A human leukemia cell culture system for testing new antifols: differential sensitivity of lymphoid and nonlymphoid cell lines to unconjugated and methotrexate-conjugated polymers of basic amino acids.

A human cell culture system is described for biological testing of potent new folate-targeted antileukemic drugs that are poorly transported. Basic amino acid (lysine and ornithine) polymers were employed as carriers for increasing the uptake of folate analogs by human leukemia cell lines. In growth inhibition assays, the lymphocytic CCRF-CEM line displayed sensitivities to covalent methotrexate (MTX) conjugates of poly-L-lysine (Mr = 15,000, 50,000, or 100,000) or poly-L-ornithine (Mr = 35,000) which were identical to the sensitivities of these cells to the unconjugated polymers during continuous (120 hr) and pulse (24 hr) exposures; both polymers and conjugates were 50-fold less toxic than unconjugated MTX. The growth inhibitory effects of the polymers or MTX-conjugates were not reversed by simultaneous inclusion of leucovorin, while those of MTX were reversed. In contrast, the nonlymphocytic K562 line showed toxicity by the MTX-conjugates at nontoxic levels of the polymers during continuous, but not pulse, exposures. During continuous exposure the conjugates were only 10-fold less toxic than unconjugated MTX. Toxicities of the MTX-conjugates for the K562 line under continuous exposure conditions were reversed by the simultaneous presence of leucovorin or the lysosomotropic agent leupeptin and thus appeared to be a true antifolate effect which required uptake and lysosomal degradation. This human cell line is thus a suitable system in which to study the effects of antifolates which can be coupled to basic polymers.