Selective protection against the cytotoxicity of methotrexate and methotrexate-poly(lysine) by thiamine pyrophosphate, heparin and leucovorin

Effect of liposomes sensitized with methotrexate-gamma-dimyristoylphosphatidylethanolamine on cells that are resistant to methotrexate

This study compares the ability of methotrexate and liposomes, in which the drug is anchored to the lipid bilayers via methotrexate-gamma-dimyristoylphosphatidylethanolamine, to inhibit proliferation of human leukemic cells (CEM/O) and cells derived from this line that are resistant to methotrexate because of either a defective transport system (CEM/MTX cells) or elevated levels of dihydrofolate reductase (CEM/R1 cells). Whereas CEM/O and CEM/MTX cells show a 120-fold difference in their susceptibility to methotrexate (as measured by the incorporation of tritiated deoxyuridine into DNA), both lines are equally sensitive to the liposomes. In contrast, proliferation of CEM/MTX cells is not inhibited significantly by methotrexate-gamma-glycerophosphorylethanolamine (MTX-gamma-glyceroPE), the water-soluble analog of MTX-gamma-DMPE. Both the ability of the liposomes to circumvent the transport defect, and the inability of MTX-gamma-glyceroPE to do so, were anticipated on the basis of previous experiments which show that thiamine pyrophosphate could antagonize inhibition of mouse 3T3 and L1210 cell proliferation by methotrexate and MTX-gamma-glyceroPE, but not inhibition by liposomes. Human cells (CEM/O) behave similarly. The present experiments also suggest that liposomes prepared with MTX-gamma-DMPE can partially reverse the methotrexate resistance of CEM/R1 cells that is due to overproduction of the target enzyme.

Binding sites and endocytosis of heparin and polylysine are changed when the two molecules are given as a complex to Chinese hamster ovary cells

This paper characterizes the complex formed in vitro between polylysine and heparin in the presence of heparin excess, and investigates the interaction of this complex with the surface of Chinese hamster ovary cells. It examines the kinetics of surface binding and cellular uptake of the complex and shows that both processes can be distinguished from those of free heparin and free polylysine. The view that these three ligands bind to different surface sites is further supported by the fact that their interaction with cells is influenced differently by cell detachment with trypsin, detachment with EGTA or exposure to acid pH. Membrane transport of the complex is a saturable process suggestive of receptor-mediated endocytosis. It is, however, less effective than would be expected on the basis of the binding kinetics. Only 40% of the complex bound at 0 degrees C is internalized during a 2 h reincubation period at 37 degrees C, suggesting some degree of uncoupling between binding and endocytosis. These data confirm prior results obtained with methotrexate-polylysine conjugates. We had shown that the addition of heparin to a medium containing a methotrexate-polylysine conjugate leads unexpectedly to a marked cellular uptake of drug conjugate, which is capable of killing cells that are otherwise resistant to free methotrexate (Shen, W.-C. and Ryser, H.J.-P. (1981) Proc. Natl. Acad. Sci. USA 78, 7589-7593). The polylysine X heparin complex is therefore of interest as a potential carrier for intracellular drug delivery through endocytosis.

Poly(L-lysine) has different membrane transport and drug-carrier properties when complexed with heparin

Methotrexate (MTX) conjugated to a Mr 3000 poly(L-Lys) markedly inhibits the growth of Pro-3 MtxRII5-3 Chinese hamster ovarian cells, a mutant line known to be drug resistant because of defective MTX transport. In these cells, membrane transport of [3H]MTX-poly(Lys) is sharply decreased by addition of 0.5- to 2.5-fold heparin but remains at 15-20% of control in 2.5- to 50-fold heparin excess. Heparin addition at first markedly inhibits but, at high concentration, restores the growth inhibitory effect of MTX-poly(Lys). In excess heparin, MTX-poly(Lys) is transported as a heparin complex. Because reduced transport (15-20%) is sufficient to cause a 90% inhibition of cell growth, MTX-poly(Lys) apparently gains pharmacologic potency when compared to heparin. This gain can be related to a greater inhibitory effect on dihydrofolate reductase and to a different mode of transport. The inhibitory effect of MTX-poly(Lys) on dihydrofolate reductase in vitro is increased nearly 100-fold in the presence of excess heparin but remains less than that of free MTX. Unlike that of MTX-poly(Lys), the transport of MTX-poly(Lys)-heparin has the characteristics and efficiency of a receptor-mediated process. It proceeds by endocytosis but is not, as in the case of uncomplexed conjugate, followed by the intracellular generation of pharmacologically active breakdown products that would account for cytotoxicity. These observations raise the possibility that at least part of the MTX-poly(Lys)-heparin reaches cellular dihydrofolate reductase in the form of macromolecular complexes that escape from entrapment in endocytotic structures. Our data illustrate a way to overcome drug resistance by taking advantage of the specific uptake of a macromolecular drug carrier. They offer a method of drug delivery in which heparin improves selectivity and decreases the unwanted toxicity inherent to polycationic carriers.