First-in-Human Evaluation of 18F-PF-06445974, a PET Radioligand That Preferentially Labels Phosphodiesterase-4B.

Phosphodiesterase-4 (PDE4), which metabolizes the second messenger cyclic adenosine monophosphate (cAMP), has 4 isozymes: PDE4A, PDE4B, PDE4C, and PDE4D. PDE4B and PDE4D have the highest expression in the brain and may play a role in the pathophysiology and treatment of depression and dementia. This study evaluated the properties of the newly developed PDE4B-selective radioligand 18F-PF-06445974 in the brains of rodents, monkeys, and humans. Methods: Three monkeys and 5 healthy human volunteers underwent PET scans after intravenous injection of 18F-PF-06445974. Brain uptake was quantified as total distribution volume (V T) using the standard 2-tissue-compartment model and serial concentrations of parent radioligand in arterial plasma. Results: 18F-PF-06445974 readily distributed throughout monkey and human brain and had the highest binding in the thalamus. The value of V T was well identified by a 2-tissue-compartment model but increased by 10% during the terminal portions (40 and 60 min) of the monkey and human scans, respectively, consistent with radiometabolite accumulation in the brain. The average human V T values for the whole brain were 9.5 ± 2.4 mL ⋅ cm-3 Radiochromatographic analyses in knockout mice showed that 2 efflux transporters-permeability glycoprotein (P-gp) and breast cancer resistance protein (BCRP)-completely cleared the problematic radiometabolite but also partially cleared the parent radioligand from the brain. In vitro studies with the human transporters suggest that the parent radioligand was a partial substrate for BCRP and, to a lesser extent, for P-gp. Conclusion: 18F-PF-06445974 quantified PDE4B in the human brain with reasonable, but not complete, success. The gold standard compartmental method of analyzing brain and plasma data successfully identified the regional densities of PDE4B, which were widespread and highest in the thalamus, as expected. Because the radiometabolite-induced error was only about 10%, the radioligand is, in the opinion of the authors, suitable to extend to clinical studies.

Optimizing the Benefit/Risk of Acetyl-CoA Carboxylase Inhibitors through Liver Targeting.

Preclinical and clinical data suggest that acetyl-CoA carboxylase (ACC) inhibitors have the potential to rebalance disordered lipid metabolism, leading to improvements in nonalcoholic steatohepatitis (NASH). Consistent with these observations, first-in-human clinical trials with our ACC inhibitor PF-05175157 led to robust reduction of de novo lipogenesis (DNL), albeit with concomitant reductions in platelet count, which were attributed to the inhibition of fatty acid synthesis within bone marrow. Herein, we describe the design, synthesis, and evaluation of carboxylic acid-based ACC inhibitors with organic anion transporting polypeptide (OATP) substrate properties, which facilitated selective distribution of the compounds at the therapeutic site of action (liver) relative to the periphery. These efforts led to the discovery of clinical candidate PF-05221304 (12), which selectively inhibits liver DNL in animals, while demonstrating considerable safety margins against platelet reduction in a nonhuman primate model.

Quantitative Analysis of 18F-PF-06684511, a Novel PET Radioligand for Selective ß-Secretase 1 Imaging, in Nonhuman Primate Brain.

