Recent data indicate a role for a pathway involved in controlling cell proliferation as a potential site for therapeutic intervention in breast cancer.
Mateusz Opyrchal, MD, PhD
Recent data indicate a role for a pathway involved in controlling cell proliferation as a potential site for therapeutic intervention in breast cancer (BC). The concept of intervening in cell cycle control in cancer is not new, but has been borne out in preclinical studies, and is now offering real possibilities for treatment in women with BC who fail a commonly used endocrine therapy.Unrestrained cell proliferation is a defining feature in cancer, and it is understood that, under normal conditions, this process is tightly regulated at many levels, one of which is the cell cycle.1,2In a critical cell cycle ‘checkpoint’ mitogenic signals cause upregulation of cyclin D1, which binds to, and activates, cyclindependent kinases (CDKs) 4 and 6. CDKs 4 and 6, when activated, phosphorylate the retinoblastoma (Rb) protein, an event which causes its release from the E2F transcription factor.2,3The release of E2F results in the transcriptional regulation of genes involved in the subsequent transition from the G1 phase of the cycle to the DNA synthesis (S) phase.2,3Additional control is provided by inhibitors of CDK 4/6, such as p21 and p16.2
Mateusz Opyrchal, MD, PhD, assistant professor of oncology at the Roswell Park Cancer Institute, Buffalo, New York, noted the considerable effort under way to examine the role of CDKs in oncogenesis and progression of cancers.
“CDKs play an important role in regulating cell division, and they guide many processes before and during the cell division, making this a highly regulated process. In cancers, aberrations in CDK function, and specifically in the CDK 4/6 and Rb pathway, which regulates initiation of gene transcription, lead to increased cell proliferation and tumor growth, as well as resistance to treatments,” Opyrchal said. Early evidence for a role of the CDK 4/6 pathway in BC came from key findings, such as cyclin D1 overexpression and dysregulation of Rb function in BC.4,5
Breast cancers are grouped by hormone receptor status (ie, estrogen receptor [ER]) and human epidermal growth factor receptor 2 (HER2) expression, and these classifications guide many treatment options (eg, tamoxifen, aromatase inhibitors for ER+ disease; trastuzumab for HER2+ disease).1Therapies that antagonize activity in the ER, such as tamoxifen and aromatase inhibitors (in postmenopausal women), are the mainstay for patients with ER+ disease, but resistance to these therapies is common.1,6,7We now know that an important component of efficacy for antiestrogen therapies is cell cycle control by the Rb-E2F pathway, and this could be at least one mechanism for resistance.6Initial data on the potential benefit of targeting CDK 4/6 in BC were reported in 2009 by Finn et al, who evaluated the effect of palbociclib (PD0332991), an orally active, specific inhibitor of CDK 4 and 6, to inhibit the growth of a range of ER+ BC cell lines.3They found that luminaltype ER+ breast cancers were most sensitive to growth inhibition by palbociclib, inclusive of those that were HER2 amplified, while those of the nonluminal/basal subtype were largely resistant. Moreover, by comparing large-scale gene expression profiles between palbociclib-sensitive and palbociclib-resistant cells, some 450 differentially expressed genes were identified; interestingly, p16 expression was decreased, and phosphorylated Rb (pRb) and cyclin D1 were upregulated in the most sensitive cell lines.
