Nonhematopoietic cells represent a more rational target of in vivo hedgehog signaling affecting normal or acute myeloid leukemia progenitors

Niche-directed therapy in acute myeloid leukemia: optimization of stem cell competition for niche occupancy

Acute myeloid leukemia (AML) is an aggressive malignancy of stem cell origin that contributes to significant morbidity and mortality. The long-term prognosis remains dismal given the high likelihood for primary refractory or relapsed disease. An essential component of relapse is resurgence from the bone marrow. To date, the murine hematopoietic stem cell (HSC) niche has been clearly defined, but the human HSC niche is less well understood. The design of niche-based targeted therapies for AML must account for which cellular subsets compete for stem cell occupancy within respective bone marrow microenvironments. In this review, we highlight the principles of stem cell niche biology and discuss translational insights into the AML microenvironment as of 2021. Optimization of competition for niche occupancy is important for the elimination of measurable residual disease (MRD). Some of these novel therapeutics are in the pharmacologic pipeline for AML and may be especially useful in the setting of MRD.

New Therapeutic Approaches for Acute Myeloid Leukaemia

Acute myeloid leukaemia (AML) is a genetically heterogeneous haematopoietic neoplasm characterised by the accumulation of transformed immature blood progenitors in bone marrow. Since 1973, the backbone treatment has relied on the combination of cytarabine and an anthracycline, followed by allogeneic haematopoietic transplant if eligible. Therefore, the treatment decisions have largely revolved around chemotherapy drug intensity. Despite advances in our understanding of the underlying biology over the past decades, AML remains a therapeutic challenge as the overall survival is poor and treatment options are limited for relapsed/refractory AML or for unfit patients. After four decades without substantial changes, eight new noncytostatic drugs have been granted approval: vyxeos, enasidenib, gilteritinib, glasdegib, gemtuzumab ozogamicin, ivosidenib, midostaurin, and venetoclax. Despite promising preliminary results, some indications are based on early efficacy data, obtained in single-arm nonrandomised trials, highlighting the necessity for further validation in extended clinical trials. Interestingly, several druggable targets have been identified recently, associated with specific target-directed drugs. Based on the preclinical data available, great impact on clinical outcomes for patients with AML is expected, potentially increasing the therapeutic landscape for this disease.

Cell signaling pathways as molecular targets to eliminate AML stem cells

Acute myeloid leukemia (AML) remains the most lethal of leukemias and a small population of cells called leukemic stem cells (LSCs) has been associated with disease relapses. Some cell signaling pathways play an important role in AML survival, proliferation and self-renewal properties and are abnormally activated or suppressed in LSCs. This includes the NF-κB, Wnt/β-catenin, Hedgehog, Notch, EGFR, JAK/STAT, PI3K/AKT/mTOR, TGF/SMAD and PPAR pathways. This review aimed to discuss these pathways as molecular targets for eliminating AML LSCs. Herein, inhibitors/activators of these pathways were summarized as a potential new anti-AML therapy capable of eliminating LSCs to guide future researches. The clinical use of cell signaling pathways data can be useful to enhance the anti-AML therapy.

Glasdegib in the treatment of acute myeloid leukemia

Pharmacologic inhibition of the Hedgehog pathway significantly enhanced the sensitivity of leukemic cells to cytotoxic drugs. Glasdegib (PF-04449913; DAURISMO™) is a potent and selective oral inhibitor of the Hedgehog signaling pathway with clinical activity in patients with acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS), particularly in combination with chemotherapy. The results of Phase Ib/II studies evaluating safety and efficacy of glasdegib combined with chemotherapy in previously untreated patients with AML or high-risk myelodysplastic syndrome have recently been published. In the BRIGHT AML 1003 study, glasdegib in combination with low-dose cytarabine (LDAC) was well tolerated and demonstrated a significant 54% reduction in mortality compared with LDAC for AML patients. In 2018, the US FDA approved glasdegib in combination with LDAC for the treatment of newly diagnosed patients with AML who are 75 years old or older or who have co-morbidities that preclude use of intensive induction chemotherapy.

