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.[32] 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.[33] 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).[33][34] 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.[33] 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.[24] In monkey studies, the administration of increasing doses 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.[34] Dopamine D2 and D3 receptor occupancy in humans has been summarized as, "In healthy volunteers, single-dose cariprazine of 0.5 mg occupied up to 12% of striatal D2/D3 receptors, while striatal D2/D3 occupancy after multiple dosing up to cariprazine 1.0 mg/d ranged from 63 to 79% [39]. In an open-label, fixed-dose, 2-week trial in eight males with schizophrenia, PET scans of dorsal striatal regions (caudate nucleus and putamen) and ventral striatum (nucleus accumbens) showed maximum occupancy (‡ 90%) at a 3-mg target dose of cariprazine following 14 d of treatment [40,41]. After 14 d of cariprazine 1.5 mg/d, receptor occupancy was 69% in the caudate nucleus, 69% in the nucleus accumbens, and 75% in the putamen".[24]
Cariprazine, as well as other third generation antipsychotics, possesses a lower chance of exacerbating extrapyramidal symptoms. However the ability to induce akathasia remains relatively high. This may be mediated through a lack of anticholinergic effects (as agents of this class are sometimes used to treat akathisia), as well as a lack of a balanced dopaminergic(D2)/serotonergic(5-HT2A) ratio .[35][36][37] 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.[38][39]
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.[40][41][42]
