Synergistic cytotoxicity of cytosine arabinoside and mitoxantrone for K562 and CFU-GM.

High-dose cytosine arabinoside (Ara-C) plus mitoxantrone (MX) have proved to be effective in the treatment of refractory acute leukemia. The optimal sequence of drug administration was tested in a clonogenic assay with the leukemic myeloid cell line K562, and with CFU-GM of normal human bone marrow. The exposure times were 24 h for Ara-C and 1 h for MX with 1 h delay between incubations. Either order of the drugs and a wide range of drug concentrations were tested. Cytotoxicity was quantified by the survival fraction (fs) of colonies scored on day 7 (K562) or day 14 (CFU-GM). Drug synergism was evaluated by a cooperative index (CI). CI less than 1 indicates synergism, CI = 1 summation, and CI greater than 1 antagonism of the cytotoxic drugs. In K562 the sequence Ara-C much greater than MX was significantly more toxic (3.68 logs cell kill, CI = 0.02) than MX much greater than Ara-C (2.64 logs kill, CI = 0.31). The highest synergism was found by adding MX during the last hour of a 24 h Ara-C exposure. For CFU-GM, Ara-C much greater than MX showed higher synergistic toxicity (2.24 log cell kill, CI = 0.23) than MX much greater than Ara-C (1.44 logs, CI = 1.11). The clinical high dose Ara-C/MX protocol was transformed into an in vitro model and tested on K562. The highest synergism was found after the sequence of 3 h Ara-C followed by 0.5 h MX after 8.5 h delay (1.73 logs kill, whole sequence 2.01 logs kill), thus supporting the clinically applied sequence.