A methylthioadenosine phosphorylase (MTAP) fusion transcript identifies a new gene on chromosome 9p21 that is frequently deleted in cancer

Homozygous deletions of human chromosome 9p21 occur frequently in malignant cell lines, and are also common in primary gliomas, lung cancers, and leukemias. Moving from the centromere to the telomere, this complex region encodes the tumor suppressor genes p15INK4B (CDKN2B), p14ARF, p16INK4A (CDKN2A), and the housekeeping gene methylthioadenosine phosphorylase (MTAP). However, not all chromosome 9p21 deletions in tumors include these tumor suppressor genes. Here we describe the partial sequence and the exact localization of a new gene on chromosome 9p21 centromeric of p15INK4B, that formed an in frame fusion transcript with MTAP in a glioma xenograft, and that is homozygously deleted in various malignant cell lines. Northern blot revealed corresponding 1.5 kb transcript in non-deleted cell lines as well as in normal lymphocytes. Using a RNA master blot membrane including 50 different tissues, we could show that this new transcript is expressed in all tissues of the adult but not or only at very low levels in most of the fetal tissues tested. The expression pattern is similar to that of p16INK4A. The localization as well as the deletion pattern makes this transcript a candidate for a new tumor suppressor gene.

Deoxyadenosine analogs induce programmed cell death in chronic lymphocytic leukemia cells by damaging the DNA and by directly affecting the mitochondria

Adenine deoxynucleosides induce apoptosis in quiescent lymphocytes and are thus useful drugs for the treatment of indolent lymphoproliferative diseases. To explain why deoxyadenosine and its analogs are toxic to a cell that is not undergoing replicative DNA synthesis, several mechanisms have been proposed, including the direct binding of dATP to the pro-apoptotic factor Apaf-1 and the activation of the caspase-9 and -3 pathways. In this study it is shown, by means of several assays on whole cells and isolated mitochondria, that 2-chloro-2'-deoxyadenosine (2CdA) and 2-choloro-2'-ara-fluorodeoxyadenosine (CaFdA) disrupt the integrity of mitochondria from primary chronic lymphocytic leukemia (B-CLL) cells. The nucleoside-induced damage leads to the release of the pro-apoptotic mitochondrial proteins cytochrome c and apoptosis-inducing factor. The other adenine deoxynucleosides tested displayed comparable DNA-damaging potency but did not affect mitochondrial function. Interference with mitochondrial integrity, thus, may be a factor in the potent cytotoxic effects of 2CdA and CaFdA toward nondividing lymphocytes.

Rational design, synthesis and structure-activity relationships of antitumor (E)-2-benzylidene-1-tetralones and (E)-2-benzylidene-1-indanones

Novel substituted 6,7-dimethoxy-1-tetralones and 5,6-dimethoxy-1-indanones have been synthesized and evaluated for their cytotoxicity. Compounds with 3'-lipophilic, 3',5'-dilipophilic, or 3',5'-dilipophilic-4'-hydrophilic substituents on (E)-2-benzylidene moiety showed highly cytotoxic effects. The unique structure of 42 possibly matches the pharmacophore features for these cytotoxic compounds.

Indanocine, a microtubule-binding indanone and a selective inducer of apoptosis in multidrug-resistant cancer cells

BACKGROUND

Certain antimitotic drugs have antitumor activities that apparently result from interactions with nontubulin components involved in cell growth and/or apoptotic cell death. Indanocine is a synthetic indanone that has been identified by the National Cancer Institute's Developmental Therapeutics Program as having antiproliferative activity. In this study, we characterized the activity of this new antimitotic drug toward malignant cells.

METHODS

We tested antiproliferative activity with an MTT [i.e., 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] assay, mitochondrial damage and cell cycle perturbations with flow cytometry, caspase-3 activation with fluorometry, alterations of the cytoskeletal components with immunofluorescence, and antimicrotubule activity with a tubulin polymerization assay.

RESULTS/CONCLUSIONS

Indanocine is a cytostatic and cytotoxic indanone that blocks tubulin polymerization but, unlike other antimitotic agents, induces apoptotic cell death in stationary-phase multidrug-resistant cancer cells at concentrations that do not impair the viability of normal nonproliferating cells. Of the seven multidrug-resistant cell lines tested, three (i.e., MCF-7/ADR, MES-SA/DX5, and HL-60/ADR) were more sensitive to growth inhibition by indanocine than were their corresponding parental cells. Confluent multidrug-resistant cells (MCF-7/ADR), but not drug-sensitive cancer cells (MCF-7) or normal peripheral blood lymphocytes, underwent apoptotic cell death 8-24 hours after exposure to indanocine, as measured by sequential changes in mitochondrial membrane potential, caspase activity, and DNA fragmentation. Indanocine interacts with tubulin at the colchicine-binding site, potently inhibits tubulin polymerization in vitro, and disrupts the mitotic apparatus in dividing cells.

IMPLICATIONS

The sensitivity of stationary multidrug-resistant cancer cells to indanocine suggests that indanocine and related indanones be considered as lead compounds for the development of chemotherapeutic strategies for drug-resistant malignancies.

Nucleotide requirements for the in vitro activation of the apoptosis protein-activating factor-1-mediated caspase pathway

Adenine deoxynucleosides, such as 2-chlorodeoxyadenosine (2CdA) and fludarabine, induce apoptosis in quiescent lymphocytes, and are thus useful drugs for the treatment of indolent lymphoproliferative diseases. We previously demonstrated that that the 5'-triphosphate metabolite of 2CdA (2CdATP), similar to dATP, can cooperate with cytochrome c and apoptosis protein-activating factor-1 (APAF-1) to trigger a caspase pathway in a HeLa cell-free system. We used a fluorometry-based assay of caspase activation to extend the analysis to several other clinically relevant adenine deoxynucleotides in B-chronic lymphocytic leukemia extracts. The nucleotide-induced caspase activation displayed typical Michaelis-Menten kinetics. As estimated by the V(max)/K(m) ratios, the relative efficiencies of different nucleotides were Ara-ATP > 9-fluoro-9-beta-D-arabinofuranosyladenine 5'-triphosphate > dATP > 2CdATP > 9-beta-D-arabinofuranosylguanine 5'-triphosphate > dADP > ATP. In contrast to dADP, both ADP and its nonhydrolyzable alpha, beta-methylphosphonate analog were strong inhibitors of APAF-1-dependent caspase activation. The hierarchy of nucleotide activation was confirmed in a fully reconstituted system using recombinant APAF-1 and recombinant procaspase-9. These results suggest that the potency of adenine deoxynucleotides as co-factors for APAF-1-dependent caspase activation is due both to stimulation by the 5'-triphosphates and lack of inhibition by the 5'-diphosphates. The capacity of adenine deoxynucleoside metabolites to activate the apoptosome pathway may be an additional biochemical mechanism that plays a role in the chemotherapy of indolent lymphoproliferative diseases.

