Olaparib is an FDA-approved PARP inhibitor (PARPi) that has shown promise as a 
synthetic lethal treatment approach for BRCA-mutant castration-resistant 
prostate cancer (CRPC) in clinical use. However, emerging data have also shown 
that even BRCA-mutant cells may be resistant to PARPi. The mechanistic basis for 
these drug resistances is poorly understood. Polo-like kinase 1 (Plk1), a 
critical regulator of many cell-cycle events, is significantly elevated upon 
castration of mice carrying xenograft prostate tumors. Herein, by combination 
with Plk1 inhibitor BI2536, we show a robust sensitization of olaparib in 22RV1, 
a BRCA1-deficient CRPC cell line, as well as in CRPC xenograft tumors. 
Mechanistically, monotherapy with olaparib results in an override of the G1-S 
checkpoint, leading to high expression of Plk1, which attenuates olaparib's 
overall efficacy. In BRCA1 wild-type C4-2 cells, Plk1 inhibition also 
significantly increases the efficacy of olaparib in the presence of p53 
inhibitor. Collectively, our findings not only implicate the critical role of 
Plk1 in PARPi resistance in BRCA-mutant CRPC cells, but also shed new light on 
the treatment of non-BRCA-mutant patient subgroups who might also respond 
favorably to PARPi. Mol Cancer Ther; 16(3); 469-79. ©2017 AACR.