Kinetics of Ion-Pair Absorption

Abstract

According to the ‘pH-partition’ theory, drugs in their ionized form cannot readily diffuse through biological membrane. However, it is well known that the permeation of many ionized drugs can be enhanced by the presence of anionic or cationic agents. One of the explanations for this observation is that the charged molecules of drug interact with the agents and form lipophillic ion-pair species, and as a result, the absorption of ionized molecules increases. In the present paper, the kinetic relationships of the formation of the ion-pair as a function of ion-pair agent and the type of the biological membrane have been discussed and analyzed. Criteria have been developed for the distinguishing the following two cases: 1) when the absorption is the rate limiting step in the process of permeation of the ion-pair, and 2) when the ion-pair crosses the membrane as though it were a fine sieve.