Long-Term Exposure of Cells to Cdk4 Inhibitor Palbociclib Leads to Chromosomal Aberrations

Breast cancers are often driven by mutations, alterations and activation of cell cycle regulatory proteins, including the retinoblastoma tumor suppressor protein (Rb), cyclin E and cyclin-dependent kinases (Cdks), especially cyclin D:Cdk4/6 complexes. There are currently three FDA approved Cdk4/6 inhibitors (Cdk4i) for treating breast cancer. The standard treatment protocol is 21 days of continuous Cdk4i treatment, followed by a 7 day cessation period and then repeating the 28 day protocol. We asked the question of what happens to cells that reenter the cell cycle during the 7 day Cdk4i cessation period. Using RPE1 cells containing visual reporter endogenous histone 2B and p27 genes tagged with eGFP and mCherry, we treated the cells with a Cdk4i, Palbociclib for 1 to 42 days that spanned the clinical exposure, followed by drug release (washout) and video microscopic analysis. Surprisingly, we found that as little as 4 days of Cdk4i treatment and release resulted in a significant increase in micronuclei and multinucleated cells that had reentered the cell cycle. The peak chromosomal aberration occurred between 14 and 35 days, a timing that spans the clinical dosing regimen.These observations raise questions concerning the potential that cycling patients on and off of Cdk4 inhibitors may generate gross chromosomal changes to tumor cells that reenter the cell cycle during the 7 day clinical cessation (washout) period and thereby increase the potential to initiate secondary oncogenic events.