Brain Penetration of the ROS1/ALK Inhibitor Lorlatinib Confirmed by PET.

The Massachusetts General Hospital Radiochemistry Program, in collaboration with Pfizer, has developed unique 11C and 18F-labeling strategies to synthesize isotopologs of lorlatinib (PF-06463922) which is undergoing phase III clinical trial investigations for treatment of non-small-cell lung cancers with specific molecular alterations. A major goal in cancer therapeutics is to measure the concentrations of this drug in the brain metastases of patients with lung cancer, and penetration of the blood-brain barrier is important for optimal therapeutic outcomes. Our recent publication in Nature Communications employed radiolabeled lorlatinib and positron emission tomography (PET) studies in preclinical models including nonhuman primates (NHPs) that demonstrated high brain permeability of this compound. Our future work with radiolabeled lorlatinib will include advanced PET evaluations in rodent tumor models and normal NHPs with the goal of clinical translation.

Synthesis and preliminary PET imaging of 11C and 18F isotopologues of the ROS1/ALK inhibitor lorlatinib.

Lorlatinib (PF-06463922) is a next-generation small-molecule inhibitor of the orphan receptor tyrosine kinase c-ros oncogene 1 (ROS1), which has a kinase domain that is physiologically related to anaplastic lymphoma kinase (ALK), and is undergoing Phase I/II clinical trial investigations for non-small cell lung cancers. An early goal is to measure the concentrations of this drug in brain tumour lesions of lung cancer patients, as penetration of the blood-brain barrier is important for optimal therapeutic outcomes. Here we prepare both 11C- and 18F-isotopologues of lorlatinib to determine the biodistribution and whole-body dosimetry assessments by positron emission tomography (PET). Non-traditional radiolabelling strategies are employed to enable an automated multistep 11C-labelling process and an iodonium ylide-based radiofluorination. Carbon-11-labelled lorlatinib is routinely prepared with good radiochemical yields and shows reasonable tumour uptake in rodents. PET imaging in non-human primates confirms that this radiotracer has high brain permeability.

First-in-Human Assessment of the Novel PDE2A PET Radiotracer 18F-PF-05270430.

UNLABELLED: This was a first-in-human study of the novel phosphodiesterase-2A (PDE2A) PET ligand (18)F-PF-05270430. The primary goals were to determine the appropriate tracer kinetic model to quantify brain uptake and to examine the within-subject test-retest variability. METHODS: In advance of human studies, radiation dosimetry was determined in nonhuman primates. Six healthy male subjects participated in a test-retest protocol with dynamic scans and metabolite-corrected input functions. Nine brain regions of interest were studied, including the striatum, white matter, neocortical regions, and cerebellum. Multiple modeling methods were applied to calculate volume of distribution (VT) and binding potentials relative to the nondisplaceable tracer in tissue (BPND), concentration of tracer in plasma (BPP), and free tracer in tissue (BPF). The cerebellum was selected as a reference region to calculate binding potentials. RESULTS: The dosimetry study provided an effective dose of less than 0.30 mSv/MBq, with the gallbladder as the critical organ; the human target dose was 185 MBq. There were no adverse events or clinically detectable pharmacologic effects reported. Tracer uptake was highest in the striatum, followed by neocortical regions and white matter, and lowest in the cerebellum. Regional time-activity curves were well fit by multilinear analysis-1, and a 70-min scan duration was sufficient to quantify VT and the binding potentials. BPND, with mean values ranging from 0.3 to 0.8, showed the best intrasubject and intersubject variability and reliability. Test-retest variability in the whole brain (excluding the cerebellum) of VT, BPND, and BPP were 8%, 16%, and 17%, respectively. CONCLUSION: (18)F-PF-05270430 shows promise as a PDE2A PET ligand, albeit with low binding potential values.

The phosphodiesterase 10 positron emission tomography tracer, [18F]MNI-659, as a novel biomarker for early Huntington disease.

