Discovery of novel (+)-Usnic acid derivatives as potential anti-leukemia agents with pan-Pim kinases inhibitory activity

Usnic acid (UA) is the main secondary metabolite isolated from lichens, with moderate anticancer activity. A small group of ( + )-UA derivatives characterized with flavanone moiety was designed and synthesized, and their anticancer activities were evaluated in leukemia cells. It was demonstrated that ( + )-UA derivatives 6a-6g inhibited the proliferation of leukemia cells HL-60 and K562 with low micromolar IC50 values. Mechanisms of action were investigated to find that 6g induced apoptosis in HL-60 and K562 cell lines, and affected the expression of MNK/eIF4E axis-related proteins, such as Mcl-1, p-eIF4E, p-4E-BP1. Finally, kinase inhibition assay suggested 6g was a potential inhibitor of pan-Pim kinases. Meanwhile, the blocking of phosphorylation of BAD and 4E-BP1 by 6g, together with the proposed binding mode of 6g with Pim-1 further confirmed its Pim inhibition effects. Our finding provides the sight towards the potential mechanism of (+ )-UA derivatives 6g as anti-leukemia agents.