Apoptosis of CD34+ cells after incubation with sera of leukopenic patients with systemic lupus erythematosus.

Leukopenia and anaemia are observed in about a fifth of all patients with systemic lupus erythematosus (SLE) and may be due either to the destruction of blood cells or their decreased production. The former may be humoral or cell-mediated or result from apoptosis of peripheral blood cells. Several observations suggest the occurrence of the latter reduced in vitro proliferation of pluripotent bone marrow progenitors from the bone marrow aspirates of SLE patients,reduced counts of CD34+ cells in bone marrow aspirates in SLE patients, apoptosis of lymphopoietic progenitors and apoptosis of bone marrow cells. The aim of our study was to investigate whether humoral factors may induce suppression of haematopoiesis by increased apoptosis of CD34+ cells. For this purpose, we incubated allogeneic CD34+-enriched cells with sera of 18 leukopenic SLE patients. Apoptosis was induced by four of 18 sera. This effect was independent of complement-inhibition and FAS-blockade. Although reduced proliferation of autologous pluripotent bone marrow progenitors has been attributed to an IgG serum inhibitor, removal of IgG from these four proapoptotic sera had no effect on apoptosis of allogeneic CD34+ cells. The proapoptotic effect was associated with high titres of anti-dsDNA antibodies and low haemoglobin concentrations, but not with high titres of antinuclear antibodies, TNF-alpha and IFN-alpha of the sera tested.

New rearrangement pattern after treatment of hairy-cell leukemia with 2-chlorodeoxyadenosine.

Leukemic hairy cells are clonally proliferating B-lymphoid cells with clonal rearrangements of genes for immunoglobulin chains. We describe a patient with a new hairy-cell clone after treatment with 2-chlorodeoxyadenosine (2-CdA). In this patient, a single course of 2-CdA resulted in good partial remission of hairy-cell leukemia, but Southern blot analysis of bone marrow biopsies and polymerase chain reaction using seminested amplifications with consensus primers revealed a new rearranged band 4 months after therapy with 2-CdA. Four years after therapy, the patient is in complete clinical remission and both bands disappeared during follow-up. The new rearranged band might have been related to prior treatment of hairy-cell leukemia with 2-CdA.

Differential effects of cladribine and gemcitabine on erythroid and granulocytic progenitors from patients with chronic myeloid leukemia.

Chronic myeloid leukemia (CML) is a clonal neoplastic disease that originates in a pluripotent stem cell. Selection of normal progenitors by graft-purging may improve the outcome after autologous transplantation. In our methylcellulose assays, the nucleoside analogs cladribine (2-CdA) and gemcitabine (dFdC) showed more prominent inhibitory effects on CML than normal bone marrow (BM) progenitors. For dFdC, however, long-term incubations were necessary to achieve complete inhibition. Deoxycytidine kinase, the key enzyme of both 2-CdA and dFdC metabolisms, was only partially responsible for this differential sensitivity. We suggest that 2-CdA and dFdC might be helpful in purging of CML BM cells before autologous BM transplantation. Further studies on more primitive cells are warranted.

Gemcitabine--a novel immunosuppressive agent--prevents rejection in a rat cardiac transplantation model.

BACKGROUND: 2',2' -difluorodeoxycytidine (dFdC, gemcitabine) is a pyrimidine antimetabolite with antineoplastic activity against a wide range of solid tumors. The immunosuppressive activities of this compound have not been described to date. METHODS: The in vitro effects on activated T lymphocytes were studied with a lymphocyte colony-forming assay in a microagar culture system. Heart transplantations were performed in the fully allogeneic Lewis/ Brown Norway combination. dFdC was administered once daily at various dosages from the time of surgery until day 50. RESULTS: Phytohemagglutinin-induced lymphocyte proliferation was inhibited 50% by dFdC at a concentration of 3.25+/-0.9 nmol/L. Allografts of untreated animals survived for 7.5 (7-8) days and those with 25, 50, and 75 microg/kg body weight dFdC for 7.3 (7-8), 9.3 (8-10), and 16.3 (10-38) days, respectively. Treatment with 100 or 125 microg/kg body weight of dFdC, however, prolonged allograft survival until day 152.8 (129-178). Dose-dependent leukopenia was the main toxicity. CONCLUSIONS: DFdC is a new immunosuppressive agent that can successfully prevent cardiac rejection in a rat transplantation model.

