Significance of lipoproteins in serum binding variations of amitriptyline, nortriptyline, and quinidine

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Abstract
By using isotope technique, the serum binding of amitriptyline (AT), nortriptyline (NT) and quinidine (Q) was measured by equilibrium dialysis in sera containing varying amounts of lipoproteins. Sera were obtained from 10 fasting subjects with normal to grossly elevated levels of cholesterol, triglycerides or both. When the lipoproteins were removed from 8 of the sera by a standard ultracentrifugation technique, the ratio bound/unbound (B/F) AT decreased an average of 47% (range 30-68%), NT an average of 54% (range 39-67%) and Q an average of 6% (range 0-16%). This decrease in the ratio B/F correlated linearly with the sum of serum concentrations of cholesterol and triglycerides for AT (r = 0.88) and NT (r = 0.82), but not for Q (r = 0.15). In 3 lipoprotein-depleted sera resuspended with lipoproteins at 8 different concentrations ranging from 0-100% of the original content, there was a linear correlation between the ratio B/F for AT and NT and the lipoproteins, as evidenced by cholesterol or triglycerides concentrations (r = -0.97-0.99), but not for Q (r = -0.17-0.36). In the original 10 serum samples, there was a linear correlation between the ratio B/F and the serum lipoproteins (sum of cholesterol and triglycerides) for AT (r = 0.89) and NT (r = 0.68), whereas there was no such relationship for Q (r = -0.15). Basic drugs differ in binding characteristics (probably depending on lipophilicity).

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