Synergistic imipridone‐based drug combinations for treatment of pediatric H3K27M mutant diffuse intrinsic pontine glioma (DIPG)

Abstract

The H3K27M oncohistone mutation has been identified in approximately 80% of diffuse intrinsic pontine gliomas (DIPG), and now represents a potential target for urgently needed new therapies. ONC201, a first-in-class imipridone therapy, is known to selectively induce apoptosis of cancer cells independent of p53, and has shown efficacy against H3K27M mutant DIPG. We sought to identify synergy between ONC201 or second generation imipridones (ONC206 and ONC212), and other chemotherapeutics with known preclinical activity against DIPG. We hypothesized that second generation imipridones would demonstrate superior activity to ONC201, and that the combination of imipridones and other chemotherapeutics may be superior to single agent treatment in H3K27M mutant DIPG. Seven patient derived DIPG cell lines (SU-DIPG-IV, SU-DIPG-13, SU-DIPG-25, SUDIPG-27, SU-DIPG-29, SU-DIPG-36, SF8628) were grown in culture and exposed to imipridones as a monotherapy and in combination with other FDA approved chemotherapeutics (histone de-acetylase inhibitors [HDACi], marizomib, etoposide, and temozolomide). Cell viability was measured by CellTiter-Glo. Dose dependent response to imipridone therapy was noted in all cell lines with half maximal inhibitory concentration (IC50) of 1.46 µM, 0.11 µM, and 0.03 µM, in ONC201, ONC206, and ONC212 respectively. Synergy was identified via combination analyses, with combination indices of <1 indicating some degree of synergy, and <0.5 indicating very strong synergy. Strong synergy was identified between ONC201 and HDACi Panobinostat (CI 0.01), Romidepsin (CI 0.08) and protease inhibitor Marizomib (CI 0.19). Lesser synergy was demonstrated between ONC201 and Entinostat (CI 0.71) or Etoposide (CI 0.54). Importantly, temozolomide and ONC201 showed a lack of combinational efficacy against H3K27M mutant DIPG, consistent with the literature to date. Second generation imipridones showed similar synergistic effects with Panobinostat, Romidepsin, and Marizomib. Additionally, induction of apoptosis as measured by the induction of poly (ADP-ribose) polymerase (PARP) cleavage via immunoblotting, was demonstrated with a combination of imipridones and Panobinostat. These results suggest increased sensitivity of H3K27M mutant DIPG cell lines to second generation imipridone therapies, as compared to ONC201. Additionally, there is evidence of promising synergy between imipridones and Panobinostat, Romidepsin, or Marizomib, which should be taken into consideration for future clinical trials.