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Discovery and Optimization of a Series of Pyrimidine-Based Phosphodiesterase 10A (PDE10A) Inhibitors through Fragment Screening, Structure-Based Design, and Parallel Synthesis William D. Shipe,*,† Steven S. Sharik,†,⊥ James C. Barrow,†,# Georgia B. McGaughey,‡,∇ Cory R. Theberge,†,○ Jason M. Uslaner,§ Youwei Yan,∥ John J. Renger,§ Sean M. Smith,§ Paul J. Coleman,† and Christopher D. Cox† Departments of †Discovery Chemistry, ‡Chemistry Modeling and Informatics, §Neuroscience, and ∥Structural Chemistry, Merck Research Laboratories, West Point, Pennsylvania 19486, United States S Supporting Information *

ABSTRACT: Screening of a fragment library for PDE10A inhibitors identified a low molecular weight pyrimidine hit with PDE10A Ki of 8700 nM and LE of 0.59. Initial optimization by catalog followed by iterative parallel synthesis guided by X-ray cocrystal structures resulted in rapid potency improvements with minimal loss of ligand efficiency. Compound 15h, with PDE10A Ki of 8.2 pM, LE of 0.49, and >5000-fold selectivity over other PDEs, fully attenuates MK-801-induced hyperlocomotor activity after ip dosing.

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INTRODUCTION Schizophrenia is a mental disorder that represents a substantial unmet medical need. An estimated 0.3−0.7% of the population is affected by schizophrenia, and the societal costs are high.1 The current treatment of schizophrenia relies on the use of atypical antipsychotic agents that target dopamine and serotonin receptors, a pharmacology associated with undesirable side effects including weight gain, hyperglycemia, dyslipidemia, and extrapyramidal symptoms.2,3 Dysregulation of the glutamate pathway has also been implicated in the cognitive deficits seen in schizophrenia.4,5 Recently, there has been increased interest in phosphodiesterase 10A (PDE10A) inhibitors as a means to treat schizophrenia.6,7 PDE10A is a dual substrate phosphodiesterase, highly expressed in the striatal region of the brain, that catalyzes hydrolysis of the second messengers cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP). Since striatal dysfunction is implicated in the pathophysiology of schizophrenia and PDE10A acts downstream of the dopamine and glutamate receptors in their respective signaling pathways, direct inhibition of PDE10A may raise levels of cGMP and cAMP in the striatum and correct the behavioral control and cognitive deficits associated with schizophrenia without the side effects associated with current © XXXX American Chemical Society

standard of care. The PDE superfamily includes 11 families of PDEs that produce more than 50 functionally unique enzymes. However, the amino acid similarity between PDE families ranges from 25% to 35%, making the development of selective PDE10A inhibitors an achievable goal.8 On the basis of this evidence, we began a campaign to identify a small molecule inhibitor of PDE10A as a novel therapeutic. In addition to high-throughput screening (HTS) and mass spectrometry based affinity selection approaches, we conducted a fragment screen in the search for chemical matter to serve as starting points for lead optimization. The fragment approach, in which low molecular weight, weakly binding fragment hits are evolved toward druglike compounds, has been more frequently practiced in recent years,9 with several reported examples applied to PDE10A.10,11 While large, druglike lead compounds may bind with higher affinity, they can require more time in the optimization phase to address liabilities inherent in the lead structure. Smaller leads can be harder to detect owing to lower affinity for the target but are more likely to bind efficiently. Various metrics have been used for the assessment of chemical leads, ligand efficiency (LE) Received: June 24, 2015

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among them,12,13 which we employed as a “guidepost” in our efforts to optimize fragment leads. The fragment approach is especially powerful when cocrystal structure determination is used to provide rationale for design decisions during the optimization phase. In this article, we describe the evolution of a PDE10A fragment screening hit into a potent and selective inhibitor that demonstrated efficacy in a preclinical model of schizophrenia.

rapid advancement of fragment hit 5 above others that we identified.

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RESULTS AND DISCUSSION In our initial efforts to identify PDE10A inhibitors, several structural series were pursued on the basis of HTS, represented by optimized compounds 1−3 (Figure 1). While 1 served as a Figure 2. Crystal structure of pyrimidine fragment hit 5 in the PDE10A catalytic domain.

