Cariprazine {RGH-188; trans-N-[4-[2-[4-(2,3-dichlorophenyl)piperazin-1-yl]ethyl]cyclohexyl]-N′,N′-dimethylurea hydrochloride}, a novel candidate antipsychotic, demonstrated approximately 10-fold higher affinity for human D₃ versus human D_2L and human D_2S receptors (pKi 10.07, 9.16, and 9.31, respectively). It displayed high affinity at human serotonin (5-HT) type 2B receptors (pK_i 9.24) with pure antagonism. Cariprazine had lower affinity at human and rat hippocampal 5-HT_1A receptors (pK_i 8.59 and 8.34, respectively) and demonstrated low intrinsic efficacy. Cariprazine displayed low affinity at human 5-HT_2A receptors (pK_i 7.73). Moderate or low affinity for histamine H₁ and 5-HT_2C receptors (pK_i 7.63 and 6.87, respectively) suggest cariprazine's reduced propensity for adverse events related to these receptors. Cariprazine demonstrated different functional profiles at dopamine receptors depending on the assay system. It displayed D₂ and D₃ antagonism in [³⁵S]GTPγS binding assays, but stimulated inositol phosphate (IP) production (pEC₅₀ 8.50, E_max 30%) and antagonized (±)-quinpirole-induced IP accumulation (pK_b 9.22) in murine cells expressing human D_2L receptors. It had partial agonist activity (pEC₅₀ 8.58, E_max 71%) by inhibiting cAMP accumulation in Chinese hamster ovary cells expressing human D₃ receptors and potently antagonized R(+)-2-dipropylamino-7-hydroxy-1,2,3,4-tetrahydronaphtalene HBr (7-OH-DPAT)-induced suppression of cAMP formation (pK_b 9.57). In these functional assays, cariprazine showed similar (D₂) or higher (D₃) antagonist–partial agonist affinity and greater (3- to 10-fold) D₃ versus D₂ selectivity compared with aripiprazole. In in vivo turnover and biosynthesis experiments, cariprazine demonstrated D₂-related partial agonist and antagonist properties, depending on actual dopaminergic tone. The antagonist–partial agonist properties of cariprazine at D₃ and D₂ receptors, with very high and preferential affinity to D₃ receptors, make it a candidate antipsychotic with a unique pharmacological profile among known antipsychotics.

Dopamine (DA), as one of the major neurotransmitters in the central nervous system (CNS) and periphery, exerts its actions through five types of receptors which belong to two major subfamilies such as D1-like (i.e., D1 and D5 receptors) and D2-like (i.e., D2, D3 and D4) receptors. Dopamine D3 receptor (D3R) was cloned 30 years ago, and its distribution in the CNS and in the periphery, molecular structure, cellular signaling mechanisms have been largely explored. Involvement of D3Rs has been recognized in several CNS functions such as movement control, cognition, learning, reward, emotional regulation and social behavior. D3Rs have become a promising target of drug research and great efforts have been made to obtain high affinity ligands (selective agonists, partial agonists and antagonists) in order to elucidate D3R functions. There has been a strong drive behind the efforts to find drug-like compounds with high affinity and selectivity and various functionality for D3Rs in the hope that they would have potential treatment options in CNS diseases such as schizophrenia, drug abuse, Parkinson’s disease, depression, and restless leg syndrome. In this review, we provide an overview and update of the major aspects of research related to D3Rs: distribution in the CNS and periphery, signaling and molecular properties, the status of ligands available for D3R research (agonists, antagonists and partial agonists), behavioral functions of D3Rs, the role in neural networks, and we provide a summary on how the D3R-related drug research has been translated to human therapy.

Introduction: Cariprazine, a dopamine D₃-preferring D₃/D₂ receptor partial agonist and serotonin 5-HT_1A receptor partial agonist, has two major human metabolites, desmethyl-cariprazine (DCAR) and didesmethyl-cariprazine (DDCAR). The metabolite pharmacology was profiled to understand the contribution to cariprazine efficacy. Methods: In vitro receptor binding and functional assays, electrophysiology, animal models, microdialysis, and kinetic-metabolism approaches were used to characterize the pharmacology of DCAR and DDCAR. Results: Similar to cariprazine, both metabolites showed high affinity for human D₃, D_2L, 5-HT_1A, 5-HT_2A, and 5-HT_2B receptors, albeit with higher selectivity than cariprazine for D₃ versus D₂ receptors. In [³⁵S]GTPγS binding assays, cariprazine and DDCAR were antagonists in membranes from rat striatum and from cells expressing human D₂ and D₃ receptors, and were partial agonists in membranes from rat hippocampus. In cAMP signaling assays, cariprazine, DCAR, and DDCAR acted as partial agonists at D₂ and D₃ receptors; cariprazine and DDCAR were full agonists, whereas DCAR was a partial agonist at 5-HT_1A receptors. Cariprazine, DCAR, and DDCAR were pure antagonists at human 5-HT_2B receptors. Cariprazine and DDCAR increased rat striatal dopamine and reduced cortical serotonin turnover. Cariprazine and DDCAR showed similar in vivo D₃ receptor occupancy in rat brain; however, cariprazine was more potent for D₂ receptor occupancy. Both cariprazine and DDCAR dose-dependently but partially suppressed the spontaneous activity of midbrain dopaminergic neurons in rats, with the parent compound being more potent but shorter acting than its metabolite. Consistent with the D₂ receptor occupancy profile, DDCAR was 3- to 10-fold less potent than cariprazine in rodent models of antipsychotic-like activity. Following acute cariprazine administration, DDCAR was detected in the rodent brain but at much lower levels than cariprazine. Conclusion: Overall, in vitro and in vivo pharmacological profiles of DCAR and DDCAR demonstrated high similarity with cariprazine, suggesting that the major metabolites of cariprazine contribute significantly to its clinical efficacy.

Dysfunctions of the dopaminergic system are believed to play a major role in the core symptoms of schizophrenia such as positive, negative, and cognitive symptoms. The first line of treatment of schizophrenia are antipsychotics, a class of medications that targets several neurotransmitter receptors in the brain, including dopaminergic, serotonergic, adrenergic and/or muscarinic receptors, depending on the given agent. Although the currently used antipsychotics display in vitro activity at several receptors, majority of them share the common property of having high/moderate in vitro affinity for dopamine D₂ receptors (D₂Rs) and D₃ receptors (D₃Rs). In terms of mode of action, these antipsychotics are either antagonist or partial agonist at the above-mentioned receptors. Although D₂Rs and D₃Rs possess high degree of homology in their molecular structure, have common signaling pathways and similar in vitro pharmacology, they have different in vivo pharmacology and therefore behavioral roles. The aim of this review, with summarizing preclinical and clinical evidence is to demonstrate that while currently used antipsychotics display substantial in vitro affinity for both D₃Rs and D₂Rs, only very few can significantly occupy D₃Rs in vivo. The relative importance of the level of endogenous extracellular dopamine in the brain and the degree of in vitro D₃Rs receptor affinity and selectivity as determinant factors for in vivo D₃Rs occupancy by antipsychotics, are also discussed.