BIOLOGICAL-ACTIVITY AND INTRACELLULAR METABOLISM OF ZD1694 IN HUMAN LEUKEMIA-CELL LINES WITH DIFFERENT RESISTANCE MECHANISMS TO ANTIFOLATE DRUGS

The biological activity and cellular metabolism of ZD1694, a novel fol ate-based thymidylate synthase (TS) inhibitor, were analyzed in a huma n leukemia cell line, MOLT-3, and its antifolate-resistant sublines wi th different mechanisms of resistance to methotrexate (MTX), trimetrex ate (TMQ) and N-10-propargyl-5,8-dideazafolic acid (CB3717), MOLT-3/CB 3717(40), which was selected for CB3717 resistance, demonstrated impai red membrane drug transport via reduced folate carrier (RFC) and lower accumulation of [H-3]ZD1694-polyglutamates in the cells with a shift in the polyglutamate distribution profile to shorter chain length poly glutamates, indicating an alteration in polyglutamation capacity in th is subline. Impaired RFC and reduced rate of polyglutamation could exp lain the cross-resistance (12-fold) of this subline to ZD1694, On the other hand, there was little or no cross-resistance to this drug in a subline (MOLT-3/TMQ(800)) reportedly resistant to TMQ through impaired membrane transport for TMQ and an increase in dihydrofolate reductase (DHFR) activity, Total amount of ZD1694 polyglutamated to a level hig her than diglutamate was approximately 1.7-fold higher in the TMQ-resi stant cells than that in the parent cells, but a low degree of increas e in TS activity in the cells counteracted the supposed increase in se nsitivity to ZD1694, MOLT-3/TMQ(800)-MTX(10000) cells, which were esta blished cells, which were established by sequential exposure of the TM Q-resistant cells to MTX and were previously shown to amplify mutated DHFR with low affinity for MTX, showed a decreased accumulation of pol yglutamated ZD1694 as compared with the parent line and this was consi stent with cross-resistance to ZD1694 in this subline. Overproduction of variant DHFR scarcely influenced the sensitivity to this drug. Thes e results indicate that ZD1694 could overcome antifolate resistance th rough a mechanism such as amplified DHFR activity, and the biological activity of this drug against the cells paralleled the amount of polyg lutamated drug inside the cells, Determination of polyglutamation capa city in tumor cells may allow prediction of sensitivity to this drug.