Palbociclib for HR Positive / HER2-negative Isolated Locoregional Recurrence of Breast Cancer

Local or regional recurrence of breast cancer after mastectomy or lumpectomy indicates a poor prognosis, and accompanies or precedes distant metastasis in a high proportion of patients. Patients with isolated locoregional recurrences (ILRR), without evidence of distant metastasis hold a substantial risk of developing subsequent distant metastasis, with 5-year survival probabilities ranging between 45% and 80% after locoregional recurrence. These outcomes show the powerful negative prognostic importance of ILRR events and the need for treatments beyond surgical removal of the ILRR. Adjuvant chemotherapy and endocrine therapies reduce the risk of relapse and death in patients with primary breast cancer. However, few data are available to inform the recommendation of systemic treatment for locoregional recurrence. The International Breast Cancer Studies Group carried out the CALOR trial, Chemotherapy as Adjuvant for Locally Recurrent breast cancer (IBCSG 27-02 / BIG 1-02 / NSABP B-37), in collaboration with the Breast International Group (BIG) and the National Surgical Adjuvant Breast and Bowel Project (NSABP), to establish whether chemotherapy improves the outcome of patients with ILRR. An updated, final analysis of CALOR after median follow-up of about 9 years was published in the Journal of Clinical Oncology in April 2018, which confirmed chemotherapy benefitted patients with resected ER-negative ILRR and did not support the use of chemotherapy for ER-positive ILRR. CALOR results strongly suggest that tailoring treatment according to the disease characteristics of the recurrent lesion, in this case ILRR, provides a better indication of the possible responsiveness to treatment than relying on the characteristics of the primary tumor. Palbociclib has been granted FDA approval in the U.S. for the treatment of HR-positive/HER2-negative advanced breast cancer in combination with the hormonal treatments letrozole and fulvestrant given the unprecedented results in terms of efficacy of two pivotal clinical trials (PALOMA-2 and PALOMA-3). Palbociclib and other CDK4/6 inhibitors have also shown a good toxicity profile and therefore are ideal candidates for combination with hormonal therapy. CDK4/6 pathway activation is a well-known mechanism of resistance to endocrine therapy, indeed CDK4/6 inhibitors have shown activity in cellular models of acquired resistance to endocrine therapies. The reason for prolonged duration of palbociclib in the adjuvant setting (2 years) comes from the evidence of preclinical studies where cell senescence was investigated as an appealing mechanism of cell death and was indeed observed in vitro after exposure of breast cancer cells and tumors to a combination of endocrine therapy and palbociclib. It is therefore hypothesized that the longer patients receive combined treatment with palbociclib and an antiestrogen, the more likely they may derive prolonged clinical benefit. Based on the results of the CALOR trial and on strong evidence of activity of the combination of CDK4/6 inhibitors and endocrine therapy, the hypothesis of the POLAR trial is that the CDK4/6 inhibitor palbociclib in combination with endocrine therapy may be active as adjuvant therapy in patients with HR-positive/HER2-negative resected isolated locoregional recurrence of breast cancer.