ICI 204,636: a new atypical antipsychotic drug.

The therapeutic effects of ‘classic’ (typical) antipsychotic agents lie in their ability to block central dopaminergic receptors – a property that is also responsible for the frequent occurrence of undesirable extrapyramidal side-effects (EPS). In contrast to these typical agents, clozapine alone has distinguished itself in humans – by virtue of its enhanced antipsychotic action and lack of concurrent EPS – as an atypical antipsychotic. However, the use of clozapine has been limited by the occurrence of agranulocytosis and, to a lesser extent, seizures (Alvir et al, 1993; Haring et al, 1994). The mechanism underpinning the atypical profile of clozapine remains elusive. One hypothesis suggests that it lies in clozapine's higher serotonin 5-HT2: D2 binding ratio, when compared with typical agents – a factor being considered as a predictor of atypicality (Meltzer, this issue; Meltzer et al, 1989). However, an emerging view is that it is not a single pharmacological action, but rather multiple properties that may define an atypical, clozapine-like compound (Lieberman, 1993).