THYMIDYLATE SYNTHASE AS A TARGET FOR GROWTH-INHIBITION IN METHOTREXATE-SENSITIVE AND METHOTREXATE-RESISTANT HUMAN HEAD-AND-NECK-CANCER AND LEUKEMIA-CELL LINES

Thymidylate synthase (TS) inhibitor effects on growth of human head an d neck squamous cell carcinoma (HNSCC) cell lines and CCRF-CEM human l eukemia cells and sublines with acquired methotrexate (2,4-diamino-10- methylpteroylglutamic acid) (MTX) resistance were studied. During 120- h treatment, HNSCC cell lines A253 and FaDu are equally sensitive to M TX, whereas the polyglutamylatable TS inhibitors ZD1694 and BW1843U89 are 5- to 35-fold more potent than MTX and the lipophilic AG331 is app roximate to 10(2)-fold less potent than MTX. A253 is intrinsically res istant to intermittent (24 h) MTX and BW1843U89 exposure (higher EC50 values and shallower slopes of concentration-response curves relative to FaDu); AG331 and ZD1694 largely overcome this intrinsic resistance to intermittent exposure. Thymidine (TdR) protects against growth inhi bition by these inhibitors, confirming that TS is their target in HNSC C; at high AG331 levels, TdR only partially protects, implying that a second site of action exists. Growth inhibition of HNSCC by ZD1694 and BW1843U89 is protected by leucovorin (LV) at greater than or equal to 10(-7) and > 10(-5) M, respectively; 10(-4) M LV cannot protect HNSCC cells against AG331. Results similar to protection studies are obtain ed if LV addition is delayed less than or equal to 24 h after ZD1694 o r BW1843U89 exposure. CCRF-CEM sublines with acquired MTX resistance r esulting from dihydrofolate reductase (DHFR) overexpression, defective MTX transport, or defective MTX polyglutamylation retain full sensiti vity to AG331. Cells with defective MTX transport are highly cross-res istant to ZD1694 and BW1843U89, implicating the reduced folate/MTX car rier in their transport. Minor cross-resistance of the DHFR overexpres sing line to ZD1694 and BW1843U89 is observed. A subline with highly d efective MTX polyglutamylation is cross-resistant to 120-h exposure to ZD1694, but not to BW 1843U89, suggesting a profound contribution of polyglutamylation to the mechanism of action of ZD1694.