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

Geminal Diheteroatomic Motifs: Some Applications of Acetals, Ketals, and Their Sulfur and Nitrogen Homologues in Medicinal Chemistry and Drug Design

Acetals and ketals and their nitrogen and sulfur homologues are often considered to be unconventional and potentially problematic scaffolding elements or pharmacophores for the design of orally bioavailable drugs. This opinion is largely a function of the perception that such motifs might be chemically unstable under the acidic conditions of the stomach and upper gastrointestinal tract. However, even simple acetals and ketals, including acyclic molecules, can be sufficiently robust under acidic conditions to be fashioned into orally bioavailable drugs, and these structural elements are embedded in many effective therapeutic agents. The chemical stability of molecules incorporating geminal diheteroatomic motifs can be modulated by physicochemical design principles that include the judicious deployment of proximal electron-withdrawing substituents and conformational restriction. In this Perspective, we exemplify geminal diheteroatomic motifs that have been utilized in the discovery of orally bioavailable drugs or drug candidates against the backdrop of understanding their potential for chemical lability.

Phase I/II multisite trial of optimally dosed clofarabine and low-dose TBI for hematopoietic cell transplantation in acute myeloid leukemia

Clofarabine is an immunosuppressive purine nucleoside analog that may have better anti-leukemic activity than fludarabine. We performed a prospective phase I/II multisite trial of clofarabine with 2 Gy total body irradiation as non-myeloablative conditioning for allogeneic hematopoietic cell transplantation in adults with acute myeloid leukemia who were unfit for more intense regimens. Our main objective was to improve the 6-month relapse rate following non-myeloablative conditioning, while maintaining historic rates of non-relapse mortality (NRM) and engraftment. Forty-four patients, 53 to 74 (median: 69) years, were treated with clofarabine at 150 to 250 mg/m² , of whom 36 were treated at the maximum protocol-specified dose. One patient developed multifactorial acute kidney injury and another developed multiorgan failure, but no other grade 3 to 5 non-hematologic toxicities were observed. All patients fully engrafted. The 6-month relapse rate was 16% (95% CI, 5%-27%) among all patients and 14% (95% CI, 3%-26%) among high-risk patients treated at the maximum dose, meeting the pre-specified primary efficacy endpoint. Overall survival was 55% (95% CI, 40%-70%) and leukemia-free survival was 52% (95% CI, 37%-67%) at 2 years. Compared to a historical high-risk cohort treated with the combination of fludarabine at 90 mg/m² and 2 Gy TBI, protocol patients treated with the clofarabine-TBI regimen had lower rates of overall mortality (HR of 0.50, 95% CI, 0.28-0.91), disease progression or death (HR 0.48, 95% CI, 0.27-0.85), and morphologic relapse (HR 0.30, 95% CI, 0.13-0.69), and comparable NRM (HR 0.85, 95% CI 0.36-2.00). The combination of clofarabine with TBI warrants further investigation in patients with high-risk AML.

Fluorinated nucleosides as an important class of anticancer and antiviral agents

Fluorine-containing nucleoside analogs (NAs) represent a significant class of the US FDA-approved chemotherapeutics widely used in the clinic. The incorporation of fluorine into drug-like agents modulates lipophilic, electronic and steric parameters, thus influencing pharmacodynamic and pharmacokinetic properties of drugs. Fluorine can block oxidative metabolism of drugs and the formation of undesired metabolites by changing H-bonding interactions. In this review, we focus our attention on chemical fluorination reagents and methods used in the NAs field, including positron emission tomography radiochemistry. We briefly discuss both the cellular biology and clinical properties of FDA-approved and fluorine-containing nucleoside/nucleotide analogs in development as well as common resistance mechanisms associated with their use. Finally, we emphasize pronucleotide strategies used to improve therapeutic outcome of NAs in the clinic.

Determination and quantification of intracellular fludarabine triphosphate, cladribine triphosphate and clofarabine triphosphate by LC-MS/MS in human cancer cells

Purine nucleoside analogues are widely used in the treatment of haematological malignancies, and their biological activity is dependent on the intracellular accumulation of their triphosphorylated metabolites. In this context, we developed and validated a liquid chromatography tandem mass spectrometry (LC-MS/MS) method to study the formation of 5'-triphosphorylated derivatives of cladribine, fludarabine, clofarabine and 2'-deoxyadenosine in human cancer cells. Br-ATP was used as internal standard. Separation was achieved on a hypercarb column. Analytes were eluted with a mixture of hexylamine (5 mM), DEA (0.4%, v/v, pH 10.5) and acetonitrile, in a gradient mode at a flow rate of 0.3mLmin^-1. Multiple reactions monitoring (MRM) and electrospray ionization in negative mode (ESI-) were used for detection. The application of this method to the quantification of these phosphorylated cytotoxic compounds in a human follicular lymphoma cell line, showed that it was suitable for the study of relevant biological samples.

Metabolism, Biochemical Actions, and Chemical Synthesis of Anticancer Nucleosides, Nucleotides, and Base Analogs

Nucleoside, nucleotide, and base analogs have been in the clinic for decades to treat both viral pathogens and neoplasms. More than 20% of patients on anticancer chemotherapy have been treated with one or more of these analogs. This review focuses on the chemical synthesis and biology of anticancer nucleoside, nucleotide, and base analogs that are FDA-approved and in clinical development since 2000. We highlight the cellular biology and clinical biology of analogs, drug resistance mechanisms, and compound specificity towards different cancer types. Furthermore, we explore analog syntheses as well as improved and scale-up syntheses. We conclude with a discussion on what might lie ahead for medicinal chemists, biologists, and physicians as they try to improve analog efficacy through prodrug strategies and drug combinations.

Review : Purine nucleoside analogues: Fludarabine, pentostatin, and cladribine

Purpose. An introductory template for an extensive discussion of the pharmacology, pharmacokinetics, clinical use and adverse effects of the currently ap proved adenosine analogues: fludarabine, pentostatin, and cladribine is profiled. This is part one of a three-part series.

Data Sources. We reviewed the literature through a MEDLINE search from 1986 to 1996. Relevant articles cited in literature obtained by MEDLINE searching were also considered. We searched the following terms: fludarabine, cladribine, pentostatin, apoptosis and adenosine ana logues. The search was restricted to the English language. We have incorporated pricing information from our prac tice sites as well as the average wholesale price for the purpose of cost comparison.

Data Extraction. We have reviewed the current literature with regard to the chemistry, mechanisms of action and pharmacology, pharmacokinetics, clinical use, adverse effects, drug interactions, indications, formulation, dosage, administration, pharmaceutical issues and a cost comparison of the currently approved adenosine ana logues, fludarabine, pentostatin, and cladribine.

Data Synthesis. The adenosine analogues are struc turally similar agents used in the management of hemato logical malignancies. Fludarabine and cladribine exhibit significant activity in CLL (chronic lymphocytic leukemia) and NHL (non-Hodgkin's lymphoma) and pentostatin and cladribine are both active in the treatment of hairy cell leukemia. There are no comparative clinical trials between the agents and we have provided comparisons based on pharmacology, clinical experience, adverse effects and cost as well as reviewing the clinical use of these agents.