ß-secretase 1 (BACE1) is a key enzyme in the generation of ß-amyloid, which is accumulated in the brain of Alzheimer disease patients. PF-06684511 was identified as a candidate PET ligand for imaging BACE1 in the brain and showed high specific binding in an initial assessment in a nonhuman primate (NHP) PET study using 18F-PF-06684511. In this effort, we aimed to quantitatively evaluate the regional brain distribution of 18F-PF-06684511 in NHPs under baseline and blocking conditions and to assess the target occupancy of BACE1 inhibitors. In addition, NHP whole-body PET measurements were performed to estimate the effective radiation dose. Methods: Initial brain PET measurements were performed at baseline and after oral administration of 5 mg/kg of LY2886721, a BACE1 inhibitor, in 2 cynomolgus monkeys. Kinetic analysis was performed with the radiometabolite-corrected plasma input function. In addition, a wide dose range of another BACE1 inhibitor, PF-06663195, was examined to investigate the relationship between the brain target occupancy and plasma concentration of the drug. Finally, the effective radiation dose of 18F-PF-06684511 was estimated on the basis of the whole-body PET measurements in NHPs. Results: Radiolabeling was accomplished successfully with an incorporation radiochemical yield of 4%-12% (decay-corrected) from 18F ion. The radiochemical purity was greater than 99%. The whole-brain uptake of 18F-PF-06684511 peaked (∼220% SUV) at approximately 20 min and decreased thereafter (∼100% SUV at 180 min). A 2-tissue-compartment model described the time-activity curves well. Pretreatment with LY2886721 reduced the total distribution volume of 18F-PF-06684511 by 48%-80% depending on the brain region, confirming its in vivo specificity. BACE1 occupancy of PF-06663195, estimated using the Lassen occupancy plot, showed a dose-dependent increase. The effective dose of 18F-PF-06684511 was 0.043 mSv/MBq for humans. Conclusion: 18F-PF-06684511 is the first successful PET radioligand for BACE1 brain imaging that demonstrates favorable in vivo binding and brain kinetics in NHPs.

Design and Synthesis of Clinical Candidate PF-06751979: A Potent, Brain Penetrant, ß-Site Amyloid Precursor Protein Cleaving Enzyme 1 (BACE1) Inhibitor Lacking Hypopigmentation.

A major challenge in the development of ß-site amyloid precursor protein cleaving enzyme 1 (BACE1) inhibitors for the treatment of Alzheimer's disease is the alignment of potency, drug-like properties, and selectivity over related aspartyl proteases such as Cathepsin D (CatD) and BACE2. The potential liabilities of inhibiting BACE2 chronically have only recently begun to emerge as BACE2 impacts the processing of the premelanosome protein (PMEL17) and disrupts melanosome morphology resulting in a depigmentation phenotype. Herein, we describe the identification of clinical candidate PF-06751979 (64), which displays excellent brain penetration, potent in vivo efficacy, and broad selectivity over related aspartyl proteases including BACE2. Chronic dosing of 64 for up to 9 months in dog did not reveal any observation of hair coat color (pigmentation) changes and suggests a key differentiator over current BACE1 inhibitors that are nonselective against BACE2 in later stage clinical development.

Late-Stage Microsomal Oxidation Reduces Drug-Drug Interaction and Identifies Phosphodiesterase 2A Inhibitor PF-06815189.

Late-stage oxidation using liver microsomes was applied to phosphodiesterase 2 inhibitor 1 to reduce its clearance by cytochrome P450 enzymes, introduce renal clearance, and minimize the risk for victim drug-drug interactions. This approach yielded PF-06815189 (2) with improved physicochemical properties and a mixed metabolic profile. This example highlights the importance of C-H diversification methods to drug discovery.

Identification of a Novel Positron Emission Tomography (PET) Ligand for Imaging ß-Site Amyloid Precursor Protein Cleaving Enzyme 1 (BACE-1) in Brain.

Alzheimer's disease (AD) is characterized by accumulation of ß-amyloid (Aß) plaques and neurofibrillary tau tangles in the brain. ß-Site amyloid precursor protein cleaving enzyme 1 (BACE1) plays a key role in the generation of Aß fragments via extracellular cleavage of the amyloid precursor protein (APP). We became interested in developing a BACE1 PET ligand to facilitate clinical assessment of BACE1 inhibitors and explore its potential in the profiling and selection of patients for AD trials. Using a set of PET ligand design parameters, compound 3 (PF-06684511) was rapidly identified as a lead with favorable in vitro attributes and structural handles for PET radiolabeling. Further evaluation in an LC-MS/MS "cold tracer" study in rodents revealed high specific binding to BACE1 in brain. Upon radiolabeling, [18F]3 demonstrated favorable brain uptake and high in vivo specificity in nonhuman primate (NHP), suggesting its potential for imaging BACE1 in humans.

