A series of substituted 1H-indolyl carboxylic acid amides that contain a N-(2-methoxyphenyl)piperazine or N-(2-fluoroethoxy)piperazine group were synthesized and their affinities for human dopamine D2, D3, and D4 receptors were determined. Two of these compounds, 14a and 14b, displayed high binding affinity at D3 (Ki = 0.18 and 0.4 nM, respectively), and selectivity for D3
vs. D2 receptors (87-fold and 60-fold, respectively). These two compounds had low binding affinity at D4 receptors and σ receptor sites. The intrinsic activity of these compounds at D2 and D3 receptors was determined using a forskolin-dependent adenylyl cyclase inhibition assay; both 14a and 14b were found to be partial agonists. Furthermore, for compound 14a, the log D value of 2.85 suggested it has suitable lipophilicity for crossing the blood–brain-barrier.
Peroxisome proliferator-activated receptor alpha (PPAR-α) is a ligand-activated nuclear receptor transcription factor that regulates the fatty acid β-oxidation. An in vitro assay identified the p-methoxy phenyl ureido thiobutyric acid derivative KSM-01 (IC50=0.28±0.09 nM) having a higher affinity to activate PPAR-α than the PPAR-α agonist GW7647 ((IC50=0.46±0.19 nM). In this study, we report the synthesis and initial in vivo evaluation of [11C]KSM-01. The radiosynthesis was carried out by first alkylating the corresponding p-phenol precursor with [11C]MeI in DMF using NaOH, followed by deprotection of the t-butyl ester group by TFA, yielding [11C]KSM-01. SUV analysis of dynamic micro PET/CT imaging data showed that [11C]KSM-01 accumulation was ~2.0-fold greater in cardiac-specific PPAR-α overexpressing transgenic mice compared to wild-type littermates. The post-PET biodistribution studies were consistent with these results and demonstrated 2.5-fold greater radiotracer uptake in the heart of transgenic mice compared to the wild-type littermates. These results demonstrate the potential utility of PPAR-α agonists as PET radiopharmaceuticals.
peroxisome proliferator-activated receptor alpha; (PPAR-α); β-oxidation; PET imaging; Cardiomyopathy; Ureido thioisobutyric acid (TiBA)
The sigma-2 receptor is expressed in higher density in proliferating (P) tumor cells versus quiescent (Q) tumor cells, thus providing an attractive target for imaging the proliferative status (i.e., P:Q ratio) of solid tumors. Here we evaluate the utility of the sigma-2 receptor ligand 2-(2-[18F]fluoroethoxy)-N-(4-(3,4-dihydro-6,7-dimethoxyisoquinolin-2(1H)-yl)butyl)-5-methyl-benzamide, [18F]ISO-1, in two different rodent models of breast cancer. In the first study, small animal Positron Emission Tomography (PET) imaging studies were conducted with [18F]ISO-1 and 18FDG in xenografts of mouse mammary tumor 66 and tracer uptake was correlated with the in vivo P:Q ratio determined by flow cytometric measures of BrdU-labeled tumor cells. The second model utilized a chemically-induced (N-methyl-N-nitrosourea [MNU]) model of rat mammary carcinoma to correlate measures of [18F]ISO-1 and FDG uptake with MR-based volumetric measures of tumor growth. In addition, [18F]ISO-1 and FDG were used to assess the response of MNU-induced tumors to bexarotene and Vorozole therapy. In the mouse mammary 66 tumors, a strong linear correlation was observed between the [18F]ISO-1 tumor: background ratio and the proliferative status (P:Q ratio) of the tumor (R = 0.87). Similarly, measures of [18F]ISO-1 uptake in MNU-induced tumors significantly correlated (R = 0.68, P<0.003) with changes in tumor volume between consecutive MR imaging sessions. Our data suggest that PET studies of [18F]ISO-1 provide a measure of both the proliferative status and tumor growth rate, which would be valuable in designing an appropriate treatment strategy.
