Discovery, Characterization, and Antiparkinsonian Effect of Novel Positive Allosteric Modulators of Metabotropic Glutamate Receptor 4

Abstract

Parkinson's disease (PD) is caused by the death of dopamine neurons in the basal ganglia and results in motor symptoms such as tremor and bradykinesia. Activation of metabotropic glutamate receptor 4 (mGluR4) has been shown to modulate neurotransmission in the basal ganglia and results in antiparkinsonian effects in rodent PD models. N-Phenyl-7-(hydroxyimino)cyclopropa [b]chromen-1a-carboxamide (PHCCC) is a positive allosteric modulator (PAM) of mGluR4 that has been used to further validate the role of mGluR4 in PD, but the compound suffers from a lack of selectivity, relatively low potency, and poor solubility. Via high-throughput screening, we discovered more than 400 novel PAMs of mGluR4. Compounds derived from a novel chemical scaffold were characterized in vitro at both rat and human mGluR4 using two distinct assays of mGluR4 function. The lead compound was approximately 8-fold more potent than PHCCC, enhanced the potency of glutamate at mGluR4 by 8-fold, and did not show any significant potentiator or antagonist activity at other mGluR subtypes. Resolution of the regioisomers of the lead revealed that the cis regioisomer, (Ϯ)-cis-2-(3,5-dichlorphenylcarbamoyl)cyclohexanecarboxylic acid (VU0155041), contained the majority of the mGluR4 PAM activity and also exhibited partial agonist activity at mGluR4 at a site that was distinct from the glutamate binding site, suggesting that this compound is a mixed allosteric agonist/PAM of mGluR4. VU0155041 was soluble in an aqueous vehicle, and intracerebroventricular administration of 31 to 316 nmol of VU0155041 dose-dependently decreased haloperidol-induced catalepsy and reserpine-induced akinesia in rats. These exciting results provide continued support for mGluR4 as a therapeutic target in PD.Metabotropic glutamate receptors (mGluRs) play important roles in a broad range of central nervous system functions and have therapeutic potential in a variety of neurological and psychiatric disorders (Niswender et al., 2005). mGluRs are G protein-coupled receptors (GPCRs) classified into three major groups, groups I, II, and III, based on their sequence homology, signal transduction profile, and ligand binding specificity. The group III mGluRs (mGluRs 4, 6, 7,

Recent Progress on the Identification of Metabotropic Glutamate 4 Receptor Ligands and Their Potential Utility as CNS Therapeutics

“…Progress has been made on these varying degrees of efficacy; however, the optimal combination is not yet apparent from the current state of the art (vide supra). Indeed, the proper mix of EC 50 , E max , and FS may have vastly different requirements for different disease states, and a significant volume of data and progress are needed in the field before this important issue can be resolved.…”

“…50 Separation of the isomers showed that the cis-regioisomer 15 (VU0155041) was solely responsible for the potency. Subsequent chiral chromatographic separation of the cis-isomers apparently showed no enantiomeric preference for the receptor.…”

“…In vivo efficacy studies (via icv administration) in two rat models of PD (reversal of haloperidol-induced catalepsy and reserpineinduced akinesia) demonstrated its antiparkinsonian potential, further supporting the therapeutic potential of mGlu4 PAMs. 50 Subsequent work by the Vanderbilt group identified additional leads of varying chemical diversity (16,17) (Scheme 3) and potency at mGlu4. 53,54 The noteworthy feature of both of these ligands 16 VU0080241 and 17 VU0001171 was their rather large increase in fold shift (16, EC 50 4.6 μM, FS 28; 17, EC 50 0.65 μM, FS 36), a feature not previously reported.…”

“…59 Utilizing the SAR from the amide series, the furyl group was exchanged for the optimized 2-pyridyl moiety, providing 23 (VU0361591), with a concomitant improvement in activity of more than 30 fold (EC 50 159 nM). A focused SAR optimization of the remaining aryl groups afforded the 2-chlorophenyl derivative 24 (VU0364439), the most potent mGlu4 PAM reported at that time (EC 50 20 nM). An examination of the pharmacokinetic parameters of this series unfortunately showed them to be unstable in a microsomal incubation assay, limiting their use as in vivo tool molecules.…”

“…65 The identification of 34 (VU0105737), shown in Scheme 8, a moderately potent derivative (EC 50 7.5 μM), containing a homopiperazinyl group, served as the starting point for the ensuing SAR investigation. Unfortunately, after preparation of a variety of derivatives examining the various regions, the SAR again showed limited scope and depth.…”