Relationship between alpha 1-acid glycoprotein and plasma binding of disopyramide and mono-N-dealkyldisopyramide.

Highly purified serum albumin did not bind either disopyramide (DP) or mono-N-dealkyldisopyramide (MND). The unbound fraction of DP and MND in highly purified serum alpha 1-acid glycoprotein (AAG) at 0.5 g/l was 57 and 62 and at 2.0 g/l 19 and 30% respectively. Unbound DP and MND were measured in spiked plasma (10 mumol/l of DP or MND), from 60 patients, having AAG concentrations varying from 0.4 to 3.0 g/l. Unbound drug varied from 13 to 58 and from 24 to 62% for DP and MND, respectively, and was inversely related to the plasma concentration of AAG (r = -0.9016, r = -0.9157). A linear relationship was found between the binding ratio (moles bound divided by moles unbound) and the plasma concentration of AAG for both DP (r = 0.9199) and MND (r = 0.9270), whereas no relationship was found between the binding ratios of DP or MND and the plasma concentrations of total protein, albumin, haptoglobin, alpha 1-antitrypsin or the immunoglobulins IgG, IgA or IgM. In patients on DP maintenance therapy, a linear relationship was found between percent unbound DP and the plasma concentration of DP in samples with similar AAG concentrations. Furthermore, a linear relationship was found between the binding ratio of DP and the plasma concentration of AAG in samples with similar DP concentrations. The present findings support the concept that AAG is the major serum protein responsible for the binding of DP and MND.