In vitro functional characteristics of dopamine D2 receptor partial agonists in second and third messenger-based assays of cloned human dopamine D2Long receptor signalling

Aripiprazole, (+)terguride, OPC-4392 and (-)3-PPP have been classified as dopamine D-2 receptor partial agonists based largely on their activity in second messenger-based assays of dopamine D-2 receptor signalling. Nevertheless, signal transduction amplification might result in these compounds behaving as dopamine D-2 receptor full agonists at a more downstream level of signalling. We compared the intrinsic activity (E-max' expressed as a percentage of the maximal effect of dopamine) of aripiprazole, (+)terguride, OPC-4392 and (-)3-PPP using second (calcium (Ca2+) mobilization) and third (extracellular signal-regulated kinase 2 (ERK2) phosphoprotein expression) messenger readouts of cloned human dopamine D-2Long (hD(2L)) receptor signalling in CHO cells. These compounds were all less potent and displayed lower intrinsic activity in the Ca2+ assay (aripiprazole = 24.3%, (+)terguride 56.9%, OPC-4392 = 58.6% and (-)3-PPP = 75.1%), and aripiprazole (E-max = 54.5%) displayed a substantially lower intrinsic activity than (+)terguride (E-max = 92.3%), OPC-4392 (E-max = 93.1%) and (-)3-PPP (E-max = 101.1%) in the more downstream-based ERK2 phosphoprotein expression assay. These drug effects on Ca2+ mobilization and ERK2 phosphoprotein expression were mediated through dopamine hD(2L) receptors, as they all were blocked by (-)raclopride, whereas (-)raclopride and other dopamine D-2 receptor antagonists (haloperidol, risperidone, ziprasidone, olanzapine, clozapine and quetiapine) were inactive on their own in both assays. These data are consistent with clinical evidence that only dopamine D-2 receptor partial agonists with a sufficiently tow enough intrinsic activity will prove effective against the positive symptoms of schizophrenia, and also highlight the importance of using downstream-based assays in the discovery of novel D-2 receptor partial agonist therapeutics.