Deriving the therapeutic concentrations for clozapine and haloperidol: The apparent dissociation constant of a neuroleptic at the dopamine D2 or D4 receptor varies with the affinity of the competing radioligand

The apparent dissociation constant, K_i, for a neuroleptic at the dopamine D₂ or D₄ receptor was consistently higher when competed against [³H]nemonapride than against [³H]spiperone which was in turn higher than that against [³H]raclopride. This finding obtained for all four types of dopamine receptors studied, including the native dopamine D₂ receptor in the anterior pituitary tissue, the human D_2long receptor, the human D_2short receptor and the human D_4.4 receptor. Some neuroleptics revealed a difference of over 10-fold between the K_i using [³H]nemonapride and the K_i using [³H]raclopride. The K_D values of the three ³H-ligands and the neuroleptic K_i values were lower when using a much lower concentration of tissue, indicating that depletion of ligand presumably accounted for the phenomenon. The K_i values of each neuroleptic were related to the the tissue/buffer partition coefficients of the three ³H-ligands. Extrapolating the neuroleptic K_i value down to a tissue/buffer partition coefficient of unity or zero led to a K_i value for competition versus a water-soluble ligand such as dopamine. Clozapine extrapolated to a K_i value of 1.3 nM. Direct measurement gave a K_i value of 1.6 nM for [³H]clozapine at the dopamine D₄ receptor. When competing versus endogenous dopamine, this clozapine value of 1.6 nM would rise to 20 nM for the blockade of 75% of dopamine D₄ receptors, matching the observed therapeutic concentration of 18 nM. These data also explain why clozapine occupies 48% of the D₂ receptors in patients when measured with [¹¹C]raclopride, but between 0% and 22% when measured with [¹⁸F]methylspiperone or [¹⁸F]fluoroethylspiperone.