The fast-off hypothesis revisited: A functional kinetic study of antipsychotic antagonism of the dopamine D2 receptor.

Newer, "atypical" antipsychotics carry a lower risk of motor side-effects than older, "typical" compounds. It has been proposed that a ~100-fold faster dissociation from the dopamine D2 receptor (D2R) distinguishes atypical from typical antipsychotics. Furthermore, differing antipsychotic D2R affinities have been suggested to reflect differences in dissociation rate constants (koff), while association rate constants (kon) were assumed to be similar. However, it was recently demonstrated that lipophilic accumulation of ligand in the cell interior and/or membrane can cause underestimation of koff, and as high-affinity D2R antagonists are frequently lipophilic, this may have been a confounding factor in previous studies. In the present work, a functional electrophysiology assay was used to measure the recovery of dopamine-mediated D2R responsivity from antipsychotic antagonism, using elevated concentrations of dopamine to prevent the potential bias of re-binding of lipophilic ligands. The variability of antipsychotic kon was also reexamined, capitalizing on the temporal resolution of the assay. kon was estimated from the experimental recordings using a simple mathematical model assumed to describe the binding process. The time course of recovery from haloperidol (typical antipsychotic) was only 6.4- to 2.5-fold slower than that of the atypical antipsychotics, amisulpride, clozapine, and quetiapine, while antipsychotic kons were found to vary more widely than previously suggested. Finally, affinities calculated using our kon and koff estimates correlated well with functional potency and with affinities reported from radioligand binding studies. In light of these findings, it appears unlikely that typical and atypical antipsychotics are primarily distinguished by their D2R binding kinetics.