Cannabinoid CB2 Receptors in a Mouse Model of Aβ Amyloidosis: Immunohistochemical Analysis and Suitability as a PET Biomarker of Neuroinflammation

In Alzheimer's disease (AD), one of the early responses to Aβ amyloidosis is recruitment of microglia to areas of new plaque. Microglial receptors such as cannabinoid receptor 2 (CB2) might be a suitable target for development of PET radiotracers that could serve as imaging biomarkers of Aβ-induced neuroinflammation. Mouse models of amyloidosis (J20APPswe/ind and APPswe/PS1ΔE9) were used to investigate the cellular distribution of CB2 receptors. Specificity of CB2 antibody (H60) was confirmed using J20APPswe/ind mice lacking CB2 receptors. APPswe/PS1ΔE9 mice were used in small animal PET with a CB2-targeting radiotracer, [11C]A836339. These studies revealed increased binding of [11C]A836339 in amyloid-bearing mice. Specificity of the PET signal was confirmed in a blockade study with a specific CB2 antagonist, AM630. Confocal microscopy revealed that CB2-receptor immunoreactivity was associated with astroglial (GFAP) and, predominantly, microglial (CD68) markers. CB2 receptors were observed, in particular, in microglial processes forming engulfment synapses with Aβ plaques. In contrast to glial cells, neuron (NeuN)-derived CB2 signal was equal between amyloid-bearing and control mice. The pattern of neuronal CB2 staining in amyloid-bearing mice was similar to that in human cases of AD. The data collected in this study indicate that Aβ amyloidosis without concomitant tau pathology is sufficient to activate CB2 receptors that are suitable as an imaging biomarker of neuroinflammation. The main source of enhanced CB2 PET binding in amyloid-bearing mice is increased CB2 immunoreactivity in activated microglia. The presence of CB2 immunoreactivity in neurons does not likely contribute to the enhanced CB2 PET signal in amyloid-bearing mice due to a lack of significant neuronal loss in this model. However, significant loss of neurons as seen at late stages of AD might decrease the CB2 PET signal due to loss of neuronally-derived CB2. Thus this study in mouse models of AD indicates that a CB2-specific radiotracer can be used as a biomarker of neuroinflammation in the early preclinical stages of AD, when no significant neuronal loss has yet developed.

Brown Adipose Tissue Response Dynamics: In Vivo Insights with the Voltage Sensor 18F-Fluorobenzyl Triphenyl Phosphonium

Brown adipose tissue (BAT) thermogenesis is an emerging target for prevention and treatment of obesity. Mitochondria are the heat generators of BAT. Yet, there is no noninvasive means to image the temporal dynamics of the mitochondrial activity in BAT in vivo. Here, we report a technology for quantitative monitoring of principal kinetic components of BAT adaptive thermogenesis in the living animal, using the PET imaging voltage sensor ¹⁸F-fluorobenzyltriphenylphosphonium (¹⁸F-FBnTP). ¹⁸F-FBnTP targets the mitochondrial membrane potential (ΔΨm)—the voltage analog of heat produced by mitochondria. Dynamic ¹⁸F-FBnTP PET imaging of rat’s BAT was acquired just before and during localized skin cooling or systemic pharmacologic stimulation, with and without administration of propranolol. At ambient temperature, ¹⁸F-FBnTP demonstrated rapid uptake and prolonged steady-state retention in BAT. Conversely, cold-induced mitochondrial uncoupling resulted in an immediate washout of ¹⁸F-FBnTP from BAT, which was blocked by propranolol. Specific variables of BAT evoked activity were identified and quantified, including response latency, magnitude and kinetics. Cold stimulation resulted in partial washout of ¹⁸F-FBnTP (39.1%±14.4% of basal activity). The bulk of ¹⁸F-FBnTP washout response occurred within the first minutes of the cold stimulation, while colonic temperature remained nearly intact. Drop of colonic temperature to shivering zone did not have an additive effect. The ß3-adrenergic agonist CL-316,243 elicited ¹⁸F-FBnTP washout from BAT of kinetics similar to those caused by cold stimulation. Thus, monitoring ΔΨm in vivo using ¹⁸F-FBnTP PET provides insights into the kinetic physiology of BAT. ¹⁸F-FBnTP PET depicts BAT as a highly sensitive and rapidly responsive organ, emitting heat in short burst during the first minutes of stimulation, and preceding change in core temperature. ¹⁸F-FBnTP PET provides a novel set of quantitative metrics highly important for identifying novel therapeutic targets at the mitochondrial level, for developing means to maximize BAT mass and activity, and assessing intervention efficacy.

