Inhibition of DNA synthesis by cytosine arabinoside: Relation to response of acute non-lymphocytic leukemia to remission induction therapy and to stage of the disease

A Short-Term Chemosensitivity Test with Different Labeled Precursors of DNA or Protein Synthesis: Correlation with Clinical Response

Blood or bone marrow samples from 15 patients with newly diagnosed acute myeloblastic leukemia undergoing remission induction treatment with daunorubicin, cytosine-arabinoside and 6-thioguanine were tested in vitro. Leukemic cells were incubated for 24 h at 37 °C with or without the drugs alone or in combination. A 3-h pulse with labelled precursors of DNA synthesis (3H-thymidine) or protein synthesis (3H-leucine) was then given separately. In vitro growth, expressed as the percentage ratio between labeled precursor uptake in treated cells and in control cells, was compared with the clinical results obtained. Three patients were not considered evaluable (death occurred too early), 8 had a complete response (CR), and 4 were disease resistant to chemotherapy. Leukemic cells of resistant-disease patients showed a significantly lower growth inhibition than cells taken from CR patients, with each drug alone or in combination, when measured with thymidine. Inhibition of leucine uptake was not related to the clinical outcome.

Effects of fludarabine and gemcitabine on human acute myeloid leukemia cell line HL 60: direct comparison of cytotoxicity and cellular Ara-C uptake enhancement

This study was designed to compare the effects of fludarabine and gemcitabine on cytosine arabinoside (Ara-C) uptake and retention, and their specific cytotoxicity on HL 60 human acute myeloid leukemia cells. The leukemic blasts were exposed to either drug at equimolar concentrations (10 microM) for 3 h and further incubated with Ara-C (5 microM), added immediately (day 0) or after an interval of 24 h in cells were kept in a drug free medium (day 1). On day 0, leukemic cells exposed to fludarabine 10 microM had a significant (P<0.01) increase in Ara-C uptake (297 +/- 11 pmol/10(7) cells) with respect to control cells (not pre-treated: 195 +/- 10 pmol/10 (7) cells). After treatment of leukemic cells with fludarabine, cytoplasmic Ara-C peaked after 180 min of exposure, as well as nuclear bound Ara-C. At the same time, a significant decrease in the number of S-phase leukemic cells, consistent with depressed [3 H]TdR uptake was observed. Although on day 0 gemcitabine 10 microM did not have potentiating effects on Ara-C uptake, it showed a high degree of intrinsic cytotoxicity as a single agent(clear from cell cycle distribution, [3H]TdR uptake, plating efficiency (PE) data and percentage of apoptotic cells). Cells exposed to gemcitabine, on the other hand, showed on day 1 a significant increase in Ara-C uptake (2.4 x control values in the cytoplasmic and 3x in the nuclear fractions) and a reduced number of S-phase blasts, [3H]TdR uptake and PEs, as well as an increased apoptotic cell death. Evidently, it is possible to modulate Ara-C uptake by leukemic cells with gemcitabine. Although this effect is similar to that demonstrated with fludarabine, its kinetics and time of efficacy are different and also, because of its intrinsic higher cytoxicity and lack of important side effects, gemcitabine could be considered a suitable candidate for Ara-C association therapy in acute leukemia.

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.

Effects of rGM-CSF on leukemia cell proliferation and on the incorporation of cytosine arabinoside into DNA

In vitro studies of the effects of recombinant granulocyte macrophage-colony stimulating factor (rGM-CSF) on freshly obtained human leukemia cells were conducted to determine if there is a relationship between the effects of this growth factor on the proliferative characteristics of leukemia cells and on their incorporation of cytosine arabinoside (araC) into DNA. While rGM-CSF was found to be able to stimulate both leukemia cell proliferation and araC incorporation, for individual leukemia specimens there was no consistent relationship among these effects. In some specimens proliferation was stimulated without an increase in araC incorporation. The reverse was also observed. These studies demonstrate the difficulty in identifying assays capable of predicting the clinical effects of growth factors on leukemia cells in patients since the effect in vitro vary with the assay.

Abrogation of the prognostic significance of low leukemic cell retention of cytosine arabinoside triphosphate by intensification of therapy and by alteration in the dose and schedule of administration of cytosine arabinoside

The ability of leukemic cells to phosphorylate cytosine arabinoside (araC) and retain the triphosphate form of the drug (araCTP) is strongly predictive of remission duration for patients with acute nonlymphocytic leukemia who are treated with araC-based maintenance therapy. An increase in the intensity of therapy improves the overall median duration of remission, the increased intensity of therapy being especially beneficial for patients whose leukemic cells do not retain araCTP. This alteration in therapy reduces the prognostic significance of leukemic cell araCTP retention. Further, it seems that the use of high-dose araC as intensive consolidation therapy and the administration of conventional-dose araC by continuous infusion make it possible to further reduce or even abrogate the adverse prognostic significance of low leukemic cell retention of araCTP.

Predictors of response of acute nonlymphocytic leukemia

Given the time and effort expended by investigators and given the large numbers of patients studied, there are disappointingly few "predictors" of response. From the clinical perspective, aside from performance status, age, prior history of toxic exposure, and grossly abnormal organ function, there are essentially no reliable indicators of the likelihood of a patient surviving remission induction therapy. The absence of such indicators might reflect the fact that without grossly abnormal organ function, all patients generally begin with an equal possibility of survival and that events which occur during therapy determine survival. One thing is certain, death directly attributable to leukemic cell overgrowth despite chemotherapy is an extremely rare event. Hence, therapeutic inadequacy in this sense, at the time of initial diagnosis, is not a common cause of treatment failure. Studies aimed at the prediction of leukemic cell responsiveness to therapy have been plagued by two general problems. The first is that the drug sensitivity assays have been quite primitive. For example, the ability of a cell to take up a drug is not synonymous with sensitivity to that drug. Additionally, tests which are dependent upon assays capable of making measurements in only a small subpopulation of cells, such as in vitro clonogenic assays, are likely to have only limited applicability. On the other hand, assays which measure the properties of the leukemic cell population as a whole are incapable of recognizing arabinoside therapy have provided data which suggest that in addition to patient survival three conditions must be satisfied if a complete remission is to occur: the pretherapy leukemic cell mass must be moderate or low, an adequate number of cells must be synthesizing DNA, and cytosine arabinoside must produce significant inhibition of DNA synthesis in vitro. Each factor is consistent with what is known about cytosine arabinoside: it is an S-phase-specific agent which must be incorporated into DNA in order to kill leukemic cells. When the relationship between these same factors and response to combination chemotherapy were studied, not unexpectedly, no relationship was discerned. Finally, in this setting pharmacokinetic studies have demonstrated that the amount of araCTP formed in leukemic cells in vivo when doses of 2 g and 3 g/m2 are administered are indistinguishable, thereby explaining the clinical equivalence of these two dosage levels.(ABSTRACT TRUNCATED AT 400 WORDS)