imaging modality.

Protein expression signatures for inhibition of epidermal growth factor receptor-mediated signaling

Analysis of cellular signaling networks typically involves targeted measurements of phosphorylated protein intermediates. However, phosphoproteomic analyses usually require affinity enrichment of phosphopeptides and can be complicated by artifactual changes in phosphorylation caused by uncontrolled preanalytical variables, particularly in the analysis of tissue specimens. We asked whether changes in protein expression, which are more stable and easily analyzed, could reflect network stimulation and inhibition. We employed this approach to analyze stimulation and inhibition of the epidermal growth factor receptor (EGFR) by EGF and selective EGFR inhibitors. Shotgun analysis of proteomes from proliferating A431 cells, EGF-stimulated cells, and cells co-treated with the EGFR inhibitors cetuximab or gefitinib identified groups of differentially expressed proteins. Comparisons of these protein groups identified 13 proteins whose EGF-induced expression changes were reversed by both EGFR inhibitors. Targeted multiple reaction monitoring analysis verified differential expression of 12 of these proteins, which comprise a candidate EGFR inhibition signature. We then tested these 12 proteins by multiple reaction monitoring analysis in three other models: 1) a comparison of DiFi (EGFR inhibitor-sensitive) and HCT116 (EGFR-insensitive) cell lines, 2) in formalin-fixed, paraffin-embedded mouse xenograft DiFi and HCT116 tumors, and 3) in tissue biopsies from a patient with the gastric hyperproliferative disorder Ménétrier's disease who was treated with cetuximab. Of the proteins in the candidate signature, a core group, including c-Jun, Jagged-1, and Claudin 4, were decreased by EGFR inhibitors in all three models. Although the goal of these studies was not to validate a clinically useful EGFR inhibition signature, the results confirm the hypothesis that clinically used EGFR inhibitors generate characteristic protein expression changes. This work further outlines a prototypical approach to derive and test protein expression signatures for drug action on signaling networks.

A kinome-wide screen identifies the insulin/IGF-I receptor pathway as a mechanism of escape from hormone dependence in breast cancer

Estrogen receptor α (ER)-positive breast cancers adapt to hormone deprivation and become resistant to antiestrogens. In this study, we sought to identify kinases essential for growth of ER(+) breast cancer cells resistant to long-term estrogen deprivation (LTED). A kinome-wide siRNA screen showed that the insulin receptor (InsR) is required for growth of MCF-7/LTED cells. Knockdown of InsR and/or insulin-like growth factor-I receptor (IGF-IR) inhibited growth of 3 of 4 LTED cell lines. Inhibition of InsR and IGF-IR with the dual tyrosine kinase inhibitor OSI-906 prevented the emergence of hormone-independent cells and tumors in vivo, inhibited parental and LTED cell growth and PI3K/AKT signaling, and suppressed growth of established MCF-7 xenografts in ovariectomized mice, whereas treatment with the neutralizing IGF-IR monoclonal antibody MAB391 was ineffective. Combined treatment with OSI-906 and the ER downregulator fulvestrant more effectively suppressed hormone-independent tumor growth than either drug alone. Finally, an insulin/IGF-I gene expression signature predicted recurrence-free survival in patients with ER(+) breast cancer treated with the antiestrogen tamoxifen. We conclude that therapeutic targeting of both InsR and IGF-IR should be more effective than targeting IGF-IR alone in abrogating resistance to endocrine therapy in breast cancer.

ERα-dependent E2F transcription can mediate resistance to estrogen deprivation in human breast cancer

Most estrogen receptor α (ER)-positive breast cancers initially respond to antiestrogens, but many eventually become estrogen-independent and recur. We identified an estrogen-independent role for ER and the CDK4/Rb/E2F transcriptional axis in the hormone-independent growth of breast cancer cells. ER downregulation with fulvestrant or small interfering RNA (siRNA) inhibited estrogen-independent growth. Chromatin immunoprecipitation identified ER genomic binding activity in estrogen-deprived cells and primary breast tumors treated with aromatase inhibitors. Gene expression profiling revealed an estrogen-independent, ER/E2F-directed transcriptional program. An E2F activation gene signature correlated with a lesser response to aromatase inhibitors in patients' tumors. siRNA screening showed that CDK4, an activator of E2F, is required for estrogen-independent cell growth. Long-term estrogen-deprived cells hyperactivate phosphatidylinositol 3-kinase (PI3K) independently of ER/E2F. Fulvestrant combined with the pan-PI3K inhibitor BKM120 induced regression of ER(+) xenografts. These data support further development of ER downregulators and CDK4 inhibitors, and their combination with PI3K inhibitors for treatment of antiestrogen-resistant breast cancers.