Hedgehog pathway inhibition as a therapeutic target in acute myeloid leukemia

Introduction: The Hedgehog (HH) pathway constitutes a collection of signaling molecules which critically influence embryogenesis. In adults, however, the HH pathway remains integral to the proliferation, maintenance, and apoptosis of adult stem cells including hematopoietic stem cells. Areas covered: We discuss the current understanding of the HH pathway as it relates to normal hematopoiesis, the pathology of acute myeloid leukemia (AML), the rationale for and data from combination therapies including HH pathway inhibitors, and ultimately the prospects that might offer promise in targeting this pathway in AML. Expert opinion: Efforts to target the HH pathway have been focused on impeding this disposition and restoring chemosensitivity to conventional myeloid neoplasm therapies. The year 2018 saw the first approval of a HH pathway inhibitor (glasdegib) for AML, though for an older population and in combination with an uncommonly-used therapy. Several other clinical trials with agents targeting modulators of HH signaling in AML and MDS are underway. Further study and understanding of the interplay between the numerous aspects of HH signaling and how it relates to the augmented survival of AML will provide a more reliable substrate for therapeutic strategies in patients with this poor-risk disease.

Conditional deletion of Ahr alters gene expression profiles in hematopoietic stem cells

The aryl hydrocarbon receptor (AHR) is a ligand activated bHLH transcription factor that belongs to the Per-Arnt-Sim (PAS) superfamily of proteins involved in mediating responses to cellular environment regulating normal physiological and developmental pathways. The AHR binds a broad range of naturally derived and synthetic compounds, and plays a major role in mediating effects of certain environmental chemicals. Although our understanding of the physiological roles of the AHR in the immune system is evolving, there is little known about its role in hematopoiesis and hematopoietic diseases. Prior studies demonstrated that AHR null (AHR-KO) mice have impaired hematopoietic stem cell (HSC) function; they develop myeloproliferative changes in peripheral blood cells, and alterations in hematopoietic stem and progenitor cell populations in the bone marrow. We hypothesized mice lacking AHR expression only within hematopoietic cells (AHRVav1 mice) would develop similar changes. However, we did not observe a complete phenocopy of AHR-KO and AHRVav1 animals at 2 or 18 months of age. To illuminate the signaling mechanisms underlying the alterations in hematopoiesis observed in these mice, we sorted a population of cells highly enriched for HSC function (LSK cells: CD34-CD48-CD150+) and performed microarray analyses. Ingenuity Pathway and Gene Set Enrichment Analyses revealed that that loss of AHR within HSCs alters several gene and signaling networks important for HSC function. Differences in gene expression networks among HSCs from AHR-KO and AHRVav1 mice suggest that AHR in bone marrow stromal cells also contributes to HSC function. In addition, numerous studies have suggested a role for AHR in both regulation of hematopoietic cells, and in the development of blood diseases. More work is needed to define what these signals are, and how they act upon HSCs.

Glasdegib in combination with cytarabine and daunorubicin in patients with AML or high-risk MDS: Phase 2 study results

Glasdegib is a Hedgehog pathway inhibitor. This ongoing, open‐label, phase 2 study (NCT01546038) evaluated glasdegib plus cytarabine/daunorubicin in patients with untreated acute myeloid leukemia (AML) or high‐risk myelodysplastic syndromes (MDS). Patients received glasdegib 100 mg orally, once daily in continuous 28‐day cycles from day −3, with intravenous cytarabine 100 mg/m² on days 1‐7 and daunorubicin 60 mg/m² on days 1‐3. Patients in remission then received consolidation therapy (2‐4 cycles of cytarabine 1 g/m² twice daily on days 1, 3, 5 of each cycle), followed by maintenance glasdegib (maximum 6 cycles). Primary endpoint was complete remission (CR) in patients aged ≥55 years. Secondary endpoints included overall survival (OS), safety and outcome by mutational status. Patients had a median (range) age of 64.0 (27‐75) years, 60.0% were male, and 84.5% were white. In 69 evaluable patients, 46.4% (80% confidence interval [CI]: 38.7‐54.1) achieved investigator‐reported CR. Among patients ≥55 years old (n = 60), 40.0% (80% CI 31.9‐48.1) achieved CR. Among all 69 patients, median OS was 14.9 (80% CI 13.4‐19.3) months, with 12‐month survival probability 66.6% (80% CI 58.5‐73.4). The most common treatment‐related adverse events (≥50% patients) were diarrhea and nausea. There were no significant associations between mutational status (12 genes) and clinical response, suggesting potential benefit across diverse molecular profiles. Glasdegib plus cytarabine/daunorubicin was well tolerated and associated with clinical activity in patients with untreated AML or high‐risk MDS. A randomized phase 3 trial of glasdegib in combination with chemotherapy (7 + 3 schedule) is ongoing.

