The overexpression of estrogen receptor (ER) α is not only closely related to the development of ER⁺ breast cancer, but is also an important biomarker for clinical diagnosis and treatment. Herein, we report several ERα targeting hypoxia turn-on fluorescent probes with antitumor activity for breast cancer cells. Among them, probes 3 and 5 displayed good ERα targeting ability and favorable hypoxia turn-on response in MCF-7 cells. Moreover, the probes 3 and 5 exhibited good antiproliferative activity towards MCF-7 cells (IC₅₀ = 8.5 μM, 10.3 μM) and a much lower cytotoxicity to normal cells compared with the positive control. It is expected that these novel fluorescent probes may provide useful tools for the theranostics of ER⁺ breast cancer.

中文翻译：

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合理设计具有抗增殖活性的针对缺氧的ERα乳腺癌探针。

雌激素受体（ER）α的过表达不仅与ER ⁺乳腺癌的发展密切相关，而且还是临床诊断和治疗的重要生物标志物。本文中，我们报道了几种针对缺氧的ERα靶向荧光探针，这些探针具有针对乳腺癌细胞的抗肿瘤活性。其中，探针3和5在MCF-7细胞中显示出良好的ERα靶向能力和良好的缺氧开启反应。此外，探针3和5对MCF-7细胞具有良好的抗增殖活性（IC ₅₀= 8.5μM，10.3μM），与阳性对照相比，对正常细胞的细胞毒性要低得多。预期这些新颖的荧光探针可以为ER ⁺乳腺癌的治疗诊断提供有用的工具。