Anti-CD33 monoclonal antibody and etoposide/cytosine arabinoside combinations for the ex vivo purification of bone marrow in acute nonlymphocytic leukemia. Pharmacologic and immunologic methods of ex-vivo bone marrow (BM) purging for acute nonlymphocytic leukemia (ANLL) were combined to augment the effect of either method alone. Etoposide (VP16; 20 to 30 micrograms/mL) with or without cytosine arabinoside (Ara C; 10 mg/mL) was used in tandem with the anti-CD33 monoclonal antibody (MoAb), MY9, chosen because CD33 is found on the stem cell pool in the majority of patients with ANLL. The agents were tested singly or sequentially, with a 1-hour incubation of the drugs preceding complement-mediated lysis using MY9. VP16 combined with Ara C killed up to 3.9 +/- 0.3 and 5.11 +/- 0.4 logs of the human ANLL cell lines HL60 and K562 at drug concentrations that killed only 1.2 +/- 0.1 logs of normal committed granulocyte/macrophage stem cells (CFU-GM). Adding a single exposure of the MY9 and complement (C') to the drug-treated cells, greater than 5.4 logs of HL60 were killed. Similar to other pharmacologic agents, no differential kill for clonagenic leukemic cells (colony-forming unit-leukemia; CFU-L) from patients with ANLL was seen for drug only treated blasts versus normal CFU-granulocyte-macrophage (CFU-GM), with less than 1 log CFU-L kill at drug concentrations that spared 1 log of CFU-GM. Similarly, only 1.1 +/- 0.3 logs of ANLL CFU-L were eliminated using MY9 and C'. However, with the sequential VP16/Ara C----MY9 + C' treatment, synergy was demonstrated and 2.6 +/- 0.3 logs of CFU-L were eliminated. Because CD33 is also found on the normal CFU-GM pool, two-stage long-term BM cultures were performed to determine pluripotent stem cell elimination by the drug/MoAb purging combination. No difference of CFU-GM or BFU-E production at 4 to 6 weeks of culture for VP16/Ara C, MY9 + C', or VP16/AraC----My9 + C' treated cells was seen compared with untreated controls indicating sparing of early progenitor cells. Sequential ex vivo treatment of human ANLL CFU-L with VP16/Ara C followed by complement-mediated lysis using MY9 synergistically kills CFU-L while sparing early normal hematopoietic progenitor cells, and thus may be a more effective way to purge BM than either alone. 