Abstract 2633: Vitamin D3 analog inecalcitol synergizes with tyrosine kinase inhibitors (TKI) and selectively inhibit the growth of chronic myeloid leukemia (CML) progenitors: Development of a clinically applicable leukemic stem cell targeting strategy

Proceedings: AACR 106th Annual Meeting 2015; April 18-22, 2015; Philadelphia, PA Tyrosine kinase inhibitor (TKI) therapies have profoundly changed the natural history of chronic myeloid leukemia (CML) and prolonged survival. However, in vitro and in vivo data suggest strongly that the eradication of the most primitive CML stem cells will not be possible by the use of TKI therapies alone. The mechanisms of this inefficiency might involve cell autonomous (activation of alternate signaling, reduced BCR-ABL expression) or non-cell autonomous (niche-related) pathways. It would therefore be of major interest to determine if compounds targeting CML progenitors and stem cells can be used in combination with TKI. Few targeted therapies have been so far shown to be clinicaly acceptable. We have used for this purpose Inecalcitol (19, nor 14 epi 23-yne-1,25 (OH)2D3) (ICC),a vitamin D3 analog, which has been shown to exert antiproliferative effects in several types of cancer cell lines. CD34+ cells isolated from CML patients at diagnosis (n = 15) were tested in clonogenic assays (500 CD34+ cells / dish in triplicate). Interestingly, ICC alone is highly efficient to inhibit the clonogenic growth in the majority of the CML patients at diagnosis (10/ 15 patients). The combination of ICC with either Imatinib Mesylate (IM), Dasatinib (DA) or Nilotinib (NIL) in clonogenic assays showed a synergistic effect for the inhibition of CFC growth (10 - 25% CFC survival) using IM (n = 15 patients). In the same conditions, we have not observed any significant inhibitory effect of IM and ICC combination in cord-blood derived progenitors (n = 3). To determine the effects of these combinations in the most primitive stem cells, we have performed long-term cultures initiating cell (LTC-IC) assays using purified CD34+ cells (4.104 cells / dish) from CML patients with half medium changes each week for 5 weeks. The combination was tested in 6 (ICC+ IM) and 4 (ICC + DA / NIL) CML samples. As a control, CD34+ cells from cord blood were used. In CML samples tested with the combination of either IM / ICC (n = 6) or DA/ NIL and ICC (n = 4), CML LTC-IC derived progenitors were highly inhibited (ICC and IM) or undetectable (ICC and DA or NIL). Short-term cultures of CML CD34+ cells in the presence of 5 growth factors with or without ICC showed that ICC induced the expression of myeloid markers and highly favored the appearance of double positive CD14/CD15 cells. Experiments are underway to determine if ICC interferes with the expression of the components of SHH pathway (Smo, Ptched, Gli). as well as the gene expression profiling of CML cells treated with ICC. Thus, these results establish that ICC, a clinically used derivative of vitamin D3 has a clear activity in CML progenitors by itself and a major synergistic effect with TKI. A clinical phase 2 trial aiming to confirm the synergistic effect of ICC and TKI is ongoing in CML patients treated with imatinib. Citation Format: Ali G. Turhan, Hyacinthe Johnson Ansah, Patricia Hugues, Camille Debord, Remi Delansorne, Agnes Guerci-Bresler, Jean Francois Dufour Lamartinie, Annelise Bennaceur-Griscelli. Vitamin D3 analog inecalcitol synergizes with tyrosine kinase inhibitors (TKI) and selectively inhibit the growth of chronic myeloid leukemia (CML) progenitors: Development of a clinically applicable leukemic stem cell targeting strategy. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2633. doi:10.1158/1538-7445.AM2015-2633