Variation in sensitivity of DNA synthesis to ara-C in acute myeloid leukaemia

Oral cytarabine ocfosfate pharmacokinetics and assessment of leukocyte biomarkers in normal dogs

BACKGROUND

Cytosine arabinoside (Ara-C) is a nucleoside analog prodrug utilized for immunomodulatory effects mediated by its active metabolite Ara-CTP. Optimal dosing protocols for immunomodulation in dogs have not been defined. Cytarabine ocfosfate (CO) is a lipophilic prodrug of Ara-C that can be administered PO and provides prolonged serum concentrations of Ara-C.

OBJECTIVES

Provide pharmacokinetic data for orally administered CO and determine accumulation and functional consequences of Ara-CTP within peripheral blood leukocytes.

ANIMALS

Three healthy female hound dogs and 1 healthy male Beagle.

METHODS

Prospective study. Dogs received 200 mg/m2 of CO PO q24h for 7 doses. Serum and cerebrospinal fluid (CSF) CO and Ara-C concentrations were measured by liquid chromatography-tandem mass spectroscopy (LC-MS/MS). Complete blood counts, flow cytometry, and leukocyte activation assays were done up to 21 days. Incorporation of Ara-CTP within leukocyte DNA was determined by LC-MS/MS.

RESULTS

Maximum serum concentration (Cmax ) for Ara-C was 456.1-724.0 ng/mL (1.88-2.98 μM) and terminal half-life was 23.3 to 29.4 hours. Cerebrospinal fluid: serum Ara-C ratios ranged from 0.54 to 1.2. Peripheral blood lymphocyte concentrations remained within the reference range, but proliferation rates poststimulation were decreased at 6 days. Incorporation of Ara-CTP was not saturated and remained >25% of peak concentration at 13 days.

CONCLUSIONS AND CLINICAL IMPORTANCE

Oral CO may produce prolonged serum Ara-C half-lives at concentrations sufficient to induce functional changes in peripheral leukocytes and is associated with prolonged retention of DNA-incorporated Ara-CTP. Application of functional and active metabolite assessment is feasible and may provide more relevant data to determine optimal dosing regimens for Ara-C-based treatments.

Intracellular Pharmacokinetics of Pyrimidine Analogues used in Oncology and the Correlation with Drug Action

Pyrimidine analogues can be considered as prodrugs, like their natural counterparts, they have to be activated within the cell. The intracellular activation involves several metabolic steps including sequential phosphorylation to its monophosphate, diphosphate and triphosphate. The intracellularly formed nucleotides are responsible for the pharmacological effects. This review provides a comprehensive overview of the clinical studies that measured the intracellular nucleotide concentrations of pyrimidine analogues in patients with cancer. The objective was to gain more insight into the parallels between the different pyrimidine analogues considering their intracellular pharmacokinetics. For cytarabine and gemcitabine, the intracellular pharmacokinetics have been extensively studied over the years. However, for 5-fluorouracil, capecitabine, azacitidine and decitabine, the intracellular pharmacokinetics was only very minimally investigated. This is probably owing to the fact that there were no suitable bioanalytical assays for a long time. Since the advent of suitable assays, the first exploratory studies indicate that the intracellular 5-fluorouracil, azacitidine and decitabine nucleotide concentrations are very low compared with the intracellular nucleotide concentrations obtained during treatment with cytarabine or gemcitabine. Based on their pharmacology, the intracellular accumulation of nucleotides appears critical to the cytotoxicity of pyrimidine analogues. However, not many clinical studies have actually investigated the relationship between the intracellular nucleotide concentrations in patients with cancer and the anti-tumour effect. Only for cytarabine, a relationship was demonstrated between the intracellular triphosphate concentrations in leukaemic cells and the response rate in patients with AML. Future clinical studies should show, for the other pyrimidine analogues, whether there is a relationship between the intracellular nucleotide concentrations and the clinical outcome of patients. Research that examined the intracellular pharmacokinetics of cytarabine and gemcitabine focused primarily on the saturation aspect of the intracellular triphosphate formation. Attempts to improve the dosing regimen of gemcitabine were aimed at maximising the intracellular gemcitabine triphosphate concentrations. However, this strategy does not make sense, as efficient administration also means that less gemcitabine can be administered before dose-limiting toxicities are achieved. For all pyrimidine analogues, a linear relationship was found between the dose and the plasma concentration. However, no correlation was found between the plasma concentration and the intracellular nucleotide concentration. The concentration-time curves for the intracellular nucleotides showed considerable inter-individual variation. Therefore, the question arises whether pyrimidine analogue therapy should be more individualised. Future research should show which intracellular nucleotide concentrations are worth pursuing and whether dose individualisation is useful to achieve these concentrations.

