Mass effect of injected dose in small rodent imaging by SPECT and PET

Type 1 cannabinoid receptor mapping with [18F]MK-9470 PET in the rat brain after quinolinic acid lesion: a comparison to dopamine receptors and glucose metabolism

PURPOSE

Several lines of evidence imply early alterations in metabolic, dopaminergic and endocannabinoid neurotransmission in Huntington's disease (HD). Using [18F]MK-9470 and small animal PET, we investigated cerebral changes in type 1 cannabinoid (CB1) receptor binding in the quinolinic acid (QA) rat model of HD in relation to glucose metabolism, dopamine D2 receptor availability and amphetamine-induced turning behaviour.

METHODS

Twenty-one Wistar rats (11 QA and 10 shams) were investigated. Small animal PET acquisitions were conducted on a Focus 220 with approximately 18 MBq of [18F]MK-9470, [18F]FDG and [11C]raclopride. Relative glucose metabolism and parametric CB1 receptor and D2 binding images were anatomically standardized to Paxinos space and analysed voxel-wise using Statistical Parametric Mapping (SPM2).

RESULTS

In the QA model, [18F]MK-9470 uptake, glucose metabolism and D2 receptor binding were reduced in the ipsilateral caudate-putamen by 7, 35 and 77%, respectively (all p<2.10(-5)), while an increase for these markers was observed on the contralateral side (>5%, all p<7.10(-4)). [18F]MK-9470 binding was also increased in the cerebellum (p=2.10(-5)), where it was inversely correlated to the number of ipsiversive turnings (p=7.10(-6)), suggesting that CB1 receptor upregulation in the cerebellum is related to a better functional outcome. Additionally, glucose metabolism was relatively increased in the contralateral hippocampus, thalamus and sensorimotor cortex (p=1.10(-6)).

CONCLUSION

These data point to in vivo changes in endocannabinoid transmission, specifically for CB1 receptors in the QA model, with involvement of the caudate-putamen, but also distant regions of the motor circuitry, including the cerebellum. These data also indicate the occurrence of functional plasticity on metabolism, D2 and CB1 neurotransmission in the contralateral hemisphere.

[(18)F]Fallypride dopamine D2 receptor studies using delayed microPET scans and a modified Logan plot

[(18)F]Fallypride PET studies can be used to estimate the nondisplaceable binding potential (BP(ND)) in vivo of dopamine D2/D3 receptor-rich regions of the brain. These studies often take considerable time, up to >or=2 h, limiting the throughput. In this work, we investigated whether limited-duration scans performed subsequent to tracer administration yielded stable BP(ND) estimates. In particular, we applied a modified version of the Logan plot method on the last 60 min of 120-min data and compared the results to those from analysis of the full data set.

METHODS

Fourteen male Sprague-Dawley rats were injected with [(18)F]fallypride intravenously while under isoflurane anesthesia, and dynamic data were acquired on the microPET Focus 220 scanner for 120 min. The distribution volume ratio (DVR=BP(ND)+1) was calculated from a Logan plot using 120 min of data and from a modified version using only the last 60 min. Three of these rats were imaged again on a second day to test the reproducibility. A two-tissue compartment model also was used to fit the time-activity curves (TACs) of the 120-min scans to estimate the parameters K(1), k(2), k(on), k(4) and B(max). These parameters were then used to simulate similar TACs while changing k(on) to reflect changes in the dopaminergic system. The simulated TACs were used as a means for exploring the differences in DVR estimates between the last 60 min only and the full 120 min of simulated data.

RESULTS

The average DVR from the full 120-min scans was 13.8+/-0.9, whereas the average DVR estimated from only the last 60 min of data (DVR') was 16.3+/-1.0. The DVR estimates showed good reproducibility in the three rats (mean DVR=13.8+/-1.5 on Day 1 and DVR=13.8+/-0.9 on Day 2). The simulations showed that the relationship between DVR' and DVR estimates follows a semilinear form with varying k(on).

CONCLUSION

Although the BP(ND) estimates are slightly overestimated in a delayed scan mode (i.e., no initial radiotracer uptake measurements) compared to a full scan, this overestimation depends primarily on k(3) (approximately k(on) x B(max)) and has been evaluated in this work for a wide range of k(on) values using simulated TACs. In particular, the sensitivity of DVR' to changes in k(on) is similar to that of DVR. This method of delayed scans eliminates the necessity of imaging during the initial uptake of the radiotracer and, thus, can be used to increase the throughput of studies.