IMPORTANCE: In Huntington disease (HD) striatal neuron loss precedes and predicts motor signs or symptoms. Current imaging biomarkers lack adequate sensitivity for assessing the early stages of HD. Developing an imaging biomarker for HD spanning the time of onset of motor signs remains a major unmet research need. Intracellular proteins whose expression is altered by the mutant huntingtin protein may be superior markers for early HD stages. OBJECTIVE: To evaluate whether [18F]MNI-659 (2-(2-(3-(4-(2-[18F]fluoroethoxy)phenyl)-7-methyl-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione), a novel phosphodiesterase 10 positron emission tomography (PET) ligand, is a sensitive marker for striatal changes in early HD. DESIGN, SETTING, AND PARTICIPANTS: A cohort of individuals with HD, including premanifest (pre-HD) or manifest with motor signs (mHD), underwent clinical assessments, genetic determination, [18F]MNI-659 PET imaging, and brain magnetic resonance imaging. Age-matched healthy volunteers (HVs) also received clinical assessments and PET and magnetic resonance imaging. MAIN OUTCOMES AND MEASURES: Binding potentials (BPnds) were estimated for brain regions of interest, specifically within the basal ganglia, and compared between participants with HD and the HVs and correlated with markers of HD severity and atrophy of basal ganglia nuclei. RESULTS: Eleven participants with HD (8 mHD and 3 pre-HD) and 9 HVs participated. Ten of 11 HD participants had known huntingtin CAG repeat length, allowing determination of a burden of pathology (BOP) score. One individual with HD declined CAG determination. All participants with mHD had relatively early-stage disease (4 with stage 1 and 4 with stage 2) and a Unified Huntington's Disease Rating Scale (UHDRS) total Motor subscale score of less than 50. The HD cohort had significantly lower striatal [18F]MNI-659 uptake than did the HV cohort (mean, -48.4%; P < .001). The HD cohort as a whole had a reduction in the basal ganglia BPnd to approximately 50% of the level in the HVs (mean, -47.6%; P < .001). The 3 pre-HD participants had intermediate basal ganglia BPnds. Striatal [18F]MNI-659 uptake correlated strongly with the severity of disease measured by the clinical scale (UHDRS Motor subscale; R = 0.903; P < .001), the molecular marker (BOP; R = 0.908; P < .001), and regional atrophy (R = 0.667; P < .05). CONCLUSIONS AND RELEVANCE: As a promising striatal imaging biomarker, [18F]MNI-659 is potentially capable of assessing the extent of disease in early mHD. Furthermore, [18F]MNI-659 may identify early changes in medium spiny neurons and serve as a marker to predict conversion to mHD. Additional studies with larger, stratified cohorts of patients with HD and prospective studies of individuals with pre-HD are warranted.

Molecular imaging of neurovascular cell death in experimental cerebral stroke by PET.

UNLABELLED: Clinical molecular imaging of apoptosis is a highly desirable yet unmet challenge. Here we provide the first report on (18)F-labeled 5-fluoropentyl-2-methyl-malonic acid ((18)F-ML-10), a small-molecule, (18)F-labeled PET tracer for the imaging of apoptosis in vivo; this report includes descriptions of the synthesis, radiolabeling, and biodistribution of this novel apoptosis marker. We also describe the use of (18)F-ML-10 for small-animal PET of neurovascular cell death in experimental cerebral stroke in mice. METHODS: (18)F-ML-10 was synthesized by nucleophilic substitution from the respective mesylate precursor, and its biodistribution was assessed in healthy rats. Permanent occlusion of the middle cerebral artery (MCA) was induced in mice, and small-animal PET was performed 24 h later. RESULTS: Efficient radiolabeling of ML-10 with (18)F was achieved. Biodistribution studies with (18)F-ML-10 revealed rapid clearance from blood (half-life of 23 min), a lack of binding to healthy tissues, and rapid elimination through the kidneys. No significant tracer metabolism in vivo was observed. Clear images of distinct regions of increased uptake, selectively in the ischemic MCA territory, were obtained in the in vivo small-animal PET studies. Uptake measurements ex vivo revealed 2-fold-higher uptake in the affected hemisphere and 6- to 10-fold-higher uptake in the region of interest of the infarct. The cerebral uptake of (18)F-ML-10 was well correlated with histologic evidence of cell death. The tracer was retained in the stroke area but was cleared from blood and from intact brain areas. CONCLUSION: (18)F-ML-10 is useful for noninvasive PET of neurovascular histopathology in ischemic cerebral stroke in vivo. Such an assessment may assist in characterization of the extent of stroke-related cerebral damage and in the monitoring of disease course and effect of treatment.

