Pharmacologic Targeting of LIN28/Let-7 in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a genetically heterogeneous malignancy with high relapse rates and mortality due to the outgrowth of chemotherapy-resistant leukemic stem cells (LSCs). Thus, the development of novel therapeutic strategies capable of eradicating human AML represents a major area of unmet medical need. The RNA binding protein, LIN28, is a known driver of many cancer stem cells, AML included, wherein overexpression of LIN28 correlates with reduced patient survival. LIN28 blocks the function of the let-7 microRNA family, which exert tumor suppressive effects by repressing oncogenes and cell cycle regulators including MYC, RAS and CyclinD. Thus, LIN28 is an attractive mechanistic target for the purpose of inhibiting AML LSCs. Using a targeted high-throughput screen, we identified a class of small molecules which selectively block the LIN28/let-7 interaction (Roos et al., ACS Chem Biol, 2016). Preliminary studies demonstrate that a lead small molecule markedly impairs the proliferation and clonogenic capacity of human AML cell lines and primary patient AML samples. In vivo, systemic administration of a lead small molecule LIN28/let-7 inhibitor decreases leukemic tumor burden, reduces LSC numbers and significantly improves animal survival. Mechanistic studies revealed that targeted inhibition of the LIN28/let-7 axis restores let-7 microRNA levels in AML LSCs and subsequently inhibits a panoply of key oncogenic driver genes, including the NF-ĸB pathway, a hallmark for LSC proliferation. Furthermore, AML cell lines and primary patient cells treated with the LIN28/let-7 small molecule inhibitor showed a block at the G1/S phase interface and significantly decreased cell cycle progression. Consequently, LIN28/let-7 inhibition leads to LSC differentiation and ultimately leukemic cell death. In summary, we demonstrate for the first time the drugability of the LIN28/let-7 axis in vivo and reveal a novel pharmacological means to suppress a multitude of oncogenic driver genes in human AML. These results suggest that small molecule inhibition of LIN28/let-7 has high therapeutic potential as a new class of targeted therapies for AML.