Use of alanosine as a methylthioadenosine phosphorylase-selective therapy for T-cell acute lymphoblastic leukemia in vitro

Methylthioadenosine phosphorylase (MTAP) is an important enzyme for the salvage of adenine and methionine and is deficient in a variety of cancers including T-cell acute lymphocytic leukemia (T-ALL). Previously, we reported that the MTAP gene was deleted in over 30% of T-ALL patients at both diagnosis and relapse. We now report that MTAP-primary T-ALL cells are more sensitive to the toxicity of L-alanosine, an inhibitor of de novo AMP synthesis, than are MTAP+ primary T-ALL cells. As measured by [3H]thymidine incorporation, DNA synthesis in all seven MTAP-primary T-ALL cells was inhibited by L-alanosine with a mean IC50 of 4.8+/-5.3 ILM (range, 0.3-11.3 microM). On the other hand, the IC50 for 60% (12 of 20) of MTAP+ primary T-ALL was 19+/-18 microM (range, 1.7-67 microM; P = 0.02), whereas the remaining 40% (8 of 20) had an IC50 of >80 microM4. Furthermore, normal lymphocytes and MTAP+ primary T-ALL cells were rescued from L-alanosine toxicity by the MTAP substrate 5'-deoxyadenosine, but MTAP-T-ALL cells were not. These results indicate that normal cells, which are intrinsically MTAP+, would be protected from L.-alanosine toxicity, whereas MTAP-tumor cells would be killed. Thus, our results support the use of L-alanosine alone or in combination with a salvage agent as a MTAP-selective therapy and therefore lay the foundation for the initiation of clinical trials for the treatment of T-ALL and other MTAP-deficient malignancies with L-alanosine.

Homozygous deletions of methylthioadenosine phosphorylase (MTAP) are more frequent than p16INK4A (CDKN2) homozygous deletions in primary non-small cell lung cancers (NSCLC)

Homozygous deletions of the tumor suppressor gene p16INK4A and deficiency of methylthioadenosine phosphorylase (MTAP), both located on chromosome 9p21, have been independently reported in non-small cell lung cancer (NSCLC). To determine the frequency of co-deletion of these two genes, we investigated 50 samples of primary NSCLC using a quantitative PCR-ELISA. All specimens were fixed in formalin, paraffin embedded and stored until assayed. Histologic subtypes included 25 adenocarcinomas (50%), 21 squamous cell carcinomas (42%) and four large cell carcinomas (8%). Homozygous deletions of MTAP exon 8 could be detected in 19 of 50 NSCLC samples (38%). Adenocarcinoma (11 of 25, 44%) showed a higher deletion frequency than squamous cell carcinoma (six of 21, 29%). In contrast, homozygous p16INK4A deletions were detected in only nine of 50 (18%) samples using specific primers for p16INK4A exon 1alpha. No difference between the histological subtypes and p16INK4A deletion frequency was observed. We further investigated the ten samples with MTAP deletions but intact p16INK4A exon 1alpha with primers specific for p16INK4A exon 3, the exon nearest to MTAP exon 8. Interestingly, none of the ten samples had deletion of the p16INK4A exon 3 coding region. Fine mapping analysis performed in ten samples showed a frequent breakpoint between MTAP exon 4 and exon 5. In addition, p16 protein expression could not be detected in five out of six samples with intact p16 but deleted MTAP locus. These data show a high frequency of homozygous MTAP deletions in NSCLC which is associated with detectable co-deletion of p16INK4A in only half of the cases. This result suggests the existence either of another tumor suppressor gene telomeric of p16INK4A or of deletions involving 3'-untranslated (3'-UTR) regulatory regions of p16INK4A that can interfere with its expression or function.

Induction of an apoptotic program in cell-free extracts by 2-chloro-2'-deoxyadenosine 5'-triphosphate and cytochrome c

Adenine deoxynucleosides, such as 2-chloro-2'-deoxyadenosine (2CdA) induce apoptosis in quiescent lymphocytes, and are thus useful drugs for the treatment of indolent lymphoproliferative diseases. However, it has remained puzzling why deoxyadenosine and its analogs are toxic to a cell that is not undergoing replicative DNA synthesis. The present experiments demonstrate that the 5'-triphosphate metabolite of 2CdA (2CdA-5'-triphosphate), similar to dATP, can cooperate with cytochrome c and Apaf-1 to activate caspase-3 in a cell free system. Chronic lymphocytic leukemia cells and normal peripheral blood lymphocytes expressed both caspase-3 and apoptotic protease activating factor 1. Incubation of the lymphocytes with 2CdA induced caspase-3 activation prior to DNA degradation and cell death. Stimulation of the caspase proteolytic cascade by 2CdA-5'-triphosphate, in the context of DNA strand break formation, may provide an explanation for the potent cytotoxic effects of 2CdA toward nondividing lymphocytes.

The methylthioadenosine phosphorylase gene is frequently co-deleted with the p16INK4a gene in acute type adult T-cell leukemia

Adult T-cell leukemia (ATL) is a retrovirus-associated leukemia with poor prognosis and often has deletions of the p16INK4a and p15INK4b genes on chromosome 9p21. The gene for methylthioadenosine phosphorylase (MTAP), a purine and methionine metabolic enzyme, resides approximately 100 Kb telomeric to the p16INK4a gene and is frequently co-deleted with the tumor suppressor gene in a variety of cancers. This enzyme deficiency can be exploited for selective chemotherapy with de novo purine synthesis inhibitors and/or methionine depletion. To determine whether ATL can be a candidate for selective chemotherapy based on genetic alterations on chromosome 9p21, we analyzed the MTAP gene in 41 samples from ATL patients (27 acute type and 14 chronic type ATL) and 3 cell lines established from ATL patients. Five samples from the acute type had deletions of the MTAP gene (4 total deletions and 1 partial deletion of exons 6-8). The MTAP gene was always co-deleted with p16INK4a. No deletion of the MTAP gene was detected in samples from the chronic type. Of 3 cell lines, 2 showed partial deletions of exons 5-8 of the MTAP gene, and 1 lost all exons. The p16INK4a gene was deleted in all cell lines. In conclusion, deletions of the MTAP gene were found in 5 of 27 acute type ATL samples. Acute type ATL with MTAP deficiency can be a good candidate for selective chemotherapy by depleting purines and/or methionine.