Retrospective comparison of fludarabine in combination with intermediate-dose cytarabine versus high-dose cytarabine as consolidation therapies for acute myeloid leukemia

This retrospective study compared efficacy and safety of fludarabine combined with intermediate-dose cytarabine (FA regimen) versus high-dose cytarabine (HiDAC regimen) as consolidation therapy in acute myeloid leukemia (AML) patients who achieved complete remission. Disease-free survival (DFS) and overall survival (OS) based on age (≥ 60, <60 years) and cytogenetics were evaluated from data between January 2005 and March 2013. Total 82 patients (FA, n = 45; HiDAC, n = 37; 14-65 years) were evaluated. Five-year DFS was 32.0% and 36.2% for FA and HiDAC groups, respectively (P = 0.729), and 5-year OS was 39.5% and 47.8% (P = 0.568), respectively. Among older patients (≥ 60 years), 3-year DFS was 26.0% for FA group and 12.5% for HiDAC group (P = 0.032), and 3-year OS was 34.6% and 12.5%, respectively (P = 0.026). In FA group, hematological toxicities were significantly lower. FA regimen was as effective as HiDAC regimen in patients with good/intermediate cytogenetics and significantly improved DFS and OS in older patients.

Excretion of cytosine arabinoside in saliva after its administration at high doses

High-dose cytosine arabinoside (cytarabine) is widely used, either alone or in combination with other chemotherapeutic agents, for the treatment of refractory hematological malignancies. Its pharmacology in plasma and cerebrospinal fluid has been extensively examined. In this study, we measured the concentration of cytarabine in saliva of nine patients with hematological malignancies who received high-dose cytarabine. Cytarabine at a dose of 3 g/m was administered i.v. over 2 h. Saliva samples were collected before initiating cytarabine infusion, within 15 min after the completion of infusion and 2 or 4 h after infusion. The concentration of cytarabine was measured by HPLC methods. All nine patients showed a detectable level of cytarabine in saliva within 15 min after the completion of infusion (0.58+/-0.48 microg/ml), which was equivalent to 5% of its plasma concentration; however, the drug was no longer detectable in saliva thereafter. These findings suggest that cytarabine is excreted in saliva during and shortly after its administration at a high dose.

Cyclopentenyl cytosine increases the phosphorylation and incorporation into DNA of 1-beta-D-arabinofuranosyl cytosine in a human T-lymphoblastic cell line

The cytotoxic effect of 1-beta-D-arabinofuranosyl cytosine (araC) depends on the intracellular phosphorylation into its active compound araCTP, on the degree of degradation of araCTP and on the incorporation of araCTP into DNA. Deoxycytidine triphosphate (dCTP) inhibits the phosphorylation of araC (by feedback inhibition of the enzyme deoxycytidine kinase) and the incorporation of araCTP into DNA (by competition for DNA polymerase). In a T-lymphoblastic cell line, we studied whether the cytotoxicity of araC (2 nM-50 microM) could be enhanced by decreasing the concentration of dCTP, using the nucleoside-analogue cyclopentenyl cytosine (CPEC), an inhibitor of the enzyme CTP synthetase. Preincubation of the cells with CPEC (100-1,600 nM) for 2 hr increased the concentrations of araCMP 1.6-9.5-fold, which was significant for each concentration of CPEC used. The concentration of araCDP remained low, whereas the concentration of araCTP changed depending on the concentration of araC used. With 2-15 microm of araC and a preincubation with 400 nM of CPEC, the araCTP concentration increased by 4-15% (not significant), and the total amount of araC nucleotides increased significantly by 21-45%. When using a concentration of araC of 2 nM after a preincubation with CPEC of 100 nM, the concentration of araCMP increased by 60% (p = 0.015), whereas that of araCTP decreased by 10% (p = 0.008). This was compensated by an increase of 41% (p = 0.005) of araCTP incorporation into DNA, which represented 43% of all araC metabolites. Moreover, by performing pulse/chase experiments with 400 nM of CPEC and 2 microM of araC, the retention of cytosolic araCTP and the incorporated amount of araCTP into DNA were increased by CPEC. The modulation by CPEC of araC metabolism was accompanied by a synergistic increase of araC-induced apoptosis and by an additive effect on the araC-induced growth inhibition.