In vitro and in vivo binding of neuroactive steroids to the sigma-1 receptor as measured with the positron emission tomography radioligand [18F]FPS.

Sigma-1 receptors are widely expressed in the mammalian brain and also in organs of the immune, endocrine and reproductive systems. Based on behavioral and pharmacological assessments, sigma-1 receptors are important in memory and cognitive processes, and are thought to be involved in specific psychiatric illnesses, including schizophrenia, depression, and drug addiction. It is thought that specific neuroactive steroids are endogenous ligands for these sites. In addition, several sigma-1 receptor binding steroids including progesterone, dihydroepiandrosterone (DHEA), and testosterone are being examined clinically for specific therapeutic purposes; however, their mechanisms of action have not been clearly defined. We previously described the high affinity sigma-1 receptor selective PET tracer [(18)F]FPS. This study examines the effect of neuroactive steroids on [(18)F]FPS binding in vitro and in vivo. Inhibition constants were determined in vitro for progesterone, testosterone, DHEA, estradiol, and estriol binding to the [(18)F]FPS labeled receptor. The affinity order (K(i) values) for these steroids ranged from 36 nM for progesterone to >10,000 nM for estrodiol and estriol. Biodistribution studies revealed that i.v. coadministration of progesterone (10 mg/kg), testosterone (20 mg/kg), or DHEA (20 mg/kg) significantly decreased [(18)F]FPS uptake (%ID/g) by up to 50% in nearly all of eight brain regions examined. [(18)F]FPS uptake in several peripheral organs that express sigma-1 receptors (heart, spleen, muscle, lung) was also reduced (54-85%). These studies clearly demonstrate that exogenously administered steroids can occupy sigma-1 receptors in vivo, and that [(18)F]FPS may provide an effective tool for monitoring sigma-1 receptor occupancy of specific therapeutic steroids during clinical trials.

Imaging sigma receptors: applications in drug development.

Sigma receptors have been implicated in a myriad of cellular functions, biological processes and diseases. While the precise biological functions of sigma receptors have not been elucidated, recent work has shed some light on to these enigmatic systems. Sigma receptors have recently been a target of drug development related to psychiatric and neurological disorders. Sigma ligands have also been shown to modulate endothelial cell proliferation and can control angiogenesis which makes them a promising target for oncology applications. Other areas currently being investigated include treatment of gastrointestinal, cardiovascular, endocrine and immune system disorders. Of interest is that the human sigma-1 receptor gene contains a steroid binding component, and several gonadal steroids, including progesterone, testosterone and dehydroepiandrosterone (DHEA), interact with sigma-1 receptors. Of the steroids examined thus far, progesterone binds with the highest affinity to human sigma-1 receptors, with a reported affinity (Ki) as high as 30 nM while the other steroids exhibit lower affinity. For this and other reasons, sigma-1 receptors have been proposed as a link between the central nervous system and the endocrine and reproductive systems. Taken together, the above information highlights an important yet largely unexplored but promising area of research to examine the biological function and therapeutic potential of sigma receptors. This review provides an overview of the current knowledge of these sites with a focus on specific areas where in vivo sigma receptor imaging is currently being investigated.

