Targeting Polo-like Kinase in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) carrying fms-like tyrosine kinase 3 (FLT3)-internal tandem duplication (ITD) is associated with poor prognosis when treated with conventional chemotherapy and allogeneic hematopoietic stem cell transplantation (HSCT). Despite much interest in tyrosine kinase inhibitor (TKI) targeting FLT3 activation, drug resistance is invariable and acquisition of secondary point mutation in the tyrosine kinase domain (TKD) of FLT3 was frequently reported. We have shown that genes encoding cell cycle regulators including polo-like kinase 1 (PLK1), cell division cycle 25 homolog A (CDC25A), cyclin B2 (CCNB2), and cyclin E1 (CCNE1) were up-regulated in sorafenib-resistant primary AML samples. In particular, PLK1, a member of the polo-like kinase family, has been found to express at several checkpoints critical for cell cycle progression. PLK1 inhibitors have recently been exploited for the treatment of both solid organ and haematological cancers. We hypothesized that aberrant cell cycle progression mediated by increased PLK1 expression might confer survival advantage to drug resistant AML cells and be targetable by PLK1 inhibition. In vitro treatment with PLK1 inhibitors volasertib and BI 2536 significantly inhibited the growth of 8 AML cell lines (KG-1, ML2, MOLM-13, MV4-11, Kasumi-1, NB4, THP-1 and OCI-AML3) with IC₅₀ (all in nM) ranging from 48.7 - 98.4 (volasertib) and 35.1 - 81.6 (BI 2536). The growth inhibitory effects of PLK1 inhibition on two FLT3-ITD+ cell lines, MOLM-13 and MV411, correlated with induction of apoptosis and cell cycle arrest at G2/M phase. Moreover, both PLK1 inhibitors significantly suppressed the growth of a sorafenib resistant MOLM-13R cell line and sorafenib naïve MOLM-13N cell line. Introduction of FLT3-ITD alone or FLT3-ITD and TKD double mutations into Ba/F3 cell lines sensitized them to the growth inhibitory effects of PLK1 inhibitors. Primary FLT3-ITD+ AML cells obtained from patients at TKI resistance were shown to be more sensitive to PLK1 inhibitors than those obtained before treatment. The results suggested that the TKI resistant clones could be effectively targeted by PLK1 inhibition, providing an insight to the design of combination treatment with FLT3 inhibitors in FLT3-ITD+ AML.