Acute myeloid leukaemia disrupts endogenous myelo-erythropoiesis by compromising the adipocyte bone marrow niche

Acute myeloid leukaemia (AML) is distinguished by the generation of dysfunctional leukaemic blasts, and patients characteristically suffer from fatal infections and anaemia due to insufficient normal myelo-erythropoiesis. Direct physical crowding of bone marrow (BM) by accumulating leukaemic cells does not fully account for this haematopoietic failure. Here, analyses from AML patients were applied to both in vitro co-culture platforms and in vivo xenograft modelling, revealing that human AML disease specifically disrupts the adipocytic niche in BM. Leukaemic suppression of BM adipocytes led to imbalanced regulation of endogenous haematopoietic stem and progenitor cells, resulting in impaired myelo-erythroid maturation. In vivo administration of PPARγ agonists induced BM adipogenesis, which rescued healthy haematopoietic maturation while repressing leukaemic growth. Our study identifies a previously unappreciated axis between BM adipogenesis and normal myelo-erythroid maturation that is therapeutically accessible to improve symptoms of BM failure in AML via non-cell autonomous targeting of the niche.

GLI3 repressor determines Hedgehog pathway activation and is required for response to SMO antagonist glasdegib in AML

The Hedgehog (Hh) signaling pathway is activated in many cancers and is a promising target for therapeutic development. Deletions in the receptor Patched (PTCH) or activating mutations in Smoothened (SMO) have been reported in basal cell carcinoma and medulloblastoma, but are largely absent in most tumor types. Therefore, the mechanism of pathway activation in most cancers, including hematological malignancies, remains unknown. In normal tissues, Hh pathway activation via PTCH/SMO causes an increase in the downstream transcriptional activator GLI1 and a decrease in the GLI3 transcriptional repressor (GLI3R). In this article, we confirm that the Hh pathway is active in acute myeloid leukemia (AML), however, this activity is largely independent of SMO. Epigenetic and gene expression analysis of The Cancer Genome Atlas AML data set reveals that GLI3 expression is silenced in most AML patient samples, and the GLI3 locus is abnormally methylated. We show that GLI3R is required for the therapeutic effect of SMO antagonists in AML samples and restoration of GLI3R suppresses the growth of AML. We additionally demonstrate that GLI3R represses AML growth by downregulating AKT expression. In summary, this study provides the first evidence that GLI3R plays an essential role in SMO-independent Hh signaling in AML, and suggests that GLI3R could serve as a potential biomarker for patient selection in SMO antagonist clinical trials. Furthermore, these data support rational combinations of hypomethylating agents with SMO antagonists in clinical trials.

Small-molecule Hedgehog inhibitor attenuates the leukemia-initiation potential of acute myeloid leukemia cells