Targeting murine heart and brain: visualisation conditions for multi-pinhole SPECT with (99m)Tc- and (123)I-labelled probes

PURPOSE

The study serves to optimise conditions for multi-pinhole SPECT small animal imaging of (123)I- and (99m)Tc-labelled radiopharmaceuticals with different distributions in murine heart and brain and to investigate detection and dose range thresholds for verification of differences in tracer uptake.

METHODS

A Triad 88/Trionix system with three 6-pinhole collimators was used for investigation of dose requirements for imaging of the dopamine D(2) receptor ligand [(123)I]IBZM and the cerebral perfusion tracer [(99m)Tc]HMPAO (1.2-0.4 MBq/g body weight) in healthy mice. The fatty acid [(123)I]IPPA (0.94 +/- 0.05 MBq/g body weight) and the perfusion tracer [(99m)Tc]sestamibi (3.8 +/- 0.45 MBq/g body weight) were applied to cardiomyopathic mice overexpressing the prostaglandin EP(3) receptor.

RESULTS

In vivo imaging and in vitro data revealed 45 kBq total cerebral uptake and 201 kBq cardiac uptake as thresholds for visualisation of striatal [(123)I]IBZM and of cardiac [(99m)Tc]sestamibi using 100 and 150 s acquisition time, respectively. Alterations of maximal cerebral uptake of [(123)I]IBZM by >20% (116 kBq) were verified with the prerequisite of 50% striatal of total uptake. The labelling with [(99m)Tc]sestamibi revealed a 30% lower uptake in cardiomyopathic hearts compared to wild types. [(123)I]IPPA uptake could be visualised at activity doses of 0.8 MBq/g body weight.

CONCLUSION

Multi-pinhole SPECT enables detection of alterations of the cerebral uptake of (123)I- and (99m)Tc-labelled tracers in an appropriate dose range in murine models targeting physiological processes in brain and heart. The thresholds of detection for differences in the tracer uptake determined under the conditions of our experiments well reflect distinctions in molar activity and uptake characteristics of the tracers.

Utility of small-animal positron emission tomographic imaging of rats for preclinical development of drugs acting on the serotonin transporter

Visualization of neurotransmission components in living small animals using positron emission tomography (PET) has the potential of contributing to the preclinical development of neuroactive drugs, although it is yet to be examined whether quantitative animal PET data on candidate compounds can be extrapolated to humans. Here, we investigated the comparability of the occupancies of serotonin transporter (5-HTT) by therapeutic agents in rat PET studies with our predetermined data from ex- vivo animal experiments and clinical PET scans. Rats were treated with varying doses of fluvoxamine and a newly developed compound, (2S)-1-[4-(3,4-dichlorophenyl) piperidin-1-yl]-3-[2-(5-methyl-1,3,4-oxadiazol-2-yl)benzo[b]furan-4-yloxy]propan-2-ol monohydrochloride (Wf-516), and underwent PET scans with [11C]3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile ([11C]DASB), a selective radioligand for in-vivo quantification of 5-HTT. PET images indicated a reduction of [11C]DASB binding to 5-HTT as a function of the doses and/or plasma concentrations of fluvoxamine and Wf-516. The doses of these drugs at half-maximal effect (15.2 mg/kg and 3.1 mg/kg, respectively), determined that using binding potentials for [11C]DASB, were comparable to those estimated by our previous ex-vivo measurements in rats (4.5 mg/kg and 1.1 mg/kg, respectively), as there was only a 3-fold difference between these results. Moreover, the plasma concentration of fluvoxamine needed for 50% occupancy of central 5-HTT (6.1 ng/ml) was almost equivalent to the value determined in human PET studies (4.6 ng/ml). These findings support the view that the conjunctive use of small-animal PET and [11C]DASB facilitates a quantitative comparison of in-development drugs targeting 5-HTT with established inhibitors and a predictive estimation of their plasma concentrations exerting therapeutic effects in humans.

