SUMMARY Post-transcriptional adenosine-to-inosine RNA editing mediated by adenosine deaminase acting on RNA1 (ADAR1) promotes cancer progression and therapeutic resistance. However, ADAR1 editase-dependent mechanisms governing leukemia stem cell (LSC) generation have not been elucidated. In blast crisis chronic myeloid leukemia (BC CML), we show that increased JAK2 signaling and BCR-ABL1 amplification activate ADAR1. In a humanized BC CML mouse model, combined JAK2 and BCR-ABL1 inhibition prevents LSC self-renewal commensurate with ADAR1 downregulation. Lentiviral ADAR1 wild-type, but not an editing-defective ADAR1E912 mutant, induces self-renewal gene expression and impairs biogenesis of stem cell regulatory let-7 microRNAs. Combined RNA sequencing, qRT-PCR, CLIP-ADAR1, and pri-let-7 mutagenesis data suggest that ADAR1 promotes LSC generation via let-7 pri-microRNA editing and LIN28B upregulation. A small molecule tool compound antagonizes ADAR1’s effect on LSC self-renewal in stromal co-cultures and restores let-7 biogenesis. Thus, ADAR1 activation represents a unique therapeutic vulnerability in LSC with active JAK2 signaling.

BackgroundDormant leukemia stem cells (LSC) promote therapeutic resistance and leukemic progression as a result of unbridled activation of stem cell gene expression programs. Thus, we hypothesized that 1) deregulation of the hedgehog (Hh) stem cell self-renewal and cell cycle regulatory pathway would promote dormant human LSC generation and 2) that PF-04449913, a clinical antagonist of the GLI2 transcriptional activator, smoothened (SMO), would enhance dormant human LSC eradication.MethodsTo test these postulates, whole transcriptome RNA sequencing (RNA-seq), microarray, qRT-PCR, stromal co-culture, confocal fluorescence microscopic, nanoproteomic, serial transplantation and cell cycle analyses were performed on FACS purified normal, chronic phase (CP) chronic myeloid leukemia (CML), blast crisis (BC) phase CML progenitors with or without PF-04449913 treatment.ResultsNotably, RNA-seq analyses revealed that Hh pathway and cell cycle regulatory gene overexpression correlated with leukemic progression. While lentivirally enforced GLI2 expression enhanced leukemic progenitor dormancy in stromal co-cultures, this was not observed with a mutant GLI2 lacking a transactivation domain, suggesting that GLI2 expression prevented cell cycle transit. Selective SMO inhibition with PF-04449913 in humanized stromal co-cultures and LSC xenografts reduced downstream GLI2 protein and cell cycle regulatory gene expression. Moreover, SMO inhibition enhanced cell cycle transit and sensitized BC LSC to tyrosine kinase inhibition in vivo at doses that spare normal HSC.ConclusionIn summary, while GLI2, forms part of a core HH pathway transcriptional regulatory network that promotes human myeloid leukemic progression and dormant LSC generation, selective inhibition with PF-04449913 reduces the dormant LSC burden thereby providing a strong rationale for clinical trials predicated on SMO inhibition in combination with TKIs or chemotherapeutic agents with the ultimate aim of obviating leukemic therapeutic resistance, persistence and progression.Electronic supplementary materialThe online version of this article (doi:10.1186/s12967-015-0453-9) contains supplementary material, which is available to authorized users.

Conditionally active COBRA™ (COnditional Bispecific Redirected Activation) T cell engagers are engineered to overcome the limitations of inherently active first-generation T cell engagers, which are unable to discern between tumor and healthy tissues. Designed to be administered as prodrugs, COBRAs target cell surface antigens upon administration, but engage T cells only after they are activated within the tumor microenvironment (TME). This allows COBRAs to be preferentially turned on in tumors while safely remaining inactive in healthy tissue. Here, we describe the development of the COBRA design and the characterization of these conditionally active T cell engagers. Upon administration COBRAs are engineered to bind to tumor-associated antigens (TAAs) and serum albumin (to extend their half-life in circulation), but are inhibited from interacting with the T cell receptor complex signaling molecule CD3. In the TME, a matrix metalloproteinase (MMP)-mediated linker cleavage event occurs within the COBRA construct, which rearranges the molecule, allowing it to co-engage TAAs and CD3, thereby activating T cells against the tumor. COBRAs are conditionally activated through cleavage with MMP9, and once active are highly potent, displaying sub-pM EC 50 s in T cell killing assays. Studies in tumor-bearing mice demonstrate COBRA administration completely regresses established solid tumor xenografts. These results strongly support the further characterization of the novel COBRA design in preclinical development studies.

T cells have a unique capability to eliminate cancer cells and fight malignancies. Cancer cells have adopted multiple immune evasion mechanisms aimed at inhibiting T cells. Dramatically improved patient outcomes have been achieved with therapies genetically reprogramming T cells, blocking T-cell inhibition by cancer cells, or transiently connecting T cells with cancer cells for redirected lysis. This last modality is based on antibody constructs that bind a surface antigen on cancer cells and an invariant component of the T-cell receptor. Although high response rates were observed with T-cell engagers specific for CD19, CD20, or BCMA in patients with hematologic cancers, the treatment of solid tumors has been less successful. Here, we developed and characterized a novel T-cell engager format, called TriTAC (for Trispecific T-cell Activating Construct). TriTACs are engineered with features to improve patient safety and solid tumor activity, including high stability, small size, flexible linkers, long serum half-life, and highly specific and potent redirected lysis. The present study establishes the structure/activity relationship of TriTACs and describes the development of HPN424, a PSMA- (FOLH1-) targeting TriTAC in clinical development for patients with metastatic castration-resistant prostate cancer.