Consistent with its mechanism of action, pRb was decreased in the cell lines that were sensitive to palbociclib inhibition, and cells sensitive to palbociclib were arrested in the G0/G1 phase of the cell cycle. Most notably, palbociclib acted synergistically with both tamoxifen (in ER+ cell lines), as well as with trastuzumab (in HER2 amplified cell lines), and cells conditioned to be resistant to ER antagonists (ie, tamoxifen) could be partly resensitized by cotreatment with palbociclib.3
Further preclinical findings were reported by Thangavel et al in 2011, who demonstrated an uncoupling of cell cycle control and the functional (active/inactive) state of the ER in BC cell lines.6They found that luminal B cancers, which exhibit poor durability in response to endocrine therapy, exhibited elevated expression of a gene signature indicative of loss of Rb function. Thus, despite retaining Rb gene expression, there is a disruption in the function of Rb in this BC subtype.6
Their findings also suggested that Rb transcriptional control is a key component of the response to therapies that antagonize the ER. Moreover, in models of acquired antiestrogen resistance, there was a failure of therapies such as tamoxifen to active Rb-mediated transcriptional repression, with only sensitive cell lines exhibiting a downregulation of cyclin D1 RNA and protein levels. Treatment with palbociclib resulted in dephosphorylation of Rb, the attenuation of downstream target gene transcription, and inhibition of cell cycle progression.6
Collectively, these findings suggest that CDK 4/6 inhibition is a means by which sensitivity to antiestrogen therapy can be restored in tumors failing endocrine therapy through a dysregulation of Rb phosphorylation.6A further intriguing finding from this study was that, unlike antiestrogen therapy, prolonged exposure to palbociclib-induced cell cycle arrest and a phenotype consistent with cellular senescence, a state whereby cells remain growth arrested but are capable of long-term survival.2,6A phase II clinical trial (PALOMA-1) has investigated the efficacy of palbociclib, with or without letrozole, an aromatase inhibitor, in postmenopausal women with ER+/HER2 BC. Findings of the study, initially presented at the San Antonio Breast Cancer Symposium in 2012, showed a remarkable improvement in progression-free survival (PFS), the primary endpoint, for patients randomized to the combination versus letrozole alone (26.2 vs 7.5 months; hazard ratio [HR] = 0.32;P<.001). The treatment was well tolerated, with neutropenia, leukopenia, anemia, and fatigue being the most common events in the combination arm of the trial.8
In a recent panel discussion, Hope S. Rugo, MD, professor of medicine and director of the Breast Oncology, Clinical Trials, and Education Program, at the University of California San Francisco Comprehensive Cancer Center, commented on the findings of PALOMA-1, noting, “We’ve all been very excited because the PFS was [some] 3 1/2 times longer in the experimental arm…”9
Richard S. Finn, MD
PALOMA-2 is a phase III study evaluating the use of palbociclib in combination with letrozole versus letrozole alone in postmenopausal women with ER+, HER2 BC who have not received treatment for advanced disease, with the primary outcome of PFS.10 The study will also evaluate potential tumor tissue biomarkers of response, and/ or resistance to treatment. Speaking on the use of ER positivity as a selection criteria for PALOMA-2, Richard S. Finn, MD, from the division of hematology/oncology at the Jonsson Comprehensive Cancer Center at the University of California in Los Angeles, noted: “We are not looking at any other selection marker other than that, because the data we have to date from the phase II study show that the best predictive marker is just being ER positive.”11
Looking toward second-line treatment, the phase III PALOMA-3 trial is investigating palbociclib in combination with endocrine therapy (fulvestrant) versus placebo + fulvestrant in women with ER+/HER2 BC that has progressed after a prior endocrine therapy, with the primary outcome of PFS.12 Nicholas Turner, PhD, consultant medical oncologist at The Royal Marsden Hospital in London, stated: “In the PALOMA-3 trial, we’re specifically targeting women who progressed on their first-line hormone therapy. There are a number of options [in this setting], but one of the drugs that has the best data is fulvestrant, given at the 500-mg dose, and that’s why we chose to partner that with palbociclib… in this study.”
According to Turner, there was no evidence of any drug interaction between palbociclib and fulvestrant, and very strong data that endocrine-resistant cancers can still be sensitive to palbociclib, possibly extending the length of time that the cancer can be controlled.13In addition to palbociclib, 2 other inhibitors of the CDK 4/6 pathway are undergoing clinical evaluation, LEE011 and LY2835219.14Studies are evaluating the use of these agents in advanced breast cancer, either alone or in combination with hormonal agents.15,16As with many other targeted therapies, however, there is a need to identify prospectively those patients most likely to benefit from these therapies, and those who will not, or are no longer benefitting.
“The one disappointing result of the PALOMA-1 trial was that it failed to identify biomarkers to select patients who are best suited for the therapy,” Oprychal added, noting that several rationally chosen biomarkers (eg, amplified cyclin D1 and/or loss of p16) “did not have any influence on whether the patients received benefit from palbociclib.” Efforts will therefore continue to identify potential biomarkers in breast and other cancers, and their potential for use in combination with other targeted therapies.14