The influence of 2-chloro-2'-deoxyadenosine on metabolism of deoxyadenosine in human primary CNS lymphoma

New insights into the synergism of nucleoside analogs with radiotherapy

Nucleoside analogs have been frequently used in combination with radiotherapy in the clinical setting, as it has long been understood that inhibition of DNA repair pathways is an important means by which many nucleoside analogs synergize. Recent advances in our understanding of the structure and function of deoxycytidine kinase (dCK), a critical enzyme required for the anti-tumor activity for many nucleoside analogs, have clarified the mechanistic role this kinase plays in chemo- and radio-sensitization. A heretofore unrecognized role of dCK in the DNA damage response and cell cycle machinery has helped explain the synergistic effect of these agents with radiotherapy. Since most currently employed nucleoside analogs are primarily activated by dCK, these findings lend fresh impetus to efforts focused on profiling and modulating dCK expression and activity in tumors. In this review we will briefly review the pharmacology and biochemistry of the major nucleoside analogs in clinical use that are activated by dCK. This will be followed by discussions of recent advances in our understanding of dCK activation via post-translational modifications in response to radiation and current strategies aimed at enhancing this activity in cancer cells.

Two Cases of Cerebral Involvement in Malignant Lymphoma (CD20+) That Responded to Combination Therapy with Rituximab and Cladribine

Cerebral involvement frequently occurs in association with progression or relapse of malignant lymphoma. Chemotherapy with cyclophosphamide, doxorubicin, vincristine, and prednisolone, the standard chemotherapy for malignant lymphoma, is an ineffective treatment for cerebral involvement because these drugs cannot cross the blood-brain barrier. Therefore, various alternative strategies have been attempted. Although high-dose methotrexate combined with whole-brain radiotherapy is widely used to treat primary central nervous system lymphoma, there is no standard therapy to treat cerebral involvement in malignant lymphoma. Furthermore, high-dose methotrexate in combination with whole-brain radiotherapy is not always effective, and high rates of neurotoxicity are often observed, particularly in the elderly. To expand the therapeutic options for central nervous system involvement in recent years, systemic chemotherapies, including rituximab, high-dose methotrexate, and other agents that act during the S, G2, and M phases of the cell cycle, have been attempted. In our hospital, cladribine, a purine analogue with a cytocidal effect on resting malignant cells (G0/G1 phase of the cell cycle), has been used in combination with rituximab, which exhibits antitumor effects on nodal and extranodal lesions of relapsed and/or refractory B cell lymphomas, particularly cerebral lesions. Here, we report 2 representative cases of patients who were treated with cladribine plus rituximab and survived for 30 months (died of sepsis) and 52 months (still alive), respectively. The outcomes of these cases suggest that cladribine plus rituximab combination therapy with whole-brain radiotherapy may be very useful as salvage therapy for secondary central nervous system lymphoma and as initial therapy for primary central nervous system lymphoma.

Purine metabolism in leukocytes and erythrocytes in Graves' or Hashimoto's disease

INTRODUCTION

Adenosine deaminase (ADA), purine nucleoside phosphorylase (PNPase), S-adenosylhomocysteine hydrolase (SAHH), 5'-nucleotidase (5N), and deoxycytidine kinase (dCK) are involved in purine salvage metabolism. Changes of the activities of the above enzymes have been observed in blood cells in patients with immunological disorders.

MATERIALS AND METHODS

The activities of ADA, PNPase, SAHH, 5'N, and dCK in lysates of leukocytes and erythrocytes, obtained from patients with Graves' or Hashimoto's disease, were measured, using chromatographic analysis. Serum concentrations of antithyroglobulin (Tg Ab) and antithyroperoxidase (TPO Ab) antibodies were measured by an immunoenzymatic method.

RESULTS

(1) ADA activity in leukocytes, obtained from patients with Hashimoto's disease, was significantly higher than in control leukocytes, as well as in leukocytes from patients with Graves' disease; (2) dCK activities in leukocytes from patients with both Graves' and Hashimoto's diseases were approximately four and five times higher, respectively, than in leukocytes of control subjects; (3) a positive correlation was observed between dCK activity in leukocytes and serum Tg Ab concentration in patients with Graves' disease. In conclusion, the increased ADA and dCK activities in leukocytes from patients with Graves' and Hashimoto's diseases may be regarded as indicators of autoimmunological thyroid diseases.

Induction of apoptosis by 2-chloro-2'deoxyadenosine (2-CdA) alone and in combination with other cytotoxic drugs: synergistic effects on normal and neoplastic lymphocytes by addition of doxorubicin and mitoxantrone

2-CdA is active as a single agent in the treatment of low-grade lymphomas. We analyzed the induction of apoptosis by 2-CdA alone (n=5) and in combination with other drugs in peripheral lymphocytes from 25 patients with leukemic low-grade lymphomas and from 25 healthy volunteers. 2-CdA was tested in 4 escalating concentrations (0.05 microg/ml to 0.4 microg/ml). Linear regressions showed a dose dependent apoptosis rate of 0.29 x microg 2-CdA/ml + 0.11 (r2=0.88, p=0.006) in normal cells and 0.41 x microg 2-CdA/ml + 0.15 (r2=0.88, p=0.005) in leukemic cells. Intracellular metabolization of 2-CdA into 2-CdA-5'mono-, -di- and the active metabolite -triphosphate was analyzed by HPLC and paralleled the dose dependent increase of apoptosis. The combination of 2-CdA with doxorubicin or mitoxantrone had a synergistic effect on the induction of apoptosis (p<0.001) in both normal and neoplastic lymphocytes, whereas 2-CdA plus etoposide or cytosine arabinoside were only additive. Due to the flat slope of the dose response of 2-CdA concentration on apoptosis we assume that higher in vivo dosages of 2-CdA in the treatment of low-grade lymphomas may not result in a higher clinical efficacy. The synergistic lymphocytotoxic effect of 2-CdA combined with doxorubicin or mitoxantrone may be relevant for new treatment approaches.

