Design and Selection Parameters to Accelerate the Discovery of Novel Central Nervous System Positron Emission Tomography (PET) Ligands and Their Application in the Development of a Novel Phosphodiesterase 2A PET Ligand

To accelerate the discovery of novel small molecule central nervous system (CNS) positron emission tomography (PET) ligands, we aimed to define a property space that would facilitate ligand design and prioritization, thereby providing a higher probability of success for novel PET ligand development. Toward this end, we built a database consisting of 62 PET ligands that have successfully reached the clinic and 15 radioligands that failed in late-stage development as negative controls. A systematic analysis of these ligands identified a set of preferred parameters for physicochemical properties, brain permeability, and nonspecific binding (NSB). These preferred parameters have subsequently been applied to several programs and have led to the successful development of novel PET ligands with reduced resources and timelines. This strategy is illustrated here by the discovery of the novel phosphodiesterase 2A (PDE2A) PET ligand 4-(3-[¹⁸F]­fluoroazetidin-1-yl)-7-methyl-5-{1-methyl-5-[4-(trifluoromethyl)­phenyl]-1<i>H</i>-pyrazol-4-yl}­imidazo­[5,1-<i>f</i>]­[1,2,4]­triazine, [¹⁸F]­PF-05270430 (<b>5</b>)

Design and Selection Parameters to Accelerate the Discovery of Novel Central Nervous System Positron Emission Tomography (PET) Ligands and Their Application in the Development of a Novel Phosphodiesterase 2A PET Ligand

To accelerate the discovery of novel small molecule central nervous system (CNS) positron emission tomography (PET) ligands, we aimed to define a property space that would facilitate ligand design and prioritization, thereby providing a higher probability of success for novel PET ligand development. Toward this end, we built a database consisting of 62 PET ligands that have successfully reached the clinic and 15 radioligands that failed in late-stage development as negative controls. A systematic analysis of these ligands identified a set of preferred parameters for physicochemical properties, brain permeability, and nonspecific binding (NSB). These preferred parameters have subsequently been applied to several programs and have led to the successful development of novel PET ligands with reduced resources and timelines. This strategy is illustrated here by the discovery of the novel phosphodiesterase 2A (PDE2A) PET ligand 4-(3-[¹⁸F]­fluoroazetidin-1-yl)-7-methyl-5-{1-methyl-5-[4-(trifluoromethyl)­phenyl]-1<i>H</i>-pyrazol-4-yl}­imidazo­[5,1-<i>f</i>]­[1,2,4]­triazine, [¹⁸F]­PF-05270430 (<b>5</b>)

The Discovery of a Novel Phosphodiesterase (PDE) 4B-Preferring Radioligand for Positron Emission Tomography (PET) Imaging

As part of our effort in identifying phosphodiesterase (PDE) 4B-preferring inhibitors for the treatment of central nervous system (CNS) disorders, we sought to identify a positron emission tomography (PET) ligand to enable target occupancy measurement in vivo. Through a systematic and cost-effective PET discovery process, involving expression level (<i>B</i>_max) and biodistribution determination, a PET-specific structure–activity relationship (SAR) effort, and specific binding assessment using a LC-MS/MS “cold tracer” method, we have identified <b>8</b> (PF-06445974) as a promising PET lead. Compound <b>8</b> has exquisite potency at PDE4B, good selectivity over PDE4D, excellent brain permeability, and a high level of specific binding in the “cold tracer” study. In subsequent non-human primate (NHP) PET imaging studies, [¹⁸F]<b>8</b> showed rapid brain uptake and high target specificity, indicating that [¹⁸F]<b>8</b> is a promising PDE4B-preferring radioligand for clinical PET imaging

The Discovery of a Novel Phosphodiesterase (PDE) 4B-Preferring Radioligand for Positron Emission Tomography (PET) Imaging

As part of our effort in identifying phosphodiesterase (PDE) 4B-preferring inhibitors for the treatment of central nervous system (CNS) disorders, we sought to identify a positron emission tomography (PET) ligand to enable target occupancy measurement in vivo. Through a systematic and cost-effective PET discovery process, involving expression level (<i>B</i>_max) and biodistribution determination, a PET-specific structure–activity relationship (SAR) effort, and specific binding assessment using a LC-MS/MS “cold tracer” method, we have identified <b>8</b> (PF-06445974) as a promising PET lead. Compound <b>8</b> has exquisite potency at PDE4B, good selectivity over PDE4D, excellent brain permeability, and a high level of specific binding in the “cold tracer” study. In subsequent non-human primate (NHP) PET imaging studies, [¹⁸F]<b>8</b> showed rapid brain uptake and high target specificity, indicating that [¹⁸F]<b>8</b> is a promising PDE4B-preferring radioligand for clinical PET imaging

