(R)-MRT199665 is an isomer of MRT199665. MRT199665 is a potent and ATP-competitive, selective MARK / SIK / AMPK inhibitor with IC 50 s of 2/2/3/2 nM, 10/10 nM, and 110/12/43 nM for MARK1/MARK2/MARK3/MARK14 , AMPKα1/AMPKα2 , and SIK1/SIK2/SIK3 , respectively. MRT199665 causes apoptosis in MEF2C-activated human acute myeloid leukemias (AML) cells. MRT199665 inhibits the phosphorylation of SIK substrate CRTC3 at S370.