The goal of this study was to evaluate the 18F-labeled nonnatural amino acid (S)-2-amino-3-[1-(2-18F-fluoroethyl)-1H-[1,2,3]triazol-4-yl]propanoic acid (18F-AFETP) as a PET imaging agent for brain tumors and to compare its effectiveness with the more-established tracers O-(2-18F-fluoroethyl)-L-tyrosine (18F-FET) and 18F-FDG in a murine model of glioblastoma. The tracer 18F-AFETP is a structural analog of histidine and is a lead compound for imaging cationic amino acid transport, a relatively unexplored target for oncologic imaging.
18F-AFETP was prepared using the click reaction. BALB/c mice with intracranially implanted delayed brain tumor (DBT) gliomas (n = 4) underwent biodistribution and dynamic small-animal PET imaging for 60 min after intravenous injection of 18F-AFETP. Tumor and brain uptake of 18F-AFETP were compared with those of 18F-FDG and 18F-FET through small-animal PET analyses.
18F-AFETP demonstrated focally increased uptake in tumors with good visualization. Peak tumor uptake occurred within 10 min of injection, with stable or gradual decrease over time. All 3 tracers demonstrated relatively high uptake in the DBTs throughout the study. At late time points (47.5–57.5 min after injection), the average standardized uptake value with 18F-FDG (1.9 ± 0.1) was significantly greater than with 18F-FET (1.1 ± 0.1) and 18F-AFETP (0.7 ± 0.2). The uptake also differed substantially in normal brain, with significant differences in the standardized uptake values at late times among 18F-FDG (1.5 ± 0.2), 18F-FET (0.5 ± 0.05), and 18F-AFETP (0.1 ± 0.04). The resulting average tumor-to-brain ratio at the late time points was significantly higher for 18F-AFETP (7.5 ± 0.1) than for 18F-FDG (1.3 ± 0.1) and 18F-FET (2.0 ± 0.3).
18F-AFETP is a promising brain tumor imaging agent, providing rapid and persistent tumor visualization, with good tumor–to–normal-brain ratios in the DBT glioma model. High tumor-to-brain, tumor-to-muscle, and tumor-to-blood ratios were observed at 30 and 60 min after injection, with higher tumor-to-brain ratios than obtained with 18F-FET or 18F-FDG. These results support further development and evaluation of 18F-AFETP and its derivatives for tumor imaging.
click reaction; amino acid; glioma; 18F
This first study in humans was designed to evaluate the safety and dosimetry of a cellular proliferative marker, N-(4-(6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)butyl)-2-(2-18F-fluoroethoxy)-5-methylbenzamide (18F-ISO-1), and evaluate the feasibility of imaging tumor proliferation by PET in patients with newly diagnosed malignant neoplasms.
Patients with biopsy-proven lymphoma, breast cancer, or head and neck cancer underwent 18F-ISO-1 PET. Tumor 18F-ISO-1 uptake was assessed semiquantitatively by maximum standardized uptake value, ratios of tumor to normal tissue and tumor to muscle, and relative distribution volume ratio. The PET results were correlated with tumor Ki-67 and mitotic index, from in vitro assays of the tumor tissue. The biodistribution of 18F-ISO-1 and human dosimetry were evaluated.
Thirty patients with primary breast cancer (n = 13), head and neck cancer (n = 10), and lymphoma (n = 7) were evaluated. In the entire group, tumor maximum standardized uptake value and tumor-to-muscle ratio correlated significantly with Ki-67 (τ = 0.27, P = 0.04, and τ = 0.38, P = 0.003, respectively), but no significant correlation was observed between Ki-67 and tumor–to–normal-tissue ratio (τ = 0.07, P = 0.56) or distribution volume ratio (τ = 0.26, P = 0.14). On the basis of whole-body PET data, the gallbladder is the dose-limiting organ, with an average radiation dose of 0.091 mGy/MBq. The whole-body and effective doses were 0.012 mGy/MBq and 0.016 mSv/MBq, respectively. No adverse effects of 18F-ISO-1 were encountered.
The presence of a significant correlation between 18F-ISO-1 and Ki-67 makes this agent promising for evaluation of the proliferative status of solid tumors. The relatively small absorbed doses to normal organs allow for the safe administration of up to 550 MBq, which is sufficient for PET imaging in clinical trials.