Presence of methylthioadenosine phosphorylase (MTAP) in hematopoietic stem/progenitor cells: its therapeutic implication for MTAP (-) malignancies

Methylthioadenosine phosphorylase (MTAP) is important for the salvage of adenine and methionine. Recently, we found frequent deletion of MTAP in T-cell acute lymphoblastic leukemia (T-ALL) patients both at diagnosis and at relapse (A. Batova et al., Blood, 88: 3083-3090, 1996). In addition, MTAP deficiency has been reported in other cancers. Thus, MTAP deficiency in cancer may offer opportunities for developing selective therapy, which would spare normal cells. It is therefore important to document the presence of MTAP activity in hematopoietic stem/progenitor cells. Our approach was to investigate whether hematopoietic stem/progenitor cells can be rescued from the cytotoxicity of an AMP synthesis inhibitor, L-alanosine, by 5'-deoxyadenosine, a process that requires MTAP. Erythroid burst-forming unit, granulocyte/monocyte colony-forming unit, or granulocyte/erythrocyte/macrophage/megakaryocyte colony-forming unit progenitors and the primitive high proliferative potential colony-forming cells in the purified CD34(+) cells were cultured in horse serum-containing medium, and their colony growth was found to be suppressed by incubation with 5 microM or greater concentrations of L-alanosine. However, in the presence of 5-10 microM of 5'-deoxyadenosine, colony formation of hematopoietic stem/primitive progenitors was restored. On the other hand, 5'-deoxy-5'-methylthioadenosine, the endogenous substrate of MTAP, was toxic to hematopoietic stem/progenitors (ID50 < 1 microM), presumably due to inhibition of methylation reactions or polyamine synthesis. We also compared the effects of L-alanosine and 5'-deoxyadenosine on MTAP (+) and MTAP (-) T-ALL cell lines. Treatment of MTAP (+) Molt 4 and MTAP (-) CEM cell lines with L-alanosine in the presence of 5'-deoxyadenosine resulted in killing of MTAP (-), but not MTAP (+) cells. Therefore, our findings demonstrate the presence of MTAP in human hematopoietic stem/progenitor cells and support the possibility of targeting MTAP in the design of an enzyme-selective therapy for T-ALL and other MTAP-deficient malignancies.

Methylthioadenosine phosphorylase cDNA transfection alters sensitivity to depletion of purine and methionine in A549 lung cancer cells

Methylthioadenosine phosphorylase (MTAP), an enzyme involved in purine and methionine metabolism, is present in all normal tissues but is frequently deficient in a variety of cancers. It has been suggested that this metabolic difference between normal and cancer cells may be exploited to selectively treat MTAP-negative cancers by inhibiting de novo purine synthesis and by depleting L-methionine. However, these therapeutic strategies have only been tested in naturally occurring MTAP-positive and -negative cell lines, which might have additional genetic alterations that affect chemotherapeutic sensitivity. Therefore, it is of importance to examine the feasibility of enzyme-selective treatment using paired cell lines that have an identical genotype except for MTAP status. MTAP-negative A549 lung cancer cells were transfected with eukaryotic expression vectors encoding MTAP cDNA in sense and antisense orientations. The resultant stable transfectomas were treated with inhibitors of de novo purine synthesis such as methotrexate, 5,10-dideazatetrahydrofolate, and L-alanosine and by methionine depletion. The A549 cells transfected with an antisense construct (antisense transfectoma) expressed no MTAP protein and were more sensitive to both purine and methionine depletion than were cells expressing MTAP protein (sense transfectoma). Methylthioadenosine was able to completely rescue the sense transfectoma but not the antisense transfectoma from growth inhibition by depletion of purine and methionine. These results prove that MTAP deficiency contributes directly to the sensitivity of cancer cells to purine or methionine depletion. Inhibition of de novo purine synthesis, combined with methionine depletion in the presence of methylthioadenosine, is a highly selective treatment for MTAP-negative cancers.

Oral 2-chlorodeoxyadenosine in psoriatic arthritis. A preliminary report

OBJECTIVE

To determine if weekly oral 2-chlorodeoxyadenosine (2-CdA) can induce selective lymphocytopenia, and reduce inflammation, in patients with refractory psoriatic arthritis.

METHODS

Seven patients with psoriatic arthritis were treated with oral 2-CdA at weekly dosages of 0.3 mg/kg to 0.45 mg/kg for 12 weeks, followed by monthly maintenance therapy. The patients were evaluated after 6 months.

RESULTS

The drug treatment produced selective lymphocytopenia, and reduced lymphocyte infiltration into involved skin. One patient did not complete 12 weeks of therapy because of perceived lack of efficacy. Four of the 6 remaining patients had improved joint disease, and 5 of 6 had improved psoriasis.

CONCLUSION

Weekly oral 2-CdA appears to be a well-tolerated regimen for the inducement of peripheral lymphocytopenia in patients with psoriatic arthritis. Larger-scale, controlled trials may be warranted.

Purine metabolism of lymphocytes. Targets for chemotherapy drug development

The unique metabolic profile that renders lymphoid cells sensitive to purine deoxynucleosides also accounts for the response of chronic lymphoid malignancies to purine analogues. Consistent with earlier observations in children with adenosine deaminase deficiency, a profound and relatively selective lymphocyte depletion results from treatment with drugs that elevate or mimic deoxyadenosine. Three such agents available for clinical use are 2-chlorodeoxyadenosine, 2'-deoxycoformycin, and fludarabine phosphate. In addition to a review of the relevant biochemistry and cellular pharmacology of these agents in target lymphoid cells, this article reviews the current clinical response data in leukemias and lymphoma.

Marrow suppression produced by repeated doses of cladribine

2-Chlorodeoxyadenosine (cladribine, Leustatin) is being used extensively in the treatment of hematologic malignancies, but relatively little is known regarding its toxicity to the normal marrow. Long-term serial hematologic observations have been made on 29 patients with multiple sclerosis undergoing experimental therapy with monthly courses of cladribine, each of which consisted of 0.087-0.1 mg/kg per day for 7 days. The characteristic hematologic responses of the patients consisted of acute transient monocytopenia, prolonged, profound lymphopenia especially of CD4-positive cells, and modest lowering of the granulocyte count and hemoglobin with development of long-lasting macrocytosis. Two patients developed severe aplastic anemia, requiring transfusion both of red cells and platelets. One of these had previously received extensive therapy with chlorambucil, while the other had received carbamazepine (Tegretol) and was ingesting phenytoin (Dilantin) at the time of cladribine therapy. Both patients recovered after several months of marrow suppression.

2-Chlorodeoxyadenosine dose escalation in nonhematologic malignancies

PURPOSE

We performed a dose-escalation study of 2-chlorodeoxyadenosine (2-CdA) in solid tumors to determine the maximum-tolerated dose (MTD) and define its toxicity profile at higher doses.

PATIENTS AND METHODS

Twenty-one patients, seven with malignant astrocytoma, twelve with metastatic melanoma, and two with metastatic hypernephroma, were enrolled onto the study. Patients were entered onto cohorts that received 0.10, 0.15, or 0.20 mg/kg/d of 2-CdA by continuous intravenous infusion for 7 days every 28 days. 2-CdA levels were determined by radioimmunoassay. In tumor tissue samples, deoxycytidine kinase (dCK) levels were measured by both enzyme activity and immunoreactive protein analysis.

