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PubMed2003

Pharmacokinetics and pharmacodynamics of plasma clofarabine and cellular clofarabine triphosphate in patients with acute leukemias.

PURPOSE: The purpose of our study was to investigate the pharmacology of clofarabine and its triphosphate and the pharmacodynamic actions in circulating blasts obtained from acute leukemia patients who entered a Phase I clinical trial of clofarabine. EXPERIMENTAL DESIGN: Adults with refractory acute leukemias including lymphoblastic (ALL), myelogenous (AML) and chronic myelogenous leukemia in blastic phase (CML-BP) received clofarabine from 4 mg/m(2) to 55 mg/m2/day for 5 days as a 1-h i.v. infusion. A total of 26 of the 32 patients were studied for pharmacological investigations. RESULTS: The maximum tolerated dose was 40 mg/m2/day for 5 days. Plasma pharmacology studies done in 25 patients indicate a linear increase in the plasma clofarabine concentration with increasing doses. At 40 mg/m2 the median plasma clofarabine level was 1.5 micro M (range, 0.42-3.2 micro M; n = 7). Cellular pharmacokinetic studies done at the end of the first clofarabine infusion in 26 patients appeared dose proportional but showed a wide variation in the concentrations of clofarabine triphosphate. At the maximum tolerated dose, the concentration was a median 19 micro M (range, 3-52 micro M). In the majority of cases, more than 50% of the analog triphosphate was present at 24 h after infusion. Compared with clofarabine triphosphate concentration, the endogenous level of dATP was low, resulting in a favorable ratio of analog triphosphate:normal deoxynucleoside triphosphate (dNTP) for incorporation into DNA. In association with the accumulation of triphosphate, there was a decrease in DNA synthesis. At 40- and 55-mg/m2 doses, the inhibition of DNA synthesis was maintained to 24 h. CONCLUSIONS: Clofarabine at the maximum tolerated dose was effective with regard to inhibition of DNA synthesis and decline in circulating leukemia blasts. Given the clinical activity of clofarabine in adult acute leukemias, it is of interest to conduct a detailed characterization of the cellular pharmacology of clofarabine triphosphate and its relationship to clinical responses.

Adenine Nucleotides↗
PubMed2005

Clofarabine.

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Adenine Nucleotides↗
PubMed2004

Clofarabine.