In vivo tomographic imaging studies of neurodegeneration and neuroprotection: a review.

Noninvasive tomographic imaging methods including positron emission tomography (PET) and single photon emission computed tomography (SPECT) are extremely sensitive and are capable of measuring biochemical processes that occur at concentrations in the nanomolar range. Inherent to neurodegenerative processes is neuronal loss. Thus, PET or SPECT monitoring of biochemical processes altered by neuronal loss (changes in neurotransmitter turnover, alterations in receptor, transporter or enzyme concentrations) can provide unique information not attainable by other methods. Such imaging techniques can also be used to longtitudinally monitor the effects of neuroprotective treatments. This review highlights current imaging probes used to evaluate patients with specific neurodegenerative disorders (e.g., Alzheimer's Disease, Parkinson's Disease, Huntington's Chorea), including those that image receptors of the dopaminergic, cholinergic and glutamatergic systems. Areas of future research focus are also defined. It is clear that monitoring the progression of neurodegenerative disorders and the impact of neuroprotective treatments are two different but related goals for which noninvasive imaging via PET and SPECT methods plays a powerful and unique role.

Preclinical acute toxicity studies and dosimetry estimates of the novel sigma-1 receptor radiotracer, [18F]SFE.

[(18)F]1-(2-Fluoroethyl)-4-[(4-cyanophenoxy)methyl]piperidine ([(18)F]SFE) is a novel, selective, high-affinity sigma-1 receptor radioligand that has been preclinically well characterized in rodents. To support an investigational new drug (IND) application for the first evaluation of [(18)F]SFE in humans, single-organ and whole-body radiation adsorbed doses associated with [(18)F]SFE injection were estimated from rat distribution data. In addition, single- and multiple-dose toxicity studies were conducted in rabbits and in dogs. Multiple-dose toxicity studies in rabbits and single-dose toxicity studies in beagles suggest at least a 100-fold safety margin for humans studies at a mass dose limit of 4.0 mug per intravenous injection, based on the combined no observable adverse effect levels (NOAEL, mg/m(2)) measured in these species. Radiation dosimetry estimates obtained from rat biodistribution analyses of [(18)F]SFE suggest that most tissues would receive about 0.010-0.020 mGy/MBq, while the adrenal glands, brain, bone, liver, lungs, and spleen would receive slightly higher doses (0.024-0.044 mGy/MBq). The adrenal glands were identified as the critical organ, because they received the highest adsorbed radiation dose. The total exposure resulting from a 5 mCi administration of [(18)F]SFE is well below the FDA-defined limits for yearly cumulative and per-study exposures to research participants. These combined results support the expectation that [(18)F]SFE will be safe for use in human positron emission tomography (PET) imaging studies with the administration of 5 mCi and a mass dose equal to or less than 4.0 mug SFE per injection.

[123I]TPCNE--a novel SPET tracer for the sigma-1 receptor: first human studies and in vivo haloperidol challenge.

[123I]TPCNE (1(trans-[123I]iodopropen-2-yl)-4-[(4-cyanophenoxy)methyl] piperidine; Ki = 0.67 nM; log P = 3.36) is a novel sigma-1 receptor SPET ligand. In this study, we developed an optimized labeling method for [123I]TPCNE and investigated the kinetics, binding characteristics, and whole-body distribution of this tracer for the first time in humans. We also performed a challenge with the sigma-1 receptor antagonist haloperidol against [123I]TPCNE. Seven healthy volunteers were recruited. Dynamic brain SPET scans were performed following i.v. administration of 185 MBq [123I]TPCNE in all seven subjects. Three of the subjects were given oral haloperidol (2.5 mg) approximately 1 h before the scan. The dynamic data were analyzed with both reversible and irreversible compartmental models.[123I]TPCNE showed high uptake in brain and liver. All non-haloperidol-treated subjects showed a high whole-brain uptake (average: 8.7% of injected activity). No significant clearance of the tracer was seen up to 30 h post injection. In the haloperidol-treated subjects, the time-activity curves clearly demonstrated clearance of the tracer from the brain. Regional radioactivity concentrations were reduced by haloperidol from 42% in the cerebellum to 73% in the thalamus.[(123)I]TPCNE demonstrated high brain uptake, with highest binding found in the posterior cingulate. A region in which binding was unaffected by haloperidol pretreatment could not be identified, and the time-activity data were best described by an irreversible model.

