- A168 Acute myeloid leukemia (AML) is the most common acute leukemia in adults, for which there is no effective therapy at this time. Differentiation therapy with vitamin D 3 derivatives (VDDs) is a promising potential approach to treat AML. However, most VDDs induce severe hypercalcemia at pharmacologically active doses. We found that carnosic acid, the major rosemary polyphenolic antioxidant, as well as the carnosic acid-rich extract of rosemary leaves synergize with very low doses of different VDDs in the antiproliferative and differentiation effects in human and murine AML cells, without inducing cytotoxicity. These effects were associated with a decrease in the intracellular levels of reactive oxygen species, activation of the Nrf2/antioxidant response element transcription system, and elevation of glutathione content. In the in vivo study, we tested the hypothesis that the synergy between rosemary antioxidants and VDDs can be exploited for differentiation therapy of AML. The leukemic tumor and systemic AML models in syngeneic Balb/c mice were established by intraperitoneal (i.p.) and intravenous, respectively, inoculation of WEHI-3B D - murine myelomonocytic leukemia cells. The tumor- or systemic leukemia-bearing mice were separately treated with rosemary extract (mixed with food), low-calcemic VDDs such as nor-gemini, 1,25-dihydroxy-21(3-hydroxy-3-methyl-butyl)-19-nor-cholecalciferol or 1,25-dihydroxy-20,16-ene-5,6-trans-cholecalciferol (injected i.p.), and their combinations. The combined treatments resulted in at least an additive strong inhibition of tumor growth as well as in a substantial synergistic increase in the life-span of the systemic leukemia-bearing mice. These cooperative effects of rosemary extract and VDDs in vivo were not accompanied by hypercalcemia or general drug toxicity. Our results suggest novel therapeutic and preventive strategies against AML and may have a carry-over significance in other types of cancers. We thank Dr. M. Uskokovic (BioXell, Inc., Nutley, NJ) for providing vitamin D derivatives and Dr. Zohar Nir (LycoRed Natural Products Industries, Beer-Sheva, Israel) for donating rosemary extract.