Factors influencing uptake and retention of amino-containing drugs in large unilamellar vesicles exhibiting transmembrane pH gradients

The level of uptake and retention of amino-containing drugs in large unilam ellar vesicles (LUVs) following uptake in response to a transmembrane pH gr adient (Delta pH) can vary dramatically depending on the drug. For example, the anticancer drugs doxorubicin and epirubicin can be readily retained, w hereas the anticancer drug vincristine and the antibiotic ciprofloxacin ten d to leak out rapidly. In this investigation, we examine the influence of t he hydrophobicity of the entrapped amines (that induce the Delta pH) and th e anionic lipid content of the LUV on drug retention. It is shown that entr apment of increasingly hydrophobic monoamines (methylamine to amylamine) al l lead to an induced Delta pH of 3 units and essentially complete drug upta ke under the conditions employed, but do not lead to improved retention of vincristine and ciprofloxacin. However, significantly improved retention co uld be achieved by substitution of the anionic lipid distearoylphosphatidyl glycerol (DSPG) for distearoylphosphatidylcholine (DSPC) in the LUV bilayer . Further, it is shown that if the induced Delta pH is reduced to 1.4 units (driven by entrapped diamine) nearly 100% accumulation of doxorubicin and epirubicin could be achieved, whereas only 25% loading for vincristine and ciprofloxacin was observed. Taken together these results provide methodolog y for improving tweak base) drug retention in liposomes and indicate that d rugs that can partition into the lipid bilayer exhibit improved uptake and retention characteristics. (C) 1999 Elsevier Science B.V. All rights reserv ed.