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.