A review of imaging agent development

This educational review highlights the processes, opportunities, and challenges encountered in the discovery and development of imaging agents, mainly positron emission tomography and single-photon emission computed tomography tracers. While the development of imaging agents parallels the drug development process, unique criteria are needed to identify opportunities for new agents. Imaging agent development has the flexibility to pursue functional or nonfunctional targets as long as they play a role in the specific disease or mechanism of interest and meet imageability requirements. However, their innovation is tempered by relatively small markets for diagnostic imaging agents, intellectual property challenges, radiolabeling constraints, and adequate target concentrations for imaging. At the same time, preclinical imaging is becoming a key translational tool for proof of mechanism and concept studies. Pharmaceutical and imaging industries face a common bottleneck in the form of the limited number of trials one company can possibly perform. However, microdosing and theranostics are evidence that partnerships between pharmaceutical and imaging companies can accelerate clinical translation of tracers and therapeutic interventions. This manuscript will comment on these aspects to provide an educational review of the discovery and development processes for imaging agents.

Kinetic modelling in small animal imaging with PET

Small animal imaging with positron emission tomography has undergone a major evolution. This has been driven by technical improvements and the development of dedicated PET camera's for small animals. The focus has shifted from detection of tracer uptake and visualization of the tracer distribution towards the quantification of the physiological parameters necessary to use this technique for kinetic modelling of tracers. At the moment there are still several issues which need further research and evaluation before we can fully employ the possibilities of PET as an in-vivo measurement of underlying molecular biology. These issues relate to improved quantification of measurements, improved image reconstruction and processing, and the use of blood plasma data in combination with kinetic models. Besides the more technical issues, there are two more issues which need further clarification: the effect of the anaesthesia, and the effect of radiation on the experiment itself. In this review, we will give an overview of how the technique can be used but we will also discuss the issues mentioned above. The focus will be on the three major parts of the imaging procedure: acquisition, reconstruction of images, and kinetic modelling of the data.

Developing new molecular imaging probes for PET

Positron emission tomography (PET) is a fully translational molecular imaging technique that requires specific probes radiolabelled with short-lived positron emitting radionuclides. This review discusses relevant methods which are applied throughout the different steps in the development of new PET probes for in vivo visualization of specific molecular targets related to diagnosis or important for drug development.

Optimised labeling, preclinical and initial clinical aspects of CCK-2 receptor-targeting with 3 radiolabeled peptides

Medullary thyroid carcinoma (MTC) expresses CCK-2 receptors. (111)In-labeled DOTA-DGlu-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH(2) (DOTA-MG11), DOTA-DAsp-Tyr-Nle-Gly-Trp-Nle-Asp-Phe-NH(2) (DOTA-CCK), and (99m)Tc-labeled N(4)-Gly-DGlu-(Glu)(5)-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH(2) ((99m)Tc-Demogastrin 2) are analogs developed for CCK-2 receptor-targeted scintigraphy. All 3 radiolabeled analogs were selected on the basis of their high CCK-2 receptor affinity and their good in vitro serum stability, with in vitro serum t(1/2) values of several hours. Radiolabeling of DOTA-peptides with (111)In requires a heating procedure, typically in the range of 80 degrees -100 degrees C up to 30 min. Following this procedure with DOTA-MG11 resulted in a >98 % incorporation of (111)In, however, with a radiochemical purity (RCP) of <50 %. The decrease in RCP was found to be due to oxidation of the methionine residue in the molecule. Moreover, this oxidized compound lost its CCK-2 receptor affinity. Therefore, conditions during radiolabeling were optimised: labeling of DOTA-MG11 and DOTA-CCK with (111)In involved 5 min heating at 80 degrees C and led to an incorporation of (111)In of >98 %. In addition, all analogs were radiolabeled in the presence of quenchers to prevent radiolysis and oxidation resulting in a RCP of >90 %. All 3 radiolabeled analogs were i.v. administered to 6 MTC patients: radioactivity cleared rapidly by the kidneys, with no significant differences in the excretion pattern of the 3 radiotracers. All 3 radiolabeled analogs exhibited a low in vivo stability in patients, as revealed during analysis of blood samples, with the respective t(1/2) found in the order of minutes. In patient blood, the rank of radiopeptide in vivo stability was: (99m)Tc-Demogastrin 2 (t(1/2) 10-15 min)>(111)In-DOTA-CCK (t(1/2) approximately 5-10 min)>(111)In-DOTA-MG11 (t(1/2)<5 min).

