Chronodisruption and Ambulatory Circadian Monitoring in Cancer Patients: Beyond the Body Clock

Recent studies in circadian biology bear Benjamin Franklin, the sage of colonial America, out, on his advice “Early to bed and early to rise, makes a man healthy, wealthy, and wise.” Staying in synchrony with the 24-h light–dark cycle of Earth has indeed proven benefits to human health and brain function [1].

Circadian biology studies the biochemical clocks that keep time in our brains and most cells in our bodies. Evidence is accumulating that misalignment of these clocks with the daily light–dark cycle of our environment can have profound impact on physiology, raising the risk of disease. Circadian rhythms control a remarkable variety of metabolic and physiological functions in the human body. Accumulating epidemiological and genetic evidence in the last years indicates that the disruption of circadian rhythms, or chronodisruption (CD), may be directly linked to cancer, due to impairment of cellular functions important for tumor suppression including cell proliferation, senescence, metabolism, and DNA damage response (DDR) [1, 2 ••, 3]. Perturbations of these processes are hallmarks of cancer and chronic circadian rhythm disruption predisposes to tumor development. Indeed, several molecular gears constituting the circadian clock machinery have been found to establish functional interplays with regulators of the cell cycle, and alterations in clock function could lead to aberrant cellular proliferation [4]. In addition, connections between the circadian clock and cellular metabolism, regulated by chromatin remodeling, have been identified, thus suggesting that abnormal metabolism in cancer could also be a consequence of a disrupted circadian clock [3, 5 ••].

Circadian rhythms are endogenously generated rhythms that occur with a periodicity of approximately 24 h. In humans, as in all animals, circadian rhythms regulate hundreds of activities, from sleep patterns to body temperature, or digestion. The machinery of the molecular clock in mammals, thoroughly described in literature [1, 3], is based on auto-regulatory transcriptional feedback loops driven by the heterodimer of transcription factors BMAL1/CLOCK which binds DNA recognition sequences called E-boxes and activates the expression of a wide number of genes (clock-controlled genes or CCGs, representing 3–10% of all mRNAs expressed in any tissue).

Among all CCGs, the heterodimer BMAL1/CLOCK activates the expression of their downstream transcriptional repressor targets Cryptochrome (Cry1, 2) and Period (Per1, 2, 3) at the beginning of the circadian day. The accumulation of PER and CRY proteins in the cytoplasm by the end of the circadian day leads to the formation of a PER/CRY repressor complex that translocates into the nucleus at the beginning of the circadian night and inhibits the activity of BMAL1/CLOCK heterodimers. In addition, there is a second feedback loop by which the transcription of Bmal1 is alternatively regulated by two of its own transcription targets, the nuclear receptors REV-ERBα/β (the repressors) and RORα (the activator) [6]. The multiple interlocked auto-regulatory feedback loops result in a robust circadian variation in the expression and activity of Bmal1 over a 24-h period, providing a driving force for circadian oscillation of a wide number of genes, some of which are key regulators of cell proliferation, metabolism, senescence, and DDR.

Although circadian rhythms are autonomously generated at the cell level due to the existence of the molecular clock, these rhythms also respond to an internal time, which is set by a master clock or central pacemaker. In humans, this role is played by the suprachiasmatic nucleus (SCN), a hypothalamus region that through physical (temperature swings), humoral (melatonin and cortisol), and nervous signals (autonomic nervous system), synchronizes the peripheral clocks. The SCN is inherently sensitive to environmental cues, so-called zeitgebers (German for time-givers). The most powerful zeitgeber is the 24-h light/dark cycle, which resets circadian functions by acting on neurons within the SCN via a direct connection with the melanopsin-expressing ganglion cells in the retina [6]. Therefore, environmental light “sets the time” of the central clock or circadian pacemaker, which in turn synchronizes the billions of peripheral clocks located in every single nucleated cell and drives the expression of key genes for cancer development.

