The Novel Imipridone ONC212 Induces Pronounced Anti-Leukemia Effects in Vitro and In Vivo and Is Highly Synergistic with the BCL-2 Inhibitor ABT-199

Imipridones constitute a novel class of molecules that includes the first-in-class anti-cancer compound ONC201. ONC201 has shown promising clinical activity against hematological and solid malignancies (Allen et al., Oncotarget, 2016). ONC212 is a second-generation imipridone with higher potency than ONC201. We first confirmed the overall in vitro anti-tumor effects of ONC212 using the collection of 1,088 human cancer cell lines available from the Genomic of Drug Sensitivity in Cancer Project. Ranking the sensitivity of these cancer cell lines to ONC212 by tumor type revealed that leukemias and lymphomas were especially sensitive. In fact, ONC212 exerted prominent apoptogenic effects in AML cell lines with > 20-fold potency compared to ONC201 (ED50s at 72 hr; 6.4 μM vs 258.7 nM, respectively for OCI-AML3 cells). Similar results were observed in primary AML cells, but there was no activity against normal bone marrow (BM) cells. We previously demonstrated that ONC201 induces apoptosis via an atypical integrated stress response (ISR, Ishizawa et al., Sci Signal, 2016). Similarly, ONC212 showed ISR activation accompanying its apoptogenic effects in AML cells which was agnostic to the mutation status of p53. However, because of the higher sensitivity of wild-type p53 cells to ONC212, we speculated that this effect could also reflect a p53-dependent apoptosis mechanism. Indeed, p53 knockdown reduced the ONC212-induced apoptosis in MV4;11 and MOLM13 cells, suggesting both, p53-dependent and -independent, mechanisms.

We next investigated the effects of ONC212 in vivo in a systemic AML xenograft model using OCI-AML3 cells expressing luciferase. Biweekly oral administration of 50 mg/kg ONC212 markedly inhibited AML expansion and prolonged overall survival (p = 0.0003). Median survival increased from 43 d in controls to 49 d in the ONC212-treated group (+14%). For in vivo functional assessment of ONC212's anti-tumor effects against leukemia stem and progenitor cells, we utilized our patient-derived xenograft (PDX) mouse model. We treated primary PDX cells with ONC212 (250 nM, 36 hr) ex vivo, and then injected identical numbers of viable cells (treated or untreated) into recipient NSG mice. After one month, the percentages of human CD45+ cells in the peripheral blood, spleen, and BM were significantly decreased in the ONC212-treated group. The median survival in this group was remarkably different: 82 d compared to 36 d for controls (+128%, p<0.0001). These results indicated that ONC212 targets AML stem cells with potential to eradicate the disease.

Because BCL-2 is generally considered protective against ISR-mediated apoptosis and ONC212 induced p53-mediated apoptosis as demonstrated above, we hypothesized that the BCL-2 inhibitor ABT-199 or the p53 activator nutlin-3a could further sensitize AML cells to ONC212. Indeed, the in vitro combination of ONC212 plus ABT-199 or nutlin-3a synergistically induced apoptosis in AML cells. Furthermore, the combination of ONC212 and ABT-199 showed highly significant synergistic anti-leukemia effects in vivo in a MOLM-13 xenograft mouse model. The combinatorial treatment prolonged overall median survival from 20 d for untreated controls to 21 d for each agent as monotherapy to 30 d for the combination.

Since the G-protein-coupled receptor (GPCR) dopamine receptor D2 is the putative target of first-generation imipridone ONC201, we performed a PathHunter β-arrestin screening to determine if ONC212 also bound GPCRs. ONC212 did not engage DRD2 but instead specifically engaged the orphan GPCR GPR132, which is a putative tumor suppressor when activated. Interestingly, expression of GPR132 mRNA was highly correlated with sensitivity to ONC212. Furthermore, GPR132 overexpression induced cell death and ONC212 treatment induced GPR132 mRNA expression in AML cell lines, suggesting that GPR132 could be a potential therapeutic target in hematological malignancies.

Taken together, ONC212, and its putative target GPR132, represent a promising novel anti-cancer strategy for AML therapy that warrants further development.