Cladribine in the remission induction of adult acute myeloid leukemia: where do we stand?

The combination of cytarabine and an anthracycline has been the standard of care for the induction of remission in acute myeloid leukemia (AML). The response to treatment and survival of adult patients with AML are still variable and depend on multiple factors. Therefore, there have been many efforts to improve the response to treatment and survival rates by either increasing the cytarabine dose or adding a third agent to the standard induction chemotherapy regimen. Unfortunately, attempts to improve response and survival have been mostly unsuccessful. Recent clinical trials and retrospective studies explored the addition of cladribine to standard induction chemotherapy for AML. Some of these studies showed higher rates of complete remission, and one showed improved survival. In this review, we will discuss the antileukemic properties of cladribine and summarize the recent clinical data regarding its incorporation into the induction therapy for adult AML.

Revisiting the role of cladribine in acute myeloid leukemia: an improvement on past accomplishments or more old news?

Originally studied in lymphoid diseases, cladribine (CdA) is an adenosine deaminase resistant analog of adenosine that was later discovered to induce myeloid cell apoptosis. The activity of CdA in myeloid malignancies was first reported in relapsed/refractory (RR) pediatric acute myeloid leukemia (AML) with complete response (CR) rates of up to 47%. Consequently, several studies have confirmed the efficacy of single agent CdA or CdA combination regimens in AML. Established CR rates for combination regimens in RR adults are approximately 50%, while CR rates for newly diagnosed (ND) adults are approximately 70% and show similar toxicity profiles to previously used regimens. Despite these promising data, many centers have yet to adopt CdA combination regimens for these difficult to treat populations. We review the pharmacology, pharmacokinetics, clinical data, and safety of CdA monotherapy and combination regimens for the management of pediatric and adult ND and RR-AML.

Phospholipid derivatives of cladribine and fludarabine: synthesis and biological properties.

Phospholipid derivatives of anticancer nucleosides cladribine and fludarabine (F-ara-A) bearing 1,2- and 1,3-diacylglycerol moieties have been prepared by the H-phosphonate approach using 1,1,3,3-tetraisopropyldisiloxane-1,3-diyl protecting group for cladribine and a combination of tert-butyldimethylsilyl and levulinyl protecting groups for 2-fluoroadenine nucleosides. The synthesized conjugates exhibited lower in vitro antiproliferative activity against human tumor cell lines in comparison with the same concentrations of the parent cladribine and fludarabine phosphate. In the course of biokinetic study, it was found that intragastric administration of phospholipid F-ara-A derivatives to Wistar rats and ICR outbred male mice led to a slow release of F-ara-A into the bloodstream, a smooth increase in nucleoside concentration, and prolonged serum circulation of liberated nucleoside. The oral bioavailability of F-ara-A from 1,2-dimyristoylglycerophosphate derivative 29 was similar to its oral bioavailability from fludarabine phosphate.

Beyond hairy cell: the activity of cladribine in other hematologic malignancies.

Before the contemporary development of rationally designed antineoplastic therapies, cladribine was identified as a lymphocyte-specific agent. Its profound impact on the natural history of hairy cell leukemia, with responses approaching 100% and a median duration of response of nearly a decade after only a single 7-day course, is well known and revolutionized the treatment of hairy cell leukemia. However, cladribine's impressive activity in other lymphoproliferative disorders has been generally underappreciated. Multiple single-arm phase 2 trials have demonstrated cladribine's potency across the full spectrum of lymphoid malignancies. In a limited number of phase 3 trials and cross-study analyses, cladribine compared favorably with fludarabine, another purine nucleoside analog that is more commonly used in the treatment of indolent lymphoid malignancies. Cladribine has been noted to have particular activity among lymphoid disorders with few effective therapies, specifically, chronic lymphocytic leukemia, lymphoplasmacytic lymphoma, marginal zone lymphoma, and mantle cell lymphoma. Recently approved novel agents may act in synergy with cladribine for these conditions and should be incorporated into future clinical studies.

Model-informed assessment of ethnic sensitivity and dosage justification for Asian populations in the global clinical development and use of cladribine tablets.

