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