Influence of rhGM-CSF on Ara-C sensitivity of patients with acute myeloid leukemia in relapse: a flow cytometry study

Twenty-three patients with acute myelogenous leukemia (AML) in first relapse were treated with high-dose cytosine-arabinoside (Ara-C) and amsacrine or idarubicin. To prime the cells, the patients were given rhGM-CSF. We studied the influence of 48-h infusion of rhGM-CSF on proliferation and Ara-C sensitivity of leukemic cells both ex vivo and in vitro. We found that a 48-h infusion of rhGM-CSF increased both white blood cell counts and peripheral blood blast cell percentages. Using a Bromodeoxyuridine/DNA (BrdUrd/DNA) staining in flow cytometry, we found an non-constant increase in cells in the S-phase. Ex vivo 48-h culture of leukemic cells with or without rhGM-CSF, with or without other hematopoietic growth factors (HGFs), showed a greater increase of the cells in the S-phase with GF but no correlation with the ex vivo results. We used a method of quantitation of the DNA synthesis previously described (Lacombe F., et al. (1992) Cytometry 13, 730) to monitor the Ara-C sensitivity of the cells in S-phase before and after 48-h infusion with rhGM-CSF. We observed a great variation in the Ara-C sensitivity of the leukemic cells before and after infusion with rhGM-CSF from one patient to another. The BrdUrd/DNA method seems a convenient method to study the influence of HGFs on Ara-C sensitivity of the patients.

The role of systemic high-dose cytarabine in the treatment of central nervous system leukemia. Clinical results in 46 patients

BACKGROUND

Given the good penetration of systemic high-dose cytarabine (HDara-C) into the cerebrospinal fluid (CSF), this approach was used to treat patients with central nervous system (CNS) leukemia, either isolated or with concurrent extraneurologic disease (END).

METHODS

From 1983 to 1991, 46 adults with CNS involvement were treated with systemic HDara-C: 25 had acute lymphoblastic leukemia (ALL), 15 had high-grade non-Hodgkin lymphoma (NHL), 5 had acute myelogenous leukemia (AML), and 1 had lymphoid blast crisis of chronic myelogenous leukemia. Induction consisted of HDara-C 3 g/m2 every 12 hours, by 3-hour infusion, for 8 doses (30 patients), or 6 doses (16 patients), followed by 4 doses at day 21.

RESULTS

Of 46 patients, 29 (63%) achieved complete remission (CR): 15/15 with isolated CNS leukemia, and 14/31 (45%) with CNS and concurrent marrow or lymph node disease. Of 17 patients not meeting CR criteria because of persistent END, 11 showed complete CNS response. The first 10 remitters were consolidated with monthly 4-dose courses of HDara-C. The remaining 19 received postinduction multidrug chemotherapy (including vincristine, doxorubicin, cyclophosphamide, L-asparaginase, etoposide plus intermediate-dose ara-C, mitoxantrone plus HDara-C) and intrathecal methotrexate (MTX) +/- cranial radiation therapy. One patient underwent autologous and one allogeneic bone marrow transplant. Median CR duration was 7 months (range, 2-56+): 8 months for patients with isolated CNS leukemia, and 4 months for those with concurrent END: In only two patients was CNS the primary site of relapse. Three patients with isolated CNS leukemia are disease-free at 23, 40, and 56 months. The main toxicity was myelosuppression. No patient showed dose-limiting neurologic toxicity.

CONCLUSIONS

Systemic HDara-C appears effective therapy for CNS leukemia, maximally in cases with isolated CNS involvement. HDara-C may be combined safely with cranial radiation therapy and intrathecal MTX. This approach for CNS leukemia, however, needs to be combined with additional treatments to eradicate residual disease in extraneurologic compartments.

Quantitation of resistance of leukemic cells to cytosine arabinoside from BrdUrd/DNA bivariate histograms

The cytotoxicity of ara-C derives from an inhibition of DNA synthesis after incorporation of ara-CTP into DNA. The rate of DNA synthesis can be determined from the amount of bromodeoxy-uridine (BrdUrd) incorporated into cells after a short exposure to BrdUrd. We developed a computer program to quantify the inhibition of the rate of DNA synthesis by analysis of the distribution of BrdUrd/DNA. Inhibition was evaluated in ara-C-sensitive and resistant cells after incubation with different doses of ara-C. An index of resistance to ara-C (RI) was expressed as the ratio of the amount of BrdUrd incorporated into S phase cells incubated with ara-C to that incorporated in the absence of ara-C. In the ara-C-sensitive and resistant HL60 cells, a linear relationship between RI and log ara-C concentration was observed. Small numbers of slightly resistant cells in mixtures of ara-C-sensitive and resistant cells could be determined using this method, making it suitable for clinical use to test the resistance of leukemic cells to ara-C.

