Effects of diastereoisomers of 5-formyltetrahydrofolate on cellular growth, sensitivity to 5-fluoro-2'-deoxyuridine, and methylenetetrahydrofolate polyglutamate levels in HCT-8 cells.

We investigated the biological activities of the natural and unnatural diastereoisomers of 5-formyltetrahydrofolate [(6S)- and (6R)-5-HCO-H4PteGlu, respectively, both 99.99% pure], using a human ileocecal carcinoma cell line (HCT-8). Optimal cell growth could be supported by (6S)-5-HCO-H4PteGlu at concentrations as low as 1 nM. (6R)-5-HCO-H4PteGlu did not support growth. Modulation of the in vitro cytotoxicity of 5-fluoro-2'-deoxyuridine (FdUrd) and intracellular (6R)-5,10-methylenetetrahydrofolates [(6R)-CH2H4PteGlun] pools by (6S)- and (6R)-5-HCO-H4PteGlu was determined with cells growing in 1 nM (6S)-5-HCO-H4PteGlu. For the control cells, the concentration of FdUrd inhibiting growth by 50% was 179 nM and the total (6R)-CH2H4PteGlun was 2.3 pmol/10(6) cells. When cells were treated with (6S)-5-HCO-H4PteGlu for 24 h, the 50% inhibition concentration of FdUrd decreased with increasing concentrations of (6S)-5-HCO-H4PteGlu, and reached a plateau of 36 nM when (6S)-5-HCO-H4PteGlu was greater than or equal to 1 microM. The total (6R)-CH2H4PteGlun pools were augmented by (6S)-5-HCO-H4PteGlu dose dependently up to 6.8 pmol/10(6) cells at 1 microM (6S)-5-HCO-H4PteGlu. (6S)-5-HCO-H4PteGlu at 10 microM did not further increase the total (6R)-CH2H4PteGlun, but induced a marked shift in the polyglutamate chain length distribution, with an increase in tri- and tetra-, and a decrease in penta-, hexa-, and heptaglutamate. The down-shift of (6R)-CH2H4-PteGlun polyglutamate chain length observed after (6S)-5-HCO-H4PteGlu treatment did not impair the modulation of FdUrd cytotoxicity. Thus shorter chain (6R)-CH2H4PteGlua (n = 3-4) function as well as longer ones (n = 5-7). (6R)-5-HCO-H4PteGlu, at 200 microM, had no effect on the cytotoxicity of FdUrd, the total (6R)-CH2H4PteGlun level, or chain length distribution in the presence or absence of additional (6S)-5-HCO-H4PteGlu. These results suggest that the high plasma (6R)-5-HCO-H4PteGlu concentrations (up to 200 microM) achieved in patients following i.v. administration of high doses of (6R,S)-5-HCO-H4PteGlu probably do not have adverse effects on the modulation of antitumor activity of FdUrd or 5-fluorouracil. Since the optimal dose and schedule of (6S)-5-HCO-H4PteGlu for modulation of fluoropyrimidines may vary from one cell type to another, introducing high doses of (6R,S)-5-HCO-H4PteGlu in patients so that the plasma concentration of the natural isomer reaches 10 microM is still recommended.