Detection and quantification of the evolution dynamics of apoptosis using the PET voltage sensor 18F-fluorobenzyl triphenyl phosphonium

Apoptosis is a key mechanism in numerous pathologies. However, there are no effective noninvasive means available for an early detection and quantitative assessment of evolution dynamics of the apoptotic process. Here, we have characterized the ability of the novel PET voltage sensor (18)F-fluorobenzyl triphenyl phosphonium ((18)F-FBnTP) to quantify the time-dependent apoptotic action of the taxanes paclitaxel and docetaxel in vitro and in vivo.

METHODS

The duration-dependent treatment effect of paclitaxel on (18)F-FBnTP uptake was assayed in human MDA-MB-231 breast carcinoma cells. The expression of the proapoptotic Bax and antiapoptotic Bcl-2 mitochondrial proteins, release of the apoptogen cytochrome c, and activation of executioner caspase-3 were determined by Western blotting. The fraction of viable cells was determined using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. The effect of docetaxel on (18)F-FBnTP and (18)F-FDG uptake in orthotopic prostate tumors in mice was compared.

RESULTS

(18)F-FBnTP cellular uptake in viable cells declined linearly with the increasing duration of paclitaxel treatment, from 3 to 24 h, and plateaued at 48 h. The extent of decrease of (18)F-FBnTP correlated strongly with the Bax-to-Bcl-2 ratio (R(2) = 0.83) and release of cytochrome c (R(2) = 0.92), but preceded in time the caspase-3 cleavage. The P-glycoprotein blocker verapamil did not interfere with (18)F-FBnTP cellular uptake. (18)F-FBnTP prostate tumor contrast was greater than (18)F-FDG prostate tumor contrast. Docetaxel caused a marked decrease (52.4%) of (18)F-FBnTP tumor uptake, within 48 h, whereas (18)F-FDG was much less affected (12%).

CONCLUSION

The voltage sensor (18)F-FBnTP is a viable means for quantification of paclitaxel pharmacodynamics. (18)F-FBnTP permits the detection of paclitaxel apoptotic action in vivo earlier than does (18)F-FDG. (18)F-FBnTP may afford a novel approach for early detection and quantitative assessment of the cumulative-effect kinetics of proapoptotic drugs and conditions using PET.

Characterization of membrane potential-dependent uptake of the novel PET tracer 18F-fluorobenzyl triphenylphosphonium cation

PURPOSE

Mitochondrial dysfunction has been attributed a critical role in the etiology and pathogenesis of numerous diseases, and is manifested by alterations of the organelle's membrane potential (Deltapsi(m)). This suggests that Deltapsi(m) measurement can be highly useful for diagnostic purposes. In the current study, we characterized the capability of the novel PET agent (18)F-fluorobenzyl triphenylphosphonium ((18)F-FBnTP) to assess Deltapsi(m), compared with the well-established voltage sensor (3)H-tetraphenylphosphonium ((3)H-TPP).

METHODS

(18)F-FBnTP and (3)H-TPP uptake under conditions known to alter Deltapsi(m) and plasma membrane potential (Deltapsi(p)) was assayed in the H345 lung carcinoma cell line. (18)F-FBnTP biodistribution was assessed in CD1 mice using dynamic PET and ex vivo gamma well counting.