In vivo evaluation in rats of [(18)F]1-(2-fluoroethyl)-4-[(4-cyanophenoxy)methyl]piperidine as a potential radiotracer for PET assessment of CNS sigma-1 receptors.

INTRODUCTION: Sigma-1 receptors are expressed throughout the mammalian central nervous system (CNS) and are implicated in several psychiatric disorders, including schizophrenia and depression. We have recently evaluated the high-affinity (K(D)=0.5+/-0.2 nM, log P=2.9) sigma-1 receptor radiotracer [(18)F]1-(3-fluoropropyl)-4-(4-cyanophenoxymethyl)piperidine, [(18)F]FPS, in humans. In contrast to appropriate kinetics exhibited in baboon brain, in the human CNS, [(18)F]FPS does not reach pseudoequilibrium by 4 h, supporting the development of a lower-affinity tracer [Waterhouse RN, Nobler MS, Chang RC, Zhou Y, Morales O, Kuwabara H, et al. First evaluation of the sigma-1 receptor radioligand [(18)F]1-3-fluoropropyl-4-((4-cyanophenoxy)-methyl)piperidine ([(18)F]FPS) in healthy humans. Neuroreceptor Mapping 2004, July 15-18th, Vancouver, BC Canada 2004]. We describe herein the in vivo evaluation in rats of [(18)F]1-(2-fluoroethyl)-4-[(4-cyanophenoxy)methyl]piperidine ([(18)F]SFE) (K(D)=5 nM, log P=2.4), a structurally similar, lower-affinity sigma-1 receptor radioligand. METHODS: [(18)F]SFE was synthesized (n=4) as previously described in good yield (54+/-6% EOB), high specific activity (2.1+/-0.6 Ci/micromol EOS) and radiochemical purity (98+/-1%) and evaluated in awake adult male rats. RESULTS: Similar to [(18)F]FPS, regional brain radioactivity concentrations [percentage of injected dose per gram of tissue (%ID/g), 15 min] for [(18)F]SFE were highest in occipital cortex (1.86+/-0.06 %ID/g) and frontal cortex (1.76+/-0.38 %ID/g), and lowest in the hippocampus (1.01+/-0.02%ID/g). Unlike [(18)F]FPS, [(18)F]SFE cleared from the brain with approximately 40% reduction in peak activity over a 90-min period. Metabolite analysis (1 h) revealed that [(18)F]SFE was largely intact in the brain. Blocking studies showed a large degree (>80%) of saturable binding for [(18)F]SFE in discrete brain regions. CONCLUSIONS: We conclude that [(18)F]SFE exhibits excellent characteristics in vivo and may provide a superior PET radiotracer for human studies due to its faster CNS clearance compared to [(18)F]FPS.

In vivo evaluation of [123I]-4-iodo-N-(4-(4-(2-methoxyphenyl)-piperazin-1-yl)butyl)-benzamide: a potential sigma receptor ligand for SPECT studies.

