Induction of apoptosis using 2 ',2 ' difluorodeoxycytidine (gemcitabine) in combination with antimetabolites or anthracyclines on malignant lymphaticand myeloid cells. Antagonism or synergism depends on incubation schedule and origin of neoplastic cells

Induction of apoptosis in vitro using gemcitabine (dFdC) in combination wit h cladribine (2-CdA) and other cytotoxic drugs on malignant mononuclear cel ls (MNCs) of patients with acute myeloid leukemia (AML, n = 20) and chronic lymphocytic leukemia (CLL, n = 20) in myeloid (HL60, HEL) and lymphatic ce ll lines (HUT78, JURKAT) was investigated using different incubation condit ions (simultaneous and consecutive). Furthermore, the influence of dFdC on the level of intracellular metabolites of 2-CdA was studied using high-perf ormance liquid chromatography (HPLC). Apoptosis was evaluated using flow cy tometry with 7-aminoactinomycin D. In MNCs of patients with CLL, dFdC + 2-C dA showed an antagonistic effect when applied simultaneously. This antagoni sm was reduced by consecutive application. The combination of dFdC with dox orubicin was synergistic, independent of incubation schedule. In blasts fro m newly diagnosed patients with de novo AML, all drug combinations (dFdC 2-CdA, doxorubicin, or cytosine arabinoside) were antagonistic by simultane ous incubation. Reduced antagonism or even synergism was shown (P < 0.001) by consecutive incubation. The simultaneous combination of dFdC with 2-CdA in all tested cell lines resulted in a competitive inhibition on the rate o f apoptosis. By changing the incubation period to a consecutive schedule, t he antagonism was diminished or synergism of apoptosis was measured (P < 0. 001). Using similar incubation conditions, these experiments were supported by HPLC measurement of intracellular metabolites of 2-CdA influenced by dF dC application. In conclusion, we demonstrated that the efficacy of dFdC in vitro in combination with other cytotoxic drugs depends on the incubation condition and on the origin of neoplastic cells (lymphatic vs myeloid). The data suggest that simultaneous combination therapy with purine and pyrimid ine analogues may not improve the clinical efficacy of one or the other dru g administered alone.