Amino Acid-Triggered Activation of Double-Masked Gossyzid-Metal Conjugates: Metal-Dependent Stability, Photophysical Properties, and Anticancer Activity.

Double-masking strategies that layer classical prodrug chemistry with metal coordination offer a powerful route to widen the therapeutic window of reactive small molecules. Building on gossyzid, a pH-activatable imine prodrug of the polyphenolic aldehyde gossypol, we introduce a family of gossyzid-metal conjugates (GMCs) that implement orthogonal masking: gossypol is first protected as gossyzid, then "locked" within coordination environments formed with first-row transition metal ions (Mn, Fe, Co, Ni, Cu, and Zn). Spectroscopic analyses, powder x-ray diffraction, and thermogravimetric profiling support the formation of coordination architectures, rationalizing the mass-spectrometric intractability of these species. Photophysical studies reveal metal-dependent binding stoichiometries and affinities, establishing a tunable stability series across the GMCs. Using amino acids as competitive chelators, we demonstrate selective, triggerable de-chelation of the metal node that regenerates gossyzid under tumor-mimicking conditions; subsequent tumor-microenvironment (TME)-relevant acidolysis completes its activation to gossypol. Notably, only specific amino acids effectively unmask defined GMC subsets, enabling control over activation kinetics and extent. In triple-negative breast cancer models, GMC cytotoxicity correlates with intrinsic metal-ligand binding, thus showing the contributions of double masking. To our knowledge, this is the first systematic demonstration of amino-acid-triggered activation in double-masked metal-prodrug conjugates, with direct implications for the design of next-generation, TME-responsive prodrugs.