Initial fragment optimization was executed in part in an “SAR by catalog” approach utilizing similar compounds from the Merck sample collection; this effort identified pyrimidine 6, with an improved PDE10A Ki of 2200 nM, and accordingly enhanced ligand efficiency (0.65). The X-ray cocrystal structure of 6 with PDE10A,21 nearly identical to that of the PDE10A−5 complex, indicated that the binding mode was maintained and that the 6-amino group projected toward a pocket in the active site. Tolerance of an amino group at the 6-position of the pyrimidine suggested an avenue for further elaboration that might lead to PDE10A potency gains. In addition to being the original screening hit, 4,6-dichloropyrimidine 5 served as a useful synthetic intermediate. Its synthesis commenced with the cyclocondensation of cyclopropylamidine (7) with dimethyl 2methylmalonate to generate 4,6-dihydroxypyrimidine 8 (Scheme 1). Treatment with phosphoryl chloride provided 5. Nucleophilic aromatic substitution with amines proceeded with selective monoaddition to produce 6-amino inhibitors 9.

Figure 1. Merck PDE10A inhibitors 1−3 derived from HTS leads. Pfizer PDE10A clinical compound 4.

useful tool compound,14−16 it suffered from low solubility and poor PDE selectivity (it did not access the PDE “selectivity pocket”; vide infra). Compound 2 served as a platform for PET tracer development17 but suffers from poor pharmacokinetic properties, limiting its potential as a drug candidate. Compound 3 lacks PDE selectivity and possesses other liabilities that precluded its further development, including low solubility and inhibition of CYPs. The LE value for all three compounds is 0.42 or less. Compound 4 (PF-2545920), with a slightly higher LE of 0.46, is one of the furthest advanced PDE10A inhibitors, having progressed to phase II clinical trials and benefits from engagement of the selectivity pocket.18 Fragment screening of a proprietary collection of 1600 soluble, low molecular weight compounds against PDE10A in a biochemical assay at high concentration identified 60 hits showing >80% inhibition at 200 μM. Nine of these hits were soaked into apo-PDE10A after confirmative titration.19 Seven of the fragment hits crystallized in the PDE10A catalytic domain and displayed the expected interactions with key residues in the active site, including a π-stacking interaction with Phe719 and a hydrogen bond with Gln716.20 Pyrimidine 5, a PDE10A inhibitor with a Ki of 8700 nM, was one of several fragment hits chosen as a starting point for optimization on the basis of its excellent ligand efficiency (0.59) and observed binding mode (Figure 2). The synthetic tractability of the pyrimidine scaffold also offered a practical advantage, aiding the

Scheme 1. Synthesis of 6-Aminopyrimidines 9

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Table 1. Survey of Amino Substitutions at the Pyrimidine 6-Position

a

LE = ligand efficiency. bEach Ki value reported is an average of at least two measurements with a 10-point dose−response curve.

corresponding linear analogs (9e vs 9f; 9g vs 9h). The npropyl compound 9h, with a PDE10A Ki of 880 nM and LE of 0.55, was selected for further characterization by X-ray cocrystallization. The structure revealed that the pyrimidine core maintained interactions with Gln716 and Phe719 and that the alkyl substituent was oriented toward a pocket prominently featuring the aromatic residues Phe686 and Tyr514.21 While aromatic analogs such as 9i and 9j were not as ligand-efficient as some of the other compounds tested, they and the 2-

A wide array of amino substitutions were surveyed using this chemistry in a parallel format, beginning with a representative set of low molecular weight amines (Table 1, entries 9a−k). Consistent with the PDE10A X-ray cocrystal structures of 5 and 6, a tether of specific length was preferred for access to the pocket toward which the 6-amino group projected. Substitution on the tether was poorly tolerated; tertiary amines showed generally inferior PDE10A potency (9a−d), and C-branched analogs were likewise less active in comparison to the C

