Fludarabine Inhibits K_V1.3 Currents in Human B Lymphocytes

Fludarabine (F-ara-A) is a purine analog commonly used in the treatment of indolent B cell malignancies that interferes with different aspects of DNA and RNA synthesis. K_V1.3 K⁺ channels are membrane proteins involved in the maintenance of K⁺ homeostasis and the resting potential of the cell, thus controlling signaling events, proliferation and apoptosis in lymphocytes. Here we show that F-ara-A inhibits K_V currents in human B lymphocytes. Our data indicate that K_V1.3 is expressed in both BL2 and Dana B cell lines, although total K_V1.3 levels were higher in BL2 than in Dana cells. However, K_V currents in the plasma membrane were similar in both cell lines and were abrogated by the specific K_V1.3 channel inhibitor PAP-1, indicating that K_V1.3 accounts for most of the K_V currents in these cell lines. F-ara-A, at a concentration (3.5 μM) similar to that achieved in the plasma of fludarabine phosphate-treated patients (3 μM), inhibited K_V1.3 currents by 61 ± 6.3% and 52.3 ± 6.3% in BL2 and Dana B cells, respectively. The inhibitory effect of F-ara-A was concentration-dependent and showed an IC value of 0.36 ± 0.04 μM and a n value of 1.07 ± 0.15 in BL2 cells and 0.34 ± 0.13 μM (IC ) and 0.77 ± 0.11 (n ) in Dana cells. F-ara-A inhibition of plasma membrane K_V1.3 was observed irrespective of its cytotoxic effect on the cells, BL2 cells being sensitive and Dana cells resistant to F-ara-A cytotoxicity. Interestingly, PAP-1, at concentrations as high as 10 μM, did not affect the viability of BL2 and Dana cells, indicating that blockage of K_V1.3 in these cells is not toxic. Finally, F-ara-A had no effect on ectopically expressed K_V1.3 channels, suggesting an indirect mechanism of current inhibition. In summary, our results describe the inhibitory effect of F-ara-A on the activity of K_V1.3 channel. Although K_V1.3 inhibition is not sufficient to induce cell death, further research is needed to determine whether it might still contribute to F-ara-A cytotoxicity in sensitive cells or be accountable for some of the clinical side effects of the drug.