Binding of phenothiazine neuroleptics to plasma proteins

The binding of chlorpromazine, trifluoperazine, perphenazine, desipramine, propranolol and salicylic acid to human plasma and isolated plasma proteins was studied using equilibrium dialysis. Unlike salicylic acid, an acidic compound only bound to human serum albumin, the basic drugs were bound to all plasma protein fractions studied (albumin, alpha 1-acid glycoprotein, lipoproteins, gamma-globulins) with alpha 1-acid glycoprotein an important binding protein for each of them. The interaction of chloropromazine, perphenazine and trifluoperazine with alpha 1-acid glycoprotein was studied using Scatchard analysis. The primary class of binding sites revealed a low capacity (n = 0.5-1) and a high affinity (K = 10(5)-10(6) M-1) for the phenothiazines. The interaction of chlorpromazine, perphenazine and trifluoperazine with albumin was of the high capacity-low affinity type. In binding studies using plasma obtained from healthy volunteers, alpha 1-acid glycoprotein was found to be a very important binding protein for the basic drug studied with the exception of desipramine. This shows that results derived from binding studies using isolated protein fractions should be interpreted with caution.