Conclusion. The adenosine analogues, fludarabine, pentostatin, and cladribine, represent an important ad vance in the treatment of indolent lymphoid malignancies. Although response rates for fludarabine and cladribine in chronic lymphocytic leukemia and for pentostatin and cladribine in hairy cell leuemia are improved over standard therapy, the true clinical impact of these agents has not yet been realized. Additional studies in larger populations of both previously treated and untreated patients, as well as comparative trials between the deoxyadenosine analogues themselves need to be carried out. Moreover, combination chemotherapy trials with deoxyadenosine analogues and other cytotoxic agents need to be performed to determine the efficacy and toxicity of these combinations in various lymphoid malignancies.

Simultaneous determination of clofarabine and cytarabine in human plasma by LC-MS/MS

Combination of cytostatic agents is a basic principle in the treatment of cancer. For the treatment of acute myeloid leukemia (AML), purine analogs, like clofarabine and cytarabine act synergistically. Little is known, however, on their interaction in vivo. We developed a method for the simultaneous determination of clofarabine and cytarabine in human plasma. The substances were extracted from plasma samples by protein precipitation with acetonitrile. Cladribine was the internal standard (IS). The analytes were separated on Synergi HydroRP column (150mm×2.0mm, 4μm) and a triple-quadrupole mass spectrometry with an electrospray ionisation (ESI) source was applied for detection. The mobile phase consisted of acetonitrile, ammonium acetate 2mM and 0.5% formic acid in a gradient mode at a flow rate of 0.5ml/min. The injection volume was 10μl and the total run time was 6.0min. Retention times were 2.46min for clofarabine, 0.97min for cytarabine and 2.43min for the IS. Calibration ranges were 8-1000ng/ml for clofarabine and 20-2500ng/ml for cytarabine. The intra-day and inter-day precision was less than 15% and the relative standard deviation was all within ±15%. This new method allows a rapid and simple determination of both clofarabine and cytarabine in human plasma. It was applied to a pharmacokinetic investigation within a hematological trial in adult patients with AML.

Control of relapsed or refractory acute myeloid leukemia by clofarabine in preparation for allogeneic stem cell transplant

Allogeneic stem cell transplant is indicated for patients with refractory or relapsed acute myeloid leukemia (AML). Since elimination of the leukemic load is thought to be a prerequisite for treatment success, we here investigate toxicity and anti-leukemic activity of a clofarabine-AraC salvage protocol preceding transplant. In this retrospective analysis, we observed induction of objective remissions in 86% of patients receiving clofarabine-AraC as compared to 83% with sequential high dose AraC/mitoxantrone (S-HAM) and 50% after mitoxantrone/topotecane/AraC (MTC) salvage strategies. In addition, clofarabine conferred anti-leukemic activity to some patients who failed initial MTC or S-HAM therapy. For overall and leukemia-free survival, we identified cytogenetically defined adverse risk markers but not response to therapy to be a strong predictor. In summary, the clofarabine-AraC salvage strategy combines pronounced anti-leukemic activity with an acceptable toxicity profile and allows the majority of patients with relapsed or refractory AML to proceed to allo-SCT, even in cytogenetically defined high risk situations.

Phenotypic screening in cancer drug discovery - past, present and future

There has been a resurgence of interest in the use of phenotypic screens in drug discovery as an alternative to target-focused approaches. Given that oncology is currently the most active therapeutic area, and also one in which target-focused approaches have been particularly prominent in the past two decades, we investigated the contribution of phenotypic assays to oncology drug discovery by analysing the origins of all new small-molecule cancer drugs approved by the US Food and Drug Administration (FDA) over the past 15 years and those currently in clinical development. Although the majority of these drugs originated from target-based discovery, we identified a significant number whose discovery depended on phenotypic screening approaches. We postulate that the contribution of phenotypic screening to cancer drug discovery has been hampered by a reliance on 'classical' nonspecific drug effects such as cytotoxicity and mitotic arrest, exacerbated by a paucity of mechanistically defined cellular models for therapeutically translatable cancer phenotypes. However, technical and biological advances that enable such mechanistically informed phenotypic models have the potential to empower phenotypic drug discovery in oncology.

Phase I study of oral clofarabine consolidation in adults aged 60 and older with acute myeloid leukemia

Clofarabine has shown activity and tolerability in older patients with acute myeloid leukemia (AML). We investigated the safety and tolerability of an oral formulation of clofarabine for consolidation therapy of patients aged 60 and older with AML. In this phase I study, twenty-two patients older than 60 years with AML in first complete remission were treated once daily with oral clofarabine for 14 or 21 days of a 28-day cycle, for up to five cycles. Dose escalation from 1 mg to 6 mg daily using a 3 + 3 design was used to determine dose-limiting toxicities (DLT), the maximum tolerated dose (MTD), and tolerability of oral clofarabine. No DLTs or Grade 3-4 nonhematologic toxicities were observed. The primary toxicities were hematologic, including uncomplicated grade 3-4 neutropenia (50%) and thrombocytopenia (50%). Given that myelosuppression necessitating dose delays/reductions was observed more commonly at higher doses, the recommended phase II dose is 2 mg daily for 21 of 28 days. At doses equal to or greater than 2 mg, the median relapse-free survival was 28.35 months. Oral clofarabine was well-tolerated with encouraging activity in patients older than 60 years. Further investigation of oral clofarabine as a consolidation and/or maintenance therapy in AML for older individuals is warranted. (ClinicalTrials.gov:NCT00727766).

Clofarabine targets the large subunit (α) of human ribonucleotide reductase in live cells by assembly into persistent hexamers

Clofarabine (ClF) is a drug used in the treatment of leukemia. One of its primary targets is human ribonucleotide reductase (hRNR), a dual-subunit, (α(2))(m)(β(2))(n), regulatory enzyme indispensable in de novo dNTP synthesis. We report that, in live mammalian cells, ClF targets hRNR by converting its α-subunit into kinetically stable hexamers. We established mammalian expression platforms that enabled isolation of functional α and characterization of its altered oligomeric associations in response to ClF treatment. Size exclusion chromatography and electron microscopy documented persistence of in-cell-assembled-α(6). Our data validate hRNR as an important target of ClF, provide evidence that in vivo α's quaternary structure can be perturbed by a nonnatural ligand, and suggest small-molecule-promoted, persistent hexamerization as a strategy to modulate hRNR activity. These studies lay foundations for documentation of RNR oligomeric state within a cell.

Clofarabine in the treatment of myelodysplastic syndromes

INTRODUCTION

Clofarabine is a second-generation purine nucleoside analog approved in 2004 for the treatment of pediatric patients with relapsed or refractory acute lymphocytic leukemia (ALL) following failure of at least two prior regimens. Clofarabine is a hybrid of fludarabine and cladribine, designed to overcome the pharmacologic limitations associated with its predecessors, while retaining their beneficial properties. In addition to providing a valuable treatment option for pediatric patients with ALL, clofarabine alone and in combination with cytarabine (Ara-C) has demonstrated substantial activity against myelodysplastic syndrome (MDS), thus rendering this agent a potential therapeutic option for MDS.

AREAS COVERED

This review focuses on the pharmacology and clinical activity of clofarabine in MDS, as well as its emerging role in the treatment of MDS. Publications in English were selected from the MEDLINE (PubMed) database, as well articles of interest from bibliographies and abstracts based on the publication of meeting materials.

