Oral low-dose cyclophosphamide with endocrine therapy through effects on Tregs in TLSs may improve clinical response in elderly metastatic breast cancer patients

Background: Despite limited research on refractory and/or endocrine therapy failure in elderly metastatic breast cancer (MBC) patients, a prior study did show that low-dose oral cyclophosphamide (CY) can improve the overall survival rate of MBC, possibly through immunoregulation of regulatory T cells (Tregs). We performed a preliminary investigation of the combination of endocrine therapy (ET) with oral low-dose CY as salvage therapy in elderly patients with peripheral blood regulatory T cell analyses. In addition, we evaluated the association of tumor tertiary lymphoid structures (TLSs) with therapeutic outcomes. Patients and Methods: HR+/HER2 advanced breast cancer patients who received low-dose CY combined with ET or ET only from April 2015 to August 2021 were enrolled in this retrospective study. The primary outcome was the clinical control rate (CCR), and the secondary outcome was progression-free survival (PFS). Circulating T lymphocyte subpopulations represented by Tregs were monitored during treatment by flow cytometry methods. TLS diagnosis was confirmed by hematoxylin–eosin staining of pretreatment specimens, and CD3, CD4, and Foxp3 were stained using Opal multicolor immunofluorescence. Results:85 patients received CY + ET and 50 patients received ET only were enrolled, CCR was 73% (62/85) vs. 70% (45/50), and objective response rate (ORR) was 28% (24/85) vs. 24% (12/50). No deaths occurred during the study period. The median PFS time was 13 vs. 11 months (P = 0.03). In the CY + ET group, the decreases in CD4+/CD25+/Foxp3+ T cells (P<0.001) were favorable for both clinical control and prolonged PFS (P < 0.001), compared with patients without TLSs, those with TLSs were more likely to have better clinical control and PFS (mean time=6 months), and a higher level of Treg cells in TLSs pretreatment correlated with longer PFS (P=0.043). Conclusions: Oral low-dose CY combined with standard ET exerts immunological regulation by decreasing Treg to achieve improved clinical responses. Moreover, patients with TLSs might benefit more from such therapy than those without TLSs, and a high Treg count in TLSs before treatment predicts better therapy efficacy.