Cladribine tablets have been approved in many countries for the treatment of patients with various forms of relapsing multiple sclerosis (MS). Cladribine has a unique pharmacokinetic/pharmacodynamic (PK/PD) profile with a short elimination half-life (~ 1 day) relative to a prolonged PD effect on specific immune cells (most notably a reversible reduction in B and T lymphocyte counts). This results in a short dosing schedule (up to 20 days over 2 years of treatment) to sustain efficacy for at least another 2 years. Global clinical studies were conducted primarily in White patients, in part due to the distinctly higher prevalence of MS in White patients. Given the very low prevalence in Asian countries, MS is considered as a rare disease there. In spite of the limited participation of Asian patients, to demonstrate favorable benefit/risk profile in the treatment of MS demanded application of a Totality of Evidence approach to assess ethnic sensitivity for informing regulatory filings in Asian countries and supporting clinical use of cladribine in Asian patients. Population PD modeling and simulation of treatment-related reduction in absolute lymphocyte count, as a mechanism-related biomarker of drug effect, confirmed consistent PDs in Asian and non-Asian patients with MS, supporting absence of ethnic sensitivity and a common dosage across populations. Through this example, we demonstrate the value of holistic integration of all available data using a model-informed drug development (MIDD) framework and a Totality of Evidence mindset to evaluate ethnic sensitivity in support of Asia-inclusive development and use of the drug across populations.

Phase I/II and pharmacokinetic study of cladribine with 2-h infusion in Japanese patients with relapsed indolent B-cell lymphoma mostly pretreated with rituximab.

We conducted a phase I/II study to investigate the toxicity, pharmacokinetics, and efficacy profiles of cladribine with 2-h intravenous infusion for five consecutive days every four weeks in Japanese patients with relapsed indolent B-cell lymphoma. This was a dose-escalation study to confirm the safety of the doses which have been recommended for Caucasian patients (phase I), and to further evaluate the efficacy and safety (phase II). In the phase I portion for nine patients, no dose-limiting toxicities were observed at levels 1 (0.09 mg/kg/day, n = 3) and 2 (0.12 mg/kg/day, n = 6). No appreciable accumulation of plasma cladribine concentration was suggested. We enrolled a total of 20 patients, and an additional 14 patients in the phase II portion at level 2 (0.12 mg/kg/day). Eighteen patients, including 13 with follicular lymphoma, were eligible for efficacy evaluation, and 15 (83%) were pretreated with rituximab. The overall response rate was 50% (9/18; 80% confidence interval, 35-65%), with 11% (2/18) complete response. With a median follow-up of 296 days, the estimated median time to progression for 18 eligible patients was 382 days. The most frequent adverse events were hematologic toxicities, including grade 4 neutropenia. Non-hematologic toxicities were mild. In conclusion, cladribine with 2-h intravenous infusion for five consecutive days every four weeks is effective with acceptable toxicities for Japanese patients with relapsed indolent B-cell lymphoma, including those pretreated with rituximab.

The Clinical Pharmacology of Cladribine Tablets for the Treatment of Relapsing Multiple Sclerosis.

Cladribine Tablets (MAVENCLAD®) are used to treat relapsing multiple sclerosis (MS). The recommended dose is 3.5 mg/kg, consisting of 2 annual courses, each comprising 2 treatment weeks 1 month apart. We reviewed the clinical pharmacology of Cladribine Tablets in patients with MS, including pharmacokinetic and pharmacometric data. Cladribine Tablets are rapidly absorbed, with a median time to reach maximum concentration (Tmax) of 0.5 h (range 0.5-1.5 h) in fasted patients. When administered with food, absorption is delayed (median Tmax 1.5 h, range 1-3 h), and maximum concentration (Cmax) is reduced by 29% (based on geometric mean). Area under the concentration-time curve (AUC) is essentially unchanged. Oral bioavailability of cladribine is approximately 40%, pharmacokinetics are linear and time-independent, and volume of distribution is 480-490 L. Plasma protein binding is 20%, independent of cladribine plasma concentration. Cladribine is rapidly distributed to lymphocytes and retained (either as parent drug or its phosphorylated metabolites), resulting in approximately 30- to 40-fold intracellular accumulation versus extracellular concentrations as early as 1 h after cladribine exposure. Cytochrome P450-mediated biotransformation of cladribine is of minor importance. Cladribine elimination is equally dependent on renal and non-renal routes. In vitro studies indicate that cladribine efflux is minimally P-glycoprotein (P-gp)-related, and clinically relevant interactions with P-gp inhibitors are not expected. Cladribine distribution across membranes is primarily facilitated by equilibrative nucleoside transporter (ENT) 1, concentrative nucleoside transporter (CNT) 3 and breast cancer resistance protein (BCRP), and there is no evidence of any cladribine-related effect on heart rate, atrioventricular conduction or cardiac repolarisation (QTc interval prolongation). Cladribine Tablets are associated with targeted lymphocyte reduction and durable efficacy, with the exposure-effect relationship showing the recommended dose is appropriate in reducing relapse risk.