Magnetic resonance in the era of molecular imaging of cancer

Magnetic resonance imaging (MRI) has played an important role in the diagnosis and management of cancer since it was first developed, but other modalities also continue to advance and provide complementary information on the status of tumors. In the future, there will be a major continuing role for noninvasive imaging in order to obtain information on the location and extent of cancer, as well as assessments of tissue characteristics that can monitor and predict treatment response and guide patient management. Developments are currently being undertaken that aim to provide improved imaging methods for the detection and evaluation of tumors, for identifying important characteristics of tumors such as the expression levels of cell surface receptors that may dictate what types of therapy will be effective and for evaluating their response to treatments. Molecular imaging techniques based mainly on radionuclide imaging can depict numerous, specific, cellular and molecular markers of disease and have unique potential to address important clinical and research challenges. In this review, we consider what continuing and evolving roles will be played by MRI in this era of molecular imaging. We discuss some of the challenges for MRI of detecting imaging agents that report on molecular events, but highlight also the ability of MRI to assess other features such as cell density, blood flow and metabolism which are not specific hallmarks of cancer but which reflect molecular changes. We discuss the future role of MRI in cancer and describe the use of selected quantitative imaging techniques for characterizing tumors that can be translated to clinical applications, particularly in the context of evaluating novel treatments.

18FDG-PET predicts pharmacodynamic response to OSI-906, a dual IGF-1R/IR inhibitor, in preclinical mouse models of lung cancer

PURPOSE

To evaluate 2-deoxy-2-[(18)F]fluoro-d-glucose positron emission tomography imaging ((18)FDG-PET) as a predictive, noninvasive, pharmacodynamic (PD) biomarker of response following administration of a small-molecule insulin-like growth factor-1 receptor and insulin receptor (IGF-1R/IR) inhibitor, OSI-906.

EXPERIMENTAL DESIGN

In vitro uptake studies of (3)H-2-deoxy glucose following OSI-906 exposure were conducted evaluating correlation of dose with inhibition of IGF-1R/IR as well as markers of downstream pathways and glucose metabolism. Similarly, in vivo PD effects were evaluated in human tumor cell line xenografts propagated in athymic nude mice by (18)FDG-PET at 2, 4, and 24 hours following a single treatment of OSI-906 for the correlation of inhibition of receptor targets and downstream markers.

RESULTS

Uptake of (3)H-2-deoxy glucose and (18)FDG was significantly diminished following OSI-906 exposure in sensitive tumor cells and subcutaneous xenografts (NCI-H292) but not in an insensitive model lacking IGF-1R expression (NCI-H441). Diminished PD (18)FDG-PET, collected immediately following the initial treatment agreed with inhibition of pIGF-1R/pIR, reduced PI3K (phosphoinositide 3-kinase) and MAPK (mitogen activated protein kinase) pathway activity, and predicted tumor growth arrest as measured by high-resolution ultrasound imaging.

CONCLUSION

(18)FDG-PET seems to serve as a rapid, noninvasive PD marker of IGF-1R/IR inhibition following a single dose of OSI-906 and should be explored clinically as a predictive clinical biomarker in patients undergoing IGF-1R/IR-directed cancer therapy.