Pharmacokinetics of ara-C and ara-U in plasma and CSF after high-dose administration of cytosine arabinoside

Cytosine arabinoside (ara-C) and uracil arabinoside (ara-U) levels were measured in the plasma, cerebrospinal fluid (CSF), and urine of 10 patients exhibiting primary central nervous system lymphoma who received 31 infusions of high-dose ara-C (3 g/m2) as part of their treatment regimen. Peak plasma and CSF ara-C levels were 10.8 and 1.5 micrograms/ml, respectively. Ara-C was cleared more rapidly from plasma than from CSF. Ara-U appeared rapidly in both plasma and CSF, reaching a peak that was 10 times higher than the corresponding ara-C concentration (104 and 11.2 micrograms/ml, respectively). Only 4%-6% of the dose was excreted unchanged in the urine, but 63%-73% of it appeared as ara-U within the first 24 h. The presence of leptomeningeal lymphoma did not affect the CSF level of ara-C or ara-U.

Saturation of 2',2'-difluorodeoxycytidine 5'-triphosphate accumulation by mononuclear cells during a phase I trial of gemcitabine

The plasma and cellular pharmacology of 2',2'-difluorodeoxycytidine (dFdC, Gemcitabine) was studied during a phase I trial. The steady-state concentration of dFdC in plasma was directly proportional to the dFdC dose, which ranged between 53 and 1,000 mg/m2 per 30 min. The cellular pharmacokinetics of an active metabolite, dFdC 5'-triphosphate (dFdCTP), were determined in mononuclear cells of 22 patients by anion-exchange high-pressure liquid chromatography. The rate of dFdCTP accumulation and the peak cellular concentration were highest at a dose rate of 350 mg/m2 per 30 min, during which steady-state dFdC levels of 15-20 microM were achieved in plasma. A comparison of patients infused with 800 mg/m2 over 60 min with those receiving the same dose over 30 min demonstrated that the dFdC steady-state concentrations were proportional to the dose rate, but that cellular dFdCTP accumulation rates were similar at each dose rate. At the lower dose rate, the AUC for dFdCTP accumulation was 4-fold that observed at the higher dose rate. Consistent with these observations, the accumulation of dFdCTP by mononuclear cells incubated in vitro was maximal at 10-15 microM dFdC. These studies suggest that the ability of mononuclear cells to use dFdC for triphosphate formation is saturable. In the design of future protocols, a dose rate should be considered that produces maximal nucleotide analogue formation, with increased intensity being achieved by prolonging the duration of infusion.

Intermediate-dose cytosine arabinoside and amsacrine. An effective regimen with low toxicity in refractory acute nonlymphocytic leukemia

Results of remission induction therapy in refractory acute nonlymphocytic leukemia (ANLL) has been improved since the introduction of high-dose cytosine arabinoside. However, the toxicity of these regimens attributes to an early death rate of about 20% to 30%. The authors treated 37 poor-risk patients with ANLL with intermediate-dose cytosine arabinoside and amsacrine for remission induction. One consolidation course and no maintenance therapy was given. Eleven of 19 patients with a first relapse entered complete remission (58%); ten of 15 patients in this group older than 50 years were complete responders (67%). Median duration of second remission was 8.2 months (range, 2-14). Three of 13 patients with primarily resistant disease had a complete remission (23%), but there was no response in five patients with a myeloid blastic phase of chronic myelogenous leukemia. Side effects of this remission induction regimen were mild; no cardiac, pulmonary, or central nervous system toxicity was observed. Five patients (14%) died during the remission induction phase, three from complications during aplasia and two from refractory leukemia.

Clinical pharmacology of N4-palmitoyl-1-beta-D-arabinofuranosylcytosine in patients with hematologic malignancies