The distribution of D2/D3 receptor binding in the adolescent rhesus monkey using small animal PET imaging

PET imaging of the neuroreceptor systems in the brain has earned a prominent role in studying normal development, neuropsychiatric illness and developing targeted drugs. The dopaminergic system is of particular interest due to its role in the development of cognitive function and mood as well as its suspected involvement in neuropsychiatric illness. Nonhuman primate animal models provide a valuable resource for relating neurochemical changes to behavior. To facilitate comparison within and between primate models, we report in vivo D2/D3 binding in a large cohort of adolescent rhesus monkeys.

METHODS

In this work, the in vivo D2/D3 dopamine receptor availability was measured in a cohort of 33 rhesus monkeys in the adolescent stage of development (3.2-5.3 years). Both striatal and extrastriatal D2/D3 binding were measured using [F-18]fallypride with a high resolution small animal PET scanner. The distribution volume ratio (DVR) was measured for all subjects and group comparisons of D2/D3 binding among the cohort were made based on age and sex. Because two sequential studies were acquired from a single [F-18]fallypride batch, the effect of competing (unlabeled) ligand mass was also investigated.

RESULTS

Among this cohort, the rank order of regional D2/D3 receptor binding did not vary from previous studies with adult rhesus monkeys, with: putamen>caudate>ventral striatum>amygdala approximately substantia nigra>medial dorsal thalamus>lateral temporal cortex approximately frontal cortex. The DVR coefficient of variation ranged from 14%-26%, with the greatest variance seen in the head of the caudate. There were significant sex differences in [F-18]fallypride kinetics in the pituitary gland, but this was not observed for regions within the blood-brain barrier. Furthermore, no regions in the brain showed significant sex or age related differences in DVR within this small age range. Based on a wide range of injected fallypride mass across the cohort, significant competition effects could only be detected in the substantia nigra, thalamus, and frontal cortex, and were not evident above intersubject variability in all other regions.

CONCLUSION

These data represent the first report of large cohort in vivo D2/D3 dopamine whole brain binding in the adolescent brain and will serve as a valuable comparison for understanding dopamine changes during this critical time of development and provide a framework for creating a dopaminergic biochemical atlas for the rhesus monkey.

In vivo imaging of vesicular monoamine transporter 2 in pancreas using an (18)F epoxide derivative of tetrabenazine

OBJECTIVES

Development of imaging agents for pancreatic beta cell mass may provide tools for studying insulin-secreting beta cells and their relationship with diabetes mellitus. In this paper, a new imaging agent, [(18)F](+)-2-oxiranyl-3-isobutyl-9-(3-fluoropropoxy)-10-methoxy-2,3,4,6,7,11b-hexahydro-1H-pyrido[2,1-a]isoquinoline [(18)F](+)4, which displays properties targeting vesicular monoamine transporter 2 (VMAT2) binding sites of beta cells in the pancreas, was evaluated as a positron emission tomography (PET) agent for estimating beta cell mass in vivo. The hydrolyzable epoxide group of (+)4 may provide a mechanism for shifting biodistribution from liver to kidney, thus reducing the background signal.

METHODS

Both (18)F- and (19)F-labeled (+) and (-) isomers of 4 were synthesized and evaluated. Organ distribution was carried out in normal rats. Uptake of [(18)F](+)4 in pancreas of normal rats was measured and correlated with blocking studies using competing drugs, (+)dihydrotetrabenazine [(+)-DTBZ] or 9-fluoropropyl-(+)dihydro tetrabenazine [FP-(+)-DTBZ, (+)2].

RESULTS

In vitro binding study of VMAT2 using rat brain striatum showed a K(i) value of 0.08 and 0.15 nM for the (+)4 and (+/-)4, respectively. The in vivo biodistribution of [(18)F](+)4 in rats showed the highest uptake in the pancreas (2.68 %ID/g at 60 min postinjection). In vivo competition experiments with cold FP-(+)-DTBZ, (+)2, (3.5 mg/kg, 5 min iv pretreatment) led to a significant reduction of pancreas uptake (85% blockade at 60 min). The inactive isomer [(18)F](-)4 showed significantly lower pancreas uptake (0.22 %ID/g at 30 min postinjection). Animal PET imaging studies of [(18)F](+)4 in normal rats demonstrated an avid pancreatic uptake in rats.