RESULTS

Of seven patients treated with 2-CdA at 0.1 mg/kg/d, one experienced grade 3 or 4 myelotoxicity. Of 11 patients treated at 0.15 mg/kg/d, four experienced myelotoxicity, two after a single course of 2-CdA. All three patients who received 2-CdA at 0.2 mg/kg/d experienced myelosuppression. Neurologic events occurred in two patients, both with malignant melanoma. Two of seven patients (28.6%) with astrocytomas obtained partial responses with a median duration of 8 months. 2-CdA penetrated the blood-brain barrier. An association was found between dCK levels as measured by enzymatic activity and immunoreactive proteins, but this did not correlate with 2-CdA tumor responsiveness.

CONCLUSION

The MTD for 2-CdA delivered as a 7-day intravenous infusion in patients with nonhematologic malignancies was determined to be 0.1 mg/kg/d, the same as the MTD for patients with hematologic malignancies. There was no clinical correlation with dCK expression and response to 2-CdA. The responses noted in patients with malignant astrocytoma warrant further phase II study.

Relationship of deoxycytidine kinase and cytoplasmic 5'-nucleotidase to the chemotherapeutic efficacy of 2-chlorodeoxyadenosine

The agent 2-chlorodeoxyadenosine (2-CdA) has chemotherapeutic activity in hairy cell leukemia (HCL) and in refractory chronic lymphocytic leukemia (CLL). The cytotoxic activity of 2-CdA requires the intracellular accumulation of 2-CdA nucleotides. Deoxycytidine kinase (dCK) and cytoplasmic 5'-nucleotidase (5'-NT) are the principal enzymes that phosphorylate 2-CdA and dephosphorylate 2-CdA 5'-monophosphate, respectively. The net accumulation of 2-CdA nucleotides may therefore depend on both dCK and 5'-NT. The purpose of the present experiments was to determine if there is a relationship between pretreatment levels of dCK and 5'-NT in HCL and in CLL cells, and the clinical outcome of 2-CdA treatment. As measured by a direct immunoassay for dCK in 25 CLL patients, and by a 5'-NT activity assay in 23 patients, mean dCK levels were significantly higher in 2-CdA responders than in nonresponders (P < .01), whereas mean 5'-NT levels were significantly lower in 2-CdA responders than in nonresponders (P < .05). Mean dCK levels were higher in six HCL 2-CdA responders than in one nonresponder, whereas mean 5'-NT levels were lower in the 2-CdA responders than in the nonresponder. These results suggest that both dCK and 5'-NT are determinants of 2-CdA responsiveness.

Quantitative immunoassay of human deoxycytidine kinase in malignant cells

Deoxycytidine kinase (dCK) is necessary for the activity of several nucleosides used for the chemotherapy of cancer and AIDS. However, the measurement of dCK catalytic activity in crude cell extracts may be imprecise, due to the presence of phosphatases and nucleotidases that degrade the enzyme products. We describe a simple immunoassay for dCK that can measure accurately as little as 5 ng enzyme protein in crude tissue extracts. The assay enabled us to show (i) that mutant cells deficient in dCK activity lack immunoreactive dCK protein, (ii) that dCK catalytic activity and immunoreactivity correlate closely in human tumors, and (iii) that immunoreactive dCK is particularly high in lymphocytes and lymphoid malignancies, although certain solid tumors may also contain the enzyme. The immunoassay of dCK could prove useful in the selection and monitoring of patients who are being treated with nucleosides that are activated by this enzyme.

2-Chlorodeoxyadenosine: an active agent in the treatment of cutaneous T-cell lymphoma

Cutaneous T-cell lymphomas are disfiguring malignant lymphoproliferative disorders for which standard therapy has been principally palliative. 2-Chlorodeoxyadenosine (2-CdA), a new purine analogue resistant to degradation by adenosine deaminase that has substantial activity against lymphoid neoplasms, was administered to 16 patients with cutaneous involvement by T-cell lymphoma. All patients had failed topical treatment modalities and/or systemic therapies. Fifteen patients were evaluable; one patient was not evaluable due to incomplete therapy and follow-up. The overall response rate was 47%. Three of 15 patients (20%) achieved complete responses and four of 15 patients (27%) achieved partial responses. The median duration of response was 5 months. One patient remains in unmaintained complete remission at 52+ months. Therapy was well tolerated. Myelosuppression was the principal toxicity encountered, occurring in 8 of 15 (53%) patients. 2-CdA is an effective new agent for the treatment of cutaneous T-cell lymphoma and warrants further study both as a single agent and in combination regimens.

Oral antilymphocyte activity and induction of apoptosis by 2-chloro-2'-arabino-fluoro-2'-deoxyadenosine

2-Chlorodeoxyadenosine (CdA) is active in chronic lymphocytic leukemia, hairy-cell leukemia, and low-grade lymphomas. In part, this spectrum of activity may be attributable to the selective toxicity of CdA to nondividing lymphocytes and monocytes. However, CdA is unstable at acidic pH and is degraded by bacterial nucleoside phosphorylases. The present experiments demonstrate that the 2'-arabino-fluoro derivative of CdA, designated CAFdA, is also directly toxic to quiescent lymphocytes and macrophages. Unlike CdA, CAFdA was stable at pH 2 and resisted degradation by Escherichia coli nucleoside phosphorylase. Cell killing was preceded by the formation of DNA strand breaks and could be prevented by supplementation of the medium with deoxycytidine. The initial DNA damage initiated the pattern of oligonucleosomal DNA fragmentation characteristic of apoptosis. Mutant lymphoblasts, deficient in deoxycytidine kinase, with elevated cytoplasmic 5'-nucleotidase, or with expanded deoxynucleotide pools secondary to increased ribonucleotide reductase activity, were cross-resistant to both CAFdA and CdA toxicity. One-week oral treatment with CAFdA (1 mg/ml in drinking water) achieved an average plasma concentration of 0.56 microM and eliminated 90% of chronic lymphocytic leukemia cells transplanted into severe combined immunodeficiency (scid) mice. Under the same conditions, CdA was much less active. Collectively, these results suggest that CAFdA could be effective as an oral agent in indolent lymphoproliferative diseases and in autoimmune diseases where lymphocyte and monocyte depletion is desirable.

2-Chlorodeoxyadenosine treatment of low-grade lymphomas

PURPOSE

Because of the need to identify effective new agents in the treatment of non-Hodgkin's lymphoma and because of the high activity of the purine analog 2-chlorodeoxyadenosine (2-CdA) against chronic lymphocytic leukemia and hairy cell leukemia, a phase II trial of 2-CdA was initiated in patients with low-grade lymphocytic lymphomas.

PATIENTS AND METHODS

Forty patients with low-grade lymphocytic lymphomas including diffuse small lymphocytic, follicular small-cleaved, and follicular mixed histologies were enrolled onto the study. Conventional therapies had failed in all patients, and six patients had lymph node biopsies showing evidence of histologic evolution to a higher-grade lymphoma. A total of 107 courses of 2-CdA were administered. There were 27 males and 13 females. The median age was 59 years (range, 37 to 80 years). Patients had received a median of three prior therapies (range, one to six therapies).

