Effects of pH on the Transport of 5-Fluorouracil across a Fibroin Membrane

The transport of 5-fluorouracil (an anti-cancer medicine, 5FU) through a protein membrane was investigated. The membrane was prepared from silk fibroin. The permeability coefficients of 5FU were measured at different pH values ranging from 3.0 to 9.0. The protein membrane was composed of both weak acid and weak base groups—a so-called amphoteric membrane. The isoelectric point of this membrane is about pH 4.0. The measured results showed that the permeability coefficient of 5FU abruptly decreased above about pH 7 because 5FU became anionic in an alkaline solution and the membrane become a cation exchanger above pH 4.0. The abrupt decrease in permeation can be theoretically explained in terms of the Donnan equilibrium and the Nernst-Planck equation. Fibroin membranes are suggested to be potentially very useful in drug delivery systems.