István Laszlovszky

Cariprazine (RGH-188), a Dopamine D3 Receptor-Preferring, D3/D2 Dopamine Receptor Antagonist–Partial Agonist Antipsychotic Candidate: In Vitro and Neurochemical Profile

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 D2L and human D2S receptors (pKi 10.07, 9.16, and 9.31, respectively). It displayed high affinity at human serotonin (5-HT) type 2B receptors (pKi 9.24) with pure antagonism. Cariprazine had lower affinity at human and rat hippocampal 5-HT1A receptors (pKi 8.59 and 8.34, respectively) and demonstrated low intrinsic efficacy. Cariprazine displayed low affinity at human 5-HT2A receptors (pKi 7.73). Moderate or low affinity for histamine H1 and 5-HT2C receptors (pKi 7.63 and 6.87, respectively) suggest cariprazines 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, Emax 30%) and antagonized (±)-quinpirole-induced IP accumulation (pKb 9.22) in murine cells expressing human D2L receptors. It had partial agonist activity (pEC50 8.58, Emax 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 (pKb 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.

Discovery of cariprazine (RGH-188): a novel antipsychotic acting on dopamine D3/D2 receptors.

Medicinal chemistry optimization of an impurity isolated during the scale-up synthesis of a pyridylsulfonamide type dopamine D(3)/D(2) compound (1) led to a series of new piperazine derivatives having affinity to both dopamine D(3) and D(2) receptors. Several members of this group showed excellent pharmacokinetic and pharmacodynamic properties as demonstrated by outstanding activities in different antipsychotic tests. The most promising representative, 2m (cariprazine) had good absorption, excellent brain penetration and advantageous safety profile. Based on its successful clinical development we are looking forward to the NDA filing of cariprazine in 2012.

Effects of RGH-237 [N-{4-[4-(3-Aminocarbonyl-phenyl)-piperazin-1-yl]-butyl}-4-bromo-benzamide], an Orally Active, Selective Dopamine D3 Receptor Partial Agonist in Animal Models of Cocaine Abuse

Dopamine D3 receptor partial agonism has been suggested as a potential therapeutic intervention in cocaine addiction. RGH-237 [N-{4-[4-(3-aminocarbonyl-phenyl)-piperazin-1-yl]-butyl}-4-bromo-benzamide] was identified as a novel selective dopamine D3 receptor partial agonist and used for testing this hypothesis in animal models. The compound showed nanomolar affinity to human (Ki = 6.7 nM) and rat (Ki = 1.6 nM) D3 receptors with an intrinsic activity of approximately 50%. It possessed several hundredfold selectivity over the D2 receptor. The molecule bound with moderate (100–250 nM) affinity to 5-hydroxytryptamine 1A (5-HT1A) and nonselectively labeled opiate receptors. RGH-237 proved to be practically inactive on more than 40 other targets, including monoaminergic, cholinergic, GABAergic, and glutamatergic receptors. In rats orally administered RGH-237 was well and rapidly absorbed yielding 41% oral bioavailability. At its pharmacologically active dose (10 mg/kg p.o.), the brain concentration of RGH-237 reached 110 ng/g. Its blood and brain levels were sustained for 3 h. RGH-237 at the oral dose of 10 mg/kg moderately but significantly inhibited the acquisition of cocaine-induced place preference, although by itself, it had no place-conditioning effect. The compound did not affect fixed ratio 1 cocaine self-administration. In a reinstatement paradigm of cocaine self-administration, the compound potently and dose-dependently blocked the cue-induced cocaine-seeking behavior of rats at 10 and 30 mg/kg oral doses. RGH-237 did not affect seeking activity for natural rewards, such as sucrose and water. It did not exert notable effect on spontaneous motor activity of rats. Our results demonstrate that selective D3 partial agonists may be an effective therapeutic means in the treatment of cocaine abuse.