A shift in the synchronizing environmental cues, for example, as a consequence of a flight crossing different time zones, or working night shifts, induces a change in the central clock phase that directly affects the peripheral clocks that reside in various tissues throughout the body [7–9]. If these changes become chronic, something that is happening nowadays as a consequence of modern lifestyle, a separation between our biological time (internal time set by the SCN), our habits (social time), and the environmental synchronizers (external time) may take place, which can in turn cause a relevant impairment of the circadian organization of physiology, endocrinology, metabolism, and behavior in the human body. This phenomenon, recently denominated with the term chronodisruption (CD) [10], can manifest as a reduction in the amplitude of circadian rhythms, as a complete asynchrony, an advance, or a delay of the peripheral clocks, compared to the SCN phase or even as a phase inversion of the rhythms [11]. It has been proven that CD is closely linked to chronic processes such as aging and cancer [2 ••, 5 ••, 12, 13, 14 ••, 15, 16 •]. Of interesting note, in 2007 (and revised in 2019), the International Agency for Research on Cancer (IARC) from the World Health Organization (WHO) classified the “shift-work that involves circadian disruption,” which includes both working at night and working in a job that involves rapidly crossing many time zones, as probably carcinogenic to humans [17].

Alterations caused by cancer itself or by anti-cancer therapy at cell and/or tissue levels may disrupt CS functioning [18, 19]. Cancer, or the organism response to stress caused by cancer, leads to an increased release of pro-inflammatory cytokines that affect the hypothalamus–pituitary–adrenal axis and cause CS deregulation [20]. Impairment of CS correlates to several symptoms in cancer patients, such as reduced capability to cope with daily-life activities, insomnia, appetite loss, and fatigue, all of them diminishing quality of life (QoL) [19, 21–25].

Alternatively, several studies in humans have evidenced that CD may not only be a consequence of tumor development, but also may cause cancer and promote its progression. This has been particularly shown in metastatic colorectal cancer [26–29], in advanced breast cancer [24, 30–36], and in lung cancer [21, 37–39]. Besides, circadian rhythm dysfunction is also linked to lower anti-cancer treatment efficacy and, overall, to reduce survival [22, 23, 27, 29, 31].

Erren and Reiter [10] proposed that CD may play a critical role in the expression and development of disease through two related ways: firstly, as a possible cause of chronic disease by representing the adverse split of a physiological nexus of internal and external times, and secondly, by being a critical link in the chain of causation leading to chronic disease, including cancer [40], via a relevant disturbance of the circadian organization of physiology, endocrinology, metabolism, and behavior.

In this sense, we can define chronodisruptors as exogenous and endogenous “exposures” or “effectors” which are chronobiologically active and can thus disrupt the timing and order, i.e., temporal organization of physiologic functions and hierarchies. In principle, whatever allows the establishment of temporal organizational order in organisms should also be capable of disrupting such order or temporal program when present, or applied in excess or deficit and, most importantly, at unusual and inappropriate times, especially when combined with further agonistic or antagonistic chronobiological effectors [41]. An endogenous chronodisruptor may be a mutation in a clock gene or the alteration in cell signaling as a consequence of tumor development. In this case, the tumor is able to impact the molecular clock functioning and cause CD.

Alternatively, one key exogenous or external chronodisruptor is light at night (LAN). Light, when received at unusual times, can powerfully disrupt the circadian rhythmicity of our biology, thus leading to CD and to an increase in the risk of developing certain diseases. In this case, CD would happen independently of the tumor and would be a risk factor for its genesis and progression. Epidemiologic studies have shown that LAN may contribute to higher breast cancer rates in women with normal sight [42–44], while the opposite occurs in visually impaired women who are insensitive to light changes in the environment, and thus depend largely on free-running endogenous circadian clocks to synchronize daily physiology [45, 46]. Several studies suggest that totally blind women are half as likely to have breast cancer as women who are not totally blind [46]. Thus, further research is needed in order to elucidate the impact on CD in these blind patients and how these factors may relate to breast cancer risk. Besides, working night shifts and/or flying frequently across different time zones may also be associated to increased risk of developing many forms of cancer [47], and melatonin suppression and/or disruption of its adequate timing under chronodisruptive conditions may be an important factor for increasing cancer risk [47–49].

