In B-CLL, non-proliferating B cells accumulate due to defective apoptosis. Cytotoxic therapies trigger apoptosis and deregulation of apoptotic pathways contributes to chemoresistance. Loss of the apoptosis-promoting Bax has been implicated in resistance to cytotoxic therapy. We therefore evaluated ex vivo drug sensitivity of CLL, producing chemoresponse data which are prognostic indicators for B-CLL, in particular in the case of purine nucleoside analogs. To analyze the underlying mechanisms of drug resistance, we compared endogenous Bax and Bcl-2 expression to ex vivo response to eight drugs, and to survival in 39 B-CLL patients. We found that reduced Bax levels correlated well with ex vivo resistance to traditional B-CLL therapies -anthracyclines, alkylating agents and vincristine (all P Ͻ 0.04). Surprisingly, no such relationship was observed for the purine nucleoside analogs or corticosteroids (all P Ͼ 0.5). Mutational analysis of p53 could not explain the loss of Bax protein expression. Levels of Bcl-2 were not associated with sensitivity to any drug. In contrast to the ex vivo data, neither Bax or Bcl-2 expression nor doxorubicin sensitivity were associated with increased survival whereas sensitivity to fludarabine correlated with better overall survival (P = 0.031). These findings suggest that the resistance to purine nucleoside analogs and corticosteroids in B-CLL is due to inactivation of pathways different from those activated by anthracyclines, vinca alkaloids and alkylating agents and may be the molecular rationale for the efficacy of purine analogs in this disease.

With a mean age at diagnosis for chronic lymphocytic leukaemia (CLL) of 65 years, development of optimal therapeutic regimens has been hampered by the advanced age of patients. In general, because of comorbidity older patients are not treated with the intent of achieving a complete response and so do not attain the quality of response of younger patients and do not survive as long. We have investigated whether or not ex vivo cellular sensitivity to cytotoxic drugs could be an underlying biological basis for this age differential in response and survival by comparing ex vivo drug response with age in untreated CLL patients. Cells from 365 untreated CLL patients aged 31.1-87.1 years (average 65.3 years) were tested for drug response by differential staining cytotoxicity (DiSC) assay with a panel of 10 drugs. An average of 280 results (range 196-361) obtained for each drug was compared with patient age. For chlorambucil, cyclophosphamide, prednisolone, vincristine, doxorubicin, epirubicin, fludarabine, cladribine and methylprednisolone, no relationship was found between ex vivo drug response and age (r Ͻ 0.12). For pentostatin, a possible but very weak relationship (r = 0.18; n = 210; P = 0.06) was found. We conclude that cellular sensitivity to cytotoxic drugs does not support the differential treatment of older and younger CLL patients. Leukemia (2000) 14, 712-715.