Distinct inhibitory effects of 2-chloro-2'-deoxyadenosine and 9-beta-D-arabinosyl-2-fluoroadenine on DNA methyltransferase in human T-lymphocytes

The effect of 2-chloro-2'-deoxyadenosine and 9-beta-D-arabinosyl-2-fluoroadenine on DNA methyltransferase activity in stimulated human T-lymphocytes was estimated. In comparative studies 5-aza-deoxycytidine and deoxyadenosine plus deoxycoformycin were used. These antileukemic compounds demonstrated different effects; both 2CdA and dAdo plus dCF, like 5-aza-dCyt, inhibited the enzyme activity by 85-90% after 72 hours activation of lymphocytes, while the effect of F-ara-A, under the same conditions, was insignificant.

Activation of deoxycytidine kinase during inhibition of DNA synthesis by 2-chloro-2'-deoxyadenosine (Cladribine) in human lymphocytes

Deoxycytidine kinase (dCK, EC.2.7.1.74), a key enzyme in intracellular metabolism of many antileukemic drugs, was shown to be activated during treatment of lymphocytes by 2-chloro-2'-deoxyadenosine (Cl-dAdo, cladribine), a potent inhibitor of DNA synthesis. While 5-[3H]-thymidine (TdR) incorporation into DNA was decreased by 80-90%, dCK activity was doubled as a consequence of incubating the cells with 1 microM 2-chloro-2'-deoxyadenosine. Thymidine kinase (dTK, EC.2.7.1.21) activity was slightly decreased under the same conditions, similarly to 5-[3H]-thymidine incorporation. dCK activation could not be prevented by cycloheximide, and neither the amount of dCK protein nor its mRNA level was increased after 2-chloro-2'-deoxyadenosine treatment. These results suggest a post-translational activation of dCK protein during inhibition of DNA synthesis.

Apoptosis by 2-chloro-2'-deoxy-adenosine and 2-chloro-adenosine in human peripheral blood mononuclear cells

Adenosine has profound effects on immune cells and has been implicated in the intrathymic apoptotic deletion of T-cells during development. In order to characterize adenosine effects on quiescent peripheral blood mononuclear cells (PBMC), we have evaluated the ability of the previously characterized adenosine receptor agonist 2-chloro-adenosine (2CA; Ceruti, Barbieri et al., 1997) and of the antineoplastic drug 2-chloro-2'-deoxy-adenosine (2CdA, cladribine) to trigger apoptosis of PBMC. Apoptosis was assessed by morphological changes, DNA fragmentation by agarose gel electrophoresis and appearance of hypodiploid DNA peak by flow cytometry. 2CA (10 microM) and 2CdA (1 microM) induced apoptosis in human PBMC, which are relatively insensitive to apoptosis. For both agents, the effect was concentration- and time-dependent, although 2CdA induced apoptosis more potently than 2CA. Evaluation of mitochondrial function in parallel samples using the mitochondrial membrane-potential-specific dye JC-1 showed that mitochondrial damage followed the same kinetics as apoptosis, hence an early damage of mitochondria is likely not responsible for adenosine-induced death of PBMC. The effect of 2CA was partially prevented by addition of dipyridamole (DP), a nucleoside transport inhibitor, hence some of the apoptotic effect of this nucleoside is, at least in part, due to intracellular action. Alternatively, DP did not affect 2CdA-induced apoptosis, suggesting that 2CdA may enter cells via a DP-insensitive transporter. 5-Iodotubercidin (5-Itu), a nucleoside kinase inhibitor, was also able to partially prevent the action of 2CA and was not able to affect 2CdA-induced apoptosis, suggesting a different role for phosphorylation in 2CA- vs 2CdA-induced apoptosis. To test the role of P1 receptors, agonists and antagonists selective at various P1 receptor subtypes were used. Data suggest that, for 2CA, apoptosis is partially sustained by activation of the A2A receptor subtype, whereas no role is exerted by P1 receptors in 2CdA-dependent apoptosis. Moreover, in these cells, apoptosis could also be triggered through intense activation of the A3 receptor via selective agonists such as 2-chloro-N6-(3-iodobenzyl)adenosine-5'-N-methyluronamide (Cl-IB-MECA), but this mechanism plays no role in either 2CA- or 2CdA-induced apoptosis. On the whole, our results suggest that 2CA and 2CdA follow different pathways in inducing apoptosis of immune cells. Moreover, our data also suggest that there are at least three different ways by which adenosine derivatives may induce apoptosis of human PBMC: (i) through an A2A-like extracellular membrane receptor; (ii) through entry of nucleosides into cells and direct activation of intracellular events involved in the apoptotic process; or (iii) through activation of the A3 receptor.