EXPERT OPINION

DNA-methyltransferase inhibitors are the mainstay of therapy for many patients with MDS who require treatment. Although these agents are very well tolerated and represent a significant advancement in the treatment of MDS by improving transfusion requirements and prolonging survival in various subgroups of patients, response rates are modest and the duration of response is short. In addition to providing a valuable treatment option for pediatric ALL patients, clofarabine has substantial activity against MDS and is well tolerated by elderly patients, thus rendering it a potential therapeutic option.

Cytotoxicity of chemotherapeutic agents in glyceraldehyde-3-phosphate dehydrogenase-depleted human lung carcinoma A549 cells with the accelerated senescence phenotype

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) plays a central role in glycolysis. Because cancer cells rely on aerobic glycolysis rather than oxidative phosphorylation, GAPDH-depleting agents have a therapeutic potential to impede cancer cell proliferation. Knockdown of GAPDH by RNA interference induced the accelerated senescent phenotype in A549 cells, suggesting that GAPDH is a potential molecular target for combination chemotherapy. The cytotoxic effects of a panel of anticancer drugs, 5-fluorouracil, 5-fluorouridine, 5-fluorodeoxyuridine, 6-thioguanine, cytarabine, fludarabine, cladribine, clofarabine, 2-chloroadenosine, and doxorubicin, were assessed in GAPDH-depleted A549 cells using a cell proliferation assay. GAPDH-depleted A549 cells, when compared with control cells, exhibited increased chemoresistance to several antimetabolite agents including cytarabine [inhibitory concentration 50 (IC50) 1.7±0.3 vs. 0.03±0.02 μmol/l], 2-chloroadenosine (IC50 7.1±1.8 vs. 1.5±0.6 μmol/l), 6-thioguanine (IC50 7.5±1.6 vs. 1.4±0.5 μmol/l), 5-fluorouracil (IC50 13.2±2.5 vs. 3.0±0.7 μmol/l), and 5-fluorodeoxyuridine (IC50 >100 vs. 3.7±0.9 μmol/l), which we designated as group A agents. In contrast, GAPDH-deficient and GAPDH-proficient cells were equally sensitive to group B agents including doxorubicin (IC50 0.05±0.02 vs. 0.04±0.02 μmol/l), fludarabine (IC50 18.5±2.3 vs. 15.7±2.8 μmol/l), 5-fluorouridine (IC50 0.1±0.03 vs. 0.1±0.03 μmol/l), clofarabine (IC50 0.7±0.3 vs. 0.5±0.3 μmol/l), and cladribine (IC50 0.5±0.1 vs. 0.5±0.2 μmol/l). After treatment with group B agents at concentrations equivalent to 7-10-fold the IC50 value, the fraction of apoptotic cells in GAPDH-depleted, senescent A549 cells was similar to that in GAPDH-proficient cells. Our study identified the antimetabolite drugs active in senescent cells that can be used in combination with GAPDH inhibitors in cancer treatment. GAPDH-targeted combination therapy is a novel strategy to control the proliferation of tumor cells.

The role of clofarabine in acute myeloid leukemia

Clofarabine is a second-generation purine nucleoside analog that has been synthesized to overcome the limitations and incorporate the best qualities of fludarabine and cladribine. Clofarabine acts by inhibiting ribonucleotide reductase and DNA polymerase, thereby depleting the amount of intracellular deoxynucleoside triphosphates available for DNA replication. Compared to its precursors, clofarabine has an increased resistance to deamination and phosphorolysis, and hence better stability as well as higher affinity to deoxycytidine kinase (dCyd), the rate-limiting step in nucleoside phosphorylation. Since the initiation of the first phase I study of clofarabine in 1993 in patients with hematologic and solid malignancies, clofarabine has demonstrated single-agent antitumor activity in adult acute leukemia, including acute myeloid leukemia (AML). Due to its unique properties of biochemical modulation when used in combination with other chemotherapy drugs, mainly cytarabine, combination regimens containing clofarabine have been evaluated. A review of the English literature was performed that included original articles and related reviews from the MEDLINE (PubMed) database and from abstracts based on the publication of meeting materials. This review describes the development, pharmacology and clinical activity of clofarabine, as well as its emerging role in the treatment of adult patients with AML and myelodysplastic syndrome.

Oral clofarabine for relapsed/refractory non-Hodgkin lymphomas: results of a phase 1 study

We conducted a phase 1 trial evaluating the oral nucleoside analog clofarabine in patients with relapsed/refractory non-Hodgkin lymphoma. Patients were treated once daily on days 1 through 21 of a 28-day cycle for a maximum of six cycles. The study was conducted with a 3 + 3 design with 10 additional patients treated at the recommended phase 2 dose. Thirty patients were enrolled including indolent B-cell lymphomas (n = 21), mantle cell lymphoma (n = 6) and diffuse large B-cell lymphoma (n = 3). The primary toxicities were hematologic including grade 3-4 neutropenia (53%) and thrombocytopenia (27%). Three milligrams was determined to be the recommended phase 2 dose. Tumor volume was reduced in 70% of patients, and the overall response rate was 47% including 27% complete remissions. Responses were seen in indolent B-cell lymphomas and mantle cell lymphoma. At a median follow-up of 17 months, 68% of responding patients remain in ongoing remission. Oral clofarabine was well tolerated with encouraging efficacy in indolent B-cell lymphomas and mantle cell lymphomas, warranting further investigation.

Cytarabine and clofarabine after high-dose cytarabine in relapsed or refractory AML patients

Clofarabine has been shown to be effective in AML patients, either as single agent or, mainly, in association with intermediate dose cytarabine. Based on these reports, we conducted a preliminary study combining clofarabine and intermediate dose cytarabine in AML patients who relapsed or failed to respond to at least two induction therapies. We treated 47 patients affected by relapsed/refractory AML with a regimen including clofarabine at 22.5 mg/m(2) daily on days 1-5, followed after 3 hr by cytarabine at 1 g/m(2) daily on days 1-5. Ten patients received a further consolidation cycle with clofarabine at 22.5 mg/m(2) and cytarabine at 1 g/m(2) day 1-4. Among the 47 patients, 24/47 (51%) achieved a complete remission, 5/47 (10.5%) a partial response, 10/47 (21%) had a resistant disease, and 6/47 (13%) died of complications during the aplastic phase. The most frequent nonhematologic adverse events were vomiting, diarrhea, transient liver toxicity, febrile neutropenia, and infections microbiologically documented. Among the 24 patients who obtained a CR 13 underwent allogeneic bone marrow transplantation. In 14 patients, complete remission duration was shorter than 12 months, whereas 10 patients experienced longer complete remission duration. These very preliminary results suggest that clofarabine-cytarabine regimen is effective in this particularly poor prognosis category of patients, representing a potential "bridge" toward bone marrow transplant procedures. Safety data were consistent with previously reported salvage therapies. Further studies and a longer follow up are warranted.

