Structure–Activity Relationship Studies of Substituted 2-Phenyl-1,2,4-triazine-3,5(2<i>H</i>,4<i>H</i>)-dione Analogues: Development of Potent eEF2K Degraders against Triple-Negative Breast Cancer

eEF2K, an atypical alpha-kinase, is responsible for regulating protein synthesis and energy homeostasis. Aberrant eEF2K function has been linked to various human cancers, including triple-negative breast cancer (TNBC). However, limited cellular activity of current eEF2K modulators impedes their clinical application. Based on the 2-phenyl-1,2,4-triazine-3,5(2<i>H</i>,4<i>H</i>)-dione scaffold of our hits <b>I4</b> and <b>C1</b>, structure-activity relationship analysis led to the discovery of several more active derivatives (e.g., <b>19</b>, <b>34</b>, and <b>36</b>) in inhibiting the viability of TNBC cell line MDA-MB-231. Moreover, the most potent compound <b>36</b> significantly suppresses the viability, proliferation, and migration of both MDA-MB-231 and HCC1806 cell lines. Mechanistically, compound <b>36</b> has a high binding affinity for the eEF2K protein and effectively induces its degradation. Additionally, <b>36</b> exerts a comparable tumor-suppressive effect to paclitaxel in an MDA-MB-231 cell xenograft mouse model with no obvious toxicity, demonstrating that compound <b>36</b> could be developed as a potential novel therapeutic for TNBC treatment.