Rapamycin -- a Potential Mechanistically Targeted Therapeutic for Juvenile Myelomonocytic Leukemia.

Juvenile myelomonocytic leukemia (JMML) is a mixed myelodysplastic /myeloproliferative disorder (MDS/MPD) of infancy and early childhood. It is characterized by monocytosis, leukocytosis, elevated fetal hemoglobin, hypersensitivity to granulocyte-macrophage colony-stimulating factor (GM-CSF), a low percentage of myeloblasts in the bone marrow, and absence of the Philadelphia chromosome or the BCR/ABL fusion gene. The pathogenesis of JMML has been clearly and definitively linked to dysregulated signal transduction through the RAS signaling pathway. A series of studies conducted over the last decade have shown that mutations or other abnormalities in RAS, NF1, and PTPN11, are potentially responsible for the pathogenesis of JMML in up to 75% of cases. Treatment has been very difficult. There is no effective therapy for JMML. Only allogeneic stem cell transplantation (SCT) can extend survival. However, the relapse rate from allogeneic SCT is inordinately high in JMML (28–55%), with 5-year disease-free survival rates of 25-40%. Rapamycin is a macrolide antibiotic with established clinical applications in organ transplantation. Recent studies have proved that the Mammalian Target of Rapamycin (mTOR) plays an important role in cytokine receptor signaling and induction of apoptosis. Numerous studies have suggested that mTOR functions as a nutritional checkpoint and is connected to energy sensing through AMP-dependent kinase (AMPK) which senses the AMP: ATP ratio in cells. Its function is regulated by the RAS/PI3-kinase pathway. In searching for novel mechanistically-targeted reagents to treat JMML, we conducted an in vitro pilot study with JMML cells. The CFU-GM formation assay was used to test the therapeutic sensitivity of rapamycin to JMML cells. Mononuclear cells (MNCs) from peripheral blood of 9 JMML patients were collected and plated on 0.3% agar medium with rapamycin at a concentration of 1-8nM(0.91-7.28_μg/L) and carrier (DMSO). Greater than 50% inhibition of spontaneous CFU-GM growth was observed in all cultures in a dose-dependent fashion, with the exception of one patient sample which had colonies resistant to rapamycin. The effective concentrations in our cultures are equivalent to the safe and tolerable whole blood concentrations achieved in organ transplant patients in clinical settings (5-30_μg/L). Our data suggests that rapamycin may be considered as a potentially safe and effective reagent to treat JMML, but that in vitro sensitivity testing might be recommended since one patient sample demonstrated complete resistance to rapamycin in vitro. Further studies are ongoing to explore the mechanism of rapamycin in inhibiting hypersensitivity of JMML cells to GM-CSF.