PET; cell proliferation; 18F-ISO-1; cancer
Peroxisome proliferator activated-receptor gamma (PPARγ) transcriptionally modulates fat metabolism and also plays a role in pathological conditions such as cancer, neurodegenerative disease, and inflammation. PPARγ imaging agents are potential tools for investigating these diseases.
Four analogs of GW9662, a PPARγ antagonist, with different fluorine-containing substituents at the para-position of the aniline ring were synthesized and evaluated using two different receptor binding assays for measuring PPARγ affinity. MicroPET imaging studies were performed in a transgenic mouse model having a heart specific overexpression of PPARγ.
All four analogs were found to have binding affinities that were comparable to or better than the reference antagonist, GW9662, using a scintillation proximity assay. However, only the chloro-based analogs (compounds 3 and 4) had activity in a whole cell assay measuring activation of the PPARγ / RXR receptor complex. MicroPET imaging studies in a MHC-PPARγ transgenic mouse model showed high uptake and PPARγ specific binding for the irreversible antagonist [18F]3, whereas the corresponding reversible methoxy analog ([18F]5) displayed only nonspecific uptake in heart.
The results of this preliminary study show that the irreversible antagonist [18F]3 may represent a novel strategy for imaging PPARγ in vivo with PET. 1.
PPARγ; peroxisome proliferator-activated receptor-γ; antagonist; imaging agent
The sigma-2 (σ2) receptor is a potential biomarker of proliferative status of solid tumors. Specific synthetic probes using N-substituted-9-azabicyclo[3.3.1]nonan-3α-yl carbamate analogs have been designed and implemented for experimental cancer diagnosis and therapy.
We employed the fluorescently-labeled σ2 receptor probe, SW120, to evaluate σ2 receptor expression in human stem cells (SC), including: bone marrow stromal (BMSC), neural progenitor (NPC), amniotic fluid (AFSC), hematopoetic (HSC) and embryonic stem cells (ESC). We concurrently evaluated the intensity of SW120 and 5-ethynyl-2′-deoxyuridine (EdU) relative to passage number and multipotency.
We substantiated significantly higher σ2 receptor density among proliferating SC relative to lineage-restricted cell types. Additionally, cellular internalization of the σ2 receptor in SC was consistent with receptor-mediated endocytosis and confocal microscopy indicated SW120 specific co-localization with a fluorescent marker of lysosomes in all SC imaged.
These results suggest that σ2 receptors may serve to monitor stem cell differentiation in future experimental studies.
sigma 2 receptor; embryonic stem cell; amniotic fluid stem cell; bone marrow stromal cells; hematopoietic stem cell; differentiation; proliferation; EdU; Ki 67
The sigma-2 receptor, whose gene remains to be cloned, has been validated as a biomarker for tumor cell proliferation. Here we report the use of a novel photoaffinity probe, WC-21, to identify the sigma-2 receptor binding site. WC-21, a sigma-2 ligand containing both a photoactive moiety azide and a fluorescein isothiocyanate group, irreversibly labels sigma-2 receptors in rat liver; the membrane-bound protein was then identified as PGRMC1 (progesterone receptor membrane component-1). Immunocytochemistry reveals that both PGRMC1 and SW120, a fluorescent sigma-2 receptor ligand, colocalizes with molecular markers of the endoplasmic reticulum and mitochondria in HeLa cells. Overexpression and knockdown of the PGRMC1 protein results in an increase and a decrease in binding of a sigma-2 selective radioligand, respectively. The identification of the putative sigma-2 receptor binding site as PGRMC1 should stimulate the development of unique imaging agents and cancer therapeutics that target the sigma-2 receptor/PGRMC1 complex.