Development and validation of a sensitive and specific HPLC assay of cladribine for pharmacokinetics studies in rats.

PURPOSE: To develop and validate a sensitive and specific HPLC assay for cladribine (CdA) in plasma for pharmacokinetic studies in rats. METHODS: CdA and the internal standard AZT were purchased from Sigma-Aldrich Chem. The HPLC system consisted of a Shimadzu LC-9A pump, a 3 im, 250 x 2.0 mm I.D. high speed C18 column (Jupitertrade mark), preceded by a 5 im 4 4 mm I.D. C18 guard column (Licrocarttrade mark), an Agilent Model 1050 UV-VIS detector and a 3395 Integrator. The mobile phase was made up of 0.01M KH2PO4 (pH 5): methanol: acetonitrile 90:5:5). The system was operated at ambient temperature with a flow rate of 0.3 mL/min, and UV wavelength at 265 nm, and an operating pressure of ca. 1.56 kpsi. Extraction of cladribine and AZT from plasma was achieved by solid phase extraction using 100 mg/mL C18 SPE columns Extra-septrade mark). The assay was validated for sensitivity, precision, specificity and application for pharmacokinetic study in rats. RESULTS: Under these conditions, the average retention times of CdA and AZT were 13.5 and 21 min, respectively, and recoveries were between 80 - 95%. Standard curve constructed from plasma standards was linear from 0.1 ug/mL to 1 ug/mL with regression coefficient (r2) 0.99 or greater. Sensitivity assessed by on column injection was < 1 ng. Using a 50-uL plasma sample size, the mean intra assay variations 0.1 ug/mL were 7%, and inter assay variations over a period of 3 months for 5 separate batches were less than 20%. The assay was used to study a single dose pharmacokinetic study of CdA in rats after a 2 mg/kg subcutaneous injection. CONCLUSION: The described HPLC assay has adequate sensitivity and specificity to study pharmacokinetics of CdA in rats, and could be adapted also to clinical pharmacokinetic studies.

Population Pharmacokinetics of Cladribine in Patients with Multiple Sclerosis.

PURPOSE: The aims of this study were to characterize the concentration-time course of cladribine (CdA) and its main metabolite 2-chloroadenine (CAde), estimate interindividual variability in pharmacokinetics (PK), and identify covariates explaining variability in the PK of CdA. METHODS: This population PK analysis was based on the combined dataset from four clinical studies in patients with multiple sclerosis (MS): three phase I studies, including one food and one drug-drug interaction study, and one phase III clinical study. Plasma and urine concentration data of CdA and CAde were modeled simultaneously. RESULTS: The analysis comprised a total of 2619 CdA and CAde plasma and urine concentration observations from 173 patients with MS who received an intravenous infusion or oral tablet doses of CdA as a single agent or in combination with interferon (IFN) ß-1a. CdA PK data were best described by a three-compartment model, while a one-compartment model best described the PK of CAde. CdA renal clearance (CLR) was correlated with creatinine clearance (CLCR), predicting a decrease in the total clearance of 19%, 30% and 40% for patients with mild (CLCR = 65 ml/min), moderate (CLCR = 40 ml/min) and severe (CLCR = 20 ml/min) renal impairment, respectively. Food decreased the extent of CdA absorption by 11.2% and caused an absorption delay. Coadministration with IFNß-1a was found to increase non-CLR (CLNR) by 21%, resulting in an increase of 11% in total clearance. CONCLUSIONS: Both CdA and CAde displayed linear PK after intravenous and oral administration of CdA, with CdA renal function depending on CLCR. Trial registration number for study 25643: NCT00213135.

