Plasma protein binding of ritodrine at parturition and in nonpregnant women.

The binding of ritodrine HCl in whole plasma from healthy nonpregnant women, parturients and matched umbilical venous plasma and in solutions of HSA and alpha 1-AGP has been studied by equilibrium dialysis. Binding to plasma from nonpregnant subjects and HSA was independent of ritodrine concentration over a wide range. The free fraction in plasma was high and significantly different between groups of nonpregnant (alpha = 0.64), parturient (alpha = 0.68) and matched umbilical venous plasma (alpha = 0.75). It would seem that variability in transplacental transfer of ritodrine, as a result of plasma binding fluctuations, will be minor.