Purpose
To investigate the impact of 5-formytetrahydrofolate on the activities of pralatrexate, as compared to methotrexate (MTX), in vitro.
Methods
Cells were exposed to (6S)5-formyltetrahydrofolate (5-formylTHF) for 24h, before or after a 6h exposure to antifolates following which the cellular accumulation and activities of the drugs were evaluated in HeLa cells.
Results
A 24h delay between a 6h exposure to antifolates and a subsequent 24h exposure to 4 μM 5-formylTHF sustained the full activities of both antifolates. A 72h interval was required between a single exposure of up to 4 μM 5-formylTHF and subsequent exposure to drugs to sustain activities of the antifolates. When cells were incubated with 4 μM 5-formylTHF for 24h weekly, for 4 weeks, there was no significant increase in the IC50 for pralatrexate, but the MTX IC50 increased 2.5-fold as compared to cells growing continuously in 25nM 5-formylTHF. This cyclical exposure to 5-formylTHF increased the cell folate pool by 16%, had no significant effect on the intracellular pralatrexate level, but decreased intracellular MTX by 15%. An extracellular concentration of methotrexate 50-fold higher than that of pralatrexate was required to achieve an intracellular level, and growth inhibition, comparable to that of pralatrexate.
Conclusions
Cyclical exposures to 5-formylTHF at levels in excess of what is achieved in most clinical “rescue” regimens do not affect pralatrexate accumulation nor antitumor activity in HeLa cells, in contrast to MTX. An important element in preserving pralatrexate activity is achieving a sufficient interval between exposure to 5-formylTHF and the next dose of antifolate.