Cladribine: an investigational immunomodulatory agent for multiple sclerosis.

OBJECTIVE: To review the pharmacology, pharmacokinetics, efficacy, and safety of cladribine, a purine analog undergoing Phase III trials for approval of its use in the treatment of multiple sclerosis (MS). DATA SOURCES: A MEDLINE search (1966-September 2006) was conducted using the key words cladribine and multiple sclerosis. No limits were placed on the search. STUDY SELECTION AND DATA EXTRACTION: Studies and review articles related to cladribine and MS were reviewed. The trials examining the role of cladribine in MS were analyzed. DATA SYNTHESIS: Cladribine is a purine analog that demonstrates lymphocytotoxic activity. Recent data suggest that cladribine may have a role in the treatment of relapsing-remitting and the progressive forms of MS. In these studies, cladribine has shown mixed results in decreasing neurologic disability, as measured by various rating scales, but has consistently shown positive results in reducing the number of enhancing lesions, which reflects a measure of disease activity. To date, there is one ongoing study examining the role of oral cladribine in the treatment of relapsing-remitting MS. The incidence of adverse effects with cladribine has been significantly greater than with placebo, with the most common being myelosuppression. CONCLUSIONS: While data do not support its use as a first-line MS treatment, cladribine may be a promising agent for refractory patients with secondary progressive MS. Further studies are warranted.

Molecular and biochemical mechanisms of fludarabine and cladribine resistance in a human promyelocytic cell line.

2F-Adenine arabinoside (fludarabine, Fara-A) and 2-chloro-2'-deoxyadenosine (cladribine, CdA) are nucleoside analogues with antineoplastic activity in vitro and in vivo. Lack of clinical resistance between CdA and Fara-A has been demonstrated in patients with chronic lymphocytic leukemia (G. Juliusson et al., N. Engl. J. Med., 327: 1056-1061, 1992). To clarify the differences in mechanism of resistance to CdA and Fara-A in vitro, we developed two stable, resistant cell lines, HL60/CdA and HL60/ Fara-A, by exposure to increasing concentrations of analogues over a period of 8 months. Resistant cells tolerated >8,000 and 5-fold higher concentrations of CdA and Fara-A, respectively. The specific activity of the nucleoside phosphorylating enzyme (using deoxycytidine as substrate) in cell extracts from HL60/CdA and HL60/Fara-A mutants was about 10 and 60%, respectively, compared with the parental cell line. Western blot analysis using a polyclonal antibody showed no detectable deoxycytidine kinase (dCK) protein in CdA-resistant cells, whereas in Fara-A-resistant cells, it was at the same level as in the parental cells. The mitochondrial enzyme deoxyguanosine kinase was not altered in resistant cell lines. The HL60/CdA cells showed cross-resistance to 2-chloro-2'-arabino-fluoro-2'-deoxyadenosine, Fara-A, arabinofuranosyl cytosine, difluorodeoxyguanosine, and difluorodeoxycytidine toxicity, most likely because of the decreased phosphorylation of these analogues by dCK. Using real-time quantitative PCR, the mRNA levels of dCK and cytosolic 5'-nucleotidase (5'-NT), a major nucleoside dephosphorylating enzyme, were measured. It was shown that the dCK mRNA levels in both CdA- and Fara-A resistant cells were decreased in parallel with the activity. The expression of 5'-NT mRNA was not significantly elevated in CdA- and Fara-A resistant cells, as compared with the parental cells. Ribonucleotide reductase maintains a balanced supply of deoxynucleotide triphosphate pools in the cell and may also be a major cellular target for CdA and Fara-A nucleotides. Except for the deoxycytidine triphosphate level, the intracellular deoxynucleotide triphosphate pools were significantly higher in Fara-A-resistant cells compared with the parental cell line. This might be a consequence of mutation or altered regulation of ribonucleotide reductase activity and may explain the 2-5-fold cross-resistance to several nucleoside analogues observed with HL60/Fara-A cells. It is likely that the resistance for CdA was mainly attributable to a dCK deficiency, and Fara-A-resistant cells might have another contributing factor to the resistance beyond the dCK deficiency.

