Bright rRNA-Associated Merocyanine Probes Highlight Tumor Extensions in Ovarian Cancer

We report a structure-activity relationship of merocyanine quinolinium dyes optimized as fluorogenic rRNA-associated probes. A series of analogs (11a-h) bearing electron-withdrawing groups (EWGs) or electron-donating groups (EDGs) on the phenolic "push" ring (ortho vs meta; mono vs disubstitution) was synthesized to evaluate electronic and steric effects on photophysical performance. Emission extended to 658 nm, fluorescence turn-on reached 570-fold, and brightness to 8 mM-1 cm-1. ortho-EWGs and meta-EDGs enhanced fluorescence, whereas steric hindrance (e.g., meta-dichloro) reduced planarity and diminished performance, consistent with DFT analysis. Dye 11f (meta-dimethoxy) emerged as the lead compound, combining strong fluorogenic response, brightness, stability, rapid membrane permeability, and low cytotoxicity. Enzymatic and pharmacologic perturbation experiments supported preferential cellular association of 11f with rRNA-rich compartments. U2OS cancer cells showed higher fluorescence intensity than HFF primary fibroblasts, consistent with increased rRNA-rich compartments in cancer cells. In ex vivo ovarian tumor specimens, 11f demonstrated enhanced visualization of malignant regions and small tumor extensions within surrounding adipose tissue.