Clofarabine in pediatric acute leukemia: current findings and issues

Clofarabine is a second-generation purine nucleoside analog and has significant anti-leukemic activity as a single agent. It is approved by the United States Food and Drug Administration (FDA) for the treatment of relapsed or refractory acute lymphoblastic leukemia (ALL) in children. Combinations of clofarabine with purine nucleoside analogs or DNA-damaging agents have been investigated utilizing synergistic effects and now tested in a number of studies including a frontline study. In this article, we review the development of clofarabine, rationale and history of combination regimens, and their potential roles and toxicities in the treatment of pediatric ALL that are important to treating clinicians.

Phase 1 dose-escalation trial of clofarabine followed by escalating dose of fractionated cyclophosphamide in adults with relapsed or refractory acute leukaemias

The prognosis of patients with relapsed and refractory acute leukaemia (RRAL) is very poor. Forty patients with RRAL were enroled [28 acute myeloid leukaemia (AML), 12 acute lymphoblastic leukaemia (ALL)] in this Phase 1 dose-escalation trial of daily-infused clofarabine (CLO) followed by cyclophosphamide (CY) for four consecutive days (CLO-CYx4). The median age was 48·5 years. The median number of prior regimens was 2 (range 1-5), and 6/40 patients (15%) had prior allogeneic haematopoietic stem cell transplant. 28/40 patients (70%) had adverse genetic features. 6/40 patients (15%) died within 60 d of induction (two infections, four progressive disease). The average time to neutrophil recovery (absolute neutrophil count ≥0·5 × 10(9) /l was 34 d, (range, 17-78). The overall response rate (ORR) was 33% (13/40), with seven complete remissions (18%), four complete remissions with incomplete recovery of blood counts (10%), and two partial remissions (5%). ORR was 25% (7/28), and 50% (6/12), for AML and ALL respectively. Notably, the clinical responses were independent of dose level. 7/17 patients (41%) exhibited CLO-mediated enhancement of CY-induced DNA, which was associated with, but not necessary for, improved clinical outcomes. In summary, the CLO-CYx4 regimen was well tolerated and had activity in patients with RRAL, especially relapsed ALL. Therefore, CLO-CYx4 can be considered a salvage therapy for adults with RRALs, and warrants further investigations.

Targeting of polo-like kinases and their cross talk with Aurora kinases--possible therapeutic strategies in human acute myeloid leukemia?

INTRODUCTION

Five human polo-like kinases (PLKs) have been identified, and PLK1 - 4 seem to interact with Aurora kinases and act as cell cycle regulators in both normal and malignant human cells.

AREAS COVERED

The present review describes i) experimental evidence for a role for PLKs and Aurora kinases in human leukemogenesis and ii) the results from clinical studies of PLK and Aurora kinase inhibitors in the treatment of human acute myeloid leukemia (AML). The review was based on searches in the PubMed and the ClinicalTrials.gov databases. These inhibitors have antiproliferative and proapoptotic effects in AML cells. Hematological and gastrointestinal toxicities are frequently dose limiting, and this may limit the use of these agents in combination with conventional AML therapy. Aurora kinase inhibitors seem to be most effective for patients with high expression of the target kinases, and the same may be true for PLK inhibitors.

EXPERT OPINION

PLK inhibition is a promising strategy for the treatment of AML. Future clinical studies have to clarify i) whether this strategy is most effective for certain subsets of patients; ii) whether multikinase inhibitors targeting several cell cycle regulators should be preferred; and iii) how this therapeutic strategy eventually should be combined with conventional antileukemic chemotherapy.

Synthesis of 2'-deoxy-2'-[18F]fluoro-9-β-D-arabinofuranosylguanine: a novel agent for imaging T-cell activation with PET

PURPOSE

9-(β-D-Arabinofuranosyl)guanine (AraG) is a guanosine analog that has a proven efficacy in the treatment of T-cell lymphoblastic disease. To test the possibility of using a radiofluorinated AraG as an imaging agent, we have synthesized 2'-deoxy-2'-[(18)F]fluoro-9-β-D-arabinofuranosylguanine ([(18)F]F-AraG) and investigated its uptake in T cells.

PROCEDURE

We have synthesized [(18)F]F-AraG via a direct fluorination of 2-N-acetyl-6-O-((4-nitrophenyl)ethyl)-9-(3',5'-di-O-trityl-2'-O-trifyl-β-D-ribofuranosyl)guanine with [(18)F]KF/K.2.2.2 in DMSO at 85°C for 45 min. [(18)F]F-AraG uptake in both a CCRF-CEM leukemia cell line (unactivated) and activated primary thymocytes was evaluated.

RESULTS

We have successfully prepared [(18)F]F-AraG in 7-10% radiochemical yield (decay corrected) with a specific activity of 0.8-1.3 Ci/μmol. Preliminary cell uptake experiments showed that both a CCRF-CEM leukemia cell line and activated primary thymocytes take up the [(18)F]F-AraG.

CONCLUSION

For the first time to the best of our knowledge, [(18)F]F-AraG has been successfully synthesized by direct fluorination of an appropriate precursor of a guanosine nucleoside. This approach maybe also useful for the synthesis of other important positron emission tomography (PET) probes such as [(18)F]FEAU, [(18)F]FMAU, and [(18)F]FBAU which are currently synthesized by multiple steps and involve lengthy purification. The cell uptake studies support future studies to investigate the use of [(18)F]F-AraG as a PET imaging agent of T cells.

Long-term stability study of clofarabine injection concentrate and diluted clofarabine infusion solutions

PURPOSE

The aim of this study was to investigate the physicochemical stability of clofarabine (CAFdA) injection concentrate and ready-to-use CAFdA infusion solutions over a prolonged period of 28 days.

METHODS

To determine the stability of CAFdA infusion solutions, the injection concentrate (Evoltra®, 1 mg/mL, Genzyme) was diluted either with 0.9% sodium chloride or 5% glucose infusion solution. The resulting concentrations of 0.2 mg/mL or 0.6 mg/mL, respectively, were chosen to represent the lower and upper limit of the ordinary concentration range. Test solutions were stored under refrigeration (2-8°C) or at room temperature either light protected or exposed to light. CAFdA concentrations and pH values were determined at different time intervals throughout a 28-day storage period. Compatibility of diluted CAFdA infusion solutions (0.1-0.4 mg/mL) with different container materials (polyvinyl chloride (PVC), glass, and polypropylene/polyethylene (PP/PE)) was tested over a 48-h storage period. CAFdA concentrations were measured by a stability-indicating reversed phase high-performance liquid chromatography (HPLC) assay with ultraviolet detection.

RESULTS

CAFdA injection concentrate and CAFdA infusion solutions remained physicochemically stable (>90% CAFdA) for 4 weeks. Results are independent of storage conditions, drug concentrations (0.2, 0.6, and 1.0 mg/mL) and diluents (0.9% sodium chloride, 5% glucose infusion solution). Adsorption of CAFdA to container material can be excluded.

CONCLUSIONS

CAFdA injection concentrate and diluted infusion solutions in commonly used vehicles are stable for at least 28 days either refrigerated or at room temperature. Physicochemical stability favors pharmacy-based centralized preparation. Due to microbiological reasons, strict aseptic handling and storage of the products under refrigeration is recommended.