A series of isatin analogs containing a hydrophilic group, including a pyridine ring, ethylene glycol group, and a triazole ring, have been synthesized, and their inhibition potency for caspase-3 was measured both in vitro (i.e. recombinant enzyme) and in whole cells (HeLa cells). The analogs having a hydrophilic group, including 12, 13, 16, 38, and 40, have dramatically increased activity in vitro and in HeLa cells compared to the corresponding unsubstituted N-phenyl isatin analogs.
caspase-3; apoptosis; cell death
Alfentanil is a validated probe for hepatic, first-pass, and intestinal cytochrome P450 (CYP) 3A activity, using plasma clearances, single-point concentrations and noninvasive pupil diameter change (miosis). Assessing intravenous and oral drug disposition typically requires separate dosing. This investigation evaluated concurrent administration of oral deuterated and intravenous unlabeled alfentanil, to assess both intestinal and hepatic CYP3A, and compare sequential and simultaneous dosing. Alfentanil disposition was evaluated after strong hepatic and/or intestinal CYP3A induction and inhibition by rifampin, ketoconazole, and grapefruit juice. Using plasma alfentanil concentrations and area under the curve, clearance, or single-point concentrations, both simultaneous and sequential dosing provided equivalent results and detected hepatic and intestinal CYP3A induction and inhibition. Miosis better detected CYP3A modulation with sequential vs simultaneous dosing. These results show that concurrent oral deuterated and intravenous alfentanil, administered either sequentially or simultaneously, is an efficient and effective approach to assessing hepatic and intestinal CYP3A activity.
alfentanil; cytochrome P450 3A; CYP3A; in vivo probe; phenotyping
4-(Dimethylamino)-N-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)-benzamide (WC-10), a N-phenyl piperazine analog, displays high affinity and moderate selectivity for dopamine D3 receptors versus dopamine D2 receptors (Chu et al.  Bioorg Med Chem 13:77–87). In this study, WC-10 was radiolabeled with tritium (specific activity = 80 Ci/mmol), and quantitative autoradiography studies were conducted using rhesus monkey and Sprague-Dawley rat brain sections. Kd values for the binding of [3H]WC-10 to D3 receptors obtained from quantitative autoradiography with rhesus monkey and rat brain sections are in agreement with Kd values obtained from cloned human and rat receptors (Xu et al.  Synapse 63:717-728). The D2 selective antagonist [3H]raclopride binds with 11-fold higher affinity to human HEK D2L (Kd = 1.6 nM) than HEK D3 (Kd = 18 nM) receptors; [3H]raclopride binds to rat Sf9 rD2L receptors with a Kd of 6.79 nM, a value that is 4-fold lower than binding to human HEK D2L receptors and 2.5-fold higher than binding to rat Sf9 rD3 receptors. In vitro quantitative autoradiography studies with [3H]WC-10 and [3H]raclopride were conducted on adult rat and rhesus monkey brain sections. A mathematical model for calculating the absolute densities of dopamine D2 and D3 receptors based on the in vitro receptor binding data of [3H]WC-10 and [3H]raclopride was developed.
dopamine; DA D3/D2 receptors; quantitative autoradiography
15-(4-(2-[18F]fluoroethoxy)phenyl)pentadecanoic acid ([18F]7) was synthesized as a PET probe for assessing myocardial fatty acid metabolism. The radiosynthesis of [18F]7 was accomplished using a two step reaction, starting with the corresponding tosylate ester, methyl 15-(4-(2-(tosyloxy)ethoxy)phenyl)pentadecanoate (5) and gave the radiolabeled fatty acid, [18F]7 in a radiolabeling yield of 55 – 60% and a specific activity of > 2,000 Ci/mmol (decay corrected to EOB). The biological evaluation of [18F]7 in rats displayed high uptake in heart (1.94%.ID/g at 5 min), which was higher than the uptake (%ID/g) in blood, lung, muscle, pancreas and brain. MicroPET studies of [18F]7 in Sprague-Dawley rats demonstrated excellent images of the myocardium when compared with [11C]palmitate images in the same animal. Moreover, the tracer kinetics of [18F]7 paralleled those seen with [11C]palmitate, with an early peak followed by biphasic washout. When compared to [11C]palmitate, [18F]7 exhibited a slower early clearance (0.17 ± 0.01 vs. 0.30 ± 0.02, P < 0.0001) and a significantly higher late clearance (0.0030 ± 0.0005 vs. 0.0006 ± 0.00013, P < 0.01). These initial studies suggest that [18F]7 could be a potentially useful clinical PET tracer to assess abnormal myocardial fatty acid metabolism.
