Analysis of the actions of the novel dopamine receptor-directed compounds (S)-OSU6162 and ACR16 at the D2 dopamine receptor

Abstract

BACKGROUND AND PURPOSE; The two phenylpiperidines, OSU6162 and ACR16, have been proposed as novel drugs for the treatment of brain disorders, including schizophrenia and Huntington's disease, because of their putative dopamine stabilizing effects. Here we evaluated the activities of these compounds in a range of assays for the D(2) dopamine receptor in vitro.

Experimental approach: The affinities of these compounds for the D(2) dopamine receptor were evaluated in competition with [(3) H]spiperone and [(3) H]NPA. Agonist activity of these compounds was evaluated in terms of their ability to stimulate [(35) S]GTPγS binding.

Key results: Both compounds had low affinities for inhibition of [(3) H]spiperone binding (pK(i) vs. [(3) H]spiperone, ACR16: <5, OSU6162: 5.36). Neither compound was able to stimulate [(35) S]GTPγS binding when assayed in the presence of Na(+) ions, but if the Na(+) ions were removed, both compounds were low-affinity, partial agonists (E(max) relative to dopamine: ACR16: 10.2%, OSU6162:54.3%). Schild analysis of the effects of OSU6162 to inhibit dopamine-stimulated [(35) S]GTPγS binding indicated Schild slopes of ∼0.9, suggesting little deviation from competitive inhibition. OSU6162 was, however, able to accelerate [(3) H]NPA dissociation from D(2) dopamine receptors, indicating some allosteric effects of this compound.

Conclusions and implications: The two phenylpiperidines were low-affinity, low-efficacy partial agonists at the D(2) dopamine receptor in vitro, possibly exhibiting some allosteric effects. Comparing their in vitro and in vivo effects, the in vitro affinities were a reasonable guide to potencies in vivo. However, the lack of in vitro-in vivo correlation for agonist efficacy needs to be further addressed.

Linked articles: This article is part of a themed section on Analytical Receptor Pharmacology in Drug Discovery. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2010.161.issue-6.

Figure 1

Structures of the phenylpiperidines ACR16 and OSU6162.

Figure 2

Binding of ACR16 and OSU6162 to membranes of CHO cells expressing D₂ dopamine receptors. Binding of the two compounds was determined in competition with [³H]spiperone (A) or [³H]NPA (B).

Figure 3

Stimulation of [³⁵S]GTPγS binding by dopamine, ACR16 and OSU6162. Stimulation of [³⁵S]GTPγS binding to membranes of CHO cells expressing D₂ dopamine receptors was determined in the presence of Na⁺ ions (A) or NMDG (B).

Figure 4

Schild analysis of the effect of OSU6162 to inhibit dopamine stimulation of [³⁵S]GTPγS binding. Dopamine-stimulation of [³⁵S]GTPγS binding to membranes of CHO cells expressing D₂ dopamine receptors was determined in the presence of different concentrations of OSU6162, in the presence of Na⁺ ions (A,C) or NMDG (B,D). Panels A and B show the stimulation curves and panels C and D show the derived Schild plots.

Figure 5

Dissociation of [³H]NPA from D₂ dopamine receptors expressed in membranes of CHO cells. [³H]NPA dissociation was determined as described. Dissociation curves were best described by one-phase (butaclamol) or two-phase models (dopamine and OSU6162).