Luis F. Gomez

[18F]T807, a novel tau positron emission tomography imaging agent for Alzheimer's disease

We wished to develop a highly selective positron emission tomography (PET) imaging agent targeting PHF‐tau in human Alzheimers disease (AD) brains.

A Highly Selective and Specific PET Tracer for Imaging of Tau Pathologies

Senile plaques and neurofibrillary tangles are prominent neuropathological hallmarks in Alzheimers disease and are considered to be targets for therapeutic intervention as well as biomarkers for diagnostic in vivo imaging agents. While there are a number of amyloid-β positron emission tomography (PET) tracers currently in different stages of clinical development and commercialization, there have been very few reports on imaging agents selectively targeting tau aggregates. In search of [18F]-PET tracers that possess great binding affinity and selectivity toward tau tangles, we tested more than 900 compounds utilizing a unique screening process. A competitive autoradiography assay was set up to test compounds for binding to native tau tangles and amyloid-β plaques on human brain tissue sections. In our in vitro assays, the 18F labeled compound [18F]-T808 displayed a high level of binding affinity and good selectivity for tau aggregates over amyloid-β plaques. [18F]-T808 showed rapid uptake and washout in rodent brains. Our in vitro and preclinical in vivo studies suggest that [18F]-T808 possesses suitable properties and characteristics to be a specific and selective PET probe for imaging of paired helical filament tau in human brains.

Insulin causes hyperthermia by direct inhibition of warm-sensitive neurons.

OBJECTIVE Temperature and nutrient homeostasis are two interdependent components of energy balance regulated by distinct sets of hypothalamic neurons. The objective is to examine the role of the metabolic signal insulin in the control of core body temperature (CBT). RESEARCH DESIGN AND METHODS The effect of preoptic area administration of insulin on CBT in mice was measured by radiotelemetry and respiratory exchange ratio. In vivo 2-[18F]fluoro-2-deoxyglucose uptake into brown adipose tissue (BAT) was measured in rats after insulin treatment by positron emission tomography combined with X-ray computed tomography imaging. Insulin receptor–positive neurons were identified by retrograde tracing from the raphe pallidus. Insulin was locally applied on hypothalamic slices to determine the direct effects of insulin on intrinsically warm-sensitive neurons by inducing hyperpolarization and reducing firing rates. RESULTS Injection of insulin into the preoptic area of the hypothalamus induced a specific and dose-dependent elevation of CBT mediated by stimulation of BAT thermogenesis as shown by imaging and respiratory ratio measurements. Retrograde tracing indicates that insulin receptor–expressing warm-sensitive neurons activate BAT through projection via the raphe pallidus. Insulin applied on hypothalamic slices acted directly on intrinsically warm-sensitive neurons by inducing hyperpolarization and reducing firing rates. The hyperthermic effects of insulin were blocked by pretreatment with antibodies to insulin or with a phosphatidylinositol 3–kinase inhibitor. CONCLUSIONS Our findings demonstrate that insulin can directly modulate hypothalamic neurons that regulate thermogenesis and CBT and indicate that insulin plays an important role in coupling metabolism and thermoregulation at the level of anterior hypothalamus.

Noninvasive Molecular Imaging of Apoptosis in a Mouse Model of Anthracycline-Induced Cardiotoxicity

Background—Anthracycline-induced cardiotoxicity and myocardial dysfunction may be associated with apoptosis. Caspase 3 catalyzes a terminal step in apoptosis, and its expression may serve as a marker of cardiomyocyte apoptosis. We synthesized 18F-CP18, a caspase-3 substrate and evaluated cardiac 18F-CP18 uptake in a mouse model of anthracycline cardiotoxicity. Methods and Results—For 12 weeks, mice were injected with doxorubicin, 3 mg/kg/week, or vehicle (control). Left ventricular fractional shortening was quantified by echocardiography. CP18 uptake after intravenous injection of 250 &mgr;Ci of 18F-CP18, 24 hours post-doxorubicin treatment was quantified by microPET, autoradiography, and gamma counting. Apoptosis was assessed by enzymatic assay of myocardial caspase 3 and TUNEL staining of tissue sections. Compared with controls, at 6 and 12 weeks of doxorubicin treatment, fractional shortening was reduced (20.7%±2.5% versus 31%±3.5%, P=0.010; and 20.3%±3.1% versus 32.4%±2.1%, P=0.011). Doxorubicin treatment was associated with increased 18F-CP18 uptake in %ID/g by gamma counting from 0.36±0.01 (week 1) to 0.78±0.01 (week 12), P=0.003. A similar increase in 18F-CP18 uptake was observed by microPET (0.41±0.04 versus 0.73±0.1, P=0.014) and autoradiography (1.1±0.3 versus 2.8±0.2 P=0.001). Caspase 3 enzymatic activity and apoptosis by TUNEL staining were also increased after 12 weeks of doxorubicin compared with weeks 1 and 3. CP18 uptake in controls was relatively unchanged at weeks 1, 3, and 12. Conclusions—In a mouse model of cardiotoxicity, doxorubicin treatment is associated with increased myocardial caspase 3 expression and an increase in CP18 uptake. 18F-CP18 may be useful for detection of anthracycline-induced myocardial apoptosis.

