Research Interests: Preclinical and clinical PET imaging, novel tracer validation in vivo, B cell role in multiple sclerosis and pharmacokinetic modelling

Samantha is a multidisciplinary research assistant with extensive experience in multi-modal molecular imaging of neurological diseases and a strong background in synthetic chemistry. She helped validate both a novel sigma-1 radiotracer, [18F]FTC-146, for its ability to detect molecular markers of neuropathic pain non-invasively and a [64Cu]hCD19-mAb radiotracer for imaging therapy-induced B cell depletion. Currently, her research focuses on evaluating novel therapeutics and Positron Emission Tomography (PET) imaging agents in rodent models with the ultimate goal of clinically translating these approaches and enhancing our ability to monitor and treat neurological diseases. She is well-versed in pharmacokinetic modeling, receptor occupancy studies, autoradiography, and cell-based assays. Moreover, is an expert in PET/MR imaging of neonatal mice.


Honors & Awards

  • Women in Molecular Imaging Network (WIMIN) scholar award, World Molecular Imaging Society (WMIS)/World Molecular Imaging Congress (WMIC) (2021)
  • Student travel stipend award, World Molecular Imaging Society (WMIS)/World Molecular Imaging Congress (WMIC) (2021)

Service, Volunteer and Community Work

  • Science education volunteer, Science is Elementary (2015 - Present)

    Teach science lessons to students from pre-school-6th grade in under-served elementary schools


    Palo Alto, CA

All Publications

  • Brain cell signaling abnormalities are detected in blood in a murine model of Fragile X syndrome and corrected by Sigma-1 receptor agonist Blarcamesine. American journal of medical genetics. Part A Cogram, P., Deacon, R. M., Klamer, D., Rebowe, N., Sprouse, J., Reyes, S. T., Missling, C. U., Kaufmann, W. E. 2022

    View details for DOI 10.1002/ajmg.a.62853

    View details for PubMedID 35661397

  • Radiosynthesis and initial preclinical evaluation of [11C]AZD1283 as a potential P2Y12R PET radiotracer. Nuclear medicine and biology Jackson, I. M., Buccino, P. J., Azevedo, E. C., Carlson, M. L., Luo, A. S., Deal, E. M., Kalita, M., Reyes, S. T., Shao, X., Beinat, C., Nagy, S. C., Chaney, A. M., Anders, D. A., Scott, P. J., Smith, M., Shen, B., James, M. L. 2022


    INTRO: Chronic neuroinflammation and microglial dysfunction are key features of many neurological diseases, including Alzheimer's Disease and multiple sclerosis. While there is unfortunately a dearth of highly selective molecular imaging biomarkers/probes for studying microglia in vivo, P2Y12R has emerged as an attractive candidate PET biomarker being explored for this purpose. Importantly, P2Y12R is selectively expressed on microglia in the CNS and undergoes dynamic changes in expression according to inflammatory context (e.g., toxic versus beneficial/healing states), thus having the potential to reveal functional information about microglia in living subjects. Herein, we identified a high affinity, small molecule P2Y12R antagonist (AZD1283) to radiolabel and assess as a candidate radiotracer through in vitro assays and in vivo positron emission tomography (PET) imaging of both wild-type and total knockout mice and a non-human primate.METHODS: First, we evaluated the metabolic stability and passive permeability of non-radioactive AZD1283 in vitro. Next, we radiolabeled [11C]AZD1283 with radioactive precursor [11C]NH4CN and determined stability in formulation and human plasma. Finally, we investigated the in vivo stability and kinetics of [11C]AZD1283 via dynamic PET imaging of naive wild-type mice, P2Y12R knockout mouse, and a rhesus macaque.RESULTS: We determined the half-life of AZD1283 in mouse and human liver microsomes to be 37 and>160min, respectively, and predicted passive CNS uptake with a small amount of active efflux, using a Caco-2 assay. Our radiolabeling efforts afforded [11C]AZD1283 in an activity of 12.69±10.64mCi with high chemical and radiochemical purity (>99%) and molar activity of 1142.84±504.73mCi/mumol (average of n=3). Of note, we found [11C]AZD1283 to be highly stable in vitro, with >99% intact tracer present after 90min of incubation in formulation and 60min of incubation in human serum. PET imaging revealed negligible brain signal in healthy wild-type mice (n=3) and a P2Y12 knockout mouse (0.55±0.37%ID/g at 5min post injection). Strikingly, high signal was detected in the liver of all mice within the first 20min of administration (peak uptake=58.28±18.75%ID/g at 5min post injection) and persisted for the remaining duration of the scan. Ex vivo gamma counting of mouse tissues at 60min post-injection mirrored in vivo data with a mean %ID/g of 0.9%±0.40, 0.02%±0.01, and 106±29.70% in the blood, brain, and liver, respectively (n=4). High performance liquid chromatography (HPLC) analysis of murine blood and liver metabolite samples revealed a single radioactive peak (relative area under peak: 100%), representing intact tracer. Finally, PET imaging of a rhesus macaque also revealed negligible CNS uptake/binding in monkey brain (peak uptake=0.37 Standard Uptake Values (SUV)).CONCLUSION: Despite our initial encouraging liver microsome and Caco-2 monolayer data, in addition to the observed high stability of [11C]AZD1283 in formulation and human serum, in vivo brain uptake was negligible and rapid accumulation was observed in the liver of both naive wildtype and P2Y12R knockout mice. Liver signal appeared to be independent of both metabolism and P2Y12R expression due to the confirmation of intact tracer in this tissue for both wildtype and P2Y12R knockout mice. In Rhesus Macaque, negligible uptake of [11C]AZD1283 brain indicates a lack of potential for translation or its further investigation in vivo. P2Y12R is an extremely promising potential PET biomarker, and the data presented here suggests encouraging metabolic stability for this scaffold; however, the mechanism of liver uptake in mice should be elucidated prior to further analogue development.

