The interaction of the antipsychotic drug trifluoperazine (TFP) with membranes was investigated in terms of lipid phase perturbation. TFP partition coefficients (P) were measured by phase separation between octanol/water and model membranes/water. The profile of P values at pH 7.4 was: microsomes (7172 +/- 1229) > liposomes (1916 +/- 341) > erythrocyte ghosts (1380 +/- 429)> octanol (452 +/- 55). Hemolytic experiments showed a biphasic, protective (at lower concentrations) and hemolytic effect above the CMC (42 mu M at pH 7.4) of the phenothiazine. By applying classical treatments for surface active compounds to the hemolytic curves, we could calculate P values in whole erythrocyte cells. The preferential binding of uncharged to charged TFP in the membrane was discussed, since it results in a ionization constant (PKapp) different from that observed in the aqueous phase (pK). The TFP ionization constant was decreased from 8.1 (in water) to 7.62 in the presence of membranes and almost the same ratio of charged/uncharged TFP species is present at physiologic pH. Taking into account the Delta pK, we calculated the average TFP partition coefficient between egg phosphatidylcholine liposomes and water, at pH 7.4 (P-average = 1432), which was well correlated with the measured one (p(lip) = 1916). P-average is highly influenced by the uncharged TFP species and the real base/acid ratio under physiologic conditions was discussed in terms of its possible role in the biological activity of TFP. (C) 1998 Elsevier Science B.V. All rights reserved.13732332340