Clofarabine [Clofarex] is a purine nucleoside in development with Bio-envision, the Southern Research Institute and ILEX Oncology as an anticancer agent. Clofarabine's nucleoside structure is such that both the purine and ribose rings are halogenated, which allows it to inhibit DNA synthesis at two critical junctures: DNA polymerase I and RNA reductase. An intravenous infusion and an oral formulation are undergoing clinical development. Clofarabine was originated by the Southern Research Institute. In August 1998 Bioenvision signed a co-development agreement with the Southern Research Institute, under which it obtained the right to manufacture, market and distribute clofarabine worldwide, except Japan and Southeast Asia. In addition, the company appears to have licensed rights from the Institute that cover the development and marketing of other purine nucleoside analogues that have relevance in the treatment of leukaemia and lymphoma. Bioenvision will pay royalties to the Southern Research Institute for sales of clofarabine. Bioenvision extended its option in May 2004 to manufacture, market and distribute clofarabine in Japan and Southeast Asia, and is seeking a co-marketing partner to convert the option into a license agreement following the terms agreed upon between Bioenvision and the Southern Research Institute. Bioenvision and ILEX Products (a wholly owned subsidiary of ILEX Oncology) signed an agreement in February 2004 that converted ILEX's option (agreed in March 2001) to market and distribute clofarabine in the US and Canada. As part of the deal, Bioenvision received a $US3.5 million payment from ILEX in December 2003. In March 2004, Genzyme Corporation announced that it had signed a merger agreement with ILEX Oncology under which ILEX shareholders will receive shares of Genzyme common stock valued at approximately $US1 billion in equity value. Genzyme's business combination with ILEX is expected to be completed by the middle of 2004, Genzyme will, therefore, acquire a considerable boost to its product portfolio. Bioenvision obtained the exclusive option from the Southern Research Institute in September 2003 to manufacture, market and distribute clofarabine in Japan and Southeast Asia. Bioenvision stated it was actively seeking a co-marketing partner to convert this option into a license. Bioenvision announced in June 2003 that it had formed two separate agreements with Ferro Pfanstiehl Laboratories. The agreements cover worldwide development and supply of clofarabine, excluding the US and Canada. Ferro Pfanstiehl has more than 25 years of experience in potent compound manufacturing. The US FDA granted clofarabine fast-track designation for the treatment of refractory or relapsed acute lymphoblastic leukaemia in children in September 2003. Clofarabine has also been granted orphan drug status by the US FDA for the treatment of adult and paediatric patients with acute lymphocytic leukaemia (ALL) or acute myeloid leukaemia (AML). In December 2001, clofarabine was granted orphan drug status in the EU for the treatment of adult and paediatric patients with ALL. A single-agent phase II study has been completed in patients with acute leukaemia and myelodysplastic syndromes. Results of a phase II study of clofarabine in the treatment of acute myelogenous leukaemia in older adults who are not considered suitable for intensive chemotherapy have been very positive, with a 64% response rate in these patients being reported. In May 2004, Bioenvision announced that it had decided to stop enrollment at 25 evaluable patients (initially anticipated to be approximately 37 patients) because of the encouraging interim results. It said the trial would conclude earlier than expected and be completed by the end of June 2004. The pivotal trial will enroll approximately 65 patients with AML considered unsuitable for intensive chemotherapy. Bioenvision currently has phase II trials ongoing in adult and paediatric patients with acute leukaemia and chronic lymphocytic leukaemia (CLL). In addition, Bioenvision-sponsored phase I/II clinical trials of clofarabine in patients with CLL and non-Hodgkin's lymphoma are underway in Europe. In July 2002, ILEX began two US multicentre, open-label, phase II trials in children with relapsed or refractory AML or ALL. Children enrolled in the studies receive an intravenous infusion of clofarabine over 2 hours for five consecutive days every 2-6 weeks. In June 2003, at the 39th Annual Meeting of the American Society of Clinical Oncology (ASCO-2003), an overall response rate of 28% was reported for clofarabine therapy in heavily pretreated children with acute leukaemia. In September 2003, a multicentre European phase II trial (BIOV-111) was initiated in children with relapsed/refractory ALL. In December 2003, the first of 65 patients received treatment. As part of the global development programme, Bioenvision and ILEX are also conducting a phase II study in adult patients with AML. The companies are planning to investigate the potential use of clofarabine in combination with DNA-damaging agents, because clofarabine has been shown to inhibit DNA repair and may, therefore, potentiate the effects of DNA damaging drugs. A phase I/II trial of clofarabine in combination with cytarabine (Ara-C) in adult patients with first relapse AML, ALL, CML blast crisis and myelodysplastic syndrome was initiated at the University of Texas MD Anderson Cancer Centre in October 2002. Clofarabine has completed US phase I trials, and has reported favourable results in patients with leukaemia and solid tumours, including breast, colorectal and prostate cancers. A phase I/II trial in patients with solid tumours was initiated in July 2002. In addition, ILEX said it intended to develop an oral formulation of clofarabine for the treatment of colorectal cancer.

Adenine Nucleotides↗
PubMed2004

Biochemical modulation of cytarabine triphosphate by clofarabine.

