Synergistic interaction between 1-beta-D-arabinofuranosylcytosine, thymidine, and hydroxyurea against human B cells and leukemic blasts in vitro

Ribonucleotide reductase inhibitors suppress SAMHD1 ara-CTPase activity enhancing cytarabine efficacy

The deoxycytidine analogue cytarabine (ara-C) remains the backbone treatment of acute myeloid leukaemia (AML) as well as other haematological and lymphoid malignancies, but must be combined with other chemotherapeutics to achieve cure. Yet, the underlying mechanism dictating synergistic efficacy of combination chemotherapy remains largely unknown. The dNTPase SAMHD1, which regulates dNTP homoeostasis antagonistically to ribonucleotide reductase (RNR), limits ara-C efficacy by hydrolysing the active triphosphate metabolite ara-CTP. Here, we report that clinically used inhibitors of RNR, such as gemcitabine and hydroxyurea, overcome the SAMHD1-mediated barrier to ara-C efficacy in primary blasts and mouse models of AML, displaying SAMHD1-dependent synergy with ara-C. We present evidence that this is mediated by dNTP pool imbalances leading to allosteric reduction of SAMHD1 ara-CTPase activity. Thus, SAMHD1 constitutes a novel biomarker for combination therapies of ara-C and RNR inhibitors with immediate consequences for clinical practice to improve treatment of AML.

A phase II study of cisplatin preceded by a 12-h continuous infusion of concurrent hydroxyurea and cytosine arabinoside (Ara-C) for adult patients with malignant gliomas (Southwest Oncology Group S9149)

Inhibition of DNA excision repair can modulate resistance to cisplatin. Cytosine arabinoside (Ara-C) and hydroxyurea (HU), in combination, inhibit the excision-repair system and removal of platinum-DNA adducts. Marked cytotoxic synergy had been demonstrated in vitro at clinically achievable levels. The three-drug regimen was found to be feasible in clinical pilot studies. A Phase II study in patients with relapsed or progressive anaplastic astrocytoma (AA) or glioblastoma multiforme (GBM) was performed in the Southwest Oncology Group. The primary end point was 6 month survival, historically about 42%. A loading dose of HU 1,260 mg/m2 IV over 1 h was followed by Ara-C 1,200 mg/m2 plus HU 5,040 mg/m2 IV over 12 h, followed by cisplatin 100 mg/m2 IV over 1 h. A total of 76 patients were registered. The GBM stratum registered 56 patients in a two-stage accrual. Among 51 eligible GBM patients, the 6-month survival probability was 41% (95% CI 28-55%), and median overall survival was 5 months (95% CI 4-6 months). The 6-month progression-free survival probability was 25% (95% CI 14-37%), and median progression-free survival was 2 months (95% CI 2-4 months). One patient achieved a partial response (2%, 95% CI 0-10%), 13 patients had stable disease (25%, 95% CI 14-39%). Twenty-two patients progressed, and 14 were not assessable for response. The AA stratum was closed early after 20 patients due to slow accrual. Among 19 eligible patients, the 6-month survival probability was 58% (95% CI 36-80%), and median overall survival was 7 months (95% CI 7-14 months). The 6-month progression-free survival probability was 26% (95% CI 6-46%), and median progression-free survival was 3 months (95% CI 2-5 months). No responses were seen. Six patients (32%) had stable disease (95% CI 13-57%), 11 progressed, and 2 were not assessable for response. Of the 70 patients evaluable for toxicity, two died of infection. Twenty-three patients (33%) experienced Grade 4 toxicities, primarily hematological. Cisplatin combined with HU and Ara-C did not improve the 6 month survival rate in patients with relapsed or progressive AA or GBM. Significantly more hematological toxicity was seen than expected from cisplatin alone. Although benefit might be possible in a more platinum-sensitive tumor type, further clinical trials with this regimen for patients with glioblastoma multiforme or anaplastic astrocytoma are not justified.

