A Systematic Comparison of 18F-C-SNAT to Established Radiotracer Imaging Agents for the Detection of Tumor Response to Treatment

PURPOSE: An early readout of tumor response to therapy through measurement of drug or radiation-induced cell death may provide important prognostic indications and improved patient management. It has been shown that the uptake of (18)F-C-SNAT can be used to detect early response to therapy in tumors by positron emission tomography (PET) via a mechanism of caspase-3-triggered nanoaggregation. EXPERIMENTAL DESIGN: Here, we compared the preclinical utility of (18)F-C-SNAT for the detection of drug-induced cell death to clinically evaluated radiotracers, (18)F-FDG, (99m)Tc-Annexin V, and (18)F-ML-10 in tumor cells in culture, and in tumor-bearing mice in vivo. RESULTS: In drug-treated lymphoma cells, (18)F-FDG, (99m)Tc-Annexin V, and (18)F-C-SNAT cell-associated radioactivity correlated well to levels of cell death (R(2) > 0.8; P 0.05). A similar pattern of response was observed in two human NSCLC cell lines following carboplatin treatment. EL-4 tumor uptake of (99m)Tc-Annexin V and (18)F-C-SNAT were increased 1.4- and 2.1-fold, respectively, in drug-treated versus naïve control animals (P < 0.05), although (99m)Tc-Annexin V binding did not correlate to ex vivo TUNEL staining of tissue sections. A differential response was not observed with either (18)F-FDG or (18)F-ML-10. CONCLUSIONS: We have demonstrated here that (18)F-C-SNAT can sensitively detect drug-induced cell death in murine lymphoma and human NSCLC. Despite favorable image contrast obtained with (18)F-C-SNAT, the development of next-generation derivatives, using the same novel and promising uptake mechanism, but displaying improved biodistribution profiles, are warranted for maximum clinical utility

Synthesis and pre-clinical evaluation of a [18F] fluoromethyl-tanaproget derivative for imaging of progesterone receptor expression

The estrogen receptor (ER) and progesterone receptor (PR) are over-expressed in ∼50% of breast cancer lesions, and used as biomarkers to stratify patients for endocrine therapy. Currently, immunohistochemical (IHC) assessment of these lesions from a core-needle biopsy in deep-sited metastases has limitations associated with sampling error and lack of standardization. An alternative solution is positron emission tomography (PET)-based probes, which are inherently quantitative and capable of imaging the entire tumor, including metastases. This work features the synthesis and biological evaluation of a novel fluorinated derivative of tanaproget, a high affinity non-steroidal PR ligand, as a candidate for imaging PR expression in vivo. Radiolabeling of the candidate was achieved in a 15% ± 4 radiochemical yield (non-decay corrected) in one step from [18F]fluoromethyltosylate in 30 min. Cell uptake studies showed a significant difference between the radioligand uptake in PR+ and PR- cell lines; however, in vivo imaging was confounded by defluorination hypothesized to occur via iminium salt formation. Investigation into high affinity, metabolically stable non-steroidal PR ligands is currently ongoing

PET imaging of tumor glycolysis downstream of hexokinase through noninvasive measurement of pyruvate kinase M2

Cancer cells reprogram their metabolism to meet increased biosynthetic demands, commensurate with elevated rates of replication. Pyruvate kinase M2 (PKM2) catalyzes the final and rate-limiting step in tumor glycolysis, controlling the balance between energy production and the synthesis of metabolic precursors. We report here the synthesis and evaluation of a positron emission tomography (PET) radiotracer, [(11)C]DASA-23, that provides a direct noninvasive measure of PKM2 expression in preclinical models of glioblastoma multiforme (GBM). In vivo, orthotopic U87 and GBM39 patient-derived tumors were clearly delineated from the surrounding normal brain tissue by PET imaging, corresponding to exclusive tumor-associated PKM2 expression. In addition, systemic treatment of mice with the PKM2 activator TEPP-46 resulted in complete abrogation of the PET signal in intracranial GBM39 tumors. Together, these data provide the basis for the clinical evaluation of imaging agents that target this important gatekeeper of tumor glycolysis

