Efficient utilization of the reduced folate carrier in CCRF-CEM human leukemic lymphoblasts by the potent antifolate N-alpha-(4-amino-4-deoxypteroyl)-N-delta-hemiphthaloyl-L-ornithine (PT523) and its B-ring analogues

The potent nonpolyglutamatable dihydrofolate reductase inhibitor N-alpha-(4-amino-4-deoxypteroyl) N-delta-hemiphthaloyl-L-ornithine (PT523) and six of its B-ring (5-deaza, 8-deaza, and 5,8-dideaza) analogues were compared in terms of their ability to: (a) inhibit the growth of CCRF-CEM human leukemic lymphoblasts, and (b) utilize the reduced folate carrier (RFC) in these cells as measured in a competition assay of [H-3]methotrexate ([H-3]MTX) influx. The Ic(50) values of the hemiphthaloylornithine derivatives against CCRF-CEM cells after 72 hr of drug exposure varied from 0.64 to 1.3 nM as compared with 14 nM for MTX and 4.4 nM for aminopterin (AMT). The K-i Values of these compounds in the [3H]MTX influx assay were in the 0.3 to 0.7 mu M range as compared with a K-i of 5.4 mu M for AMT and a K-t of 7.1 mu M for MTX. As a group, the affinities of these compounds for the RFC were approximately 10-fold greater than those of their respective glutamate analogues. These results indicate that, in addition to their previously reported tight binding to dihydrofolate reductase, a property contributing to the high potency of PT523 and its B-ring analogs as inhibitors of tumor cell growth is their strong affinity for the RFC. BIOCHEM PHARMACOL 60;1:41-46, 2000. (C) 2000 Elsevier Science Inc.