Targeting the Hedgehog Signaling Pathway By PF-04449913 Limits the Self-Renewal of MDS-Derived Induced Potent Stem Cells (iPSC): Molecular Mechanisms

Background:

Myelodysplastic syndromes (MDS) are clonal hematopoietic disorders characterized by no efficient hematopoiesis and frequent progression to acute myeloid leukemia (AML). Even in low risk MDS, clonal hematopoiesis already dominates at diagnosis, and clones found in secondary AML originate from the MDS stage of disease, highlighting the need to specifically target the MDS-initiating clone. PF-0449913 is a potent and selective hedgehog pathway inhibitor that act by binding Smoothened (SMO) and blocking signal transduction. In xenograft models of human coloirectal and pancreatic cancer, treatment with PF-04449913 in combination with other anticancer agents reduced the tumor growth. Furthermore, PF-04449913 demonstrated preliminary antitumor activity in a phase I trial, when given as monotherapy in patients with several hematopoietic malignancy.  In the present study, we investigated the molecular mechanisms by which PF-04449913 regulate the self-renewal of MDS-derived iPS cells (iPSCs) in vivo.

Methods:

We generated iPSCs from bone marrow mononuclear cells of two MDS patients (RAEB1 and RAEB2 by WHO clssification) with chromosome 5 deletion and complex karyotypic abnormalities, respectivly. Karyotyping analysis revealed that MDS-derived iPSCs have identical abnormalities to primary MDS cells. We also generated iPSCs from bone marrow mononuclear cells of normal volunteer as control. To investigate the effects of PF-04449913 on self-renewal and the relevance as a therapeutic target in MDS initiating cells, we examined the activity of PF-04449913 against MDS-derived iPSCs transferred NOD/SCID mice in vivo. NOD/SCID mice were injected sucutaneously with MDS-derived iPSCs or normal iPSCs then treated with PF-04449913 (100 mg/kg; p.o.) from day 10 for 28 days. We also used MDS-L, a myelodysplastic cell line establised from MDS patient with del(5q) and complex karyotypic abnormalities for in vitro studies.

Results:

Both MDS-derived iPSCs transferred NOD/SCID mice and normal iPSCs transferred  NOD/SCID mice demonstrated the engraftment of CD34+CD38- positive cells by flow cytometry. However, the treatment with PF-04449913 reduced the population of CD34+CD38- positive cells in MDS-derived iPSCs transferred NOD/SCID mice. We isolated human CD45+ cells from the spleen of mice from each treatment group and injected equivalent numbers of CD45+ cells into secondary recipients. Following 50 days, all mice treated with vehicle engrafted with CD34+CD38- positive cells. In contrast, CD34+CD38- positive cells engraftment was not detected in recipient mice (n=3) from PF-04449913-treated donors. These results demonstrate the persistent effects of PF-0449913 on long term self-renewing MDS-initiating cells. We further examined the effects of Nanog pathway modulation on in vitro clonogenic growth.  CD34+CD38- cells from MDS-derived iPSCs transferred NOD/SCID mice and MDS-L cells were treated with 2 mM of PF-04449913 for 72 hrs, washed free of drugs, and plated in quadruplicate in methylcellulose. At 14 days, colonies were counted as initial plating. The representative plate was then washed and cells were re-suspended and re-plated. After an additional 14 days, colonies were counted as secondary re-plating. Clonogenic recovery of untreated cells was normalized to 100% and plating results from all treatment groups were expressed as % control.  PF-04449913 had only minimum effects on colony formation after initial plating over control cells. However, upon serial re-plating, secondary colony formations were significantly inhibited by PF-04449913 (p<0.001). To identify the mechanisms that limit the self-renewal of MDS-initiating ells by PF-04449913, NOD/SCID mice engrafted with CD34+CD38- fractions from MDS-derived iPSCs were treated with PF-04449913 (100 mg/kg; p.o.) for 14 days. PF-04449913 induced the expressions of p21Cip1, cleaved PARP and reduced the expression of BMI-1, c-Myc, Nanog, and Bcl-XL. 

Conclusion:

Our preclinical results indicate that PF-04449913 have potential as an important option for controlling the drug-resistant MDS-initiating cells.  It is expected that the PF-04449913 may become extremely useful therapeutic interventions in a number of hematological neoplasms, including MDS, where the persistence of cancer stem cells.