CONCLUSION

The preliminary results suggest that the epoxide, [(18)F](+)4, is highly selective in binding to VMAT2 and it has an excellent uptake in the pancreas of rats. The liver uptake was significantly reduced through the use of the epoxide group. Therefore, it may be potentially useful for imaging beta cell mass in the pancreas.

Targeted imaging: an important biomarker for understanding disease progression in the era of personalized medicine

The key to applying targeted imaging to personalized medicine is the choice of the right radiolabeled probe for the right target for the right disease following the lead of pharmaceutical development. The imaging approach differs depending on whether the target is a single disease control point (e.g. a specific receptor or transport protein linked to the mechanistic activity of a drug) or a general disease control point applicable to a number of treatment paradigms (e.g. proliferation, angiogenesis, inflammation). But in either case, the number of control points must be small given the time constraints on molecular imaging procedures in the clinic. Regardless of the choice, the radiotracer must be validated as binding to the target with the appropriate pharmacokinetics and pharmacodynamics for effective external imaging. Such an imaging agent developed in concert with drug development has a built in synergy that will accelerate the drug development process, targeted imaging and personalized medicine as well.

Small-animal preclinical nuclear medicine instrumentation and methodology

Molecular medicine enhances the clinician's ability to accurately diagnose and treat disease, and many technological advances in diverse fields have made the translation of molecular medicine to the clinic possible. Nuclear medicine encompasses 2 technologies--single-photon emission computed tomography (SPECT) and positron emission tomography (PET)--that have driven the field of molecular medicine forward. SPECT and PET, inherently molecular imaging techniques, have been at the forefront of molecular medicine for several decades. These modalities exploit the radioactive decay of nuclides with specific decay properties that make them useful for in vivo imaging. As recently as the mid-1990s, SPECT and PET were mostly restricted to use in the clinical setting because their relatively coarse spatial resolution limited their usefulness in studying animal (especially rodent) models of human disease. About a decade ago, several groups began making significant strides in improving resolution to the point that small-animal SPECT and PET as a molecular imaging technique was useful in the study of rodent disease models. The advances in these 2 techniques progressed as the result of improvements in instrumentation and data reconstruction software. Here, we review the impact of small-animal imaging and, specifically, nuclear medicine imaging techniques on the understanding of the biological basis of disease and the expectation that these advances will be translated to clinical medicine.

What were they thinking? Cognitive states may influence [11C]raclopride binding potential in the striatum

[(11)C]Raclopride ([(11)C]RAC) is a selective dopamine D(2)/D(3) antagonist that is commonly used in positron emission tomography (PET) studies to assess both basal levels of receptor availability and changes in availability caused by alterations in striatal dopamine concentration. When designing [(11)C]RAC studies, it is important to understand what variables may affect the results. Here, we examined differences in baseline striatal [(11)C]RAC binding potential (BP(ND)) under two different "rest" conditions. Thirteen subjects received [(11)C]RAC scans. Eight subjects were aware prior to initiation of scanning that they would receive a "baseline" scan, and that no additional procedures would take place during the scan ("certain rest" group, CER). Five subjects were informed that they might or might not receive an i.v. alcohol infusion during the scan ("uncertain rest" group, UNC). This group was informed five min after scan start that they would not receive alcohol. Voxel-wise analyses of binding potential (BP(ND)) images generated for both "rest" conditions indicated that receptor availability was higher in UNC than in CER. This result was confirmed by a region-of-interest analysis, which indicated that the average BP(ND) in right and left putamen was statistically higher in UNC. There were no differences in groups with respect to age or raclopride mass dose that could account for the difference in D(2)/D(3) receptor availability. Our findings suggest that even slight differences in cognitive states between groups can have an effect on BP(ND), presumably mediated by changes in endogenous dopamine concentration.

