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In the present study the effects of the 48-hour administration of granulocyte-macrophage colony-stimulating factor (GM-CSF) (100 U/mL) or interleukin-3 (IL-3) (100 U/mL) on the proliferative activity of leukemic cells and on the intracellular metabolism and cytotoxic efficacy of a subsequent 12-hour application of cytosine arabinoside (ara-C) at doses of 0.1, 1.0, 10.0, and 100.0 mumol/L were evaluated on bone marrow cells from 17 patients with acute myeloid leukemia. After GM-CSF or IL-3, a 1.2- to 2.4-fold increase in S-phase cells was observed in nine of 14 GM-CSF and seven of 11 IL-3 cases. 3H-Cytosine arabinoside incorporation into the DNA was enhanced 1.33- to 18.3-fold over respective controls in 14 of 17 patients. While in control specimens are ara-C dose-dependent increase in 3H-ara-C uptake was accompanied by a corresponding rise in intracellular ara-C-5' triphosphate (ara-CTP) levels, ara-CTP concentrations were not increased after GM-CSF or IL-3 exposure, resulting in a higher ara-C to ara-CTP ratio over controls. This finding may be explained by a stimulatory effect of GM-CSF and IL-3 on ara-C phosphorylating enzymes and a more rapid incorporation of ara-CTP into the DNA of leukemic blasts. These effects translated into a 2.2- to 229.0-fold increase in the cytotoxic activity of ara-C against clonogenic leukemic cells after GM-CSF or IL-3 pretreatment. Hence, GM-CSF and IL-3 enhance the intracellular metabolism of ara-C and its incorporation into the DNA of leukemic cells leading to a higher antileukemic activity of ara-C on clonogenic leukemic cells (CFU-L).

Inhibitors of DNA synthesis, such as 1-beta-D-arabinofuranosylcytosine, have been proven useful in the management of acute myelogenous leukemia. Despite significant improvement of response rates, long-term disease-free survival can be presently achieved only in a small percentage of patients. As S-phase--specific drugs, such as the inhibitors of DNA synthesis, only reach the cycling part of their target populations, dormant cells are left mainly unaffected. Their survival, however, is largely responsible for later disease relapse. This paper reports experiments that show that cell kinetic manipulations by hematopoietic growth factors may help to overcome the problem of cytokinetic and pharmacologic resistance. Hematopoietic growth factors synergize with inhibitors of DNA synthesis given at low doses to induce a more mature phenotype or to enhance leukemia cell kill when combined with high doses of DNA inhibitors. In addition, we present data to unravel several aspects of the molecular mechanism underlying this synergism.

Although the mechanism of action responsible for the effects of low-dose ara-C remains unclear, certain insights are available concerning the interaction of this agent with DNA. Ara-C incorporates into DNA, and the extent of (ara-C)DNA formation correlates significantly with loss of clonogenic survival. The inhibition of DNA replication by ara-C also results in DNA fragmentation and terminal differentiation of leukemic cells. Other studies have demonstrated that inhibition of DNA replication by ara-C results in an aberrant form of DNA synthesis with certain segments of DNA being replicated more than once within a single cell cycle. The additional copies of certain segments of DNA could lead to the accumulation of DNA fragments and alterations in gene expression. It is of interest that other inhibitors of S-phase DNA replication such as aphidicolin and hydroxyurea can also induce similar phenotypic changes in HL-60 and K562 leukemia cells. Although the in vitro data support the concept that ara-C is capable of inducing leukemic cell differentiation, there is no evidence to suggest that this agent induces differentiation of human leukemic cells in vivo. Drug levels achieved by administration of low-dose ara-C (42-64 nmol/L) result in the incorporation of ara-C into bone marrow mononuclear preparations from patients with preleukemia syndromes. This concentration of ara-C (5 X 10(-8) mol/L) slows DNA replication of human leukemic cells in vitro. Thus, the clinical use of low doses of ara-C that achieve plasma concentrations of 10(-8) to 10(-7) mol/L could theoretically induce maturational effects by partially inhibiting DNA synthesis. At the present time we have no available data to support this contention. On the basis of chromosomal analyses, low-dose ara-C apparently maintains sufficient drug levels to suppress more "malignant" clones, but even "clonal" selection may represent elimination of leukemic cells by either cytotoxicity or induction of terminal differentiation. Further studies will be necessary to define the mechanism of action of low-dose ara-C in preleukemia.

