Liposome-dependent delivery of pteridine antifolates: a two-compartment growth inhibition assay for evaluating drug leakage and metabolism.

We have developed a two-compartment growth inhibition assay that can provide information about leakage, metabolism and delivery of liposome-dependent drugs under cell culture conditions, and at drug concentrations that are relevant to drug delivery. Two cell lines are grown in separate compartments separated from each other by a 0.1 micron polycarbonate membrane. The membrane allows free drugs to diffuse rapidly from one compartment to another, and does not allow liposomes to diffuse through. Liposomes are added to the first compartment, which contains target cells. The extent of leakage caused by these cells is determined by the growth inhibition of non-target cells in the second compartment. We show that methotrexate and methotrexate-gamma-aspartate leak rapidly and almost completely when encapsulated in phosphatidylglycerol/cholesterol (67:33) liposomes. In contrast, there is only 42% leakage when the drugs are encapsulated in distearoylphosphatidylglycerol/cholesterol (67:33) liposomes. We also demonstrate that the target cells (CV1-P) may partially degrade encapsulated methotrexate-gamma-aspartate to methotrexate. Therefore, methotrexate-gamma-aspartate may be a lysosomally cleaved pro-drug of methotrexate.