Interaction of antipsychotic drugs with neurotransmitter receptor sites in vitro and in vivo in relation to pharmacological and clinical effects: role of 5HT₂ receptors

Abstract

In the introductory section an overview is given of the strategies which have been proposed in the search for side-effect free antipsychotics. Special attention is paid to the role of predominant 5HT₂ receptor blockade over D₂ blockade. Whereas D₂ receptor blockade seems to be essential for the treatment of positive symptoms of schizophrenia, it also underlies the induction of extrapyramidal side effects (EPS). Predominant 5HT₂ receptor blockade may reduce the EPS liability and can ameliorate negative symptoms of schizophrenia. We further report a nearly complete list of neuroleptics that are on the European market and eight new antipsychotics that recently entered clinical trial, 5HT₂ and D₂ receptor binding affinity (K_i values) and the rank order in affinity for various neurotransmitter receptor subtypes are also discussed. For the eight new antipsychotics and for six reference compounds the complete receptor binding profile (including 33 radioligand receptor binding and neurotransmitter uptake models) is reported. Furthermore, for a series of 120 compounds the relative affinity for D₂ receptors and D₃ receptors (a recently cloned new dopamine receptor subtype) is compared. Finally, original findings are reported for the new antipsychotic risperidone and for haloperidol and clozapine on the in vivo occupation of neurotransmitter receptors in various brain areas after systemic treatment of rats or guinea pigs. The receptor occupation by the drugs was measured ex vivo by quantitative receptor autoradiography. The receptor occupancy was related to the motor activity effects of the test compounds (measurements were done in the same animals) and to the ability of the drugs to antagonize various 5HT₂ and D₂ receptor mediated effects. With risperidone a high degree of central 5HT₂ receptor occupation was achieved before other neurotransmitter receptors became occupied. This probably co-underlies the beneficial clinical properties of the drug. Antagonism of the various D₂ receptor-mediated effects was achieved at widely varying degrees of D₂ receptor occupancy, from just about 10% to more than 70%. For therapeutic application it may be of prime importance to carefully titrate drug dosages. Antipsychotic effects may be achieved at a relatively low degree of D₂ receptor occupancy at which motor disturbances are still minimal. With drugs such as risperidone that produce shallow log dose-effect curves, differentiation between the various D₂ receptor mediated effects may be made more easily, allowing EPS-free maintenance therapy of schizophrenic patients.