Discovery of Phosphodiesterase 10A (PDE10A) PET Tracer AMG 580 to Support Clinical Studies.

We report the discovery of PDE10A PET tracer AMG 580 developed to support proof of concept studies with PDE10A inhibitors in the clinic. To find a tracer with higher binding potential (BPND) in NHP than our previously reported tracer 1, we implemented a surface plasmon resonance assay to measure the binding off-rate to identify candidates with slower washout rate in vivo. Five candidates (2-6) from two structurally distinct scaffolds were identified that possessed both the in vitro characteristics that would favor central penetration and the structural features necessary for PET isotope radiolabeling. Two cinnolines (2, 3) and one keto-benzimidazole (5) exhibited PDE10A target specificity and brain uptake comparable to or better than 1 in the in vivo LC-MS/MS kinetics distribution study in SD rats. In NHP PET imaging study, [(18)F]-5 produced a significantly improved BPND of 3.1 and was nominated as PDE10A PET tracer clinical candidate for further studies.

A selective prostaglandin E2 receptor subtype 2 (EP2) antagonist increases the macrophage-mediated clearance of amyloid-beta plaques.

A high-throughput screen resulted in the discovery of benzoxazepine 1, an EP2 antagonist possessing low microsomal stability and potent CYP3A4 inhibition. Modular optimization of lead compound 1 resulted in the discovery of benzoxazepine 52, a molecule with single-digit nM binding affinity for the EP2 receptor and significantly improved microsomal stability. It was devoid of CYP inhibition and was ∼4000-fold selective against the other EP receptors. Compound 52 was shown to have good PK properties in CD-1 mice and high CNS permeability in C57Bl/6s mice and Sprague-Dawley rats. In an ex vivo assay, it demonstrated the ability to increase the macrophage-mediated clearance of amyloid-beta plaques from brain slices in a dose-dependent manner.

Radiosynthesis and initial characterization of a PDE10A specific PET tracer [18F]AMG 580 in non-human primates.

INTRODUCTION: Phosphodiesterase 10A (PDE10A) is an intracellular enzyme responsible for the breakdown of cyclic nucleotides which are important second messengers for neurotransmission. Inhibition of PDE10A has been identified as a potential target for treatment of various neuropsychiatric disorders. To assist drug development, we have identified a selective PDE10A positron emission tomography (PET) tracer, AMG 580. We describe here the radiosynthesis of [(18)F]AMG 580 and in vitro and in vivo characterization results. METHODS: The potency and selectivity were determined by in vitro assay using [(3)H]AMG 580 and baboon brain tissues. [(18)F]AMG 580 was prepared by a 1-step [(18)F]fluorination procedure. Dynamic brain PET scans were performed in non-human primates. Regions-of-interest were defined on individuals' MRIs and transferred to the co-registered PET images. Data were analyzed using two tissue compartment analysis (2TC), Logan graphical (Logan) analysis with metabolite-corrected input function and the simplified reference tissue model (SRTM) method. A PDE10A inhibitor and unlabeled AMG 580 were used to demonstrate the PDE10A specificity. KD was estimated by Scatchard analysis of high and low affinity PET scans. RESULTS: AMG 580 has an in vitro KD of 71.9 pM. Autoradiography showed specific uptake in striatum. Mean activity of 121 ± 18 MBq was used in PET studies. In Rhesus, the baseline BPND for putamen and caudate was 3.38 and 2.34, respectively, via 2TC, and 3.16, 2.34 via Logan, and 2.92, and 2.01 via SRTM. A dose dependent decrease of BPND was observed by the pre-treatment with a PDE10A inhibitor. In baboons, 0.24 mg/kg dose of AMG 580 resulted in about 70% decrease of BPND. The in vivo KD of [(18)F]AMG 580 was estimated to be around 0.44 nM in baboons. CONCLUSION: [(18)F]AMG 580 is a selective and potent PDE10A PET tracer with excellent specific striatal binding in non-human primates. It warrants further evaluation in humans.

AMG 580: a novel small molecule phosphodiesterase 10A (PDE10A) positron emission tomography tracer.

