Cyclin-dependent Kinase 4/6 Inhibitor Palbociclib in Combination with Ralaniten Analogs for the Treatment of Androgen Receptor–positive Prostate and Breast Cancers

Androgen receptor (AR) has essential roles in the growth of prostate cancer and some breast cancers. Inhibition of AR transcriptional activity by targeting its N-terminal domain with ralaniten or an analog such as EPI-7170 causes accumulation of cells in the G(1)-phase of the cell cycle. Inhibition of cyclin-dependent kinases 4/6 with palbociclib also leads to accumulation of cells in the G(1)-phase. Here, a combination of EPI-7170 with palbociclib attenuated the in vivo growth of human castration-resistant prostate cancer xenografts that are resistant to antiandrogens. Cell-cycle tracing experiments in cultured cells revealed that EPI-7170 targeted cells in the S-phase, possibly through inducing DNA damage or impairing the DNA damage response, whereas palbociclib targeted the G(1)–S transition to delay the cell cycle. Combination treatment prevented cells in G(1) and G(2)–M from progressing in the cell cycle and caused a portion of cells in the S-phase to arrest, which contributed to a twofold increase in doubling time to >63 hours compared with 25 hours in control cells. Importantly, sequential combination treatments with palbociclib administered first then followed by EPI-7170, resulted in more cells accumulating in G(1) and less cells in the S-phase than concomitant combination which was presumably because each inhibitor has a unique mechanism in modulating the cell cycle in cancer cells. Together, these data support that the combination therapy was more effective than individual monotherapies to reduce tumor growth by targeting different phases of the cell cycle.