A phase I study in adults of clofarabine combined with high-dose melphalan as reduced-intensity conditioning for allogeneic transplantation

Clofarabine is a novel purine nucleoside analog with immunosuppressive and antileukemia activity. We performed a phase I study of the combination of clofarabine plus melphalan as a reduced-intensity conditioning regimen for allogeneic stem cell transplantation in patients with acute myelogenous leukemia. Patients over age 18 in complete remission or with active disease (up to 50% marrow blasts) who had a matched related or unrelated donor were eligible. The conditioning regimen consisted of escalating doses of clofarabine plus melphalan, followed by allogeneic stem cell transplantation. Sixteen patients (median age, 63 years) were treated at 3 dose levels; 4 of these patients had primary induction failure, and 3 were in first relapse. One patient at dose level 2 and 1 patient at dose level 3 died of multiorgan toxicity; no other dose-limiting toxicities were seen. All other patients at both doses of clofarabine studied demonstrated complete engraftment by day 30, with a median time to absolute neutrophil count recovery of 14 days, and 16 days for platelet recovery. With a median follow-up of 17 months, only 2 patients relapsed, and 4 patients died. Clofarabine plus melphalan at dose level 2 is a well-tolerated conditioning regimen with activity in patients with advanced acute myelogenous leukemia.

Treating refractory leukemias in childhood, role of clofarabine

Approximately 4000 children and adolescents under the age of 20 years develop acute leukemia per year in the US. Acute lymphoblastic leukemia (ALL) is the most common pediatric cancer. Despite impressive improvements in outcome, relapsed ALL is the fourth most common pediatric malignancy. Therapy for relapsed ALL remains unsatisfactory, and the majority of relapse patients still succumb to leukemia. Between one-third and one-half of patients with acute myelogenous leukemia (AML) relapse, and no standard therapy is recognized for patients with relapsed and/or refractory AML. Novel therapeutic agents are needed to improve the cure rate for relapsed ALL and AML. Clofarabine is a next-generation nucleoside analog, designed to incorporate the best features and improve the therapeutic index of cladribine and fludarabine. Clofarabine inhibits both DNA polymerase and ribonucleotide reductase, leading to impaired DNA synthesis and repair, and directly induces apoptosis. Phase I and II single-agent trials in children have shown that clofarabine is safe and active in both myeloid and lymphoid relapsed/refractory acute leukemias. Clofarabine has been approved by the FDA for pediatric patients with relapsed/refractory ALL after at least 2 prior therapeutic attempts. Rational combinations of clofarabine with other active agents in refractory leukemias are currently under investigation.

Clofarabine 5'-di and -triphosphates inhibit human ribonucleotide reductase by altering the quaternary structure of its large subunit

Human ribonucleotide reductases (hRNRs) catalyze the conversion of nucleotides to deoxynucleotides and are composed of α- and β-subunits that form active α(n)β(m) (n, m = 2 or 6) complexes. α binds NDP substrates (CDP, UDP, ADP, and GDP, C site) as well as ATP and dNTPs (dATP, dGTP, TTP) allosteric effectors that control enzyme activity (A site) and substrate specificity (S site). Clofarabine (ClF), an adenosine analog, is used in the treatment of refractory leukemias. Its mode of cytotoxicity is thought to be associated in part with the triphosphate functioning as an allosteric inhibitor of hRNR. Studies on the mechanism of inhibition of hRNR by ClF di- and triphosphates (ClFDP and ClFTP) are presented. ClFTP is a reversible inhibitor (K(i) = 40 nM) that rapidly inactivates hRNR. However, with time, 50% of the activity is recovered. D57N-α, a mutant with an altered A site, prevents inhibition by ClFTP, suggesting its A site binding. ClFDP is a slow-binding, reversible inhibitor ( K(i)*; t(1/2) = 23 min). CDP protects α from its inhibition. The altered off-rate of ClFDP from E•ClFDP* by ClFTP (A site) or dGTP (S site) and its inhibition of D57N-α together implicate its C site binding. Size exclusion chromatography of hRNR or α alone with ClFDP or ClFTP, ± ATP or dGTP, reveals in each case that α forms a kinetically stable hexameric state. This is the first example of hexamerization of α induced by an NDP analog that reversibly binds at the active site.

Deoxycytidine kinase modulates the impact of the ABC transporter ABCG2 on clofarabine cytotoxicity

Purine nucleoside antimetabolites, such as clofarabine, are effective antileukemic agents. However, their effectiveness depends on an initial activation step in which they are monophosphorylated by deoxycytidine kinase (dCK). Some purine nucleoside antimetabolites and their monophosphate derivatives are exported by the ABC transporter ABCG2. Because clofarabine is a dCK substrate, and we show substantial variation in dCK and ABCG2 in myeloid leukemia, we hypothesized that the activity of dCK may modulate ABCG2-mediated resistance to clofarabine by regulating the formation of clofarabine monophosphate. We show that ABCG2 influence on clofarabine cytotoxicity was markedly influenced by dCK activity. When dCK expression was reduced by siRNA, clofarabine cytotoxicity was strongly reduced by enhanced ABCG2-mediated efflux. Conversely, dCK overexpression blunted ABCG2-mediated efflux of clofarabine by increasing the formation of clofarabine nucleotides. The use of an ABCG2 inhibitor confirmed that ABCG2 export of clofarabine is maximal when dCK levels are minimal. Analysis of intracellular clofarabine metabolites suggested that ABCG2 exported clofarabine more readily than clofarabine monophosphate. That ABCG2 primarily effluxes clofarabine, but not chlorfarabine-monophosphate, was confirmed by HPLC analysis of drug exported from ABCG2-overexpressing cells. Because the level and function of dCK and ABCG2 vary substantially among other types of cancer, these findings have important implications not only for clofarabine therapy but for purine nucleoside therapy in general. Therefore, we propose that addition of ABCG2 inhibitors would effectively increase the antitumor efficacy of purine nucleosides by blocking drug efflux that may be a significant mode of resistance when dCK levels are low.

The synergistic cytotoxicity of clofarabine, fludarabine and busulfan in AML cells involves ATM pathway activation and chromatin remodeling

DNA alkylating agents alone or with ionizing radiation have been the preferred conditioning treatment in allogeneic hematopoietic stem cell transplantation (allo-HSCT). In search of less toxic alternatives, we hypothesized that combination of busulfan (Bu), fludarabine (Flu) and clofarabine (Clo) would provide superior efficacy. At low concentrations, these drugs show synergistic cytotoxicity in Bu-resistant AML KBM3/Bu250(6) cells. Similar molecular responses were observed in other AML cell lines and in primary explanted AML cells. The [Clo+Flu+Bu] combination activates an intense DNA damage response through the ATM pathway, leading to cell cycle checkpoint activation and apoptosis. Phosphorylations of SMC1 and SMC3, and methylations of histones 3 and 4, are much more pronounced in cells exposed to [Clo+Flu+Bu] than [Clo+Flu], suggesting their relevance in the efficacy of the triple-drug combination. A possible mechanism for these observed synergistic effects involves the capability of [Clo+Flu] to induce histone methylations and subsequent chromatin remodeling, which may render the genomic DNA more accessible to Bu alkylation. The Bu-mediated DNA cross-linking may provide a feedback loop which perpetuates the DNA damage response initiated by [Clo+Flu] and commits the cells to apoptosis. Our results provide a conceptual mechanistic basis for exploring this triple-drug combination in pretransplant conditioning therapy for allo-HSCT.