2-(2'-((dimethylamino)methyl)-4'-(fluoroalkoxy)-phenylthio)benzenamine derivatives as serotonin transporter imaging agents

A novel series of ligands with substitutions at the 5-position on phenyl ring A and at the 4'-position on phenyl ring B of 2-(2'-((dimethylamino)methyl)-4'-(fluoroalkoxy)phenylthio)benzenamine (4'-2-fluoroethoxy derivatives 28-31 and 4'-3-fluoropropoxy derivatives 40-42) were prepared and tested as serotonin transporter (SERT) imaging agents. The new ligands displayed high binding affinities to SERT (Ki ranging from 0.03 to 1.4 nM). The corresponding 18F labeled compounds, which can be prepared readily, showed excellent brain uptake and retention after iv injection in rats. The hypothalamus region showed high uptake values between 0.74% and 2.2% dose/g at 120 min after iv injection. Significantly, the hypothalamus to cerebellum ratios (target to nontarget ratios) at 120 min were 7.8 and 7.7 for [18F]28 and [18F]40, respectively. The selective uptake and retention in the hypothalamus, which has a high concentration of SERT binding sites, demonstrated that [18F]28 and [18F]40 are promising positron emission computed tomography imaging agents for mapping SERT binding sites in the brain.

Challenge by the murine brain: multi-pinhole SPECT of 123I-labelled pharmaceuticals

This protocol presents an improved method for SPECT imaging based on multi-pinhole techniques, applied to the visualisation of neurotracers in small animal models. Three types of collimators with 6-pinhole apertures adapted to special requirements for the imaging of the brain of mice and rats and to full body imaging in mice are employed in the experiments. A conventional triple-headed TRIAD/Trionix SPECT system was upgraded with pyramidal supports and shieldings onto the multi-pinhole collimators were installed. The system was employed for the assessment of the uptake of [123I]FP-CIT and [123I]IBZM, well known tracers of dopamine transport and dopamine D2/D3 receptors, respectively. Requirements regarding the applied radioactivity are reported, as well as further conditions determining the effectiveness of the detection of the uptake of [123I]FP-CIT and [123I]IBZM. The measurements in mice required only 20-25% of the activity described in previous studies. Dynamic measurements are presented, with a time resolution as high as 10 min in the brain of rats. Due to the lower signal intensity obtained for mice, the time resolution was 42min for [123I]FP-CIT, with a ratio ROI/background of 5.4, and 17 min for [123I]IBZM, with the ratio ROI/background of 4.5 (1.6-7.4).

Imaging the availability of serotonin transporter in rat brain with 123I-ADAM and small-animal SPECT

BACKGROUND

Imaging serotonin transporters during antidepressant treatment in small animals is a useful tool for preclinical study during drug development. In this work, we aimed to demonstrate the feasibility of using 123I-ADAM and small-animal SPECT to monitor serotonin transporter availabilities in rat brains prior to and after administration of a selective serotonin re-uptake inhibitor.

METHODS

Male Sprague-Dawley rats with and without administration of citalopram (4 mg x kg body weight) were examined in this study. During the process rat brains were scanned using a double-headed microSPECT system equipped with pinhole collimators. SPECT tomographic images and X-ray computed tomography (CT) were acquired after introducing 123I-ADAM via the tail vein. The 123I-ADAM specific binding was assessed by SPECT/CT fused image to draw regions of interest in the midbrain and cerebellum. Ex-vivo autoradiography was carried out as a parallel investigation to validate the SPECT technique.

RESULTS

SPECT images displayed specific binding ratio in midbrain to be 0.91+/-0.30 averaged from three rats. Drug occupancies (95.47+/-1.56)% were shown after administration of citalopram in a dosage of 4 mg x kg.

CONCLUSION

This study demonstrated that the serotonin transporter availability during antidepressant treatment in small animals can be assessed semi-quantitatively by using 123I-ADAM and SPECT.

Biomedical imaging research opportunities workshop IV: a white paper

The Fourth Biomedical Imaging Research Opportunities Workshop (BIROW IV) was held on February 24-25, 2006, in North Bethesda, MD. The workshop focused on opportunities for research and development in four areas of imaging: imaging of rodent models; imaging in drug development; imaging of chronic metabolic disease: diabetes; and image guided intervention in the fourth dimension-time. These topics were examined by four keynote speakers in plenary sessions and then discussed in breakout sessions devoted to identifying research opportunities and challenges in the individual topics. This paper synthesizes these discussions into a strategy for future research directions in biomedical imaging.