RESULTS

An overall response rate of 43% was achieved, with eight patients experiencing complete responses (CRs) and nine patients experiencing partial responses (PRs). The duration of responses ranged from 1 to greater than 33 months without maintenance therapy (median duration of response, 5 months). Histology and prior therapy history did not seem to correlate with responses. Significant toxicity was limited to bone marrow suppression; 18% of patients developed neutropenia, and 30% developed thrombocytopenia.

CONCLUSIONS

This phase II trial demonstrates that 2-CdA is an effective antilymphocyte, antineoplastic agent with significant activity as a single agent in patients with recurrent or refractory low-grade lymphocytic lymphoma. Responses were achieved with an acceptable toxicity profile. Further trials of this agent in previously untreated patients and in combination regimens are indicated and will be developed.

Dependence of cell survival on DNA repair in human mononuclear phagocytes

Mononuclear phagocytes play a central role in the pathogenesis of chronic inflammatory diseases. It is therefore important to define chemotherapeutically exploitable metabolic pathways that distinguish monocytes from other cell types. Blood monocytes do not synthesize deoxynucleotides de novo, and their transformation to macrophages occurs without cell division. Whether or not monocytes can repair DNA damage, and whether or not DNA repair is necessary for their survival, is unknown. The present experiments demonstrate that normal human monocytes, unlike neutrophils, rapidly repair DNA strand breaks induced by gamma-irradiation. Monocyte extracts contain functional immunoreactive DNA polymerase-alpha. DNA repair synthesis in normal monocytes is blocked by aphidicolin, an inhibitor of DNA polymerase-alpha with respect to dCTP. Aphidicolin is also directly toxic to normal monocytes, but has no effect on nondividing lymphocytes or fibroblasts. Compared to most other cell types, monocytes and macrophages have very low dCTP pools, but abundant deoxycytidine kinase activity. This suggests that dCTP derived from salvage pathways is important for DNA repair in these cells. Consistent with this notion, exogenous deoxycytidine could partially protect monocytes from aphidicolin killing. The unexpected toxicity of aphidicolin toward normal human monocytes may be attributable to their high rate of spontaneous DNA strand break formation, to the importance of DNA polymerase-alpha for DNA repair in these cells, and to their minute dCTP pools.

Dependence of cell survival on DNA repair in human mononuclear phagocytes

Mononuclear phagocytes play a central role in the pathogenesis of chronic inflammatory diseases. It is therefore important to define chemotherapeutically exploitable metabolic pathways that distinguish monocytes from other cell types. Blood monocytes do not synthesize deoxynucleotides de novo, and their transformation to macrophages occurs without cell division. Whether or not monocytes can repair DNA damage, and whether or not DNA repair is necessary for their survival, is unknown. The present experiments demonstrate that normal human monocytes, unlike neutrophils, rapidly repair DNA strand breaks induced by gamma-irradiation. Monocyte extracts contain functional immunoreactive DNA polymerase-alpha. DNA repair synthesis in normal monocytes is blocked by aphidicolin, an inhibitor of DNA polymerase-alpha with respect to dCTP. Aphidicolin is also directly toxic to normal monocytes, but has no effect on nondividing lymphocytes or fibroblasts. Compared to most other cell types, monocytes and macrophages have very low dCTP pools, but abundant deoxycytidine kinase activity. This suggests that dCTP derived from salvage pathways is important for DNA repair in these cells. Consistent with this notion, exogenous deoxycytidine could partially protect monocytes from aphidicolin killing. The unexpected toxicity of aphidicolin toward normal human monocytes may be attributable to their high rate of spontaneous DNA strand break formation, to the importance of DNA polymerase-alpha for DNA repair in these cells, and to their minute dCTP pools.

Deoxyadenosine-resistant human T lymphoblasts with elevated 5'-nucleotidase activity

Although several different enzymes with 5'-nucleotidase activity have been described in mammalian cells, their functions in nucleotide metabolism have not been clearly distinguished. In the present experiments, a mutant human T lymphoblastoid cell line (CEM-dAdoR) was selected specifically for resistance to deoxyadenosine toxicity. Compared to parental CEM cells, the variant had 4-fold elevated ATP-activated cytosolic 5'-nucleotidase activity. Other enzymes of potential importance for deoxyadenosine metabolism were indistinguishable in the two cell types. In medium supplemented with the adenosine deaminase inhibitor deoxycoformycin, the T cells with increased 5'-nucleotidase accumulated less nucleotides from exogenously added deoxyadenosine, or 9-beta-D-arabinofuranosyladenine, than did parental T lymphocytes. These metabolic changes were associated with resistance to the growth inhibitory effects of these nucleosides, and also to deoxyguanosine and to 9-beta-D-arabinofuranosylguanine. The T cells with elevated 5'-nucleotidase activity formed more 2',3'-dideoxyadenosine than did parental cells, in deoxycoformycin-supplemented medium. The accumulation of 2',3'-dideoxyadenosine 5'-triphosphate from 2',3'-dideoxyinosine was similarly augmented in the mutant. These data establish the importance of the cytosolic 5'-nucleotidase for the metabolism of purine 2'-deoxyribonucleosides, arabinonucleosides and 2',3'-dideoxyribonucleosides in T lymphoblasts.

Potent toxicity of 2-chlorodeoxyadenosine toward human monocytes in vitro and in vivo. A novel approach to immunosuppressive therapy

Lymphoid cells were thought to be uniquely susceptible to excess 2'-deoxyadenosine (dAdo), when exposed to inhibitors of adenosine deaminase (ADA). However, we now find that human monocytes are as sensitive as lymphocytes to dAdo or to the ADA-resistant congener 2-chloro-2'-deoxyadenosine (CldAdo). Monocytes exposed in vitro to CldAdo, or to dAdo plus deoxycoformycin rapidly developed DNA strand breaks. Both the DNA damage and the toxicity of CldAdo or dAdo toward monocytes were blocked by deoxycytidine, but not by inhibitors of poly(ADP-ribose) polymerase. A partial decrease in RNA synthesis and a gradual decline of cellular NAD were early biochemical events associated with monocyte DNA damage. Low CldAdo concentrations (5-20 nM) inhibited monocyte phagocytosis and reduced the release of interleukin 6. Higher CldAdo concentrations led to a dose- and time-dependent loss of monocyte viability. Circulating monocytes disappeared within 1 wk in patients with cutaneous T cell lymphoma or with rheumatoid arthritis during continuous CldAdo infusion. The marked sensitivity of human monocyte function and survival to CldAdo in vitro, together with the monocyte depletion in patients receiving CldAdo chemotherapy, suggests that CldAdo or other dAdo analogues offer a novel therapeutic strategy for chronic inflammatory and autoimmune diseases characterized by inappropriate monocyte deployment or function.