PURPOSE: Clofarabine has proven to be effective in the treatment of adult and pediatric acute myelogenous leukemia (AML). To investigate if clofarabine could be used with success in biochemical modulation strategies, we investigated the biochemical modulation of cytarabine triphosphate (ara-CTP) by clofarabine in a myeloid leukemia cell line and the effect of this combination on cytotoxicity. EXPERIMENTAL DESIGN: K562 cells were incubated with clofarabine and ara-C either sequentially or simultaneously to evaluate the combination effect on their phosphorylated metabolites. Clonogenic assays were used to determine the cytotoxicity of each agent alone and in combination. Deoxynucleotide analysis was performed to assess the effect of clofarabine on dNTPs. RESULTS: Clofarabine added either simultaneously or in sequence increased ara-CTP accumulation. The maximal modulation of ara-CTP accumulation occurred with 1 microM clofarabine. This level was achieved at the maximum tolerated dose for adult and pediatric patients with AML. With 10 microM ara-C alone, 86 microM ara-CTP had accumulated after 3 h. The optimal sequence for the drug combination, i.e., clofarabine followed 4 h later by ara-C, resulted in 248 microM ara-CTP at 3 h. Clofarabine accumulated maximally in the monophosphate form. Preincubation with ara-C did not affect the triphosphate form, but it lowered clofarabine monophosphate. Clofarabine resulted in the intracellular decrease of dATP and dGTP levels. Clonogenic assays revealed that the combination of clofarabine and ara-C produced synergistic killing of myeloid leukemia cells. CONCLUSIONS: These findings demonstrate that combination of clofarabine followed by ara-C results in a biochemical modulation of ara-CTP and synergistic cell kill. These studies provide a compelling rationale for clinical trials using this combination regimen for adult and pediatric patients with AML.

Adenine Nucleotides↗
PubMed2006

Clinical and pharmacokinetic study of clofarabine in chronic lymphocytic leukemia: strategy for treatment.

PURPOSE: Based on its mechanistic similarity to fludarabine and cladribine and the success of these analogues for treatment of chronic lymphocytic leukemia (CLL), we hypothesized that clofarabine would be effective for indolent leukemias. The present study was conducted to determine the efficacy and cellular pharmacology during clinical trials of single-agent clofarabine in CLL. EXPERIMENTAL DESIGN: Previously treated patients with relapsed/refractory CLL were eligible for this study. Clofarabine was infused over 1 hour daily for 5 days. Most patients received 3 or 4 mg/m2/d x 5 days, whereas the other two were treated with 15 mg/m2/d x 5 days. Clinical outcome and associated pharmacologic end points were assessed. RESULTS: Myelosuppression limited the maximum tolerated dose of clofarabine to 3 mg/m2/d on this schedule. Cellular pharmacokinetic studies showed a median clofarabine triphosphate concentration in CLL lymphocytes of 1.5 micromol/L (range, 0.2-2.3 micromol/L; n = 9). In the majority of cases, >50% of the analogue triphosphate was present 24 hours after infusion, indicating prolonged retention of the triphosphate in CLL cells. Although cytoreduction was observed, no patients achieved a response. In vitro clofarabine incubation of leukemic lymphocytes from 29 CLL patients showed that clofarabine monophosphate accumulated to a higher concentration compared with the triphosphate. Nonetheless, the triphosphate increased in a dose-dependent fashion and upon successive clofarabine infusions, suggesting benefit from greater doses given at less frequent intervals. CONCLUSION: Levels of clofarabine triphosphate at higher doses and prolonged maintenance of clofarabine triphosphate in leukemic lymphocytes provide a rationale to treat CLL in a weekly clofarabine schedule.

Adenine Nucleotides↗
PubMed2004

Results of a phase 1-2 study of clofarabine in combination with cytarabine (ara-C) in relapsed and refractory acute leukemias.

