aphidicolin and Preleukemia

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.

Topics: Adult; Aged; Aphidicolin; Bone Marrow; Cell Differentiation; Cell Division; Cell Line; Cytarabine; Diterpenes; DNA; DNA Replication; Dose-Response Relationship, Drug; Female; Hemoglobins; Humans; Karyotyping; Kinetics; Leukemia; Male; Middle Aged; Preleukemia

Numerous agents induce differentiation and maturation of neoplastic and dysplastic myeloid cells in vitro and some of these agents have been used with limited success in the treatment of patients with myelodysplastic syndromes (MDS) and myeloid leukaemias. We recently proposed that physiological and pharmacological agents which enhance differentiation and maturation in vitro act by two fundamentally different routes: (1) by hastening the progression through various differentiation/maturation steps; (2) by slowing proliferation (usually by inhibition of DNA synthesis). In order to test this thesis we looked for synergistic effects on differentiation using pairs of agents from the two groups in cultures of cells from myelodysplastic and acute myeloid leukaemia (AML) patients and from normal marrow donors. The results with three MDS, two AML and three normal samples show that combinations of differentiation inducing agents (retinoic acid, N-methylformamide) with DNA synthesis inhibitors (6-mercaptopurine, cytosine arabinoside and aphidicolin) produce a differentiation inducing effect equivalent to that of 10-100, or even 1000 fold higher concentrations of single agents. Myelotoxic effects in vitro were not synergistic. The use of these synergistic combinations should greatly enhance the usefulness of differentiation inducers in the therapy of MDS and myeloid leukaemia.

Topics: Antineoplastic Combined Chemotherapy Protocols; Aphidicolin; Bone Marrow; Bone Marrow Diseases; Cell Differentiation; Cells, Cultured; Cytarabine; Diterpenes; DNA; Drug Synergism; Formamides; Humans; Leukemia, Myeloid, Acute; Mercaptopurine; Preleukemia; Syndrome; Tretinoin