BISPECIFIC ANTIBODY THERAPY OF HUMAN AML IN A SCID MOUSE MODEL

We have previously reported the development of a bispecific antibody ( BsAb 251x3G8) combining reactivity against CD33 (an antigen expressed on myeloid leukemia cells) and CD16 (Fc gamma RIII). This BsAb augment ed in vitro killing of acute myeloid leukemia (AML) targets by periphe ral blood lymphocytes (PBL), purified resting NK (R-NK) cells or IL-2- activated NK (A-NK) cells. We now report further findings supporting t he therapeutic potential of this BsAb in an in vitro purging model and in an in vivo scid/hu AML model. The killing of fresh AML targets med iated by IL-2-activated PBL (A-PBL) in the presence of BsAb 251x3G8 wa s 5-7 fold greater than that mediated by A-PBL alone in 4 h Cr-51- rel ease cytotoxicity assays. The depletion of leukemia colony forming cel ls (L-CFC) by A-PBL and A-NK cells was also enhanced Ti-fold by the ad dition of BsAb 251x3G8. In additional experiments designed to examine the potential of purging small numbers of leukemia cells, L-CFC were r educed by greater than 3 logs when compared to A-NK and IL-2 treatment without BsAb. In a scid/hu leukemia model of NB4 AML cells, intraperi toneal injection of A-NK cells pre-incubated with BsAb 251x3G8 and IL- 2 prevented AML cell proliferation and the development of tumor masses in 5/5 C.B/17-scid mice at four weeks. In contrast, 5/5 mice receivin g no therapy and 2/5 receiving A-NK cells plus IL-2 alone were found t o have gross and microscopic AML growth. A two-log depletion of perito neal tumor cells was observed with the addition of BsAb in Nod-scid wh en compared to treatment with A-NK cells alone at 24 days after NB4 ce lls inoculation, Thus, BsAb-armed A-NK cells were more effective than unarmed A-NK cells in the killing of AML cells in this in vivo model. This study bespeaks the clinical potential of targeted A-NK immunother apy of leukemia by BsAb 251x3G8 both in vitro and in vivo.