Clofarabine (2-chloro-2'-fluoro-deoxy-9-beta-D-arabinofuranosyladenine) is a second-generation nucleoside analog with activity in acute leukemias. As clofarabine is a potent inhibitor of ribonucleotide reductase (RnR), we hypothesized that clofarabine will modulate ara-c triphosphate accumulation and increase the antileukemic activity of cytarabine (ara-C). We conducted a phase 1-2 study of clofarabine plus ara-C in 32 patients with relapsed acute leukemia (25 acute myeloid leukemia [AML], 2 acute lymphoblastic leukemia [ALL]), 4 high-risk myelodysplastic syndrome (MDS), and 1 blast-phase chronic myeloid leukemia (CML).(1) Clofarabine was given as a 1-hour intravenous infusion for 5 days (days 2 through 6) followed 4 hours later by ara-C at 1 g/m(2) per day as a 2-hour intravenous infusion for 5 days (days 1 through 5). The phase 2 dose of clofarabine was 40 mg/m(2) per day for 5 days. Among all patients, 7 (22%) achieved complete remission (CR), and 5 (16%) achieved CR with incomplete platelet recovery (CRp), for an overall response rate of 38%. No responses occurred in 3 patients with ALL and CML. One patient (3%) died during induction. Adverse events were mainly less than or equal to grade 2, including transient liver test abnormalities, nausea/vomiting, diarrhea, skin rashes, mucositis, and palmoplantar erythrodysesthesias. Plasma clofarabine levels generated clofarabine triphosphate accumulation, which resulted in an increase in ara-CTP in the leukemic blasts. The combination of clofarabine with ara-C is safe and active. Cellular pharmacology data support the biochemical modulation strategy.

Acute Disease↗
PubMed2005

Plasma and cerebrospinal fluid pharmacokinetics of clofarabine in nonhuman primates.

INTRODUCTION: Clofarabine (2-chloro-2'fluoro-2'-deoxy-9-beta-d-arabinofuranosyladenine) is a purine nucleoside analogue that is active in the treatment of acute leukemia. We studied the pharmacokinetics and cerebrospinal fluid penetration of clofarabine in a nonhuman primate model. METHODS: A dose of 2.3 mg/kg of clofarabine was given i.v. over 2 hours to each of four animals. Plasma and cerebrospinal fluid (CSF) samples were obtained at specified intervals and the clofarabine concentration determined by reverse-phase high-pressure liquid chromatography with mass spectroscopy. RESULTS: The median clofarabine clearance was 17 mL/min/kg (range, 15-20), the median plasma area under the concentration-time curve was 452 mumol/L minutes (range, 380-487), and the median terminal half-life was 105 minutes (range, 78-138). Concentrations of clofarabine in CSF could not be modeled reliably because the terminal rate constant was not well defined. The median CSF penetration was 5% (range, 3-26%). CONCLUSION: Clofarabine penetrates into the CSF only modestly, but the concentrations obtained may approach those that are cytotoxic in vitro. Evaluation of the contribution of clofarabine to central nervous system preventive therapy should be considered in future studies.

Adenine Nucleotides↗
PubMed2005

Clofarabine in the treatment of acute myeloid leukaemia and acute lymphoblastic leukaemia: a review.

Clofarabine, a synthesised adenosine nucleoside, has recently demonstrated single-agent activity in the acute leukaemias. Originally developed to capture the best qualities of cladribine and fludarabine, clofarabine contains halogenated carbons, rendering it resistant to inactivating enzymes and maintaining its stability in acidic environments. Like other adenosine nucleosides, clofarabine acts by inhibiting ribonucleotide reductase and DNA polymerase, thereby depleting the amount of intracellular deoxynucleoside triphosphates available for DNA replication and also resulting in premature DNA chain termination. Clofarabine has also been shown to induce apoptosis in transformed cell lines, indicating that clofarabine results in cell death in both cycling and non-cycling cells. Interest in the development of clofarabine was initially hampered by the availability of other active nucleoside analogues for the treatment of haematological malignancies. However, the results of several early-phase trials evaluating the use of clofarabine in acute leukaemias in adults and children have rekindled enthusiasm for further investigation into its use. This article describes the development, pharmacology, toxicity and clinical activity of clofarabine, as well as discuss its potential role in the treatment of acute leukaemia.