Multispectral fluorescence imaging to assess pH in biological specimens

Simple, quantitative assays to measure pH in tissue could improve the study of complicated biological processes and diseases such as cancer. We evaluated multispectral fluorescence imaging (MSFI) to quantify extracellular pH (pHe) in dye-perfused, surgically-resected tumor specimens with commercially available instrumentation. Utilizing a water-soluble organic dye with pH-dependent fluorescence emission (SNARF-4F), we used standard fluorimetry to quantitatively assess the emission properties of the dye as a function of pH. By conducting these studies within the spectroscopic constraints imposed by the appropriate imaging filter set supplied with the imaging system, we determined that correction of the fluorescence emission of deprotonated dye was necessary for accurate determination of pH due to suboptimal excitation. Subsequently, employing a fluorimetry-derived correction factor (CF), MSFI data sets of aqueous dye solutions and tissuelike phantoms could be spectrally unmixed to accurately quantify equilibrium concentrations of protonated (HA) and deprotonated (A-) dye and thus determine solution pH. Finally, we explored the feasibility of MSFI for high-resolution pHe mapping of human colorectal cancer cell-line xenografts. Data presented suggest that MSFI is suitable for quantitative determination of pHe in ex vivo dye-perfused tissue, potentially enabling measurement of pH across a variety of preclinical models of disease.

Molecular imaging metrics to evaluate response to preclinical therapeutic regimens

Molecular imaging comprises a range of techniques, spanning not only several imaging modalities but also many disease states and organ sites. While advances in new technology platforms have enabled a deeper understanding of the cellular and molecular basis of malignancy, reliable non-invasive imaging metrics remain an important tool for both diagnostics and patient management. Furthermore, the non- invasive nature of molecular imaging can overcome shortcomings associated with traditional biological approaches and provide valuable information relevant to patient care. Integration of information from multiple imaging techniques has the potential to provide a more comprehensive understanding of specific tumor characteristics, tumor status, and treatment response.

Quantitative, preclinical PET of translocator protein expression in glioma using 18F-N-fluoroacetyl-N-(2,5-dimethoxybenzyl)-2-phenoxyaniline

Translocator protein (TSPO), also referred to as peripheral benzodiazepine receptor (PBR), is a crucial 18-kDa outer mitochondrial membrane protein involved in numerous cellular functions, including the regulation of cholesterol metabolism, steroidogenesis, and apoptosis. Elevated expression of TSPO in oncology correlates with disease progression and poor survival, suggesting that molecular probes capable of assaying TSPO levels may have potential as cancer imaging biomarkers. In preclinical PET studies, we characterized a high-affinity aryloxyanilide-based TSPO imaging ligand, 18F-N-fluoroacetyl-N-(2,5-dimethoxybenzyl)-2-phenoxyaniline (18F-PBR06), as a candidate probe for the quantitative assessment of TSPO expression in glioma.

METHODS

Glioma-bearing rats were imaged with 18F-PBR06 in a small-animal PET system. Dynamic images were acquired simultaneously on injection of 18F-PBR06 (70-100 MBq/0.2 mL). Over the course of scanning, arterial blood was collected to derive the input function, with high-performance liquid chromatography radiometabolite analysis performed on selected samples for arterial input function correction. Compartmental modeling of the PET data was performed using the corrected arterial input function. Specific tumor cell binding of PBR06 was evaluated by radioligand displacement of 3H-PK 11195 with PBR06 in vitro and by displacement of 18F-PBR06 with excess PBR06 in vivo. Immediately after imaging, tumor tissue and adjacent healthy brain were harvested for assay of TSPO protein levels by Western blotting and immunohistochemistry.

RESULTS

18F-PBR06 was found to preferentially accumulate in tumors, with modest uptake in the contralateral brain, facilitating excellent contrast between tumor and adjacent tissue. Infusion with PBR06 (10 mg/kg) displaced 18F-PBR06 binding by approximately 75%. The accumulation of 18F-PBR06 in tumor tissues and adjacent brain agreed with the ex vivo assay of TSPO protein levels by Western blotting and quantitative immunohistochemistry.

CONCLUSION

These preclinical studies illustrate that 18F-PBR06 is a promising tracer for visualization of TSPO-expressing tumors. Importantly, the close correlation between 18F-PBR06 uptake and TSPO expression in tumors and normal tissues, coupled with the high degree of displaceable binding from both tumors and the normal brain, represents a significant improvement over other TSPO imaging ligands previously evaluated in glioma. These data suggest the potential of 18F-PBR06 to elucidate the role of TSPO in oncology, as well as its potential development as a cancer imaging biomarker.