A phase II study of low dose intravenous clofarabine for elderly patients with myelodysplastic syndrome who have failed 5-azacytidine

The treatment options for patients with myelodysplastic syndrome (MDS) who have failed DNA hypomethylating agents are limited. In this study, we set out to investigate the efficacy of low dose clofarabine in 10 patients with MDS (four intermediate-2/high risk disease) who had failed 5-azacytidine. The median age was 73 years (range 65-78) and median cycles of clofarabine received were 2 (range 1-4). Nine patients were evaluable for response. An overall response rate of 44% was observed (one CR, one PR, and two HI). All responders had low risk disease. The median duration of response was 12 months (range 6.5-15.5). Although the doses of clofarabine administered were only 12.5-25% of that used in other studies, significant hematologic toxicities were observed. Severe and prolonged pancytopenia occurred in all 10 patients. One patient who had a history of thrombocytopenic gastrointestinal bleed died due to an intracranial bleed despite aggressive platelet support. Low dose clofarabine may, therefore, induce response, but with significant toxicities, in patients with low risk MDS who fail 5-azacytidine. Future work involving a larger patient population is needed to establish the role of low dose clofarabine in low risk MDS.

Deconstructing nucleotide binding activity of the Mdm2 RING domain

Mdm2, a central negative regulator of the p53 tumor suppressor, possesses a Really Interesting New Gene (RING) domain within its C-terminus. In addition to E3 ubiquitin ligase activity, the Mdm2 RING preferentially binds adenine base nucleotides, and such binding leads to a conformational change in the Mdm2 C-terminus. Here, we present further biochemical analysis of the nucleotide–Mdm2 interaction. We have found that MdmX, an Mdm2 family member with high sequence homology, binds adenine nucleotides with similar affinity and specificity as Mdm2, suggesting that residues involved in nucleotide binding may be conserved between the two proteins and adenosine triphosphate (ATP) binding may have similar functional consequences for both Mdm family members. By generating and testing a series of proteins with deletions and substitution mutations within the Mdm2 RING, we mapped the specific adenine nucleotide binding region of Mdm2 to residues 429–484, encompassing the minimal RING domain. Using a series of ATP derivatives, we demonstrate that phosphate coordination by the Mdm2 P-loop contributes to, but is not primarily responsible for, ATP binding. Additionally, we have identified the 2′ and 3′ hydroxyls of the ribose and the C6 amino group of the adenine base moiety as being essential for binding.

Interstrand crosslink inducing agents in pretransplant conditioning therapy for hematologic malignancies

Despite successful molecularly targeted, highly specific, therapies for hematologic malignancies, the DNA interstrand crosslinking agents, which are among the oldest and least specific cytotoxic drugs, still have an important role. This is particularly true in stem cell transplantation, where virtually every patient receives conditioning therapy with a DNA-alkylating agent-based program. However, due to concern about serious additive toxicities with combinations of different alkylating drugs, the last several years have seen nucleoside analogs, whose cytotoxic action follows vastly different molecular pathways, introduced in combination with alkylating agents. The mechanistic differences paired with different metabolic pathways for the respective drugs have clinically translated into increased safety without appreciable loss of antileukemic activity. In this report, we review pre-clinical evidence for synergistic antileukemic activity when nucleoside analog(s) and DNA-alkylating agent(s) are combined in the most appropriate manner(s), without a measurable decrease in clinical efficacy compared with the more established alkylating agent combinations. Data from our own laboratory using combinations of fludarabine, clofarabine, and busulfan as prototype representatives for these respective classes of cytotoxic agents are combined with information from other investigators to explain how the observed molecular events will result in greatly enhanced synergistic cytotoxicity. We further present possible mechanistic pathways for such desirable cytotoxic synergism. Finally, we propose how this information-backed hypothesis can be incorporated in the design of the next generation conditioning therapy programs in stem cell transplantation to optimize antileukemic efficacy while still safeguarding patient safety.

Use of clofarabine for acute childhood leukemia

A second-generation of purine nucleoside analogs, starting with clofarabine, has been developed in the course of the search for new therapeutic agents for acute childhood leukemia, especially for refractory or relapsed disease. Clofarabine is a hybrid of fludarabine and cladribine, and has shown to have antileukemic activity in acute lymphoblastic leukemia as well as in myeloid disorders. As the only new antileukemic chemotherapeutic agent to enter clinical use in the last 10 years, clofarabine was approved as an orphan drug with the primary indication of use in pediatric patients. Toxicity has been tolerable in a heavily pretreated patient population, and clofarabine has been demonstrated to be safe, both as a single agent and in combination therapies. Liver dysfunction has been the most frequently observed adverse event, but this is generally reversible. Numerous Phase I and II trials have recently been conducted, and are still ongoing in an effort to find the optimal role for clofarabine in various treatment strategies. Concomitant use of clofarabine, cytarabine, and etoposide was confirmed to be safe and effective in two independent trials. Based on the promising results when used as a salvage regimen, clofarabine is now being investigated for its potential to become part of frontline protocols.

Oral clofarabine in the treatment of patients with higher-risk myelodysplastic syndrome

PURPOSE

Efficacy and toxicity profile of orally administered clofarabine were evaluated in patients with higher-risk myelodysplastic syndrome (MDS).

PATIENTS AND METHODS

Thirty-two patients were treated, of whom 22 had intermediate-2 or high-risk disease (International Prognostic Scoring System). Median age was 70 years (range, 53 to 86), nine patients had secondary MDS, and 20 patients experienced prior therapy failure with hypomethylating agents. Three doses of clofarabine were evaluated: 40 mg/m(2), 30 mg/m(2), and 20 mg/m(2) daily for 5 days. Courses were repeated every 4 to 8 weeks.

RESULTS

Eight patients (25%) achieved complete remission (CR), three had (9%) hematologic improvement (HI), and three had (9%) clinical benefit (CB; overall response rate, 43%). Responses in patients who experience treatment failure with hypomethylating agents included CR in two (10%), HI in two (10%), and CB in two patients (10%). No patients died within 6 weeks of induction. Renal failure occurred in four patients in the context of myelosuppression-associated infectious complications. Common adverse events were gastrointestinal and hepatic. Myelosuppression was common, but prolonged myelosuppression (> 42 days) was rare. The toxicity profile was better with lower doses of clofarabine, whereas response rates did not differ significantly.

CONCLUSION

Oral clofarabine has achieved a response rate of 43% in patients with higher-risk MDS. The optimal dose and schedule and the appropriate patient population for such therapy remain to be further defined.

Clofarabine for the treatment of adult acute myeloid leukemia

Clofarabine, a next-generation deoxyadenosine analog, has demonstrated significant activity in patients with acute myeloid leukemia (AML). The single-agent activity compares favorably with that demonstrated by the current standard antileukemia agents. Clofarabine has been safely and effectively combined with other agents and will probably become an integral part of induction and/or consolidation regimens in AML. Current studies are underway to better define the role of clofarabine in younger and elderly patients with AML, and also explore development strategies for an oral formulation.