Fatty acid; PET imaging; F-18
One major challenge in the development of cancer therapeutics is the selective delivery of the drugs to their cellular targets. In the case of pancreatic cancer, the σ-2 receptor is a unique target that triggers apoptosis upon activation. We have previously developed a series of chemical compounds with high affinity for the σ-2 receptor and demonstrated rapid internalization of the ligands. One particular specific ligand of the σ-2 receptor, SV119, binds to pancreatic cancer cells and induces target cell death in vitro and in vivo. In this study, we characterized the ability of SV119 to selectively deliver other death-inducing cargos to augment the cytotoxic properties of SV119 itself. When conjugated to SV119, small molecules that are known to interfere with intracellular pro-survival pathways retained their ability to induce cell death, the efficiency of which was enhanced by the combinatorial effect of SV119 delivered with its small molecule cargo.
Our findings define a simple platform technology to increase the tumor-selective delivery of small molecule therapeutics via sigma-2 ligands, permitting chemotherapeutic synergy that can optimize efficacy and patient benefit.
apoptosis; drug delivery; pancreatic neoplasms; σ-2 receptor; sigma-2 conjugates
Noninvasive imaging methods that can distinguish apoptosis from necrosis may be useful in furthering our understanding of diseases characterized by apoptotic dysregulation as well as aiding drug development targeting apoptotic pathways. We evaluated the ability of radiolabeled isatins to quantify caspase-3 activity induced by the activation of the extrinsic apoptotic pathway by the anti-Fas antibody in mice.
The behavior of three different radiolabeled isatins ([18F]WC-II-89, [18F]WC-IV-3, and [11C]WC-98), was characterized in mice with and without anti-Fas antibody treatment by microPET imaging and biodistribution studies. The activity of [18F]WC-II-89 was also compared with [99mTc]mebrofenin. The effect of pan-caspase inhibition with Q-VD-OPH on [18F]WC-II-89 uptake was studied. Caspase-3 activity was confirmed by a fluorometric enzyme assay.
All three tracers behaved similarly in microPET and biodistribution studies. Increased retention of all tracers was observed in the livers of treated animals and several other organs, all of which demonstrated increased caspase-3 enzyme activity; however, impaired hepatobiliary excretion made attribution of these findings to caspase-3 activity difficult. The isatin [18F]WC-II-89 was retained at statistically significantly higher levels in the organs after anti-Fas antibody treatment while [99mTc]mebrofenin activity cleared, suggesting specific binding to activated caspase-3, but the magnitude of increased binding was still relatively low. Caspase inhibition with Q-VD-OPH partially blocked [18F]WC-II-89 retention but completely blocked caspase-3 enzyme activity in the liver.
The radiolabeled isatins appear to bind specifically to caspase-3 in vivo, but their sensitivity is limited. Further optimization is required for these tracers to be useful for clinical applications.
Attention system abnormalities represent a significant barrier to scholastic achievement in children with neurofibromatosis-1 (NF1). Using a novel mouse model of NF1-associated attention deficit (ADD), we demonstrate a presynaptic defect in striatal dopaminergic homeostasis and leverage this finding to apply [11C]-raclopride positron-emission tomography (PET) in the intact animal. While methylphenidate and L-Deprenyl correct both striatal dopamine levels on PET imaging and defective attention system function in Nf1 mutant mice, pharmacologic agents that target de-regulated cyclic AMP and RAS signaling in these mice do not. These studies establish a robust preclinical model to evaluate promising agents for NF1-associated ADD.
dopamine; behavior; neurofibromin; cyclic AMP; RAS
A series of analogues were synthesized by optimizing the structure of papaverine. The in vitro PDE10A binding affinity ( IC50) values for these new analogues were measured; for compounds that have IC50 value less than 60 nM for PDE10A, the binding affinities (IC50 value) for PDE3A and PDE3B were tested. Of these analogues, compounds 6a, 6b, 6n, 8b, 8c and 11 displayed relatively higher PDE10A potency with IC50 value in the range of 28–60 nM. The most potent compound 1-(4-(2-(2-fluoroethoxy)ethoxy)-3-methoxybenzyl)-6,7-dimethoxyisoquinoline (8c) has the IC50 value of 28 ± 1.2 nM for PDE10A, 2200 ± 437 nM for PDE3A and 2520 ± 210 nM for PDE3B. Compared to papavarine, compound 8c displayed similar PDE10A potency but improved selectivity to PDE10A versus PDE3A and PDE3B. To identify high potent PDE10A inhibitor, further optimization of the structures of these analogues is necessary.