Microwave-assisted Organic Synthesis of a High-affinity Pyrazolo-pyrimidinyl TSPO Ligand

We herein report a dramatically improved total synthesis of the high-affinity translocator protein (TSPO) ligand DPA-714, featuring microwave-assisted organic synthesis (MAOS). Compared with previously described approaches, our novel MAOS method dramatically reduces overall reaction time without adversely effecting reaction yields. We envision that the described MAOS protocol may be suitably applied to high-throughput, diversity-oriented synthesis of novel compounds based on the pyrazolo-pyrimidinyl scaffold. Such an approach could accelerate the development of focused libraries of novel TSPO ligands with potential for future development as molecular imaging and therapeutic agents.

A novel in vitro assay to assess phosphorylation of 3'-[(18)F]fluoro-3'-deoxythymidine

PURPOSE

3'-[(18)F]fluoro-3'-deoxythymidine ([(18)F]FLT) is phosphorylated by thymidine kinase 1 (TK-1), a cell cycle regulated enzyme. Appropriate use of [(18)F]FLT tracer requires validation of the TK-1 activity. Here, we report development of a novel phosphoryl-transfer assay to assess phosphorylation of [(18)F]FLT both in tumor cell lysates and tumor cells.

PROCEDURES

The intrinsic F-18 radioactivity was used to quantify both substrate and phosphorylated products using a rapid thin layer chromatography method. Phosphorylation kinetics of [(18)F]FLT in SW480 and DiFi tumor cell lysates and cellular uptake were measured.

RESULTS

The apparent Michaelis-Menten kinetic parameters for [(18)F]FLT are K(m) = 4.8 ± 0.3 μM and V(max) = 7.4 pmol min(-1) per 1 × 10(6) cells with ~2-fold higher TK-1 activity in DiFi versus SW480 lysates.

CONCLUSIONS

The apparent K (m) of [(18)F]FLT was comparable to the value reported with purified recombinant TK-1. The uptake of [(18)F]FLT by SW480 cells is inhibited by nitrobenzylthioinosine or dipyridamole indicating that uptake is mediated predominantly by the equilibrative nucleoside transporters in these tumor cells.

Volume of preclinical xenograft tumors is more accurately assessed by ultrasound imaging than manual caliper measurements

OBJECTIVE

The volume of subcutaneous xenograft tumors is an important metric of disease progression and response to therapy in preclinical drug development. Noninvasive imaging technologies suitable for measuring xenograft volume are increasingly available, yet manual calipers, which are susceptible to inaccuracy and bias, are routinely used. The goal of this study was to quantify and compare the accuracy, precision, and inter-rater variability of xenograft tumor volume assessment by caliper measurements and ultrasound imaging.

METHODS

Subcutaneous xenograft tumors derived from human colorectal cancer cell lines (DLD1 and SW620) were generated in athymic nude mice. Experienced independent reviewers segmented 3-dimensional ultrasound data sets and collected manual caliper measurements resulting in tumor volumes. Imaging- and caliper-derived volumes were compared with the tumor mass, the reference standard, determined after resection. Bias, precision, and inter-rater differences were estimated for each mouse among reviewers. Bootstrapping was used to estimate mean and confidence intervals of variance components, intraclass correlation coefficients (ICCs), and confidence intervals for each source of variation.

RESULTS

The average deviation from the true volume and inter-rater differences were significantly lower for ultrasound volumes compared with caliper volumes (P = .0005 and .001, respectively). Reviewer ICCs for ultrasound and caliper measurements were similarly low (1%), yet caliper volume variance was 1.3-fold higher than for ultrasound.

CONCLUSIONS

Ultrasound imaging more accurately, precisely, and reproducibly reflects xenograft tumor volume than caliper measurements. These data suggest that preclinical studies using the xenograft burden as a surrogate end point measured by ultrasound imaging require up to 30% fewer animals to reach statistical significance compared with analogous studies using caliper measurements.

