High p16 expression and heterozygous RB1 loss are biomarkers for CDK4/6 inhibitor resistance in ER+ breast cancer

CDK4/6 inhibitors combined with endocrine therapy have demonstrated higher antitumor activity than endocrine therapy alone for the treatment of advanced estrogen receptor-positive breast cancer. Some of these tumors are de novo resistant to CDK4/6 inhibitors and others develop acquired resistance. Here, we show that p16 overexpression is associated with reduced antitumor activity of CDK4/6 inhibitors in patient-derived xenografts (n = 37) and estrogen receptor-positive breast cancer cell lines, as well as reduced response of early and advanced breast cancer patients to CDK4/6 inhibitors (n = 89). We also identified heterozygous RB1 loss as biomarker of acquired resistance and poor clinical outcome. Combination of the CDK4/6 inhibitor ribociclib with the PI3K inhibitor alpelisib showed antitumor activity in estrogen receptor-positive non-basal-like breast cancer patient-derived xenografts, independently of PIK3CA, ESR1 or RB1 mutation, also in drug de-escalation experiments or omitting endocrine therapy. Our results offer insights into predicting primary/acquired resistance to CDK4/6 inhibitors and post-progression therapeutic strategies.

Targeting the PI3K/AKT/mTOR Pathway in Hormone-Positive Breast Cancer

Approximately 70% of invasive breast cancers have some degree of dependence on the estrogen hormone for cell proliferation and growth. These tumors have estrogen and/or progesterone receptors (ER/PR+), generally referred to as hormone receptor positive (HR+) tumors, as indicated by the presence of positive staining and varying intensity levels of estrogen and/or progesterone receptors on immunohistochemistry. Therapies that inhibit ER signaling pathways, such as aromatase inhibitors (letrozole, anastrozole, exemestane), selective ER modulators (tamoxifen), and ER down-regulators (fulvestrant), are the mainstays of treatment for hormone-receptor-positive breast cancers. However, de novo or acquired resistance to ER targeted therapies is present in many tumors, leading to disease progression. The PI3K/AKT/mTOR pathway is implicated in sustaining endocrine resistance and has become the target of many new drugs for ER+ breast cancer. This article reviews the function of the phosphoinositide 3-kinase (PI3K)/AKT/mTOR pathway and the various classes of PI3K pathway inhibitors that have been developed to disrupt this pathway signaling for the treatment of hormone-receptor-positive breast cancer.

Treatment of Luminal Metastatic Breast Cancer beyond CDK4/6 Inhibition: Is There a Standard of Care in Clinical Practice?

BACKGROUND

CDK4/6 inhibitors have become the standard for first-line treatment of metastatic luminal breast cancer based on consistent data from several phase 3 trials demonstrating clinically meaningful improvement of progression-free as well as overall survival. In addition, they are about to become a part of adjuvant treatment for patients with high-risk luminal disease based on positive results from the first randomized phase 3 trial on abemaciclib. Nevertheless, the majority of patients with advanced or metastatic luminal breast cancer and prospectively a relevant proportion of patients treated in the adjuvant setting will eventually develop resistance to this endocrine based combination within 12-36 months, depending on the line of treatment.

CONCLUSION

Potential subsequent therapies include PI3K inhibitors, mTOR inhibitors, endocrine monotherapy, PARP inhibitors, and chemotherapy. However, these therapies have mainly been developed in the pre-CDK4/6 inhibitor era and little is known about potential cross-resistance. The concept of continuing CDK4/6 inhibition beyond progression is supported by some preclinical data, but to date there is very limited clinical evidence to support this strategy. Therefore, treatment of metastatic luminal breast cancer after progression on CDK4/6 inhibitors remains a challenge.

KEY MESSAGES

Here we review current evidence from pro- and retrospective studies and give an outlook on future developments with respect to novel therapeutic agents, including oral SERD and AKT inhibitors, which have the potential to change the therapeutic landscape in the future. Furthermore, clinical treatment algorithms and current research will also be discussed.