Lasting remissions in hairy-cell leukemia induced by a single infusion of 2-chlorodeoxyadenosine

2-Chlorodeoxyadenosine is a simple purine nucleoside that has previously been shown to be effective in the treatment of low-grade malignant disorders of lymphoid tissue, including chronic lymphocytic leukemia and non-Hodgkin's lymphoma. Because of these encouraging results, we treated 12 patients with another low-grade B-cell neoplasm, hairy-cell leukemia. The patients received 2-chlorodeoxyadenosine (0.1 mg per kilogram of body weight per day) by continuous infusion for seven days. All the patients responded to treatment. Eleven had complete remissions characterized by the normalization of peripheral blood and bone marrow and disappearance of tumor masses. The longest remission has been 3.8 years. None of the patients have relapsed, and the median duration of remission has been 15.5 months. No serious toxic reactions occurred as a result of 2-chlorodeoxyadenosine therapy. These results suggest that 2-chlorodeoxyadenosine may be the most effective therapy available for hairy-cell leukemia. The administration of 2-chlorodeoxyadenosine resulted in a higher rate of complete remission than is observed with interferon alfa, and it required no maintenance therapy. Its toxicity may be lower than that of deoxycoformycin, and the responses were achieved with single courses of treatment.

Profound toxicity of deoxyadenosine and 2-chlorodeoxyadenosine toward human monocytes in vitro and in vivo

Deoxyadenosine is known to be toxic to both proliferating and resting lymphocytes that lack adenosine deaminase (ADA) activity. We now show that human monocytes are also highly sensitive in vitro to nanomolar concentrations of deoxyadenosine plus the ADA inhibitor deoxycoformycin, and to the ADA-resistant analogue 2-chlorodeoxyadenosine (CdA). Monocytes exposed to deoxyadenosine or to CdA in vitro accumulate massive DNA damage detectable within 1 hour. The DNA damage in monocytes exposed to CdA is associated with a decrease in protein synthesis and with inhibitions of phagocytosis and IL-6 secretion. However, unlike lymphocytes with similar DNA damage, the monocytes show no significant NAD or ATP depletion until cell viability declines. The selective toxicity of CdA to monocytes was confirmed by in vivo studies. In almost all patients receiving CdA infusion chemotherapy for cutaneous lymphoma, the blood monocytes counts fell to near 0 during one week of therapy. Our results suggest that CdA and related compounds may have potential clinical use in the therapy of immune disorders associated with monocyte/macrophage activation.

2-Chlorodeoxyadenosine: an effective new agent for the treatment of chronic lymphocytic leukemia

2-Chlorodeoxyadenosine, a new lymphocyte-selective, anti-neoplastic drug was administered to 18 patients with advanced chronic lymphocytic leukemia of B-cell origin. All patients were resistant to conventional treatment. A total of 44 courses of 2-chlorodeoxyadenosine were completed with minimal toxicity. An overall response rate of 55% was achieved with four of 18 patients demonstrating partial response and six of 18 patients experiencing clinical improvement. Only minor bone marrow suppression occurred during administration of the drug, indicating a high degree of lymphocyte selectivity. Reduction of lymphocyte infiltration in bone marrow occurred in treated patients including one patient who experienced normalization of the bone marrow. Three of four patients with concurrent autoimmune hemolytic anemia experienced resolution of hemolysis, as indicated by elimination of transfusion requirement, fall in reticulocyte count, elevation of hemoglobin, and ability to taper prednisone without recurrence of hemolysis. Duration of responses ranged from 2 to 15 months without maintenance therapy.

Metabolism and anti-human immunodeficiency virus-1 activity of 2-halo-2',3'-dideoxyadenosine derivatives

Both 2',3'-dideoxyadenosine and 2',3'-dideoxyinosine have been shown (Mitsuya, H., and Broder, S. (1987) Nature 325, 773-778) to have in vitro activity against the human immunodeficiency virus-1 (HIV). However, these dideoxynucleosides may be catabolized by human T cells, even when adenosine deaminase is inhibited by deoxycoformycin. To overcome this problem, we have synthesized the 2-fluoro-, 2-chloro-, and 2-bromo-derivatives of 2',3'-dideoxyadenosine. The metabolism and anti-HIV activity of the 2-halo-2',3'-dideoxyadenosine derivatives and of 2',3'-dideoxyadenosine were compared. The 2-halo-2',3'-dideoxyadenosine derivatives were not deaminated significantly by cultured CEM T lymphoblasts. Experiments with 2-chloro-2',3'-dideoxyadenosine showed that the T cells converted the dideoxynucleoside to the 5'-monophosphate, 5'-diphosphate, and 5'-triphosphate metabolites. At concentrations lower than those producing cytotoxicity in uninfected cells (3-10 microM), the 2-halo-2',3-dideoxyadenosine derivatives inhibited the cytopathic effects of HIV toward MT-2 T lymphoblasts, and retarded viral replication in CEM T lymphoblasts. Experiments with a deoxycytidine kinase-deficient mutant CEM T cell line showed that this enzyme was necessary for the phosphorylation and anti-HIV activity of the 2-chloro-2',3'-dideoxyadenosine. In contrast, 2',3'-dideoxyadenosine was phosphorylated by the deoxycytidine kinase-deficient mutant and retained anti-HIV activity in this cell line. Thus, the 2-halo derivatives of 2',3'-dideoxyadenosine, in contrast to 2',3'-dideoxyadenosine itself, are not catabolized by T cells. Their anti-HIV and anti-proliferative activities are manifest only in cells expressing deoxycytidine kinase. The in vivo implications of these results for anti-HIV chemotherapy are discussed.

Programmed cell death and adenine deoxynucleotide metabolism in human lymphocytes

Agents that cause the accumulation of DNA strand breaks are directly cytotoxic to non-dividing normal human peripheral blood lymphocytes, and to chronic lymphocytic leukemia (CLL) cells. Activation of poly(ADP-ribose) polymerase (ADPRP), and the resultant consumption of NAD, play an essential role in mediating the toxicity of these agents. Human peripheral blood lymphocytes contain a substantial number of alkali-sensitive DNA sites, reflecting ongoing DNA strand breakage and repair. However, resting lymphocytes have a limited capacity to synthesize NAD. Pulse-chase experiments indicate that approximately 75% of their NAD turnover is due to ADPRP activity. Exposure of the cells in vitro to deoxyadenosine, or to 2-chlorodeoxyadenosine (CdA, an adenosine deaminase resistant deoxyadenosine congener), caused an increase in DNA strand breaks, rapid NAD consumption, ATP depletion and cell death. Supplementation of the medium with inhibitors of poly(ADP-ribose) polymerase blocks the fall in cellular NAD and ATP, and protects the lymphocytes from the toxicity of DNA damaging agents. Slowly dividing malignant lymphocytes from patients with CLL are also susceptible to lethal NAD depletion following DNA damage. 2-chlorodeoxyadenosine (CdA) induced massive DNA strand break formation in CLL cells in vitro and a fall in NAD and ATP pools. In an initial clinical trial, several CLL patients, and two patients with hairy cell leukemia, have responded to treatment with CdA, with minimal toxicity. Thus, the suicidal activation of ADPRP in response to DNA damage has been rationally exploited in the treatment of chronic lymphoid malignancies.

