Circumvention of the methotrexate transport system by methotrexate-phosphatidylethanolamine derivatives: effect of fatty acid chain length.

Methotrexate has been conjugated (amide bond) via either the alpha or gamma, or both alpha and gamma, glutamyl carboxyl groups to the amino function of dihexanoylphosphatidylethanolamine (C6C6PE) and 1-tetradecanoyl-2-hexanoylphosphatidylethanolamine (C14C6PE). These phospholipid prodrugs (either free or incorporated into liposomes) were compared with the corresponding ditetradecanoylphosphatidylethanolamine (C14C14PE) conjugates, some of whose properties have been described previously, for their ability to inhibit the proliferation of human leukemic cells (CEM/O) or cells derived therefrom (CEM/MTX) that are resistant to methotrexate because of a defective drug transport system. Regardless of chain length, the gamma conjugates were more effective than either the alpha or the alpha, gamma conjugates, in inhibiting growth of the parent cells, confirming initial experiments with mouse cells. Chain length had, however, a pronounced influence on the capacity of the various gamma derivatives to circumvent the transport defect. For example, CEM/MTX cells were 120-fold less susceptible than CEM/O cells to inhibition by either methotrexate or methotrexate-gamma-C6C6PE, whereas both cell lines were equally sensitive to methotrexate-gamma-C14C14PE. Although less potent than either of the foregoing, methotrexate-gamma-C14C6PE could partially by-pass the defective transport system. These results suggest that methotrexate-gamma-PE derivatives with appropriate acyl residues might be useful probes to investigate the mechanism by which phospholipids in general are able to traverse cell membranes.