Further studies on the interaction of nonpolyglutamatable aminopterin analogs with dihydrofolate reductase and the reduced folate carrier as determinants of in vitro antitumor activity

Thirteen structural analogs of the potent nonpolyglutamatable dihydrofolate reductase inhibitor Nα-(4-amino-4-deoxypteroyl)-Nδ-hemiphthaloyl-l-ornithine (PT523) with modifications in the side chain, the para-aminobenzoyl moiety, or the 9,10-bridge were evaluated for the ability to inhibit human recombinant dihydrofolate reductase (DHFR), to utilize the reduced folate carrier (RFC) for influx, and to inhibit the growth of CCRF-CEM human leukemia cells in culture. In spectrophotometric assays of the kinetics of the reduction of dihydrofolate by DHFR in the presence of NADPH, these compounds had Ki values ranging from 0.2 to 1.3pM, and thus were not greatly different in potency from the parent drug PT523. By comparison, the Ki values of aminopterin (AMT), methotrexate (MTX), and 10-ethyl-10-deazaaminopterin (EDX) were 3.7, 4.8, and 11pM. In assays of competitive inhibition of [3H]MTX influx into CCRF-CEM cells, the Ki values ranged from 0.21 to 7.3μM, as compared with 0.71, 5.4, and 1.1μM for PT523, AMT, and EDX. The Kt for MTX was also re-analyzed and found to be 4.7μM, in better agreement with the literature than our previously reported value of 7.1μM. The ic50 values of these compounds as inhibitors of the growth of CCRF-CEM cells after 72hr of drug exposure ranged from 0.53 to 55nM, and were qualitatively consistent with the other results.