molecules Article Enantioenriched Positive Allosteric Modulators Display Distinct Pharmacology at the Dopamine D1 Receptor Tim J. Fyfe 1, Peter J. Scammells 1, J. Robert Lane 2,3,* and Ben Capuano 1,* 1 Medicinal Chemistry, Monash Institute of Pharmaceutical Sciences, Monash University, 381 Royal Parade, Parkville, VIC 3052, Australia; tim.fyfe@griffithhack.com (T.J.F.); [email protected] (P.J.S.) 2 Drug Discovery Biology, Monash Institute of Pharmaceutical Sciences, Monash University, 381 Royal Parade, Parkville, VIC 3052, Australia 3 School of Life Sciences, Queen’s Medical Centre, University of Nottingham, Nottingham NG7 2UH, UK * Correspondence: [email protected] (J.R.L.); [email protected] (B.C.) Abstract: (1) Background: Two first-in-class racemic dopamine D1 receptor (D1R) positive allosteric modulator (PAM) chemotypes (1 and 2) were identified from a high-throughput screen. In particular, due to its selectivity for the D1R and reported lack of intrinsic activity, compound 2 shows promise as a starting point toward the development of small molecule allosteric modulators to ameliorate the cognitive deficits associated with some neuropsychiatric disease states; (2) Methods: Herein, we describe the enantioenrichment of optical isomers of 2 using chiral auxiliaries derived from (R)- and (S)-3-hydroxy-4,4-dimethyldihydrofuran-2(3H)-one (D- and L-pantolactone, respectively); (3) Results: We confirm both the racemate and enantiomers of 2 are active and selective for the D1R, but that the respective stereoisomers show a significant difference in their affinity and magnitude of positive allosteric cooperativity with dopamine; (4) Conclusions: These data warrant further investigation Citation: Fyfe, T.J.; Scammells, P.J.; of asymmetric syntheses of optically pure analogues of 2 for the development of D1R PAMs with Lane, J.R.; Capuano, B. superior allosteric properties. Enantioenriched Positive Allosteric Modulators Display Distinct Keywords: dopamine D1 receptor; positive allosteric modulator; PAM; G protein-coupled recep- Pharmacology at the Dopamine D1 tors; synthetic medicinal chemistry; pharmacological evaluation; enantioenriched; chiral auxiliary; Receptor. Molecules 2021, 26, 3799. cAMP BRET https://doi.org/10.3390/ molecules26133799 Academic Editor: Jianguo Fang 1. Introduction Schizophrenia (SCZ) is a debilitating neuropsychiatric illness characterised by three Received: 26 May 2021 distinct symptoms domains [1,2]. Positive symptoms describe manifestations of psychosis Accepted: 17 June 2021 such as delusions, whereas negative symptoms are defined as alterations in drive and Published: 22 June 2021 volition [3]. Cognitive deficits are a central feature of SCZ, and include deficits in working memory, attention, learning, and executive functioning [4]. Numerous studies have demon- Publisher’s Note: MDPI stays neutral strated an association between the severity of cognitive impairment and functional, social, with regard to jurisdictional claims in and occupational outcomes in SCZ. Thus, the development of therapeutics that address published maps and institutional affil- this symptom domain are desirable [5]. Unfortunately, current clinical antipsychotic drugs iations. (APDs) fail to address these cognitive symptoms [6,7]. Hypodopaminergic function in the dorsolateral prefrontal cortex (dlPFC), an area associated with cognitive control and executive functions including working memory and selective attention [8], is thought to be related to negative symptoms and cognitive Copyright: © 2021 by the authors. deficits [9,10]. Indeed, both dopamine receptor (DR) antagonists and dopamine (DA) Licensee MDPI, Basel, Switzerland. depletion in the dlPFC impair cognitive function [11–13]. There is mounting evidence This article is an open access article to suggest that D R agonists can reverse these deficits [14,15], although excessive D R distributed under the terms and 1 1 stimulation may impede cognitive function [16,17]. Many orthosteric D R agonists that conditions of the Creative Commons 1 Attribution (CC BY) license (https:// compete with DA for the orthosteric binding site display a lack of subtype selectivity creativecommons.org/licenses/by/ (relative to other DRs) as well as poor pharmacokinetic properties. The benzazepine class 4.0/). of D1R agonists have poor bioavailability [18] and has the propensity to lower seizure Molecules 2021, 26, 3799. https://doi.org/10.3390/molecules26133799 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x FOR PEER REVIEW 2 of 19 Molecules 2021, 26, 3799 2 of 19 poor bioavailability [18] and has the propensity to lower seizure thresholds [19]. Similarly, the D1R agonist dihydrexidine displays poor oral bioavailability and is rapidly metabo- lised in vivo [20]. Orthosteric