Novel antipsychotics and extrapyramidal side effects. Theory and reality

Novel antipsychotics are less likely to produce extrapyramidal symptoms (EP S). The superior EPS profile of these drugs compared with that of low-poten cy classic antipsychotics does not seem to be associated with an antimuscar inergic effect. Several other mechanisms could account for this typical fea ture of novel antipsychotics, which combines a strong antipsychotic effect with low liability to produce EPS. The hypotheses are based on either the r eceptor affinity or the topic selectivity of the drugs, but these effects a re closely interconnected and it is not possible to separate them. For cloz apine treatment, its very low D occupancy could explain the absence of EPS, but this does not account for the low EPS with other novel antipsychotics. The direct topic selectivity for extrastriatal D-2 receptors and different distribution of the drug in the brain is less probable. The most plausible hypotheses are based on the concept of indirect topic selectivity for extr astriatal D-2 receptors, mediated by high affinity for extrastriatal D-2 re ceptors (D-3, D-4), 5-HT2A antagonism, alpha(1) antagonism or muscarinergic antagonism at the limbic level. The D-1 and indirect GABAergic antagonisms may play a role in the prevention of tardive dyskinesia. This paper briefl y outlines the hypotheses and observations and summarizes the current knowl edge of the occurrence of and risk for extrapyramidal side effects with nov el antipsychotics.