Potentiation of in vitro ara-C cytotoxicity by ribonucleotide reductase inhibitors, cyclin-dependent kinase modulators and the DNA repair inhibitor aphidicolin in paediatric acute myeloid leukaemia

RNA Targeting in Acute Myeloid Leukemia

Nucleosides and their analogues constitute an essential family of anticancer drugs. DNA has been the presumptive target of the front-line prodrug for acute myeloid leukemia (AML), cytarabine (ara-C), since the 1980s. Here, the biomolecular targeting of ara-C was evaluated in primary white blood cells using the ara-C mimic "AzC" and azide-alkyne "click" reactions. Fluorescent staining and microscopy revealed that metabolic incorporation of AzC into primary white blood cells was unexpectedly enhanced by the DNA polymerase inhibitor aphidicholine. According to RNaseH digestion and pull-down-and-release experiments, AzC was incorporated into short RNA fragments bound to DNA in peripheral blood monocytes (PBMCs) collected from all six healthy human donors tested. Samples from 22 AML patients (French-American-British classes M4 and M5) exhibited much more heterogeneity, with 27% incorporating AzC into RNA and 55% into DNA. The overall survival of AML patients whose samples incorporated AzC into RNA was approximately 3-fold higher as compared to that of the DNA cohort (p ≤ 0.056, χ² = 3.65). These results suggest that the RNA primers of DNA synthesis are clinically favorable targets of ara-C, and that variable incorporation of nucleoside drugs into DNA versus RNA may enable future patient stratification into treatment-specific subgroups.

Modified cladribine, cytarabine, and G-CSF as a salvage regimen in patients with relapsed/refractory acute myeloid leukemia: a bridge to myeloablative allogeneic hematopoietic stem cell transplantation

Patients with primary refractory or early relapsed acute myeloid leukemia (AML) have a dismal prognosis, and the treatment options for these patients are limited. The present study retrospectively examined the efficacy and toxicities of the combination of cladribine 5 mg/m² per day and intermediate-dose cytarabine 1 g/m² per day for 5 days and granulocyte colony-stimulating factor (G-CSF) as a salvage treatment in 36 patients with relapsed/refractory AML. Among these, 32 patients had de novo AML, and the remaining 4 patients had secondary AML. The median age for the study cohort was 45.8 years. According to the European LeukemiaNet prognostic index, 5 patients had favorable risk, 18 had intermediate risk, and 11 had poor risk. The complete remission was achieved in 58% of the patients with tolerable toxicities. Fifteen patients underwent stem cell transplantation later. Patients who underwent allogeneic hematopoietic stem cell transplantation had a significantly improved 1-year overall survival compared with those who did not (73% vs. 29%, P < 0.001). The results suggested that, as a salvage regimen, modified cladribine, cytarabine, and G-CSF were effective and well tolerated for patients with relapsed/refractory AML, especially for patients who underwent subsequent stem cell transplantation.

Targeting DNA repair with aphidicolin sensitizes primary chronic lymphocytic leukemia cells to purine analogs

Purine analogs are among the most effective chemotherapeutic drugs for the treatment of chronic lymphocytic leukemia (CLL). However, chemoresistance and toxicity limit their clinical use. Here, we report that the DNA polymerase inhibitor aphidicolin, which displayed negligible cytotoxicity as a single agent in primary CLL cells, markedly synergizes with fludarabine and cladribine via enhanced apoptosis. Importantly, synergy was recorded regardless of CLL prognostic markers. At the molecular level, aphidicolin enhanced purine analog-induced phosphorylation of p53 and accumulation of γH2AX, consistent with increase in DNA damage. In addition, aphidicolin delayed γH2AX disappearance that arises after removal of purine analogs, suggesting that aphidicolin causes an increase in DNA damage by impeding DNA damage repair. Similarly, aphidicolin inhibited UV-induced DNA repair known to occur primarily through the nucleotide excision repair (NER) pathway. Finally, we showed that fludarabine induced nuclear import of XPA, an indispensable factor for NER, and that XPA silencing sensitized cell lines to undergo apoptosis in response to fludarabine. Together, our data indicate that aphidicolin potentiates the cytotoxicity of purine analogs by inhibiting a DNA repair pathway that involves DNA polymerases, most likely NER, and provide a rationale for manipulating it to therapeutic advantage.

Assaying pharmacodynamic endpoints with targeted therapy: flavopiridol and 17AAG induced dephosphorylation of histone H1.5 in acute myeloid leukemia

Histone H1 is commonly used to assay kinase activity in vitro. As many promising targeted therapies affect kinase activity of specific enzymes involved in cancer transformation, H1 phosphorylation can serve as potential pharmacodynamic marker for drug activity within the cell. In this study we utilized a phosphoproteomic workflow to characterize histone H1 phosphorylation changes associated with two targeted therapies in the Kasumi-1 acute myeloid leukemia cell line. The phosphoproteomic workflow was first validated with standard casein phosphoproteins and then applied to the direct analysis of histone H1 from Kasumi-1 nuclear lysates. Ten H1 phosphorylation sites were identified on the H1 variants, H1.2, H1.3, H1.4, H1.5 and H1.x. LC MS profiling of intact H1s demonstrated global dephosphorylation of H1.5 associated with therapy by the cyclin-dependent kinase inhibitor, flavopiridol and the Heat Shock Protein 90 inhibitor, 17-(Allylamino)-17-demethoxygeldanamycin. In contrast, independent treatments with a nucleotide analog, proteosome inhibitor and histone deacetylase inhibitor did not exhibit decreased H1.5 phosphorylation. The data presented herein demonstrate that potential of histones to assess the cellular response of reagents that have direct and indirect effects on kinase activity that alters histone phosphorylation. As such, this approach may be a highly informative marker for response to targeted therapies influencing histone phosphorylation.

