title: On the interaction between cytosine arabinoside and etoposide in vivo and in vitro.
abstract: Cytosine arabinoside (ara-C) and etoposide are often used in combination in the treatment of acute myelocytic leukemia (AML). The intracellular phosphorylation of ara-C to its 5'-triphosphate (ara-CTP) is a prerequisite for its cytotoxic effects. It has been shown in vitro that etoposide can impair the formation of ara-CTP in leukemia cells. The present study was undertaken in order to elucidate whether this interaction may be of clinical importance. Leukemia cells were isolated from 3 patients with acute myelocytic leukemia and incubated in medium (RPMI-1640) with or without 10% fetal calf serum or in human plasma. When the cells were incubated in RPMI-1640 with ara-C (10 mumol/l) and etoposide during 2 h, the formation of ara-CTP was decreased to 71 +/- 18 (mean +/- S.D.) and 30 +/- 15% of control at 1 and 10 micrograms/ml etoposide, respectively. When the cells were incubated in human plasma, the formation of ara-CTP was not influenced by the presence of etoposide (101 +/- 6 and 103 +/- 20% at 1 and 10 micrograms/ml etoposide). When incubated in RPMI supplemented with 10% fetal calf serum, the corresponding figures were 81 +/- 8 and 70 +/- 20%. Six patients with AML were therefore treated with ara-C 0.5 or 1.0 g/m2 as a 2-h infusion every 12 h and, during 1 h before the second ara-C infusion, 100 or 200 mg/m2 etoposide was administered. The median change in the AUC of cellular ara-CTP between the first and second ara-C dose was 0% (-37 to +21%). The corresponding median change in rate of accumulation of ara-CTP in leukemia cells was 12% (-26 to +110%). The concentration of etoposide in plasma during the ara-C infusion was 18.7 +/- 5.1 micrograms/ml while the non-protein bound etoposide was 0.73 +/- 0.34 micrograms/ml. Thus, despite exposure to higher etoposide concentrations in vivo than in vitro, no impairment of ara-CTP formation was seen in the patients. This corresponds to the results obtained when leukemic cells were incubated in plasma. It is concluded that the inhibition of ara-CTP formation by etoposide seen in vitro is offset by the high protein binding of etoposide in plasma (96%) and that etoposide does not impair the formation of ara-CTP in leukemia cells in vivo during treatment with standard-dose etoposide.
authors.affiliation: Department of Medicine, Karolinska Institute, Huddinge hospital, Sweden.
authors.forename: J
authors.initials: J
authors.lastname: Liliemark
authors.forename: E
authors.initials: E
authors.lastname: Knochenhauer
authors.forename: A
authors.initials: A
authors.lastname: Gruber
authors.forename: B
authors.initials: B
authors.lastname: Pettersson
authors.forename: M
authors.initials: M
authors.lastname: Björkholm
authors.forename: C
authors.initials: C
authors.lastname: Peterson
mesh_terms: D000971:Antineoplastic Combined Chemotherapy Protocols / Q000627:therapeutic use; D001085:Arabinofuranosylcytosine Triphosphate / Q000378:metabolism; D003561:Cytarabine / Q000008:administration & dosage / Q000494:pharmacology* / Q000627:therapeutic use; D004347:Drug Interactions; D005047:Etoposide / Q000008:administration & dosage / Q000494:pharmacology* / Q000627:therapeutic use; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D015470:Leukemia, Myeloid, Acute / Q000188:drug therapy*; D010766:Phosphorylation
chemical_list: D003561:Cytarabine; D001085:Arabinofuranosylcytosine Triphosphate; D005047:Etoposide