Phosphodiesterase 10A (PDE10A) inhibitors have therapeutic potential for the treatment of psychiatric and neurologic disorders, such as schizophrenia and Huntington's disease. One of the key requirements for successful central nervous system drug development is to demonstrate target coverage of therapeutic candidates in brain for lead optimization in the drug discovery phase and for assisting dose selection in clinical development. Therefore, we identified AMG 580 [1-(4-(3-(4-(1H-benzo[d]imidazole-2-carbonyl)phenoxy)pyrazin-2-yl)piperidin-1-yl)-2-fluoropropan-1-one], a novel, selective small-molecule antagonist with subnanomolar affinity for rat, primate, and human PDE10A. We showed that AMG 580 is suitable as a tracer for lead optimization to determine target coverage by novel PDE10A inhibitors using triple-stage quadrupole liquid chromatography-tandem mass spectrometry technology. [(3)H]AMG 580 bound with high affinity in a specific and saturable manner to both striatal homogenates and brain slices from rats, baboons, and human in vitro. Moreover, [(18)F]AMG 580 demonstrated prominent uptake by positron emission tomography in rats, suggesting that radiolabeled AMG 580 may be suitable for further development as a noninvasive radiotracer for target coverage measurements in clinical studies. These results indicate that AMG 580 is a potential imaging biomarker for mapping PDE10A distribution and ensuring target coverage by therapeutic PDE10A inhibitors in clinical studies.

Synthesis and preliminary biological evaluation of potent and selective 2-(3-alkoxy-1-azetidinyl) quinolines as novel PDE10A inhibitors with improved solubility.

We report the discovery of a novel series of 2-(3-alkoxy-1-azetidinyl) quinolines as potent and selective PDE10A inhibitors. Structure-activity studies improved the solubility (pH 7.4) and maintained high PDE10A activity compared to initial lead compound 3, with select compounds demonstrating good oral bioavailability. X-ray crystallographic studies revealed two distinct binding modes to the catalytic site of the PDE10A enzyme. An ex vivo receptor occupancy assay in rats demonstrated that this series of compounds covered the target within the striatum.

Discovery of clinical candidate 1-(4-(3-(4-(1H-benzo[d]imidazole-2-carbonyl)phenoxy)pyrazin-2-yl)piperidin-1-yl)ethanone (AMG 579), a potent, selective, and efficacious inhibitor of phosphodiesterase 10A (PDE10A).

We report the identification of a PDE10A clinical candidate by optimizing potency and in vivo efficacy of promising keto-benzimidazole leads 1 and 2. Significant increase in biochemical potency was observed when the saturated rings on morpholine 1 and N-acetyl piperazine 2 were changed by a single atom to tetrahydropyran 3 and N-acetyl piperidine 5. A second single atom modification from pyrazines 3 and 5 to pyridines 4 and 6 improved the inhibitory activity of 4 but not 6. In the in vivo LC-MS/MS target occupancy (TO) study at 10 mg/kg, 3, 5, and 6 achieved 86-91% occupancy of PDE10A in the brain. Furthermore, both CNS TO and efficacy in PCP-LMA behavioral model were observed in a dose dependent manner. With superior in vivo TO, in vivo efficacy and in vivo PK profiles in multiple preclinical species, compound 5 (AMG 579) was advanced as our PDE10A clinical candidate.

Discovery of Novel Imidazo[4,5-b]pyridines as Potent and Selective Inhibitors of Phosphodiesterase 10A (PDE10A).

We report the discovery of novel imidazo[4,5-b]pyridines as potent and selective inhibitors of PDE10A. The investigation began with our recently disclosed ketobenzimidazole 1, which exhibited single digit nanomolar PDE10A activity but poor oral bioavailability. To improve oral bioavailability, we turned to novel scaffold imidazo[4,5-b]pyridine 2, which not only retained nanomolar PDE10A activity but was also devoid of the morpholine metabolic liability. Structure-activity relationship studies were conducted systematically to examine how various regions of the molecule impacted potency. X-ray cocrystal structures of compounds 7 and 24 in human PDE10A helped to elucidate the key bonding interactions. Five of the most potent and structurally diverse imidazo[4,5-b]pyridines (4, 7, 12b, 24a, and 24b) with PDE10A IC50 values ranging from 0.8 to 6.7 nM were advanced into receptor occupancy studies. Four of them (4, 12b, 24a, and 24b) achieved 55-74% RO at 10 mg/kg po.

Initial characterization of a PDE10A selective positron emission tomography tracer [11C]AMG 7980 in non-human primates.