Atherosclerotic plaque uptake of a novel integrin tracer 18F-Flotegatide in a mouse model of atherosclerosis

IntroductionRupture of unstable atherosclerotic plaque that leads to stroke and myocardial infarction may be induced by macrophage infiltration and neovessel formation. A tracer that selectively binds to integrin αvβ3 a protein expressed by macrophages and neovascular endothelium may identify rupture prone plaque.Methods18F-labeled “R-G-D” containing tripeptide (Flotegatide), a click chemistry derived radiotracer that binds to integrin αvβ3 was injected in ApoE knockout mice fed a high fat diet. Uptake of Flotegatide by atherosclerotic plaque was visualized by micro-PET, autoradiography, and correlated to histologic markers of inflammation and angiogenesis.ResultsWe found that Flotegatide preferentially binds to aortic plaque in an ApoE knockout mouse model of atherosclerosis. The tracer’s uptake is strongly associated with presence of histologic markers for macrophage infiltration and integrin expression. There is a weaker but detectable association between Flotegatide uptake and presence of an immunohistochemical marker for neovascularization.DiscussionWe hypothesize that Flotegatide may be a useful tracer for visualization of inflamed plaque in clinical subjects with atherosclerosis and may have potential for detecting vulnerable plaque.

Evaluation of [18F]-CP18 as a PET Imaging Tracer for Apoptosis

PurposeWe identified and validated [18F]-CP18, a DEVD (the caspase 3 substrate recognition motif) containing substrate-based compound as an imaging tracer for caspase-3 activity in apoptotic cells.ProceduresCP18 was radiolabeled with fluorine-18 using click chemistry. The affinity and selectivity of CP18 for caspase-3 were evaluated in vitro. The biodistribution and metabolism pattern of [18F]-CP18 were assessed in vivo. [18F]-CP18 positron emission tomography (PET) scans were performed in a dexamethasone-induced thymic apoptosis mouse model. After imaging, the mice were sacrificed, and individual organs were collected, measured in a gamma counter, and tested for caspase-3 activity.ResultsIn vitro enzymatic caspase-3 assay demonstrated specific cleavage of CP18. In vivo, [18F]-CP18 is predominantly cleared through the kidneys and urine, and is rapidly eliminated from the bloodstream. There was a sixfold increase in caspase activity and a fourfold increase of [18F]-CP18 retention in the dexamethasone-induced thymus of treated versus control mice.ConclusionsWe report the use [18F]-CP18 as a PET tracer for imaging apoptosis. Our data support further development of this tracer for clinical PET applications.

In Vitro and In Vivo Evaluation of the Caspase-3 Substrate-Based Radiotracer [ 18 F]-CP18 for PET Imaging of Apoptosis in Tumors

PurposeA novel caspase-3 substrate-based probe [18F]-CP18 was evaluated as an in vivo positron emission tomography (PET) imaging agent for monitoring apoptosis in tumors.MethodsUptake of [18F]-CP18 in cell assays and tumors was measured. Caspase-3/7 activities in cell lysates and tumor homogenates were determined. Autoradiography,Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), and cleaved caspase-3 immunostaining were performed on adjacent tumor sections to identify areas of apoptosis.ResultsThe in vitro cell assays showed caspase-3-dependent uptake of [18F]-CP18 in tumor cells when treated with an apoptosis inducer. The in vivo microPET imaging signal of [18F]-CP18 in xenograft tumors correlated with the ex vivo caspase-3/7 activities in these tumors. Furthermore, tumor autoradiographies of [18F]-CP18 in tumor sections matched adjacent sections stained by TUNEL and caspase-3 immunohistochemistry (IHC).Conclusions[18F]-CP18 demonstrated high affinity and selectivity for activated caspase-3 both in vitro and in vivo, and the results support [18F]-CP18 as a promising new PET imaging agent for apoptosis.

