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Introduction: 3-Ingenyl angelate (PEP005) is a selective small molecule activator of Protein Kinase C (PKC) extracted from the plant Euphorbia peplus (Kedei et al, 2004), which has been shown to have anti-tumor effects on skin cancers when applied topically (Ogbourne et al, 2004). We wanted to determine whether the anti-tumor effects of PEP005 might extend to systemic tumors and investigated its efficacy against leukemic cell lines and primary acute myeloid leukemic (AML) cells. Methods: PEP005 at nanomolar concentrations was added to cultures of 4 leukemic cell lines (HL60, U937, NB4 and KG1a) and to primary AML cells and differentiation and apoptosis were measured. The role of PKC and MAP kinase signalling pathways in PEP005 actions was assessed using selective pharmacological inhibitors. Results: PEP005 had potent anti-leukemic effects, inducing differentiation and apoptosis in 3 of the myeloid leukemia cell lines (HL60, U937 and NB4), with maximal effects at 20 nM PEP005. PEP005 also induced apoptosis in 7 out of 8 primary AML cell samples. Importantly PEP005 did not induce apoptosis in normal CD34+ve cord blood myeloblasts at up to two log concentrations higher than those required to induce cell death in primary AML cells. The effects of PEP005 were PKC dependent and PEP005 efficacy correlated with expression of PKC-delta. Transfection of KG1a leukemia cells, that did not express PKC-delta or respond to PEP005, with GFP-PKC-delta restored sensitivity to induction of apoptosis by PEP005. The pro-apoptotic effects of PEP005 were also found to be dependent on p38 MAP kinase, whereas the differentiation effects were p42/44 dependent. Conclusion: Our data show that the diterpene ester 3 ingenyl angelate is able to activate both PKC and MAP kinase signalling pathways and suggest that activation of PKC-delta signaling pathways in particular provides a novel approach for treatment of AML. References Kedei N, Lundberg DJ, Toth A, Welburn P, Garfield SH, et al (2004). Characterization of the interaction of ingenol 3-angelate with protein kinase C. Cancer Res. 64:3243-3255. Ogbourne SM, Suhrbier A, Jones B, Cozzi SJ, Boyle GM, et al (2004). Antitumor activity of 3-ingenyl angelate: Plasma membrane and mitochondrial disruption and necrotic cell death. Cancer Res. 64:2833-2839.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]