PDE10A; CNS; Schizophrenia
A series of microPET imaging studies were conducted in anesthetized rhesus monkeys using the dopamine D3-selective partial agonist, [18F]5. There was variable uptake in regions of brain known to express a high density of D3 receptors under baseline conditions. Pretreatment with lorazepam (1 mg/kg, i.v. 30 min) to reduce endogenous dopamine activity prior to tracer injection resulted in a dramatic increase in uptake in the caudate, putamen, and thalamus, and an increase in the binding potential (BP) values, a measure of D3 receptor binding in vivo. These data indicate that there is a high level of competition between [18F]5 and endogenous dopamine for D3 receptors in vivo.
D3 receptors; Positron Emission Tomography; endogenous dopamine
The sigma-2 (σ2) receptor is proving to be an important protein in the field of cancer biology. The observations that σ2 receptors have a 10-fold higher density in proliferating tumor cells than in quiescent tumor cells, and that σ2 receptor agonists are capable of killing tumor cells via apoptotic and non-apoptotic mechanisms, indicate that this receptor is an important molecular target for the development of radiotracers for imaging tumors using techniques such as Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) and for the development of cancer chemotherapeutic agents. In spite of recent promising results towards achieving these goals, research in this field has been hampered by the fact that the molecular identity of the protein sequence of the σ2 receptor is currently not known. Consequently, most of what is known about this protein has been obtained using either radiolabeled or fluorescent probes for this receptor, or biochemical analysis of the effect of σ2 selective ligands on cells growing under tissue culture conditions. This article provides a review of the development and use of σ2 receptor ligands, and how these ligands have been used with a variety of in vitro and in vivo models to gain a greater understanding of the role this receptor plays in cancer.
Sigma-2 receptors; cell proliferation; positron emission tomography
Two different strategies have been developed for imaging the proliferative status of solid tumors with the functional imaging technique, Positron Emission Tomography (PET). The first strategy uses carbon-11 labeled thymidine and/or, more recently, fluorine-18 labeled thymidine analogs. These agents are a substrate for the enzyme thymidine kinase-1 (TK-1) and provide a pulse label of the number of cells in S phase. The second method for imaging the proliferative status of a tumor uses radiolabeled ligands that bind to the sigma-2 receptor which has a 10-fold higher density in proliferating (P) tumor cells versus quiescent (Q) tumor cells. This article compares and contrasts the two different strategies for imaging the proliferative status of solid tumors, and describes the strengths and weaknesses of each approach.
Cell proliferation; thymidine analogs; sigma-2 receptors; PET imaging
The effects of sleep deprivation on dopaminergic systems remain elusive, in part due to the lack of selective ligands for dopamine receptor subtypes. We examined D1, D2 and D3 receptor density in the mouse brain after sleep deprivation by receptor autoradiography using [3H]SCH 23390 for D1R, [3H]raclopride for D2R, and [3H]WC-10 for D3R (a novel D3R-selective compound developed in our laboratory, not previously reported in mouse). Sleep-deprived mice showed a significant decrease in D1R, no change in D2R, and a significant increase in D3R binding in striatum. This pattern of dopamine receptor changes was not seen in mice subjected to restraint stress, suggesting specificity to sleep. These data provide evidence that brain dopaminergic circuits are remodeled after sleep deprivation.
Dopamine; Sleep Deprivation; Receptors, Dopamine D1; Receptors, Dopamine D2; Receptors, Dopamine D3; Autoradiography
Four benzamide analogs having a high affinity and selectivity for D3 versus D2 receptors were radiolabeled with 11C or 18F for in vivo evaluation.