The relationship between endogenous thymidine concentrations and [F-18]FLT uptake in a range of preclinical tumour models

BACKGROUND: Recent studies have shown that 3′-deoxy-3′-[18F] fluorothymidine ([18F]FLT)) uptake depends on endogenous tumour thymidine concentration. The purpose of this study was to investigate tumour thymidine concentrations and whether they correlated with [18F]FLT uptake across a broad spectrum of murine cancer models. A modified liquid chromatography-mass spectrometry (LC-MS/MS) method was used to determine endogenous thymidine concentrations in plasma and tissues of tumour-bearing and non-tumour bearing mice and rats. Thymidine concentrations were determined in 22 tumour models, including xenografts, syngeneic and spontaneous tumours, from six research centres, and a subset was compared for [18F]FLT uptake, described by the maximum and mean tumour-to-liver uptake ratio (TTL) and SUV. RESULTS: The LC-MS/MS method used to measure thymidine in plasma and tissue was modified to improve sensitivity and reproducibility. Thymidine concentrations determined in the plasma of 7 murine strains and one rat strain were between 0.61 ± 0.12 μM and 2.04 ± 0.64 μM, while the concentrations in 22 tumour models ranged from 0.54 ± 0.17 μM to 20.65 ± 3.65 μM. TTL at 60 min after [18F]FLT injection, determined in 14 of the 22 tumour models, ranged from 1.07 ± 0.16 to 5.22 ± 0.83 for the maximum and 0.67 ± 0.17 to 2.10 ± 0.18 for the mean uptake. TTL did not correlate with tumour thymidine concentrations. CONCLUSIONS: Endogenous tumour thymidine concentrations alone are not predictive of [18F]FLT uptake in murine cancer models

Rapid Imaging of Tumor Cell Death in vivo using the C2A domain of Synaptotagmin-I

Cell death is an important target for imaging the early response of tumors to treatment. We describe here validation of a phosphatidylserine-binding agent for detecting tumor cell death in vivo based on the C2A domain of Synaptotagmin-I. Methods: The capability of near infrared fluorophore-labeled and 99mTechnetium- and 111Indium-labeled derivatives of C2Am for imaging tumor cell death, using planar near infrared fluorescence (NIRF) imaging and single photon computed tomography (SPECT) respectively, was evaluated in implanted and genetically engineered mouse models of lymphoma and in a human colorectal xenograft. Results: The fluorophore labeled C2Am derivative showed predominantly renal clearance and high specificity and sensitivity for detecting low levels of tumor cell death (2-5%). There was a significant correlation (R>0.9, P<0.05) between fluorescently-labeled C2Am binding and histological markers of cell death, including cleaved caspase-3, whereas there was no such correlation with a site-directed mutant of C2Am (iC2Am) that does not bind phosphatidylserine. 99mTc-C2Am and 111In-C2Am also showed favorable biodistribution profiles, with predominantly renal clearance and low non-specific retention in liver and spleen at 24 h after probe administration. 99mTc-C2Am and 111In-C2Am generated tumor-to-muscle ratios in drug-treated tumors of 4.3× and 2.2× respectively at two hours and 7.3× and 4.1× respectively at twenty-four hours after administration. Conclusion: Given the favorable biodistribution profile of 99mTc- and 111In-labelled C2Am, and their ability to produce rapid and cell death-specific image contrast, these agents have potential for clinical translation.This work was supported by a Cancer Research UK programme grant to K.M.B. S.F. was the recipient of a Ph.D. studentship from the Cambridge Biomedical Research Centre of the National Institute of Health Research with financial support from GlaxoSmithKline UK. T.B.R. was in receipt of Intra-European Marie Curie (FP7-PEOPLE-2009-IEF, Imaging Lymphoma) and Long-term EMBO (EMBO-ALT-1145-2009) fellowships