Anti proliferative activity of ELACY (CP-4055) in combination with cloretazine (VNP40101M), idarubicin, gemcitabine, irinotecan and topotecan in human leukemia and lymphoma cells

This study evaluated combination drug partners for CP-4055, the C18:1^Δ9,trans unsaturated fatty acid ester of cytarabine in HL-60 and U937 cells. Growth inhibition was assessed by ATP assay and drug interaction by the combination index and three dimensional methods. Synergy was observed in HL-60 cells for simultaneous combinations of CP-4055 with gemcitabine, irinotecan and topotecan, while combinations with cloretazine (VNP40101M) and idarubicin were additive. In U937 cells, synergy was observed with gemcitabine and additivity for the other drugs. In HL-60, the IC50 concentration of CP-4055 could be reduced 10-fold and that of gemcitabine 3-fold in combination versus the agents alone, an interaction that was independent of drug sequence, ratio and exposure time. In contrast, interactions of CP-4055 with the topoisomerase inhibitors became antagonistic when the drugs were administered 24 h prior to CP-4055 and at certain drug ratios, particularly in U937 cells. In summary, CP-4055 produced additive to synergistic anti proliferative activity when combined simultaneously with drugs from four mechanistic classes in cell culture models of human leukemia and lymphoma. The impact of drug sequence and ratio on the interactions argues for incorporation of these parameters into the design of combination chemotherapy regimens.

Cladribine combined with high doses of arabinoside cytosine, mitoxantrone, and G-CSF (CLAG-M) is a highly effective salvage regimen in patients with refractory and relapsed acute myeloid leukemia of the poor risk: a final report of the Polish Adult Leukemia Group

OBJECTIVES

Patients with primary refractory AML and with early relapses have unfavorable prognoses and require innovative therapeutic approaches. Purine analogs fludarabine (FA) and cladribine (2-CdA) increase cytotoxic effect of Ara-C in leukemic blasts and inhibit DNA repair mechanisms; therefore its association with Ara-C and mitoxantrone (MIT) results in a synergistic effect. In the current report, we present the final results of multi-center phase II study evaluating the efficacy and toxicity of CLAG-M salvage regimen in poor risk refractory/relapsed AML patients.

METHODS

The induction chemotherapy consisted of 2-CdA 5 mg/m2, Ara-C 2 g/m2, MIT 10 mg/m2, and granulocyte-colony stimulating factor. In the case of PR, a second CLAG-M was administered. Patients in CR received consolidation courses based on high doses of Ara-C and MIT with or without 2-CdA.

RESULTS

One hundred and eighteen patients from 11 centers were registered; 78 primary resistant and 40 relapsed. Sixty-six patients (58%) achieved CR after one or two courses of CLAG-M, 49 (35%) were refractory, and 8 (7%) died early. WBC >10 g/L and age >34 yr were factors associated with increased risk of treatment failure. Hematological toxicity was the most prominent toxicity of this regimen. The probability of OS at 4 yr was 14% (95% CI 4-23%). OS was influenced by age, WBC >10 g/L and poor karyotype in both univariate and multivariate analyses. The probability of 4 yr DFS was 30% for all 66 patients in CR (95% CI 11-49%). Poor karyotype was the only factor associated with decreased probability of DFS.

CONCLUSIONS

We conclude that CLAG-M is a well-tolerated and highly effective salvage regimen in poor risk refractory/relapsed AML.

Effect of cytarabine and decitabine in combination in human leukemic cell lines

PURPOSE

1-beta-D-Arabinofuranosylcytosine (cytarabine; ara-C) is the most active agent in myeloid leukemia. 5-Aza-2'-deoxycytidine (DAC) is a cytosine analogue that inhibits DNA methylation and also has activity in myeloid leukemia. Therefore, we investigated combining these two drugs in human leukemia cell lines in vitro.

EXPERIMENTAL DESIGN

We initially examined the effects of ara-C and DAC on human leukemia cell lines HL60, ML-1, RAji, and Jurkat. We measured IC(50) of DAC and ara-C in these cell lines and calculated a combination index of these two drugs given either simultaneously or sequentially. In searching for mechanisms relative to epigenetic regulation for this effect, we examined DNA methylation of LINE and Alu repetitive elements as a surrogate for global genomic DNA methylation. In addition, we sorted Annexin V positive and negative cells and measured differences in LINE methylation between them.

