44P - Unravelling mechanisms of resistance to CDK4/6 inhibitors using triple negative breast cancer (TNBC).

Abstract

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Introduction:

CDK4/6 inhibition is a proven therapy for hormone receptor positive breast cancers, yet TNBC are largely resistant.

Objectives:

The aims of this study were to understand mechanisms of resistance to CDK4/6 inhibitors and identify novel drug combinations in TNBC

Methods:

Sensitivity to palbociclib and ribociclib in 18 TNBC cell lines was quantified using in vitro assays. In vivo drug toxicity, efficacy and pharmacodynamics studies were performed in mouse MDAMB453 xenografts. Single cell phenotypic analysis using a CDK2 live cell reporter and time-lapse imaging technology was assessed. Molecular determinants of sensitivity were explored with immunofluorescence, western blotting and immunohistochemistry (IHC).

Results:

The luminal androgen receptor (LAR) subtype of TNBC was highly sensitive to CDK4/6 inhibition although basal-like cancers were resistant [p<0.001 LAR vs. basal subtype Mann Whitney]. In vivo validation using MDAMB453 xenografts confirmed palbociclib sensitivity with 7 partial responses (n=10 palbociclib 50mg/kg; n=10 vehicle) and reduction in tumour pRB levels at 4 hours assessed by IHC.

Phenotypic single cell analysis in 3 palbociclib sensitive and 3 palbociclib resistant TNBC cell lines highlighted distinct cell cycle dynamics. Palbociclib sensitive cells exited mitoses in a CDK2^low quiescent state requiring CDK4/6 activity for cell cycle re-entry. Palbociclib resistant cells exited mitoses directly into a CDK2^high proliferative state with rapid transition to S phase and shorter cell cycles. Resistant CDK2^high cells had higher cyclin E1 levels during early G1 phase than palbociclib sensitive CDK2^low cells, and silencing cyclin E1 sensitise resistant cells to palbociclib. Synergistic drug combinations with CDK4/6 inhibitors were observed with PI3kinase inhibitors in PIK3CA mutant TNBC cell lines, extending palbociclib sensitivity to cell lines of additional TNBC subtypes.

Conclusion:

The LAR subgroup of TNBC have a high sensitivity to CDK4/6 inhibtion in pre-clinical models. Dysregulation of cyclin E1 expression is a key determinant of sensitivity to CDK4/6 inhibtion. An on going therapeutic trial is assessing CDK4/6 inhibition in TNBC.

Clinical trial identification

NCT02389842

PIPA: A Phase Ib study to assess the safety, tolerability and activity of the PI3K inhibitor taselisib (GDC-0032), in combination with PAlbociclib, with the subsequent addition of fulvestrant in PIK3CA-mutant breast cancers or letrozole in advanced breast cancers.