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Cancer is a heterogeneous systemic disease that is strongly influenced by dynamic interactions with the tumour microenvironment (TME). Despite major advances in understanding spatial and molecular tumour heterogeneity, the temporal dynamics of tumours have received far less attention. Growing evidence has linked circadian clocks to cancer risk, progression, and treatment response, including in breast cancer. However, temporal regulation has yet to be recognized as a cancer hallmark, and its interaction with the TME remains poorly understood. This review examines how circadian rhythms organize breast cancer biology through bidirectional interactions with the TME. Circadian clocks coordinate proliferation, DNA damage responses, metabolism, and immune surveillance. Ageing, chronic stress, and obesity, all of which are established breast cancer risk modifiers, disrupt these rhythms and are reciprocally exacerbated by circadian dysfunction, establishing feed-forward loops that accelerate disease. Within the TME, the extracellular matrix (ECM) plays a central role in mediating this bidirectional control. Stiffened fibrotic stroma dampens epithelial clock amplitude, while circadian rhythms in turn shape collagen turnover and ECM remodelling. These dynamics can foster inflammation, stem cell expansion, and metastatic dissemination, including time-of-day-dependent release of circulating breast tumour cells. Systemically, circadian clocks gate immune cell trafficking, creating predictable windows of immunosurveillance and therapeutic vulnerability. By integrating insights from mechanobiology, metabolism, immune regulation, and ageing, we position circadian timing as a unifying layer that connects cell-intrinsic programmes with the evolving breast TME. Understanding these connections opens new opportunities for chronotherapeutic strategies in which treatment timing is aligned with circadian rhythms to improve outcomes.

The circadian clock, a fundamental biological regulator, governs essential cellular processes in health and disease. Circadian-based therapeutic strategies are increasingly gaining recognition as promising avenues. Aligning drug administration with the circadian rhythm can enhance treatment efficacy and minimize side effects. Yet, uncovering the optimal treatment timings remains challenging, limiting their widespread adoption. In this work, we introduce a high-throughput approach integrating live-imaging and data analysis techniques to deep-phenotype cancer cell models, evaluating their circadian rhythms, growth, and drug responses. We devise a streamlined process for profiling drug sensitivities across different times of the day, identifying optimal treatment windows and responsive cell types and drug combinations. Finally, we implement multiple computational tools to uncover cellular and genetic factors shaping time-of-day drug sensitivity. Our versatile approach is adaptable to various biological models, facilitating its broad application and relevance. Ultimately, this research leverages circadian rhythms to optimize anti-cancer drug treatments, promising improved outcomes and transformative treatment strategies. The circadian rhythm has been linked to cancer cell sensitivity to therapy but tools to understand this further are limited. Here, by combining live-cell imaging and computational tools, the authors develop a high-throughput deep-phenotyping approach to evaluate circadian rhythms and use it to determine time-of-day drug sensitivity in cancer cell lines.

The circadian clock plays a pivotal role in regulating various aspects of cancer, influencing tumor growth and treatment responses. There are significant changes in drug efficacy and adverse effects when drugs are administered at different times of the day, underscoring the importance of considering the time of day in treatments. Despite these well-established findings, chronotherapy approaches in drug treatment have yet to fully integrate into clinical practice, largely due to the stringent clinical requirements for proving efficacy and safety, alongside the need for deeper mechanistic insights. In this study, we employ a combined mathematical and experimental approach to systematically investigate the factors influencing time-of-day drug sensitivity in human cells. Here we show how circadian and drug properties independently shape time-of-day profiles, providing valuable insights into the temporal dynamics of treatment responses. Understanding how drug efficacy fluctuates throughout the day holds immense potential for the development of personalized treatment strategies aligned with an individual’s internal biological clock, revolutionizing cancer treatment by maximizing therapeutic benefits. Moreover, our framework offers a promising avenue for refining future drug screening efforts, paving the way for more effective and targeted therapies across diverse tissue types. Many cancer drugs change their effects at different times of the day; here the authors combine experiments with general computational models to show how circadian and drug properties shape time-of-day sensitivity, offering insights for personalized cancer treatments.