Clofarabine in leukemia

Clofarabine is a second-generation purine nucleoside analogue that has been synthesized to overcome the limitations and incorporate the best qualities of fludarabine and cladribine. Clofarabine acts by inhibiting ribonucleotide reductase and DNA polymerase, thereby depleting the amount of intracellular deoxynucleoside triphosphates available for DNA replication. Compared with its precursors, clofarabine has an increased resistance to deamination and phosphorolysis, hence better stability, as well as higher affinity to deoxycytidine kinase (dCyd), the rate-limiting step in nucleoside phosphorylation. In 1993, the first Phase I study was initiated in patients with hematologic and solid malignancies. Since then, clofarabine has demonstrated single-agent antitumor activity in pediatric and adult acute leukemia. Owing to its unique properties of biochemical modulation when used in combination with other established antileukemic drugs, mainly cytarabine, combination regimens containing clofarabine are being evaluated. A review of the English literature was performed that included original articles and related reviews from the MEDLINE (PubMed) database and from abstracts based on the publication of meeting materials. This article describes the development, pharmacology and clinical activity of clofarabine, as well as its emerging role in the treatment of acute leukemia, myelodysplastic syndrome and solid tumors.

Impaired S-phase arrest in acute myeloid leukemia cells with a FLT3 internal tandem duplication treated with clofarabine

PURPOSE

Acute myeloid leukemia cells with an internal tandem duplication mutation of FLT3 (FLT3-ITD) have effective DNA repair mechanisms on exposure to drugs. Despite this, the phenotype is not associated with primary resistant disease. We show defects in the response of mutant FLT3 AML cells to the S-phase drug clofarabine that could account for the apparent contradiction.

EXPERIMENTAL DESIGN

We studied responses of AML cells to clofarabine in vitro.

RESULTS

When treated with a short pulse of clofarabine, FLT3-ITD-harboring MOLM-13 and MV4.11 cells undergo similar damage levels (gammaH2AX foci) to wild-type cells but have a better repair capability than wild-type cells. However, whereas the wild-type cells undergo rapid S-phase arrest, the S-phase checkpoint fails in mutant cells. Cell cycle arrest in response to DNA damage in S phase is effected via loss of the transcriptional regulator cdc25A. This loss is reduced or absent in clofarabine-treated FLT3 mutant cells. Furthermore, cdc25A message levels are maintained by the FLT3-ITD, such that message is reduced by 87.5% on exposure to FLT3 small interfering RNA. Primary FLT3-ITD samples from untreated patients also display impaired cell cycle arrest and show enhanced sensitivity on prolonged treatment with clofarabine compared with wild-type samples.

CONCLUSION

There is a reversal of phenotype in mutant FLT3 cells dependent on the length of exposure to clofarabine. Efficient DNA repair may render the cells resistant to a short pulse of the drug, but a failure of cell cycle checkpoint(s) in S phase renders the cells sensitive to prolonged exposure.

Clofarabine: emerging role in leukemias

Clofarabine is a second-generation purine nucleoside analogue. It works mainly by inhibiting ribonucleotide reductase and incorporating into DNA. Clofarabine has shown efficacy in selected pediatric leukemias. It has also shown significant efficacy alone and in combination with other drugs in treating adult myeloid leukemias and high-risk myelodysplastic syndromes. Further, there is significant promise for clofarabine in the treatment of older patients with acute myeloid leukemia who are unlikely to benefit from standard induction chemotherapy due to unfavorable baseline prognostic factors. An oral formulation of clofarabine is also currently under development.

Mechanisms of anti-cancer action and pharmacology of clofarabine

Clofarabine, a next-generation deoxyadenosine analogue, was developed on the basis of experience with cladribine and fludarabine in order to achieve higher efficacy and avoid extramedullary toxicity. During the past decade this is the only drug granted approval for treatment of pediatric acute leukemia. Recent clinical studies have established the efficacy of clofarabine in treating malignancies with a poor prognosis, such as adult, elderly, and relapsed pediatric leukemia. The mechanisms of its anti-cancer activity involve a combination of direct inhibition of DNA synthesis and ribonucleotide reductase and induction of apoptosis. Due to this broad cytotoxicity, this drug is effective against various subtypes of leukemia and is currently being tested as an oral formulation and for combination therapy of both leukemias and solid tumors. In this review we summarize current knowledge pertaining to the molecular mechanisms of action and pharmacological properties of clofarabine, as well as clinical experiences with this drug with the purpose of facilitating the evaluation of its efficacy and the development of future therapies.

Clofarabine induces hypomethylation of DNA and expression of Cancer-Testis antigens

In this study, treatment of lymphoid tumor cells with low dose clofarabine upregulated the expression of Sp17 and SPAN-Xb. This was associated with an increase in hypomethylated CpG dinucleotides and a decrease in global DNA methylation, as demonstrated by decreases in the percent of methylated Alu repeats. The most optimal concentration of clofarabine to induce DNA hypomethylation and CT antigen expression was between 1x10(-9) and 1x10(-8)M. Above this, clofarabine resulted in tumor cell growth inhibition and apoptosis. Our results provide the first evidence for the CT antigen-inducing and DNA hypomethylating property of low concentration clofarabine.

The use of higher dose clofarabine in adults with relapsed acute lymphoblastic leukemia

The standard dose of clofarabine is 52 mg/m2 for pediatrics and 40 mg/m2 in adults. Clofarabine dosed at 52 mg/m2 was used in adult patients with refractory ALL to maximize response before allo-HSCT. All patients had a significant response to therapy. Published pharmacokinetic analysis revealed no difference in peak plasma or intracellular concentrations at clofarabine dosed above 40 mg/m2, yet inhibition of replication in leukemia cells was only sustained over 24 hr at 55 mg/m2. Despite this, there have been no reports of high dose clofarabine used in this setting. Our experience implies that there may be a niche role for clofarabine in reducing disease burden before allo-HSCT for adults with relapsed ALL.

Studies on the hydrolytic stability of 2'-fluoroarabinonucleic acid (2'F-ANA)

The stability of 2'-deoxy-2'-fluoroarabinonucleic acid (2'F-ANA) to hydrolysis under acidic and basic conditions was compared to that of DNA, RNA and 2'F-RNA. In enzyme-free simulated gastric fluid (pH approximately 1.2), 2'F-ANA was found to have dramatically increased stability (virtually no cleavage observed after 2 days) with respect to both DNA (t(1/2) approximately 2 min) and RNA (t(1/2) approximately 3 h (PO) or 3 days (PS)). These results were observed for both phosphodiester and phosphorothioate backbones and with multiple mixed-base sequences. Under basic conditions, 2'F-ANA also showed good stability. In 1 M NaOH at 65 degrees C, 2'F-ANA had a t(1/2) of approximately 20 h, while RNA was entirely degraded in a few minutes. Furthermore, the nuclease cleavage of phosphorothioate 2'F-ANA and DNA by snake venom phosphodiesterase was studied in detail. One diastereomer of the PS-2'F-ANA linkage was found to be much more vulnerable to enzymatic cleavage than the other, which is parallel to the properties observed for PS-DNA. Additional studies of 2'F-ANA-containing oligonucleotides are warranted based on the excellent stability properties described here.