Lack of cross-resistance with gemcitabine and cytarabine in cladribine-resistant HL60 cells with elevated 5'-nucleotidase activity.

Cross-resistance patterns between chemotherapeutic agents have implications for the treatment of hematologic and other diseases. Previous in vitro models have shown cross-resistance between the purine analog 2-chlorodeoxyadenosine (cladribine) and the pyrimidine analogs 2',2'-difluorodeoxycytidine (gemcitabine) and 1-beta-D-arabinofuranosylcytosine (cytosine arabinoside, cytarabine) with reduced deoxycytidine kinase (dCK) activity as the underlying determinant of resistance. In this study, we continuously exposed the human promyelocytic leukemia cell line HL60 to as much as 1024 nM cladribine. After limiting dilution, the cladribine concentrations that caused 50% growth inhibition (IC50) of the two clones R13 and R23 were 33.3- and 18.7-fold, respectively, higher than the IC50 of the parental HL60 cells (8.7+/-1.3 nM). These cladribine-resistant clones, however, showed no cross-resistance to gemcitabine and only 3.3- and 2.7-fold resistance to cytarabine, respectively. Characterization of both clones revealed stably elevated levels of purine-specific "high-Michaelis constant (Km)" 5'-nucleotidase (5'-NT) messenger RNA expression and specific activity, whereas pyrimidine-specific "low-Km" 5'-NT activity was undetectable, and dCK activity was only marginally decreased in R13. Thus, the ratio of dCK (specific for cladribine) to high-Km 5'-NT activity in R13 and R23 was reduced to 65.3% and 63.7%, respectively. These results show that changes of high-Km 5'-NT activity can induce cladribine resistance, without cross-resistance to gemcitabine.

Antitumor effect of the nucleoside analogs 2-chlorodeoxyadenosine and 2',2'-difluorodeoxycytidine on human hepatoma HepG2 cells.

BACKGROUND/AIMS: Hepatocellular carcinoma is one of the most malignant tumors in the world. Although a wide range of therapeutic options is available, the efficacy of these methods and the prognosis of hepatocellular carcinoma are still very poor. The nucleoside analogs 2-chlorodeoxyadenosine (Cladribine, 2-CdA) and 2',2'-difluorodeoxycytidine (Gemcitabine, dFdC) have shown potent cytotoxic effects on various human tumor cell lines in vitro and marked therapeutic efficacy in the treatment of lymphoproliferative disorders and several solid tumors in vivo. In the present study we evaluated the antitumor effect of 2-CdA and dFdC on human hepatoma HepG2 cells. METHODS: HepG2 cells were grown in the absence and presence of increasing concentrations of 2-CdA and dFdC. Antitumor activity was assessed by inhibition of cell growth, evaluated by counting cell numbers in a hemocytometer and by 3H-thymidine uptake, and by reduction of cell viability as determined by exclusion of 0.1% trypan blue. For rescue experiments, the natural pyrimidine deoxycytidine (dCyd) was added simultaneously or delayed. RESULTS: A strong antitumor activity was observed for both compounds. dFdC showed a more pronounced effect with an inhibition constant (IC50) of 3.98+/-0.03 nM in comparison to 2-CdA with an IC50 of 16.66+/-0.40 nM. Both drugs achieved their half-maximal antitumor activity after 31 h. With respect to dFdC, fractionated daily administrations showed a distinctly greater antitumor activity than a single transient administration. The cytotoxic effects of 2-CdA and dFdC were completely reversed by simultaneous addition of dCyd. CONCLUSION: In this paper we show strong antitumor effects of the nucleoside analogs 2-CdA and dFdC on the human hepatoma cell line HepG2. These findings suggest that both compounds, but in particular dFdC, are promising substances for further evaluations in the treatment of hepatocellular carcinoma.

2-Chlorodeoxyadenosine (cladribine) in combination with low-dose cyclosporin prevents rejection after allogeneic heart and liver transplantation in the rat.