Discovery and Preclinical Characterization of 1‑Methyl‑3-(4‑methylpyridin‑3‑yl)‑6‑(pyridin‑2‑ylmethoxy)‑1<i>H</i>‑pyrazolo-[3,4‑<i>b</i>]­pyrazine (PF470): A Highly Potent, Selective, and Efficacious Metabotropic Glutamate Receptor 5 (mGluR5) Negative Allosteric Modulator

A novel series of pyrazolopyrazines is herein disclosed as mGluR5 negative allosteric modulators (NAMs). Starting from a high-throughput screen (HTS) hit (<b>1</b>), a systematic structure–activity relationship (SAR) study was conducted with a specific focus on balancing pharmacological potency with physicochemical and pharmacokinetic (PK) properties. This effort led to the discovery of 1-methyl-3-(4-methylpyridin-3-yl)-6-(pyridin-2-ylmethoxy)-1<i>H</i>-pyrazolo­[3,4-<i>b</i>]­pyrazine (PF470, <b>14</b>) as a highly potent, selective, and orally bioavailable mGluR5 NAM. Compound <b>14</b> demonstrated robust efficacy in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-rendered Parkinsonian nonhuman primate model of l-DOPA-induced dyskinesia (PD-LID). However, the progression of <b>14</b> to the clinic was terminated because of a potentially mechanism-mediated finding consistent with a delayed-type immune-mediated type IV hypersensitivity in a 90-day NHP regulatory toxicology study

Discovery and Characterization of (<i>R</i>)‑6-Neopentyl-2-(pyridin-2-ylmethoxy)-6,7-dihydropyrimido[2,1‑<i>c</i>][1,4]oxazin-4(9<i>H</i>)‑one (PF-06462894), an Alkyne-Lacking Metabotropic Glutamate Receptor 5 Negative Allosteric Modulator Profiled in both Rat and Nonhuman Primates

We previously observed a cutaneous type IV immune response in nonhuman primates (NHP) with the mGlu₅ negative allosteric modulator (NAM) <b>7</b>. To determine if this adverse event was chemotype- or mechanism-based, we evaluated a distinct series of mGlu₅ NAMs. Increasing the sp³ character of high-throughput screening hit <b>40</b> afforded a novel morpholinopyrimidone mGlu₅ NAM series. Its prototype, (<i>R</i>)-6-neopentyl-2-(pyridin-2-ylmethoxy)-6,7-dihydropyrimido­[2,1-<i>c</i>]­[1,4]­oxazin-4­(9<i>H</i>)-one (PF-06462894, <b>8</b>), possessed favorable properties and a predicted low clinical dose (2 mg twice daily). Compound <b>8</b> did not show any evidence of immune activation in a mouse drug allergy model. Additionally, plasma samples from toxicology studies confirmed that <b>8</b> did not form any reactive metabolites. However, <b>8</b> caused the identical microscopic skin lesions in NHPs found with <b>7</b>, albeit with lower severity. Holistically, this work supports the hypothesis that this unique toxicity may be mechanism-based although additional work is required to confirm this and determine clinical relevance

Discovery and Characterization of (<i>R</i>)‑6-Neopentyl-2-(pyridin-2-ylmethoxy)-6,7-dihydropyrimido[2,1‑<i>c</i>][1,4]oxazin-4(9<i>H</i>)‑one (PF-06462894), an Alkyne-Lacking Metabotropic Glutamate Receptor 5 Negative Allosteric Modulator Profiled in both Rat and Nonhuman Primates

We previously observed a cutaneous type IV immune response in nonhuman primates (NHP) with the mGlu₅ negative allosteric modulator (NAM) <b>7</b>. To determine if this adverse event was chemotype- or mechanism-based, we evaluated a distinct series of mGlu₅ NAMs. Increasing the sp³ character of high-throughput screening hit <b>40</b> afforded a novel morpholinopyrimidone mGlu₅ NAM series. Its prototype, (<i>R</i>)-6-neopentyl-2-(pyridin-2-ylmethoxy)-6,7-dihydropyrimido­[2,1-<i>c</i>]­[1,4]­oxazin-4­(9<i>H</i>)-one (PF-06462894, <b>8</b>), possessed favorable properties and a predicted low clinical dose (2 mg twice daily). Compound <b>8</b> did not show any evidence of immune activation in a mouse drug allergy model. Additionally, plasma samples from toxicology studies confirmed that <b>8</b> did not form any reactive metabolites. However, <b>8</b> caused the identical microscopic skin lesions in NHPs found with <b>7</b>, albeit with lower severity. Holistically, this work supports the hypothesis that this unique toxicity may be mechanism-based although additional work is required to confirm this and determine clinical relevance