Isomer-driven supramolecular polymorphism of 4-hydroxytamoxifen modulating nanostructure-dependent cellular uptake and cytotoxicity.

Geometric isomerism in small-molecule drugs has traditionally been studied in the context of receptor binding or metabolic stability. However, its influence on supramolecular behavior and downstream biological function remains underexplored. Here, we investigate how the (Z)- and (E)-isomers of 4-hydroxytamoxifen (4-OHT) self-assemble in aqueous environments and how this supramolecular divergence impacts anticancer activity. The (Z)-isomer forms small, amorphous nanoflake assemblies that display enhanced colloidal dispersibility and efficient cellular uptake, while the (E)-isomer assembles into large, anisotropic crystalline ribbons with limited intracellular delivery. These differences in aggregate morphology arise spontaneously in water, without surfactants or templating agents, and are driven solely by molecular geometry. Spectroscopic, microscopic, and computational analyses reveal distinct packing modes consistent with J- and H-type aggregation for the (Z)- and (E)-forms, respectively. In vitro assays show that (Z)-4-OHT exhibits significantly higher cytotoxicity in ER-positive MCF-7 cells compared to its (E)-counterpart, as a result of both superior receptor affinity and greater intracellular accumulation enabled by its self-assembled form. By systematically linking molecular configuration, self-assembly behavior, and biological efficacy, this study introduces a dual-axis model of drug performance in which both molecular recognition and supramolecular organization are critical design parameters. These insights open new opportunities for the rational design of isomeric therapeutic agents through the co-optimization of receptor binding and nanoscale morphology.