The purine analogue 2-chlorodeoxyadenosine (2-CDA) has been shown to possess synergistic immunosuppressive properties when given together with cyclosporin (CSA) in a rat small bowel transplant model. The present study investigated the immunosuppressive potency of 2-CDA alone and in combination after liver or heart transplantation in a fully allogeneic rat model with 5 animals in each group. Immunosuppression was provided with CSA 10 mg/kg body weight (BW)/day orally or 2-CDA 0.1 mg/kg/BW day intravenously or both compounds together in the dosages mentioned. Animals were sacrificed on day 10 following transplantation, and graft histology was assessed. In addition, cardiac graft function was evaluated by palpation immediately prior to sacrificing the animal. CSA given alone was able to mitigate but not prevent rejection. 2-CDA alone did not exhibit any detectable immunosuppressive effect. When CSA was combined with 2-CDA, no rejection was seen in 80% of the liver allografts and in 60% of heart allografts, and only mild rejection was observed in the remaining animals. All hearts of the combined treatment group, however, beat strongly. From these findings it is concluded that 2-CDA alone has no, but together with CSA a strong immunosuppressive effect in preventing solid organ allograft rejection.

Urticaria pigmentosa: a clinical, hematopathologic, and serologic study of 30 adults.

Urticaria pigmentosa (UP) is the most common form of cutaneous mastocytosis and may be associated with systemic involvement, most often of the bone marrow. The incidence of systemic involvement is not yet well established, however. To address this question, we subjected a group of 30 adults with histologically proved UP to a retrospective study that included history, physical examination, laboratory tests including cytokine measurements, radiologic examinations, and bone marrow biopsies. The most frequently associated clinical symptoms were recurrent flush episodes in 16 of 30 patients, alcohol intolerance in 13, pruritus in 10, and gastrointestinal problems in 11 (recurrent diarrhea, 8 patients; gastritis, 2 patients; and history of peptic ulcer, 1 patient). Of the 30 patients, 18 (60%) had mast cell infiltrates of the bone marrow (nodular type, 10 patients; diffuse interstitial type, 8 patients). Bone marrow involvement was not correlated with massive cutaneous mast cell infiltration, clinically or histologically, or with the incidence of clinical symptoms and associated hematologic disorders. None of the patients had experienced progression of clinical symptoms, skin or organ involvement, or development of hematologic malignant neoplasms since UP was first diagnosed (10 years on average). Urticaria pigmentosa was found associated with mast cell infiltration of the bone marrow in 18 patients (60%). However, bone marrow involvement does not seem to predict adverse clinical course.

A comparison of the antiproliferative properties of recombinant human IFN-alpha 2 and IFN-omega in human bone marrow culture.

We compared the antiproliferative effects in vitro of recombinant preparations of interferon-alpha 2c (IFN-alpha 2) and IFN-omega on the formation of colonies from bone marrow progenitor cells. Both IFNs led to statistically indistinguishable dose-dependent inhibitory effects when tested on bone marrow cells derived from 8 normal donors or from 7 patients with chronic myelogenous leukemia (CML). With both IFNs, the cells from CML patients appeared slightly but not significantly more sensitive to inhibition than the cells from normal donors. These results suggest that under some circumstances, IFN-omega may prove an effective treatment for CML, for example, in those becoming resistant to IFN-alpha 2 because of the formation of neutralizing antibodies.

T cells and natural killer cells after treatment of hairy cell leukaemia with 2-chlorodeoxyadenosine.

We report that continuous suppression of CD4+ lymphocyte subsets does not necessarily lead to an increased rate of infections more than 6 months after treatment of hairy cell leukemia (HCL) with 2-chlorodeoxyadenosine (cladribine; 2-CdA). In a retrospective analysis of 17 consecutive patients with HCL treated with 2-CdA and followed thereafter for an average of 440 days (ranging from 71 to 1,046 days), all lymphocyte subsets significantly decreased after therapy. However, natural killer (NK) cells increased to 203 cells/microliter during the following 4 months, whereas CD3+ and CD4+ T cell subsets did not reach pretreatment values even more than 1 year after 2-CdA therapy. In our study group, 5 opportunistic infections occurred during the first 2 weeks after treatment with 2-CdA, and no severe infections were registered after this early leukopenic phase. A review of the current literature revealed that at least 56 infections occurred in 158 patients repeatedly treated with 2CdA for low-grade lymphoproliferative diseases other than HCL (= 35%). In patients with HCL, the rapid increase of NK cell counts after 6 months might be responsible for reducing the risk of severe infections after the early leukopenic phase despite CD4+ cell suppression.

Local application of antimycotics in mucormycosis cerebri: a case report.