Alpelisib in the treatment of metastatic HR+ breast cancer with PIK3CA mutations

Since the US FDA approval of everolimus/exemestane in July 2012, and of the first CDK 4/6 inhibitor, palbociclib, combined with endocrine treatment in February 2015, a third class of therapeutic compounds, the PI3K inhibitors, has been introduced to the arsenal of targeted therapies overcoming endocrine resistance in hormone receptor-positive metastatic breast cancer. Alpelisib (PIQRAY®) is the first of these novel agents yielding promising clinical results, giving an impetus to further development of tailored endocrine anticancer treatments. Herein, we review its pharmacodynamic and pharmacokinetic properties, safety and efficacy data, as well as Phase III SOLAR-1 trial, prompting FDA approval of alpelisib in hormone receptor-positive metastatic breast cancer harboring PIK3CA mutations. Furthermore, implications for clinical use and current research will also be discussed.

A pharmacokinetic evaluation of alpelisib for the treatment of HR+, HER2-negative, PIK3CA-mutated advanced or metastatic breast cancer

Introduction: In most cases, metastatic breast cancer remains an incurable disease. A PIK3CA mutation is detected in 30-40% of all hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) advanced breast cancers. PIK3CA activating mutations have been linked to endocrine resistance. PI3K inhibitors therefore offer promising new therapeutic options for this disease. Areas covered: This review discusses the pharmacologic properties, preclinical development, clinical efficacy, and safety profile of alpelisib, a PI3K inhibitor indicated in HR+/HER2 - PIK3CA-mutated advanced breast cancer, describing current therapeutic indication and open questions. Expert opinion: Following results of the SOLAR-1 trial, alpelisib became the first PI3K inhibitor approved by the U.S. Food and Drug Administration, in combination with fulvestrant, for postmenopausal women and men with HR+/HER2 - PIK3CA-mutated advanced breast cancer following progression on or after an endocrine-based regimen. This trial showed a substantial improvement in progression-free survival. However, given the side effects of alpelisib, the treatment decision should follow a thorough benefit-risk assessment. The BYLieve trial suggests alpelisib-fulvestrant benefit after progression on CDK 4/6 inhibitors. The identification of patients that are likely to benefit the most from PI3K inhibitors is still eagerly sought.

Alpelisib: First Global Approval

Alpelisib (Piqray™)-an orally available phosphatidylinositol 3-kinase (PI3K) inhibitor with specific activity against PI3K alpha (PI3Kα)-is being developed by Novartis for the treatment of breast cancer. Alpelisib has demonstrated efficacy in combination with fulvestrant as treatment for hormone receptor (HR)-positive, human epidermal growth factor receptor-2 (HER2)-negative breast cancer in patients with a PIK3CA mutation and was recently approved for this indication in the USA. This article summarizes the milestones in the development of alpelisib leading to this first approval.

Practical Treatment Strategies and Future Directions After Progression While Receiving CDK4/6 Inhibition and Endocrine Therapy in Advanced HR+/HER2- Breast Cancer

Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) in combination with backbone endocrine therapy have markedly improved progression-free survival and overall survival over endocrine therapy alone in advanced hormone receptor-positive, HER2-negative (HR⁺/HER2^-) breast cancer and are the standard of care in the first- or second-line setting. There are few data to drive decision making for subsequent treatment strategies after inevitable disease progression after CDK4/6i. Information about the genomic landscape of CDK4/6i-resistant disease is emerging. Resistance mechanisms appear to be varied, but mutations in PIK3CA and ESR1, which can be acquired while receiving treatment, are frequent. Activating PIK3CA mutations are present in up to 35% of patients and are now the most actionable genomic alteration in HR⁺/HER2^- advanced breast cancer with the recent approval of alpelisib and fulvestrant. Everolimus-based combinations and chemotherapy appear to have continued efficacy after progression while receiving CDK4/6i, although historical data on benefit include CDK4/6i-naive patients. Use of selective estrogen down-regulators over aromatase inhibitors is best once the patient has an acquired ESR1 mutation. Tumor biopsy with genomic sequencing and repeat biomarker analysis in patients with CDK4/6i- and endocrine-resistant disease will be integral to guide subsequent treatment strategies and to inform clinical trial eligibility. Promising novel therapeutics in CDK4/6i-resistant disease including oral selective estrogen down-regulators, fibroblast growth factor receptor antagonists, and immunotherapy will be discussed.