RESULTS

(18)F-FBnTP and (3)H-TPP demonstrated similar uptake kinetics and plateau concentrations in H345 cells. Stepwise membrane depolarization resulted in a linear decrease in (18)F-FBnTP cellular uptake, with a slope (-0.58+/-0.06) and correlation coefficient (0.94+/-0.07) similar (p>0.17) to those measured for (3)H-TPP (-0.63+/-0.06 and 0.96+/-0.05, respectively). Selective collapse of Deltapsi(m) caused a substantial decrease in cellular uptake for (18)F-FBnTP (81.6+/-8.1%) and (3)H-TPP (85.4+/-6.7%), compared with control. Exposure to the proapoptotic staurosporine, known to collapse Deltapsi(m), resulted in a decrease of 68.7+/-10.1% and 71.5+/-8.4% in (18)F-FBnTP and (3)H-TPP cellular uptake, respectively. (18)F-FBnTP accumulated mainly in kidney, heart and liver.

CONCLUSION

(18)F-FBnTP is a mitochondria-targeting PET radiopharmaceutical responsive to alterations in membrane potential with voltage-dependent performance similar to that of (3)H-TPP. (18)F-FBnTP is a promising new voltage sensor for detection of physiological and pathological processes associated with mitochondrial dysfunction, such as apoptosis, using PET.

Assessment of Severity of Coronary Artery Stenosis in a Canine Model Using the PET Agent 18F-Fluorobenzyl Triphenyl Phosphonium: Comparison with 99mTc-Tetrofosmin

Myocardial perfusion imaging plays an important role in clinical management of coronary artery disease, but the most commonly used radionuclides significantly underestimate the severity of coronary artery stenosis. The objective of this study was to evaluate the potential clinical utility of the PET compound (18)F-fluorobenzyl triphenyl phosphonium ((18)F-FBnTP) and characterize its capacity to assess the severity of coronary artery stenosis in a canine model in vivo and ex vivo.

METHODS

(18)F-FBnTP myocardial uptake was measured in 17 dogs with various degrees of stenosis of the left anterior descending (LAD) or circumflex (LCx) coronary arteries during adenosine vasodilation, using dynamic PET and gamma-well counting. True myocardial blood flow in ischemic (IS) and nonischemic (NIS) beds of the left ventricle was determined with radioactive microspheres. (18)F-FBnTP and (99m)Tc-tetrofosmin activities were compared in 8 dogs ex vivo.

RESULTS

The quantitative assessment of the perfusion defect was significantly (P < 0.03) more accurate with (18)F-FBnTP than with (99m)Tc-tetrofosmin, in mild (IS/NIS; 0.72 +/- 0.08, 0.93 +/- 0.07, respectively, mean +/- SE) and severe stenosis (0.42 +/- 0.05, 0.64 +/- 0.08, respectively), compared with microsphere flow (mild, 0.43 +/- 0.06; severe, 0.22 +/- 0.04). The IS/NIS ratio of both radionuclides correlated linearly with microsphere flow disparity with a similar slope. Flow defect contrast was 2.7 times greater for (18)F-FBnTP than for (99m)Tc-tetrofosmin, as inferred from the regression line intercept (0.14 vs. 0.38, respectively). The (18)F-FBnTP PET IS/NIS ratio (mild, 0.70 +/- 0.04; severe, 0.46 +/- 0.02), did not differ statistically (P >or= 0.330) from that measured ex vivo. A nearly identical qualitative and quantitative estimate of stenosis severity was obtained by early, short (5-15-min) and delayed, prolonged (30-60-min) (18)F-FBnTP PET scans. The stenotic area measured by PET was 16% smaller than that defined by tissue staining.

CONCLUSION

(18)F-FBnTP PET is a promising new technology for rapid noninvasive detection and assessment of perfusion defect severity in the myocardium.

Imaging delta- and mu-opioid receptors by PET in lung carcinoma patients

In the present study, we measured the kinetics and distribution in vivo of the selective delta-opioid antagonist 11C-methylnaltrindole (11C-MeNTI) and the mu-opioid agonist 11C-carfentanil (11C-CFN) in patients with lung carcinoma using PET.

