Dendrogenin_A : A Natural Liver X Receptor Modulator for the Treatment of Acute Myeloid Leukemia

Acute Myeloid Leukemia (AML) is the most common type of leukemia in adults. Despite intensive research, current treatments remain unsatisfactory with only 40% of younger (<60 years) and less than 10% of older (>60 years) AML patients achieving long-term complete remission. Consequently, drugs with novel mechanism of action are urgently needed to improve the outcome of these patients. We have recently identified Dendrogenin A (DDA) as a cholesterol metabolite present in normal cells but undetectable in various cancer cell lines including AML (de Medina et al, Nat Commun, 2013). DDA, the first steroidal alkaloid identified in mammals, exhibited strong anticancer effects against different tumor models in vitro and in vivo. In this study, we investigated the antileukemic potency of DDA in AML.

We demonstrated that DDA exerts potent cytotoxic effect in a large panel of AML cell lines and cytogenetically and molecularly diverse primary AML patient samples (n=50) with a median IC₅₀ of 3.3 µM (range 1.2-10 µM). We determined that DDA triggers both apoptosis and cytotoxic autophagy on AML cells. Macroautophagy was characterized by the accumulation of autophagic vacuoles and the stimulation of autophagic flux.

As opposed to conventional chemotherapies, the antileukemic effect of DDA was similarly efficient in both immature stem/progenitor CD34⁺CD38^-CD123⁺ subpopulation and leukemic bulk. Interestingly, the antileukemic activity of DDA on AML patient samples was not correlated to usual prognostic factors such as adverse cytogenetic risk karyotype, clonogenic ability, white blood cells count and FLT3-ITD or NPM status.

Pharmacokinetic studies revealed that both per os (PO) and intraperitoneal (IP) administration led to a good absorption with calculated bioavailability of 74% (PO) and 48% (IP), showing that these modes of administration are relevant to in vivo preclinical studies.

We then examined the in vivo anti-leukemic efficacy of DDA in NOD/SCID mice injected subcutaneously with HL60 and KG1 cells. We demonstrated that daily administration of DDA (20 mg/kg IP or 40 mg/kg PO) significantly reduced KG1 and HL60 tumor growth. Immunohistochemical analysis revealed that AML xenografts from mice exposed to DDA display a 3.5 fold increase of LC3 punctated cells and a decreased P62 level highlighting that DDA induces autophagy in vivo. Furthermore, DDA significantly kills AML cells in bone marrow and brain (55±5.6% reduction of viable CD45⁺ cells), and strongly reduces (57±7.8%) the total cell tumor burden in bone marrow and spleen in established disease models (eg. orthotopically engraftment of HL60 cells and three primary AML patient cells via tail vein injection in NOD/SCID/IL2Rγc-deficient mice). In addition, we showed that DDA is well tolerated in mice at effective dose and spares normal hematopoietic stem/progenitor cells from healthy donor.

Mechanistic studies revealed that DDA is a natural modulator of the Liver X Receptor (LXR), a nuclear receptor involved in cholesterol homeostasis, immunity and proliferation. We found that the silencing of LXRβ gene prevents the capacity of DDA to trigger both cell death and autophagy on AML cells in vitro. In addition, DDA failed to block tumor development and to trigger autophagy on LXRβ-invalidated KG1 cells xenografted on NOD/SCID mice. Moreover, DDA strongly stimulates the expression of the myeloid leukemogenesis tumor suppressors Nur77 and Nor1 through an LXRβ-dependent mechanism. Interestingly, DDA triggers the relocation of Nur77 to the mitochondria, a process associated with both apoptosis and autophagic cell death. This study provides a strong rationale to bring DDA in clinical trials for patients with AML.