ONC201 Depletes Cancer Stem Cells in Refractory Cancer Patient Samples

ONC201 is a small molecule that is being developed as a novel anticancer drug based on its strong anti-tumor efficacy in preclinical models of advanced cancer (Allen et al, Sci Transl Med, 2013). ONC201 has demonstrated strong antitumor effects in vitro and in vivo in a variety of solid and liquid tumors. Cancer stem cells (CSCs) are known to be responsible for tumor propagation and maintenance as well as driving resistance to many cancer therapies. Based on the potent efficacy of ONC201 in preclinical models, we investigated the therapeutic impact of ONC201 on CSCs to rationalize its strong antitumor effects and realize the extent of its therapeutic potential.

In primary refractory AML patient samples, ONC201 induced very high levels of apoptosis in leukemia stem/progenitor cells (CD45 dim+/ CD34+/ CD38-) to a similar extent observed in the bulk tumor population. In addition to this effect in hematological malignancies, ONC201 has also exhibited strong anti-CSC efficacy in solid tumors such as colorectal cancer (CRC) and glioblastoma multiforme (GBM). ONC201 eradicated CSC-enriched 3-dimensional neurosphere culture models of primary GBM samples, including newly diagnosed and recurrent samples. ONC201 potently inhibited in vitro cell proliferation of all 5 neurosphere lines tested, with IC50 values of 433 nM (MGG18), 1.46 µM (MGG4), 1.09 µM (MGG8), 3.97 µM (MGG67R) and 688 nM (MGG152). In CRC, ONC201 significantly depleted CD133, CD44 and Aldefluor positive CSCs in HCT116, DLD1 and SW480 cells. ONC201 significantly inhibited colonosphere formation of unsorted and sorted Aldefluor positive 5-Fluorouracil-resistant DLD1 and SW480 cells. ONC201 significantly reduced tumor growth of CSC-initiated tumors and prevented the passage of these tumors. CD44 and CD133 expression was reduced in ONC201-treated tumors. ONC201-treatment decreased tumor initiation relative to 5-Fluorouracil treatment in a limiting dilution tumor initiation assay. The ONC201-mediated anti-CSC effect was significantly blocked by the TRAIL sequestering antibody RIK-2. ONC201-mediated TRAIL-induction was also observed in CSC-initiated tumors. ONC201 inhibited Akt and ERK leading to Foxo3a activation and surface TRAIL induction in sorted CSCs and non-CSCs. Overexpression of myristoylated Akt significantly reduced the induction of surface TRAIL by ONC201 in CSCs and prevented ONC201-mediated depletion of CSCs and colonosphere inhibition.

In conclusion, ONC201 cytotoxicity is extended to CSCs in hematological and solid malignancies as well as the bulk tumor population. The ability of ONC201 to deplete CSCs suggests the clinical potential for ONC201 to reduce the incidence of disease relapse, which is frequently a fatal event in hematological malignancies, in addition to its immediately apparent antitumor effects.