In this study, in vivo evaluation in mice and rabbits of [123I]-4-iodo-N-(4-(4-(2-methoxyphenyl)-piperazin-1-yl)butyl)-benzamide ([123I]-BPB), a potential radioligand for visualisation of the sigma receptor by single photon emission computed tomography (SPECT), is reported. The compound possesses appropriate lipophilicity (log P=2.2) and binds sigma-1 and sigma-2 receptors (pKi=6.51 and 6.79, respectively). In mice, this new radioiodinated tracer exhibited high brain uptake (4.99% ID/g tissue at 10 min postinjection) and saturable binding (3.06% ID/g tissue at 10 min postinjection) as determined by pretreatment with unlabeled [123I]-BPB. A metabolite study demonstrated no (less than 5%) labeled metabolites in the brain. In rabbits, regional brain distribution was investigated and the tracer displayed high, homogeneous central nervous system uptake. Selectivity was assessed by competition experiments with known sigma ligands. Metabolite analysis showed no (less than 8%) labeled metabolites in the rabbit brain. In conclusion, our findings indicate that [123I]-BPB is not a suitable tracer for visualisation of D3 receptors while its potential for sigma receptor imaging is severely hampered by its affinity for dopamine receptors.

In vivo evaluation of [11C]N-(2-chloro-5-thiomethylphenyl)-N'-(3-methoxy-phenyl)-N'-methylguanidine ([11C]GMOM) as a potential PET radiotracer for the PCP/NMDA receptor.

The development of imaging methods to measure changes in NMDA ion channel activation would provide a powerful means to probe the mechanisms of drugs and device based treatments (e.g., ECT) thought to alter glutamate neurotransmission. To provide a potential NMDA/PCP receptor PET tracer, we synthesized the radioligand [11C]GMOM (ki = 5.2 +/-0.3 nM; log P = 2.34) and evaluated this ligand in vivo in awake male rats and isoflurane anesthetized baboons. In rats, the regional brain uptake of [11C]GMOM ranged from 0.75+/-0.13% ID/g in the medulla and pons to 1.15+/-0.17% ID/g in the occipital cortex. MK801 (1 mg/kg i.v.) significantly reduced (24-28%) [11C]GMOM uptake in all regions. D-serine (10 mg/kg i.v.) increased [11C]GMOM %ID/g values in all regions (10-24%) reaching significance in the frontal cortex and cerebellum only. The NR2B ligand RO 25-6981 (10 mg/kg i.v.) reduced [11C]GMOM uptake significantly (24-38%) in all regions except for the cerebellum and striatum. Blood activity was 0.11+/-0.03 %ID/g in the controls group and did not vary significantly across groups. PET imaging in isoflurane-anesthetized baboons with high specific activity [11C]GMOM provided fairly uniform regional brain distribution volume (VT) values (12.8-17.1 ml g(-1)). MK801 (0.5 mg/kg, i.v., n = 1, and 1.0 mg/kg, i.v., n = 1) did not significantly alter regional VT values, indicating a lack of saturable binding. However, the potential confounding effects associated with ketamine induction of anesthesia along with isoflurane maintenance must be considered because both agents are known to reduce NMDA ion channel activation. Future and carefully designed studies, presumably utilizing an optimized NMDA/PCP site tracer, will be carried out to further explore these hypotheses. We conclude that, even though [11C]GMOM is not an optimized PCP site radiotracer, its binding is altered in vivo in awake rats as expected by modulation of NMDA ion channel activity by MK801, D-serine or RO 25-6981. The development of higher affinity NMDA/PCP site radioligands is in progress.

Imaging the PCP site of the NMDA ion channel.

The N-methyl-D-aspartate (NMDA) ion channel plays a role in neuroprotection, neurodegeneration, long-term potentiation, memory, and cognition. It is implicated in the pathophysiology of several neurological and neuropsychiatric disorders including Parkinson's Disease, Huntington's Chorea, schizophrenia, alcoholism and stroke. The development of effective radiotracers for the study of NMDA receptors is critical for our understanding of their function, and their modulation by endogenous substances or therapeutic drugs. Since the NMDA/PCP receptor lies within the channel, it is a unique target and is theoretically accessible only when the channel is in the active and "open" state, but not when it is in the inactive or "closed" state. The physical location of the NMDA/PCP receptor not only makes it an important imaging target but also complicates the development of suitable PET and SPECT radiotracers for this site. An intimate understanding of the biochemical, pharmacological, physiological and behavioral processes associated with the NMDA ion channel is essential to develop improved imaging agents. This review outlines progress made towards the development of radiolabeled agents for PCP sites of the NMDA ion channel. In addition, the animal and pharmacological models used for in vitro and in vivo assessment of NMDA receptor targeted agents are discussed.

