Dear editor,

I read with great interest the Bun et al. study [1], in which investigated the association between cytokines and immune cells, such as myeloid-derived suppressor cells (MDSCs) and cytotoxic and regulatory T cells, to explore how these cytokines might affect the immune microenvironment in metastatic breast cancer (MBC) patients treated with eribulin. They concluded that baseline interleukin (IL)-6 is an important prognostic factor in patients with MBC treated with eribulin. Their results show that high IL-6 is associated with higher levels of MDSCs which suppress anti-tumor immunity, such as CD8+ cells. It appears that eribulin is not particularly effective in patients with high IL-6 due to a poor tumor immune microenvironment. IL-6 plays a major role in inflammatory processes. It modulates the transcription of several liver-specific genes during acute inflammatory states, particularly C-reactive protein (CRP). The prognostic impacts of preoperative CRP and interleukin (IL)-6 expression levels in patients with breast cancer remain controversial [2]. The authors did not measure serum CRP level. It would be expected to see a positive association between IL-6 and CRP levels. Moreover, CRP measurement is also more cost effective than IL-6 measurement in routine practice. This issue merits further investigation.