D1R agonists have also been shown to increase incidence of thresholds [19]. Similarly, the D1R agonist dihydrexidine displays poor oral bioavailability drug-induced hypotension, potentially mediated by D1Rs expressed in the periphery [21], and is rapidly metabolised in vivo [20]. Orthosteric D1R agonists have also been shown to as well as a rapid acquisition of tolerance [22,23]. There is clearly an unmet need for the increase incidence of drug-induced hypotension, potentially mediated by D1Rs expressed indevelopment the periphery of [selective,21], as well bioavailable as a rapid acquisitionsmall molecule of tolerance D1R ligands [22,23 to]. further There is interrogate clearly an their potential utility to treat cognitive deficits associated with SCZ. unmet need for the development of selective, bioavailable small molecule D1R ligands to furtherThere interrogate has been their tremendous potential recent utility progress to treat cognitive in the development deficits associated of novel withD1R positive SCZ. allosteric modulators (PAMs) [24–29] in concert with X-ray crystal [30] and cryo-EM struc- There has been tremendous recent progress in the development of novel D1R positive allosterictures [31,32] modulators that have (PAMs) revealed [24 fascinating–29] in concert structural with X-ray insights crystal into [30 the] and therapeutic cryo-EM poten- struc- turestial of [31 the,32 ]D that1R. haveThis revealedhas culminated fascinating in structuralthe clinical insights evaluation into the of therapeutic the first D potential1R PAM ofLY3154207 the D1R. This(Mevidalen) has culminated [33] which in theis currently clinical evaluation in phase 2 offor the the first amelioration D1R PAM of LY3154207 cognition (Mevidalen)associated with [33] Lewy which body is currently dementias. in phase PAMs 2 forrepresent the amelioration an alternative of cognition approach associated to target- withing the Lewy D1R body and act dementias. to modulate PAMs the represent affinity and/or an alternative efficacy approach of DA from to targetinga topographically the D1R anddistinct act tobut modulate conformationally the affinity linked and/or binding efficacy site. of DAThe fromengagement a topographically of a less conserved distinct but al- conformationallylosteric binding site linked may binding confer site.greater The subtype engagement selectivity of a less than conserved orthosteric allosteric D1R agonists. binding siteA D may1R PAM confer that greater displays subtype positive selectivity allosteric than orthostericcooperativity D1R but agonists. lacks Aintrinsic D1R PAM efficacy that displayswhich, in positive its own allosteric right, might cooperativity maintain but the lacks temporal intrinsic and efficacy spatial which,patterns in of its DA own neuro- right, mighttransmission maintain [34,35]. the temporal and spatial patterns of DA neurotransmission [34,35]. LewisLewis etet al.al. recentlyrecently identifiedidentified twotwo racemicracemic DD11R PAM chemotypeschemotypes (Compound(Compound A,A, ((11)) andand CompoundCompound B,B, ((racrac--22),), FigureFigure1 1))[ [36].36]. 11 waswas shownshown toto bebe aa DD11RR PAMPAM butbut alsoalso actedacted asas anan agonistagonist atat thethe DD22RR andand thusthus waswas notnot investigatedinvestigated further.further. AsAs DD22RR agonismagonism isis knownknown toto exacerbateexacerbate the the positive positive symptoms symptoms of of SCZ, SCZ, achieving achieving selectivity selectivity for for the the D1 DR1 versusR versus the the D2 RD is2R paramountis paramount for for the the development development of efficacious of efficacious cognitive-enhancing cognitive-enhancing therapeutics. therapeutics.rac-2 racwas-2 shownwas shown to have to have superior superior potency potency compared compared to 1 whilst to 1 whilst being being selective selective for the for human the human D1R. ToD1 ourR. To knowledge our knowledge there is there no reported is no reported chemical ch synthesisemical synthesis and biological and biological characterisation characteri- of opticalsation of isomers optical of isomers2. Herein, of 2 we. Herein, report thewe synthesisreport the and synthesis pharmacological and pharmacological characterisation char- ofacterisation (rac)-2, and of enantioenriched(rac)-2, and enantioenriched optical isomers optical of 2 isomers(herein of denoted 2 (herein as (denotedS)-2 and as (R ()-S2)-).2 Theseand (R enantioenriched)-2). These enantioenriched samples were samples accessed were by employingaccessed by various employing chiral various auxiliaries chiral in anauxiliaries asymmetric in an Diels asymmetric Alder (ADA) Diels cycloadditionAlder (ADA) cycloaddition reaction. reaction. FigureFigure 1.1. High-throughputHigh-throughput screening screening hits