# Endocrine therapy, CDK4/6, and why resistance still happens 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. {% hint style="info" %} **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. URL: {% endhint %} *** ### 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. {% hint style="info" %} 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. {% endhint %} *** ### 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. {% hint style="warning" %} 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. {% endhint %} {% hint style="info" %} © 2026 Abbey Mitchell. All rights reserved. Please share by URL rather than copying page text. {% endhint %} --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://myhealingcommunity.gitbook.io/myhealingcommunity-docs/breast-cancer/er-positive-her2-negative/endocrine-therapy-resistance-and-dormancy/autophagy-and-senescence-in-antiestrogen-resistance/endocrine-therapy-cdk4-6-and-why-resistance-still-happens.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.