Abstract 5244: Preclinical, real-time assessment of early pharmacodynamic effects for the IGF-1R/IR inhibitor OSI-906 by [18F]-FDG-PET

The insulin-like growth factor-1 receptor (IGF-1R) and insulin receptor (IR) are transmembrane receptor tyrosine kinases that serve as key regulators of the IGF signaling axis. Activation of IGF-1R and IR results in proliferative and antiapoptotic effects via the ERK and PI3K/AKT pathways. Activated IGF-1R and IR also directly affect glucose metabolism by decreasing gluconeogenesis, increasing glycogen formation and increasing translocation of GLUT transporters, thereby increasing overall glucose uptake in tissues. Dysregulated IGF-1R/IR signaling has been implicated in many human cancers and has been implicated in resistance to anticancer therapies including cytotoxic chemotherapy, hormonal agents and radiation therapy. IGF-1R/IR overexpression has been demonstrated in several tumor types including breast, prostate, lung, ovarian and colon cancers. Moreover, increased activation and overexpression of IGF-1R/IR is associated with an increased propensity for invasion and metastasis correlating with poor survival. Consequently, IGF-1R and IR have been identified as attractive targets for the generation of novel anticancer agents. OSI-906 is a potent and selective dual inhibitor of IGF-1R/IR and is currently in clinical development. The radiotracer [18F]-FDG is used clinically to stage disease or monitor treatment response by virtue of measuring glucose uptake as a surrogate for metabolic activity in tumors. Since IGF-1R and IR signaling directly affect glucose metabolism we evaluated the utility of [18F]-FDG-PET as a potential means to non-invasively monitor pharmacodynamic (PD) effects for OSI-906. In vitro the IGF-1R/IR positive cell line, H292, responded rapidly to OSI-906 treatment by significantly reducing uptake of 3H-glucose. Radiotracer uptake was inhibited in a dose dependent manner and occurred within minutes of drug treatment correlating with inhibition of the IGF-1R/IR pathways and other downstream signaling events. In vivo xenografts bearing the same cell line demonstrated significantly reduced uptake of [18F]-FDG at 2 and 4 hours post treatment which correlated with substantial (&gt;80%) target inhibition of both phospho-IGF1-R and phospho-IR in the tumor lysates. Western blot analysis of in vivo tumor lysates confirmed significant inhibition of phospho-AKT and other markers associated with altered glycolysis. In contrast, in the IGF-1R/IR-negative H441 xenograft model no significant change in uptake of [18F]-FDG was observed over the same time course. Our data indicate that [18F]-FDG-PET may serve as a rapid, non-invasive PD marker for OSI-906 in the clinical setting where accurate assessment of PD effects is often limited by the lack of readily accessible tumor samples.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5244.

Inhibition of mammalian target of rapamycin is required for optimal antitumor effect of HER2 inhibitors against HER2-overexpressing cancer cells

PURPOSE

A significant fraction of HER2-overexpressing breast cancers exhibit resistance to the HER2 antibody trastuzumab. Hyperactivity of the phosphatidylinositol 3-kinase (PI3K)/AKT pathway confers trastuzumab resistance, and mammalian target of rapamycin (mTOR) is a major downstream effector of PI3K/AKT. Therefore, we examined whether mTOR inhibitors synergize with trastuzumab.

EXPERIMENTAL DESIGN

Immunocompetent mice bearing HER2(+) mammary tumors were treated with trastuzumab, the mTOR inhibitor rapamycin, or the combination. Mice were imaged for tumor cell death using an optical Annexin-V probe and with [(18)F]FDG positron emission tomography. The signaling and growth effects of the mTOR inhibitor RAD001 on HER2(+) cells treated with trastuzumab or lapatinib were evaluated.

RESULTS

Treatment of mice with trastuzumab plus rapamycin was more effective than single-agent treatments, inducing complete regression of 26 of 26 tumors. The combination induced tumor cell death (Annexin-V binding) and inhibited FDG uptake. Rapamycin inhibited mTOR and tumor cell proliferation as determined by phosphorylated S6 and Ki-67 immunohistochemistry, respectively. In culture, the combination of RAD001 plus trastuzumab inhibited cell growth more effectively than either drug alone. Trastuzumab partially decreased PI3K but not mTOR activity. Knockdown of TSC2 resulted in HER2-independent activation of mTOR and dampened the response to trastuzumab and lapatinib. Treatment with the HER2 inhibitor lapatinib decreased phosphorylated S6 and growth in TSC2-expressing cells but not in TSC2-knockdown cells.