Determination of lipophilicity and its use as a predictor of blood-brain barrier penetration of molecular imaging agents.

Compound lipophilicity is a fundamental physicochemical property that plays a pivotal role in the absorption, distribution, metabolism, and elimination (ADME) of therapeutic drugs. Lipophilicity is expressed in several different ways, including terms such as Log P, clogP, delta Log P, and Log D. Often a parabolic relationship exists between measured lipophilicity and in vivo brain penetration of drugs, where those moderate in lipophilicity often exhibit highest uptake. Reduced brain extraction of more lipophilic compounds is associated with increased non-specific binding to plasma proteins. More lipophilic compounds can also be more vulnerable to P450 metabolism, leading to faster clearance. Very polar compounds normally exhibit high water solubility, fast clearance through the kidneys, and often contain ionizable functional groups that limit blood-brain barrier (BBB) penetration. The brain penetration and specific to non-specific binding ratios exhibited in vivo by positron emission tomography (PET) and single photon emission computed tomography (SPECT) radiotracers involves a complex interplay between many critical factors, including lipophilicity, receptor affinity, metabolism, molecular size and shape, ionization potential, and specific binding to BBB efflux pumps or binding sites on albumin or other plasma proteins. This paper explores situations in which lipophilicity is a good predictor of BBB penetration, as well as those where this correlation is poor. The more commonly used methods for measuring lipophilicity are presented, and the various terms often found in the literature outlined. An attempt is made to describe how this information can be used in optimizing the development of PET and SPECT tracers that target the central nervous system (CNS).

Preclinical acute toxicity studies and rodent-based dosimetry estimates of the novel sigma-1 receptor radiotracer [(18)F]FPS.

[(18)F]1-(Fluoropropyl)-4-[(4-cyanophenoxy)methyl]piperidine ([(18)F]FPS) is a novel high affinity (KD = 0.5 nM) sigma receptor radioligand that exhibits saturable and selective in vivo binding to sigma receptors in rats, mice and non-human primates. In order to support an IND application for the characterization of [(18)F]FPS through PET imaging studies in humans, single organ and whole body radiation adsorbed doses associated with [(18)F]FPS injection were estimated from distribution data obtained in rats. In addition, acute toxicity studies were conducted in rats and rabbits and limited toxicity analyses were performed in dogs. Radiation dosimetry estimates obtained using rat biodistribution analysis of [(18)F]FPS suggest that most organs would receive around 0.012-0.015 mGy/MBq. The adrenal glands, brain, kidneys, lungs, and spleen would receive slightly higher doses (0.02-0.03 mGy/MBq). The adrenal glands were identified as the organs receiving the greatest adsorbed radiation dose. The total exposure resulting from a 5 mCi administration of [(18)F]FPS is well below the FDA defined limits for yearly cumulative and per study exposures to research participants. Extended acute toxicity studies in rats and rabbits, and limited acute toxicity studies in beagle dogs suggest at least a 175-fold safety margin in humans at a mass dose limit of 2.8 microg per intravenous injection. This estimate is based on the measured no observable effect doses (in mg/m(2)) in these species. These data support the expectation that [(18)F]FPS will be safe for use in human PET imaging studies at a maximum administration of 5 mCi and a mass dose equal to or less than 2.8 microg FPS per injection.

Synthesis and in vivo evaluation of [11C]zolpidem, an imidazopyridine with agonist properties at central benzodiazepine receptors.