RESULTS

The combination of DAC and ara-C showed additive induction of cell death in ML-1 and synergistic induction in HL60, Raji, and Jurkat. Sequentially, DAC followed by ara-C was a synergistic combination in all cell lines. Low-dose DAC induced more hypomethylation than high doses of the drug, whereas ara-C had no effects on methylation. The combination of ara-C with DAC either together or DAC followed by ara-C resulted in inhibition of LINE demethylation in HL60. The RIL gene, which is silenced by DNA hypermethylation, was activated by DAC, but the addition of ara-C to DAC reduced RIL gene activation. DAC treatment increased H3 Lys(9) acetylation of Alu elements, whereas ara-C had no effect, and the addition of ara-C to DAC inhibited this effect. Finally, we showed that after DAC exposure, Annexin V positive cells were more hypomethylated than Annexin V negative cells.

CONCLUSION

The combination of DAC and ara-C showed additive or synergistic effects on cell death in four human leukemia cell lines in vitro, but antagonism in terms of epigenetic effects. One possible explanation for these paradoxical observations is that hypomethylated cells are sensitized to cell killing by ara-C. These data suggest that DAC used in combination with ara-C has clinical potential in the treatment of acute myeloid leukemia.

The synergistic interaction of gemcitabine and cytosine arabinoside with the ribonucleotide reductase inhibitor triapine is schedule dependent

Gemcitabine and ara-C have multiple mechanisms of action: DNA incorporation and for gemcitabine also ribonucleotide reductase (RNR) inhibition. Since dCTP competes with their incorporation into DNA, dCTP depletion can potentiate their cytotoxicity. We investigated whether additional RNR inhibition by Triapine (3-AP), a new potent RNR inhibitor, enhanced cytotoxicity of gemcitabine and ara-C in four non-small-cell-lung-cancer (NSCLC) cell lines, using the multiple-drug-effect analysis. Simultaneous and sequential exposure (preexposure to 3-AP for 24h) in a constant molar ratio of 3-AP and gemcitabine was antagonistic/additive in all cell lines. Preexposure to 3-AP at an IC(25) concentration for 24h before variable concentrations of gemcitabine was synergistic. RNR inhibition by 3-AP resulted in a more synergistic interaction in combination with ara-C, which does not inhibit RNR. Two cell lines with pronounced synergism (SW1573) or antagonism (H460) for gemcitabine/3-AP, were evaluated for accumulation of the active metabolites, dFdCTP and ara-CTP. Simultaneous exposure induced no or a small increase, but ara-CTP increased after pretreatment with 3-AP, 4-fold in SW1573 cells, but not in H460 (<1.5 fold). Ara-C and gemcitabine incorporation into DNA were more pronounced (about 2-fold increased) for sequential treatment in SW1573 compared to H460 cells (<1.5 fold). This was not related to the activity and expression of deoxycytidine kinase and the M2 subunit of RNR. In conclusion, RNR inhibition by 3-AP prior to gemcitabine or ara-C exposure stimulates accumulation of the active metabolites and incorporation into DNA. The combination 3-AP/Ara-C is more synergistic than 3-AP/gemcitabine possibly because gemcitabine already inhibits RNR, but ara-C does not.

Improvement of esophageal adenocarcinoma cell and xenograft responses to radiation by targeting cyclin-dependent kinases

BACKGROUND AND PURPOSE

Concurrent chemo-radiotherapy before surgery is standard treatment protocol for esophageal cancer with a less than 30% complete response due to resistance to therapy. The aim of this study was to determine whether molecular targeting approach using an inhibitor of cyclin-dependent kinases, flavopiridol, can help overcome the resistance to radiotherapy.

MATERIALS AND METHODS

SEG-1 cells (human esophageal adenocarcinoma) were exposed to gamma-rays with and without flavopiridol treatment and assayed for clonogenic survival, apoptosis, cell cycle distribution, and Western blot analysis. Efficacy of flavopiridol in enhancing tumor response to radiation was determined by tumor growth delay assay using SEG-1 tumor xenografts generated in nude mice.

RESULTS

The clonogenic cell survival assay data showed that flavopiridol (300 nM, 24h), when given either before or after radiation, significantly enhanced the radiosensitivity of SEG-1 cells. The cells were accumulated at G1 phase of the cell cycle by flavopiridol that was associated with downregulation of p-cdk-1, p-cdk-2, cyclin D1 and p-Rb expression. Flavopiridol by itself induced apoptosis in SEG-1 cells and also enhanced the radiation-induced apoptosis, associated with an increase in cleaved poly ADP-ribose polymerase. Reduction in phosphorylation of RNA polymerase II by flavopiridol suggested that flavopiridol inhibited the transcriptional activity. In vivo studies with SEG-1 tumor xenografts showed that flavopiridol, either given before or after radiation, greatly enhanced the effect of tumor irradiation.

CONCLUSIONS

Flavopiridol treatment significantly enhanced SEG-1 cell radiosensitivity as well as the radioresponse of SEG-1 tumor xenografts. The underlying mechanisms are multiple, including cell cycle redistribution, apoptosis, and transcriptional inhibition. These preclinical data suggest that flavopiridol has the potential to increase the radioresponse of esophageal adenocarcinomas.