    View details for DOI 10.1016/j.nucmedbio.2022.05.001

    View details for PubMedID 35680502

  • A Clinical PET Imaging Tracer ([18F]DASA-23) to Monitor Pyruvate Kinase M2 Induced Glycolytic Reprogramming in Glioblastoma. Clinical cancer research : an official journal of the American Association for Cancer Research Beinat, C., Patel, C. B., Haywood, T., Murty, S., Naya, L., Castillo, J. B., Reyes, S. T., Phillips, M., Buccino, P., Shen, B., Park, J. H., Koran, M. E., Alam, I. S., James, M. L., Holley, D., Halbert, K., Gandhi, H., He, J. Q., Granucci, M., Johnson, E., Liu, D. D., Uchida, N., Sinha, R., Chu, P., Born, D. E., Warnock, G. I., Weissman, I., Hayden Gephart, M., Khalighi, M. M., Massoud, T. F., Iagaru, A., Davidzon, G., Thomas, R., Nagpal, S., Recht, L. D., Gambhir, S. S. 2021


    PURPOSE: Pyruvate kinase M2 (PKM2) catalyzes the final step in glycolysis, a key process of cancer metabolism. PKM2 is preferentially expressed by glioblastoma (GBM) cells with minimal expression in healthy brain. We describe the development, validation, and translation of a novel positron emission tomography (PET) tracer to study PKM2 in GBM. We evaluated 1-((2-fluoro-6-[18F]fluorophenyl)sulfonyl)-4-((4-methoxyphenyl)sulfonyl)piperazine ([18F]DASA-23) in cell culture, mouse models of GBM, healthy human volunteers, and GBM patients.EXPERIMENTAL DESIGN: [18F]DASA-23 was synthesized with a molar activity of 100.47 {plus minus} 29.58 GBq/mol and radiochemical purity >95%. We performed initial testing of [18F]DASA-23 in GBM cell culture and human GBM xenografts implanted orthotopically into mice. Next we produced [18F]DASA-23 under FDA oversight, and evaluated it in healthy volunteers, and a pilot cohort of glioma patients.RESULTS: In mouse imaging studies, [18F]DASA-23 clearly delineated the U87 GBM from surrounding healthy brain tissue and had a tumor-to-brain ratio (TBR) of 3.6 {plus minus} 0.5. In human volunteers, [18F]DASA-23 crossed the intact blood-brain barrier and was rapidly cleared. In GBM patients, [18F]DASA-23 successfully outlined tumors visible on contrast-enhanced magnetic resonance imaging (MRI). The uptake of [18F]DASA-23 was markedly elevated in GBMs compared to normal brain, and it identified a metabolic non-responder within 1-week of treatment initiation.CONCLUSIONS: We developed and translated [18F]DASA-23 as a new tracer that demonstrated the visualization of aberrantly expressed PKM2 for the first time in human subjects. These results warrant further clinical evaluation of [18F]DASA-23 to assess its utility for imaging therapy-induced normalization of aberrant cancer metabolism.