Aberrant activation of the Hedgehog signaling pathway has been implicated in the maintenance of leukemia stem cell populations in several model systems. PF‐04449913 (PF‐913) is a selective, small‐molecule inhibitor of Smoothened, a membrane protein that regulates the Hedgehog pathway. However, details of the proof‐of‐concept and mechanism of action of PF‐913 following administration to patients with acute myeloid leukemia (AML) are unclear. This study examined the role of the Hedgehog signaling pathway in AML cells, and evaluated the in vitro and in vivo effects of the Smoothened inhibitor PF‐913. In primary AML cells, activation of the Hedgehog signaling pathway was more pronounced in CD34⁺ cells than CD34⁻ cells. In vitro treatment with PF‐913 induced a decrease in the quiescent cell population accompanied by minimal cell death. In vivo treatment with PF‐913 attenuated the leukemia‐initiation potential of AML cells in a serial transplantation mouse model, while limiting reduction of tumor burden in a primary xenotransplant system. Comprehensive gene set enrichment analysis revealed that PF‐913 modulated self‐renewal signatures and cell cycle progression. Furthermore, PF‐913 sensitized AML cells to cytosine arabinoside, and abrogated resistance to cytosine arabinoside in AML cells cocultured with HS‐5 stromal cells. These findings imply that pharmacologic inhibition of Hedgehog signaling attenuates the leukemia‐initiation potential, and also enhanced AML therapy by sensitizing dormant leukemia stem cells to chemotherapy and overcoming resistance in the bone marrow microenvironment.

Non-Canonical Hh Signaling in Cancer-Current Understanding and Future Directions

As a major regulatory pathway for embryonic development and tissue patterning, hedgehog signaling is not active in most adult tissues, but is reactivated in a number of human cancer types. A major milestone in hedgehog signaling in cancer is the Food and Drug Administration (FDA) approval of a smoothened inhibitor Vismodegib for treatment of basal cell carcinomas. Vismodegib can block ligand-mediated hedgehog signaling, but numerous additional clinical trials have failed to show significant improvements in cancer patients. Amounting evidence indicate that ligand-independent hedgehog signaling plays an essential role in cancer. Ligand-independent hedgehog signaling, also named non-canonical hedgehog signaling, generally is not sensitive to smoothened inhibitors. What we know about non-canonical hedgehog signaling in cancer, and how should we prevent its activation? In this review, we will summarize recent development of non-canonical hedgehog signaling in cancer, and will discuss potential ways to prevent this type of hedgehog signaling.

Deficiency in Aryl Hydrocarbon Receptor (AHR) Expression throughout Aging Alters Gene Expression Profiles in Murine Long-Term Hematopoietic Stem Cells

Dysregulation of hematopoietic stem cell (HSC) signaling can contribute to the development of diseases of the blood system. Lack of aryl hydrocarbon receptor (AhR) has been associated with alterations in gene expression related to HSC function and the subsequent development of a myeloproliferative disorder in aging female mice. We sorted the most primitive population of HSCs with the highest stem cell potential (Long-term, or LT-HSCs) from 18-month-old AhR-null-allele (AhR-KO) and WT mice and analyzed gene expression using microarray to determine alterations in gene expression and cell signaling networks in HSCs that could potentially contribute to the aging phenotype of AhR-KO mice. Comparisons with previous array data from 8-week old mice indicated that aging alone is sufficient to alter gene expression. In addition, a significant number of gene expression differences were observed in aged LT-HSCs that are dependent on both aging and lack of AhR. Pathway analysis of these genes revealed networks related to hematopoietic stem cell activity or function. qPCR was used to confirm the differential expression of a subset of these genes, focusing on genes that may represent novel AhR targets due to the presence of a putative AhR binding site in their upstream regulatory region. We verified differential expression of PDGF-D, Smo, Wdfy1, Zbtb37 and Zfp382. Pathway analysis of this subset of genes revealed overlap between cellular functions of the novel AhR targets and AhR itself. Lentiviral-mediated knockdown of AhR in lineage-negative hematopoietic cells was sufficient to induce changes in all five of the candidate AhR targets identified. Taken together, these data suggest a role for AhR in HSC functional regulation, and identify novel HSC AhR target genes that may contribute to the phenotypes observed in AhR-KO mice.