Successful treatment of recalcitrant psoriasis with a combination of infliximab and hydroxyurea

We report the use of a combination of the tumour necrosis factor alpha (TNF alpha) inhibitor infliximab and hydroxyurea to achieve control of disabling psoriasis and psoriatic arthritis. Our patient had psoriasis that proved resistant to conventional therapy including vitamin D analogues, topical steroids, dithranol, crude coal tar, narrow band UVB, bath PUVA and acitretin. She subsequently responded to hydroxyurea 1 g daily combined with infliximab infusions repeated at three monthly intervals which led to satisfactory control of her psoriasis and psoriatic arthritis. She has not reported any side-effects from this treatment regimen and her full blood count has remained normal.

Modulation of ara-CTP levels by fludarabine and hydroxyurea in leukemic cells

The rate of ara-cytosine triphosphate (ara-CTP) accumulation and its retention has been correlated with 1-beta-D-arabinofuranosylcytosine (ara-C)-mediated toxicity and clinical outcome in childhood and adult leukemia. We tested to what extent preincubation with the ribonucleotide reductase inhibitors fludarabine (F-ara-A) and hydroxyurea (HU) enhanced ara-CTP levels in two human myeloid (HL-60, CMK) and two lymphoblastic leukemia cell lines (MOLT-4, BLIN-1) and also in blasts from 28 children with acute leukemia (AML: 14, ALL: 14). Incubation experiments carried out with cell lines showed F-ara-A and HU to be equipotent in increasing ara-CTP levels. The highest increase was observed in HL-60 cells whereas preincubation had no modulatory effect in MOLT-4 cells. Accordingly, modulation of intracellular ara-CTP levels differed between the subtypes of childhood acute leukemia: whereas in T-ALL (five) preincubation with F-ara-A and HU had no effect on intracellular ara-C metabolism, increased ara-CTP levels were seen in some cases of pre-B-ALL (seven). In myelogenous blasts (12) clinically relevant enhancement of ara-C toxification was regularly obtained with both, F-ara-A (1.9-fold) and HU (1.5-fold). In conclusion, our data suggest that combinations of ara-C and ribonucleotide reductase inhibitors are apt to increase ara-CTP levels depending on the individual cell type and its sensitivity towards ara-C modulators.

The subcellular location of nucleoside analog phosphorylation is a determinant of synergistic effects of hydroxyurea

The ribonucleotide reductase inhibitor hydroxyurea exhibits synergistic pharmacological activity with several nucleoside analogs used in antiviral and anticancer chemotherapy. We have used a cell model system where a deoxycytidine kinase (dCK)-deficient cell line was reconstituted with genetically engineered dCK targeted to the cytosol, the nucleus, or the mitochondria to investigate how the subcellular location of nucleoside analog phosphorylation affected the synergistic effects of a ribonucleotide reductase inhibitor. Hydroxyurea showed synergistic cytotoxicity with the nucleoside analogs 1-beta-d-arabinofuranosylcytosine and 2-chloro-2'-deoxyadenosine when dCK was expressed in the cytosol or in the nucleus, but not when dCK was expressed in the mitochondria. These data indicate that the synergistic effect of ribonucleotide reductase inhibition is limited to nucleoside analogs phosphorylated in the cytosol or the cell nucleus.

Ara-C: cellular and molecular pharmacology

The antimetabolite cytosine arabinoside (ara-C) represents a prototype of the nucleoside analog class of antineoplastic agents and remains one of the most effective drugs used in the treatment of acute leukemia as well as other hematopoietic malignancies. The ability of ara-C to kill neoplastic cells is regulated at three distinct but interrelated levels. First, the activity of ara-C depends on conversion to its lethal triphosphate derivative, ara-CTP, a process that is influenced by multiple factors, including nucleoside transport, phosphorylation, deamination, and levels of competing metabolites, particularly dCTP. Second, the antiproliferative and lethal effects of ara-C are linked to the ability of ara-CTP to interfere with one or more DNA polymerases as well as the degree to which it is incorporated into elongating DNA strands, leading to DNA fragmentation and chain termination. Finally, the fate of the cell is ultimately determined by whether a threshold level of ara-C-mediated DNA damage is exceeded, thereby inducing apoptosis, or programmed cell death. The latter process is influenced by components of various signal transduction pathways (e.g., PKC) and expression of oncogenes (e.g., bcl-2, c-Jun), perturbations in which may significantly alter ara-C sensitivity. A better understanding of these factors could eventually lead to the development of novel therapeutic strategies capable of overcoming ara-C resistance and improving therapeutic efficacy.