CONCLUSIONS

Inhibition of PI3K and mTOR are required for the growth-inhibitory effect of HER2 antagonists. These findings collectively support the combined use of trastuzumab and mTOR inhibitors for the treatment of HER2(+) breast cancer.

Molecular imaging: 18F-FDG PET and a whole lot more

The intention of this review is to provide information about the rapidly evolving field of molecular imaging and its potential impact on the clinical practice of nuclear medicine. On completing this article the reader should be able to define molecular imaging, describe the ways in which molecular imaging can be used, identify some of the biologic processes that can be targeted with molecular imaging agents, and list the modalities that can be used for molecular imaging, along with the strengths and weaknesses of each.

Imaging biomarkers predict response to anti-HER2 (ErbB2) therapy in preclinical models of breast cancer

PURPOSE

To evaluate noninvasive imaging methods as predictive biomarkers of response to trastuzumab in mouse models of HER2-overexpressing breast cancer. The correlation between tumor regression and molecular imaging of apoptosis, glucose metabolism, and cellular proliferation was evaluated longitudinally in responding and nonresponding tumor-bearing cohorts.

EXPERIMENTAL DESIGN

Mammary tumors from MMTV/HER2 transgenic female mice were transplanted into syngeneic female mice. BT474 human breast carcinoma cell line xenografts were grown in athymic nude mice. Tumor cell apoptosis (NIR700-Annexin V accumulation), glucose metabolism [2-deoxy-2-[18F]fluoro-d-glucose positron emission tomography ([18F]FDG-PET)], and proliferation [3'-[18F]fluoro-3'-deoxythymidine-PET ([18F]FLT-PET)] were evaluated throughout a biweekly trastuzumab regimen. Imaging metrics were validated by direct measurement of tumor size and immunohistochemical analysis of cleaved caspase-3, phosphorylated AKT, and Ki67.

RESULTS

NIR700-Annexin V accumulated significantly in trastuzumab-treated MMTV/HER2 and BT474 tumors that ultimately regressed but not in nonresponding or vehicle-treated tumors. Uptake of [18F]FDG was not affected by trastuzumab treatment in MMTV/HER2 or BT474 tumors. [18F]FLT-PET imaging predicted trastuzumab response in BT474 tumors but not in MMTV/HER2 tumors, which exhibited modest uptake of [18F]FLT. Close agreement was observed between imaging metrics and immunohistochemical analysis.

CONCLUSIONS

Molecular imaging of apoptosis accurately predicts trastuzumab-induced regression of HER2+ tumors and may warrant clinical exploration to predict early response to neoadjuvant trastuzumab. Trastuzumab does not seem to alter glucose metabolism substantially enough to afford [18F]FDG-PET significant predictive value in this setting. Although promising in one preclinical model, further studies are required to determine the overall value of [18F]FLT-PET as a biomarker of response to trastuzumab in HER2+ breast cancer.

Multispectral molecular imaging of capillary endothelium to facilitate preoperative endovascular brain mapping

OBJECT

Brain mapping aims to localize neurological function to specific regions of the human brain. Preoperative endovascular brain mapping (PEBM) is a novel approach that allows clear visualization of nonfunctional (silent) brain parenchyma in real time during a resection. It has potential to improve neurosurgical guidance because brain shift does not alter the maps, and the map is visualized directly on the brain in situ rather than on a nearby image. Therefore, the risk of a new neurological deficit should be reduced. The authors report the first PEBM approach that combines selective molecular targeting of brain endothelium with multispectral (optical) imaging in preclinical animal models.

METHODS

Sprague-Dawley rats and New Zealand white rabbits were selectively catheterized, and a fluorescein isothiocyanate-derivatized tomato lectin-based imaging probe was administered into the carotid artery or posterior cerebral artery, measuring < 500 microm in diameter. After binding/uptake of the imaging probe, and removal of unbound probe, a craniotomy was performed to directly visualize the "brain map."

RESULTS

Selective localization of the imaging probe to the right hemisphere in rats or right posterior cerebral artery in rabbits was clearly visualized after craniotomy. Cross-sections of stained capillaries demonstrated that the imaging probe did not cause vascular occlusion. Gross regional selectivity of the imaging probe was documented by multispectral molecular imaging of intact brains, with discrete localization and endothelium-directed targeting validated by histological examination.

