Optimization of a Novel D2 Dopamine Receptor-Selective Antagonist into Lead Candidates for the Treatment of Neuropsychiatric Disorders

Abstract

<b>Abstract ID 19484</b> <b>Poster Board 566</b> Schizophrenia is a devastating neuropsychiatric illness impacting approximately 1% of the global population and is the 15^th leading cause of disability worldwide. Schizophrenia is characterized by positive (hallucinations, delusions, paranoia), negative (flat affect, decreased motivation) and cognitive symptoms. Current therapies treat mostly the positive symptoms and are associated with a plethora of off-target side effects such as sedation, weight gain, and diabetes, among others. All FDA-approved antipsychotic medications target the D2 dopamine receptor (D2R) but also exhibit poly-pharmacology with other receptors. Further, there are few D2R antagonists that can selectively inhibit the D2R without also antagonizing the D3 and D4 dopamine receptors (D3R, D4R, respectively). We recently identified a D2R-selective antagonist scaffold, MLS6916, from a high throughput screen of the D2R. When counter-screened against 168 GPCRs using β-arrestin recruitment as a functional readout, 10 uM MLS6916 only inhibited the D2R, and to a lesser extent, the D4R. Further, using radioligand binding competition assays, MLS6916 was &gt;200-fold D2R&gt;D3R selective and 12-fold D2R&gt;D4R selective. Despite its promising D2R selectivity, MLS6916 was found to exhibit poor metabolic stability when assayed using rat liver microsomes. Interestingly, we found high species variability with respect to metabolic stability using rat, mouse, and human liver microsomes. While many analogs exhibited poor metabolic stability in rat liver microsomes, we observed equal or worse stability in mouse liver microsomes, but dramatically higher stability in human liver microsomes. High metabolic stability in humans is essential for moving a compound into the clinic, but preclinical studies will require at least moderate metabolic stability in rodents to employ animal models that are predictive of antipsychotic efficacy and/or adverse side effects. Thus, to chemically optimize this scaffold and explore its structure-activity relationships, greater than 100 analogs were synthesized to identify modifications that might result in improved metabolic stability. All analogs were also evaluated for D2R, D3R, and D4R activities using radioligand binding competition and β-arrestin recruitment assays. Lead compounds were identified that possessed D2R Ki values of &lt;100 nM, were highly selective versus the D3R and D4R, and exhibited improved metabolic stability in mouse and/or human liver microsomes. We further determined the pharmacokinetic profiles of the most promising compounds in mice since this species has good models for predicting antipsychotic efficacy. After injecting 30 mg/kg i.p., we found that the compounds exhibited t_½ values of 5-6 hr in both plasma and brain. Importantly, the compounds exhibited 1:1 brain-plasma ratios with Cmax values &gt;10 uM indicating excellent brain penetration. In summary, we have identified lead candidate compounds that have exceptional D2R-selectivity, excellent metabolic stabilities in human liver microsomes, and sufficient metabolic stability in mice to conduct behavioral studies. Future studies will investigate preclinical antipsychotic efficacy in mouse models as well as the potential for on-target side effects such as catalepsy. These advanced leads may have the potential to treat neuropsychiatric disorders with greatly reduced off-target side effects.