Cell experiment:

Leukemia cells or primary leukemia cells (2 × 104) are seeded in 96-well plates overnight and incubated with DMSO, or increasing concentrations of RO3280 (0.05-120 μM) for 24 h. The same volume of DMSO added to the vehicle treated wells. Each drug concentration is replicated four times. Then, 10 μL CCK8 solution is added to each well, incubated at 37℃ for 2-4 h and the optical density (OD) values are measured at 450 nm using a scanning multi-well spectrophotometer. Relative survival rate is calculated from the absorbance values compared with the control group. The proliferation of cells is calculated as a percentage of the DMSO-treated control wells with 50% inhibitory concentration (IC50) values derived after plotting proliferation values on a logarithmic curve. The IC50 of PLK1 inhibitor is calculated by Graph Prism software[1].

Animal experiment:

Briefly, mice (female, 4-5 weeks of age) are used in the assay. Cells (5 × 106 cells in 150 μL) are suspended in RPMI 1640 and injected subcutaneously into the flank of each BALB/c nude mouse. On day 5, tumour size is measured, the animals are randomized into two groups (n = 15 per group), and RO3280 (40 mg/kg, once every 5 days) treatment is initiated by intraperitoneal injection. The control group is treated with vehicle (1.5% DMSO in PBS). The drug (or vehicle) treatment is performed for 40 days. The length and width of the resulting tumours (in millimetres) are measured every 3 days with callipers. The tumour diameter is measured, and the volume (length × width2 × 0.52) is calculated. The mice are humanely killed on day 45, and the tumours are dissected and weighed. Western blot and immunohistochemistry assays are also performed with these sections. Then, the tumours are fixed, embedded and cut into 3‐μm‐ hick sections, which are subsequently stained with haematoxylin and eosin to permit the observation of the tumour margin[3].

Ro3280 is a selective inhibitor of PLK1 with IC50 value of 3 nM [1].

Polo-like kinase 1 (PLK1) is an enzyme and plays an important role in regulating cell cycles as an early trigger for G2/M phase transition. It has been shown that PLK1 is over-expressed in a variety of cancer cells and thus is regarded as a promising target for cancer drugs [2].

When tested with a panel of acute leukemia cell lines, Ro3280 showed inhibitory function on U937, HL 60, NB4, K562, MV4-11, and CCRF with IC50 value of 186 nM, 175 nM, 74 nM, 797 nM, 120 nM, and 162 nM, respectively. It was interesting to notice that ALL cells (35.49-110.76 nM) were more sensitive to Ro3280 compared with AML cells (IC50, 52.80-147.50 nM). Further, it was showed that Ro3280 treatment decreased cell viability, induced apoptosis and disrupted cell cycle [3]. In H82 (lung cancer), HT-29 (colon cancer), MDA-MB-468 (breast cancer), PC3 (prostate cancer), and A375 (cutaneum carcinoma), Ro3280 treatment inhibited PLK1 with the IC50 was 5 nM, 10 nM, 19 nM, 12 nM, and 70 nM, respectively [1].

In nude mice model with HT-29 cells subcutaneous xenograft, administration of Ro3280 caused significant anti-tumor activity with 78% reduction at the dose of 40 mg/kg, once a week and then completely regressed tumor when more drug and more frequent [1].

References:
[1].  Chen, S., et al., Identification of novel, potent and selective inhibitors of Polo-like kinase 1. Bioorg Med Chem Lett, 2012. 22(2): p. 1247-50.
[2].  Czaplinski, S., et al., Polo-like kinase 1 inhibition sensitizes neuroblastoma cells for vinca alkaloid-induced apoptosis. Oncotarget, 2015.
[3].  Wang, N.N., et al., Molecular targeting of the oncoprotein PLK1 in pediatric acute myeloid leukemia: RO3280, a novel PLK1 inhibitor, induces apoptosis in leukemia cells. Int J Mol Sci, 2015. 16(1): p. 1266-92.