Resistance to multiple adenine nucleoside and methionine analogues in mutant murine lymphoma cells with enlarged S-adenosylmethionine pools

Adenosine and many adenosine analogues exert toxicity to mammalian cells at the nucleoside level. The mechanism of action of these agents is controversial. Previous experiments suggested that adenosine toxicity could be mediated by the accumulation of S-adenosylhomocysteine (AdoHcy), a potent inhibitor of S-adenosylmethionine (AdoMet) dependent methylation reactions. To analyze this question genetically, adenosine resistant, adenosine kinase deficient mutant clones of a murine T-lymphoma cell line (R1.1) have been selected and analyzed. Compared to parental lymphoma cells, the adenosine resistant mutants had severalfold elevated levels of AdoMet and an increased AdoMet:AdoHcy ratio. The activity of methionine adenosyltransferase was also raised in the mutants. The mutant cells were cross-resistant to agents postulated to cause accretion of AdoHcy, formation of AdoHcy analogues, impairment of AdoMet synthesis, or direct interference with AdoMet dependent reactions. These included 3-deazaadenosine, carbocyclic adenosine, carbocyclic 3-deazaadenosine, formycin A, 8-azaadenosine, 5'-deoxy-5'-methylthiotubercidin, 5'-deoxy-5'-methylthioadenosine, 5'-deoxy-5'-S-isobutylthioadenosine, adenine, cycloleucine, L-ethionine, seleno-DL-ethionine, and (+/-)-2-aminobicyclo[2.1.1]hexane-2-carboxylic acid. These results suggest that diverse purine nucleoside and methionine analogues may block the growth of adenosine kinase deficient cells by interference with AdoMet synthesis and degradation. An increase in AdoMet pools can render mammalian cells cross-resistant to multiple drugs affecting this essential metabolic pathway.

DNA strand breaks, NAD metabolism, and programmed cell death

An intimate relationship exists between DNA single-strand breaks, NAD metabolism, and cell viability in quiescent human lymphocytes. Under steady-state conditions, resting lymphocytes continually break and rejoin DNA. The balanced DNA excision-repair process is accompanied by a proportional consumption of NAD for poly(ADP-ribose) synthesis. However, lymphocytes have a limited capacity to resynthesize NAD from nicotinamide. An increase in DNA strand break formation in lymphocytes, or a block in DNA repair, accelerates poly(ADP-ribose) formation and may induce lethal NAD and ATP depletion. In this way, the level of DNA single-strand breaks in the lymphocyte nucleus is linked to the metabolic activity of the cytoplasm. The programmed removal of lymphocytes (and perhaps of other cells) with damaged DNA, may represent a novel physiologic function for poly(ADP-ribose)-dependent NAD cycling.

Inhibition of DNA repair by deoxyadenosine in resting human lymphocytes

Profound lymphopenia is characteristic of immunodeficient children who lack adenosine deaminase (ADA). When ADA is inactive, deoxyadenosine (dAdo) is phosphorylated by immature T lymphoblasts and inhibits cell division. However, dAdo also causes the slow accumulation of DNA strand breaks in nondividing, mature human peripheral blood lymphocytes. To explore the basis for this phenomenon, we have assessed the effects of dAdo and other deoxynucleosides on the repair of gamma-radiation induced DNA strand breaks in resting normal lymphocyte cultures. As measured by a sensitive DNA unwinding assay, most DNA strand breaks were rejoined within 2 hr after exposure of lymphocytes to 500 rad. In medium supplemented with deoxycoformycin, a tight binding ADA inhibitor, dAdo retarded DNA rejoining in a dose and time dependent manner. The inhibition required dAdo phosphorylation. Over an 8-hr period, 10 microM dAdo gradually rendered peripheral blood lymphocytes incompetent for DNA repair. Among several other compounds tested, 2-chlorodeoxyadenosine, an ADA resistant dAdo congener with anti-leukemic and immunosuppressive activity, was the most powerful inhibitor of DNA repair, exerting significant activity at concentrations as low as 100 nM. Both dAdo and 2-chlorodeoxyadenosine blocked unscheduled DNA synthesis in irradiated resting lymphocytes, as measured by [3H]thymidine uptake. On the basis of this and other data, we suggest that quiescent peripheral blood lymphocytes break and rejoin DNA at a slow and balanced rate. The accumulation of dATP progressively retards the DNA repair process and thereby fosters the time-dependent accretion of DNA strand breaks. By inhibiting DNA repair, dAdo, 2-chlorodeoxyadenosine and related compounds may substantially potentiate the toxicity of DNA damaging agents to normal and malignant lymphocytes.

Inhibition of DNA repair by deoxyadenosine in resting human lymphocytes

Profound lymphopenia is characteristic of immunodeficient children who lack adenosine deaminase (ADA). When ADA is inactive, deoxyadenosine (dAdo) is phosphorylated by immature T lymphoblasts and inhibits cell division. However, dAdo also causes the slow accumulation of DNA strand breaks in nondividing, mature human peripheral blood lymphocytes. To explore the basis for this phenomenon, we have assessed the effects of dAdo and other deoxynucleosides on the repair of gamma-radiation induced DNA strand breaks in resting normal lymphocyte cultures. As measured by a sensitive DNA unwinding assay, most DNA strand breaks were rejoined within 2 hr after exposure of lymphocytes to 500 rad. In medium supplemented with deoxycoformycin, a tight binding ADA inhibitor, dAdo retarded DNA rejoining in a dose and time dependent manner. The inhibition required dAdo phosphorylation. Over an 8-hr period, 10 microM dAdo gradually rendered peripheral blood lymphocytes incompetent for DNA repair. Among several other compounds tested, 2-chlorodeoxyadenosine, an ADA resistant dAdo congener with anti-leukemic and immunosuppressive activity, was the most powerful inhibitor of DNA repair, exerting significant activity at concentrations as low as 100 nM. Both dAdo and 2-chlorodeoxyadenosine blocked unscheduled DNA synthesis in irradiated resting lymphocytes, as measured by [3H]thymidine uptake. On the basis of this and other data, we suggest that quiescent peripheral blood lymphocytes break and rejoin DNA at a slow and balanced rate. The accumulation of dATP progressively retards the DNA repair process and thereby fosters the time-dependent accretion of DNA strand breaks. By inhibiting DNA repair, dAdo, 2-chlorodeoxyadenosine and related compounds may substantially potentiate the toxicity of DNA damaging agents to normal and malignant lymphocytes.