Pharmacokinetics of cladribine (2-chlorodeoxyadenosine) in children with acute leukemia.

Cladribine is a synthetic purine nucleoside with demonstrated activity in hairy cell leukemia and acute myeloid leukemia. We have studied the pharmacokinetics of this drug in 25 pediatric patients with acute leukemia treated with cladribine as a single agent, 8.9 mg/m2/24 h, for 5 days by continuous i.v. infusion. Twelve patients were in relapse, and acute myeloid leukemia was newly diagnosed in 13 patients. Plasma, urine, and cerebrospinal fluid cladribine concentrations were determined by a radioimmunoassay with a limit of detection of 1 nM. An open two-compartment model was fit to the plasma concentration data. The mean (SD) clearance was 39.4 (12.4) liters/h/m2 and ranged from 14.4-55.4 liters/h/m2. When clearance was normalized to body weight (liters/h/kg) it was negatively correlated with age, with older patients having slower clearances per unit of body weight. However, when clearance was normalized to body surface area, no significant correlation with age was observed. The mean (SD) steady-state plasma concentration (predicted 120-h concentration) was 37.7 (17.3) nM and ranged from 23.2-84.5 nM. The terminal phase half-life in 22 patients ranged from 14.3-25.8 h, with a mean (SD) of 19.7 (3.4) h. The volume of distribution at steady state was highly variable, with a mean (SD) of 356.6 (225.2) liters/m2. None of these parameters was significantly different between patients in relapse and patients with newly diagnosed disease. Renal clearance was determined in 7 patients and ranged from 34.6-643.6 ml/min/m2, with a mean (SD) of 317.9 (208.7) ml/min/m2. Renal clearance as a percentage of total systemic clearance ranged from 11.0-85.1%, with a mean of 51.0%. In 11 patients, the mean (SD) cerebrospinal fluid concentration was 6.1 (3.97) nM, a mean of 18.2% of the steady-state plasma concentration. The CSF:plasma concentration ratio was significantly higher on day 5 (22.7% in 7 patients) than on day 4 (7.6% in 3 patients; P = 0.03). Additional studies are needed to further define the metabolic fate of cladribine. In this paper we provide the first comprehensive description of the pharmacokinetics of this drug in children and provide data which suggest that cladribine may be useful in the treatment of patients with meningeal leukemia or malignancies of the central nervous system.

On the bioavailability of oral and subcutaneous 2-chloro-2'-deoxyadenosine in humans: alternative routes of administration.

PURPOSE: The antimetabolite 2-chloro-2'-deoxyadenosine (CdA) is a promising alternative to alkylating agents for the treatment of lymphoproliferative disorders. Its use, however, is hampered by the need for intravenous (IV) administration. The aim of the present study was to determine the bioavailability of subcutaneously (SC) and orally administered CdA, and to establish an oral dose of CdA that could supersede IV administration. PATIENTS AND METHODS: A previously developed high-performance liquid chromatography method was used for the determination of plasma CdA concentrations in 13 patients. Ten patients were treated on alternate days with 0.14 mg/kg/d CdA as a 2-hour IV infusion or by a SC injection. Three of these patients were also given 0.14 mg/kg CdA orally in enteric-coated capsules. Ten patients were administered CdA orally that was dissolved in phosphate-buffered saline (PBS) after treatment with 20 mg omeprazole 1 and 6 hours before the administration of CdA. RESULTS: The bioavailability of SC CdA was 102% +/- 28% (mean +/- SD), and the bioavailability of CdA administered in enteric-coated capsules was 19%, 24%, and 60%. In the three patients who were given 0.14 mg/kg orally dissolved in PBS, the bioavailability was 48% +/- 8%, whereas in the seven patients who received 0.28 mg/kg, the bioavailability was 55% +/- 17%. In the 10 patients who were treated with the CdA solution orally, the coefficients of variation of the areas under the curve (AUCs) after oral and IV administration were similar. Thus, oral administration did not add to the interindividual variability. CONCLUSIONS: We conclude that orally administered CdA can supersede IV infusion if the dose is doubled. SC administration gives a high peak concentration of short duration with an AUC identical to that of IV infusion. Thus, SC injection can also be used as an alternative to IV infusion.