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fluoroallyl analog 9k did offer some promise as a means to improve PDE10A potency. On the basis of the X-ray structural information as well as the results of this initial interrogation of the Phe686/Tyr514 pocket of PDE10A, we were intrigued by the possibility of additional affinity gains by exploiting the feature of sp2 character in the 6-amino substituent. Accordingly, the next broad iteration of parallel synthesis further focused on screening of the 6-amino group using the same aromatic substitution chemistry on the pyrimidine core but with an expanded set of amines, increasing the emphasis on aryl and heteroaryl substituents (Table 1, entries 9l−dd). The introduction of a heteroaryl substituent generally yielded substantial PDE10A potency gains in a ligand-efficient manner. The most preferred linker length was a single methylene group between the 6-amino group and the heterocycle (9l vs 9m), and substitution on the linking methylene was generally poorly tolerated (9n vs 9o). Tertiary 6-amino substitution was also found to have a detrimental effect on PDE10A potency (9l vs 9p; 9q vs 9r), including cyclized analogs (9s vs 9t). A general trend observed within this survey of heteroaromatic analogs was the potency-enhancing effect of a “distal” methyl substitution (9u vs 9v; 9w vs 9x). Particularly interesting was the compound 9s, containing a 2,4-dimethylthiazolyl substituent, which possessed a PDE10A Ki of 4.8 nM and ligand efficiency (0.57) comparable to the simple n-propyl compound 9h. Both of the methyl groups on the thiazole contribute to PDE10A potency; the “distally” substituted thiazole 2-methyl analog 9y is 5-fold less active than 9s, while the “proximally” substituted thiazole 4-methyl analog 9z is >300-fold less active, similar to the corresponding unsubstituted thiazole 9aa. Apart from the 2-methoxythiazole analog 9bb (PDE10A Ki of 10 nM), other thiazole 2-substituents, including isopropyl, methoxymethyl, and substituted phenyl, all suffered significant (>20-fold) losses in potency relative to 9s. Other thiazole isomers, including those linked to the pyrimidine via the thiazole 4-position (e.g., 9cc) and the thiazole 2-position (e.g., 9dd) widely varied in activity. The promising PDE10A inhibitor 9s was selected for further characterization by X-ray cocrystallization (Figure 3). The key interactions between the pyrimidine core and both Gln716 and

Phe719 were again maintained; additionally, the thiazole substituent was compellingly situated between Phe686 and Tyr514. Though the structural data suggested limited opportunities to build from 9s at the pyrimidine 4- and 5positions, we maintained a limited effort to optimize these substituents. The overall profile of all such characterized compounds demonstrated no significant advantage over the 4chloro and 5-methyl groups already in place. The X-ray structure of 9s also revealed that the 2-substituent of the pyrimidine core was ideally positioned to reach toward a region of the PDE10A active site termed the “S-pocket” or “selectivity pocket”.20 Homology modeling of all PDE isoforms combined with molecular docking of ligands led us to recognize two main factors that identified this region as key to obtaining potency and selectivity for PDE10A: (1) the uniqueness of the Tyr683 residue to only PDE2 and PDE10A and (2) the size of the Gly715 residue in PDE10A versus that of Leu715 in PDE2. Thus, there are significant PDE10A potency and PDE selectivity benefits associated with favorably interacting with Tyr683 while fully occupying the selectivity pocket. Throughout the course of the work thus far described, PDE selectivity did appear to improve from the fragment hit 5 to the 6aminopyrimidine 9s (Table 2). With the goal of further improved PDE selectivity and PDE10A potency in mind, we undertook a deeper exploration of the substitution at the pyrimidine 2-position. Table 2. PDE Selectivity Ratiosa vs PDE10A for Select Compounds

a

Selectivity ratio = (PDEx Ki)/(PDE10A Ki). b>1000-Fold selectivity for PDE10A. c100- to 1000-fold selectivity for PDE10A. d25- to 100fold selectivity for PDE10A. e1- to 25-fold selectivity for PDE10A.

To access the selectivity pocket, a new synthesis was needed to enable rapid and late-stage derivatization at the pyrimidine 2position. The synthesis began with the cyclocondensation of thiourea (10) with dimethyl 2-methylmalonate to give pyrimidine 11 (Scheme 2). Methylation of the sulfur atom followed by treatment with phosphoryl chloride provided 4,6dichloropyrimidine 12. Nucleophilic aromatic substitution with (2,4-dimethylthiazol-5-yl)methanamine produced 13. Oxidation of the thioether functionality with m-chloroperoxybenzoic acid yielded sulfone 14, which served as an electrophile in the reaction with alkoxides to give aryl ether PDE10A inhibitors 15. A large and diverse set of alkoxides was employed using this chemistry in a parallel format to explore the selectivity pocket region of the PDE10A active site (Table 3). Target molecules were selected using a combination of in silico modeling, SAR analysis, and synthetic feasibility, with a bias toward targets based on commercially available monomers. However, highly compelling compounds that required commercially unavailable monomers were pursued as well. Low molecular weight replacements for the cyclopropyl group were identified with ligand efficiency comparable to that of 9s (15a−c) but lacking breakthrough potency. There was