Mechanism of deoxyadenosine and 2-chlorodeoxyadenosine toxicity to nondividing human lymphocytes

Deoxyadenosine has been implicated as the toxic metabolite causing profound lymphopenia in immunodeficient children with a genetic deficiency of adenosine deaminase (ADA), and in adults treated with the potent ADA inhibitor deoxycoformycin. However, the biochemical basis for deoxyadenosine toxicity toward lymphocytes remains controversial. The present experiments have examined in detail the sequential metabolic changes induced in nondividing human peripheral blood lymphocytes by incubation with deoxyadenosine plus deoxycoformycin, or with 2-chlorodeoxyadenosine (CdA), an ADA resistant deoxyadenosine congener with anti-leukemic and immunosuppressive properties. The lymphotoxic effect of deoxyadenosine and CdA required their phosphorylation, and was inhibited by deoxycytidine. As early as 4 h after exposure to the deoxynucleosides, strand breaks in lymphocyte DNA began to accumulate, and RNA synthesis decreased. These changes were followed by a significant fall in intracellular NAD levels at 8 h, a drop in ATP pools at 24 h, and cell death by 48 h. Incubation of the lymphocytes with 5 mM nicotinamide, a NAD precursor and an inhibitor of poly(ADP-ribose) synthetase, prevented NAD depletion. The nicotinamide treatment also rendered the lymphocytes highly resistant to deoxyadenosine and CdA toxicity, without altering dATP formation or the accumulation of DNA strand breaks. The poly(ADP-ribose) synthetase inhibitor 3-aminobenzamide exerted a similar although less potent effect. These results suggest that NAD depletion, probably triggered by poly(ADP-ribose) formation, is the principle cause of death in normal resting human lymphocytes exposed to deoxyadenosine plus deoxycoformycin, or to CdA.

Deoxynucleoside overproduction in deoxyadenosine-resistant, adenosine deaminase-deficient human histiocytic lymphoma cells

Deoxyadenosine toxicity toward lymphocytes may produce immune dysfunction in patients with adenosine deaminase (adenosine aminohydrolase, EC 3.5.4.4) deficiency. The relationship between endogenous deoxynucleoside synthesis in adenosine deaminase-deficient cells and sensitivity to adenosine and deoxyadenosine toxicity is unclear. The human histiocytic lymphoma cell line (DHL-9) naturally lacks adenosine deaminase, and has minimal levels of thymidine kinase. Dividing DHL-9 cells excrete deoxyadenosine and thymidine into the extracellular space. The present experiments have analyzed nucleoside synthesis and excretion in a mutagenized clone of DHL-9 cells, selected for increased resistance to deoxyadenosine toxicity. The deoxyadenosine-resistant cells excreted both deoxyadenosine and thymidine at a 6-7-fold higher rate than wild-type lymphoma cells. The deoxyadenosine overproduction was accompanied by a reduced ability to form dATP from exogenous deoxyadenosine, and a 2.5-fold increase in ribonucleotide reductase activity. The pace of adenosine excretion, the growth rate, and the levels of multiple other enzymes involved in deoxyadenosine and adenosine metabolism were equivalent in the two cell types. These results suggest that the excretion of deoxyadenosine and thymidine, but not adenosine, is exquisitely sensitive to alterations in the rate of endogenous deoxynucleotide synthesis. Apparently, small changes in deoxynucleotide synthesis can significantly influence cellular sensitivity to deoxyadenosine toxicity.

Assignment of the gene for methylthioadenosine phosphorylase to human chromosome 9 by mouse-human somatic cell hybridization

The purine and polyamine metabolic enzyme methylthioadenosine (MeSAdo) phosphorylase is abundant in normal cells and tissues but is lacking from many human and murine malignant cell lines and from cells of some human leukemias in vivo. To explore the genetic control of MeSAdo phosphorylase expression, we measured levels of the enzyme in somatic cell hybrids prepared by fusing MeSAdo phosphorylase-deficient mouse L cell lines with human fibroblasts. In the hybrid clones, MeSAdo phosphorylase activity segregated concordantly with adenylate kinase 1, a marker for human chromosome 9, but not with enzyme markers for any other human chromosome. In hybrid clones derived from human fibroblasts with a reciprocal translocation between chromosomes 9 and 17, MeSAdo phosphorylase activity was confined to cells containing the 9pter----9q12 region. In every case, the enzyme-positive hybrid clones displayed bands of MeSAdo phosphorylase activity with isoelectric points characteristic of both the human and murine enzymes. These results indicate that the structural gene for human MeSAdo phosphorylase, designated MTAP, can be assigned to the 9pter----9q12 region of human chromosome 9. Furthermore, these studies with interspecies somatic cell hybrids show that the MeSAdo phosphorylase-deficient state is recessive in mouse L cell lines.

Uptake and metabolism of daunorubicin by human leukemia cells

Radiolabeled daunorubicin was used to study in vitro uptake of daunorubicin (DNR) by the human promyelocytic leukemia cell line HL-60 and by leukemic cells from five previously untreated patients with acute nonlymphocytic leukemia (ANLL). Uptake of the metabolite daunorubicinol (DOL) and the metabolism of DNR were examined using high-performance liquid chromatography (HPLC). Uptake of DNR and DOL by HL-60 and ANLL cells exhibited a similar kinetic pattern. The uptake of DOL was 35%-50% of the uptake of DNR at the same test concentration in both HL-60 and ANLL cells. Approximately 5%-10% of intracellular DNR was metabolized to DOL by HL-60 and ANLL cells after 24 h of drug exposure. Measurements of DNR or DOL derived from liquid scintillation spectrometry and HPLC permit a sensitive and accurate assessment of the pharmacokinetics of these drugs in human leukemia cells. In addition, the HL-60 cell line can be used as a model for studying in vitro pharmacokinetics of the anthracyclines.

Cellular and plasma kinetics of daunorubicin given by two methods of administration in a patient with acute leukemia

Leukemic cell and plasma concentrations of daunorubicin were examined serially in a patient with acute lymphoblastic leukemia following administration of the drug by two different regimens: regimen A-an iv bolus dose of 50 mg/m2, and regimen B-a loading dose of 15 mg/m2 followed by 35 mg/m2 infused over 4 hours. Peak plasma levels were four times higher after regimen A the B, whereas leukemic cell concentrations were not significantly different. No difference in acute toxicity or therapeutic efficacy was seen after either method of administration. However, we have demonstrated that peak plasma levels of daunorubicin can be reduced by continuous infusion without significantly altering target tissue concentrations.

Blastic transformation of a well-differentiated monocytic leukemia: changes in cytochemical and cell surface markers

The clinical course of a patient with a well-differentiated monocytic leukemia which later underwent blastic transformation is described. Cytochemical, ultrastructural and cell surface analysis data were obtained at periods throughout her illness and correlated with the blastic transformation. Although surface markers characteristic of monocytic leukemia persisted, a deficiency of peroxidase in the granules of this patient's monocytes was observed as well as loss of alpha-naphthyl butyrate esterase staining during transformation.