Subcutaneous injections of 2-chlorodeoxyadenosine for symptomatic hairy cell leukemia.

PURPOSE: To evaluate the clinical efficacy and safety of 2-chlorodeoxyadenosine (CdA) when administered by subcutaneous injection to patients with symptomatic hairy cell leukemia (HCL), and to evaluate predictive factors for response. PATIENTS AND METHODS: Seventy-three patients were given CdA as a subcutaneous injection once daily for 7 days. Complete remission (CR) required normalized blood counts and the absence of B-ly 7-positive bone marrow cells by flow cytometry. CdA concentrations in plasma following the first injection were analyzed by high-pressure liquid chromatography. RESULTS: Fifty-nine patients (81%) achieved a durable CR after one (n = 55) or two courses, and 10 had a partial remission (PR). With a median follow-up duration of 20 months, no patient had a clinical relapse. Neutropenic fever that required intravenous antibiotics occurred in 28 patients (38%). No toxicity at injection sites was observed. Incomplete response was predicted by an elevated lymphocyte count and serum beta 2-microglobulin level, and by a high percentage of hairy cells in the bone marrow. Plasma CdA levels were similar to those achieved from intravenous administration. CONCLUSION: Subcutaneous injection of CdA is safe and as effective as continuous infusion without problems associated with the mode of administration. Our schedule simplifies CdA treatment and can be generally recommended.

Pharmacokinetic study of oral and bolus intravenous 2-chlorodeoxyadenosine in patients with malignancy.

PURPOSE: This study was designed to evaluate the absolute bioavailability (F value) of 2-chlorodeoxyadenosine (cladribine; 2-CdA) after multiple oral administrations, and the intersubject variability after oral and 2-hour intravenous (IV) administration schedules in patients with malignancy. PATIENTS AND METHODS: Patients with advanced malignancies were eligible. There were two treatment cycles; during cycle 1, patients received 2-CdA solution at 0.28 mg/kg/d orally under fasting conditions for 5 consecutive days concomitantly with omeprazole, and 4 weeks later during cycle 2 patients received 2-CdA as a 2-hour IV infusion of 0.14 mg/kg/d for 5 consecutive days. Serial blood samples for 2-CdA plasma levels were obtained after drug administrations on days 1 and 5 during each treatment cycle. RESULTS: Ten patients completed cycles 1 and 2. The F value of oral 2-CdA measured on days 1 and 5 was 37.2% and 36.7%, respectively. For both oral and IV multiple administrations, there was no significant accumulation in maximum concentration (Cmax), and the intersubject variabilities (coefficient of variation [CV], approximately 40%) in Cmax and area under the concentration-time curve from 0 to 24 hours [AUC(0-24)] values were comparable for both routes on days 1 and 5. A three-compartment open model was applied to the plasma concentration data after oral and IV administrations and resulted in good agreement between observed and simulated concentration-time profiles. Neutropenia was the principal adverse event observed when 2-CdA was administered orally and IV. CONCLUSION: The F value of 2-CdA after oral administration was approximately 37% and there were no cumulative differences in bioavailability observed on multiple dosing of the drug. The absorption and disposition characteristics of oral 2-CdA were linear and predictable with this dosing regimen.

2-Chlorodeoxyadenosine dose escalation in nonhematologic malignancies.