Synthesis, radiosynthesis, in vitro and first in vivo evaluation of a new matrix metalloproteinase inhibitor based on γ-fluorinated α-sulfonylaminohydroxamic acid

BackgroundTo study MMP activity in vivo in disease, several radiolabeled MMP inhibitors functioning as radiotracers have been evaluated by means of SPECT and PET. Unfortunately, most of them suffer from metabolic instability, mainly hepatobiliary clearance and insufficient target binding. The introduction of a fluorine atom into MMPIs could contribute to target binding, enhance the metabolic stability and might shift the clearance towards more renal elimination. Recently developed α-sulfonylaminohydroxamic acid based γ-fluorinated inhibitors of MMP-2 and -9 provide promising fluorine interactions with the enzyme active site and high MMP inhibition potencies. The aim of this study is the (radio)synthesis of a γ-fluorinated MMP-2 and -9 inhibitor to evaluate its potential as a radiotracer to image MMP activity in vivo.ResultsTwo new fluorine-containing, enantiomerically pure inhibitors for MMP-2 and -9 were synthesized in a six step sequence. Both enantiomers exhibited equal inhibition potencies in the low nanomolar and subnanomolar range. LogD value indicated better water solubility compared to the CGS 25966 based analog. The most potent inhibitor was successfully radiofluorinated. In vivo biodistribution in wild type mice revealed predominantly hepatobiliary clearance. Two major radioactive metabolites were found in different organs. Defluorination of the radiotracer was not observed.Conclusion(Radio)synthesis of a CGS based γ-fluorinated MMP inhibitor was successfully accomplished. The (S)-enantiomer, which normally shows no biological activity, also exhibited high MMP inhibition potencies, which may be attributed to additional interactions of fluorine with enzyme’s active site. Despite higher hydrophilicity no significant differences in the clearance characteristics compared to non-fluorinated MMPIs was observed. Metabolically stabilizing effect of the fluorine was not monitored in vivo in wild type mice.

Abstract 5583: Response biomarkers for the hypoxia-targeted drug TH-302: imaging, plasma, and tissue.

Biomarkers that assess the therapeutic response to anticancer treatments may aid in tailoring therapeutic regimens to individuals. Ideally, the relative contributory effect of each agent in a muti-agent regimen can be discerned. Hypoxia is a prevalent feature of solid tumors, associated with treatment failure, poor prognosis, and increased metastasis. TH-302, a hypoxia-activated prodrug of the DNA cross-linker bromo isophosphoramide mustard, has demonstrated broad efficacy in preclinical models and is exhibiting promising activity in multiple ongoing clinical trials, both as a monotherapy and in combination therapy regimens. Three fundamental biomarker platforms were assessed in this preclinical study: positron emission tomography (PET) imaging employing the tracer [ 18 F]-HX4; immunohistochemistry of isolated tumor tissue employing antibodies directed against the exogenous hypoxia biomarker pimonidazole (PIMO) and the endogenous hypoxia biomarker carbonic anhydrase IX (CA-IX); and ELISA-based quantification of circulating plasma CA-IX. Both HX4 and PIMO utilize the same 2-nitroimidazole hypoxia-sensitive trigger for their activity and specificity as TH-302 utilizes. CA-IX is up-regulated by two distinct hypoxia sensing pathways: hypoxia-inducible factor 1; and the unfolded protein response. The human H460 non-small cell lung cancer human tumor ectopic xenograft model was employed for these studies. When the tumor size was 300-600 mm 3 , a single dose of TH-302 (150 mg/kg, ip) or vehicle was administered. This TH-302 treatment regimen yields a tumor growth inhibition of 58%, TGD 1000 of 10 d, and maximum body weight loss of 2%. [ 18 F]-HX4 uptake was assessed by PET/CT imaging and the tumor to muscle ratio (T/M) was determined. Animals were scanned pre- and post-treatment and the T/M ratio was significantly reduced 72 hours after TH-302 treatment (from 3.3 ± 0.3 to 2.1 ± 0.2, p 0.05). Blood was collected from the animals pre- and post- treatment via retro-orbital vein. Plasma CA-IX changed from baseline 165 ± 13 to 104 ± 9.3 pg/ml 72 hours after TH-302 dosing, a 37% reduction (p Citation Format: Jessica D. Sun, Qian Liu, Dharmendra Ahluwalia, Yan Wang, Anna-Katrin Szardenings, Luis F. Gomez, Hartmuth Kolb, Karen A. Pierce, Sheryl Brown-Shimer, Walter A. Carney, Charles P. Hart. Response biomarkers for the hypoxia-targeted drug TH-302: imaging, plasma, and tissue. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5583. doi:10.1158/1538-7445.AM2013-5583