Hydroxyurea enhances 3'-azido-3'-deoxythymidine (AZT) cytotoxicity in human chronic myeloid leukemia models

In this report we have evaluated the cytotoxic activity of 3'-azido-3'-deoxythymidine (AZT) used in combination with hydroxyurea (HU), an agent which disrupts de novo thymidylate synthesis. In 2 chronic myeloid leukemia (CML) cell lines, K562 and RWLeu4, the IC50 of AZT was 8 mumol/l and 28 mumol/l respectively, after a 5-day exposure, and the IC50 of HU was 80 mumol/l and 70 mumol/l respectively. In the presence of various concentrations of HU (1 mumol/l-100 mumol/l) the IC50 of AZT in both cell lines was significantly reduced and subsequent isobologram analysis revealed synergistic activity. Similarly, analysis of [3H]AZT incorporation into the DNA fraction of these cells indicated that exposure to AZT+HU resulted in an increased incorporation of AZT into DNA when compared to incubation in AZT alone. Biochemically, this effect appeared to be related to a decrease in dTTP pools caused by HU. The combination AZT+HU has also been demonstrated to exert a synergistic effect in inhibiting colony growth of bone marrow granulocyte-macrophage progenitors (CFU-GM) from patients affected by Ph1+ CML in chronic phase. These results are promising in view of a possible in vivo utilization of this drug combination.

Cisplatin preceded by concurrent cytarabine and hydroxyurea: a pilot study based on an in vitro model

As previously reported, cytotoxic synergy is produced when clinically achievable concentrations of cytarabine (Ara-C) and hydroxyurea (HU) are used as potential inhibitors of in vitro DNA repair in cisplatin (cis-Pt)-treated human colon carcinoma cells. This pilot study was subsequently designed to duplicate the in vitro dose and schedule and to determine the toxicity of this three-drug combination in two cohorts of patients. 21 patients had received prior chemotherapy and 19 were not previously treated. All patients had refractory solid tumors. They received monthly cycles of an oral loading dose of 800 mg/m2 HU followed every 2 h by 6 oral doses of 400 mg/m2, a 12-h continuous infusion of 200 or 250 mg/m2/h Ara-C concurrent with the HU, and then 100 mg/m2 cis-Pt over 1 h. A total of 95 cycles were given with the expected toxicities of nausea and vomiting and fatigue but not major acute toxicity observed. Thrombocytopenia was significant but transient and was dose-limiting only for patients who had received prior therapy. The median platelet nadir after one cycle was 43,000/microliters for all patients and 67,000/microliters for those who had not undergone prior treatment. Azotemia was treatment-limiting in responding and stable patients, suggesting the possibility of synergistic nephrotoxicity. Interestingly, there were early transient rises in both uric acid and lactate dehydrogenase (LDH). Partial responses were seen in 9 of 32 patients with measurable disease and there was significantly improvement in 5 of 8 patients with only evaluable disease. The responses or improvement occurred in patients with non-small-cell lung cancer, breast carcinoma, glioblastoma, ovarian carcinoma, small-cell lung cancer, and mesothelioma. Of these 14 patients, 9 had failed prior chemotherapy regimens. Significantly, responses were observed in 3 of 8 patients who had previously received cis-Pt, suggesting that the HU/Ara-C combination modulated cis-Pt resistance. Because of these encouraging results, a second pilot study has been initiated with modifications dictated by the toxicity issues raised in this trial.

