Additionally, aripiprazole was found to be associated with a lesser increase in dopamine turnover than full antagonist antipsychotics (Figure27) and decreased DAT binding density in NAc and the ventral tegmental area (VTA). The distinctive pharmacologic profile and biased agonism of this drug could be attributed to its ability to activate presynaptic D2 autoreceptors, which, as previously mentioned, regulate dopamine release via negative feedback mechanism.5,25 Cariprazine, another D2R partial agonist, has similar doubling of dopamine turnover.28
Activation of presynaptic D2S receptors ultimately leads to decreased dopamine synthesis and release, which combats or prevents the brain adaptations regarding dopamine supersensitivity and D2Rs upregulation. While TD can still occur occasionally with aripiprazole or other partial agonists,29,30 animal studies show that administration of methamphetamine significantly lowers locomotor response and the density of striatal D2Rs in a group treated with aripiprazole compared to a group treated with haloperidol.31 Aripiprazole also improved the supersensitivity parameters induced by chronic treatment with haloperidol, which suggests that it is associated with reduced dopamine supersensitivity.31 Similarly, in human studies, partial agonists appear to have a lower rate of parkinsonism and TD.32,33 One study reported that aripiprazole was associated with a significant improvement of TD in more than 50% of patients after 24 weeks of treatment.34
Lumateperone’s unique pharmacologic profile
Lumateperone is a newer antipsychotic that was FDA-approved in December 2019 for the treatment of adults with schizophrenia35 and more recently for the treatment of bipolar depression.36 It possesses a unique combination of pharmacologic properties; it is a postsynaptic D2R antagonist and a presynaptic D2R partial agonist.27
Interestingly, lumateperone has regional selectivity. It increases dopamine release in the medial prefrontal cortex (where D2R is rare) but not in the nigrostriatal pathways.27,37 It does not increase TH phosphorylation (which would increase dopamine concentration) or dopamine turnover in the striatum (Figure27). In a preclinical functional activity assay of lumateperone, the lack of change of dopamine turnover with lumateperone resembles placebo and is even less than that observed with aripiprazole (Figure27). This effect is consistent with partial agonism at the presynaptic D2S, where the stimulation of that receptor prevents the concomitant increase in dopamine synthesis and release that occurs when that receptor is blocked.
It is believed that the lack of increase in dopamine turnover is one of the reasons that lumateperone postsynaptic D2R occupancy is exceptionally low at clinically effective doses. In a positron emission tomography study analyzing posttreatment scans after approximately 2 weeks of a 60 mg/d dose, the mean peak striatal D2R occupancy was approximately 40%,38 which is remarkably lower than the 65% to 75% blockade needed for purely antagonist D2R antipsychotics.3 This low receptor occupancy appears to mediate the low incidence of parkinsonism and prolactin release seen with lumateperone.
Continue to: Take-home points