Chemotherapy De-escalation in HR +, HER2-, Intermediate-risk Early Breast Cancer Treated With Adjuvant Ribociclib

The advent of CDK4/6 inhibitors has changed the outlook of patients with metastatic hormone receptor-positive (HR+) HER2- breast cancer. Moreover, including CDK4/6 inhibitors in the adjuvant treatment regimens strategies has led to significant gains in disease-free survival.

The phase III NATALEE trial demonstrated the efficacy of an adjuvant three-year treatment with ribociclib in prolonging invasive disease-free survival (iDFS) in patients with intermediate and high-risk HR+ HER2- early breast cancer.

Contrarily to similar studies of CDK4/6 inhibitors in this setting, NATALEE included a group of patients with intermediate clinical risk (pT1-2 pN1, pT3-4 pN0 or pT2 pN0 with histological grade 3 or grade 2 with Ki67≥ 20%). These patients are usually considered for adjuvant chemotherapy based on their clinicopathological conditions or the results of a genomic signature (e.g. Oncotype Dx). Nevertheless, the benefit of adjuvant chemotherapy in these patients is uncertain (and likely small) in the context of an adjuvant treatment strategy that includes a CDK 4/6 inhibitor.

As such, a de-escalation trial could demonstrate that patients with intermediate-risk breast cancer treated with CDK4/6 inhibitors could be spared the dreaded chemotherapy side effects while ensuring similar survival outcomes.

In order to be generalizable and practice changing, a trial in this setting should aim to be as pragmatic as possible, particularly in inclusion criteria, with the required resources for patient inclusion and delivery of care being as similar as possible to those employed in usual care. As such, chemotherapy eligibility should be defined similarly to routine clinical practice in the participating centers (i.e. using routine clinicopathological parameters and/or genomic signatures).

While single-arm designs could help address non-inferiority in the previously mentioned setting, they are usually compared to historical controls and lack external validation. Moreover, in some settings, like early breast cancer, the standard of care may change relatively quickly (e.g. Oncotype Dx-based chemotherapy de-escalation or adjuvant CDK4/6 inhibitors use), rendering the comparison to historical controls challenging, limiting the study conclusions and their impact on clinical practice. Finally, there is no consensus on the optimal non-inferiority threshold in single-arm trials using historical controls as a comparator. As such, randomized controlled non-inferiority trials with a strict non-inferiority margin remain the gold standard design to prove that a de-escalated treatment regimen is safe and advantageous, and the only ones capable of producing level IA evidence according to ESMO (Trapani et al., Annals of Oncology 2022).