METHODS

Paired measurements of 11C-MeNTI and 11C-CFN binding were performed in biopsy-proven small-cell (n = 2), squamous (n = 2), and adenocarcinoma (n = 3) lung cancer patients. Dynamic PET scans of increasing duration (0.5-8 min) were acquired over 90 min after an intravenous bolus injection of 370 MBq of tracer. Time-activity curves for tumor and normal lung parenchyma binding were generated using the region-of-interest (ROI) method. The mean activity at equilibrium was measured, and the specific-to-nonspecific binding ratio (tumor - lung)/lung was calculated. Four of 7 patients underwent an additional static 18F-FDG PET scan for clinical indications. Three of 7 patients underwent surgery, and stained sections of tumor were inspected for inflammation, necrosis, and scar tissue.

RESULTS

Increased binding of 11C-MeNTI and 11C-CFN was detected in all tumor types studied. 11C-MeNTI binding in tumor and healthy lung tissue was significantly more intense than that of 11C-CFN. The average specific-to-nonspecific binding ratio across cell types for 11C-MeNTI (4.32 +/- 1.31; mean +/- SEM) was greater than that of 11C-CFN (2.42 +/- 1.17) but lower than that of 18F-FDG (7.74 +/- 0.53). Intravenous naloxone produced 50% and 44% decreases in the specific-to-nonspecific binding ratios of 11C-MeNTI and 11C-CFN, respectively.

CONCLUSION

These data provide in vivo evidence for the presence of delta- and mu-opioid receptor types in the 3 major human lung carcinomas and suggest the suitability of 11C-MeNTI and 11C-CFN as investigational probes of lung carcinoma biology.

Dopamine D2 and D3 receptor occupancy in normal humans treated with the antipsychotic drug aripiprazole (OPC 14597): a study using positron emission tomography and [11C]raclopride

Aripiprazole (OPC 14597) is an antipsychotic drug that has high affinity for dopamine D2 and D3 receptors and the dopamine autoreceptor. It is being developed for treatment of patients with schizophrenia. The purpose of this study was to determine whether a dose response following graduated doses of aripiprazole could be quantified and correlated with its occupancy of the D2 and D3 dopamine receptors in the brain of living humans. Dopamine D2 and D3 receptor occupancy in fifteen normal male human brains was measured using positron emission tomography (PET) with [11C]raclopride. PET studies were performed before and after two weeks of administration of aripiprazole. The dopamine D2 receptor occupancy was quantified with two kinetic modeling methods without using a blood input function. Administration of aripiprazole for 14 days resulted in a dose-dependent receptor occupancy between 40 - 95% after the administration of 0.5mg, 1 mg, 2 mg, 10 mg, and 30 mg per day. These results suggest that an adequate occupancy can be obtained, and this may be useful to predict an appropriate therapeutic dose for an individual patient. Interestingly, even at striatal D2 receptor occupancy values above 90%, which occurred with the higher doses, extrapyramidal side effects (EPS) were not observed. This underlines aripiprazole's unique mechanism of action as a partial dopamine receptor agonist, which might become a novel principle in the treatment of schizophrenia.

Indium-111 myosin-specific antibodies and technetium-99m pyrophosphate in the detection of acute cardiac rejection of transplanted hearts: studies in a heterotopic rat heart model

111In-labelled myosin-specific antibodies were evaluated as an indicator of early changes in acute rejection in a rat heart heterotopic transplant model. Uptake of antibodies was measured in allograft and isograft hearts of animals undergoing different regimens of cyclosporine treatment and compared with the uptake of technetium-99m pyrophosphate. The data were correlated with histological estimation of the severity of myocyte necrosis and signs of early rejection (venous cuffing and endocardial inflammation, indicators of perivascular infiltrate and intermyocyte extension, respectively). Myocyte necrosis in transplanted hearts was reflected by increases in technetium-99m pyrophosphate accumulation (r = 0.88) but was poorly correlated with labelled antibody uptake (r = 0.58). There was no positive correlation between the degree of early cardiac rejection and uptake of either of the radiopharmaceuticals: accumulation of the labelled antibodies paradoxically declined with increased histological severity scores, whereas that of technetium-99m pyrophosphate remained unchanged. Cyclosporine treatment augmented the uptake of labelled antibodies in transplanted hearts. This may be due to alterations in plasma membrane permeability brought about by the drug, resulting in a rise in antibody binding to intracellular myosin.