A 65 year old man was found to have mucormycosis cerebri during immunosuppression after treatment of hairy cell leukaemia with 2-chlorodeoxyadenosine. Although mucormycosis cerebri has a poor prognosis, the patient survived after systemic administration of high dose amphotericin B, extensive excision of the abscess, and additional local application of amphotericin B with the help of an absorbable gelatin sponge.

2',2'-Difluorodeoxycytidine (gemcitabine) induces apoptosis in myeloma cell lines resistant to steroids and 2-chlorodeoxyadenosine (2-CdA).

The paucity of effective cytotoxic agents for the treatment of steroid resistant multiple myeloma explains the ongoing search for alternative substances for chemotherapy of this disease. In the present study, the purine antagonist 2-chlorodeoxyadenosine (2-CdA, cladribine) and the pyrimidine antagonist 2',2'-difluorodeoxycytidine (gemcitabine) were tested on four myeloma cell lines (i.e., U 266, OPM 2, RPMI 8226, IM 9), one plasma cell leukemia cell line (HS Sultan) and a myeloid control cell line (HL 60), all of which are resistant to 10-6 M dexamethasone. Gemcitabine has been found to be promising in the chemotherapy of other tumors with low proliferative activity, but its effectiveness against myeloma cells has not been analyzed so far. In our tests, gemcitabine induced a significant degree of apoptosis in all cell lines investigated. After incubation for 48 h with 10 microM gemcitabine, the median numbers of apoptotic cells were in the range of 45% in the OPM 2 and 79% in the U 266 cell line. All of the investigated cell lines were responsive to concentrations of 10 microM gemcitabine even after an exposure of only 30 min, three of them (U 266, HS Sultan, IM 9) also responded to a concentration of 10 nM. Higher concentrations and longer exposure times were necessary to suppress the growth of normal hematopoietic bone marrow progenitor cells. In contrast to gemcitabine, standard concentrations of 2-CdA (i.e., 30 and 300 nM) failed to induce a significant degree of apoptosis in the cell lines investigated but inhibited the growth of myeloid progenitor cells. The results suggest that gemcitabine induces apoptosis in myeloma and plasma cell leukemia lines resistant to steroids and 2-CdA. The fact that tumor cell apoptosis was achieved at concentrations clinically achievable and tolerable, which at the same time do not inhibit the growth of normal CFU-GM progenitor cells, favors the initiation of phase I trials with this drug for the treatment of multiple myeloma.

Inhibitory effects of the nucleoside analogue gemcitabine on prostatic carcinoma cells.

Gemcitabine (2',2'difluoro-2'deoxycytidine, dFdC) is a synthetic antimetabolite of the cellular pyrimidine nucleotide metabolism. In a first series of in vitro experiments, the drug showed a strong effect on the proliferation and colony formation of the human androgen-sensitive tumor cell line LNCaP and the androgen-insensitive cell lines PC-3 and DU-145. Maximal inhibition occurred at a dFdC concentration as low as 30 nM. In contrast to the cell lines which were derived from metastatic lesions of prostate cancer patients, no inhibitory effects were found in normal primary prostatic epithelial cells at concentrations up to 100 nM. The effect of gemcitabine was reversed by co-administration of 10-100 microM of its natural analogue deoxycytidine. In view of a future clinical application of this anti-tumor drug in advanced prostatic carcinoma, we have compared the effect of gemcitabine on prostatic tumor cells with that on bone marrow granulopoietic-macrophage progenitor cells, because neutropenia is a common side effect of gemcitabine treatment. The time course of action on the two kinds of cells was markedly different. Colony formation of tumor cells was inhibited by two thirds at a gemcitabine concentration of about 3.5 nM. The same effect on granulopoietic-macrophagic progenitor cells required a concentration of 9 nM. Co-administration of deoxycytidine to gemcitabine-treated tumor cell cultures completely antagonized the effect of gemcitabine whereas addition of deoxycytidine after 48 hr of gemcitabine treatment could not prevent gemcitabine action on the tumor cells. In contrast, more than half of the granulopoietic-macrophagic progenitor cells could still be rescued by deoxycytidine administration after 48 hr. These findings and the marked difference in the susceptibility of neoplastic and normal prostatic cells suggest that gemcitabine is a promising substance which should be further evaluated as to its efficacy in the treatment of advanced prostatic carcinoma.