PURPOSE: We performed a dose-escalation study of 2-chlorodeoxyadenosine (2-CdA) in solid tumors to determine the maximum-tolerated dose (MTD) and define its toxicity profile at higher doses. PATIENTS AND METHODS: Twenty-one patients, seven with malignant astrocytoma, twelve with metastatic melanoma, and two with metastatic hypernephroma, were enrolled onto the study. Patients were entered onto cohorts that received 0.10, 0.15, or 0.20 mg/kg/d of 2-CdA by continuous intravenous infusion for 7 days every 28 days. 2-CdA levels were determined by radioimmunoassay. In tumor tissue samples, deoxycytidine kinase (dCK) levels were measured by both enzyme activity and immunoreactive protein analysis. RESULTS: Of seven patients treated with 2-CdA at 0.1 mg/kg/d, one experienced grade 3 or 4 myelotoxicity. Of 11 patients treated at 0.15 mg/kg/d, four experienced myelotoxicity, two after a single course of 2-CdA. All three patients who received 2-CdA at 0.2 mg/kg/d experienced myelosuppression. Neurologic events occurred in two patients, both with malignant melanoma. Two of seven patients (28.6%) with astrocytomas obtained partial responses with a median duration of 8 months. 2-CdA penetrated the blood-brain barrier. An association was found between dCK levels as measured by enzymatic activity and immunoreactive proteins, but this did not correlate with 2-CdA tumor responsiveness. CONCLUSION: The MTD for 2-CdA delivered as a 7-day intravenous infusion in patients with nonhematologic malignancies was determined to be 0.1 mg/kg/d, the same as the MTD for patients with hematologic malignancies. There was no clinical correlation with dCK expression and response to 2-CdA. The responses noted in patients with malignant astrocytoma warrant further phase II study.

Quantitation of synergism of arabinosylcytosine and cladribine against the growth of arabinosylcytosine-resistant human lymphoid cells.

This report presents a quantitative analysis of the synergistic interaction of arabinosylcytosine (araC) and cladribine (CdA) in human H9-lymphoid cell lines sensitive and resistant to araC (H9-araC cells). H9-araC cells obtained by cultivation of H9 cells in the presence of 0.5 microM arabinosylcytosine (araC) had lower deoxycytidine kinase (dCK) than the parental cell line. The IC50 values of araC and CdA calculated by using median-effect analysis and CalcuSyn software were: 0.55 microM and 1.16 microM for CdA and 0.0058 microM and 3.5 microM for araC in H9 and H9-araC cells, respectively. These values were reduced to 0.10 microM and 0.38 microM for CdA and to 0.004 microM and to 0.77 microM for araC when the drugs were used in combination. Computerized simulation of dose reduction index (DRI) indicated that at 50-99% growth inhibition levels, the doses of araC could be reduced by 2.0 to 11.9-fold and 2.9 to 5.3-fold and the doses of CdA by 5.9 and 183.7-fold and 3.1 to 164.8-fold in H9 and H9-araC cells, respectively, when the drugs are used in combination. Assessment by combination index (CI) analysis showed that the combination exhibited moderate to strong synergistic lympho-cytotoxic effects. CdA metabolic studies (influx and activation) in the presence of deoxyadenosine, deoxycytidine, or araC suggested that CdA enters cells by a deoxyadenosine-inhibitable transport system, which is different than that of araC and deoxycytidine transport system. Thus, in addition to the known mechanisms, other mechanisms might be involved in the metabolism of CdA. The demonstration that araC and CdA combinations exert synergistic cytotoxicity even in the resistant cells raises hope that such a combination may be useful in tumors that were found resistant to these drugs.

Intracellular pharmacokinetics of 2-chlorodeoxyadenosine in leukemia cells from patients with chronic lymphocytic leukemia.

2-Chlorodeoxyadenosine (2-CdA) is an important agent in the treatment of hairy cell leukemia and chronic lymphocytic leukemia (CLL). Others have reported that levels of 2-CdA phosphates present in human leukemia cells decline rapidly when the cells are in 2-CdA-free medium (Santana et al. J Clin Oncol 1991; 9: 416-422). In the present study, time-courses of 2-CdA loss from CLL cells were biexponential: the mean half-life of the initial phase was 0.30 +/- 0.18 h; the presence of 0.5 microM nitrobenzylthioinosine (NBMPR, a classical inhibitor of nucleoside transport) in the suspending medium, significantly decreased the initial rate of 2-CdA efflux (mean half-life, 0.43 +/- 0.22 h). As a consequence, AUCs (areas under time-course plots) were significantly higher in the NBMPR-treated cells (4.56 +/- 2.01 pmol.h/10(6) cells, n = 19) than in untreated control cells (3.83 +/- 1.74 pmol.h/10(6) cells; n = 19). 2-CdA was the principal efflux product released into the medium from 2-CdA-loaded CLL cells. We conclude that nucleoside transport processes contribute to the efflux of 2-CdA from CLL cells and that NBMPR may be useful as a retentive agent.

