Figure 2. Plk1 inhibitor-abiraterone synergy is independent of AR signaling, occurs in multiple prostate cancer cell lines, and is specific to Plk1 activity.

(A) C4-2 cells were grown in media containing FBS and subjected to increasing concentrations of abiraterone in the presence (red line) or absence (black line) of the Plk1 inhibitor BI2536 (3 nM) for 72 hours. Mean ± SEM (n = 3). Expected viability (dashed black line) was calculated according to the Bliss independence model of drug additivity. Synergy, depicted by the green area, is a decrease in viability beyond the expected additive effect.

(B) Comparison of the chemical structures of the antiandrogens abiraterone acetate, enzalutamide, galeterone, and orteronel.

(C) C4-2 cells were grown using csFBS and synergy between the antiandrogen enzalutamide and BI2536 (2.5 nM) was assessed and plotted as in (A).

(D-F) Synergy experiments between the antiandrogens orteronel, Galeterone, and ARV-110, and BI2536 (6 nM) performed and plotted as in (A).

(G) AR protein immunoblot of lysates from C4-2 cells treated with ARV-110 for 24 hours. The highest dose was 100 nM followed by four 1.5-fold serial dilutions and are the same doses used in (F).

(H) AR protein immunoblot of lysates from the indicated cell. The lower bands are AR splice variants, including AR-v7.

(I-J) LNCaP95 and 22Rv1 AR-v7+ CRPC, and DU145 and PC-3 AR-negative prostate cancer cells were assessed for synergy between abiraterone and BI2536 (4, 3, 5, and 7.5 nM, respectively) as in (A).

(K) C4-2 cells were assessed for synergy between abiraterone and BI2536 (4 nM) in the absence (top) or presence (bottom) of ARV-110 as in (A).

(L) Chemical structures of the panel of Plk1 inhibitors used.

(M) Synergy between abiraterone and volasertib (5 nM), GSK461364 (5 nM), or onvansertib (10 nM) was assessed as in (A).

(N) C4-2 cells were subjected abiraterone in the presence or absence of alisertib (20 nM, left), barasertib (200 nM, middle), or nocodazole (200 nM, right). Doses were chosen based on a ~20% decrease in viability when these antimitotic drugs were used in isolation. Synergy between these drugs was assessed as in (A).

(O) BIDMC PC4 CRPC organoids were treated with onvansertib (200 nM) and abiraterone (10 μM) for 8 days and then relative viability was assessed. LUCaP70CR CRPC organoids were treated with onvansertib (40 nM) and abiraterone (10 μM) for 5 days and then relative viability assessed. Mean ± SEM (n = 3) is shown. n.s. not significant, * p ≤ 0.05, ** p ≤ 0.01, and *** p ≤ 0.001 by Student’s two-tailed t-test.