Precursors were synthesized and the four D3 selective benzamide analogs were radiolabeled. The tissue distribution and brain uptake of the four compounds were evaluated in control rats and rats pretreated with cyclosporin A, a modulator of P-glycoprotein and an inhibitor of other ABC efflux transporters that contribute to the blood brain barrier. MicroPET imaging was carried out for [11C]6 in a control and a cyclosporin A pre-treated rat.
All four compounds showed low brain uptake in control rats at 5 and 30 min post-injection; despite recently reported rat behavioral studies conducted on analogs 6 (WC-10) and 7 (WC-44). Following administration of cyclosporin A, increased brain uptake was observed with all four PET radiotracers at both 5 and 30 min post-i.v. injection. An increase in brain uptake following modulation/inhibition of the ABC transporters was also observed in the microPET study.
These data suggest that D3 selective conformationally-flexible benzamide analogs which contain a N-2-methoxyphenylpiperazine moiety are substrates for P-glycoprotein or other ABC transporters expressed at the blood-brain barrier, and that PET radiotracers containing this pharmacophore may display low brain uptake in rodents due to the action of these efflux transporters.
D3 receptors; PET; radiotracer; P-glycoprotein
A series of compounds structurally related to aripiprazole (1), an atypical antipsychotic and antidepressant used clinically for the treatment of schizophrenia, bipolar disorder, and depression, have been prepared and evaluated for affinity at D2-like dopamine receptors. These compounds also share structural elements with the classical D2-like dopamine receptor antagonists, haloperidol, N-methylspiperone, domperidone and benperidol. Two new compounds, 7-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butoxy)-3,4-dihydroquinolin-2(1H)-one oxalate (6) and 7-(4-(4-(2-(2-fluoroethoxy)phenyl)piperazin-1-yl)butoxy)-3,4-dihydroquinolin-2(1H)-one oxalate (7) were found to (a) bind to the D2 receptor subtype with high affinity (Ki values <0.3 nM), (b) exhibit >50-fold D2 versus D3 receptor binding selectivity and (c) be partial agonists at both the D2 and D3 receptor subtype.
Dopamine D2 receptor; Aripiprazole; Dopamine partial agonist
A series of fluorine containing N-(2-methoxyphenyl)piperazine and N-(2-fluoroethoxy)piperazine analogues were synthesized and their affinities for human dopamine D2, D3 and D4 receptors were determined. Radioligand binding studies identified five compounds, 18a, 20a, 20c, 20e and 21e, which bind with high affinity at D3 (Ki = 0.17 to 5 nM) and moderate to high selectivity for D3 vs. D2 receptors (ranging from ∼25 to 163-fold). These compounds were also evaluated for intrinsic activity at D2 and D3 receptors using a forskolin-dependent adenylyl cyclase assay. This panel of compounds exhibits varying receptor subtype binding selectivity and intrinsic activity at D2 vs. D3 receptors. These compounds may be useful for behavioral pharmacology studies on the role of D2-like dopamine receptors in neuropsychiatric and neurological disorders. Furthermore, compound 20e, which has the highest binding affinity and selectivity for the D3 receptor (Ki = 0.17 nM for D3, 163-fold selectivity for D3 vs. D2 receptors) represents a candidate fluorine-18 radiotracer for in vivo PET imaging studies on the regulation of D3 receptor expression.
2-((4-(1-[11C]methyl-4-(pyridin-4-yl)-1H-pyrazol-3-yl)phenoxy)methyl)-quinoline (MP-10), a specific PDE10A inhibitor (IC50 = 0.18 nM with 100-fold selectivity over other PDEs), was radiosynthesized by alkylation of the desmethyl precursor with [11C]CH3I, ~45% yield, > 92% radiochemical purity, > 370 GBq/μmol specific activity at end of bombardment (EOB). Evaluation in Sprague-Dawley rats revealed that [11C]MP-10 had highest brain accumulation in the PDE10A enriched striatum, the 30 min striatum: cerebellum ratio reached 6.55. MicroPET studies of [11C]MP-10 in monkeys displayed selective uptake in striatum. However, a radiolabeled metabolite capable of penetrating the blood brain barrier may limit the clinical utility of [11C]MP-10 as a PDE10A PET tracer.
PDE10A; PET imaging; carbon-11; MP-10; Huntington’s disease