CONCLUSIONS

The authors have demonstrated the first molecular endothelium-targeted approach to PEBM that does not require manipulation of the intact blood-brain barrier or result in vascular occlusion. Furthermore, the presented multispectral molecular imaging technique appears to be a suitable methodology for the generation of region-selective brain maps of vascularized brain parenchyma. Further refinement of the PEBM approach, as well as the development of improved imaging probes, may result in clinical advancement of PEBM where direct visual discrimination of nonfunctional silent brain parenchyma at the time of resection could significantly improve neurosurgical outcomes.

A molecular imaging paradigm to rapidly profile response to angiogenesis-directed therapy in small animals

PURPOSE

The development of novel angiogenesis-directed therapeutics is hampered by the lack of non-invasive imaging metrics capable of assessing treatment response. We report the development and validation of a novel molecular imaging paradigm to rapidly assess response to angiogenesis-directed therapeutics in preclinical animal models.

PROCEDURES

A monoclonal antibody-based optical imaging probe targeting vascular endothelial growth factor receptor-2 (VEGFR2) expression was synthesized and evaluated in vitro and in vivo via multispectral fluorescence imaging.

RESULTS

The optical imaging agent demonstrated specificity for the target receptor in cultured endothelial cells and in vivo. The agent exhibited significant accumulation within 4T1 xenograft tumors. Mice bearing 4T1 xenografts and treated with sunitinib exhibited both tumor growth arrest and decreased accumulation of NIR800-alphaVEGFR2ab compared to untreated cohorts (p = 0.0021).

CONCLUSIONS

Molecular imaging of VEGFR2 expression is a promising non-invasive biomarker for assessing angiogenesis and evaluating the efficacy of angiogenesis-directed therapies.

Molecular imaging of therapeutic response to epidermal growth factor receptor blockade in colorectal cancer

PURPOSE

To evaluate noninvasive molecular imaging methods as correlative biomarkers of therapeutic efficacy of cetuximab in human colorectal cancer cell line xenografts grown in athymic nude mice. The correlation between molecular imaging and immunohistochemical analysis to quantify epidermal growth factor (EGF) binding, apoptosis, and proliferation was evaluated in treated and untreated tumor-bearing cohorts.

EXPERIMENTAL DESIGN

Optical imaging probes targeting EGF receptor (EGFR) expression (NIR800-EGF) and apoptosis (NIR700-Annexin V) were synthesized and evaluated in vitro and in vivo. Proliferation was assessed by 3'-[18F]fluoro-3'-deoxythymidine ([18F]FLT) positron emission tomography. Assessment of inhibition of EGFR signaling by cetuximab was accomplished by concomitant imaging of NIR800-EGF, NIR700-Annexin V, and [18F]FLT in cetuximab-sensitive (DiFi) and insensitive (HCT-116) human colorectal cancer cell line xenografts. Imaging results were validated by measurement of tumor size and immunohistochemical analysis of total and phosphorylated EGFR, caspase-3, and Ki-67 immediately following in vivo imaging.

RESULTS

NIR800-EGF accumulation in tumors reflected relative EGFR expression and EGFR occupancy by cetuximab. NIR700-Annexin V accumulation correlated with cetuximab-induced apoptosis as assessed by immunohistochemical staining of caspase-3. No significant difference in tumor proliferation was noted between treated and untreated animals by [18F]FLT positron emission tomography or Ki-67 immunohistochemistry.

CONCLUSIONS

Molecular imaging can accurately assess EGF binding, proliferation, and apoptosis in human colorectal cancer xenografts. These imaging approaches may prove useful for serial, noninvasive monitoring of the biological effects of EGFR inhibition in preclinical studies. It is anticipated that these assays can be adapted for clinical use.

Accelerating the development of novel molecular imaging probes: a role for high-throughput screening

Molecular imaging is a rapidly emerging research tool and clinical discipline aimed at noninvasive, quantitative visualization of in vivo molecular processes occurring at cellular and subcellular levels. At present, advancement of the molecular imaging field is driven by the development of improved imaging hardware for use in preclinical and clinical settings, the identification and validation of new, biologically relevant imaging targets, and the development of improved imaging probes derived from novel chemistries. Of these 3 essential facets, which comprise a majority of current molecular imaging research, hardware development and novel target discovery significantly outpace the development and clinical advancement of new molecular imaging probes, particularly with respect to cancer imaging.