Occupancy of dopamine D2 and D3 and serotonin 5-HT1A receptors by the novel antipsychotic drug candidate, cariprazine (RGH-188), in monkey brain measured using positron emission tomography

RationaleCariprazine is a novel antipsychotic drug candidate that exhibits high selectivity and affinity to dopamine D3 and D2 receptors and moderate affinity to serotonin 5-HT1A receptors. Targeting receptors other than D2 may provide a therapeutic benefit for both positive and negative symptoms associated with schizophrenia. Positron emission tomography (PET) can be used as a tool in drug development to assess the in vivo distribution and pharmacological properties of a drug.ObjectivesThe objective of this study was to determine dopamine D2/D3 and serotonin 5-HT1A receptor occupancy in monkey brain after the administration of cariprazine.MethodsWe examined three monkeys using the following PET radioligands: [11C]MNPA (an agonist at D2 and D3 receptors), [11C]raclopride (an antagonist at D2 and D3 receptors), and [11C]WAY-100635 (an antagonist at 5-HT1A receptors). During each experimental day, the first PET measurement was a baseline study, the second after a low dose of cariprazine, and the third after the administration of a high dose.ResultsWe found that cariprazine occupied D2/D3 receptors in a dose-dependent and saturable manner, with the lowest dose occupying ~5% of receptors and the highest dose showing more than 90% occupancy. 5-HT1A receptor occupancy was considerably lower compared with D2/D3 occupancy at the same doses, with a maximal value of ~30% for the raphe nuclei.ConclusionsWe conclude that cariprazine binds preferentially to dopamine D2/D3 rather than to serotonin 5-HT1A receptors in monkey brain. These findings can be used to guide the selection of cariprazine dosing in humans.

Positron emission tomographic evaluation of the putative dopamine-D3 receptor ligand, [11C]RGH-1756 in the monkey brain.

The dopamine-D3 receptor is of special interest due to its postulated role in the pathophysiology and treatment of schizophrenia and Parkinsons Disease. Increasing evidences support the assumption that the D3 receptors are occupied to a high degree by dopamine at physiological conditions. Research on the functional role of the D3 receptors in brain has however been hampered by the lack of D3 selective ligands. In the present Positron Emission Tomography (PET) study the binding of the novel, putative dopamine-D3 receptor ligand, [11C]RGH-1756 was characterized in the cynomolgus monkey brain. [11C]RGH-1756 was rather homogenously distributed in brain and the regional binding potential (BP) values ranged between 0.17 and 0.48. Pretreatment with unlabelled RGH-1756 decreased radioligand binding to the level of the cerebellum in most brain areas. The regional BP values were lower after intravenous injection of a higher mass of RGH-1756, indicating saturable binding of [11C]RGH-1756. The D2/D3 antagonist raclopride partly inhibited the binding of [11C]RGH-1756 in several brain areas, including the striatum, mesencephalon and neocortex, whereas the 5HT(1A) antagonist WAY-100635 had no evident effect on [11C]RGH-1756 binding. Despite the promising binding characteristics of RGH-1756 in vitro the present PET-study indicates that [11C]RGH-1756 provides a low signal for specific binding to the D3 receptor in vivo. One explanation is that the favorable binding characteristics of RGH-1756 in vitro are not manifested in vivo. Alternatively, the results may support the hypothesis that the dopamine-D3 receptors are indeed occupied to a high extent by dopamine in vivo and thus not available for radioligand binding.

Lack of effect of reserpine-induced dopamine depletion on the binding of the dopamine-D3 selective radioligand, [11C]RGH-1756

The effect of reserpine induced dopamine depletion on the binding of the putative dopamine-D3 receptor ligand, [(11)C]RGH-1756 was examined in the monkey brain with positron emission tomography (PET). In a previous series of experiments, we have made an attempt to selectively label D3 receptors in the monkey brain using [(11)C]RGH-1756. Despite high selectivity and affinity of RGH-1756 in vitro, [(11)C]RGH-1756 displayed only low specific binding to D3 receptors in vivo. The aim of the present study was to examine whether low specific binding of [(11)C]RGH-1756 is caused by insufficient in vivo affinity of the ligand, or by high physiological occupancy of D3 receptors by endogenous dopamine (DA). PET experiments were performed in three monkeys under baseline conditions and after administration of reserpine (0.5 mg/kg). The results of the baseline measurements corresponded well to our earlier observations with [(11)C]RGH-1756. Reserpine caused no evident change in the regional distribution of [(11)C]RGH-1756 in the monkey brain, and no conspicuous regional accumulation of activity could be observed. After reserpine treatment there was no evident increase of specific binding and binding potential (BP) of [(11)C]RGH-1756. The lack of increased [(11)C]RGH-1756 binding after reserpine treatment indicates that competition with endogenous DA is not the predominant reason for the failure of the radioligand to label D3 receptors. Therefore, the low binding of [(11)C]RGH-1756 could largely be explained by the need for very high affinity of radioligand for D3 receptors in vivo, to obtain a suitable signal for the minute densities of D3 receptors expressed in the primate brain.