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 (Bmax) 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 8 (PF-06445974) as a promising PET lead. Compound 8 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]8 showed rapid brain uptake and high target specificity, indicating that [18F]8 is a promising PDE4B-preferring radioligand for clinical PET imaging.

Chemoproteomic profiling reveals that cathepsin D off-target activity drives ocular toxicity of ß-secretase inhibitors.

Inhibition of ß-secretase BACE1 is considered one of the most promising approaches for treating Alzheimer's disease. Several structurally distinct BACE1 inhibitors have been withdrawn from development after inducing ocular toxicity in animal models, but the target mediating this toxicity has not been identified. Here we use a clickable photoaffinity probe to identify cathepsin D (CatD) as a principal off-target of BACE1 inhibitors in human cells. We find that several BACE1 inhibitors blocked CatD activity in cells with much greater potency than that displayed in cell-free assays with purified protein. Through a series of exploratory toxicology studies, we show that quantifying CatD target engagement in cells with the probe is predictive of ocular toxicity in vivo. Taken together, our findings designate off-target inhibition of CatD as a principal driver of ocular toxicity for BACE1 inhibitors and more generally underscore the power of chemical proteomics for discerning mechanisms of drug action.

Discovery of spirocyclic-diamine inhibitors of mammalian acetyl CoA-carboxylase.

A novel series of spirocyclic-diamine based, isoform non-selective inhibitors of acetyl-CoA carboxylase (ACC) is described. These spirodiamine derivatives were discovered by design of a library to mimic the structural rigidity and hydrogen-bonding pattern observed in the co-crystal structure of spirochromanone inhibitor I. The lead compound 3.5.1 inhibited de novo lipogenesis in rat hepatocytes, with an IC50 of 0.30 µM.

Utilizing structures of CYP2D6 and BACE1 complexes to reduce risk of drug-drug interactions with a novel series of centrally efficacious BACE1 inhibitors.

In recent years, the first generation of ß-secretase (BACE1) inhibitors advanced into clinical development for the treatment of Alzheimer's disease (AD). However, the alignment of drug-like properties and selectivity remains a major challenge. Herein, we describe the discovery of a novel class of potent, low clearance, CNS penetrant BACE1 inhibitors represented by thioamidine 5. Further profiling suggested that a high fraction of the metabolism (>95%) was due to CYP2D6, increasing the potential risk for victim-based drug-drug interactions (DDI) and variable exposure in the clinic due to the polymorphic nature of this enzyme. To guide future design, we solved crystal structures of CYP2D6 complexes with substrate 5 and its corresponding metabolic product pyrazole 6, which provided insight into the binding mode and movements between substrate/inhibitor complexes. Guided by the BACE1 and CYP2D6 crystal structures, we designed and synthesized analogues with reduced risk for DDI, central efficacy, and improved hERG therapeutic margins.

Discovery of a series of efficient, centrally efficacious BACE1 inhibitors through structure-based drug design.

The identification of centrally efficacious ß-secretase (BACE1) inhibitors for the treatment of Alzheimer's disease (AD) has historically been thwarted by an inability to maintain alignment of potency, brain availability, and desired absorption, distribution, metabolism, and excretion (ADME) properties. In this paper, we describe a series of truncated, fused thioamidines that are efficiently selective in garnering BACE1 activity without simultaneously inhibiting the closely related cathepsin D or negatively impacting brain penetration and ADME alignment, as exemplified by 36. Upon oral administration, these inhibitors exhibit robust brain availability and are efficacious in lowering central Amyloid ß (Aß) levels in mouse and dog. In addition, chronic treatment in aged PS1/APP mice effects a decrease in the number and size of Aß-derived plaques. Most importantly, evaluation of 36 in a 2-week exploratory toxicology study revealed no accumulation of autofluorescent material in retinal pigment epithelium or histology findings in the eye, issues observed with earlier BACE1 inhibitors.

Decreasing the rate of metabolic ketone reduction in the discovery of a clinical acetyl-CoA carboxylase inhibitor for the treatment of diabetes.

