Chemical Optimization of Advanced mGlu4 Lead Candidates
The primary pathophysiological change giving rise to the symptoms of Parkinson's disease (PD) is a loss of the dopaminergic neurons in the substantia nigra pars compacta (SNc) that are involved in modulating the function of basal ganglia (BG) nuclei. Unfortunately, traditional therapies for treatment of PD based on dopamine replacement strategies eventually fail in most patients and are more ..
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The primary pathophysiological change giving rise to the symptoms of Parkinson's disease (PD) is a loss of the dopaminergic neurons in the substantia nigra pars compacta (SNc) that are involved in modulating the function of basal ganglia (BG) nuclei. Unfortunately, traditional therapies for treatment of PD based on dopamine replacement strategies eventually fail in most patients and are associated with numerous side effects. A great deal of effort has been focused on developing a detailed understanding of the circuitry and function of the BG to develop novel, nondopaminergic, approaches for restoring normal BG function in PD patients. Exciting advances suggest that metabotropic glutamate receptors (mGlus), including the group III mGlus (mGlu4, -7 and -8), play important roles in regulating transmission through the BG and could serve as targets for novel PD therapeutics (3). For instance, mGlu4 activation reduces overactive GABA release at a specific inhibitory BG synapse (12,19,9) and reverses motor deficits in a variety of rodent PD models (12,19,10,8,16).
To more selectively activate mGlu4 and improve upon the pharmacokinetic liabilities of glutamate analogs, we and others have developed novel positive allosteric modulators (PAMs) which potentiate glutamate function at mGlu4 (11,12,14,15,4,21,7,6); several of these tool compounds exhibit antiparkinsonian and neuroprotective effects in multiple rodent PD models (12,1,14,7,6). Unfortunately, many available compounds have lacked the necessary pharmacokinetic properties required for study of mGlu4 function via systemic routes of administration. Future compounds developed should exhibit sufficient potency, efficacy, and pharmacokinetic properties, including brain penetration, to make useful probes to progress mGlu4 biology, which will undoubtedly allow the intense study of mGlu4 activation in multiple areas of neuroscience such as psychiatric disorders (18,17), cancer (5), and addiction (2).
Potency and efficacy of compounds will be determined by performing concentration-response curves (CRCs, 10 points, ranging from approximately 30 microM-1 nM at 0.3% final DMSO concentration) at human mGlu4 using a calcium assay in which the cells express the chimeric G protein Gqi5 to couple mGlu4 to calcium mobilization (13,14,7,6). PAMs with EC50 values less than 500 nM versus human mGlu4 will next be evaluated for potency versus rat mGlu4 in a thallium flux assay measuring coupling of rat mGlu4 to G Protein-coupled Inwardly Rectifying Potassium (GIRK) Channels (13,6). Following potency evaluation at human and rat mGlu4, PAMs with EC50 values less than 500 nM at rat mGlu4 will be evaluated for their ability to left-shift the CRC of glutamate for rat mGlu4 in a thallium flux assay (13,14,7,6). PAMs with a fold-shift of the glutamate CRC of > 5 will then be evaluated for Tier 1 DMPK assays including plasma protein binding (PPB), intrinsic clearance, and inhibition of cytochrome p450 enzymes (CYP) (4,6). Next, compounds demonstrating PPB > 1% free and moderate clearance will be evaluated for CNS exposure and Plasma:Brain levels using an in vivo snapshot PK paradigm and will be examined for their selectivity for mGlu4 relative to other mGlu subtypes evaluated (4,6). Novel mGlu4 PAMs showing a CNS exposure > PAM EC50, a Brain:Plasma ratio of >0.5, and at least a 50-fold or greater selectivity versus other mGlu subtypes will next be evaluated in a preclinical model of PD: haloperidol-induced catalepsy (HIC) (20). mGlu4 PAMs demonstrating activity in HIC at a dose of < 30 mg/kg will have their ancillary pharmacology fully evaluated and those compounds that show no significant off-target activity, CaCo permeability > 10 cm/s-6, and suitable solubility will be declared an MLPCN probe. The ultimate goal of this project from the PI's perspective is to generate compounds which should exhibit sufficient potency, efficacy, and pharmacokinetic properties, including brain penetration, to make useful probes to progress mGlu4 biology.
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