Aripiprazole is a full agonist at hD2 (short) receptors as assessed using the discoverx hithunter™ camp assay

K. L. Clarke, J. Watson, M. D. Wood and C. Scott. Psychiatry CEDD, GlaxoSmithKline Pharmaceuticals, Harlow, Essex, CM19 5AW.

Aripiprazole is reported to act as a low efficacy partial agonist at the dopamine D2 receptor (Kichuchi et al., 1995; Oshiro et al., 1998). However, the absolute degree of efficacy observed with aripiprazole varies (Shapiro et al, 2003) depending on the assay format. In this study, we have investigated the activity of aripiprazole at human (h) cloned D2 (short) receptors stably expressed in chinese hamster ovary (CHO) cells using the DiscoveRx HitHunter™ cAMP assay (Golla et al., 2002).

Cells were pre-incubated in increasing concentrations of forskolin (100 nM – 10 µM) for 15 mins at 37 °C prior to incubation with test compounds in Tyrodes buffer (145 mM NaCl2, 25 mM KCl, 1.2 mM MgCl2, 10 mM HEPES, 10 mM Glucose, pH 7.4) for 30 min at 37°C and the assay terminated by lysing the cells. cAMP levels were determined according to the manufacturer’s instructions. Data were expressed as % max response (i.e. inhibition of forskolin stimulated cAMP levels.)

The D2 receptor full agonist (-)-quinpirole produced a maximal inhibition of forskolin stimulated cAMP production of 28% whereas aripiprazole produced a maximal inhibition of 22%, generating an intrinsic activity in the range of 0.8-0.9 (see Table 1). Decreasing the forskolin concentration from 10 µM to 3 µM increased the maximal agonist stimulated response for both quinpirole and aripiprazole (see Table 1), without significantly altering the intrinsic activity of aripiprazole.

Table 1.

Data shown are mean ± s.e.m. of 3 independent experiments.

* Intrinsic Activity

In conclusion, these data suggest that the DiscoveRx HitHunter™ cAMP assay is a useful, novel technique for evaluating agonist activity at human (h) D2 (short) cloned receptors stably expressed in CHO cells. In this assay system aripiprazole appears to behave as a full agonist. This supports the suggestion that aripiprazole is a “functionally selective” dopamine D2 receptor agonist (Shapiro et al., 2003).

Golla, R., Seethala, R. (2002). J. Biomolecular Screening.,7 (6), 515-525.
Kichuchi, T., Morita, S. et al. (1995). JPET., 274 , 329-336.
Oshiro, Y., Nishi, T. (1998). J. Med. Chem., 41 , 658-667.
Shapiro, D.A., Renock, S. et al. (2003) Neuropsychopharmacol. 28, 1400-1411.