Structural effects of lipophilic methotrexate conjugates on model phospholipid biomembranes

Lipophilic conjugates of the antitumor drug methotrexate (MTX) with lipoami no acids (LAAs) have been previously described as a tool to enhance MTX pas sive entrance into cells, overcoming a form of transport resistance which m akes tumour cells insensitive to the antimetabolite. A knowledge of the mec hanisms of interaction of such lipophilic derivatives with cell membranes c ould be useful for planning further lipophilic MTX derivatives with an opti mal antitumour activity. To this aim, a calorimetric study was undertaken u sing a biomembrane model made from synthetic 1,2-dipalmitoyl-glycero-3-phos phocholine (DPPC) multilamellar liposomes. The effects of MTX and conjugate s on the phase transition of liposomes were investigated using differential scanning calorimetry. The interaction of pure MTX with the liposomes was limited to the outer par t of the phospholipid bilayers, due to the polar nature of the drug. Conver sely, its lipophilic conjugates showed a hydrophobic kind of interaction, p erturbing the packing order of DPPC bilayers. In particular, a reduction of the enthalpy of transition from the gel to the liquid crystal phase of DPP C membranes was observed. Such an effect was related to the structure and m ole fraction of the conjugates in the liposomes. The antitumour activity of MTX conjugates was evaluated against cultures of a CCRF-CEM human leukemic T-cell line and a related MTX resistant sub-line . The in vitro cell growth inhibitory activity was higher for bis(tetradecy l) conjugates than for both the other shorter- and longer-chain derivatives . The biological effectiveness of the various MTX derivatives correlated ve ry well with the thermotropic effects observed on the phase transition of D PPC biomembranes. (C), 2001 Elsevier Science B.V All rights reserved.