Mechanisms of resistance to chlorambucil in chronic lymphocytic leukemia

Drug resistance and DNA repair in leukaemia

Most cytotoxic agents exert their action via damage of DNA. Therefore, the repair of such lesions is of major importance for the sensitivity of malignant cells to chemotherapeutic agents. The underlying mechanisms of various DNA repair pathways have extensively been studied in yeast, bacteria and mammalian cells. Sensitive and drug resistant cancer cell lines have provided models for analysis of the contribution of DNA repair to chemosensitivity. However, the validity of results obtained by laboratory experiments with regard to the clinical situation is limited. In both acute and chronic leukaemias, the emergence of drug resistant cells is a major cause for treatment failure. Recently, assays have become available to measure cellular DNA repair capacity in clinical specimens at the single-cell level. Application of these assays to isolated lymphocytes from patients with chronic lymphatic leukaemia (CLL) revealed large interindividual differences in DNA repair rates. Accelerated O(6)-ethylguanine elimination from DNA and faster processing of repair-induced single-strand breaks were found in CLL lymphocytes from patients nonresponsive to chemotherapy with alkylating agents compared to untreated or treated sensitive patients. Moreover, modulators of DNA repair with different target mechanisms were identified which also influence the sensitivity of cancer cells to alkylating agents. In this article, we review the current knowledge about the contribution of DNA repair to drug resistance in human leukaemia.

Drug induction apoptosis assay as predictive value of chemotherapy response in patients with B-cell chronic lymphocytic leukemia

A large number of prognostic factors are available to help predict the course of the disease for patients with B-cell chronic lymphocytic leukemia (B-CLL). However, it is not clear the involvement of these well established prognostic factors in the clinical response of the patients with B-CLL to the chemotherapy. The possible association of the patient clinical-biological characteristics and the in vitro response to chemotherapic agents may serve to provide powerful predictive information to identify optimum treatment for patients. An apoptosis induction assay displays the patient in vitro responses to chemotherapy and the possible association with their clinical-biological characteristics. In this study, patients showed a significant better in vitro response to drugs when they were in the initial stages of the disease or with low beta(2) microglobulin serum level. Response to purine analogues was significantly higher in patients with long lymphocyte doubling time (LDT), few cells expressing CD38, normal karyotype or no p53 deletion, whereas there was no correspondence with ZAP-70 expression. Furthermore, a good correlation was shown between in vitro apoptosis induction assay and the patient clinical response to purine analogues. In conclusion, association between in vitro drug sensitivity and some of the markers considered as prognostic factors could help to develop personalised therapeutic regimens for patients with B-CLL.

Comparison of infrared spectra of CLL cells with their ex vivo sensitivity (MTT assay) to chlorambucil and cladribine

The drug resistance of leukemic cells from 21 patients with chronic lymphocytic leukemia (CLL) to the alkylating agent chlorambucil (CLB) and the nucleoside analog cladribine or 2-chlorodeoxyadenosine (CdA) was investigated by infrared spectroscopy. Drug sensitivities, determined in vitro with the tetrazolium dye (MTT) assay, were correlated with the infrared spectra of the CLL cells, applying linear discriminant analysis (LDA). The 63 spectra (three from each of the 21 samples), obtained before drug exposure, were successfully partitioned into drug-sensitive and drug-resistant groups; the LDA-based ex vivo prediction of the sensitivity to CdA or CLB was 85.7% and 80.3%, respectively. Similar changes in the composition/structure of DNA were observed between the spectra of the drug-sensitive and drug-resistant CLL cells for both CdA and CLB. However, CdA-resistant CLL cells could also be differentiated from CdA-sensitive CLL cells by spectral changes associated with membrane lipids; these differences were much less pronounced between CLB-resistant and CLB-sensitive CLL cells. We demonstrate here for the first time that infrared spectroscopy can be used as a new tool for predicting ex vivo drug response (sensitivity/resistance).