Adenine Nucleotides↗
PubMed2005

The role of clofarabine in hematologic and solid malignancies--development of a next-generation nucleoside analog.

Clofarabine is a new-generation nucleoside analog that has been synthesized to combine the most favorable pharmacokinetic properties of its congeners fludarabine and cladribine. In addition to inhibition of DNA polymerases and DNA synthesis, clofarabine acts as a strong inhibitor of ribonucleotide reductase (RnR), an enzyme involved in regulating intracellular deoxynucleotide pools, and has a high affinity to the enzyme deoxycytidine kinase (dCyd), the rate-limiting step in nucleoside phosphorylation.A review of the English literature was performed that included original articles and related reviews from the MEDLINE (PubMed) data base and from abstracts based on the publication of meeting materials. Although it was synthesized early in the 1980s, the development of clofarabine was stalled until 1993, when, through efforts at The University of Texas M. D. Anderson Cancer Center, animal toxicology studies were conducted, and 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 leukemias. By way of its unique metabolic properties, clofarabine also has lent itself to biochemical modulation strategies with other nucleoside analogs, such as cytarabine. Combinations of clofarabine with cytarabine have been studied in acute leukemia and currently are being evaluated in untreated elderly patients with acute myeloid leukemia. Novel schedules are being explored in lymphoproliferative disorders and solid tumors. Clofarabine is a new nucleoside analog with considerable activity and an acceptable safety profile in acute leukemias.

Adenine Nucleotides↗
PubMed2004

Clofarabine in adult acute leukemias: clinical success and pharmacokinetics.

Clofarabine is a deoxyadenosine analog synthesized with the intention of retaining the favorable mechanistic properties of fludarabine and cladribine while eliminating their undesirable characteristics. Phase I studies among 32 patients with acute leukemia defined a maximum tolerated dose (MTD) of 40 mg/m2/d given as a one hour infusion daily for 5 days. The dose limiting toxicity (DLT) was transient hepatotoxicity. In a phase II study, 62 patients with acute leukemias received clofarabine at the MTD over 1 hour daily for 5 days. Twenty patients (32%) achieved complete response (CR), 1 had a partial response (PR), and 9 had a CR but without platelet recovery (CRp), for an overall response rate of 48%. Pharmacokinetic studies in the phase I trial revealed marked heterogeneity in peak levels of clofarabine among patients at the end of infusion, however; there was a linear, dose dependent increase in clofarabine concentration in the plasma. Pharmacodynamically, at the MTD, DNA synthesis was inhibited by more than 80% at the end of infusion. In phase II studies, the relationship between the pharmacokinetics of clofarabine triphosphate accumulation and clinical response at the MTD was explored, revealing an accumulation advantage of the cytotoxic triphosphate in leukemia cells of responders. The circulating leukemia blasts of patients who respond to clofarabine therapy exhibited a favorable pharmacokinetic profile. In conclusion, clofarabine is an active agent in the treatment of acute leukemias and MDS, and cellular pharmacokinetics has prognostic significance.

Adenine Nucleotides↗
PubMed2005

The distribution, metabolism, and elimination of clofarabine in rats.

