Binding of levomepromazine and cyamemazine to human recombinant dopamine receptor subtypes

Background and Objectives: Clozapine (CLOZ) and levomepromazine (LMP) improve treatment-resistant schizophrenia. The superior efficacy of CLOZ compared with other antipsychotic agents has been attributed to an effect on D1-like and D4 receptors. We examined the binding of LMP, CLOZ and cyamemazine (CMZ), a neuroleptic analog of LMP, to human recombinant dopamine (rDA) receptor subtypes. Methods: Binding studies were performed on frozen membrane suspensions of human rDA receptor subtypes expressed in Sf9 cells. Results: (i) LMP has a high affinity (Ki, nM) for rD2 receptor subtypes (rD2L 8.6; rD2S 4.3; rD3 8.3; rD4.2 7.9); (ii) LMP and CLOZ have comparable affinities for the rD1 receptor (54.3 vs 34.6); (iii) CMZ has high affinities for rD2-like and rD1-like receptors (rD2L 4.6; rD2S 3.3; rD3 6.2; rD4.2 8.5; rD1 3.9; rD5 10.7); (iv) CMZ is 9 times more potent than CLOZ at the rD1 receptor and 5 times more potent than CLOZ at the rD4.2 receptor; (v) CMZ has high affinities for rD1 and rD5 receptor subtypes compared with LMP and CLOZ. Conclusions: If D1 and D4 receptors are important sites for the unique action of CLOZ, the present study points to a need for clinical trials comparing CMZ with CLOZ in schizophrenia and in particular, treatment-resistant schizophrenia, especially given the risk for agranulocytosis with CLOZ. Received 8 September 2008 Revised 10 March 2009 Accepted 7 April 2009 Background and Objectives The dopamine (DA) hypothesis of schizophrenia implicates an enhancement of DA function in the pathophysiology of schizophrenia1-3, at least with respect to positive symptoms4. All typical and atypical neuroleptics impair DA neurotransmission5,6. Subtypes of DA receptors have been identified, namely, D1-like (D1, D5) and D2-like (D2, D3, D4)7. Aside from reserpine, which depletes presynaptic DA, neuroleptics impair DA neurotransmission by blocking DA D2 receptors and do so in direct relation to their clinical antipsychotic potencies7. Neuroleptics show differences in their binding affinity for the various DA receptor subtypes7. Various authors have pointed to specific DA receptor subtypes as mediating the symptoms of schizophrenia and being the principle site of action of neuroleptics, namely, the D48, D39,10 or D1-like subtypes11. The superior efficacy of clozapine (CLOZ) in treating schizophrenia, especially in treatment-resistant schizophrenia12 has been attributed to antagonism at the D4 receptor8. Following two open studies13,14, it has recently been shown that levomepromazine (LMP) also improves treatment-resistant schizophrenia15. LMP, a phenothiazine neuroleptic which is structurally similar to chlorpromazine (CPZ), has a methoxy group at carbon 2 of the phenothiazine ring instead of a chlorine atom and a methyl group at carbon 2 of the aliphatic side chain (Fig. 1). Cyamemazine (CMZ), also a clinically effective neuroleptic16,17, is an analog of LMP which differs from LMP in having a cyano group at carbon 2 of the phenothiazine ring instead of a methoxy group (Fig. 1). Neither LMP nor CMZ are marketed as antipsychotics in the United States. In Canada LMP is available for the treatment of schizophrenia but when used is usually prescribed as an adjunctive agent for its sedative-hypnotic effects. In France CMZ is the most frequently prescribed neuroleptic in the treatment of schizophrenia18. Dopamine (DA) receptor binding studies with LMP19-21 or CMZ22-24 have been few and none have looked at the full spectrum of DA receptor subtypes. In view of current theories implicating specific subtypes of DA receptors in the pathophysiology of schizophrenia and the demonstration that both CLOZ and LMP improve treatment-resistant schizophrenia, we 148 LALIT K. SRIVASTAVA ET AL. Figure 1. CPZ (Chlorpromazine), 2-chloro-10-(3-[dimethylamino]-propyl)-phenothiazine; CMZ (Cyamemazine), 2-cyano-10-(3-[dimethylamino]-2-methyl-propyl)-phenothiazine; LMP (Levomepromazine), 10-(3-[dimethylamino]-2-methyl-propyl)-2-methoxy-phenothiazine. have investigated the binding of LMP and CMZ to human recombinant DA receptor subtypes and compared the binding affinities of LMP and CMZ to those of CLOZ.