    View details for DOI 10.1158/1078-0432.CCR-21-0544

    View details for PubMedID 34475101

  • Effects of the sigma-1 receptor agonist blarcamesine in a murine model of fragile X syndrome: neurobehavioral phenotypes and receptor occupancy. Scientific reports Reyes, S. T., Deacon, R. M., Guo, S. G., Altimiras, F. J., Castillo, J. B., van der Wildt, B., Morales, A. P., Park, J. H., Klamer, D., Rosenberg, J., Oberman, L. M., Rebowe, N., Sprouse, J., Missling, C. U., McCurdy, C. R., Cogram, P., Kaufmann, W. E., Chin, F. T. 2021; 11 (1): 17150


    Fragile X syndrome (FXS), a disorder of synaptic development and function, is the most prevalent genetic form of intellectual disability and autism spectrum disorder. FXS mouse models display clinically-relevant phenotypes, such as increased anxiety and hyperactivity. Despite their availability, so far advances in drug development have not yielded new treatments. Therefore, testing novel drugs that can ameliorate FXS' cognitive and behavioral impairments is imperative. ANAVEX2-73 (blarcamesine) is a sigma-1 receptor (S1R) agonist with a strong safety record and preliminary efficacy evidence in patients with Alzheimer's disease and Rett syndrome, other synaptic neurodegenerative and neurodevelopmental disorders. S1R's role in calcium homeostasis and mitochondrial function, cellular functions related to synaptic function, makes blarcamesine a potential drug candidate for FXS. Administration of blarcamesine in 2-month-old FXS and wild type mice for 2 weeks led to normalization in two key neurobehavioral phenotypes: open field test (hyperactivity) and contextual fear conditioning (associative learning). Furthermore, there was improvement in marble-burying (anxiety, perseverative behavior). It also restored levels of BDNF, a converging point of many synaptic regulators, in the hippocampus. Positron emission tomography (PET) and ex vivo autoradiographic studies, using the highly selective S1R PET ligand [18F]FTC-146, demonstrated the drug's dose-dependent receptor occupancy. Subsequent analyses also showed a wide but variable brain regional distribution of S1Rs, which was preserved in FXS mice. Altogether, these neurobehavioral, biochemical, and imaging data demonstrates doses that yield measurable receptor occupancy are effective for improving the synaptic and behavioral phenotype in FXS mice. The present findings support the viability of S1R as a therapeutic target in FXS, and the clinical potential of blarcamesine in FXS and other neurodevelopmental disorders.

    View details for DOI 10.1038/s41598-021-94079-7

    View details for PubMedID 34433831

  • BLZ945 derivatives for PET imaging of colony stimulating factor-1 receptors in the brain. Nuclear medicine and biology van der Wildt, B., Miao, Z., Reyes, S. T., Park, J. H., Klockow, J. L., Zhao, N., Romero, A., Guo, S. G., Shen, B., Windhorst, A. D., Chin, F. T. 2021; 100-101: 44-51


    BACKGROUND: The kinase colony stimulating factor-1 receptor (CSF-1R) has recently been identified as a novel therapeutic target for decreasing tumor associated macrophages and microglia load in cancer treatment. In glioblastoma multiforme (GBM), a high-grade cancer in the brain with extremely poor prognosis, macrophages and microglia can make up to 50% of the total tumor mass. Currently, no non-invasive methods are available for measuring CSF-1R expression in vivo. The aim of this work is to develop a PET tracer for imaging of CSF-1R receptor expression in the brain for future GBM patient selection and treatment monitoring.METHODS: BLZ945 and a derivative that potentially allows for fluorine-18 labeling were synthesized and evaluated in vitro to determine their affinity towards CSF-1R. BLZ945 was radiolabeled with carbon-11 by N-methylation of des-methyl-BLZ945 using [11C]CH3I. Following administration to healthy mice, metabolic stability of [11C]BLZ945 in blood and brain and activity distribution were determined ex vivo. PET scanning was performed at baseline, efflux transporter blocking, and CSF-1R blocking conditions. Finally, [11C]BLZ945 binding was evaluated in vitro by autoradiography on mouse brain sections.RESULTS: BLZ945 was the most potent compound in our series with an IC50 value of 6.9 ± 1.4 nM. BLZ945 was radiolabeled with carbon-11 in 20.7 ± 1.1% decay corrected radiochemical yield in a 60 min synthesis procedure with a radiochemical purity of >95% and a molar activity of 153 ± 34 GBq·mumol-1. Ex vivo biodistribution showed moderate brain uptake and slow wash-out, in addition to slow blood clearance. The stability of BLZ945 in blood plasma and brain was >99% at 60 min post injection. PET scanning demonstrated BLZ945 to be a substrate for efflux transporters. High brain uptake was observed, which was shown to be mostly non-specific. In accordance, in vitro autoradiography on brain sections revealed high non-specific binding.CONCLUSIONS: [11C]BLZ945, a CSF-1R PET tracer, was synthesized in high yield and purity. The tracer has high potency for the target, however, future studies are warranted to address non-specific binding and tracer efflux before BLZ945 or derivatives could be translated into humans for brain imaging.

