CHARACTERISTICS OF THE BINDING OF TACRINE TO ACIDIC PHOSPHOLIPIDS

Tacrine (1,2,3,4-tetrahydro-9-acridinamine monohydrate) is an inhibito r of acetylcholinesterase currently used in the treatment of the sympt oms of Alzheimer's disease. The present study demonstrates preferentia l binding of this drug to acidic phospholipids, as revealed by fluores cence polarization, penetration into lipid monolayers, and effects on the thermal phase behavior of dimyristoyl phosphatidic acid (DMPA). A fivefold enhancement in the polarization of tacrine emission is eviden t above the main phase transition temperature (T-m) of DMPA vesicles, whereas below T-m only a 0.75-fold increase is observed. In contrast, the binding of tacrine to another acidic phospholipid, dimyristoylphos phatidylglycerol, did not exhibit strong dependence on T-m. In accorda nce with the electrostatic nature of the membrane association of tacri ne, the extent of binding was augmented with increasing contents of eg g PG in phosphatidylcholine liposomes. Furthermore, [NaCl] > 50 mM dis sociates tacrine (albeit incompletely) from the liposomes composed of acidic phospholipids. Inclusion of the cationic amphiphile sphingosine in egg PG vesicles decreased the membrane association of tacrine unti l at 1:1 sphingosine: egg PG stoichiometry binding was no longer evide nt. Tacrine also penetrated into egg PG but not into egg PC monolayers . Together with broadening of the main transition and causing a should er on its high temperature side, the binding of tacrine to DMPA liposo mes results in a concentration-dependent reduction both in the combine d enthalpy Delta H of the above overlapping endotherms and the main tr ansition temperature T-m. Interestingly, these changes in the thermal phase behavior of DMPA as a function of the content of the drug in ves icles were strongly nonlinear. More specifically, upon increasing [tac rine], T-m exhibited stepwise decrements, Simultaneously, sharp minima in Delta H were observed at drug:lipid stoichiometries of approximate ly 2:100 and 25:100, whereas a sharp maximum in Delta H was evident at 18:100, The above results are in keeping with tacrine causing phase s eparation processes in the bilayer and may also relate to microscopic drug-induced ordering processes within the membrane.