Other exogenous effectors, such as our daily eating time schedule [50] or physical activity [51], that contribute to synchronize peripheral clocks (along with the light/dark cycle), can also be disruptors when performed at inadequate times.

Recently, several prestigious scientific journals have published about health risks produced by inadequate exposure to synchronizers [52–54] and the need to foster the development and validation of “wearable” devices, m-Health applications, and instruments suitable for population-based and big data approaches to develop predictive models and biomarker platforms for assessing circadian health [55, 56]. As a matter of fact, measuring CD is still an unresolved challenge, especially under free-living conditions. Of note, it has been proposed as a bona fide public health issue in light of the very substantial number of individuals involved [57]. The development of wearable multisensorized devices (i.e., electronic health records) and procedures to accurately quantify sleep and CD, both in clinical and nonclinical environments, is key for personalized medicine through unhealthy habit detection and assisting in the diagnose and treatment of diseases.

Sensor implementation in wearable devices (i.e., wrist watch-like) allows ACM to combine measurements of (a) endogenous variables, such as skin temperature [75]; (b) zeitnehmers (German word meaning timekeeper), such as motor activity and body position more dependent on willingness [76]; and (c) exogenous synchronizers or zeitgebers, such as light exposure and environmental temperature, providing information about lifestyle and the bidirectional crosstalk between internal time and external synchronizers [77–79], which is paramount to determine a subject suffering from CD [10]. If the environmental time (given by the light/dark cycle mainly), the internal time (marked endogenously and autonomously by the SCN), and the social time (given by the external interval for work or social activities) are in phase, we should not expect CD. The same will be anticipated when the appropriate phase relationship among overt rhythms (sleep, temperature, activity, etc.) is maintained.

ACM has been validated for sleep–wake detection (it shows higher sensitivity and specificity than actigraphy alone) compared to sleep diaries [73] and polysomnography (PSG) [74] and it can be used instead of DLMO (dim light melatonin onset) to predict the internal phase [80]. Besides, we have proven its usefulness in very different populations, such as night-shift workers [81, 82], babies [83, 84], hypertensive patients and patients with metabolic syndrome [85, 86], elderly population [84, 87], patients with mild cognitive impairment [88], or more recently in patients with sleep-disordered breathing [89], Parkinson’s disease [90, 91 •], and cancer [61] (Table 1).

Therefore, this wearable technology opens the possibility to study, on a long-term basis, a wide range of patients under real-time conditions and thus providing, at the same time, information on a single individual that can then be used to guide health-related decisions.

Since the discovery of the biological clock, cancer treatments are administered at the appropriate time-of-day, according to biological rhythms. This way, chronotherapy has been shown to improve cancer treatment efficacy [133]. In the last two decades, different experimental and clinical studies have reported positive associations between the circadian clock and drug response in cancer patients, while others have reported a negative correlation between chronodisruption during and/or after cancer treatment with survival rate in cancer animal models and patients [134]. These findings suggest a role of the circadian clock in the outcome of cancer progression and treatment response, specifically in mechanisms of resistance to chemotherapy, controlled by the circadian clock [135–137], which regulates fundamental processes in the cell, often targeted machinery of cytotoxic anticancer drugs, as well as modulating the absorption, metabolism, and elimination of these drugs. Of note, several clinical studies have shown that adverse effects experienced by cancer patients treated with cisplatin-based chemotherapy decreases when applied in a chronomodulated context [138, 139], thus proving that chronomodulated drug administration provides the potential to optimize the dosage of drugs and duration of treatments to efficiently eliminate cancer cells while reducing adverse effects and prevent early drug resistance. Also, downregulation of core clock gene Per2 in tumors predicted poor survival in chemo-naive patients with metastatic colorectal cancer. This was the first report of an association between clock gene downregulation and outcome in any cancer [140]. In rectal cancer, a prospective single-arm phase II with capecitabine and radiotherapy treatment in 85 patients with oral administration of capecitabine (60% dose at 8 am and 40% dose at noon) obtained a total tumor regression in 20% of patients, with no grade IV toxicities reported [141]. Another study on advanced non-small cell lung carcinoma randomized 41 patients with cisplatin administration at 6 pm or at 6 am vs routine chemotherapy. Lower gastrointestinal toxicity (p < 0.05) and higher clearance for cisplatin administered at 6 pm (p < 0.05) were observed in the chronotherapy group [138]. Two clinical trials compared the toxicity of two dosing times of anthracyclines and cisplatin in patients with advanced ovarian cancer. Both randomized studies demonstrated that doxorubicin or theprubicin when administered around 6 am and cisplatin between 4 and 8 pm produced significantly fewer severe hematologic suppression and renal toxicity than when given 12 h earlier, respectively [142].

