Epigenetic pathways can regulate gene expression by controlling and interpreting chromatin modifications. Cancer cells are characterized by altered epigenetic landscapes, and commonly exploit the chromatin regulatory machinery to enforce oncogenic gene expression programs1. Although chromatin alterations are, in principle, reversible and often amenable to drug intervention, the promise of targeting such pathways therapeutically has been limited by an incomplete understanding of cancer-specific dependencies on epigenetic regulators. Here we describe a non-biased approach to probe epigenetic vulnerabilities in acute myeloid leukaemia (AML), an aggressive haematopoietic malignancy that is often associated with aberrant chromatin states2. By screening a custom library of small hairpin RNAs (shRNAs) targeting known chromatin regulators in a genetically defined AML mouse model, we identify the protein bromodomain-containing 4 (Brd4) as being critically required for disease maintenance. Suppression of Brd4 using shRNAs or the small-molecule inhibitor JQ1 led to robust antileukaemic effects in vitro and in vivo, accompanied by terminal myeloid differentiation and elimination of leukaemia stem cells. Similar sensitivities were observed in a variety of human AML cell lines and primary patient samples, revealing that JQ1 has broad activity in diverse AML subtypes. The effects of Brd4 suppression are, at least in part, due to its role in sustaining Myc expression to promote aberrant self-renewal, which implicates JQ1 as a pharmacological means to suppress MYC in cancer. Our results establish small-molecule inhibition of Brd4 as a promising therapeutic strategy in AML and, potentially, other cancers, and highlight the utility of RNA interference (RNAi) screening for revealing epigenetic vulnerabilities that can be exploited for direct pharmacological intervention.

Cure rates in pediatric acute myeloid leukemia (AML) remain suboptimal. Overexpression of the surface receptor CXCR4 is associated with poor outcome in acute lymphoblastic leukemia (ALL) and AML. Certain non-chemotherapeutic agents have been shown to modulate CXCR4 expression and alter leukemia interactions with stromal cells in the bone marrow microenvironment. Because chemotherapy is the mainstay of AML treatment, we hypothesized that standard cytotoxic chemotherapeutic agents induce dynamic changes in leukemia surface CXCR4 expression, and that chemotherapy-induced upregulation of CXCR4 represents a mechanism of acquired chemotherapy resistance. Here, we show that cell lines variably upregulate CXCR4 with chemotherapy treatment. Those that showed upregulation were differentially protected from chemotherapy-induced apoptosis when co-cultured with stroma. We further explored the functional effects of chemotherapy-induced CXCR4 upregulation in an AML cell line (MOLM-14, which consistently upregulated CXCR4) and primary samples. We found enhanced stromal-cell derived factor-1α (SDF-1α)-mediated chemotaxis and stromal protection from additional chemotherapy-induced apoptosis. Further, treatment with the CXCR4 inhibitor plerixafor preferentially decreased stromal protection in cells with higher chemotherapy-induced upregulation of surface CXCR4. Upregulation of surface CXCR4 by standard chemotherapy may represent a mechanism of chemotherapy resistance in pediatric AML and may be a biomarker that can identify optimal patients for CXCR4 inhibition.

Background Ruxolitinib, an orally bioavailable JAK1/JAK2 inhibitor, may treat cancers with CRLF2 and/or JAK pathway mutations. Procedure A phase 1 trial of ruxolitinib was performed to determine the maximum-tolerated or recommended phase 2 dose, dose-limiting toxicities (DLTs), pharmacokinetics (PK), and pharmacodynamics (PD) in children with recurrent/refractory solid tumors (STs). Ruxolitinib was administered twice daily (BID) in 28-day cycles at five dose levels (15, 21, 29, 39, and 50 mg/m2/dose). PK and PD studies were performed during Cycle 1. Toxicity, preliminary efficacy, and PK/PD were also assessed in children with relapsed/refractory hematologic malignancies (HMs). Results Forty-nine patients were enrolled, 28 with STs (dose escalation cohort) and 21 with HMs. Ruxolitinib was well-tolerated with one DLT per cohort of 6 patients at dose levels (DLs) 2-5. One patient with a ST had grade 5 multi-organ failure at DL2. One patient each at DL3 and DL4 had a grade 4 neutropenia, and one patient at DL5 had a grade 4 creatinine phosphokinase elevation. No objective responses were observed in patients with STs. One patient with polycythemia vera achieved a partial response and received 18 cycles of ruxolitinib. The PK of ruxolitinib were similar to that in adults. Partial inhibition of phosphorylated JAK2, STAT5, and S6 was observed in in vitro plasma inhibitory activity PD assay. Conclusion Ruxolitinib was well-tolerated in children with refractory cancer. The recommended phase 2 dose for continuous BID oral administration is 50 mg/m2/dose. Subsequent evaluation of ruxolitinib in combination with cytotoxic chemotherapy in children, adolescents, and young adults with JAK-mutant leukemias is planned.