The distribution, metabolism, and elimination of intravenous [14C]clofarabine was studied in Fischer 344 male rats under a once daily for 5 days dosing schedule of 25 or 50 mg/kg/day. Also, the in vitro metabolism in rat, dog, and human hepatocytes was studied. Plasma radioactivity (of which clofarabine accounted for 63% to 93%) exhibited three phases of exponential elimination, with half-lives of 0.3, 1.3, and 12.8 h after administration of the 25 mg/kg/day regimen. Unscheduled deaths occurred after one to three doses with the 50 mg/kg regimen, possibly due to nonlinear pharmacokinetics; therefore, mass balance and radiokinetic profiles could not be obtained. A total of 77.1% (of which 87.2% was clofarabine) and 10.8% (of which 6.9% was clofarabine) of the dose was recovered in urine and feces, respectively. 6-ketoclofarabine, believed to be formed via adenosine deaminase, was the metabolite of greatest concentration found in urine and feces, but in each matrix, it accounted for only 7% of the daily recovery of radioactivity. 6-ketoclofarabine was also found in myocardium and liver but accounted for less than 2% of the total radioactivity in those tissues. Clofarabine was the major analyte found in myocardium (>97% region of integration) and liver (>94% region of integration). Whole body autoradiography demonstrated that the highest postdistributive concentrations of radioactivity were in the excretory organs, kidney, bladder, and gastrointestinal tract, with no remarkable suborgan distribution. In rat, dog, and human hepatocytes, 95, 96, and 99.8% [14C]clofarabine remained, respectively, after 6 h of incubation. Eleven metabolites were observed, with the largest constituting 2.5% of the radioactivity.

Adenine Nucleotides↗
PubMed2004

Population pharmacokinetics of clofarabine, a second-generation nucleoside analog, in pediatric patients with acute leukemia.

The population pharmacokinetics of plasma clofarabine and intracellular clofarabine triphosphate were characterized in pediatric patients with acute leukemias. Traditional model-building techniques with NONMEM were used. Covariates were entered into the base model using a forward selection significance level of .05 and a backwards deletion criterion of .005. Model performance, stability, and influence analysis were assessed using the nonparametric bootstrap and n-1 jackknife. Simulations were used to understand the relationship between important covariates and exposure. A 2-compartment model with weight (scaled to a 40-kg reference patient) modeled as a power function on all pharmacokinetic parameters (0.75 on clearance-related terms and 1.0 on volume-related terms) was fit to plasma clofarabine concentrations (n = 32). White blood cell (WBC) count, modeled as a power function (scaled to a WBC count of 10 x 10(3)/microL), was a significant predictor of central volume with power term 0.128 +/- 0.0314. A reference patient had a systemic clearance of 32.8 L/h (27% between-subject variability [BSV]), a central volume of 115 L (56% BSV), an intercompartmental clearance of 20.5 L/h (27% BSV), and a peripheral volume of 94.5 L (39% BSV). Intracellular clofarabine triphosphate concentrations were modeled using a random intercept model without any covariates. The average predicted concentration was 11.6 +/- 2.62 microM (80% BSV), and although clofarabine triphosphate half-life could not be definitively estimated, its value was taken to be longer than 24 hours. The results confirm that clofarabine should continue being dosed on a per-squaremeter or per-body-weight basis.

Adenine Nucleotides↗
PubMed2006

Phase II study of clofarabine in pediatric patients with refractory or relapsed acute lymphoblastic leukemia.

PURPOSE: To evaluate the efficacy and safety of clofarabine, a novel deoxyadenosine analog, in pediatric patients with refractory or relapsed acute lymphoblastic leukemia (ALL). PATIENTS AND METHODS: In a phase II, open-label, multicenter study, 61 pediatric patients with refractory or relapsed ALL received clofarabine 52 mg/m2 intravenously over 2 hours daily for 5 days, every 2 to 6 weeks. The median age was 12 years (range, 1 to 20 years), and the median number of prior regimens was three (range, two to six regimens). RESULTS: The response rate was 30%, consisting of seven complete remissions (CR), five CRs without platelet recovery (CRp), and six partial remissions. Remissions were durable enough to allow patients to proceed to hematopoietic stem-cell transplantation (HSCT) after clofarabine. Median CR duration in patients who did not receive HSCT was 6 weeks, with four patients maintaining CR or CRp for 8 weeks or more (8+, 12, 37+, and 48 weeks) on clofarabine therapy alone. The most common adverse events of grade > or = 3 were febrile neutropenia, anorexia, hypotension, and nausea. CONCLUSION: Clofarabine is active as a single agent in pediatric patients with multiple relapsed or refractory ALL. The toxicity profile is as expected in this heavily pretreated patient population. Studies exploring rational combinations of clofarabine with other agents are ongoing in an effort to maximize clinical benefit.

