Binding of antiarrhythmic drugs to human placenta in vitro

Lidocaine, procainamide and quinidine binding to human placenta was investigated in vitro. Pooled whole human placental homogenate supplemented with either non-radiolabelled lidocaine, procainamide or quinidine over the concentration range 50-5,000 x 10(-7) mol/L was submitted to equilibrium dialysis against phosphate buffer, pH 7.4, 0.1 mol/L. Post-dialysis drug concentrations were measured by enzyme immunoassay. Data were analyzed by the method of Scatchard. No binding to placenta was noted for either lidocaine or procainamide. In contrast, up to 22 percent of quinidine was bound. Two binders were defined as follows: #1 (Ka, 7.37 x 10(5) L/mol; Bo, 1.55 x 10(-7) mol/L) and #2 (Ka, 7.11 x 10(4) L/mol; Bo, 4.05 x 10(-6) mol/L). The concentrations of quinidine binding sites in moles per gram of placenta were 1.55 x 10(-9) and 4.05 x 10(-8), respectively. These data suggest that quinidine may be accumulated in human placenta.