    View details for DOI 10.1016/j.nucmedbio.2021.06.005

    View details for PubMedID 34174546

  • Evaluation of carbon-11 labeled 5-(1-methyl-1H-pyrazol-4-yl)-N-(2-methyl-5-(3-(trifluoromethyl)benzamido)phenyl)nicotinamide as PET tracer for imaging of CSF-1R expression in the brain. Bioorganic & medicinal chemistry van der Wildt, B., Nezam, M., Kooijman, E. J., Reyes, S. T., Shen, B., Windhorst, A. D., Chin, F. T. 2021; 42: 116245


    Pharmacological targeting of tumor associated macrophages and microglia in the tumor microenvironment is a novel therapeutic strategy in the treatment of glioblastoma multiforme. As such, the colony stimulating factor-1 receptor (CSF-1R) has been identified as a druggable target. However, no validated companion diagnostic marker for these therapies exists to date. Towards development of a CSF-1R PET tracer, a set of six compounds based on recently reported CSF-1R inhibitor 5-(1-methyl-1H-pyrazol-4-yl)-N-(2-methyl-5-(3-(trifluoromethyl)benzamido)phenyl)nicotinamide (Compound 5) was designed, synthesized and evaluated in vitro for potency and selectivity. The highest affinity for CSF-1R was found for compound 5 (IC50: 2.7nM). Subsequent radiosynthesis of [11C]5 was achieved in 2.0±0.2% yield (decay corrected to start of synthesis) by carbon-11 carbon monoxide aminocarbonylation in 40min after end of bombardment. In vitro autoradiography with [11C]5 on rat brain sections demonstrated high specific binding, but also strong off-target binding. Ex vivo, only intact tracer was observed in blood plasma at 90min post injection in healthy rats. PET scanning results demonstrated negligible brain uptake under baseline conditions and this brain uptake did not increase by blocking of efflux transporters using Tariquidar. To conclude, [11C]5 was successfully synthesized and evaluated in healthy rats. However, the inability of [11C]5 to cross the blood-brain-barrier excludes its use for imaging of CSF-1R expression in the brain.

    View details for DOI 10.1016/j.bmc.2021.116245

    View details for PubMedID 34119698

  • GABA Measurement in a Neonatal Fragile X Syndrome Mouse Model Using 1H-Magnetic Resonance Spectroscopy and Mass Spectrometry. Frontiers in molecular neuroscience Reyes, S. T., Mohajeri, S. n., Krasinska, K. n., Guo, S. G., Gu, M. n., Pisani, L. n., Rosenberg, J. n., Spielman, D. M., Chin, F. T. 2020; 13: 612685


    Fragile X syndrome (FXS) is the leading monogenetic cause of autism spectrum disorder and inherited cause of intellectual disability that affects approximately one in 7,000 males and one in 11,000 females. In FXS, the Fmr1 gene is silenced and prevents the expression of the fragile X mental retardation protein (FMRP) that directly targets mRNA transcripts of multiple GABAA subunits. Therefore, FMRP loss adversely impacts the neuronal firing of the GABAergic system which creates an imbalance in the excitatory/inhibitory ratio within the brain. Current FXS treatment strategies focus on curing symptoms, such as anxiety or decreased social function. While treating symptoms can be helpful, incorporating non-invasive imaging to evaluate how treatments change the brain's biology may explain what molecular aberrations are associated with disease pathology. Thus, the GABAergic system is suitable to explore developing novel therapeutic strategies for FXS. To understand how the GABAergic system may be affected by this loss-of-function mutation, GABA concentrations were examined within the frontal cortex and thalamus of 5-day-old wild type and Fmr1 knockout mice using both 1H magnetic resonance imaging (1H-MRS) and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Our objective was to develop a reliable scanning method for neonatal mice in vivo and evaluate whether 1H-MRS is suitable to capture regional GABA concentration differences at the front end of the critical cortical period where abnormal neurodevelopment occurs due to FMRP loss is first detected. 1H-MRS quantified GABA concentrations in both frontal cortex and thalamus of wild type and Fmr1 knockout mice. To substantiate the results of our 1H-MRS studies, in vitro LC-MS/MS was also performed on brain homogenates from age-matched mice. We found significant changes in GABA concentration between the frontal cortex and thalamus within each mouse from both wild type and Fmr1 knockout mice using 1H-MRS and LC-MS/MS. Significant GABA levels were also detected in these same regions between wild type and Fmr1 knockout mice by LC-MS/MS, validating that FMRP loss directly affects the GABAergic system. Thus, these new findings support the need to develop an effective non-invasive imaging method to monitor novel GABAergic strategies aimed at treating patients with FXS.

