Synthesis and Preliminary Evaluation of Tanshinone Mimic Conjugates for Mechanism of Action Studies

Human antigen R (HuR) is an RNA binding protein (RBP) belonging to the ELAV (Embryonic Lethal Abnormal Vision) family, which stabilizes mRNAs and regulates the expression of multiple genes. Its altered expression or localization is related to pathological features such as cancer or inflammation. Dihydrotanshinone I (DHTS I) is a naturally occurring, tetracyclic ortho-quinone inhibitor of the HuR-mRNA interaction. Our earlier efforts led to the identification of a synthetic Tanshinone Mimic (TM) 2 with improved affinity for HuR. Here we report five new TM probes 3-5 bearing a detection-promoting moiety (either photo affinity probe - PAP or biotin) as a para-substituent on the phenyl-sulphonamide for mechanism of action (MoA) studies. Biological and biochemical assays were used to characterize the novel TM conjugates 3-5. They showed similar toxic activity in HuR-expressing triple-negative breast cancer MDA-MB-231 cells, with micromolar CC₅₀s. REMSAs revealed that photoactivatable groups (4 a and 4 b), but not biotin (5 a and 5 b), prevented conjugates' ability to disrupt rHuR-RNA complexes. Further biochemical studies confirmed that biotinylated probes, in particular 5 a, can be used to isolate rM1 M2 from solutions, taking advantage of streptavidin-coated magnetic beads, thus being the most promising HuR inhibitor to be used for further MoA studies in cell lysates.