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

Abstract

Methotrexate has been conjugated (amide bond) via either the α or γ, or both α and γ, glutamyl carboxyl groups to the amino function of dihexanoylphosphatidylethanolamine (C 6C 6PE and 1-tetradecanoyl-2-hexanoylphosphatidylethanolamine (C 14C 6PE). These phospholipid prodrugs (either free or incorporated into liposomes) were compared with the corresponding ditetradecanoylphosphatidylethanolamine (C 14C 14 PE) 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 γ conjugates were more effective than either the a or the α, γ 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 γ 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-γ-C 6C 6PE, whereas both cell lines were equally sensitive to methotrexate-γ-C 14C 14PE. Although less potent than either of the foregoing, methotrexate-γ-C 14C 6PE could partially by-pass the defective transport system. These results suggest that methotrexate-γ-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.