Hedgehog Pathway Inhibitors: A New Therapeutic Class for the Treatment of Acute Myeloid Leukemia

Targeting Hedgehog (Hh) pathway components, such as Smoothened (SMO), is a developing strategy for the treatment of acute myeloid leukemia (AML) and for overcoming relapsed/refractory forms of this disease. Several SMO inhibitors are in clinical development for the treatment of various tumor types and the results from some clinical trials in AML have been reported. This review will discuss the role of Hh signaling in AML pathogenesis, describe the preclinical and clinical development of Hh pathway inhibitors for the treatment of AML, and examine the current evidence on Hh pathway inhibitor resistance and the implications for treatment selection in AML. Significance: In acute myeloid leukemia (AML), components of the Hedgehog (Hh) signaling pathway, such as Smoothened (SMO), have been implicated in the development, maintenance, and expansion of leukemic stem cells (LSC), as well as sensitization to chemotherapy and the development of drug resistance in AML. Observations in preclinical studies of AML, as well as from samples of patients with AML, demonstrate that Hh pathway inhibitors act primarily on the stem cell pathway as differentiation agents. The current data for hematologic malignancies indicate the potential for a synergistic effect when a Hh pathway inhibitor is administered in combination with chemotherapy or investigational agents. It is thought that Hh pathway inhibitors act as agents that reduce LSC dormancy and promote LSC differentiation, thus the newly dividing LSCs can then be targeted by other chemotherapeutic drugs.