RELEVANCE OF TUMORAL FOLYLPOLYGLUTAMATE SYNTHETASE AND REDUCED FOLATES FOR OPTIMAL 5-FLUOROURACIL EFFICACY - EXPERIMENTAL-DATA

The purpose of this study was to investigate folate-related predictors of 5-fluorouracil (5-FU) cytotoxicity in the presence or absence of l -folinic acid (l-FA). Intracellular concentrations of the reduced fola tes (tetrahydrofolate + 5,10-methylenetetrahydrofolate) and folylpolyg lutamate synthetase (FPGS) activity were determined in 14 human cancer cell lianes expressing a spontaneous sensitivity to 5-FU. On these li n cell lines grown without l-FA supplementation, a significant positiv e correlation was demonstrated between basal intracellular folate conc entration and FPGS activity. 5-FU sensitivity (IC50 range 0.6-25.4 mu M) was nea: related to the basal intracellular folate concentration, w hereas, significantly, it was linked to FPGS activity (range 2.5-11.1 pmol/min/mg protein): the higher the FPGS activity, the greater the 5- FU sensitivity. Under l-FA supplementation (0.01-300 mu M), intracellu lar reduced folates increased continuously without evidence of saturat ion in all cell lines; the pattern of accumulation was independent of the FPGS activity. l-FA enhanced 5-FU cytoxicity by a factor of 1.9-6. 4 in 12 of the 14 cell lines. In the 12 FA-sensitive cell lines, the l -FA concentrations allowing 90% of maximum 5-FU potentiation [l-FA]90 ranged between 0.7 and 107.9 mu M (median 1.9); in contrast, the intra cellular concentrations of reduced folates allowing 90% of maximum 5-F U potentiation were much less variable (range 7.6-38.3, median 24.8 pm ol/mg protein). In the presence of [l-FA]90, 5-FU sensitivity remained significantly correlated to the basal FPGS activity. in addition, red uced folates were measured in 96 tumoral samples (50 head and neck, 16 colon, 30 liver metastases from colorectal cancer) taken before treat ment. Almost all investigated tumours had folate concentrations below the median concentration required for optimal 5-FU potentiation in vit ro: median levels (range, pmol/mg protein) were 3.5 (0-17.7) for head and neck, 5.8 (2.3-12.0) for colon and 12.1 (1.7-118.5) for liver meta stases. Above all, these data establish the relevance of FPGS activity for predicting the efficacy of 5-FU modulated by FA or slot and point to the potential clinical interest of FPGS determination in human tum ours. (C) 1997 Elsevier Science Ltd.