The synthesis and evaluation of [(11)C]zolpidem, an imidazopyridine with agonist properties at central benzodiazepine receptors, is reported herein. The reaction of desmethylzolpidem with [(11)C] methyl iodide afforded the title compound [(11)C]zolpidem in a yield of 19.19 +/- 3.23% in 41 +/- 2 min in specific activities of 0.995-1.19 Ci/micromol (1.115 +/- 0.105 Ci/micromol) (n = 3; decay corrected, EOB). The amount of radioactivity in the brain after tail vein injection in male Wistar rats was low, and the regional distribution was homogeneous and not consistent with the known distribution of the central benzodiazepine receptors. The frontal cortex/cerebellum ratio was not significantly greater than one (1.007 +/- 0.266 at 5 min) and did not increase from 5 to 40 min post-injection. A PET brain imaging study in one baboon confirmed the results obtained in rats. Therefore, it can be concluded that [(11)C]zolpidem is not a suitable tracer for in vivo visualization of central benzodiazepine receptors.

Evaluation of [3H]LY341495 for labeling group II metabotropic glutamate receptors in vivo.

New glutamatergic drugs are being developed as potential therapies for neurodegenerative disorders, anxiety disorders, and psychoses. The development of effective mGluR radiotracers would provide essential tools with which to probe these sites in living humans, providing critical information about certain disease processes involving the glutamaterigic system and its regulation in humans. As a first step towards this goal, the tritiated form of the high affinity group II metabotropic glutamate receptor (mGluR) antagonist LY341495 [K(D) (mGluR(2)) = 1.67 +/- 0.20 nM, K(D) (mGluR(3)) = 0.75 +/- 0.43 nM] was evaluated to determine its potential to label mGluRs in vivo. Dissection analysis of the regional brain distribution over time of [(3)H]LY341495 in male rats revealed low brain uptake and no significant demonstrable saturable binding of this tracer. A group II mGluR tracer possessing higher affinity than [(3)H]LY341495 and an absence of carboxylic acid groups is likely required for in vivo PET imaging purposes.

In vivo evaluation of [11C]-3-[2-[(3-methoxyphenylamino)carbonyl]ethenyl]-4,6-dichloroindole-2-carboxylic acid ([11C]3MPICA) as a PET radiotracer for the glycine site of the NMDA ion channel.

Alterations in normal NMDA receptor composition, densities and function have been implicated in the pathophysiology of certain neurological and neuropsychiatric disorders such as Parkinson's Disease, Huntington's Chorea, schizophrenia, alcoholism and stroke. In our first effort to provide PET ligands for the NMDA/glycine site, we reported the synthesis of a novel high affinity glycine site ligand, 3-[2-[(3-methoxyphenylamino)carbonyl]ethenyl]-4,6-dichloroindole-2-carboxylic acid ((3MPICA), Ki = 4.8 +/- 0.9 nM) and the corresponding carbon-11 labeled PET ligand, [11C]3MPICA. We report here the in vivo evaluation of [11C]3MPICA in rats. Biodistribution analysis revealed that [11C]3MPICA exhibited low degree of brain penetration and high blood concentration. The average uptake at two minutes was highest in the cerebellum (0.19 +/- 0.04 %ID/g) and thalamus (0.18 +/- 0.05 %ID/g) and lower in the hippocampus (0.13 +/- 0.03) and frontal cortex (0.11 +/- 0.04 %ID/g). The radioactivity cleared quickly from all brain regions examined. Administration of unlabeled 3MPICA (1 mg/kg, i.v.) revealed at 60 minutes a small general reduction in regional brain radioactivity concentrations in treated animals versus controls, however, the blood radioactivity concentration was also lowered, confounding the assessment of the degree of saturable binding. Warfarin co-administration (100 mg/kg, i.v.) significantly lowered blood activity at 5 minutes post-injection (-27%, P < 0.01) but failed to significantly increase the brain uptake of the radiotracer. In view of these results, and especially considering the low brain penetration of this tracer, [11C]3MPICA does not appear to be a promising PET radiotracer for in vivo use.