Endocrine therapy, CDK4/6, and why resistance still happens
How ER-positive treatments work, where they help most, and why many cancers still find a way to survive
ER-positive breast cancer depends on estrogen signalling.
That is why endocrine therapy works at all.
The problem is not that these drugs do nothing.
The problem is that some surviving cells adapt.
Review we are unpacking
McGrath MK, Abolhassani A, Guy L, Elshazly AM, Barrett JT, Mivechi NF, Gewirtz DA, and Schoenlein PV (2024).
Autophagy and senescence facilitate the development of antiestrogen resistance in ER-positive breast cancer.
Frontiers in Endocrinology. 15:1298423.
How estrogen drives growth
Estrogen binds to ERα.
That receptor then helps switch on genes tied to cell-cycle progression.
Antiestrogen treatment tries to break that chain.
It can do that by:
blocking the receptor
degrading the receptor
lowering estrogen production upstream
Quick drug-class guide
Aromatase inhibitors such as letrozole, anastrozole, and exemestane reduce estrogen production rather than targeting the receptor directly.
SERMs such as tamoxifen block ER in breast tissue.
SERDs such as fulvestrant block ER and promote receptor degradation.
Why switching drugs can still help
Resistance to one endocrine drug does not always mean resistance to all of them.
That is why sequencing remains standard.
For example, disease that progresses on an aromatase inhibitor may still respond to fulvestrant.
This approach can extend control.
It usually does not eliminate the underlying survival problem.
Where CDK4/6 inhibitors fit
CDK4/6 inhibitors shut the gate between G1 and S phase.
They do this through the Rb–E2F pathway.
That is why palbociclib, ribociclib, and abemaciclib have become such an important part of ER-positive care.
They clearly delay progression.
They do not stop resistance forever.
McGrath et al. (2024) highlight an uncomfortable twist.
CDK4/6 inhibitors can also trigger autophagy and senescence.
That means they may help control disease while also helping create the survival states discussed in this section.
The difference between primary and acquired resistance
The paper separates resistance into two broad forms.
Primary resistance shows up early.
Cells keep proliferating despite treatment.
Acquired resistance shows up later.
Cells first respond, then survive under pressure long enough to escape.
Autophagy and senescence matter most in that second pattern.
Important nuance across aromatase inhibitors
The authors point to a useful distinction.
Exemestane appears more tied to autophagy through AMPK/mTOR signalling.
Letrozole and anastrozole appear more tied to senescence, partly through YPEL3.
That does not mean one drug is simply better.
It means the survival state may differ depending on which pressure is applied.
The paper also notes that exemestane is not identical to the non-steroidal aromatase inhibitors.
That difference may matter when thinking about treatment sequencing and escape biology.
The progesterone-receptor note
McGrath et al. (2024) briefly flag another under-discussed issue.
PR biology is not always just a passenger.
PR expression without meaningful ERα signalling may point to a more aggressive, less antiestrogen-sensitive pattern.
The paper highlights early interest in mifepristone in selected PR-related settings.
That remains exploratory.
Why resistance still happens after good early control
The paper's answer is not simply "because a mutation appeared."
It argues that treatment first creates a survival bottleneck.
Cells that enter autophagy or senescence can outlast that bottleneck.
Only later do some of them acquire the more durable changes that define full resistance.
That framing matters.
It suggests the most important window may be before resistance becomes obvious on scans or blood tests.
Bottom line
Endocrine therapy and CDK4/6 inhibition remain central in ER-positive disease.
They work.
McGrath et al. (2024) ask what the surviving cells are doing while treatment is working.
Their answer is that many of them may be buying time through autophagy, senescence, or both.
This information is for education only. It is not medical advice, diagnosis, or treatment. Please speak with a qualified clinician before making changes to care, medication, or supplement use.
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