Previous in vitro investigations demonstrated that human
leukemia cells, when incubated with hematopoietic
growth factors such as
granulocyte-colony-stimulating factor (
G-CSF), augment the accumulation of the
triphosphate 1-beta-D-arabinofuranosylcytosine (
ara-C cytarabine). To test whether
G-CSF infusion prior to
ara-C infusion would biologically modulate the accumulation of ara-9-beta-D-arabinofuranosylcytosine 5'-triphosphate (
ara-CTP) and other ara
nucleotides in the
leukemia blasts during
therapy, protocols were designed to infuse
G-CSF prior to
fludarabine (9-beta-D-arabinofuranosyl-2-fluoroadenine monophosphate) and
ara-C to increase the accumulation of the active triphosphates [9-beta-D-arabinofuranosyl-2-fluoroadenine 5'-triphosphate (
F-ara-ATP) and
ara-CTP] in
acute myelogenous leukemia (AML) blasts during
therapy. To
complement these in vivo studies, ex vivo accumulation of
ara-CTP was also investigated before and after
G-CSF infusion. Patients (n = 5) treated on the
fludarabine/
ara-C/
G-CSF regimen received a 30 mg/m2 dose of
fludarabine followed by a 2 g/m2 dose of
ara-C infused i.v. for 4 h. Beginning at 24 h, and every day, patients received a 6-h infusion of 400 microgram/m2
G-CSF. At 48 h, the
fludarabine and
ara-C couplet was repeated. Comparison of
F-ara-ATP pharmacokinetics in circulating AML cells of patients on the
fludarabine/
ara- C/
G-CSF regimen demonstrated that the area under concentration time curve (AUC) of
F-ara-ATP increased significantly (median, 1.4-fold; range, 0.9-1.5; P = 0.045) after
G-CSF infusion. This was due to an increased rate of
F-ara-ATP accumulation by AML cells. The AUC of
ara-CTP, on the other hand, was not affected (median, 1.0-fold; range, 1.0-1.2; P = 0.571) after
G-CSF infusion. Because
fludarabine potentiates the accumulation of
ara-CTP, the effect of
G-CSF on
ara-CTP metabolism may not be evident in the AML blasts of patients on the
fludarabine/
ara-C/
G-CSF regimen. To determine the effect of
G-CSF when
ara-C was infused alone, four additional patients were treated on a pilot protocol in which
ara-C (2 g/m2) was infused on days 1 and 3 and
G-CSF on day 2. The AUC of
ara-CTP accumulation in these patients decreased by a median of 48% after
G-CSF infusion. Consistent with these in vivo investigations, ex vivo
ara-CTP accumulation was decreased in the AML blasts after
G-CSF infusion. Based on these data it could be concluded that (a) infusion of
G-CSF before
fludarabine augmented the rate of
F-ara-ATP synthesis in circulating AML blasts during
therapy, suggesting that
G-CSF may benefit
fludarabine therapy by
biological modulation; (b)
G-CSF did not increase
ara-CTP accumulation, rather it may have caused it to decrease; and (c) these data imply that when
G-CSF and
ara-C are used in combination, administration of
fludarabine prior to
ara-C may maintain the
ara-CTP AUC.