Radiochemical labelling of the dopamine D3 receptor ligand RGH-1756

The dopamine D 3 receptor is expressed in low density in limbic brain areas. The receptor subtype has been proposed as a target for novel antipsychotic drugs, however no selective positron emission tomography (PET) ligand is to date available to study the receptor distribution in vivo. 1-(2-Methoxyphenyl)-4-{4-[4-(6-imidazo[2,1-b]thiazolyl)phenoxy]butyl}piperazine (RGH-1756) is a new methoxyphenylpiperazine derivative that possesses high affinity (K i : 0.119 nM) and good selectivity for the human dopamine D 3 receptor. In an attempt to develop a PET radioligand for the visualisation of the dopamine D 3 receptor in human brain we synthesised carbon-11-labelled RGH-1756. The radiolabelling was performed by reaction of the corresponding desmethyl precursor 1-(2-hydroxyphenyl)-4-{4-[4-(6-imidazo[2,1-b]thiazolyl)phenoxy]butyl}piperazine (04512626) with [ 11 C]methyl triflate in acetone and aqueous NaOH. The HPLC-determined incorporation yield was >90%. The total synthesis time was 35 min and the specific radioactivity at end of synthesis ranged from 66 to 132 GBq/μmol.

Brain uptake and distribution of the dopamine D3/D2 receptor partial agonist [11C]cariprazine: An in vivo positron emission tomography study in nonhuman primates

Cariprazine is a dopamine D3/D2 receptor partial agonist antipsychotic candidate, which binds with high affinity to dopamine D3 and D2 receptors (with ∼10‐fold higher in vitro affinity to D3 vs. D2 receptors) and with moderate affinity to 5‐HT1A receptors. The main objective of the present molecular imaging investigation was to evaluate the uptake and reversible binding of 11‐C labeled cariprazine in the nonhuman primate brain, in relation to the known distributions of dopamine D2 and D3 receptors. We examined the brains of two cynomolgus monkeys at baseline condition as well as during a pharmacological blocking condition, using unlabeled cariprazine or raclopride as blockers before injection of [11C]cariprazine. Of the total injected radioactivity, ∼7% entered the brain and ∼3–4% remained in the brain after 90 min, indicating good blood brain barrier penetration and slow washout. It was possible to block cariprazine binding with unlabeled cariprazine and raclopride indicating that [11C]cariprazine binds to dopamine D3/D2 receptors. Nondisplaceable binding potential (BPND) measurements, using a simplified reference tissue model and cerebellum as the reference region, yielded values of ∼1.5 and 0.3 in the striatum and thalamus, respectively. Striatum BPND values were reduced by 80 and 85% following pretreatment with 0.1 mg/kg IV injection of unlabeled cariprazine and 1 mg/kg IV injection of unlabeled raclopride, respectively. The data confirm that cariprazine, a novel antipsychotic drug candidate, enters the nonhuman primate brain readily and binds to dopamine D3/D2 receptors. Furthermore, in PET imaging [11C]cariprazine can effectively visualize dopamine D3/D2 receptors in the nonhuman primate brain. Synapse 67:258–264, 2013.

PET studies in cynomolgous monkeys and whole hemisphere post mortem autoradiographic studies in human brain slices with the dopamine D2/D3 receptor ligand 11C-cariprazine

Background: Earlier investigations with cariprazine (RGH-188) indicated that the compound is a D3/D2 dopamine receptor antagonist–partial agonist antipsychotic candidate. In non-human primate PET studies we demonstrated that the compound successfully occupies and blocks both the D2 and D3 receptors. The main objective of the present investigation was to demonstrate the radiolabelled compounds direct binding to the regions with high D2/D3 dopamine receptor densities in the human and the monkey brain.