Enhancement of retention and cytotoxicity of 2-chlorodeoxyadenosine in cultured human leukemic lymphoblasts by nitrobenzylthioinosine, an inhibitor of equilibrative nucleoside transport.

In leukemic cells exposed to 2-chlorodeoxyadenosine (2-CdA), levels of the nucleoside drug and its phosphate metabolites decay with time in the absence of external 2-CdA; an intrinsic part of this process is the efflux of 2-CdA. The effects of nitrobenzylthioinosine (NBMPR) and of dipyridamole (DPM), both potent inhibitors of es (e, equilibrative; s, sensitive to NBMPR) nucleoside transport processes, were studied in four lines of cultured leukemic lymphoblasts. Suspensions of 2-CdA-loaded cells were diluted 10-fold with 2-CdA-free medium to initiate the cellular 2-CdA decay processes, which followed a biexponential time course. When diluting media contained NBMPR or DPM, intracellular levels of 2-CdA and its metabolites were substantially increased (P < 0.001) compared with cells in media lacking the transport inhibitors, and 2-CdA loss followed a monoexponential time course. As a consequence, the AUCs (area under time-course plots of intracellular 2-CdA and its metabolites) were significantly (P < 0.001) lower in untreated control cells compared to inhibitor-treated cells. These results suggest that nucleoside transport processes contribute to the efflux of 2-CdA from the cultured lymphoblasts. The cytotoxicity of 1-h exposure to 2-CdA of Reh-A2 and CCRF-CEM cells was enhanced three-fold by subsequent exposure to 0.5 microM NBMPR relative to that of control cells subjected to the same manipulations without NBMPR exposure. However, before such a strategy may be considered to have a therapeutic application, careful examination of effects in normal lymphocytes and ex vivo leukemic lymphoblasts must first be undertaken. Leukemia (2000) 14, 52-60.

Pharmacokinetics of cladribine in plasma and its 5'-monophosphate and 5'-triphosphate in leukemic cells of patients with chronic lymphocytic leukemia.

The pharmacokinetic parameters of cladribine (CdA) in patient plasma and its intracellular nucleotides CdA 5'-monophosphate (CdAMP) and CdA 5'-triphosphate (CdATP) were delineated in circulating leukemia cells in 17 patients with chronic lymphocytic leukemia, after the last dose intake and up to 72 h thereafter. Patients were treated with 10 mg/m2 CdA p.o. on 3 consecutive days. A novel and specific ion-pair liquid chromatographic method, which separates the intracellular CdA nucleotides, was used. The area under the concentration versus time curve (AUC) of CdAMP in leukemia cells was generally higher (median, 47 micromol/liter x h) than the AUC of CdATP (median, 22 micromol/liter x h); however, in some patients (3 of 17), the reverse relationship was seen. The median ratio between the AUC values for CdATP and CdAMP was 0.60 (95% confidence interval, 0.4-1.0). The median half-life (t(1/2)) of CdAMP was 15 h, and that of CdATP was 10 h. The median terminal t(1/2) of CdA in plasma was 21 h. A significant correlation was found between the maximum plasma CdA and cellular CdAMP concentrations (r = 0.56, P = 0.02). There was no correlation between the AUC values of cellular CdAMP and CdATP (r = 0.224, P = 0.55). No correlation was found between deoxycytidine kinase activity and intracellular pharmacokinetic parameters of CdAMP or CdATP. The response to treatment was not significantly related to intracellular concentration of CdAMP or active metabolite CdATP. There is great heterogeneity among patients in terms of AUC and t(1/2) of CdAMP and CdATP. Furthermore, the results emphasize the differences between the pharmacokinetics of plasma CdA and those of the metabolites in circulating leukemic cells.