The pharmacokinetics of oral N4-palmitoyl-1-beta-D-arabinofuranosylcytosine (PLAC), a lipophilic and deaminase-resistant derivative of 1-beta-D-arabinofuranosylcytosine (ara-C), were determined in patients with hematologic malignancies. The concentration of ara-C and 1-beta-D-arabinofuranosyluracil (ara-U), metabolites of PLAC, were measured by radioimmunoassay and gas chromatography-mass spectrometry-mass fragmentography, respectively. The concentration of PLAC was determined by measuring ara-C, which was derived from PLAC by hydrolyzation. In six patients given an oral bolus of PLAC (300 mg/m2), the plasma-disappearance curve of PLAC corresponded to a one-compartment open model, including first-order absorption. The peak plasma level was 22.9 +/- 6.4 ng/ml, and the predicted time to reach the peak level was 2.5 +/- 1.0 h. The elimination half-life was 3.8 +/- 2.7 h. The plasma ara-C level increased slowly to 6.9 ng/ml during the 1st 2-3 h after administration and remained over 1.0 ng/ml for 12 h. Plasma ara-U was detectable for at least 24 h, with a peak concentration of 376 ng/ml at 6 h. Urinary PLAC excretion was below the limit of detection (5 ng/ml) in all cases. Prolonged urinary ara-C and ara-U excretion was detected, but the total recovery rate was low (6.7% in 24 h) and varied between patients. In spite of the lipophilic nature of the drug, the PLAC concentration in the cerebrospinal fluid, measured at 3 or 6 h, was below the limit of detection in all four patients with no meningeal involvement. This study showed low but persistent levels of PLAC in plasma and tissues, with a continuous release of small amounts of ara-C, which demonstrated antitumor activity in patients with hematologic malignancies.

Cell kinetic effect of low dose arabinosyl cytosine

Low dose arabinosyl cytosine (ARA-C) is effective for treatment of acute non-lymphocytic leukaemia (ANLL). The mechanism of action is not clearly understood and it was suggested that low doses of the drug could induce leukaemic cells to differentiate. We investigated the effect of low dose ARA-C (20 mg/m2/d, divided into two doses s.c. at 12 h intervals, x 20 d) on the cell cycle distribution of leukaemic cells in four cases of ANLL. By comparison, four other cases of ANLL were studied during treatment with standard dose ARA-C (200 mg/m2/d as a continuous i.v. infusion x 7 d). Both treatments induced an accumulation of leukaemic cells in post G1 phases, at a variable extent and rate. During treatment by low dose ARA-C, the mitotic index (MI) fell slowly to zero in two patients who achieved a complete remission (CR), while it fell but recovered during treatment in the patients who did not achieve a CR. The MI fell rapidly to zero in the four cases treated by standard dose, who achieved a CR. These data are consistent with the known cytotoxic activity of ARA-C, via inhibition and slowing of DNA synthesis leading to defective cell proliferation and to cell death.

In vitro exposure of leukemic cells to low concentration Arabinosyl Cytosine: no evidence of differentiation inducing activity

Low dose Arabinosyl Cytosine (LD ARA-C) is widely used for treatment of acute non-lymphocytic leukemia (ANLL) and myelodysplastic syndrome (MDS) in the elderly, based on a favorable response rate and on the hypothesis that LD ARA-C can induce differentiation or maturation of leukemic cells. We investigated the effect of low concentration of ARA-C on the growth of marrow cells that were obtained from 6 cases of MDS and from 11 cases of ANLL by using the in vitro culture system for normal granulo-monocyte precursors (CFU-GM). At ARA-C concentrations equal to or higher than 1 ng/ml cell growth was inhibited in a dose-dependent manner. At ARA-C concentration of 0.1 ng/ml cell growth was slightly affected, but colony number and colony cell composition were identical to control cultures. This experiment did not support the hypothesis that ARA-C can induce leukemic cells to recover any normal growth patterns but confirmed that even very low ARA-C concentrations can inhibit or slow down leukemic cell proliferation.

Preferential inhibition by cytarabine of CFU-GM from patients with chronic granulocytic leukemia

Granulocyte-macrophage colony-forming (CFU-GM) cells from peripheral blood of normal subjects and patients with chronic granulocytic leukemia (CGL) were cultured in soft agar. Drugs under study were added in a liquid overlay 2 days after initiation of cultures, providing prolonged exposure to these agents thereafter. Dose-dependent inhibition of colony growth was recorded with each of eleven agents examined, and at the higher concentrations tested, colony formation was often completely suppressed. Cytarabine showed selectivity against CFU-GM from patients in the chronic stage of CGL (P = 0.006); the median 50% inhibitory concentration for 12 such patients was 3.4 ng/ml versus 11.8 ng/ml for 15 healthy subjects. Such selectivity was not found with busulfan, hydroxyurea, mercaptopurine, thioguanine, daunorubicin, vincristine, vinblastine, methotrexate, desacetylmethylcolchicine, and trimethylcolchicinic acid. One other group has also reported a preferential effect of cytarabine against colony-forming cells from patients with CGL, and this appears to be the only drug for which such selective activity has been recorded to date.

