The ‘classical’ mechanism of action of CDK4/6 inhibitors is through the inhibition of the cell cycle. Inhibition of CDK4/6 prevents phosphorylation and inactivation of the Rb protein resulting in continued inhibition of the E2F transcription factor (Finn et al., 2016; Klein et al., 2018).
The response of tumour cells to CDK4/6 inhibition does not seem to be uniform. Depending on the cell type and transforming event, some Rb-positive cells undergo quiescence while others become senescent. The advantage of creating senescent cells is that they will not return to the cell signal following removal of the inhibitory signal and are generally resistant to proliferative signals (Klein et al., 2018). Achieving senescence requires cells to transition from quiescence and is termed seroconversion or senescence after growth arrest (SAGA). CDK4 inhibitor-induced SAGA in breast cancer cells involved the downregulation of MDM2, redistribution of the chromatin remodelling ATRX and repression of HRAS transcription (Kovatcheva et al., 2015; Kovatcheva et al., 2017). Inhibition of CDK4/6 may also promote senescence in an Rb-independent mechanism through reduced phosphorylation and increased destabilisation of the transcription factor FOXM1 – although this has been demonstrated in melanoma, but not breast cancer (Anders et al., 2011: Klein et al., 2018).
In vitro experiments have shown CDK4/6 inhibitors, often in combination with other therapies, are capable of manipulating metabolic pathways and driving cells towards apoptosis or senescence. However, these changes are likely to be context dependent and more research is required (Klein et al., 2018).
The efficacy and safety of approved CDK4/6 inhibitors are discussed in more detail in the current treatments section of the Learning Zone.
About 20% of patients with advanced HR+/HER2- breast cancer don’t respond to the available CDK4/6 inhibitors (Shah et al., 2018). Can predictive biomarkers help guide therapeutic decisions and help identify patients that are likely to be resistant to CDK4/6 inhibitors?
Currently, the best predictive marker for a response to CDK4/6 inhibitors continues to be a HR+ tumour status. A number of clinical trials have investigated a host of possible predictors; however, no predictive insights were gained from amplification of the cyclin D1 (CCND1) gene, loss of the p16 (CDKN2A) gene, mutation of the PIK3CA or ESR1 gene, baseline retinoblastoma (Rb), Ki67 and p16 protein levels or CDKN2A and CCND1 messenger RNA expression levels (Shah et al., 2018). Interestingly, while high p16ink4a expression and loss of Rb expression has been used as an exclusion criterion in some CDK4/6 clinical trials given the need for Rb for CDK4/6 to exert its effect, the relevance in breast cancer appears to be limited given that loss of Rb is rare (~3% of cases). However, more research may be required in the metastatic setting as Rb loss may occur more frequently (Knudsen and Witkiewicz, 2017).
Despite these set-backs, ongoing clinical studies are aiming to identify clinically relevant biomarkers.
The ‘Palbociclib in molecularly characterized ER-positive/HER2-negative metastatic breast cancer’ (PYTHIA) study is being conducted by the International Breast Cancer Study group. This international, multicentre, prospective single arm Phase II biomarker discovery study aims to assess the association of progression-free survival (PFS) with various gene mutations, copy number alterations and signatures in post-menopausal women with metastatic or locally relapsed HR+/HER2- breast cancer. The patients have disease that has progressed following adjuvant endocrine therapy or one line of systemic therapy and are being treated with palbociclib and fulvestrant. The study is expected to finish in January 2020.
This study is seeking to identify biomarkers that predict response to the CDK4/6 inhibitors palbociclib and ribociclib in patients with metastatic HR+/HER- breast cancer. An open-label, multicentre study, enrolled patients receive palbociclib or ribociclib plus endocrine therapy as first-line treatment for their disease. A range of biomarkers will be assessed at baseline and following disease progression to measure any changes following CDK4/6 inhibitor and endocrine therapy treatment. The primary outcome is the level of phosphorylated Rb in tumour tissue at baseline to assess its possibility as a predictive biomarker for palbociclib or ribociclib treatment. The study is expected to complete in June 2019.