The effect of imatinib on chlorambucil (CLB) cytotoxicity in chronic lymphocytic leukemia (CLL) lymphocytes was examined in vitro. Imatinib sensitizes the WSU and I83 human CLL cell lines, 10- and two-fold, respectively, to CLB. Furthermore, in primary cultures of malignant B-lymphocytes obtained from 12 patients with CLL (seven patients were untreated and five treated with CLB), imatinib synergistically sensitized these lymphocytes from two- to 20-fold to CLB. This synergistic effect was observed at concentrations of imatinib (</=10 microM), which are achievable in patients with minimal toxicity. Moreover, the combination of both drugs results in increased apoptosis in CLL cell lines. These results suggest that imatinib should be useful in improving the therapeutic index of CLB in CLL. The mechanism of action appears to involve imatinib inhibition of c-abl kinase activity with an associated decrease in CLB-induced Rad51 phosphorylation and CLB-induced Rad51 nuclear foci, suggesting that imatinib decreases Rad51-related DNA repair of CLB-induced DNA lesions. Altogether, our results suggest that imatinib is a promising adjuvant therapy to CLB treatment of CLL.