Acute Myelogenous Leukemia and its Microenvironment: A Molecular Conversation

Survival of patients with acute myelogenous leukemia (AML) depends on our ability to prevent relapse in patients that achieved complete remission after intensive chemotherapy. While studies focusing on the malignant clone brought many advances in understanding AML biology and chemoresistance, little improvement has been made in eliminating the last bastion of malignant cells, the minimal residual disease (MRD). Inspired by Sir Paget's "soil and seed" hypothesis, it is becoming more clear that there is constant feedback between the malignant clone and the leukemic microenvironment. This "molecular conversation" dictates AML behavior and holds the key to eliminating MRD. Here we review recent advances in our understanding of how leukemia cells modify their microenvironment and how these changes reinforce AML homeostasis. In addition, we outline current clinical and preclinical efforts to disrupt these interactions and to therapeutically target MRD.

The Hedgehog pathway: role in cell differentiation, polarity and proliferation

Hedgehog (Hh) is first described as a genetic mutation that has "spiked" phenotype in the cuticles of Drosophila in later 1970s. Since then, Hh signaling has been implicated in regulation of differentiation, proliferation, tissue polarity, stem cell population and carcinogenesis. The first link of Hh signaling to cancer was established through discovery of genetic mutations of Hh receptor gene PTCH1 being responsible for Gorlin syndrome in 1996. It was later shown that Hh signaling is associated with many types of cancer, including skin, leukemia, lung, brain and gastrointestinal cancers. Another important milestone for the Hh research field is the FDA approval for the clinical use of Hh inhibitor Erivedge/Vismodegib for treatment of locally advanced and metastatic basal cell carcinomas. However, recent clinical trials of Hh signaling inhibitors in pancreatic, colon and ovarian cancer all failed, indicating a real need for further understanding of Hh signaling in cancer. In this review, we will summarize recent progress in the Hh signaling mechanism and its role in human cancer.

Targeting hedgehog signaling in myelofibrosis and other hematologic malignancies

Treatment of myelofibrosis (MF), a BCR-ABL-negative myeloproliferative neoplasm, is challenging. The only current potentially curative option, allogeneic hematopoietic stem cell transplant, is recommended for few patients. The remaining patients are treated with palliative therapies to manage MF-related anemia and splenomegaly. Identification of a mutation in the Janus kinase 2 (JAK2) gene (JAK2 V617F) in more than half of all patients with MF has prompted the discovery and clinical development of inhibitors that target JAK2. Although treatment with JAK2 inhibitors has been shown to improve symptom response and quality of life in patients with MF, these drugs do not alter the underlying disease; therefore, novel therapies are needed. The hedgehog (Hh) signaling pathway has been shown to play a role in normal hematopoiesis and in the tumorigenesis of hematologic malignancies. Moreover, inhibitors of the Hh pathway have been shown to inhibit growth and self-renewal capacity in preclinical models of MF. In a mouse model of MF, combined inhibition of the Hh and JAK pathways reduced JAK2 mutant allele burden, reduced bone marrow fibrosis, and reduced white blood cell and platelet counts. Preliminary clinical data also suggest that inhibition of the Hh pathway, alone or in combination with JAK2 inhibition, may enable disease modification in patients with MF. Future studies, including one combining the Hh pathway inhibitor sonidegib and the JAK2 inhibitor ruxolitinib, are underway in patients with MF and will inform whether this combination approach can lead to true disease modification.

Targeting hedgehog signaling in cancer: research and clinical developments

Since its first description in Drosophila by Drs Nusslein-Volhard and Wieschaus in 1980, hedgehog (Hh) signaling has been implicated in regulation of cell differentiation, proliferation, tissue polarity, stem cell maintenance, and carcinogenesis. The first link of Hh signaling to cancer was established through studies of Gorlin syndrome in 1996 by two independent teams. Later, it was shown that Hh signaling may be involved in many types of cancer, including skin, leukemia, lung, brain, and gastrointestinal cancers. In early 2012, the US Food and Drug Administration approved the clinical use of Hh inhibitor Erivedge/vismodegib for treatment of locally advanced and metastatic basal cell carcinomas. With further investigation, it is possible to see more clinical applications of Hh signaling inhibitors. In this review, we will summarize major advances in the last 3 years in our understanding of Hh signaling activation in human cancer, and recent developments in preclinical and clinical studies using Hh signaling inhibitors.