Six patients with acute nonlymphoblastic leukemia (ANLL) were treated sequentially with one subcutaneous (SC) injection (50 mg/m2) and one 156 hour continuous intravenous (IV) infusion (100 mg/m2/d) of 1-beta-D-arabinofuranosylcytosine (ara-C) with an interval of 12 hours. Leukemic cells were isolated from venous blood samples. The intracellular concentration of ara-C 5'-triphosphate (ara-CTP) was determined by high-performance liquid chromatography, and in two of the patients the plasma concentration of ara-C was determined by radioimmunoassay. A rapid biphasic elimination of the drug from plasma with an initial half-life of 18 minutes was seen after SC injection. After about 1.5 hours, the concentration was lower than that obtained at steady-state during continuous IV infusion. In the leukemic cells, the concentration of ara-CTP was higher after SC injection than during continuous IV infusion for about five hours. The mean half-life was 2.1 hours. Judging from the intracellular concentration of ara-CTP, considered to be the active metabolite, the results suggest that SC administration of ara-C can mimic continuous IV infusion if the dose interval is reduced from the traditional 12 hours to four to six hours. The results of this study emphasize the differences between the plasma pharmacokinetics of the parent drug and the concentration of the active metabolite in leukemic cells.

The relationship between in vitro leukemic cell growth kinetics, intracellular biochemical pharmacology of 1-beta-D-arabinofuranosylcytosine (ara-C), and clinical response to ara-C-containing timed sequential therapy was examined in leukemic marrow populations from 62 adults with acute myelogenous leukemia (AML). Leukemic blasts from 45 previously untreated and 17 relapse patients were obtained before therapy, and cultured in autologous pretreatment serum (APS) and in serum containing drug-induced humoral stimulatory activity (HSA). While all cell populations cultured in HSA demonstrated increased proliferation, only growth-stimulated cohorts from those patients achieving complete remission with ara-C-based therapy demonstrated enhanced intracellular drug activation and active drug retention relative to cells maintained in APS, whereas cells from nonresponsive patients demonstrated no such HSA-induced increases in intracellular ara-C metabolism. In this in vitro model system, sensitive AML populations behave similarly to normal hematopoietic cohorts, with direct linkage of HSA-perturbed growth and net pharmacologic parameters, while refractory cohorts evince uncoupling of these determinants in the growth-stimulated state. This model identifies behavior patterns that may discriminate clinical sensitivity from clinical resistance and may serve to predict clinical outcome to timed sequential therapy with ara-C.

In vitro uptake and retention of 3H-cytosine arabinoside (ara-C) was studied in 68 acute myelogenous leukemia (AML) patients (ten were studied twice) treated with a regimen containing conventional (54 patients) or high doses (24) of ara-C. Drug uptake and retention after four hours were measured following 30 minutes exposure to 1 and 50 micrograms/mL of ara-C. A good correlation was observed between high uptake in the acid soluble (AS) fraction (P less than .01), or high retention in the acid insoluble (Al) fraction (P less than .03) of 1 microgram, but not 50 micrograms, of the drug and obtainment of complete remission (CR) in patients treated with conventional doses of ara-C. Patients with a high percentage of ara-C retained in the AS fraction (greater than 26%) have a significantly longer CR duration than patients with a lower level of retention (P less than .02). In 17 patients who achieved a CR in spite of a low tritiated ara-C uptake, the clonogenic leukemic (CFU-L) cells were especially sensitive to drugs (15 of 17 v three of 11 among resisting patients).

Pharmacokinetic theory predicts that a large and potentially exploitable concentration difference occurs between the peritoneal cavity and the plasma after many anticancer drugs are administered intraperitoneally in large volume. Unresolved issues remain, particularly concerning the depth of penetration of drugs into tumor nodules growing on peritoneal surfaces. Recent studies in the rat showed a steep concentration gradient of small marker molecules in a variety of normal tissues. The concentration in the stomach and small and large intestines decreased to 10% of the value at the serosal surface in about 0.5 mm or less. Human serum albumin showed deeper penetration, particularly in the diaphragm and anterior abdominal wall.