Effect of a P-glycoprotein inhibitor, Cyclosporin A, on the disposition in rodent brain and blood of the 5-HT1A receptor radioligand, [11C](R)-(-)-RWAY

Limited brain uptake of radioligands with otherwise optimal properties for imaging brain receptors can be improved by blocking the effect of P-glycoprotein (P-gp), an efflux pump at the blood-brain barrier. Using small animal positron emission tomography (PET), we investigated how P-gp and its blockade with Cyclosporin A (CsA) affect rodent brain uptake of [(11)C](R)-(-)-RWAY, a radioligand for brain 5-HT(1A) receptors. Brain uptake of radioactivity was compared in control and CsA-treated rats as well as P-gp knockout and wild type mice. Parent radioligand and radiometabolite in plasma and brain samples were determined at 30 min after injection of radioligand. PET binding potential (BP) was calculated with a reference tissue model. P-gp knockout mice had 2.5- and 2.8-fold greater brain uptake of [(11)C](R)-(-)-RWAY than wild type ones, measured by in vivo PET and ex vivo tissue sampling, respectively. Similarly, CsA increased rat brain uptake 4.9- and 5.3-fold, in vivo and ex vivo. In addition, CsA increased the plasma free fraction of [(11)C](R)-(-)-RWAY in rats by 2.7-fold. Although CsA increased brain uptake in wild type mice by 2.5-fold, it had no effect on plasma free fraction in knockout animals. Three radiometabolites of [(11)C](R)-(-)-RWAY were uniformly distributed in rat brain, suggesting they were inactive. CsA treatment increased brain uptake of [(11)C](R)-(-)-RWAY and only one of its radiometabolites. Regional rat brain BP increased 27-70% in the CsA-treated rats and the P-gp knockout mice. [(11)C](R)-(-)-RWAY is a P-gp substrate in rat and mouse. The effects of CsA in rats are mediated by both P-gp blockade and displacement of the radiotracer from plasma proteins. Studies with wild type and knockout mice showed that CsA affected only P-gp in this species.

Pharmacological prerequisites for PET ligands and practical issues in preclinical PET research

The development of PET radiopharmaceuticals for the non-invasive imaging of cancerous lesions, brain receptors, transporters and enzymes started more than 25 years ago. But till today no established algorithms exist to predict the success of a PET radiopharmaceutical. PET radioligand development is a challenging endeavor and predicting the success of PET ligand can be an elusive undertaking. A large number of PET radiopharmaceuticals have been developed for imaging, but so far only a few have found application as imaging agents in vivo in humans. Typically, the potential compound selected for development usually has the desired in vitro characteristics but unknown in vivo properties. The purpose of this chapter is to highlight some of the pharmacological constraints and prerequisites. Interspecies difference in metabolism and mass effects are discussed with examples. Finally, some of the practical issues related to laboratory animal imaging using anesthetic agents are also presented.

Comparison of [18F]FDOPA, [18F]FMT and [18F]FECNT for imaging dopaminergic neurotransmission in mice

INTRODUCTION

The clinically established positron emission tomography (PET) tracers 6-[(18)F]-fluoro-l-DOPA ([(18)F]FDOPA), 6-[(18)F]-fluoro-l-m-tyrosine ([(18)F]FMT) and 2beta-carbomethoxy-3beta-(4-chlorophenyl)-8-(2-[(18)F]-fluoroethyl)-nortropane ([(18)F]FECNT) serve as markers of presynaptic integrity of dopaminergic nerve terminals in humans. This study describes our efforts to adopt the methodology of human Parkinson's disease (PD) PET studies to mice.

METHODS

The PET imaging characteristics of [(18)F]FDOPA, [(18)F]FMT and [(18)F]FECNT were analyzed in healthy C57BL/6 mice using the dedicated small-animal PET tomograph quad-HIDAC. Furthermore, [(18)F]FECNT was tested in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD.

RESULTS

[(18)F]FDOPA and [(18)F]FMT failed to clearly visualize the mouse striatum, whereas PET experiments using [(18)F]FECNT proved that the employed methodology is capable of delineating the striatum in mice with exquisite resolution. Moreover, [(18)F]FECNT PET imaging of healthy and MPTP-lesioned mice demonstrated that the detection and quantification of striatal degeneration in lesioned mice can be accomplished.

CONCLUSIONS

This study shows the feasibility of using [(18)F]FECNT PET to analyze noninvasively the striatal degeneration in the MPTP mouse model of PD. This methodology can be therefore considered as a viable complement to established in vivo microdialysis and postmortem techniques.