In alignment with the solar day-night cycle, the circadian clock acts as a fundamental regulator of various cellular processes in health and disease. In cancer treatment, circadian rhythms can impact a drug’s pharmacokinetics and pharmacodynamics, holding potential to shape the responses of healthy and cancer cells to therapy, which is reflected in time-ofday-dependent variations in side effects and anti-tumor effectiveness. Thus, circadianbased treatment strategies, known as chronotherapy, are increasingly gaining recognition for their potential to enhance treatment outcomes. For aggressive and heterogeneous cancer subtypes that lack specific molecular targets, chronotherapy offers a promising approach, with the ability to optimize existing cytotoxic chemotherapy regimens for greater effectiveness and tolerability. However, the adoption of chronotherapy in clinical settings remains limited. Key barriers include incomplete understandings of (i) the circadian clock’s role across and within different cancer types; (ii) optimal treatment timings for specific drugs and cell types; and (iii) how cellular characteristics, such as circadian clock properties, growth dynamics, and drug sensitivity profiles, shape time-of-day-dependent drug responses. Closing these gaps could broaden the applicability of chronotherapy to a vast range of cell-drug combinations.Addressing standing challenges in the implementation of chronotherapy, this thesis provides a comprehensive approach to characterize the circadian clock landscape in cellular cancer models and elucidating its role in shaping treatment responses. We focused our work on breast cancer, which is the most prevalent cancer type among women, characterized by high molecular heterogeneity. By monitoring cellular circadian rhythms in real time over multiple days using two complementary circadian reporters, we observed diverse circadian dynamics across a broad panel of breast cancer cell lines, including several ones of the highly aggressive triple-negative breast cancer subtype. These observations led us to identify novel circadian-based subtypes among the cell line panel that challenge traditional molecular classifications. Further, we explored potential molecular interactions between the circadian clock and cellular drug sensitivity and revealed distinct dependencies between circadian properties and drug sensitivity features for numerous drugs that could be studied in chronotherapeutic settings in the future. Finally, we present a novel high-throughput approach integrating live imaging and data analysis techniques to capture and characterize time-of-day-dependent drug responses to a variety of anti-cancer agents in both healthy and cancer cell models. This approach enabled the identification of responsive drug-cell combinations and the determination of optimal treatment times for maximal or minimal benefit. Employing distinct computational tools to uncover cellular and genetic factors that shape time-of-day drug sensitivity, we further uncovered interesting relationships that depend on the specific drug studied.Collectively, this thesis illuminates the circadian clock properties of various breast cancer cell models and their influence on shaping overall drug sensitivity and time-of-daydependent response dynamics. While the findings are not yet directly translatable to clinical settings, they underscore the critical role of timing in breast cancer treatments and highlight the potential of chronotherapeutic approaches for advancing precision breast cancer therapy.

The circadian clock regulates key physiological processes, including cellular responses to DNA damage. Circadian-based therapeutic strategies optimize treatment timing to enhance drug efficacy and minimize side effects, offering potential for precision cancer treatment. However, applying these strategies in cancer remains limited due to limited understanding of the clock’s function across cancer types and incomplete insights into how the circadian clock affects drug responses. To address this, we conducted deep circadian phenotyping across a panel of breast cancer cell lines using two complementary reporters. Observing diverse circadian dynamics, we developed metrics to assess circadian rhythm strength and stability. This led to the identification of four distinct circadian-based phenotypes in breast cancer: functional, weak, unstable, and dysfunctional clocks. Furthermore, we demonstrate that the circadian clock plays a critical role in shaping pharmacological responses to various anti-cancer drugs and identify circadian features that accurately predict drug sensitivity. Collectively, our findings establish a foundation for advancing the use of chronotherapeutic strategies in breast cancer treatment, expanding their potential application to improve therapeutic outcomes in breast cancer.

Single-cell circadian oscillators exchange extracellular information to sustain coherent circadian rhythms at the tissue level. Within cells, the circadian clock and the cell cycle couple, yet the mechanisms governing this interplay remain poorly elucidated. Here, we study the role of extracellular circadian communication in the intracellular coordination between the circadian clock and the cell cycle. We demonstrate that the loss of extracellular circadian synchronization disrupts circadian and cell cycle coordination within individual cells, impeding collective tissue growth. Coherent circadian rhythms yield oscillatory growth patterns, unveiling a global timing regulator of tissue dynamics. Knocking down core circadian elements abolishes observed effects, highlighting the central role of circadian clock regulation. Our research underscores the significance of tissue-level circadian disruption in regulating proliferation, thereby linking disrupted circadian clocks with oncogenic processes. These findings illuminate the intricate interplay between circadian rhythms, cellular signaling, and tissue physiology, enhancing our understanding of tissue homeostasis and growth regulation in both health and disease contexts.

The circadian clock, a fundamental biological regulator, governs essential cellular processes in health and disease. Circadian-based therapeutic strategies are increasingly gaining recognition as promising avenues. Aligning drug administration with the circadian rhythm can enhance treatment efficacy and minimize side effects. Yet, uncovering the optimal treatment timings remains challenging, limiting their widespread adoption. In this work, we introduce a novel high-throughput approach integrating live-imaging and data analysis techniques to deep-phenotype cancer cell models, evaluating their circadian rhythms, growth, and drug responses. We devised a streamlined process for profiling drug sensitivities across different times of the day, identifying optimal treatment windows and responsive cell types and drug combinations. Finally, we implement multiple computational tools to uncover cellular and genetic factors shaping time-of-day drug sensitivity. Our versatile approach is adaptable to various biological models, facilitating its broad application and relevance. Ultimately, this research leverages circadian rhythms to optimize anti-cancer drug treatments, promising improved outcomes and transformative treatment strategies.

The circadian clock, a fundamental biological regulator, governs essential cellular processes in health and disease. Circadian-based therapeutic strategies are increasingly gaining recognition as promising avenues. Aligning drug administration with the circadian rhythm can enhance treatment efficacy and minimize side effects. Yet, uncovering the optimal treatment timings remains challenging, limiting their widespread adoption. In this work, we introduce a novel high-throughput approach integrating live-imaging and data analysis techniques to deep-phenotype cell models, evaluating their circadian rhythms, growth, and drug responses. We devised a streamlined process for profiling drug sensitivities across different times of the day, identifying optimal treatment windows and responsive cell types and drug combinations. Finally, we implement multiple computational tools to uncover cellular and genetic factors shaping time-of-day drug sensitivity. Our versatile approach is adaptable to various biological models, facilitating its broad application and relevance. Ultimately, this research leverages circadian rhythms to optimize drug treatments, promising improved outcomes and transformative treatment strategies.Competing Interest StatementThe authors have declared no competing interest.