Growth-inhibitory effect of 2-CdA on myeloid CFU-GM progenitors in normal human long-term bone marrow cultures and its compensation by G-CSF or IL-3.

As neutropenia is a common side effect of treatment with 2-chlorodeoxyadenosine (2-CdA), we investigated the myelosuppressive action of 2-CdA in Dexter-type human long-term bone marrow cultures (LTBMCs). LTBMCs were incubated with varying doses of 2-CdA (5 to 20 nM/L) during the first week. At 20 and 10 nM/L 2-CdA, we found a marked reduction in colony-forming unit-granulocyte/macrophage (CFU-GM) production throughout the culture period of 7 weeks (maximum reduction to 3.5% of untreated control cultures with 20 nM/L and to 27.2% with 10 nM/L, respectively). Even the lowest 2-CdA dose tested (5 nM/L) strongly reduced the number of CFU-GM progenitors during the first 3 weeks (maximum reduction to 52.4% of untreated controls), but this effect was transient, and values had recovered to normal within in 5 weeks. 2-CdA was also shown to cause a dose-dependent decrease in long-term culture-initiating cell (LTCIC) detections after 5 weeks in culture (49.6% of control cultures with 10 nM/L 2-CdA and 14% with 20 nM/L 2-CdA, respectively). When 2-CdA was added to LTBMCs initiated on preformed irradiated stromal feeder layers, similar results on CFU-GM production were obtained, indicating that the effects observed were not secondary to effects on the formation of a supportive layer. In addition, IL-6-concentrations in the supernatant of LTBMCs measured at various intervals after the addition of fresh medium with or without 2-CdA showed no significant decrease in cultures treated with 2-CdA. As neutropenia has been shown to be associated with a small but significant risk of fatal infection, we subsequently investigated the reversal potential of the 2-CdA effect by addition of recombinant human granulocyte colony-stimulating factor (rhG-CSF) or rh interleukin-3 (rhIL-3). The weekly addition of 100 ng/mL rhG-CSF counteracted the 2-CdA-mediated decrease in CFU-GM numbers during the entire period of 7 weeks, reaching statistical significance from weeks 3 to 7 (p < 0.05). Addition of rhIL-3 (100 ng/mL) showed an enhancement of CFU-GM output in 2-CdA-treated cultures that resulted in their numbers exceeding those in control cultures (without 2-CdA) from weeks 1 to 5 (p < 0.05) with a maximum increase of 5.1-fold over the parallel control value at week 3.(ABSTRACT TRUNCATED AT 400 WORDS)

Minimal residual disease in hairy-cell leukemia after treatment with 2-chlorodeoxyadenosine.

Some patients in apparent complete remission of hairy cell leukemia (HCL) after 2-chlorodeoxyadenosine (2-CdA) treatment may have minimal residual disease (MRD). This study examines detection of minimal residual disease by immunohistological staining using the monoclonal antibody (MoAb) B-ly 7 in 11 patients with complete remission of hairy cell leukemia (HCL) after 2-CdA therapy administered between 1990 and 1993. In all 11 cases, residual hairy cells could be detected by MoAb B-ly 7 (0.1 to 7.5%, median 0.65%). At a follow-up period of 7 - 29 months (median 19.3), 9 of these patients remained in complete remission, whereas 2 patients relapsed 22 and 27 months after 2-CdA therapy. To determine whether flow-cytometric analysis of hairy-cells in bone marrow aspirates and peripheral blood cells are comparable with results obtained by immunostaining with B-ly 7 in bone marrow biopsies, available data using CD-19/CD-11c double-staining were analyzed. In 5 of 10 cases no hairy-cells could be detected in bone marrow aspirates, and in 6 partly different cases no hairy-cells were detectable in peripheral blood using flow-cytometry, although immunostaining of bone marrow biopsies using B-ly 7 revealed hairy-cells (ranging form 0.1 to 7.5%) in these cases. These results indicate that therapy with 2-CdA does not eradicate hairy cell leukemia despite complete remission according to conventional criteria. Minimal residual disease might be responsible for subsequent relapse. We conclude that routine immunohistological examination of bone marrow biopsies with B-ly 7 should be performed for assessment of MRD. Flow cytometric investigations of mononuclear cells of bone marrow aspirate or peripheral blood seem valuable for regular long term monitoring of MRD, but does not substitute for immunostaining of bone marrow biopsies.

