BI2536

BI 2536 given i.v. once or twice per week is highly efficacious in diverse xenograft models with acceptable tolerability by inhibiting cell proliferation through a mitotic arrest, and subsequently induction of tumor-cell death. Administration of BI 2536 at 50 mg/kg once or twice per week significantly inhibits growth of HCT 116 xenografts with T/C of 15% and 0.3%, respectively. BI 2536 treatment twice-weekly also leads to excellent tumor-growth in BxPC-3 and A549 models with T/C of 5% and 14%, respectively.

BI 2536 blocks the activities of PLK2 and PLK3 with IC₅₀s of 3.5 nM and 9.0 nM, respectively. In HeLa cells, BI 2536 treatment ranging from 10-100 nM leads to the blocking of the recruitment of γ-tubulin and phosphorylation of APC6 at mitotic centrosomes, inhibition of cohesin release from chromosome arms, induction of monopolar spindles, as well as a range of other mitotic processes that are known to depend on PLK1. BI 2536 treatment leads to HeLa cell arrest in G2/M, subsequently a sub-G1 DNA peak indicative of DNA breakdown and apoptosis, and accumulated cleaved PARP p85 fragments in a concentration-dependent manner. BI 2536 inhibits the growth of a panel of 32 human cancer cell lines with EC₅₀s of 2-25 nM, while blocking the proliferation of exponentially growing hTERT-RPE1, human umbilical vein endothelial cells (HUVECs), and normal rat kidney (NRK) cells with EC₅₀s of 12-31 nM.  PLK1 inhibition by BI 2536 reduces the growth and viability of anaplastic thyroid carcinoma (ATC) cells such as CAL62, OCUT-1, SW1736, 8505C, and ACT-1 with EC₅₀ values of 1.4-5.6 nM. 

BI-2356 has been subject to kinome profiling (doi:10.1038/nbt.1990). Although it is active against >20 kinases at 1 uM, it is potent and selective for PLK1, PLK2, and to a lesser extent PLK3 at very low nM doses. CAMKK1/2 and RPS6KA4 (Kin.Dom.2-C-terminal) would be off-targets in the ~20 nM and 12 nM range, respectively.

BI2536 is a potent inhibitor of the PLK kinase family (PLK1/2/3 IC₅₀ 0.83/3.5/9.0 nM). Its selectivity was excellent in a protein kinase panel of modest size (63 other kinases). PLK4 was not a member of that panel. BI2536 is cell-permeable, causing HeLa cell-cycle block in G2 or mitosis at 10-50 nM and inhibiting a range of other cancer cell proliferation at 1-100 nM EC₅₀. It was effective in a mouse xenograft model intravenously dosed (40-50 mg/kg weekly or bi-weekly). This compound is not a pure PLK chemical probe; it is also a potent inhibitor of the BET family bromodomains (BRD4-BD1 IC₅₀ 25 nM by AlphaScreen, Kd 37 nM by ITC, IC₅₀ 100 nM vs BRD4 in a FRAP cellular assay) and has a polypharmacological profile (doi:10.1021/cb500072z, doi:10.1038/nchembio.1471) For that reason, I would not choose this compound as a selective probe for the PLK family.

BI2536 is a reasonably selective PLK inhibitor (PLK1/2/3) but BI2536 also targets BET bromodomains (10.1021/acsmedchemlett.5b00084). Despite being a PLK1 inhibitor that induces all the hallmarks of PLK1 inhibition, any observed cellular phenotypes should be interpreted with caution. BI2536 is considered a compound with polypharmacological activity; it has been superseded by Volasertib (BI6727).