Proposed Partition Mechanism of Tetracycline

Abstract

The partition mechanism of tetracycline in an n-octanol/H2O system was studied by mathematical treatment of the changes in the apparent partition coefficient determined by Colaizzi and Klink (J. Pharm. Sci., 58, 1184 (1969)) and by us, and by measuring the spectral properties of tetracycline in organic solvents. Tetracycline exists in a neutral molecular form in n-octanol, so it could be concluded that transfer of tetracycline from an aqueous to an organic phase is governed not by the amount of the zwitterionic form, but by the amount of the neutral molecular form in the aqueous phase. Results showed that the tetracycline molecule itself is hydrophobic, but that the concentration of the neutral molecular species in the aqueous phase is very small. The relationship between the apparent partition coefficient and the three macroscopic ionization constants of tetracycline at a certain pH was also clarified. From this the macroscopic ionization constants were evaluated. The values agreed very well with those determined by potentiometric titration. © 1975, The Pharmaceutical Society of Japan. All rights reserved.