Acetyl-CoA carboxylase (ACC) inhibitors offer significant potential for the treatment of type 2 diabetes mellitus (T2DM), hepatic steatosis, and cancer. However, the identification of tool compounds suitable to test the hypothesis in human trials has been challenging. An advanced series of spirocyclic ketone-containing ACC inhibitors recently reported by Pfizer were metabolized in vivo by ketone reduction, which complicated human pharmacology projections. We disclose that this metabolic reduction can be greatly attenuated through introduction of steric hindrance adjacent to the ketone carbonyl. Incorporation of weakly basic functionality improved solubility and led to the identification of 9 as a clinical candidate for the treatment of T2DM. Phase I clinical studies demonstrated dose-proportional increases in exposure, single-dose inhibition of de novo lipogenesis (DNL), and changes in indirect calorimetry consistent with increased whole-body fatty acid oxidation. This demonstration of target engagement validates the use of compound 9 to evaluate the role of DNL in human disease.

Spirolactam-based acetyl-CoA carboxylase inhibitors: toward improved metabolic stability of a chromanone lead structure.

Acetyl-CoA carboxylase (ACC) catalyzes the rate-determining step in de novo lipogenesis and plays a crucial role in the regulation of fatty acid oxidation. Alterations in lipid metabolism are believed to contribute to insulin resistance; thus inhibition of ACC offers a promising option for intervention in type 2 diabetes mellitus. Herein we disclose a series of ACC inhibitors based on a spirocyclic pyrazololactam core. The lactam series has improved chemical and metabolic stability relative to our previously reported pyrazoloketone series, while retaining potent inhibition of ACC1 and ACC2. Optimization of the pyrazole and amide substituents led to quinoline amide 21, which was advanced to preclinical development.

Discovery of triazolopyrimidine-based PDE8B inhibitors: exceptionally ligand-efficient and lipophilic ligand-efficient compounds for the treatment of diabetes.

PDE8B is a cAMP-specific isoform of the broader class of phosphodiesterases (PDEs). As no selective PDE8B inhibitors had been reported, a high throughput screen was run with the goal of identifying selective tools for exploring the potential therapeutic utility of PDE8B inhibition. Of the numerous hits, one was particularly attractive since it was amenable to rapid deconstruction leading to inhibitors with very high ligand efficiency (LE) and lipophilic ligand efficiency (LLE). These triazolopyrimidines were optimized for potency, selectivity and ADME properties ultimately leading to compound 42. This compound was highly potent and selective with good bioavailability and advanced into pre-clinical development.

Metabolism, excretion, and pharmacokinetics of ((3,3-difluoropyrrolidin-1-yl)((2S,4S)-4-(4-(pyrimidin-2-yl)piperazin-1-yl)pyrrolidin-2-yl)methanone, a dipeptidyl peptidase inhibitor, in rat, dog and human.

The disposition of 3,3-difluoropyrrolidin-1-yl{(2S,4S)-4-[4-(pyrimidin-2-yl)piperazin-1-yl]pyrrolidin-2-yl}methanone (PF-00734200), a dipeptidyl peptidase IV inhibitor that progressed to phase 3 for the treatment of type 2 diabetes, was examined in rats, dogs, and humans after oral administration of a single dose of [(14)C]PF-00734200. Mean recoveries of administered radioactivity were 97.1, 92.2, and 87.2% in rats, dogs, and humans, respectively. The majority of radioactive dose was detected in the urine of dogs and humans and in the feces of rats. Absorption of PF-00734200 was rapid in all species, with maximal plasma concentrations of radioactivity achieved within 1 h after the dose. Circulating radioactivity was primarily composed of the parent drug (79.9, 80.2, and 94.4% in rat, dog, and human, respectively). The major route of metabolism was due to hydroxylation at the 5' position of the pyrimidine ring (M5) in all species. In vitro experiments with recombinant cytochrome P450 isoforms suggested that the formation of M5 was catalyzed both by CYP2D6 and CYP3A4. Molecular docking simulations showed that the 5' position of the pyrimidine moiety of PF-00734200 can access the heme iron-oxo of both CYP3A4 and CYP2D6 in an energetically favored orientation. Other metabolic pathways included amide hydrolysis (M2), N-dealkylation at the piperazine nitrogen (M3) and an unusual metabolite resulting from scission of the pyrimidine ring (M1). Phase II metabolic pathways included the following: carbamoyl glucuronidation (M9), glucosidation (M15) on the pyrrolidine nitrogen, and conjugation with creatinine to form an unusual metabolite/metabonate (M16). The data from these studies suggest that PF-00734200 is eliminated by both metabolism and renal clearance.

