Background:
Entinostat, a selective class I histone deacetylase inhibitor, has been reported to enhance the activity of cytotoxic agents and suppress expression of PAX3-FOXO1 in alveolar rhabdomyosarcoma (ARMS).
Procedures:
Entinostat was tested against three rhabdomyosarcoma cell lines using 96-hour drug exposure. Entinostat alone or in binary combination with vincristine, actinomycin D or cyclophosphamide was tested in ARMS and two embryonal rhabdomyosarcoma (ERMS) xenograft models. Tumor growth was measured at weekly intervals. Drug-induced changes in acetylated histone H3(K9) and entinostat pharmacokinetics were determined.
Results:
In vitro, the IC50 concentration of entinostat ranged from 280 to 1300 nM. In vivo, entinostat significantly inhibited the growth of only Rh10 xenografts. For most studies, entinostat did not potentiate the activity of the cytotoxic agent. Exceptions included the vincristine and entinostat combination for Rh10 and the entinostat and actinomycin D combination for Rh10 and Rh18, although the effects were modest. For Rh18, the combination of entinostat with vincristine showed evidence of an antagonistic interaction compared with single-agent vincristine. Pharmacokinetic studies showed the average Cmax was 569.4 ng/mL (1.51 μM) with Tmax at 15 minutes, and total exposure (AUC0–12 h) was 435.6 h × ng/mL. Entinostat treatment increased acetylated histone H3.
Conclusions:
Entinostat demonstrated modest antitumor activity in only one of four models at dose and shedule that gave drug exposures relevant to human treatment. The addition of entinostat to standard-of-care cytotoxic agents was in most instances no more effective than the cytotoxic agents used alone. Entinostat demonstrated target inhibition with increased histone 2A acetylation.
Oncogenic RAS signaling is an attractive target for fusion-negative rhabdomyosarcoma (FN-RMS). Our study validates the role of the ERK MAPK effector pathway in mediating RAS dependency in a panel of H/NRASQ61X mutant RMS cells and correlates in vivo efficacy of the MEK inhibitor trametinib with pharmacodynamics of ERK activity. A screen is used to identify trametinib-sensitizing targets, and combinations are evaluated in cells and tumor xenografts. We find that the ERK MAPK pathway is central to H/NRASQ61X dependency in RMS cells; however, there is poor in vivo response to clinically relevant exposures with trametinib, which correlates with inefficient suppression of ERK activity. CRISPR screening points to vertical inhibition of the RAF–MEK–ERK cascade by cosuppression of MEK and either CRAF or ERK. CRAF is central to rebound pathway activation following MEK or ERK inhibition. Concurrent CRAF suppression and MEK or ERK inhibition, or concurrent pan-RAF and MEK/ERK inhibition (pan-RAFi + MEKi/ERKi), or concurrent MEK and ERK inhibition (MEKi + ERKi) all synergistically block ERK activity and induce myogenic differentiation and apoptosis. In vivo assessment of pan-RAFi + ERKi or MEKi + ERKi potently suppress growth of H/NRASQ61X RMS tumor xenografts, with pan-RAFi + ERKi being more effective and better tolerated. We conclude that CRAF reactivation limits the activity of single-agent MEK/ERK inhibitors in FN-RMS. Vertical targeting of the RAF–MEK–ERK cascade and particularly cotargeting of CRAF and MEK or ERK, or the combination of pan-RAF inhibitors with MEK or ERK inhibitors, have synergistic activity and potently suppress H/NRASQ61X mutant RMS tumor growth.
P had no single-agent activity, although it marginally potentiated the activity of vincristine and cisplatin in one of three models studied. However, the addition of E necessitated dose reduction of each cytotoxic agent, abrogating the enhancement observed with P alone.
Background: VTP-50469 is a potent inhibitor of the menin-MLL1 interaction and is implicated in signaling downstream of EWSR1-FLI1.Procedure: VTP-50469 was evaluated against seven Ewing sarcoma (EwS) xenograft models and in vitro against EwS cell lines.Results: VTP-50469 showed limited antitumor activity, statistically significantly slowing tumor progression in four tumor models but with no evidence of tumor regression. In vitro, the IC 50 concentration was 10 nM for the mixed lineage leukemia (MLL)-rearranged leukemia cell line MV4;11, but > 3 M for EwS cell lines.
Conclusions:In contrast to its high level of activity against MLL1-rearranged leukemia xenografts, VTP-50469 shows little activity against EwS models.
K E Y W O R D SEwing sarcoma, pediatric oncology, xenograft 1 of 4 wileyonlinelibrary.com/journal/pbc https://doi.
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