Acquired CDK6 amplification promotes breast cancer resistance to CDK4/6 inhibitors and loss of ER signaling and dependence

Abstract

Dysregulated activation of the CDK4/6 kinases is a hallmark of most mammary-derived carcinomas. ATP competitive inhibitors against this complex have been recently advanced in the clinic and shown significant activity, particularly against tumors driven by the estrogen receptor (ER). However, resistance to these compounds has begun to emerge often months to years after their initiation. We investigated potential mechanisms of resistance utilizing cell line models which are highly sensitive to this class of drugs. After prolonged exposure to the selective and potent CDK4/6 inhibitor LY2835219, clones emerged and several were found to harbor amplification of the CDK6 kinase. Amplification of CDK6 resulted in a marked increase in CDK6 expression and reduced response of the CDK4/6 target, pRb, to CDK4/6 inhibitors. Knockdown of CDK6 restored drug sensitivity while enforced overexpression of CDK6 was sufficient to mediate drug resistance. Not only did CDK6 overexpression mediate resistance to CDK4/6 inhibitors, it also led to reduced expression of the ER and progesterone receptor (PR), and diminished responsiveness to ER antagonism. The reduced ER/PR expression after CDK4/6 inhibitor resistance was additionally observed in tumor biopsy specimens from patients treated with these drugs. Alternative mechanisms of resistance to CDK4/6 inhibitors such as loss of pRb and Cyclin E1 overexpression also exhibited decreased hormone responsiveness suggesting that the clinical paradigm of sequential endocrine-based therapy may be ineffective in some settings of acquired CDK4/6 resistance.

The Strange Case of CDK4/6 Inhibitors: Mechanisms, Resistance, and Combination Strategies

“…The trial showed there was an increase in progression-free survival beyond the historical control, but since there was no control arm additional study will be required to determine if the amplification of CDK4 denotes selective sensitivity to CDK4/6 inhibitors. Selection for treatment based on CDK4 or CDK6 amplification is not without concern, as CDK6 amplification has been recently shown to drive resistance to CDK4/6 inhibitors in preclinical models[83]. …”

CDK4/6 Inhibition in Breast Cancer: Mechanisms of Response and Treatment Failure

“…Although mutations in the kinase domains of CDK4 or 6 have not been reported as mechanisms of resistance to CDK4/6 inhibitors, a recent report describes in vitro evidence of CDK6 amplification in breast cancer cell lines with acquired abemaciclib resistance(56). This amplification was sufficient to promote resistance.…”

Mechanisms of Resistance to CDK4/6 Inhibitors in Breast Cancer and Potential Biomarkers of Response

“…Finally, while a model of CAMA1 with acquired resistance to abemaciclib was found to have overexpression of CCNE1 , similar to that seen previously with palbociclib [40], a model of abemaciclib-resistant MCF7 showed an increase in CDK6 mRNA levels with concomitant increase in CDK6 protein levels. In addition, a fraction of the resistant cells showed an increased CDK6 copy number.…”

“…In addition, a fraction of the resistant cells showed an increased CDK6 copy number. Reducing CDK6 levels restored the sensitivity, while overexpression of CDK6 resulted in reduced sensitivity to abemaciclib [40], suggesting that overexpression/amplification of CDK6 might sustain abemaciclib resistance. In addition, both models showed reduced expression of ESR1 and PGR [40], which was associated with reduced sensitivity to hormonal treatments in MCF7-resistant cells [40].…”

“…Reducing CDK6 levels restored the sensitivity, while overexpression of CDK6 resulted in reduced sensitivity to abemaciclib [40], suggesting that overexpression/amplification of CDK6 might sustain abemaciclib resistance. In addition, both models showed reduced expression of ESR1 and PGR [40], which was associated with reduced sensitivity to hormonal treatments in MCF7-resistant cells [40]. Intriguingly, on a limited number of tumor biopsies from patients treated with abemaciclib or ribociclib, it appeared that tumors changed from ER+ to ER- or from PR+ to PR- [40,] supporting the preclinical data suggesting that ER might be implicated in CDK4/6 inhibitors resistance.…”