INTRODUCTION: Phosphodiesterase 10A (PDE10A) is an intracellular enzyme responsible for the breakdown of cyclic nucleotides which are important secondary messengers in the central nervous system. Inhibition of PDE10A has been identified as a potential therapeutic target for treatment of various neuropsychiatric disorders. To assist the drug development program, we have identified a selective PDE10A PET tracer, [(11)C]AMG 7980, for imaging PDE10A distribution using positron emission tomography. METHODS: [(11)C]AMG 7980 was prepared in a one-pot, two-step reaction. Dynamic PET scans were performed in non-human primates following a bolus or bolus plus constant infusion tracer injection paradigm. Regions-of-interest were defined on individuals' MRIs and transferred to the co-registered PET images. Data were analyzed using Logan graphical analysis with metabolite-corrected input function, the simplified reference tissue model (SRTM) method and occupancy plots. A benchmark PDE10A inhibitor was used to demonstrate PDE10A-specific binding. RESULTS: [(11)C]AMG 7980 was prepared with a mean specific activity of 99 ± 74 GBq/µmol (n=10) and a synthesis time of 45 min. Specific binding of the tracer was localized to the striatum and globus pallidus (GP) and low in other brain regions. Thalamus was used as the reference tissue to derive binding potentials (BPND). The BPND for caudate, putamen, and GP were 0.23, 0.65, 0.51, respectively by the graphical method, and 0.42, 0.76, and 0.75 from the SRTM method. A dose dependent decrease of BPND was observed with the pre-treatment of a PDE10A inhibitor. A bolus plus infusion injection paradigm yielded similar results. CONCLUSION: [(11)C]AMG 7980 has been successfully used for imaging PDE10A in non-human primate brain. Despite the fast brain kinetics it can be used to measure target occupancy of PDE10A inhibitors in non-human primates and potentially applicable to humans.

Comparison of phosphodiesterase 10A, dopamine receptors D1 and D2 and dopamine transporter ligand binding in the striatum of the R6/2 and BACHD mouse models of Huntington's disease.

BACKGROUND: Phosphodiesterase 10A (PDE10A) is expressed at high levels in the striatum and has been proposed both as a biomarker for Huntington's disease pathology and as a target for intervention. OBJECTIVE: PDE10A radiotracers have been successfully used to measure changes in binding density in Huntington's disease patients, but little is known about PDE10A binding in mouse models that are used extensively to model pathology and test therapeutic interventions. METHODS: Our study investigated changes in PDE10A binding using the selective tracer 3H-7980 at specific ages of two Huntington's disease transgenic mouse models: R6/2, a short-lived model carrying exon-1 of mutant HTT and BACHD, a longer-lived model carrying full-length mutant HTT. PDE10A binding was compared to binding of known markers of striatal atrophy in Huntington's disease, e.g. dopamine transporter (DAT) and dopamine receptors D1 and D2. RESULTS: We found that in the R6/2 model at 6 weeks of age, mice showed high variability of binding, however binding of all ligands was significantly decreased at 8 and 12 weeks of age. In contrast, no changes were detectable in the BACHD model at 8, 10 or 12 month of age. CONCLUSIONS: These findings suggest that radiotracer binding of PDE10A, DAT, D1 and D2 receptor in the R6/2 model may be a good indicator of striatal pathological changes that are observed in Huntington's disease patients, and that the first 12 months in the BACHD model may be more reflective of early stages of the disease.

Design, optimization, and biological evaluation of novel keto-benzimidazoles as potent and selective inhibitors of phosphodiesterase 10A (PDE10A).

Our development of PDE10A inhibitors began with an HTS screening hit (1) that exhibited both high p-glycoprotein (P-gp) efflux ratios in rat and human and poor metabolic stability. On the basis of cocrystal structure of 1 in human PDE10A enzyme, we designed a novel keto-benzimidazole 26 with comparable PDE10A potency devoid of efflux liabilities. On target in vivo coverage of PDE10A in rat brain was assessed using our previously reported LC-MS/MS receptor occupancy (RO) technology. Compound 26 achieved 55% RO of PDE10A at 30 mg/kg po and covered PDE10A receptors in rat brain in a dose-dependent manner. Cocrystal structure of 26 in PDE10A confirmed the binding mode of the novel scaffold. Further optimization resulted in the identification of keto-benzimidazole 34, which showed an increased in vivo efficacy of 57% RO in rats at 10 mg/kg po and an improved in vivo rat clearance and oral bioavailability.