Nucleoside analogs: molecular mechanisms signaling cell death

Nucleoside analogs are structurally similar antimetabolites that have a broad range of action and are clinically active in both solid tumors and hematological malignancies. Many of these agents are incorporated into DNA by polymerases during normal DNA synthesis, an action that blocks further extension of the nascent strand and causes stalling of replication forks. The molecular mechanisms that sense stalled replication forks activate cell cycle checkpoints and DNA repair processes, which may contribute to drug resistance. When replication forks are not stabilized by these molecules or when subsequent DNA repair processes are overwhelmed, apoptosis is initiated either by these same DNA damage sensors or by alternative mechanisms. Recently, strategies aimed at targeting DNA damage checkpoints or DNA repair processes have demonstrated effectiveness in sensitizing cells to nucleoside analogs, thus offering a means to elude drug resistance. In addition to their DNA synthesis-directed actions many nucleoside analogs trigger apoptosis by unique mechanisms, such as causing epigenetic modifications or by direct activation of the apoptosome. A review of the cellular and molecular responses to clinically relevant agents provides an understanding of the mechanisms that cause apoptosis and may provide rationale for the development of novel therapeutic strategies.

Clofarabine combinations as acute myeloid leukemia salvage therapy

BACKGROUND

Outcome of patients with relapsed acute myeloid leukemia (AML) remains unsatisfactory. Clofarabine is a nucleoside analog with activity in adult AML. Combinations with cytarabine in AML are feasible and effective. Idarubicin is another active AML drug, which has not yet been tested with clofarabine.

METHODS

The authors therefore designed a phase I study of clofarabine +/- cytarabine, plus idarubicin. Patients with primary refractory or first-relapse AML were assigned to either clofarabine plus idarubicin (CI) if previously exposed to cytarabine with a response lasting <12 months, or clofarabine and idarubicin plus cytarabine (CIA) for responses > or = 12 months, or if never exposed to cytarabine. A standard "3 + 3" phase 1 design was followed to define maximum tolerated dose (MTD). Forty-four patients were treated (23 CI; 21 CIA).

RESULTS

Dose-limiting toxicities were hyperbilirubinemia and hepatic transaminase elevations for CI-treated patients in addition to mucositis and diarrhea for CIA-treated patients. MTD for CI was clofarabine 22.5 mg/m(2) intravenously daily x 5 and idarubicin 10 mg/m(2) intravenously daily x 3. MTD for CIA was clofarabine 22.5 mg/m(2) intravenously x 5, idarubicin 6 mg/m(2) intravenously x 3, and cytarabine 0.75 g/m(2) intravenously x 5 days.

CONCLUSIONS

A phase 2 randomized trial is in process to compare activity between treatment arms.

Metabolism of the antineoplastic and immunosuppressive drug 2-CdA (Leustatin®) in animals and humans

1. The in vivo metabolism of the antineoplastic and immunosuppressive drug 2-CdA (Leustatin) was investigated in mice, monkeys and humans after a single subcutaneous dose of cladribine 60 mg kg(-1) to eight male and eight female mice and 10 mg kg(-1) to one male and one female monkey, and an intravenous infusion dose of cladribine 22-45 mg(-1) per subject to 12 male patients. 2. Plasma (1 h), red blood cells (1 h) and faecal samples (0-24 h) were obtained from mice and monkeys, and urine samples (0-24 h) were obtained from these species and humans. 3. Unchanged cladribine (urine: 47% of the sample in human; 60% of the sample in mouse; 73% of the sample in monkey) and 10 metabolites, consisting of four phase I metabolites (M1-3, M7) and six phase II metabolites -- five glucuronides (M4, M6, M8-10) and one sulfate (M5) -- were profiled, characterized and tentatively identified in plasma, red blood cells, and faecal and urine samples on the basis of API ionspray-mass spectrometry (MS) and MS/MS data. 4. Metabolites were formed via the following three metabolic pathways: oxidative cleavage at the adenosine and deoxyribose linkage (A); oxidation at adenosine/deoxyribose (B); and conjugation (C). 5. Pathways A and B appear to be major steps, forming four oxidative/cleavage metabolites (M1-3, M7) (each 3-20% of the sample). 6. Pathway C along or in conjunction with pathways A and B produced cladribine glucuronide, cladribine sulfate and four glucuronides of oxidative/cleavage metabolites in minor/trace quantities (each < or = 5% of the sample). 7. In addition, the in vitro metabolism of cladribine was conducted using rat and human liver microsomal fractions in the presence of an beta-nicotinamide adenine dinucleotide phosphate-generating system. Unchanged cladribine (> or = 90% of the sample) plus three minor metabolites, M1-3 (each < 8% of the sample), were profiled and tentatively identified by thin-layer chromatography and MS data. 8. Cladribine is not extensively metabolized in vitro and in vivo in all species. However, humans appear to metabolize cladribine to a greater extent than other animals.

A randomized study of clofarabine versus clofarabine plus low-dose cytarabine as front-line therapy for patients aged 60 years and older with acute myeloid leukemia and high-risk myelodysplastic syndrome

We previously reported the feasibility of clofarabine and cytarabine combinations in AML. Questions remain as to (1) the therapeutic advantage of this combination and (2) the role of lower doses of clofarabine and cytarabine in older patients. We have conducted an adaptively randomized study of lower-dose clofarabine with or without low-dose cytarabine in previously untreated patients with AML aged 60 years and older. Patients received 30 mg/m(2) clofarabine intravenously daily for 5 days with or without 20 mg/m(2) cytarabine subcutaneously daily for 14 days as induction. Consolidation consisted of 3 days of clofarabine with or without 7 days of cytarabine. Seventy patients were enrolled. The median age was 71 years (range, 60-83 years). Sixteen patients received clofarabine and 54 the combination. Overall, 56% achieved complete remission (CR). CR rate was significantly higher with the combination (63% vs 31%; P = .025). Induction mortality was 19% with the combination versus 31% with clofarabine alone (P = .276). The combination showed better event-free survival (7.1 months vs 1.7 months; P = .04), but not overall survival (11.4 months vs 5.8 months; P = .1). Clofarabine plus low-dose cytarabine has a higher response rate than clofarabine alone with comparable toxicity. This trial is registered at www.clinicaltrials.gov as no. NCT00088218.

2′F-Arabinonucleic acids (2′F-ANA) — History, properties, and new frontiers

The development of arabinonucleosides and oligoarabinonucleotides is described, focusing especially on 2′-deoxy-2′-fluoroarabinonucleosides (araF-N) and -oligonucleotides (2'F-ANA). In addition to their chemical and enzymatic synthesis, we discuss various properties of 2′F-ANA: hydrolytic stability (to nucleases, acids, and bases), binding affinity to complementary strands, structure and conformation, and optimization of RNase H activity. We also discuss the use of 2′F-ANA in gene-silencing approaches (antisense, siRNA), and in the stabilization of higher-order structures (such as triplexes and quadruplexes) including aptamers. Finally, we examine several other oligonucleotide derivatives based on 2′F-ANA and look ahead to the future of 2′-fluoroarabinonucleosides and -oligonucleotides.Key words: arabinonucleic acids, 2′F-ANA, antisense oligonucleotides, siRNA, modified oligonucleotides.