L-Glutamate is a major excitatory neurotransmitter in the central nervous system (CNS), which acts through the ligand-gated ionotropic glutamate receptor or through the G-protein coupled receptors (GPCR) called metabotropic glutamate receptors (mGluR). The mGluR5 belongs to the Group I subclass and is coupled to the phosphoinositide/Ca+2 pathway, which mainly mediates the excitatory effects of glutamate. Recently, investigation into the role of mGluR5 in drug abuse has led to speculation that this may be a new target for medication development. For example, studies using either an mGluR5 antagonist or mGluR5 knockout mice showed reduced locomotor stimulant effects induced by cocaine. Moreover, evidence that mGluR5 is involved in the rewarding effects of morphine, nicotine and ethanol has also been reported. Thus development of selective mGluR5 antagonists may provide a novel non-dopaminergic strategy toward the discovery of drug abuse medications. Additionally the mGluR5 has recently been implicated in anxiety and depression thus these antagonists might provide new therapeutic agents toward the treatment of these CNS disorders. In order to further explore structure-activity relationships (SAR) at mGluR5, the design and synthesis of a series of diaryl amides was initiated based on a putative ligand binding site at the transmembrane domain region of an mGluR5 molecular model, based on the bovine rhodopsin crystal structure. In vitro binding and functional evaluation at mGluR5 resulted in the discovery of several novel and moderately high affinity mGluR5 antagonists. Additional modifications of these amide-linked molecules focused on inducing an intramolecular hydrogen bond that might provide co-planarity achieved in the parent compound MPEP. Within this new series of compounds, several analogues showed moderate affinity for mGluR5, but a better design in which co-planarity could be achieved through a quinoline group was pursued. In this first group of quinoline analogues, clues to improve binding affinity at mGluR5 were revealed. In addition, synthesis of additional alkyne and amide analogues have been achieved to further explore SAR of the pendant aryl ring and then to combine these features with the quinoline structure. A library of >250 compounds have now been evaluated for in vitro binding and function at mGluR5. We have discovered that although the alkyne series has provided important guidance in our drug design of the amide and quinoline series, there are significant differences in which substitution is well-tolerated in the alkyne series but not in the other two templates. Hence we have focused our most recent efforts on exploring these differences and optimizing the alkyne series of compounds for further development. We have developed a customized mGluR5 binding assay using 3HMPEP in rat brain and use an ELISA-based assay to determine IP3 production induced by mGluR5 activation, as our functional screen. Full concentration response curves are generated in both in vitro assays to identify potent mGluR5 antagonists and we have also recently discovered compounds with different intrinsic activities. In addition, as anxiolysis is a hallmark behavior associated with mGluR5 antagonism, we have evaluated several of our most potent and novel compounds in mouse models of anxiety, including a novel open field, light-dark box, and marble burying. Several of these compounds have been compared to the classic mGluR5 antagonists, MPEP and MTEP and most recently, fenobam, a clinically used anxiolytic agent that is a moderately potent mGluR5 antagonist. These simple mouse models are also used to determine bioavailability and in vivo activity. One analogue, MFZ 10-7, has been selected, based on its high mGluR5 affinity (Ki<1 nM), in vitro potency and in vivo profile in the anxiety models, as a novel mGluR5 antagonist for evaluation in rat models of addiction and relapse. These studies include cocaine self-administration, incubation of craving, and reinstatement of cocaine-seeking behavior (cue and cocaine-induced.) MFZ 10-7 is being compared in these models to MTEP and fenobam. These studies are directed toward further validating the mGluR5 as a medication discovery target and providing preclinical data to support clinical trials for fenobam in human cocaine abusers.

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National Institute on Drug Abuse
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