Thermodynamics of partitioning and efflux of phenothiazines from liposomes

The partitioning of nine phenothiazines between dimyristoylphosphatidylcholine (DMPC) liposomes and 0.9% wt/vol saline at pH 6 has been studied both below and above the phase transition temperature (Tc) of the phospholipid. Higher partitioning was observed above Tc. Both the entropy and enthalpy of partitioning were positive below and above Tc, and a linear relationship between the entropy and enthalpy has been derived. In general, the partitioning and transport of alkylaminophenothiazines in DMPC liposomes over the temperature range of 5 to 40 degrees C is entropically controlled. The entropies and enthalpies of partitioning of various groups in the phenothiazine structure have been calculated. No relationship was found between particle size of the DMPC liposomes and the equilibrium partition coefficient at 25 degrees C. However, the particle size of liposomes did increase with increasing acyl chain length of the phospholipid. Using differential scanning calorimetry, the enthalpy and entropy of transition of the DMPC liposomes in the absence and presence of phenothiazines has been calculated. The temperature dependence of the first-order rate constant of trimeprazine tartrate transport in DMPC liposomes was investigated and was found to be maximum at the Tc of the phospholipid.