I have always been drawn to the idea behind personalised medicine: treatment does not work in exactly the same way for every patient. The biology of the tumour matters, of course, but so does the biological context in which that tumour exists. Different mutations can lead to different therapeutic choices, and even patients with apparently similar disease can respond differently to the same treatment. As I read more about personalised approaches, I also came across another layer of variation: treatment responses can be influenced by hormones, metabolism and lifestyle.

That made me increasingly interested in a broader question: what other biological variables might shape the way we respond to treatment?

One of the variables I kept encountering was biological sex. I had heard about sex differences in disease and treatment response, discussed the topic in study groups, attended talks and webinars, and read research exploring how sex hormones can influence different biological processes. But I realised that I was mostly encountering the conclusion — that males and females can differ — without really understanding how researchers had arrived at that conclusion in the first place.

Following discussions in a study group gave me an opportunity to look at this question from a different angle. Rather than simply reading about sex differences as an established concept, I started to think about how the field had reached that point — and, more importantly for me as someone interested in cancer biology, what this might mean for the way we think about treatment response.

This is where the oestrous cycle enters the story.

Once female animals became part of experimental research, another question became difficult to ignore: what happens to their biology across the oestrous cycle? (Prendergast 2014 Neurosci Biobehav Rev , Deecheret al. 2008 Psychoneuroendocrinology)

Hormonal fluctuations can affect metabolism, morphology and other aspects of physiology, challenging the idea that female animals are simply more variable experimental models. In the mouse ventral hippocampus, researchers have identified cycle-dependent changes in behaviour, chromatin accessibility and gene expression. ATAC-seq revealed changes in chromatin accessibility, while RNA-seq identified 172 differentially expressed genes between proestrus and diestrus. Differences in dendritic spine density and synaptic connections were also observed across cycle stages. (Jarić et al., 2019)

For me, the significance is that these findings point to the same idea: variation across the oestrous cycle can represent biological information rather than experimental noise. This raises a more important question for cancer research: could the same biological state also influence how a tumour responds to therapy?

The relevance of these hormonal fluctuations becomes even more striking when we look at the mammary gland itself. The mammary tissue responds dynamically to changes in oestrogen and progesterone, undergoing temporary changes in ductal branching and growth before returning towards its previous state as the hormonal cycle progresses. In other words, the tissue is not biologically static from one stage of the cycle to another. (Ciwinska et al Nature, 2024)

This dynamic behaviour also becomes visible when we look at early tumour development. Mutant cell populations can expand under the influence of hormonal conditions, while in the absence of those hormonal signals their capacity for growth and dissemination can be reduced (Ciwinska et al Nature, 2024). More strikingly, the evidence suggests that the tumour does not necessarily lose this relationship with the hormonal environment once it has formed. Tumour behaviour can retain dynamic features associated with the hormonal cycle. (Zomer, et al., 2013 Stem Cells)

This is one of the findings I find particularly compelling because it takes the argument beyond changes in gene expression or chromatin accessibility. We are no longer looking only at molecular signatures that change with the cycle; we are seeing changes at the level of tissue dynamics, mutant-cell populations and, eventually, the established tumour itself.

For me, this creates a natural question for cancer research. If the hormonal state can influence the dynamics of the tissue in which a tumour develops, affect the behaviour of mutant populations, and continue to shape tumour behaviour after tumour formation, why should treatment necessarily be considered independent of that biological state?

That is the point at which, for me, the question changes. It is no longer only whether the oestrous cycle changes biology. The more relevant question becomes whether those biological changes can alter the response to chemotherapy.

The question becomes more interesting when we look at treatment response across the cycle. During oestrus, increased tumour-cell proliferation and DNA replication provide a possible rationale for greater chemotherapy sensitivity, and treatment at this stage produced stronger anti-tumour effects across triple-negative, HER2-positive and luminal mammary tumour models, with reduced tumour growth and increased survival compared with diestrus. However, proliferation alone may not explain this difference. Diestrus tumours showed greater macrophage involvement, and reducing this component was associated with improved chemotherapy response, suggesting that hormonal state may influence several components of the tumour microenvironment. (Bornes et al Nature 2025)

 This raises an important question: is the cycle itself responsible for the difference, or does it reflect a broader biological state involving hormones, immune cells, metabolism and tumour behaviour? It also remains unclear whether different tumour subtypes respond through the same mechanisms, and how this relationship might interact with additional tumour mutations or treatments such as targeted therapy and immunotherapy.

Although these findings are primarily based on preclinical mouse models, preliminary evidence from two small retrospective human cohorts suggests a possible association between menstrual-cycle-related hormonal status and chemotherapy response. However, prospective clinical studies are needed before these findings can inform treatment timing in clinical practice. (Bornes et al Nature 2025)

 For me, these questions do not weaken the chemotherapy findings; they make them more interesting. The key challenge is to understand what the cycle changes within the tumour and its surrounding environment, and whether this biological context could eventually help explain differences in treatment response.

Personalised medicine continues to reveal layers of biological variation that become apparent as we look more closely. Recognising differences between males and females is only one step; understanding variation within females raises another set of questions. The oestrous cycle offers a useful starting point, but it should not become another category that we treat as a complete explanation. Animals at the same cycle stage may still differ in their hormonal environment, tumour microenvironment, immune-cell composition and proliferative signalling pathways. Understanding how these factors interact will require further investigation. Beyond these physiological differences, could future research also reveal how sex-chromosome composition contributes to variation in tumour behaviour and treatment response? The challenge is not simply to identify another biological variable, but to understand the mechanisms behind it. As personalised medicine moves towards greater precision, the questions we ask may need to become increasingly precise too.

References:

1-Jaric, I., Rocks, D., Greally, J.M. et al. Chromatin organization in the female mouse brain fluctuates across the oestrous cycle. Nat Commun 10, 2851 (2019). https://doi.org/10.1038/s41467-019-10704-0

2- Ciwinska, M., Messal, H.A., Hristova, H.R. et al. Mechanisms that clear mutations drive field cancerization in mammary tissue. Nature 633, 198–206 (2024). https://doi.org/10.1038/s41586-024-07882-3

3- Bornes, L., van Winden, L.J., Geurts, V.C.M. et al. The oestrous cycle stage affects mammary tumour sensitivity to chemotherapy. Nature 637, 195–204 (2025). https://doi.org/10.1038/s41586-024-08276-1

4- Deecher, D., Andree, T. H., Sloan, D., & Schechter, L. E. (2008). From menarche to menopause: exploring the underlying biology of depression in women experiencing hormonal changes. Psychoneuroendocrinology, 33(1), 3–17. https://doi.org/10.1016/j.psyneuen.2007.10.006

5- Prendergast, B. J., Onishi, K. G., & Zucker, I. (2014). Female mice liberated for inclusion in neuroscience and biomedical research. Neuroscience and biobehavioral reviews, 40, 1–5. https://doi.org/10.1016/j.neubiorev.2014.01.001

6- Anoek Zomer, Saskia Inge Johanna Ellenbroek, Laila Ritsma, Evelyne Beerling, Nienke Vrisekoop, Jacco Van Rheenen, Brief Report: Intravital Imaging of Cancer Stem Cell Plasticity in Mammary Tumors, Stem Cells, Volume 31, Issue 3, March 2013, Pages 602–606, https://doi.org/10.1002/stem.1296