“Braking” the Cycle of Resistance in Endocrine Therapy for Breast Cancer

Abstract

Endocrine resistance leads to recurrence and death from breast cancer. Animal models of endocrine resistance enable preclinical identification of efficacious therapeutic combinations and further our understanding of resistance. This strategy provides new insights into optimally targeting interactions between estrogen receptor (ESR-1) activity and the cell cycle by CDK4/6 inhibitors. See related article by Wardell et al., p. 5121 In this issue of Clinical Cancer Research, Wardell and colleagues utilized mouse models of endocrine therapy-resistant breast cancer to probe the relative efficacy of the cyclin-dependent kinase (CDK) 4/6 inhibitor, palbociclib, emerging endocrine agents, and their combination (1). Estrogen receptor (ER)-expressing breast cancer is the most common form of the disease, comprising nearly 80% of the more than 200,000 new cases diagnosed annually. While highly treatable by therapies that block ER signaling or prevent the production of its ligand, estrogen, resistance to endocrine therapy is universal in the metastatic setting and is also thought to be a common mechanism of recurrence in early-stage disease. The frequent occurrence of therapeutic resistance, coupled with the unparalleled high prevalence of this malignancy, mandates new approaches to overcoming endocrine resistance if continued improvements in survival are to be achieved.Current therapies targeting ER function include depletion of endogenous estrogens through aromatase-inhibition (AI), as well as blockade of estrogen binding to ER by selective estrogen receptor modulators (SERMS) and/or degraders (SERDS). Although these approaches are highly effective for some cancers, alterations resulting in estrogen-independent nuclear signaling or activation of escape pathways engender resistance. For example, activating mutations in ESR1 can render ER signaling ligandindependent (2). Similarly, changes in ER coactivators and corepressors can enable growth of ER þ cells despite low levels of estrogen.Alternative resistance mechanisms involve ER-driven activation of oncogenic signaling pathways, including the PI3K/Akt/mTOR pathway (3), and interactions with the core program that controls proliferation and cell cycle progression. These include inhibition or inactivation of negative regulators of the cell cycle, including RB family members, as well as amplification and upregulation of cell cycle-promoting oncogenes, such as cyclin D1 (CCND1) and c-myc (4). CCND1 drives RB inactivation and cell cycle progression by complexing with the cyclin-dependent kinases, CDK4 and CDK6. Given that ER activation, among other stimuli, can induce c-myc and CCND1 expression (5), CDK4 and CDK6 represent a nexus of signaling between the ER pathway and growth-promoting pathways in the cell. More recent data suggest that CDK4 and CDK6 can also regulate levels of steroid-metabolizing enzymes, including the AKR1C family, influencing the estrogenic state of breast tumors (6).Enter the new class of oral, highly selective CDK 4/6 inhibitors, including palbociclib (...

RelB sustains endocrine resistant malignancy: an insight of noncanonical NF-κB pathway into breast Cancer progression

“…Current therapeutic strategies for BCa mainly consisted of surgical reduction and radiotherapy-based locally therapy with anticancer drugs provided systemic therapy including chemotherapy, endocrine therapy and targeted therapy. However, the comprehensive treatment strategy is still limited and a large number of patients eventually developed to more aggressive malignant forms that are resistant to the most of treatments [ 6 , 7 ]. Thus, the discovery of new biomarkers for surveillance of the disease progression and novel molecular targets for improving therapeutic efficiency are urgently needed.…”

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“…Краще розуміння механізмів резистентності матиме вирішальне значення у відповіді на такі питання. Було описано декілька різних механізмів резистентності при HR+/HER2- метастатичному РМЗ, що стало основою для подальших досліджень, спрямованих на поліпшення результатів ендокринного лікування [7].…”

“…шляхів PI3K/mTOR (phosphatidylinositol 3-kinase/ mammalian target of rapamycin) і CDK4/6-Rb (cyclindependent kinase 4/6-retinoblastoma) в розвитку резистентності і використанні більш ефективних комбінацій для лікування HR+ підтипів РМЗ [6,7]. Такі комбінації ендокринної терапії з еверолімусом (інгібітор mTOR) і новим класом препаратів інгібіторів циклінзалежних кіназ CDK4/6 швидко трансформували алгоритми поточного лікування гормонозалежного поширеного РМЗ.…”

The Open Issues Regarding Cyclin-Dependent Kinase 4/6 Inhibitors in the Management of Advanced Breast Cancer

“…However, when CDK4/6 inhibitors are introduced after progression on endocrine therapy (ET) as monotherapy, it is not yet well established whether it is possible to maintain the same ET in conjunction with CDK4/6 inhibitors. Preliminary evidence suggests that CDK4/6 inhibition therapy may have the potential to reverse endocrine resistance [4], and this combination regimen would take advantage of this feature.…”