Binding of amitriptyline and nortriptyline in plasma determined from their equilibrium distributions between red cells and plasma, and between red cells and buffer solution.

In a new method for the measurement of plasma binding, amitriptyline (AT) and nortriptyline (NT) were allowed to reach equilibrium distribution between the cells and plasma of whole blood. A separate sample of the cells was equilibrated with a buffer solution containing the drugs. From the two distribution ratios for each drug, the fraction free in the plasma was calculated. Equilibria were achieved rapidly, avoiding denaturation of binding proteins. The pH was adequately controlled and the composition of the plasma was not altered by the experimental procedures. Large volumes of buffer solution gave amounts of free drug readily measurable by gas chromatography. Duplicate determinations showed coefficients of variation of 6% and 4.8% respectively for the free fractions of AT and NT in a given plasma sample. In 51 subjects the mean percentage of AT free in plasma was 4.31 +/- 0.59 SD and, of NT, 8.59 +/- 0.86. Binding was independent of drug concentration in the therapeutic range and did not differ between males and females nor between patients and normal subjects. It increased slightly with age. It was not affected by chylomicrons in the blood.