Discovery of selective biaryl ethers as PDE10A inhibitors: improvement in potency and mitigation of Pgp-mediated efflux.

We report the discovery of a novel series of biaryl ethers as potent and selective PDE10A inhibitors. Structure-activity studies improved the potency and decreased Pgp-mediated efflux found in the initial compound 4. X-ray crystallographic studies revealed two novel binding modes to the catalytic site of the PDE10A enzyme.

Use of structure based design to increase selectivity of pyridyl-cinnoline phosphodiesterase 10A (PDE10A) inhibitors against phosphodiesterase 3 (PDE3).

We report our successful effort to increase the PDE3 selectivity of PDE10A inhibitor pyridyl cinnoline 1 using a combination of computational modeling and structural-activity relationship investigations. An analysis of the PDE3 catalytic domain compared to the co-crystal structure of cinnoline analog 1 in PDE10A revealed two areas of structural differences in the active sites and suggested areas on the scaffold that could be modified to exploit those unique structural features. Once SAR established the cinnoline as the optimal scaffold, modifications on the methoxy groups of the cinnoline and the methyl group on the pyridine led to the discovery of compounds 33 and 36. Both compounds achieved significant improvement in selectivity against PDE3 while maintaining their PDE10A inhibitory activity and in vivo metabolic stability comparable to 1.

Rapid identification of a novel small molecule phosphodiesterase 10A (PDE10A) tracer.

A radiolabeled tracer for imaging therapeutic targets in the brain is a valuable tool for lead optimization in CNS drug discovery and for dose selection in clinical development. We report the rapid identification of a novel phosphodiesterase 10A (PDE10A) tracer candidate using a LC-MS/MS technology. This structurally distinct PDE10A tracer, AMG-7980 (5), has been shown to have good uptake in the striatum (1.2% ID/g tissue), high specificity (striatum/thalamus ratio of 10), and saturable binding in vivo. The PDE10A affinity (K(D)) and PDE10A target density (B(max)) were determined to be 0.94 nM and 2.3 pmol/mg protein, respectively, using [(3)H]5 on rat striatum homogenate. Autoradiography on rat brain sections indicated that the tracer signal was consistent with known PDE10A expression pattern. The specific binding of [(3)H]5 to rat brain was blocked by another structurally distinct, published PDE10A inhibitor, MP-10. Lastly, our tracer was used to measure in vivo PDE10A target occupancy of a PDE10A inhibitor in rats using LC-MS/MS technology.

Discovery of potent, selective, and metabolically stable 4-(pyridin-3-yl)cinnolines as novel phosphodiesterase 10A (PDE10A) inhibitors.

We report the discovery of 6,7-dimethoxy-4-(pyridin-3-yl)cinnolines as novel inhibitors of phosphodiesterase 10A (PDE10A). Systematic examination and analyses of structure-activity-relationships resulted in single digit nM potency against PDE10A. X-ray co-crystal structure revealed the mode of binding in the enzyme's catalytic domain and the source of selectivity against other PDEs. High in vivo clearance in rats was addressed with the help of metabolite identification (ID) studies. These findings combined resulted in compound 39, a promising potent inhibitor of PDE10A with good in vivo metabolic stability in rats and efficacy in a rodent behavioral model.

Substituted aryl pyrimidines as potent and soluble TRPV1 antagonists.

Clinical candidate AMG 517 (1) is a potent antagonist toward multiple modes of activation of TRPV1; however, it suffers from poor solubility. Analogs with various substituents at the R region of 3 were prepared to improve the solubility while maintaining the potent TRPV1 activity of 1. Compounds were identified that maintained potency, had good pharmacokinetic properties, and improved solubility relative to 1.

Pharmacological blockade of the vanilloid receptor TRPV1 elicits marked hyperthermia in humans.

