16513 Background: Chronic lymphocytic leukemia (CLL) is an indolent leukemia in which there is an accumulation of immature malignant B-lymphocytes. While treatment with chlorambucil, a nitrogen mustard analogue or fludarabine can control the disease, eventually all patients become resistant to chlorambucil/fludarabine. Chlorambucil’s cytotoxicity is mediated by the interstrand crosslinks (ICLs) introduced into DNA. In mammalian cells, two main pathways are known to be involved in double strand break(DSB) repair: homologous recombinational repair (HRR) and nonhomologous endjoining (NHEJ). Methods: Our laboratory has demonstrated that resistance to chlorambucil involved accelerated repair of the ICLs associated with enhanced DNA repair, specifically enhanced nonhomologous endjoining (NHEJ) and homologous recombinational repair (HRR). We also recently demonstrated that gleevec inhibition of c-abl with resulting inhibition of Rad51-related HRR sensitizes CLL lymphocytes in vitro to chlorambucil. This exciting result has stimulated the development of a clinical protocol to test this combination in CLL. Furthermore, our laboratory has demonstrated that wortmannin, a nonspecific inhibitor of DNA-PK (a key component of NHEJ) sensitizes CLL lymphocytes to chlorambucil. Results: We are actually evaluating the effect of NU7026, a relatively specific DNA-PK inhibitor, in the sensitivity of CLL cells to chlorambucil. Our results indicate that in a CLL cell line (I83) and primary CLL-lymphocytes chlorambucil plus NU7026 have a synergistic cytotoxic effect. Noteworthy, the NU7026 doses used in combination with chlorambucil were not toxic to the cells when used alone. Moreover, chlorambucil treatment induced DNA-PK nuclear foci which were inhibited by NU7026 suggesting that the synergy of both drugs is mediated by Nu7026-inhibition of DNA-PK. Conclusion: NU7026 plus chlorambucil should be a useful combination to treat CLL. No significant financial relationships to disclose.
In Vitro Evidence for Homologous Recombinational Repair in Resistance to Melphalan