Figure 3. Crystal structure of pyrimidine 9s in the PDE10A catalytic domain. D

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Scheme 2. Synthesis of 2-Alkoxy-6-aminopyrimidines 15

Table 3. Survey of Alkoxide Substitutions at the Pyrimidine 2-Position

clear benefit to the incorporation of an aromatic group (15d vs 15e; 15f vs 15g). Many of the alkoxide nucleophiles chosen incorporated heteroatoms seemingly capable of achieving a Hbond with the side chain of Tyr683, but it was important for PDE10A activity that the proper distance and geometry be attainable to permit such an interaction to occur.22 Subnanomolar potency was achieved by the introduction of an aryl group tethered to the pyrimidine core by a four-atom linker. A feature shared by the most active compounds (15h−l) was the presence of a N atom adjacent to the aforementioned linker within a bicyclic aromatic system. The X-ray cocrystal structure of quinoline 15h (PDE10A Ki of 8.2 pM) showed an H-bond interaction with Tyr683, and the distal aromatic ring of the bicycle exquisitely filled the bottom of the selectivity pocket (Figure 4). Inhibitor 15h demonstrated vastly improved PDE selectivity, with a selectivity ratio of >5000 over all other PDEs tested (Table 4). As 15h represented an exciting advance, it was further characterized in vivo. In the MK-801-induced hyperlocomotor activity assay, 15h demonstrated full efficacy following 1, 3, and 5 mg/kg ip doses (Figure 5).23 Plasma exposures were measured at 3.5 h after dosing 15h, which likely provides an underestimation of the true exposure during the study, given the high plasma clearance of the compound. Despite excellent PDE10A potency and PDE selectivity, as well as demonstrated in vivo efficacy, 15h suffered from several liabilities that limited its further development, including the aforementioned high in vivo clearance, poor bioavailability, ion channel inhibition, and CYP inhibition.

a

LE = ligand efficiency. bEach Ki value reported is an average of at least two measurements with a 10-point dose−response curve.

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PDE10A over all other PDEs. Robust efficacy was observed in the psychostimulant-induced hyperlocomotion assay at submicromolar plasma concentrations following ip dosing, suggesting that the pyrimidine series has potential for further development toward a treatment for schizophrenia. Subsequent efforts in this series have focused on improving pharmacokinetics and ancillary pharmacological profile and will be reported in due course.

CONCLUSION Optimization of 5, a fragment screening hit, employing library by catalog and parallel synthesis resulted in rapid potency improvements without substantial erosion of ligand binding efficiency. The use of inhibitor-bound X-ray crystal structures and in silico modeling (structure-based design) suggested modifications to lead compounds in the pyrimidine series, resulting in the identification of 15h with a PDE10A Ki of 8.2 pM. This analog represents >1 000 000-fold improvement in PDE10A potency over the initial fragment hit with a higher LE than other PDE10A inhibitors previously pursued in our group. Compound 15h also showed >5000-fold selectivity for

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EXPERIMENTAL SECTION

General. All key compounds (5, 9s, 15h) possess a purity of at least 95% as assessed by analytical reversed phase HPLC (see Supporting Information for details). All compounds produced from parallel synthesis possess a purity of at least 90%. E

