Determination of free fractions of tricyclic antidepressants

The plasma protein binding of amitriptyline, imipramine, clomipramine, and their primary demethylated metabolites were studied by means of a method combining dialysis and gas chromatography. Equilibrium in dialysis of serum containing amitriptyline and its metabolite nortriptyline was attained in about 0.5 h with the drug dissolved in the serum compartment, and in about 2 h with the drug passing from the buffer to the serum compartment. The calculation of free fractions was influenced by variations with dialysis time in the volumes of serum and buffer. Increase of pH in serum increased the protein binding of the weakly basic drugs studied, and made the Donnan distribution effects more pronounced. At pH 7.4, the Donnan effect was negligible. Binding parameters for the 6 tricyclic antidepressant substances studied were estimated for the binding to alpha 1-acid glycoprotein and for total binding in serum. For alpha 1-acid glycoprotein, the k-values ranged from 1 X 10(5) to 8 X 10(5) M-1, and for pooled serum from 0.4 X 10(5) to 8 X 10(5) M-1. The determined number of binding sites on the alpha 1-acid glycoprotein was, on average 0.87 for the 6 substances. In serum, the binding capacity was 2-14 times the concentration of alpha 1-acid glycoprotein.