Retinoic acid and arsenic synergize to eradicate leukemic cells in a mouse model of acute promyelocytic leukemia

In acute promyelocytic leukemia (APL) patients, retinoic acid (RA) triggers differentiation while arsenic trioxide (arsenic) induces both a partial differentiation and apoptosis. Although their mechanisms of action are believed to be distinct, these two drugs both induce the catabolism of the oncogenic promyelocytic leukemia (PML)/RAR alpha fusion protein. While APL cell lines resist;mt to one agent are sensitive to the other, the benefit of combining RA and arsenic in cell culture is controversial, and thus far, no data ape available in patients. Using syngenic grafts of leukemic blasts from PML/RAR alpha. transgenic mice as a model for APL, we demonstrate that arsenic induces apoptosis and modest differentiation, and prolongs mouse survival. Furthermore, combining arsenic with RA accelerates tumor regression through enhanced differentiation and apoptosis. Although IIA or arsenic alone only prolongs survival two- to threefold, associating the two drugs leads to tumor clearance after a 9-mo relapse-free period. These studies establishing RA/arsenic synergy in vivo prompt the use of combined arsenic/RA treatments in APL patients and exemplify how mouse models of human leukemia can be used to design or optimize therapies.