Maximizing lipophilic efficiency: the use of Free-Wilson analysis in the design of inhibitors of acetyl-CoA carboxylase.

This paper describes the design and synthesis of a novel series of dual inhibitors of acetyl-CoA carboxylase 1 and 2 (ACC1 and ACC2). Key findings include the discovery of an initial lead that was modestly potent and subsequent medicinal chemistry optimization with a focus on lipophilic efficiency (LipE) to balance overall druglike properties. Free-Wilson methodology provided a clear breakdown of the contributions of specific structural elements to the overall LipE, a rationale for prioritization of virtual compounds for synthesis, and a highly successful prediction of the LipE of the resulting analogues. Further preclinical assays, including in vivo malonyl-CoA reduction in both rat liver (ACC1) and rat muscle (ACC2), identified an advanced analogue that progressed to regulatory toxicity studies.

1,5-Substituted nipecotic amides: selective PDE8 inhibitors displaying diastereomer-dependent microsomal stability.

The first highly potent and selective PDE8 inhibitors are disclosed. The initial tetrahydroisoquinoline hit was transformed into a nipecotic amide series in order to address a reactive metabolite issue. Reduction of lipophilicity to address metabolic liabilities uncovered an interesting diastereomer-dependent trend in turnover by human microsomes.

1-((3S,4S)-4-amino-1-(4-substituted-1,3,5-triazin-2-yl) pyrrolidin-3-yl)-5,5-difluoropiperidin-2-one inhibitors of DPP-4 for the treatment of type 2 diabetes.

A 3-amino-4-substituted pyrrolidine series of dipeptidyl peptidase IV (DPP-4) inhibitors was rapidly developed into a candidate series by identification of a polar valerolactam replacement for the lipophilic 2,4,5-trifluorophenyl pharmacophore. The addition of a gem-difluoro substituent to the lactam improved overall DPP-4 inhibition and an efficient asymmetric route to 3,4-diaminopyrrolidines was developed. Advanced profiling of a subset of analogs identified 5o with an acceptable human DPP-4 inhibition profile based on a rat PK/PD model and a projected human dose that was suitable for clinical development.

Discovery of small molecule isozyme non-specific inhibitors of mammalian acetyl-CoA carboxylase 1 and 2.

Screening Pfizer's compound library resulted in the identification of weak acetyl-CoA carboxylase inhibitors, from which were obtained rACC1 CT-domain co-crystal structures. Utilizing HTS hits and structure-based drug discovery, a more rigid inhibitor was designed and led to the discovery of sub-micromolar, spirochromanone non-specific ACC inhibitors. Low nanomolar, non-specific ACC-isozyme inhibitors that exhibited good rat pharmacokinetics were obtained from this chemotype.

Synthesis and SAR of 1,2,3,4-tetrahydroisoquinolin-1-ones as novel G-protein-coupled receptor 40 (GPR40) antagonists.

The development of a series of novel 1,2,3,4-tetrahydroisoquinolin-1-ones as antagonists of G protein-coupled receptor 40 (GPR40) is described. The synthesis, in vitro inhibitory values for GPR40, in vitro microsomal clearance and rat in vivo clearance data are discussed. Initial hits displayed high rat in vivo clearances that were higher than liver blood flow. Optimization of rat in vivo clearance was achieved and led to the identification of 15i, whose rat oral pharmacokinetic data is reported.