Adenine Nucleotides↗
PubMed2006

Clofarabine and nelarabine: two new purine nucleoside analogs.

PURPOSE OF REVIEW: Both clofarabine and nelarabine recently received an accelerated approval by the US Food and Drug Administration for use in refractory or relapsed pediatric acute lymphoblastic leukemia and in refractory-relapsed T-cell acute lymphoblastic leukemia or T-cell lymphoblastic lymphoma. Both drugs have been studied for their metabolism and mechanisms of action in preclinical investigations and for their efficacy in clinical trials. This review will summarize these investigations and will suggest future plans with these nucleoside analogs. RECENT FINDINGS: Clofarabine and nelarabine were designed based on preclinical and clinical findings with other nucleoside analogs or normal deoxynucleotides such as dGTP. Studies in cell lines have demonstrated that triphosphate is the active metabolite for both these purine nucleoside analogs. Pharmacokinetic and pharmacodynamic investigations during clinical trials have verified the importance of triphosphate levels in achieving clinical responses. Several phase I and II clinical explorations have suggested the utility of clofarabine in acute leukemias and nelarabine in T-cell diseases. Dose-limiting toxicities were nonhematologic: hepatotoxicity for clofarabine and neurotoxicity for nelarabine. SUMMARY: Clofarabine is the first deoxyadenosine analog that shows promise in adult and pediatric acute leukemias without untoward toxicity. Nelarabine, as expected from its design, is a drug that may be directed to T-cell diseases.

Adenine Nucleotides↗
PubMed2006

The structure of human deoxycytidine kinase in complex with clofarabine reveals key interactions for prodrug activation.

Clofarabine [2-chloro-9-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-9H-purin-6-amine] is a hybrid of the widely used anticancer drugs cladribine and fludarabine. It is the precursor of an effective chemotherapeutic agent for leukemias and other hematological malignancies and received accelerated approval by the FDA for the treatment of pediatric patients with relapsed or refractory acute lymphoblastic leukemia. Clofarabine is phosphorylated intracellularly by human deoxycytidine kinase (dCK) to the 5'-monophosphate, which is the rate-limiting step in activation of the prodrug. dCK has a broad substrate specificity, with a much higher activity to deoxycytidine than to deoxyadenosine and deoxyguanosine. As a purine-nucleoside analog, clofarabine is a better substrate of dCK than deoxycytidine. The crystal structure of dCK has been solved previously in complex with pyrimidine nucleosides and ADP [Sabini et al. (2003), Nature Struct. Biol. 10, 513-519]. In the current study, the crystal structure of clofarabine- and ADP-bound dCK was solved to 2.55 angstroms by molecular replacement. It appears that the enzyme takes the same conformation as in the structures of the pyrimidine nucleoside-bound complexes. The interactions between 2-Cl and its surrounding hydrophobic residues contribute to the high catalytic efficiency of dCK for clofarabine.

Adenine Nucleotides↗
PubMed2003

Phase 2 clinical and pharmacologic study of clofarabine in patients with refractory or relapsed acute leukemia.