    View details for DOI 10.3389/fnmol.2020.612685

    View details for PubMedID 33390902

    View details for PubMedCentralID PMC7775297

  • Visualizing Nerve Injury in a Neuropathic Pain Model with [(18)F]FTC-146 PET/MRI. Theranostics Shen, B. n., Behera, D. n., James, M. L., Reyes, S. T., Andrews, L. n., Cipriano, P. W., Klukinov, M. n., Lutz, A. B., Mavlyutov, T. n., Rosenberg, J. n., Ruoho, A. E., McCurdy, C. R., Gambhir, S. S., Yeomans, D. C., Biswal, S. n., Chin, F. T. 2017; 7 (11): 2794–2805


    The ability to locate nerve injury and ensuing neuroinflammation would have tremendous clinical value for improving both the diagnosis and subsequent management of patients suffering from pain, weakness, and other neurologic phenomena associated with peripheral nerve injury. Although several non-invasive techniques exist for assessing the clinical manifestations and morphological aspects of nerve injury, they often fail to provide accurate diagnoses due to limited specificity and/or sensitivity. Herein, we describe a new imaging strategy for visualizing a molecular biomarker of nerve injury/neuroinflammation, i.e., the sigma-1 receptor (S1R), in a rat model of nerve injury and neuropathic pain. The two-fold higher increase of S1Rs was shown in the injured compared to the uninjured nerve by Western blotting analyses. With our novel S1R-selective radioligand, [(18)F]FTC-146 (6-(3-[(18)F]fluoropropyl)-3-(2-(azepan-1-yl)ethyl)benzo[d]thiazol-2(3H)-one), and positron emission tomography-magnetic resonance imaging (PET/MRI), we could accurately locate the site of nerve injury created in the rat model. We verified the accuracy of this technique by ex vivo autoradiography and immunostaining, which demonstrated a strong correlation between accumulation of [(18)F]FTC-146 and S1R staining. Finally, pain relief could also be achieved by blocking S1Rs in the neuroma with local administration of non-radioactive [(19)F]FTC-146. In summary, [(18)F]FTC-146 S1R PET/MR imaging has the potential to impact how we diagnose, manage and treat patients with nerve injury, and thus warrants further investigation.

    View details for PubMedID 28824716

  • Use of labeled tomato lectin for imaging vasculature structures. Histochemistry and cell biology Robertson, R. T., Levine, S. T., Haynes, S. M., Gutierrez, P., Baratta, J. L., Tan, Z., Longmuir, K. J. 2015; 143 (2): 225-34


    Intravascular injections of fluorescent or biotinylated tomato lectin were tested to study labeling of vascular elements in laboratory mice. Injections of Lycopersicon esculentum agglutinin (tomato lectin) (50-100 µg/100 µl) were made intravascularly, through the tail vein, through a cannula implanted in the jugular vein, or directly into the left ventricle of the heart. Tissues cut for thin 10- to 12-µm cryostat sections, or thick 50- to 100-µm vibratome sections, were examined using fluorescence microscopy. Tissue labeled by biotinylated lectin was examined by bright field microscopy or electron microscopy after tissue processing for biotin. Intravascular injections of tomato lectin led to labeling of vascular structures in a variety of tissues, including brain, kidney, liver, intestine, spleen, skin, skeletal and cardiac muscle, and experimental tumors. Analyses of fluorescence in serum indicated the lectin was cleared from circulating blood within 2 min. Capillary labeling was apparent in tissues collected from animals within 1 min of intravascular injections, remained robust for about 1 h, and then declined markedly until difficult to detect 12 h after injection. Light microscopic images suggest the lectin bound to the endothelial cells that form capillaries and endothelial cells that line some larger vessels. Electron microscopic studies confirmed the labeling of luminal surfaces of endothelial cells. Vascular labeling by tomato lectin is compatible with a variety of other morphological labeling techniques, including histochemistry and immunocytochemistry, and thus appears to be a sensitive and useful method to reveal vascular patterns in relationship to other aspects of parenchymal development, structure, and function.

    View details for DOI 10.1007/s00418-014-1301-3

    View details for PubMedID 25534591