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microwave reactor at 150 °C for 30 min. After cooling to ambient temperature, the resulting slurry was concentrated in vacuo. An amount of 340 mg (54%) of 11 was isolated as a white solid by filtration. MS (ES): m/z 159.0 [M + H]+. Dihydroxypyrimidine 11 (340 mg, 2.15 mmol) and DMF (10 mL) were placed in a 50 mL flask. Iodomethane (366 mg, 2.58 mmol) was added, and the reaction was stirred overnight. The mixture was then diluted with water (50 mL). An amount of 247 mg (67%) of the intermediate methylthioether was isolated as a white solid by filtration. MS (ES): m/z 173.2 [M + H]+. The thioether (247 mg, 1.43 mmol) was combined with phosphoryl chloride (2.6 mL, 28.7 mmol) and diisopropylethylamine (185 mg, 1.43 mmol) in a 20 mL vial. The mixture was heated in a microwave reactor at 145 °C for 30 min. After cooling to ambient temperature, excess phosphoryl chloride was removed in vacuo. The resulting mixture was then poured onto ice and extracted with dichloromethane (3 × 10 mL). The crude material was purified by elution through a silica plug, yielding 280 mg (93%) of 12 as a white, crystalline solid. MS (ES): m/z 210.1 [M + H]+. Dichloropyrimidine 12 (973 mg, 4.65 mmol) was combined with (2,4-dimethylthiazol-5-yl)methanamine (728 mg, 5.12 mmol) and triethylamine (1.2 g, 9.31 mmol) in a 20 mL vial. DMSO (5 mL) was added, and the mixture was heated in a microwave reactor at 100 °C for 20 min. After cooling to ambient temperature, the mixture was added to water, causing a precipitate to form. Filtration yielded 1.01 g (70%) of product 13 as a white solid. HRMS (ES): m/z 315.0500 [M + H]+. Chloropyrimidine 13 (1.36 g, 4.32 mmol) was placed in a 250 mL flask and diluted with dichloromethane (100 mL). To the resultant slurry was added mCPBA (1.49 g, 8.64 mmol), and the solution was stirred for 1 h. 1 N aqueous NaOH (100 mL) was then added, and the organic layer was separated and concentrated in vacuo to give 1.3 g (87%) of 14 as a white foam. MS (ES): m/z 347.8 [M + H]+. To a solution of sulfone 14 (51 mg, 0.15 mmol) and 3-(quinolin-2yl)propan-1-ol (25 mg, 0.13 mmol) in 3 mL of THF was added LiHMDS (200 μL of a 1 M solution in THF, 0.20 mmol). After stirring for 2 h, the reaction was quenched by the addition of a saturated solution of NH4Cl. The mixture was partitioned between a saturated solution of NaHCO3 and EtOAc, the layers were separated, and the aqueous layer was extracted twice with EtOAc. The combined organic layers were washed with brine, dried over Na2SO4, and concentrated in vacuo. The residue was purified by silica gel flash column chromatography (EtOAc/hexanes gradient) to provide 17 mg (28%) of 15h as an off-white solid. 1H NMR (CDCl3, 400 MHz): δH 8.1−8.0 (2H, m), 7.75 (1H, m), 7.65 (1H, m), 7.5 (1H, m), 7.35 (1H, m), 4.9 (1H, m), 4.7 (2H, d), 4.4 (2H, t), 3.15 (2H, t), 2.6 (3H, s), 2.35 (5H, m), 2.0 (3H, s). HRMS (ES): m/z 454.1455 [M + H]+.

Figure 4. Crystal structure of pyrimidine 15h in the PDE10A catalytic domain.

Table 4. PDE Selectivity Ratiosa vs PDE10A for 15h

a

Selectivity ratio = (PDEx Ki)/(PDE10A Ki). b>1000-Fold selectivity for PDE10A.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jmedchem.5b00983. General analytical experimental information and methods; synthesis and characterization of 5, 9s, and 15h, crystallographic protocols and acknowledgments for structures of PDE10A with 5, 6, 9h, 9s, 15d, and 15h; in vitro and in vivo assays and methods; supplemental figures; and expanded versions of all tables (PDF) Molecular formula strings (CSV)

Figure 5. Effect of administration of 15h in the MK-801 psychomotor activity assay. aPlasma exposures were measured at a time point of 3.5 h and are the average of two measurements. bPlasma exposure is the average of three measurements. 6-Chloro-N-((2,4-dimethylthiazol-5-yl)methyl)-5-methyl-2(3-(quinolin-2-yl)propoxy)pyrimidin-4-amine (15h). Thiourea (300 mg, 3.94 mmol) and dimethyl 2-methylpropanedioate (576 mg, 3.94 mmol) were placed in a 5 mL vial and diluted with 3 mL of methanol. A solution of sodium methoxide (710 mg, 3.94 mmol) in methanol (0.75 mL) was slowly added. The mixture was heated in a

Accession Codes

Structural coordinates have been deposited in the RCSB Protein Data Bank under the accession codes 5C1W (5), 5C28 (6), 5C29 (9h), 5C2A (9s), 5C2E (15d), and 5C2H (15h). F