In-vitro studies on phosphorylation and dephosphorylation of cytosine arabinoside in human leukemic cells

The cytotoxic effect of cytosine arabinoside (ara-C) depends on the capacity of cells to form and retain intracellularly the phosphorylated metabolite cytosine arabinoside triphosphate (ara-CTP). In this study accumulation and cellular retention of ara-CTP have been measured in vitro in the bone marrow cells of 69 patients with acute leukemia. Cells were incubated with 3H-ara-C and the amount of ara-CTP formed was determined after separation of the nucleotides by thin-layer chromatography. Phosphorylation of ara-C to ara-CTP appeared to be a saturable process. The Km-equivalents varied between 1.1 and 16.2 microM ara-C. Maximal ara-CTP formation ranged from 12 to 125 pmol ara-CTP/10(6) cells in 30 min. The phosphorylation activity did not correlate with the percentage of S-phase cells. The intracellular half-life time of ara-CTP measured in vitro ranged from 53 to 210 min. Phosphorylation of ara-C was comparable in patients with acute myeloid leukemia (n = 51) and in patients with acute lymphoblastic leukemia (n = 18). Ara-CTP elimination appeared slower in lymphoblasts than in myeloblasts. The average intracellular ara-CTP level in relapsed patients (n = 34) appeared higher than in previously untreated patients (n = 52). The less favourable outcome of second remission induction therapy with conventional doses of ara-C compared to the first remission induction treatment is not explained by an alteration in the intracellular metabolism of ara-C.

Low-dose Ara-C can cause complete remission of acute non-lymphocytic leukemia: differentiation induction?

Fourteen patients with acute nonlymphocytic leukemia were treated with low-dose arabinosylcytosine (LDAC). Thirteen patients received subcutaneous injections at a dose of 10 mg/M2 every 12 h. One patient received 25 mg intramuscularly daily. All cases received one to three courses with each course lasting 10-60 days (median 19). Complete remission was achieved in 6 (or 43%) of the patients. Three patients had only cytoreduction and 5 patients did not respond. During the therapy severe thrombocytopenia occurred in all patients while prominent other cytopenias occurred in 10. Two-thirds of the patients achieving a remission had significant myelosuppression. There was one treatment-related death. During therapy 11 patients demonstrated a decrease in leukemia cells with an associated increase in differentiated granulocytes. This included 3 of the 4 complete remitters, and 3 of the 5 nonresponders. These results seem to suggest that the therapeutic effect of low-dose Ara-C may result from a combination of differentiation induction, cytotoxicity and unusual sensitivity of the leukemic cells to this agent.

Does the metabolite uracil arabinoside inhibit cytosine arabinoside (Ara-C) penetration into the cerebrospinal fluid during high-dose Ara-C therapy?

A patient with acute leukaemia was treated with i.v. 2-h infusions of Ara-C at a dose of 3.0 g/m2 every 12 h. During 6 d of therapy the concentrations of the metabolite Ara-U in the CSF reached rather high levels of between 60 and 70 mumol/l from d 2-6 due to high levels of Ara-U in the plasma. The concentration of Ara-C in the CSF after the first infusion was 10.8 mumol/l. After repetitive doses on d 2-6 the drug concentrations increased from about 3 mumol/l just before infusion to about 8 mumol/l at the end of infusion, indicating inhibition of Ara-C influx into the CSF during prolonged treatment. We suggest that the high levels of Ara-U in the plasma interfere with Ara-C transport across the blood-brain barrier.

Low-dose cytosine arabinoside regimen induced a complete remission with normal karyotypes in a case with hypoplastic acute myeloid leukaemia with No. 8-trisomy: in vitro and in vivo evidence for normal haematopoietic recovery

Complete remission was achieved in a case of hypoplastic acute myeloid leukaemia with chromosomal aberration of No. 8-trisomy by giving very low dose of cytosine arabinoside (ara-C), 10 mg (7 X 5 mg/m2)/d, by 24 h continuous intravenous infusion for 20 d. We observed a definite cytoreduction phase in the bone marrow (BM) before normal haematopoiesis resumed. The remission BM showed only normal karyotypes in all metaphases examined. Granulocyte/macrophage colonies and erythroid bursts recovered sufficiently in numbers and cytogenetic study on single colonies and bursts revealed only normal karyotypes. These observations provide evidence that a low dose ara-C regimen can induce remission by cytoreduction which diverts the growth advantage from the leukaemic clone to the normal clones.