The vanilloid receptor TRPV1 has been identified as a molecular target for the treatment of pain associated with inflammatory diseases and cancer. Hence, TRPV1 antagonists have been considered for therapeutic evaluation in such diseases. During Phase I clinical trials with AMG 517, a highly selective TRPV1 antagonist, we found that TRPV1 blockade elicited marked, but reversible, and generally plasma concentration-dependent hyperthermia. Similar to what was observed in rats, dogs, and monkeys, hyperthermia was attenuated after repeated dosing of AMG 517 (at the highest dose tested) in humans during a second Phase I trial. However, AMG 517 administered after molar extraction (a surgical cause of acute pain) elicited long-lasting hyperthermia with maximal body temperature surpassing 40 degrees C, suggesting that TRPV1 blockade elicits undesirable hyperthermia in susceptible individuals. Mechanisms of AMG 517-induced hyperthermia were then studied in rats. AMG 517 caused hyperthermia by inducing tail skin vasoconstriction and increasing thermogenesis, which suggests that TRPV1 regulates vasomotor tone and metabolic heat production. In conclusion, these results demonstrate that: (a) TRPV1-selective antagonists like AMG 517 cannot be developed for systemic use as stand alone agents for treatment of pain and other diseases, (b) individual susceptibility influences magnitude of hyperthermia observed after TRPV1 blockade, and (c) TRPV1 plays a pivotal role as a molecular regulator for body temperature in humans.

4-Aminopyrimidine tetrahydronaphthols: a series of novel vanilloid receptor-1 antagonists with improved solubility properties.

8-(6-(4-(Trifluoromethyl)phenyl)pyrimidin-4-ylamino)-1,2,3,4-tetrahydronaphthalen-2-ol (4) and analogs (5-10) were shown to be potent inhibitors of human and rat TRPV1 in vitro with increased solubility over our previous series. Synthesis, SAR, and improvements in metabolic stability and absorption of these compounds are described herein.

Trisubstituted pyrimidines as transient receptor potential vanilloid 1 (TRPV1) antagonists with improved solubility.

A series of trisubstituted pyrimidines were synthesized to improve aqueous solubility of our first TRPV1 clinical candidate (1; AMG 517), while maintaining potent TRPV1 inhibitory activity. Structure-activity and structure-solubility studies led to the identification of compound 26. The aqueous solubility of 26 (>or=200microg/mL, 0.01 HCl; 6.7microg/mL, phosphate buffered saline (PBS); 150microg/mL, fasted-state simulated intestinal fluid (SIF)) was significantly improved over 1. In addition, compound 26 was found to be orally bioavailable (rat F(oral)=24%) and had potent TRPV1 antagonist activity (capsaicin IC(50)=1.5nM) comparable to that of 1.

Structure-activity relationship (SAR) investigations of substituted imidazole analogs as TRPV1 antagonists.

A novel series of 4,5-biarylimidazoles as TRPV1 antagonists were designed based on the previously reported 4,6-disubstituted benzimidazole series. The analogs were evaluated for their ability to block capsaicin- or acid-induced calcium influx in TRPV1-expressing CHO cells. These studies led to the identification of a highly potent and orally bioavailable TRPV1 antagonist, imidazole 33.

Nonthermal activation of transient receptor potential vanilloid-1 channels in abdominal viscera tonically inhibits autonomic cold-defense effectors.

An involvement of the transient receptor potential vanilloid (TRPV) 1 channel in the regulation of body temperature (T(b)) has not been established decisively. To provide decisive evidence for such an involvement and determine its mechanisms were the aims of the present study. We synthesized a new TRPV1 antagonist, AMG0347 [(E)-N-(7-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl)-3-(2-(piperidin-1-yl)-6-(trifluoromethyl)pyridin-3-yl)acrylamide], and characterized it in vitro. We then found that this drug is the most potent TRPV1 antagonist known to increase T(b) of rats and mice and showed (by using knock-out mice) that the entire hyperthermic effect of AMG0347 is TRPV1 dependent. AMG0347-induced hyperthermia was brought about by one or both of the two major autonomic cold-defense effector mechanisms (tail-skin vasoconstriction and/or thermogenesis), but it did not involve warmth-seeking behavior. The magnitude of the hyperthermic response depended on neither T(b) nor tail-skin temperature at the time of AMG0347 administration, thus indicating that AMG0347-induced hyperthermia results from blockade of tonic TRPV1 activation by nonthermal factors. AMG0347 was no more effective in causing hyperthermia when administered into the brain (intracerebroventricularly) or spinal cord (intrathecally) than when given systemically (intravenously), which indicates a peripheral site of action. We then established that localized intra-abdominal desensitization of TRPV1 channels with intraperitoneal resiniferatoxin blocks the T(b) response to systemic AMG0347; the extent of desensitization was determined by using a comprehensive battery of functional tests. We conclude that tonic activation of TRPV1 channels in the abdominal viscera by yet unidentified nonthermal factors inhibits skin vasoconstriction and thermogenesis, thus having a suppressive effect on T(b).