|Systematic (IUPAC) name|
|Metabolism||Hepatic via CYP3A4 & to a lesser extent, CYP2D6|
|Half-life||2-5 days (2-3 wks for active metabolite, desmethylcariprazine)|
|Mol. mass||427.411 g/mol|
Cariprazine (RGH-188) is an antipsychotic drug under development by Gedeon Richter. It acts as a D2 and D3 receptor partial agonist, with high selectivity towards the D3 receptor. Positive Phase III study results were published for schizophrenia and mania early 2012, while Phase II studies in bipolar disorder I, and for bipolar depression are in progress. Action on the dopaminergic systems makes it also potentially useful as an add-on therapy in major depressive disorder 
Forest Laboratories obtained a license on development (from the Richter - Hungary) and exclusive commercial rights in the US in 2004.
The most prevalent side effects for cariprazine include akathisia, insomnia, and weight gain. In Phase III trial for schizophrenia, 66.7 % of patients experienced these side effects, as well as other issues such as metabolic problems, extrapyramidal side effects (EPSE), and sedation. This suggests that cariprazine might not make it past Phase III testing for schizophrenia because its tolerability is lower than those of its competitors. Other medications cause many motor defects and EPSE, which are major problems for patients who did not have motor issues before taking the medicine. These motor issues are due to the fact that dopamine acts on the motor pathway as well as the reward pathway, and many antipsychotics stimulate dopamine pathway, causing excessive motor movement as well as the intended effects.
These side effects are considerably lower in the Phase III trials for bipolar disorder I (for unknown reasons).
Cariprazine acts as an antipsychotic that is effective against the positive and negative symptoms of schizophrenia. Unlike many antipsychotics that are D2 and 5-HT2A receptor antagonists, cariprazine is a D2 and D3 partial agonist. It also has a higher affinity for D3 receptors. The D2 and D3 receptors are important targets for the treatment of schizophrenia, because the overstimulation of dopamine receptors has been implicated as a possible cause of schizophrenia. Cariprazine acts to inhibit overstimulated dopamine receptors (acting as an antagonist) and stimulate the same receptors when the endogenous dopamine levels are low. Cariprazine’s high selectivity towards D3 receptors could prove to reduce side effects associated with the other antipsychotic drugs, because D3 receptors are mainly located in the ventral striatum and would not incur the same motor side effects (extrapyramidal symptoms) as drugs that act on dorsal striatum dopamine receptors. Cariprazine also acts on 5-HT1A receptors, though the affinity is considerably lower than the affinity to dopamine receptors (seen in monkey and rat brain studies). In the same studies, cariprazine has been noted to produce pro-cognitive effects, the mechanisms of which are currently under investigation. An example of pro-cognitive effects occurred in pre-clinical trials with rats: rats with cariprazine performed better in a scopolamine-induced learning impairment paradigm in a water labyrinth test. This may be due to the selective antagonist nature of D3 receptors, though further studies need to be conducted. This result could be very useful for schizophrenia, as one of the symptoms includes cognitive deficits.
Cariprazine has partial agonist as well as antagonist properties depending on the endogenous dopamine levels. When endogenous dopamine levels are high (as is hypothesized in schizophrenic patients), cariprazine acts as an antagonist by blocking dopamine receptors. When endogenous dopamine levels are low, cariprazine acts more as an agonist, increasing dopamine receptor activity. In monkey studies, the administration of increasing does of cariprazine resulted in a dose-dependent and saturable reduction of specific binding. At the highest dose (300 μg/kg), the D2/D3 receptors were 94 % occupied, while at the lowest dose (1 μg/kg), receptors were 5 % occupied.
|Receptor||Ki (nM)||Pharmacodynamic action|
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