P53, MDM-2, BAX and BCL-2 and drug resistance in chronic lymphocytic leukemia

Most antitumor agents exert their cytotoxic effect through the induction of apoptosis, and this process may be mediated through an elevation in p53 protein, with a subsequent increase in bax and decrease in bcl-2. p53 also increases mdm-2 expression and mdm-2 may then bind and inactivate p53. Cells from 31 patients with chronic lymphocytic leukemia (CLL) were treated in vitro with 2-chlorodeoxyadenosine (CdA), arabinosyl-2-fluoroadenine (F-ara-A), or chlorambucil (CLB) and drug sensitivity measured using the MTT assay. The protein levels of bax and bcl-2 were measured in CLL cells from 25 patients, and were found to be higher in leukemic cells than in normal B cells. The bcl-2 levels varied three-fold, the bax levels fifteen-fold, and the bax:bcl-2 ratios ranged from 0.44 to 2.91. The expression of mdm-2 mRNA was measured in CLL cells from 28 patients and was found to vary twenty-fold. However, no correlation was observed between drug sensitivity to CdA, F-ara-A, or CLB and the cellular levels of mdm-2 mRNA, or the protein levels of bax or bcl-2, or the bax:bcl-2 ratio. Treatment of CLL cells having wild type p53 with CdA, F-ara-A or CLB produced an increase in p53 protein and mdm-2 mRNA. This was not observed in cells having a p53 mutation, and these cells were highly resistant to both CLB and the nucleoside analogs. In contrast to the nucleoside analogs and CLB, dexamethasone and vincristine had no effect on mdm-2 mRNA levels. Treatment of CLL cells containing a wild type p53 gene with CdA, F-ara-A, or CLB, did not produce any consistent changes in bax or bcl-2. Thus, CdA, F-ara-A and CLB appear to act in CLL cells through a p53-dependent pathway, whereas this does not occur with dexamethasone or vincristine. The cellular levels of mdm-2, bcl-2, bax or the bax:bcl-2 ratios are not predictive indicators of clinical sensitivity in CLL, but an increase in mdm-2 levels after drug treatment is indicative of p53 function in these cells.

Chlorambucil in chronic lymphocytic leukemia: mechanism of action

Chronic lymphocytic leukemia (CLL) is the most common leukemia in Western countries but the clinical presentation and rate of disease progression are highly variable. When treatment is required the most commonly used therapy is the nitrogen mustard alkylating agent, chlorambucil (CLB), with or without prednisone. Although CLB has been used in the treatment of CLL for forty years the exact mechanism of action of this agent in CLL is still unclear. Studies in proliferating model tumor systems have demonstrated that CLB can bind to a variety of cellular structures such as membranes, RNA, proteins and DNA; however, DNA crosslinking appears to be most important for antitumor activity in these systems. In addition, a number of different mechanisms can contribute to CLB resistance in these tumor models including increased drug metabolism, DNA repair and CLB detoxification resulting from elevated levels of glutathione (GSH) and glutathione S-transferase (GST) activity. However, unlike tumor models in vitro, CLL cells are generally not proliferating and studies in CLL cells have raised questions about the hypothesis that DNA crosslinking is the major mechanism of antitumor action for CLB in this disease. CLB induces apoptosis in CLL cells and this appears to correlate with the clinical effects of this agent. Thus, alkylation of cellular targets other than DNA, which can also induce apoptosis, may contribute to the activity of CLB. Alterations in genes such as p53, mdm-2, bcl-2 and bax which control entry into apoptosis may cause drug resistance. Loss of wild-type p53 by mutation or deletion occurs in 10 to 15% of CLL patients and appears to correlate strongly with poor clinical response to CLB. The induction of apoptosis by CLB is paralleled by an increase in P53 and Mdm-2 but this increase in not observed in patients with p53 mutations indicating that with high drug concentrations CLB can produce cell death through P53 independent pathways. The level of Mdm-2 mRNA in the CLL cells is not a useful predictor of drug sensitivity. In addition, although Bax and Bcl-2 are important regulators of apoptosis and the levels of these proteins are elevated in CLL cells compared with normal B cells, the levels of Bax and Bcl-2, or the Bax:Bcl-2 ratio, are not important determinants of drug sensitivity in this leukemia. Finally, whereas CLB and nucleoside analogs may produce cell death in CLL by a P53 dependent pathway other agents, such as dexamethasone or vincristine, may act through P53-independent pathways.

