Abstract

Conjugation of methotrexate to poly(L-lysine) markedly increases its cellular uptake and offers a new way to overcome drug resistance related to deficient transport. Conjugates using poly(L-lysines) of molecular weights varying from 3,100 to 130,000 have comparable effects on a drug resistant CHO cell line. Conjugates using poly(D-lysine), however, have no effect on either resistant or normal CHO cells. Both the L- and D-isomeric conjugates are taken up by cells in comparable fashion. Since even the biologically active conjugates are poor inhibitors of dihydrofolate reductase (5,6,7,8-tetrahydrofolate:NADP⁺ oxidoreductase; EC 1.5.1.3) in vitro, it is concluded that the conjugate must give rise in the cell to a pharmacologically active breakdown product. Such a product can be detected in cells exposed to the L-isomeric, but not in cells exposed to the D-isomeric conjugates. Excess of poly(L-lysine) only moderately decreases methotrexate-poly(L-lysine) uptake but markedly decreases its inhibitory effect on cell growth.