Enhancing endocrine therapy activity via fasting cycles: biological rationale and clinical feasibility

ABSTRACT

We found that periodic fasting increases the anti-cancer activity of endocrine agents used to treat hormone receptor-positive breast cancer and delays acquired resistance to them by reducing blood leptin, insulin and insulin-like growth factor 1 (IGF1). Our work supports further clinical studies of fasting as an adjuvant to endocrine agents in breast cancer patients.

Breast cancer (BC) is the most frequently diagnosed malignancy with 1.7 million new diagnoses/year (23% of cancer diagnoses).¹ Seventy-five percent of BC express the estrogen receptor (ER) and/or the progesterone receptor and are collectively referred to as hormone receptor-positive (HR+) BC. Patients with HR+ BC are treated with endocrine therapy (ET), i.e., tamoxifen, fulvestrant, or aromatase inhibitors. These agents reduce the risk of relapses in patients who are treated in the adjuvant setting and delay disease progression in patients with metastatic disease.² Combining cyclin-dependent kinase 4/6 (CDK4/6) inhibitors, such as palbociclib, ribociclib or abemaciclib, with ET in patients with metastatic HR+/human epidermal growth factor receptor 2-negative (HER2-) BC, improves progression-free survival and overall survival.³ However, HR+/HER2- metastatic BC eventually progresses even under ET plus CDK4/6 inhibitors and most of these patients still die of their disease. Therefore, new strategies to improve ET efficacy are needed.

We decided to explore the value of periodic cycles of fasting in combination with ET as a means to enhance ET activity since fasting reduces circulating growth factors such as insulin and insulin-like growth factor 1 (IGF1) and blunts their signaling,⁴ which is responsible for increasing ER activity and for reducing endocrine sensitivity.⁵ We found that weekly cycles of 48-hour water-only fasting enhanced tamoxifen and fulvestrant anticancer activity in mouse xenografts of several HR+/HER2- BC cell lines.⁶ We also observed that a short-term (1 month) treatment with combined ET and fasting caused a strong carry-over anticancer effect with 14–28% of our MCF7 xenografts completely regressing during the observation period that followed the treatment (while none of the tumors growing in mice treated with just ET or fasting actually regressed). Similar effects were obtained with a vegan, low-calorie and low-protein modified fasting regime (“fasting-mimicking diet”), which we also found to prevent acquired resistance to tamoxifen in MCF7 xenografts-bearing mice. Eventually, we discovered that the circulating factors that became reduced through combined fasting and ET and that were mediating the cooperation between these interventions were IGF1, insulin and leptin. The latter is an adipokine that acts as a growth factor for BC cells, reducing ET efficacy,⁷ but whose modulation was previously never associated with the antitumor effects of fasting. In fact, it was the simultaneous downregulation of all of these factors (IGF1, insulin and leptin) that proved necessary for the potentiation of ET via fasting, as shown by the fact that the add back of any of these proteins was sufficient to fully abrogate such potentiation. We ascribed this result to the fact that all of these factors have overlapping effects in stimulating the insulin/IGF1 signaling cascade, leading to AKT and to mTORC1 stimulation.

By studying the effects of fasting, ET and their combination on insulin/IGF1 signaling in HR+ BC cells, we discovered that combined ET plus fasting were leading to the upregulation of the tumor suppressor phosphatase and tensin homolog (PTEN) and to the consequent inhibition of AKT and of mammalian target of rapamycin complex 1 (mTORC1) activity. We also found that another tumor suppressor, early growth response 1 (EGR1), was becoming upregulated in response to ET plus fasting and was responsible for the observed increase in PTEN levels, for AKT inhibition and for the anti-tumor activity of this combination. We showed that in turn, EGR1 upregulation in response to our combined treatments reflected reduced AKT activity, indicating that EGR1 and AKT negatively regulate each other in HR+ BC cells and that combining fasting with ET causes EGR1 to prevail on AKT (Figure 1).

Figure 1.

Potential mechanisms of reciprocal EGR1 and AKT regulation in HR+ BC cells. Fasting and endocrine agents, such as tamoxifen or fulvestrant, cooperate to relieve AKT-mediated inhibition of early growth response 1 (EGR1) expression in breast cancer cells. In turn, increased EGR1 raises phosphatase and tensin homolog (PTEN) levels and thereby strengthens the inhibition of AKT and of mammalian target of rapamycin complex 1 (mTORC1). Key for the ability of fasting to enhance endocrine therapy activity is the reduction in the circulating levels of insulin, insulin-like growth factor 1 (IGF1) and leptin

In subsequent experiments, that were performed in cooperation with Valter Longo’s laboratory at the Istituto FIRC di Oncologia Molecolare (IFOM) in Milan, we established that periodic modified fasting cycles were as effective as the CDK4/6 inhibitor palbociclib at delaying acquired resistance to fulvestrant. In these experiments, the triplet fulvestrant, modified fasting and palbociclib proved to be the most active regime, leading to long-lasting regressions of the MCF7 mouse xenografts. Starting cycles of modified fasting after the onset of resistance to fulvestrant plus palbociclib still led to tumor shrinkage, suggesting that periodic fasting could also be an approach to reverse acquired resistance to ET plus CDK4/6 inhibitor.

Necropsies of mice treated with tamoxifen showed the typical increase in uterus size and endometrial hyperplasia. This side effect of tamoxifen, which occurs in 5–10% of the patients, is due to its activity as an ER agonist in the uterus.⁸ It typically leads to genital bleeding and predisposes to uterus cancer. Quite remarkably, in mice receiving periodic water-only fasting or modified fasting, tamoxifen-induced endometrial hyperplasia was virtually abolished, an effect that was associated with increased Pten expression and with reduced Akt activity in the uterus.

In two, independent clinical studies (NCT03595540 and NCT03340935), monthly cycles of a modified fasting regimen (lasting 5 days) proved well tolerated in women receiving ET for HR+ BC. Modified fasting recreated the metabolic changes that were observed in animals (i.e., low leptin, insulin and IGF1) and that proved pivotal for the enhancement of ET activity. In the NCT03595540 trial, where patients also received dietary recommendations and instructions for muscle training to be followed during the “non-fasting” periods,⁹ patients achieved stable body weight, higher lean body mass and lower fat mass. Thus, these results indicate that modified fasting, when applied under a strict nutritional follow-up and in patients at low nutritional risk, can be safely introduced even in patients with advanced disease. Since none of these studies was designed to verify the antitumor activity of combined modified fasting and ET, no conclusion on the clinical usefulness of this dietary regime in combination with ET can still be drawn. Additional clinical trials assessing the efficacy of modified fasting as a means to enhance the benefit of ET in HR+ BC are necessary.

Funding Statement

This work was supported by the Associazione Italiana per la Ricerca sul Cancro [#22098]; Ministero della Salute [GR-2011-02347192]; U.S. Department of Defense [BC161452P1].

Disclosure of potential conflicts of interest

A.N. and I.C. hold intellectual property rights on clinical uses of fasting-mimicking diets.