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(12) Abad-Zapatero, C.; Metz, J. T. Ligand efficiency indices as guideposts for drug discovery. Drug Discovery Today 2005, 10, 464− 469. (13) Hopkins, A. L.; Groom, C. R.; Alex, A. Ligand efficiency: a useful metric for lead selection. Drug Discovery Today 2004, 9, 430− 431. (14) Raheem, I. T.; Breslin, M. J.; Fandozzi, C.; Fuerst, J.; Hill, N.; Huszar, S.; Kandebo, M.; Kim, S. H.; Ma, B.; McGaughey, G.; Renger, J. J.; Schreier, J. D.; Sharma, S.; Smith, S.; Uslaner, J.; Yan, Y.; Coleman, P. J.; Cox, C. D. Discovery of tetrahydropyridopyrimidine phosphodiesterase 10A inhibitors for the treatment of schizophrenia. Bioorg. Med. Chem. Lett. 2012, 22, 5903−5908. (15) Raheem, I. T.; Schreier, J. D.; Breslin, M. J. Efficient synthesis of highly functionalized tetrahydropyridopyrimidines by a novel threecomponent coupling reaction. Tetrahedron Lett. 2011, 52, 3849−3852. (16) Smith, S. M.; Uslaner, J. M.; Cox, C. D.; Huszar, S. L.; Cannon, C. E.; Vardigan, J. D.; Eddins, D.; Toolan, D. M.; Kandebo, M.; Yao, L.; Raheem, I. T.; Schreier, J. D.; Breslin, M. J.; Coleman, P. J.; Renger, J. J. The novel phosphodiesterase 10A inhibitor THPP-1 has antipsychotic-like effects in rat and improves cognition in rat and rhesus monkey. Neuropharmacology 2013, 64, 215−223. (17) Cox, C. D.; Hostetler, E.;. Flores, B. A.; Evelhoch, J. L.; Fan, H.; Gantert, L.; Holahan, M.; Eng, W.; Joshi, A.; McGaughey, G. L.; Meng, X.; Purcell, M.; Raheem, I. T.; Riffel, K.; Yan, Y.; Renger, J. J.; Smith, S. M.; Coleman, P. J. Discovery of [11C]MK-8193 as a PET tracer to measure target engagement of phosphodiesterase 10A (PDE10A) inhibitors. Bioorg. Med. Chem. Lett. 2015, 10.1016/j.bmcl.2015.05.080. (18) Verhoest, P. R.; Chapin, D. S.; Corman, M.; Fonseca, K.; Harms, J. F.; Hou, X.; Marr, E. S.; Menniti, F. S.; Nelson, F.; O’Connor, R.; Pandit, J.; Proulx-LaFrance, C.; Schmidt, A. W.; Schmidt, C. J.; Suiciak, J. A.; Liras, S. Discovery of a novel class of phosphodiesterase 10A inhibitors and identification of clinical candidate 2-[4-(1-Methyl-4-pyridin-4-yl-1H-pyrazol-3-yl)-phenoxymethyl]-quinoline (PF-2545920) for the treatment of schizophrenia. J. Med. Chem. 2009, 52, 5188−5196. (19) See Supporting Information for a description of crystallographic methods, including the “double-soaking method” used to obtain some of the X-ray cocrystal structures. (20) Chappie, T. A.; Helal, C. J.; Hou, X. Current landscape of phosphodiesterase 10A (PDE10A) inhibition. J. Med. Chem. 2012, 55, 7299−7331. (21) See Supporting Information (Figure S1) for a view of the X-ray cocrystal structures of 6 (Figure S1) and 9h (Figure S2) with PDE10A. (22) Analog 15d represented a case in which an unexpected binding mode was observed. See Supporting Information (Figure S3) for a view of the X-ray cocrystal structure of 15d with PDE10A. (23) See Supporting Information (Figure S4) for time-course data for 15h in the MK-801-induced hyperlocomotor activity assay.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (215) 652-6527. Present Addresses ⊥

S.S.S.: Exelon Nuclear, Pottstown, Pennsylvania 19464, United States. # J.C.B.: Lieber Institute for Brain Development, Department of Pharmacology and Molecular Sciences, Johns Hopkins School of Medicine, Baltimore, Maryland 21205, United States. ∇ G.B.M.: Vertex Pharmaceuticals, Boston, Massachusetts 02210, United States. ○ C.R.T.: College of Pharmacy, University of New England, Portland, Maine 04103, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by Merck & Co., Inc. The authors thank the West Point NMR and mass spectrometry facilities for structure elucidation support, the West Point purification group for automated purification of library compounds, the in vitro pharmacology group for PDE potency measurements, Sarah L. Huszar for conducting the MK-801-induced hyperlocomotor activity assay for 15h, Pravien Abeywickrema and Sujata Sharma for PDE10A protein expression and production, John C. Reid for conducting initial crystallization screens, and HuaPoo Su for refining and submitting X-ray structures to the PDB.

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REFERENCES

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