Low-dose cytosine arabinoside treatment for acute nonlymphocytic leukemia in elderly patients

Thirty patients older than 65 years of age with acute nonlymphocytic leukemia were treated with low-dose cytosine arabinoside (10 mg/m2 subcutaneously every 12 hours for 15 to 21 days). Fifteen achieved complete remission and five had partial remission. Treatment was more effective when initial bone marrow cellularity was low (P = 0.007). Four of six patients with secondary leukemia entered complete or partial remission. Therapy was well tolerated with reduced myelosuppression and few number of early deaths. Sixteen patients followed the whole treatment as outpatients. Six of 12 patients who achieved complete remission showed no evidence of post-therapeutic bone marrow aplasia. These data are consistent with the view that low-dose cytosine arabinoside acts on leukemic cells as a differentiating agent.

Low-dose ARA-C fails to enhance differentiation of leukaemic cells

Nine patients with acute myelogenous leukaemia were treated with low-dose ARA-C (10 mg/m2, q 12h) for a planned 21 d. Complete remission was attained in only one patient (11.1%). Definite cytoreductive effect was seen in four additional patients. There was one treatment-related death. Haematologic toxicity occurred in all nine patients with sever thrombocytopenia most prominent. Severe hepatotoxicity precluded further ARA-C treatment in one patient. Because of toxicity only two patients were able to complete their scheduled 3 week courses of low-dose ARA-C. No evidence of ARA-C induced differentiation of leukaemic cells was noted on follow-up bone marrow examination during or shortly after the treatment course. The utility and indication for low-dose ARA-C therapy of AML remains to be determined.

The effect of low dose Ara-C in acute nonlymphoblastic leukaemias and atypical leukaemia

Nine patients with nonlymphocytic leukaemia and one patient with refractory anaemia with excess of blasts (RAEB) were treated subcutaneously with Ara-C at a low dose of 10 mg/m2/12 h; complete remission was obtained in seven patients. In all cases severe pancytopenia was observed during treatment. We measured the concentration of Ara-C in serum, and found that the maximum concentration reached a peak (52-132 ng/ml; mean 84.2 ng/ml) at 15 min following injection. These concentrations can be considered sufficient to inhibit DNA synthesis. Primary short-term culture of human leukaemic cells with low dose Ara-C was also performed, and differentiation of leukaemic cells was observed for several types of leukaemic cells. The in vitro findings corresponded to the observed clinical effects. From the above results, the action of low dose Ara-C may have resulted from a combination of two different mechanisms, its cytostatic effect and its differentiation-inducing effect.

The use of intermediate dose cytosine arabinoside (ID Ara-C) in the treatment of acute non-lymphocytic leukaemia in relapse

Cytosine arabinoside in a high dose of 3 g/m2 (HD Ara-C), alone or in combination with doxorubicin, has been advocated for the treatment of patients with acute non-lymphocytic leukaemia (ANLL) in relapse. Although a remission rate of 65% has been reported, the toxicity was severe and was possibly related to the high plasma concentrations of Ara-C (about 100 microM) reached during 1 h infusion. It has been postulated, however, that the intracellular enzymes which convert Ara-C into the active metabolite cytosine arabinoside triphosphate (Ara-CTP), are saturated at plasma concentrations of about 10 microM. We calculated that this level could be reached with an intermediate dose of 0.5 g/m2 Ara-C, given in a 1 h infusion (ID Ara-C). Subsequently 15 patients with ANLL (12 in relapse and three refractory to conventional therapy) were treated with ID Ara-C every 12 h for 6 d in combination with doxorubicin and vincristine. The overall remission rate was 80%. The median duration of bone marrow depression was 20 d (range 14-29 d) and side effects were comparable to conventional treatment. These preliminary data suggest that the therapeutic results of this ID Ara-C regimen are not inferior to comparable schedules with HD Ara-C as reported by others while toxicity is less severe.

Clinical trial of low-dose Ara-C in the treatment of acute leukemia and myelodysplasia

In a multicenter analysis, the effect of low-dose cytosine arabinoside (Ara-C)(10 mg/ m2q 12 h subcutaneously for a minimum of 15 days) has been assessed in 13 patients with acute leukemia (10 myeloid-AML-, 3 lymphocytic-ALL-) and 7 patients with dysmyelopoietic syndromes (DMPS), conditions classified as refractory anemia with an excess of blasts ( RAEB ). Seven patients suffering from acute leukemia and 1 with DMPS in blastic transformation displayed a leukocytosis of more than 10 X 10(9)/1. Three out of 7 DMPS, 1 out of 10 AML achieved a complete remission, 1 out of 3 ALL-patients reached a partial remission twice. Seven patients showed a blast clearing in the bone marrow and peripheral blood, in another 7 instances examination of the bone marrow was not performed after therapy because of early death. The majority of patients were in their late phase of disease and refractory to conventional chemotherapy. Only 5 patients had no pretreatment at first presentation before low-dose Ara-C was initiated. At least for the DMPS-group, this therapeutic approach seems to be of some benefit.

