Deoxyribonucleoside triphosphate pools and Ara-CTP levels in P388 murine leukemic cells treated with 1-B-D-arabinofuranosylcytosine-5'-stearylphosphate which is a newly synthesized derivative of 1-B-D-arabinofuranosylcytosine

Clinical pharmacokinetics of cytarabine formulations

Cytarabine (cytosine arabinoside, Ara-C) is an effective chemotherapeutic agent for the treatment of acute myelogenous leukaemia and lymphocytic leukaemias. As cytarabine is an S-phase-specific drug, prolonged exposure of cells to cytotoxic concentrations is critical to achieve maximum cytotoxic activity. However, the activity of cytarabine is decreased by its rapid deamination to the biologically inactive metabolite uracil arabinoside. This rapid deamination is the reason for the ongoing search for effective formulations and derivatives of cytarabine that cannot be deaminated and exhibit better pharmacokinetic parameters. Protection of cytarabine from fast degradation and elimination has been investigated by encapsulating the drug into pharmaceutically acceptable carriers. Cytarabine derivatives have shown promise in vitro and in animal models. For example, ancitabine (cyclocytidine), enocitabine and cytarabine ocfosfate have been used clinically in Japan. Cytarabine encapsulated into multivesicular liposomes has been approved in several countries for the intrathecal treatment of lymphomatous meningitis. Although many compounds have been investigated, few cytarabine derivatives are currently available for clinical use. Further research is needed to improve the efficacy of cytarabine against haematological and solid tumours.

Combined oral administration of etoposide and arabinofuranosylcytosine-5'-stearylphosphate enhances the antitumor effect against P388 ascites tumors

We investigated the antitumor effect of oral administration of etoposide and arabinofuranosylcytosine-5'-stearylphosphate (C18PCA) against P388 ascites tumors in B6D2F1 mice. Etoposide (25 mg/kg) and C18PCA (5 mg/kg) were given orally on days 1-5 after tumor inoculation. The median life span of the mice treated with etoposide or C18PCA alone was 19.5 and 18 days, respectively. The combination of both drugs significantly extended the median life span to 33 days. To clarify this enhancement of the increase in median life span, we examined intracellular deoxyribonucleoside triphosphate (dNTP) pools, cell-cycle distribution, DNA fragmentation, and the time course of the plasma drug concentration. Etoposide had no effect on intracellular dNTP pools in this experimental system, whereas treatment of cells with C18PCA or with the combination of both drugs resulted in a significant increase in dTTP pools to values ranging from 1.8- to 2.0-fold higher than the control levels. There was a significant increase in cells in the S + G2/M phase when cells had been treated with both etoposide and C18PCA. Agarose-gel electrophoresis of the extracted DNA revealed that C18PCA enhanced the fragmentation of DNA, with a length of about 180 bp being induced by etoposide. The plasma peak levels of etoposide (1000 nM) and ara-C (50 nM) were observed at 20 and 30 min after the simultaneous administration of both drugs, respectively. The plasma etoposide level gradually decreased to 10% of the peak level at 240 min after administration. On the other hand, the plasma concentration of ara-C was maintained at above 20 nM at 240 min. These observations suggest that C18PCA and etoposide act on P388 murine leukemic cells by accumulating cells in the S + G2/M phase. Even if the plasma concentration of ara-C is low, the repair of DNA damage by etoposide may be hindered in the presence of ara-C following an increase in DNA fragmentation.

Decreased DNA polymerase sensitivity to 1-beta-D-arabinofuranosylcytosine 5'-triphosphate in P388 murine leukemic cells resistant to vincristine

P388 murine leukemic cells 28.6-fold resistant to vincristine were cross-resistant to doxorubicin and etoposide. Although the intracellular ara-CTP in P388 murine leukemic cells resistant to vincristine (P388/VCR) was significantly higher than that in the parent cells, the cytotoxic effect of ara-C was not significantly different between the parent and resistant cells. Therefore, we investigated the DNA synthesis in P388/VCR and its parent cell line using the permeable cell system. The DNA synthesis in P388/VCR cells was less sensitive to ara-CTP and dTTP than that in P388 parent cells. These results suggested that the characteristics of DNA polymerase in P388/VCR cells might change when the cells developed multiple drug resistance.