The immunosuppressive substance 2-chloro-2-deoxyadenosine modulates lipoprotein metabolism in a murine macrophage cell line (P388 cells).

A recently developed immunosuppressive substance, 2-chloro-2-deoxyadenosine (2-CdA), was reported to inhibit monocyte functions at low concentration. Because macrophages play a key role in the formation of atherosclerotic plaques, it was of interest to study the effect of 2-CdA on cellular lipid metabolism. For this purpose we have used a macrophage cell line (P388) to perform incubation studies in the presence of acetylated low density lipoprotein (Ac-LDL) and 2-CdA. The addition of 2-CdA, in concentrations ranging from 5-20 nM, induced a dose-dependent decrease in cellular cholesterol content and in the amount of extracellular [14C]oleic acid (OA) incorporated into the cholesteryl ester (CE) fraction. The effect was maximized at 20 nM 2-CdA with an 86% reduction in cholesterol esterification compared to controls (P < 0.008). To evaluate the mechanism of interaction of 2-CdA with cellular lipid metabolism, deoxycytidine (dCyt) and 3-methoxybenzamide (3-MOB), substances known to antagonize the effect of 2-CdA in different ways, were co-administered with 2-CdA. dCyt, a competitive inhibitor of dCyt kinase, which catalyzes phosphorylation to the active metabolite, antagonized the effects of 20 nM 2-CdA, producing significantly greater incorporation of extracellular [14C]OA into the CE fraction than in the presence of 2-CdA alone (P < 0.0086). Co-incubation with 2-CdA and the poly-ADP-ribose synthetase inhibitor 3-MOB, which is known to render cells resistant to 2-CdA toxicity by preventing cellular nicotinamide adenine dinucleotide (NAD)- and adenosine triphosphase-depletion, also reversed the effect of 2-CdA on lipid accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of deoxycytidine on 2-chloro-deoxyadenosine-mediated growth inhibition of normal human erythroid and myeloid progenitor cells.

The cytotoxic effect of chlorodeoxyadenosine (CdA) on lymphocytes and monocytes requires phosphorylation by the enzyme deoxycytidine kinase and can be antagonized by coadministration of deoxycytidine (dCyt), a competitive substrate of deoxycytidine kinase. It has also been shown for lymphocytes that coadministration of 3-aminobenzamide (3-ABA), an inhibitor of the enzyme poly-(ADP ribose) synthetase, is activated by CdA-mediated DNA strand breaks, consumes intracellular nicotinamide-dinucleotide (NAD) and can antagonize the lethal effect of CdA. Recent in vitro studies have shown that not only growth of lymphocytes and monocytes, but also colony formation by erythroid and myeloid progenitors derived from normal human bone marrow, is inhibited by CdA in a dose-dependent manner. In this study we examined the effect of various doses of dCyt (10(-6) to 10(-3) M) on CdA-mediated growth inhibition of erythroid and myeloid progenitor cells in vitro. Our results show that colony formation by human bone marrow-derived progenitor cells--CFU-E (colony-forming unit erythroid), BFU-E (burst-forming unit erythroid) and CFU-GM (colony-forming unit granulocyte/macrophage)-in semisolid medium is protected by a high, but clinically achievable and non-toxic, concentration of dCyt (> 10(-4) M) against the inhibitory effects of coadministered high concentrations of CdA. The protective effect of dCyt was markedly different on the various subclasses of progenitor cells, however. Thus, with coadministration of 10(-4) M dCyt, the CFU-E colony formation could be restored to almost 100% despite the presence of high concentrations of CdA (160 nM) compared to control cultures, whereas the colony formation of BFU-E and CFU-GM was restored to only 50%. At a concentration of 10(-3) M dCyt, colony formation of BFU-E and CFU-GM was raised to 80% of control cultures even in the presence of high concentrations of CdA (160 nM). Further experiments in which 3-ABA was coadministered to CdA-treated cultures showed that in all concentrations tested (0.3 to 5 mM) 3-ABA was not able to prevent CdA-mediated cytotoxicity on bone marrow progenitors. Based on these studies, we suggest that the CdA toxicity on CFU-E is mainly mediated by phosphorylation by deoxycytidine kinase, whereas additional mechanisms may be operative in BFU-E and CFU-GM. Considerable biochemical differences seem to exist between hematopoietic stem cells on the one hand and lymphocytes and monocytes from peripheral blood on the other.