In a phase 2 study, 62 patients with relapsed and refractory acute myeloid leukemia (AML; n = 31), myelodysplastic syndrome (MDS; n = 8), chronic myeloid leukemia in blastic phase (CMLBP; n = 11), and acute lymphocytic leukemia (ALL; n = 12) received 40 mg/m2 clofarabine intravenously over 1 hour daily for 5 days, every 3 to 6 weeks. Twenty patients (32%) achieved complete response (CR), 1 had a partial response (PR), and 9 (15%) achieved CR but without platelet recovery (CRp), for an overall response rate of 48%. In AML, responses were noted in 2 (18%) of 11 patients in first salvage with short first CR (</= 12 months), in 7 (87%) of 8 patients with longer first CR, and in 8 (67%) of 12 patients in second or subsequent salvage. Responses were observed in 4 of 8 patients with high-risk MDS (50%), in 7 (64%) of 11 with CML-BP, and in 2 (17%) of 12 with ALL. Severe reversible liver dysfunction was noted in 15% to 25%. After the first clofarabine infusion, responders accumulated more clofarabine triphosphate in blasts compared with nonresponders (median 18 vs 10 microM; P =.03). This increased only in responders (median, 1.8-fold; P =.008) after the second clofarabine infusion. In summary, clofarabine is active in acute leukemias and MDS; cellular pharmacokinetics may have prognostic significance.

Adenine Nucleotides↗
PubMed2003

Phase I clinical and pharmacology study of clofarabine in patients with solid and hematologic cancers.

PURPOSE: To define the maximum-tolerated doses (MTDs) and dose-limiting toxicities (DLTs) of clofarabine, given as a 1-hour infusion daily for 5 days, in patients with solid tumors and with acute leukemia. PATIENTS AND METHODS: The initial part of the study defined the MTD and DLT in solid tumors. The second part of the study defined the MTD and DLT in acute leukemia. RESULTS: The starting dose of clofarabine (15 mg/m(2)) was myelosuppressive, requiring several dose de-escalations to 2 mg/m(2), the dose suggested for phase II studies in solid tumors. Dose escalation in acute leukemia started at 7.5 mg/m(2), with several escalations to 55 mg/m(2). The DLT was reversible hepatotoxicity at 55 mg/m(2). The recommended dose for acute leukemia phase II studies was 40 mg/m(2). Among 32 treated patients with acute leukemia, two achieved a complete response and three had a marrow complete response without platelet recovery (hematologic improvement), for an overall response rate of 16%. At 40 mg/m(2), the median plasma clofarabine level was 1.5 micro mol/L (range, 0.42 to 3.2 micro mol/L; n = 7). Cellular and plasma pharmacokinetic studies suggested dose proportionality but showed a wide variation in intracellular concentrations of clofarabine triphosphate. CONCLUSION: This phase I study defined the following two MTDs for clofarabine given as a 1-hour infusion daily for 5 days: 2 mg/m(2) for solid tumors, the DLT being myelosuppression; and 40 mg/m(2) for acute leukemia, the DLT being hepatotoxicity. Encouraging activity was observed in acute leukemia.

Adenine Nucleotides↗
PubMed2003

Clofarabine, a novel nucleoside analog, is active in pediatric patients with advanced leukemia.

Despite progress in leukemia therapy, most children who experience relapse have a dismal prognosis. New, effective approaches are needed. We conducted a phase 1 study of a novel nucleoside analog, clofarabine, in pediatric patients with refractory and relapsed leukemia. Clofarabine was infused intravenously over 1 hour each day for 5 days. Six dose levels, between 11.25 and 70 mg/m(2) per day for 5 days, were studied in 25 patients. A modified 3 + 3 phase 1 design was followed with 30% dose escalation until the dose-limiting toxicity (DLT) was defined. The maximum tolerated dose (MTD) was 52 mg/m(2) per day for 5 days. At the end of infusion at MTD, clofarabine triphosphate levels in leukemia blasts varied between 6 microM and 19 microM, which resulted in complete and sustained inhibition of DNA synthesis. The DLT was reversible hepatotoxicity and skin rash at 70 mg/m(2) per day for 5 days. Twenty-five patients were treated. Five patients achieved complete remission (CR), and 3 achieved partial remission (PR), for an overall response rate of 32%. Clofarabine is well tolerated and shows significant antileukemic activity in heavily pretreated children. Multicenter phase 2 trials in pediatric acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML) are ongoing.

Adenine Nucleotides↗