The relationship of Ara-C metabolism in vitro to therapeutic response in acute myeloid leukaemia

Ara-C phosphorylation and Ara-C deamination was measured in vitro, using intact marrow myeloblasts from 25 patients with previously untreated acute myeloid leukaemia. At Ara-C concentrations above 10 microM there was no longer a linear relationship of phosphorylation to Ara-C concentration. Ara-U production was measured by sampling the incubation medium. This method showed greater Ara-U production than previous methods sampling the cell pellet alone. However, Ara-CTP/Ara-U ratios from intact myeloblasts were much higher than those recorded in studies using lysed myeloblasts. Using 1 microM Ara-C, a concentration representative of in vivo concentrations, deamination and phosphorylation were related to therapeutic response to Ara-C-containing drug regimens. There was no significant correlation of these variables with response, although 5/16 non-responders had low Ara-C phosphorylation (less than 1.5 pmol/10(6) cells/45 min/l pm Ara-C) compared with 0/9 responders. Measuring deaminase activity did not help in selecting non-responders. Even in patients with low phosphorylation increasing Ara-C concentration increased Ara-CTP levels proportionally, but up to 10 times conventional doses may be necessary to exceed endogenous dCTP levels.

Small doses of ARA-C in the treatment of acute myeloid leukaemia: differentiation of myeloid leukaemia cells?

Three patients with acute myeloid leukaemia were treated with small doses of ARA-C (10 mg/m2/12 h, subcutaneous injections) and complete remission was obtained. The small doses of ARA-C, the progressive evolution, the absence of aplasia before remission, the simultaneous presence of normal islets of promyelocytes and leukaemic myeloblasts, favour a differentiating role for the drug rather than an antimitotic effect.

Variations of the phosphorylation of 1-beta-D-arabinofuranosylcytosine (ARA-C) in human myeloid leukemic cells related to the cell cycle

Bone marrow cells of five patients with acute myeloid leukemia were fractionated by means of counterflow centrifugation (elutriation). The different fractions were enriched with cells belonging to subsequent stages of the cell cycle. Cytokinetic evaluation of these cell fractions was performed by [3H]thymidine autoradiography, [3H]thymidine incorporation and DNA/RNA-flow cytometry. Phosphorylation of cytosine arabinoside (ara-C, 1-beta-D-arabinofuranosylcytosine) in the different fractions was measured by incubation of the cells for 30 min with 1.07 microM [3H]ara-C. Phosphorylation of ara-C in the whole bone marrow samples ranged from 5.9 to 33.2 pmol/10(6) cells. In the fractions containing only G1-phase cells, phosphorylation ranged from 1.2 to 19.5 pmol/10(6) cells. The phosphorylation seems to increase before DNA synthesis starts. Maximal activities were found in the fractions enriched with cells in late G1- or S-phase of the cell cycle. In these fractions the ara-C phosphorylating activity was 1.5-8 times higher compared to the fractions with the lowest activity. One may therefore assume that not only S-phase cells are killed by ara-C, but that G1-phase cells which can phosphorylate ara-C, may also be doomed when they enter S-phase, since the elimination of the intracellular cytosine arabinoside tri-phosphate (ara-CTP) is a relatively slow process. The fraction of G1-phase cells phosphorylating ara-C, may be an important determinant in the extent of the cell-killing effect of ara-C treatment in the different leukemias.

Determination of 1-beta-D-arabinofuranosylcytosine and 1-beta-D-arabinofuranosyluracil in human plasma by high-performance liquid chromatography

A method is described for the determination of 1-beta-D-arabinofuranosylcytosine (Ara-C) and its metabolite 1-beta-D-arabinofuranosyluracil (Ara-U) in human plasma. After deproteinization of the plasma sample, separation is performed by reversed-phase liquid chromatography. For Ara-C concentrations exceeding 0.05 mg/l and for Ara-U concentrations exceeding 1 mg/l, injection volumes of 100 microliter are applied. For lower concentrations an injection volume of 500 microliter is used. Ara-C is detected at 280 nm with a lowest detection limit of 0.002 mg/l in plasma. Ara-U is detected at 264 nm with a lowest detection limit varying from 0.01 to 0.1 mg/l in plasma. This variation is caused by an unknown substance with the same elution properties as Ara-U and which appears to be present in plasma in variable concentrations. The coefficient of variation of the whole procedure is about 6% for Ara-C concentrations above 0.005 mg/l and for Ara-U concentrations above 0.1 mg/l. For lower concentrations the coefficient of variation is about 14%.