Drug partitioning: relationships between forward and reverse rate constants and partition coefficient.

The rate constant, k1, of drug transport from an aqueous phase to an organic phase and the rate constant, k2, of the reverse process can be described as functions of the partition coefficient, P:logk1 = log P-log (betaP + 1) + c' and logk2 = -log (betaP + 1) + c'. In a homologous series, where log P is a simple function of the number of CH2 groups, log k1 and log k2 also can be described as functions of the number of CH2 groups. The relationships between these equations and current physicochemical models of drug absorption are discussed.