The sensitivity of chronic lymphocytic leukaemia lymphocytes to irradiation in vitro

The inhibition of [3H]-thymidine incorporation into the DNA of mitogen-stimulated chronic lymphocytic leukaemia lymphocytes by chlorambucil or gamma-irradiation in vitro was measured in a series of patients, some of whom were untreated, some treated and some who were showing resistance to first-line or second-line treatment. There was evidence of resistance to irradiation developing in parallel with that to chlorambucil. The resistance to chlorambucil in chronic lymphocytic leukaemia (CLL) is not necessarily due to altered drug transport or metabolism but to a more fundamental process affecting DNA damage.

High expression of the multidrug resistance-associated protein (MRP) in chronic and prolymphocytic leukaemia

The expression of the multidrug resistance-associated protein (MRP), a new glycoprotein involved in drug resistance, was investigated in tumour samples from 80 patients with chronic B-cell malignancies by a quantitative RNase protection assay. In B-cell chronic lymphocytic leukaemia (B-CLL) (n = 32), either treated (n = 18) or untreated (n = 14), a high percentage of patients (20/32: 63%) had relatively high expression levels of the MRP gene (25U or more). In addition, hyperexpression of the MRP gene was demonstrated in 4/10 (40%) untreated patients with B-cell prolymphocytic leukaemia (B-PLL). In contrast, low MRP mRNA expression levels were detected in hairy cell leukaemia (n = 7), non-Hodgkin's lymphoma (n = 13) and multiple myeloma (n = 18). Statistical analysis of MRP expression in untreated CLL (mean +/- SD 29.2 +/- 18.5 U) versus treated CLL (mean +/- SD 26.7 +/- 13.7 U) did not show significant differences in MRP expression between the two groups. Southern blot analysis did not reveal amplification of the MRP gene in the leukaemia samples with elevated MRP mRNA levels. We conclude that B-PLL and B-CLL frequently display high MRP expression and that this hyperexpression is probably due to transcriptional activation and/or increased mRNA stability.

Multidrug resistance in leukaemia

Multidrug resistance hampers successful chemotherapy in many haematological neoplasms and is mediated by several cellular proteins. In some cases, the genes encoding these proteins have been shown to confer resistance on transfer to drug-sensitive cell lines. This is true for the efflux pump product of the MDR1 gene, P-170. Upregulation of enzymes such as GST has been observed, although the contribution of this enzyme in drug resistance expressed by malignant haematopoietic cells is still uncertain. Cells also appear to be able to downregulate enzymes which are drug targets. Examples include the decrease in Topo II which accompanies the resistance shown by cells to VP-16 and VM-26. Although many reports include both presentation and relapsed patients, there are few data on samples drawn from the same patients before and after chemotherapy. While P-170 and GST appear to be raised more often in cells from resistant and relapsed disease, it is quite clear that such mechanisms can be active in de novo malignancy and do not necessarily emerge as a consequence of prior chemotherapy. Methods of detecting drug resistance are reviewed here; these include in vitro cellular assays for drug toxicity, and molecular, immunological and functional detection of P-170 or Topo II. The clinical evaluation of such assays is only just beginning and some of the data are contradictory. To some extent, this may reflect the complex way in which the various resistance mechanisms may interact. Nevertheless, there are some encouraging early signs that the application of these assays to clinical material will yield valuable data on the relative contributions of these mechanisms and on ways in which they may be overcome. At present, much attention has focused on the potential of agents which prevent the P-170 efflux pump from exporting cytotoxics from the cell. This is likely to be only the first of new therapies arising from an improved understanding of multidrug resistance. More immediately, assays for multidrug resistance and its parameters may find their place as routine diagnostic and prognostic tools in the laboratory.