High resolution PET, SPECT and projection imaging in small animals

Positron emission tomography, single photon emission computed tomography and planar projection imaging of radioactive tracers have long been in use for detecting and diagnosing disease in human subjects. More recently, advanced versions of these same technologies have begun to be used across the breadth of modern biomedical research to study non-invasively small laboratory animals in a myriad of experimental settings. In this report, we describe some of the new instruments and techniques that make these measurements possible and illustrate, with a few examples, the potential power of these methods in modern biomedical researchPublicad

Lack of neuroinflammation in the HIV-1 transgenic rat: An [18 F]-DPA714 PET imaging study

BACKGROUND: HIV-associated neuroinflammation is believed to be a major contributing factor in the development of HIV-associated neurocognitive disorders (HAND). In this study, we used micropositron emission tomography (PET) imaging to quantify neuroinflammation in HIV-1 transgenic rat (Tg), a small animal model of HIV, known to develop neurological and behavioral problems. METHODS: Dynamic [(18)F]DPA-714 PET imaging was performed in Tg and age-matched wild-type (WT) rats in three age groups: 3-, 9-, and 16-month-old animals. As a positive control for neuroinflammation, we performed unilateral intrastriatal injection of quinolinic acid (QA) in a separate group of WT rats. To confirm our findings, we performed multiplex immunofluorescent staining for Iba1 and we measured cytokine/chemokine levels in brain lysates of Tg and WT rats at different ages. RESULTS: [(18)F]DPA-714 uptake in HIV-1 Tg rat brains was generally higher than in age-matched WT rats but this was not statistically significant in any age group. [(18)F]DPA-714 uptake in the QA-lesioned rats was significantly higher ipsilateral to the lesion compared to contralateral side indicating neuroinflammatory changes. Iba1 immunofluorescence showed no significant differences in microglial activation between the Tg and WT rats, while the QA-lesioned rats showed significant activation. Finally, cytokine/chemokine levels in brain lysates of the Tg rats and WT rats were not significantly different. CONCLUSION: Microglial activation might not be the primary mechanism for neuropathology in the HIV-1 Tg rats. Although [(18)F]DPA-714 is a good biomarker of neuroinflammation, it cannot be reliably used as an in vivo biomarker of neurodegeneration in the HIV-1 Tg rat. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12974-015-0390-9) contains supplementary material, which is available to authorized users

Fluorine-18 Labeled Fluorofuranylnorprogesterone ([18F]FFNP) and Dihydrotestosterone ([18F]FDHT) Prepared by “Fluorination on Sep-Pak” Method

To further explore the scope of our recently developed “fluorination on Sep-Pak” method, we prepared two well-known positron emission tomography (PET) tracers 21-[18F]fluoro-16α,17α-[(R)-(1′-α-furylmethylidene)dioxy]-19-norpregn-4-ene-3,20-dione furanyl norprogesterone ([18F]FFNP) and 16β-[18F]fluoro-5α-dihydrotestosterone ([18F]FDHT). Following the “fluorination on Sep-Pak” method, over 70% elution efficiency was observed with 3 mg of triflate precursor of [18F]FFNP. The overall yield of [18F]FFNP was 64–72% (decay corrected) in 40 min synthesis time with a molar activity of 37–81 GBq/µmol (1000–2200 Ci/mmol). Slightly lower elution efficiency (~55%) was observed with the triflate precursor of [18F]FDHT. Fluorine-18 labeling, reduction, and deprotection to prepare [18F]FDHT were performed on Sep-Pak cartridges (PS-HCO3 and Sep-Pak plus C-18). The overall yield of [18F]FDHT was 25–32% (decay corrected) in 70 min. The molar activity determined by using mass spectrometry was 63–148 GBq/µmol (1700–4000 Ci/mmol). Applying this quantitative measure of molar activity to in vitro assays [18F]FDHT exhibited high-affinity binding to androgen receptors (Kd~2.5 nM) providing biological validation of this method

Fluorine-18 Labeled Urea-Based Ligands Targeting Prostate-Specific Membrane Antigen (PSMA) with Increased Tumor and Decreased Renal Uptake

