Pharmacodynamics 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. Moreover, partial agonists, through their limited response triggering, ironically often have the tendency to occupy near all targeted receptors at relatively low dosages of the drug. An extreme example is aripiprazole with an average occupancy of 70% (D2) at a 2 mg dose, well below its usual antipsychotic dosage (the often cited threshold of occupancy for an antipsychotic effect is 70%). This could be another reason for akathasia from partial agonists.
Partial agonists are drugs that bind to and activate specific receptors, but they produce a weaker response than full
agonists, even when all receptors are occupied. In neuronal signaling pharmacology, their activity is often described using two models: one considers how much of the drug binds to postsynaptic receptors, which can block stronger agonists from activating the receptor, while the other looks at how the drug can activate receptors by itself, but only to a limited extent compared to a full agonist. Partial agonists can act as both weak activators and blockers, depending on the presence of natural neurotransmitters like
dopamine. When natural dopamine levels are high, partial agonists compete for the same receptors, reducing excessive signaling. When dopamine levels are low, partial agonists provide some activation, but not as much as dopamine or a full agonist would. The effectiveness of a partial agonist is often measured by the
EC50 value, which is the concentration needed to produce half of its maximum possible effect. Drugs like
aripiprazole, cariprazine, and
brexpiprazole are examples of partial agonists used as antipsychotics. They help stabilize mood and reduce psychotic symptoms by balancing dopamine activity in the brain.
Pharmacokinetics Cariprazine has high oral
bioavailability and can cross the
blood brain barrier easily in humans because it is lipophilic. In rats, the oral bioavailability was 52% (with a dose of 1 mg/kg). Cariprazine is metabolized primarily by the
cytochrome P450 3A4 isoenzyme (CYP3A4), with some minor metabolism by
CYP2D6. Cariprazine does not induce the production of CYP3A4 or CYP1A2 in the liver, and weakly, competitively inhibits CYP2D6 and CYP3A4. ==Research==