Targeted imaging of colonic tumors in smad3-/- mice discriminates cancer and inflammation

The peripheral benzodiazepine receptor (PBR) is a trans-mitochondrial membrane protein that modulates steroid biosynthesis. Recently, up-regulation and nuclear localization of PBR has been shown to be associated with colon, prostate, and breast cancer. PBR has been targeted by the exogenous synthetic ligand, PK11195, for various purposes including imaging. To capitalize on these observations, we developed a high-throughput, noninvasive, in vivo imaging approach to detect spontaneously arising colonic tumors in mice using a novel PBR-targeted molecular imaging agent (NIR-conPK11195). NIR-conPK11195 localized and was retained in colonic adenomas and carcinomas in Smad3(-/-) mice but not in non-neoplastic hamartomas or chronically inflamed colonic tissue. Using a fluorescence signal-to-noise ratio of > or =4-fold 13 h after injection of the agent, we detected colonic tumors with a sensitivity of 67% and a specificity of 86% in a cohort of 37 Smad3(-/-) mice and control littermates. Furthermore, using oral administration of dextran sulfate to induce colonic inflammation, we showed that the clearance profile of NIR-conPK11195 distinguished transient uptake in inflammatory tissue from longer term retention in tumors. Taken together, these results indicate that NIR-conPK11195 is a promising optical molecular imaging tool to rapidly screen for colonic tumors in mice and to discriminate inflammation from cancer.

A peripheral benzodiazepine receptor targeted agent for in vitro imaging and screening

We developed a molecular imaging agent (MIA), a conjugable form of PK11195 (conPK11195) coupled to a lissamine dye (Liss-ConPK11195), which targets the peripheral benzodiazepine receptor (PBR). To determine that our compound specifically binds to this 18 kDa protein, primarily expressed on the mitochondria, we performed classic binding studies on live MDA-MB-231 breast cancer cells and measured fluorescence in cell fractions of C6 glioma cells. We found that conPK11195 conjugated to the fluorophore retained significant binding to its target. Here we demonstrate the utility of the agent for in vitro imaging of live cells by specific binding to the protein of interest.

Density-functional theory structures of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid complexes for ions across the lanthanide series

The use of organically chelated lanthanides in diagnosis and treatment is a rapidly growing field in medicine. In order to gain a deeper understanding into the properties of these chelates, particularly spectroscopic, density-functional calculations have been performed on a series of lanthanide ions chelated with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid. Based on the results of these calculations, it has been concluded that the local symmetry experienced by the chelated lanthanide ion may be treated as being axial, which will make the interpretation of their spectroscopic properties greatly simplified. It has also been suggested that the so-called "capping" water molecule at the ninth coordination position of the lanthanide is hydrogen bonded to the acetate oxygens of the sidearms, rather than coordinated as the ninth ligand of the lanthanide.

Targeted molecular imaging agents for cellular-scale bimodal imaging

Molecular imaging is a powerful tool that has the ability to elucidate biochemical mechanisms and signal the early onset of disease. Overexpression of the peripheral benzodiazepine receptor (PBR) has been observed in a variety disease states, including glioblastoma, breast cancer, and Alzheimer's disease. Thus, the PBR could be an attractive target for molecular imaging. In this paper, the authors report cellular uptake and multimodal (MRI and fluorescence) imaging of PBR-overexpressing C6 glioblastoma (brain cancer) cells using a cocktail administration approach and a new PBR targeted lanthanide chelate molecular imaging agent.

Facile, efficient conjugation of a trifunctional lanthanide chelate to a peripheral benzodiazepine receptor ligand

[reaction: see text] Receptor-mediated imaging and therapy of diseased tissue is rapidly gaining favor in the medical community. The synthesis and facile aqueous/organic coupling of a peripheral-type benzodiazepine receptor ligand to a cyclen-based fluorophore is described herein. The contrast agent QM-CTMC-PK11195, when chelated with lanthanides, produces bright luminescence and good MRI contrast and can potentially serve as an imaging and demarcation agent for certain types of cancers.