Plasma protein binding-lipophilicity relationships: interspecies comparison of some organic acids

Relationships between plasma protein binding of 11 organic acids (benzoic and phenylacetic acid derivatives) and their lipophilicity were studied in man, rabbits, rats and mice. For description, the relationship fu = 1/(1 + aDb) was developed, where fu is the fraction of the unbound drug in plasma, D is the partition coefficient octanol/water and a and b are parameters. While the value of the parameter a is widely different in interspecies comparison, the value of the parameter b is very close in all species studied and is approximately equal to 1. The model used allows the simple calculation of the extent of plasma binding of structurally similar drugs from their lipophilicity, or conversion of the extent of plasma binding from one species to another.