Hydroxyurea increases the phosphorylation of 3'-fluorothymidine and 3'-azidothymidine in CEM cells

The triphosphates of the nucleoside analogues 3'-azidothymidine and 3'-fluorothymidine inhibit reverse transcriptase and are of therapeutic interest for the treatment of retrovirus infections. At equimolar concentrations 3'-fluorothymidine was more effectively transformed to the triphosphate by human CEM cells than azidothymidine which mainly accumulates as the monophosphate. Hydroxyurea, a drug that inhibits de novo synthesis of deoxyribonucleotides, considerably increased the ability of cells to phosphorylate both analogues. Addition of as little as 50 microM hydroxyurea decreased the amount of dideoxynucleoside required to attain a given intracellular concentration of its triphosphate by an order of magnitude. Hydroxyurea is known to shift the balance of substrate cycles between natural deoxynucleosides and their 5'-phosphates in the direction of synthesis and thereby to increase the import and intracellular phosphorylation of the nucleoside. The present results demonstrate a similar effect for the two analogues and raise the possibility of using this effect in therapy.

Interrelationships of protein and DNA syntheses during replication of mammalian cells

During the replication of chromatin, the syntheses of the histone protein and DNA components are closely coordinated but not totally linked. The interrelationships of total protein synthesis, histone protein synthesis, DNA synthesis, and mRNA levels have been investigated in Chinese hamster ovary cells subjected to several different types of inhibitors in several different temporal combinations. The results from these studies and results reported elsewhere can be brought together into a consistent framework which combines the idea of autoregulation of histone biosynthesis as originally proposed by W. B. Butler and G. C. Mueller (Biochim. Biophys. Acta 294:481-496, 1973] with the presence of basal histone synthesis and the effects of protein synthesis on DNA synthesis. The proposed framework obviates the difficulties of Butler and Mueller's model and may have wider application in understanding the control of cell growth.

Interrelationships of protein and DNA syntheses during replication of mammalian cells

During the replication of chromatin, the syntheses of the histone protein and DNA components are closely coordinated but not totally linked. The interrelationships of total protein synthesis, histone protein synthesis, DNA synthesis, and mRNA levels have been investigated in Chinese hamster ovary cells subjected to several different types of inhibitors in several different temporal combinations. The results from these studies and results reported elsewhere can be brought together into a consistent framework which combines the idea of autoregulation of histone biosynthesis as originally proposed by W. B. Butler and G. C. Mueller (Biochim. Biophys. Acta 294:481-496, 1973] with the presence of basal histone synthesis and the effects of protein synthesis on DNA synthesis. The proposed framework obviates the difficulties of Butler and Mueller's model and may have wider application in understanding the control of cell growth.

Thymidine as an anticancer agent, alone or in combination. A biochemical appraisal

The value of thymidine as a cytotoxic drug alone or in combination with other pyrimidine antimetabolites has received considerable attention in recent years. In this paper, the biochemical basis for the cytotoxicity of thymidine and its interaction with other pyrimidine antimetabolites is described. It is indicated that early clinical trials have largely failed to substantiate data from experimental studies that have shown thymidine to be an effective antimetabolite and capable of potentiating the antineoplastic effect of several other agents. It is suggested that tumours likely to respond to thymidine alone or in combination may be identified by measuring in clinical tumour specimens known biochemical determinants of thymidine efficacy.

Modulation of the cellular pharmacology and clinical toxicity of 1-beta-D-arabinofuranosylcytosine

The effect of thymidine (dThd) and hydroxyurea (HU) on the cellular metabolism of 1-beta-D-arabinofuranosylcytosine (Ara-C) was investigated in the human promyelocytic cell line HL-60. Both dThd and HU increased the cellular uptake and rate of formation of Ara-CTP. Measurement of ribo- and deoxyribonucleotide triphosphate pools implicated a reduction of the dCTP as the mechanism of this effect. dThd and HU had opposite effects on the incorporation of Ara-C into DNA per unit time, but both enhanced the incorporation of Ara-C per unit of newly synthesized DNA. In a Phase I trial Ara-C was given by continuous infusion for five days at 100 mg/m2, and HU by mouth every six hours with dose escalation from 0.375 to 1.78 g/m2 every six hours. Myelosuppression was the dose-limiting toxicity; the major nonhematologic toxicity was skin rash. To date responses have been observed in chronic myelogenous leukemia in blast crisis and diffuse histiocytic lymphoma.