NOVEL NON-CATECHOL D1 RECEPTOR AGONISTS EXHIBIT SUSTAINED PHARMACOLOGICAL ACTIVITY

Abstract

Increasing evidence associates dysfunctional dopamine signaling with the motor and cognitive deficits in Alzheimer's disease. While D1 receptor (D1R) specific pathologies are reported, current D1R compounds have limited therapeutic potential due to their poor CNS penetration, negligible oral bioavailability, rapid metabolism and propensity to cause receptor desensitization. We describe here a novel class of D1R agonists that show reduced receptor desensitization in vitro as well as reduced tachyphylaxis in vivo. For in vitro D1 receptor desensitization experiments cAMP activity was measured from rat striatal neurons using the Cisbio HTRF cAMP dynamic range assay kit. For in vivo testing of tachyphylaxis, Sprague-Dawley rats with unilateral 6-OHDA lesions (n=17) were individually monitored for rotational behavior using infrared cameras and dosed using a crossover design (2-week washout period) in their home cages. In each treatment period animals received six doses of either a catechol (A-77636, 0.32 mg/kg S.C.) or non-catechol (PF-2334, 10.78 mg/kg, P.O.) D1R agonist administered every 12 h followed by a single dose of the D2 agonist quinpirole (0.1 mg/kg S.C.). In vitro, pre-treatment with catecholamine agonists (dopamine, A-77636, dihydrexidine, or SKF-38393 all at 10 mM) blunted the cAMP increase induced by subsequent D1R agonist challenge by 40–60% (1 mM SKF-81297). In contrast, pretreatment with the non-catechol agonist PF-2334 did not diminish the subsequent elevation in cAMP by D1R agonist challenge. In vivo, after 72 hours of chronic treatment with a catechol agonist (A-77636), animals displayed near-zero contralateral rotational behavior. In contrast, non-catechol agonist (PF-2334) treated animals were still rotating at the 72-hour time point. The absence of rotational behavior under A-77636 was likely due to D1R tachyphylaxis since treatment with a D2 agonist caused a significant increase in the number of contralateral rotations. The persistent in vitro and in vivo pharmacodynamic response of PF-2334 supports limited receptor desensitization by non-catechol agonists and suggests durable D1R agonism by PF-2334. These data highlight the therapeutic potential of this novel class of compounds and warrants further investigation in relevant Alzheimer's disease animal models.