Harnessing the Sulfur-for-Oxygen Shift: A Magic Bullet for Dynamic Photophysical and Anticancer Activities of Indole-Barbituric Acid Construct

The development of small molecule-based drugs emerged as a cornerstone of modern drug discovery. Structural activity relationship (SAR) studies in medicinal chemistry are crucial for lead optimization, where a subtle change in the substituent can significantly alter its binding affinity with the biological target. Herein, a highly efficient single-atom substitution (SAS) approach has been developed, where sulfur for oxygen strategy is utilized as a powerful molecular editing technique to identify N-vinyl Indole-thiobarbituric acid (6a) as a novel small molecule-based scaffold with tunable photophysical and antiproliferative activities. A series of NIR-emitting indole-barbituric/thiobarbituric acid conjugates exhibiting aggregation-induced emission (AIE) were prepared, where the replacement of oxygen for sulfur strategy emerged as a magic bullet. On the evaluation of photophysical properties and chemopreventive efficacies, a significant improvement in the absorption and emission profile, cellular uptake, and antiproliferative activity was noted for sulfur counterparts. From the pool of the molecules, the lead molecule 6a unveils a 55 nm emission shift, 142-fold increased anticancer profile, and ~4-fold elevated cellular uptake. Furthermore, the colocalization experiment unravels the nuclear localization of 6a, where it causes severe DNA damage, arrests the cell cycle in the G2/M phase, and leads to the activation of p53-mediated apoptosis.