High expression of prostate-specific membrane antigen (PSMA) in prostate cancers prompted the development of the PSMA-targeted PET-imaging agent [18F]DCFPyL, which was recently approved by the FDA. Fluorine-18-labeled Lys–Urea–Glu-based oxime derivatives of [18F]DCFPyL were prepared for the comparison of their in vitro and in vivo properties to potentially improve kidney clearance and tumor targeting. The oxime radiotracers were produced by condensation of an aminooxy functionalized PSMA-inhibitor Lys–Urea–Glu scaffold with fluorine-18-labeled aldehydes. The radiochemical yields were between 15–42% (decay uncorrected) in 50–60 min. In vitro saturation and competition binding assays with human prostate cancer cells transfected with PSMA, PC3(+), indicated similar high nM binding affinities to PSMA for all radiotracers. In vivo biodistribution studies with positive control PC3(+) tumor xenografts showed that the kidneys had the highest uptake followed by tumors at 60 min. The PC3(+) tumor uptake was blocked with non-radioactive DCFPyL, and PC3(−) tumor xenograft (negative control) tumor uptake was negligible indicating that PSMA targeting was preserved. The most lipophilic tracer, [18F]2a, displayed comparable tumor-targeting to [18F]DCFPyL and a desirable alteration in pharmacokinetics and metabolism, resulting in significantly lower kidney uptake with a shift towards hepatobiliary clearance and increased liver uptake

Imaging the Met Receptor Tyrosine Kinase (Met) and Assessing Tumor Responses to a Met Tyrosine Kinase Inhibitor in Human Xenograft Mouse Models with a [ 99m

Developing an imaging agent targeting the hepatocyte growth factor receptor protein (Met) status of cancerous lesions would aid in the diagnosis and monitoring of Met-targeted tyrosine kinase inhibitors (TKIS). A peptide targeting Met labeled with [ 99m Tc] had high affinity in vitro (K d = 3.3 nM) and detected relative changes in Met in human cancer cell lines. In vivo [ 99m Tc]-Met peptide (AH-113018) was retained in Met-expressing tumors, and high-expressing Met tumors (MKN-45) were easily visualized and quantitated using singlephoton emission computed tomography or optical imaging. In further studies, MKN-45 mouse xenografts treated with PHA 665752 (Met TKI) or vehicle were monitored weekly for tumor responses by [ 99m Tc]-Met peptide imaging and measurement of tumor volumes. Tumor uptake of [“ m Tc]-Met peptide was significantly decreased as early as 1 week after PHA 665752 treatment, corresponding to decreases in tumor volumes. These results were comparable to Cy5**-Met peptide (AH-112543) fluorescence imaging using the same treatment model. [ 99m Tc] or Cy5**-Met peptide tumor uptake was further validated by histologic (necrosis, apoptosis) and immunoassay (total Met, p Met, and plasma shed Met) assessments in imaged and nonimaged cohorts. These data suggest that [ 99m Tc] or Cy5**-Met peptide imaging may have clinical diagnostic, prognostic, and therapeutic monitoring applications

Imaging Dopaminergic Dysfunction as a Surrogate Marker of Neuropathology in a Small-Animal Model of HIV

The dopaminergic system is especially vulnerable to the effects of human immunodeficiency virus (HIV) infection, rendering dopaminergic deficits early surrogate markers of HIV-associated neuropathology. We quantified dopamine D 2/3 receptors in young HIV-1 transgenic (Tg) ( n = 6) and age-matched control rats ( n = 7) and adult Tg ( n = 5) and age-matched control rats ( n = 5) using [ 18 F]fallypride positron emission tomography (PET). Regional uptake was quantified as binding potential ( BP ND ) using the two-tissue reference model with the cerebellum as the reference. Time-activity curves were generated for the ventral striatum, dorsal striatum, thalamus, and cerebellum. Whereas BP ND values were significantly lower in the ventral striatum ( p < .001) and dorsal striatum ( p = .001) in the adult Tg rats compared to controls rats, they were significantly lower only in the dorsal striatum ( p < .05) in the young rats. Tg rats had smaller striatal volumes on magnetic resonance imaging. We also found lower expression levels of tyrosine hydroxylase on immunohistochemistry in the Tg animals. Our findings suggest that progressive striatal D 2/3 receptor deficits occur in Tg rats as they age and can be detected using small-animal PET imaging. The effectiveness of various approaches in preventing or halting this dopaminergic loss in the Tg rat can thus be measured preclinically using [ 18 F]fallypride PET as a molecular imaging biomarker of HIV-associated neuropathology