Effect of uracil arabinoside on metabolism and cytotoxicity of cytosine arabinoside in L5178Y murine leukemia

Pretreatment of L5178Y murine leukemia cells with uracil arabinoside (ara-U) enhances the cytotoxicity of cytosine arabinoside (ara-C). This effect is mediated by the cytostatic effect of ara-U, which causes a delay of cell progression through S-phase. Consequently, the specific activity of enzymes that peak during S-phase increases, and deoxycytidine kinase increases 3.6-fold over untreated controls. This allows enhanced anabolism of ara-C to nucleotides, as well as increased incorporation into DNA with ultimate synergistic cytotoxicity. It is postulated that the systemic metabolism of high-dose ara-C to sustained high levels of ara-U in patients with acute leukemia may enhance the activity of subsequent doses of ara-C, and thus contribute to a means for pharmacologic self-potentiation, contributing to the unique therapeutic activity of high-dose ara-C.

DNA lysis is involved in the simplified fluorescence plus Giemsa method for differential staining of sister chromatids

The DNA labelling of the bifilarly 5-bromodeoxyuridine- to-substituted chromatid decreased relative to that of the unifilarly substituted chromatid with increasing duration of HB pretreatment (Hoechst 33258 plus black light at 55 degrees C). Sister chromatid differential staining was detected by Giemsa as well as a DNA-specific dye, ethidium bromide, after 4 s of HB pretreatment. The contrast of sister chromatid differential staining was improved with increased duration of HB pretreatment or by incubation with exonucleases. Hydrogen donors such as cysteamine, cysteine, and L-ascorbic acid inhibited the HB pretreatment, but this inhibition could be overcome by increasing the duration of HB pretreatment.

Alteration of the pharmacokinetics of high-dose ara-C by its metabolite, high ara-U in patients with acute leukemia

The pharmacokinetics of high-dose cytosine arabinoside (HiDAC) given as a three-hour intravenous infusion at 3 g/m2 were studied in five patients with acute leukemia during relapse and/or remission of their disease. Apparent steady state plasma levels of ara-C during 13 infusions averaged 115 +/- 32 microM. Upon cessation of the infusion, cytosine arabinoside (ara-C) was rapidly cleared from the plasma. The apparent postinfusion kinetics of ara-C were triexponential with a distribution half-life of 16 minutes and elimination half-lives of 1.8 hours and six hours. Total clearance averaged 86 L per hour and mean residence time averaged 0.47 hours. Disease status (relapse or remission) had no apparent effect on the pharmacokinetic characteristics of ara-C. Peak levels of ara-U averaged 310 microM and the metabolite had an average apparent elimination half-life of 3.75 hours. Despite the persistence of ara-U at about 100 microM at the time of administration of subsequent infusions of ara-C, there was no further accumulation of ara-U in the plasma with repetitive infusions of HiDAC. In vitro studies indicate that ara-U can exert an inhibitory effect on deoxycytidine (dCyd) deaminase activity. The ratio of the Ki of ara-U to the Km of ara-C for cytidine (Cyd)-dCyd deaminase is 40:1; however, during the gamma phase of ara-C elimination, the ratio of ara-U:ara-C in plasma is at least 100:1. Thus, a retardation of systemic catabolism of ara-C by ara-U is possible. Two to three hours after the termination of the HiDAC infusion, the ara-C cerebrospinal fluid: plasma ratio is 1-3:1, a feature of potential therapeutic significance. The slower elimination of ara-C from the CSF may also contribute to the plasma gamma half-life.