CDKN1B(p27kip1) enhances drug tolerant persister CTCs by restricting polyploidy following mitotic inhibitors

The mitotic inhibitor docetaxel (DTX) is often used to treat endocrine-refractory metastatic breast cancer, but initial responses are mitigated as patients eventually have disease progression. Using a cohort of ex vivo cultures of circulating tumor cells (CTCs) from patients with heavily pretreated breast cancer (n=18), we find two distinct patterns of DTX susceptibility, independent of clinical treatment history. In CTCs cultured from some patients, treatment with a single dose of DTX results in complete cell killing, associated with accumulation of non-viable polyploid (≥8N) cells arising from endomitosis. In others, a transient viable drug-tolerant persister (DTP) population emerges, ultimately enabling renewed proliferation of CTCs with preserved parental cell ploidy and DTX sensitivity. In these CTC cultures, efficient cell cycle exit generates a ≤4N drug-tolerant state dependent on CDKN1B (p27 Kip1 ). Exposure to DTX triggers stabilization of CDKN1B through AKT-mediated phosphorylation at serine 10. Suppression of CDKN1B reduces the number of persister CTCs, increases ≥8N mitotic cells and abrogates regrowth after DTX exposure. Thus, CDKN1B-mediated suppression of endomitosis contributes to a reversible persister state following mitotic inhibitors in patient-derived treatment refractory breast cancer cells. Summary in bullets Transient DTX tolerant persister cells emerge in some patient-derived cultured CTCs. DTX-tolerant persisters restrict endoreduplication and polyploidy through CDKN1 (p27 kip 1 ). DTX exposure induces CDKN1B stabilization through AKT mediated phosphorylation at serine 10. Suppression of polyploidy underlies a drug tolerant persister state specific to mitotic inhibitors.