Although the clinical relevance of chronopharmacology principles in cancer is gradually maturing with the application of clinical big data [143], in vitro pharmacology, mobile platforms and apps [144], and mathematical modeling [145] to help expand our knowledge of the molecular clock and its relevance to human disease, some challenging issues facing chronotherapeutic approaches still remain to be highlighted (such as how a given cancer affects the clock and vice versa, how a given oncotherapy and other factors, such as our lifestyle and environment (personal chronotypes), affect the clock). This is probably due to variability of individual biological rhythms.

Therefore, optimizing the timing of drugs with a narrow therapeutic index and significant circadian fluctuation could lead to significant clinical benefit from improvements in efficacy and minimization of toxicity [146, 147]. The conception of novel anticancer drug delivery systems and the combination with chronobiology may provide a powerful tool to optimize cancer therapy and represents a new challenge to improve the quality of life and survival of cancer patients through personalized cancer chronotherapeutics.

Although the results of many epidemiologic and animal studies are consistent with a role for the circadian clock in the genesis and progression of tumors, available data are still insufficient to conclude that clock disruption is generally carcinogenic. Nevertheless, both experimental and clinical data support the relevance of sleep and circadian rhythm disruption, or CD, in the expression and development of cancer. Besides, CD correlates with poor QoL and function. ACM, through wearable technology, opens the possibility to assess CD in cancer patients on a long-term basis, under real-time conditions, and provides, at the same time, objective information on a single individual that can then be used to guide health-related decisions.

ACM based on multi-variable recording continuously assesses circadian biomarkers, such as those provided by the rest/activity, body position, distal skin temperature, and light exposure monitoring computed in the integrated variable TAPL, and constitutes a unique tool to apply big-data analysis and design successful interventions in cancer patients, taking into account the status and phase of individual circadian systems. The great inter-patient variability at baseline, during and after treatment, and the differing profound effects of chemotherapy on circadian robustness, phase, and internal order synchronization confirm the interest of such multi-parametric evaluation of cancer outpatients.

The useful information provided by this concomitant TAPL monitoring is also relevant for interventional studies targeting the circadian timing system, in order to enhance or protect its function, with the aim of improving the wellbeing and outcomes of cancer patients. In this sense, ACM allows for an integrated analysis of the internal, external, and social times of the individual and thus helps to design individualized strategies to revert CD, first considering life habits (timing and regularity of bright light exposure, physical exercise, work duties, social and family life, sleep/wake routine, fasting/feeding schedule…), before addressing pharmacological approaches.

Thus, there is a certain biological plausibility that needs to be verified and studied, which could relate CD alterations and their impact on mechanisms and evolution of cancer. The objectivity that new technologies (wearables) can provide in their study needs to be further implemented in controlled and well-designed clinical trials in order to clarify and help to understand this relation. Likewise, it will be necessary to delve into the molecular mechanisms that may relate CD and tumorigenesis, especially in light of new treatments such as immunotherapy, where the tumor microenvironment is a determinant factor in the response. Future hypothesis-driven and discovery-oriented research should focus on specific interactions between clock components and carcinogenic mechanisms to realize the full clinical potential of the relationship between clocks and cancer.