MRx102, a Novel Triptolide Analogue, Has Low Nanomolar Activity Against AML Stem/Progenitor Cells in Vitro and in a Murine Model of AML

Abstract 1547

Current chemotherapeutic regimens are known to be ineffective against AML stem/progenitor cells that is responsible for disease relapses due to both intrinsic resistance of these cells and extrinsic environmental factors. Triptolide, isolated from the herb Tripterygium wilfordii, has been shown to potently inhibit growth and induce apoptosis in various malignant cells by inhibiting RNA synthesis and NF-κB activity. Previously, we showed that triptolide promotes apoptosis in AML cells via the mitochondria-mediated pathway, in part by decreasing levels of the anti-apoptotic proteins XIAP and Mcl-1. MRx102 is a novel triptolide derivative, acting by converting to triptolide. We investigated whether MRx102 effectively targets primary human AML cells, particularly AML stem/progenitor cells co-cultured with bone marrow (BM) derived mesenchymal stromal cells (MSCs), and examined the effects of MRx102 in NOD/SCID mice injected with Ba/F3-ITD cells. We show here that MRx102 potently promoted apoptosis in AML cell lines, with EC₅₀ values of low nanomolars for OCI-AML3 and MV4-11 cells. Like triptolide, MRx102 decreased XIAP and Mcl-1 protein levels and inhibited RNA synthesis in OCI-AML3 cells. Interestingly, we also found that MRx102 and triptolide decreased β-catenin protein level in OCI-AML3 cells. MRx102 effectively induced apoptosis in all primary AML samples tested, regardless of the cytogenetics or clinical responses to various therapies of the patients at the time of sampling (ED₅₀ = 132.1 ± 7.5 nM, n=12) and was less toxic to normal BM cells (ED₅₀ = 209.9 ± 56.8 nM, n=3). In addition to bulk AML cells, MRx102 also induced apoptosis in CD34⁺ progenitors, and more importantly in CD34⁺CD38⁻ stem/progenitor cells from AML patients, even when they were protected by co-culturing with BM-derived MSCs. The effectiveness of MRx102 on AML stem/progenitor cells may be in part due to its ability to decrease β-catenin and triptolide's ability to inhibit NF-κB activity as both Wnt/β-catenin and NF-κB are highly active in AML stem/progenitor cells. Furthermore, in vivo, MRx102 greatly decreased leukemic burden, reduced infiltration of leukemic cells to various organs, and increased survival of NOD/SCID mice harboring Ba/F3-ITD cells (median survival 43 days with MRx102 at 3.0 mg/kg/day compared with 36 days for the untreated mice, P = 0.027).

Collectively, we demonstrated that MRx102 has strong antileukemic activity both in vitro and in vivo in a murine model of AML, has the potential to eliminate AML stem/progenitor cells and overcome microenvironmental protection of leukemic cells, and warrants clinical investigation.