In vitro activity: Fludarabine Phosphate is converted to F-ara-ATP in cells and then incorporated into DNA in a self-limiting manner. Fludarabine Phosphate competes with dATP for incorporation into the A site of the extending DNA strand, which results in termination of DNA strand elongation. Human DNA polymerase α incorporates more Fludarabine Phosphate into DNA than polymerase δ. Fludarabine Phosphate completively inhibits DNA polymerase α and DNA polymerase δ with Ki of 1.1 μM and 1.3 μM, respectively. DNA polymerase δ is also able to excise the incorporated Fludarabine Phosphate from DNA in vitro.

Cell Assay: Cells (Human T lymphoblastoid cells) are incubated with Fludarabine Phosphate for 5 hr and washed twice with drug-free warm medium. 800 cells are mixed with 1.3 mL of 0.25% soft agar in Dulbecco’s medium supplemented with 20% fetal bovine serum (pre-warmed to 37 ℃) and incubated in a tissue culture dish for 10 days (humidified 5% CO2 , 37 ℃). At the end of the incubation period, colonies of more than 40 cells are scored under a microscope. The cytotoxic effect of the drugs is expressed as a percentage of survival relative to that of untreated control cells.