Stimulus-Responsive Prochelators for Manipulating Cellular Metals.
Metal ions are essential for a wide range of physiological processes, but they can also be toxic if not appropriately regulated by a complex network of metal trafficking proteins. Intervention in cellular metal distribution with small-molecule or peptide chelating agents has promising therapeutic potential to harness metals to fight disease. Molecular outcomes associated with forming metal-chelate interactions in situ include altering the concentration and subcellular metal distribution, inhibiting metalloenzymes, enhancing the reactivity of a metal species to elicit a favorable biological response, or passivating the reactivity of a metal species to prevent deleterious reactivity. The systemic administration of metal chelating agents, however, raises safety concerns due to the potential risks of indiscriminate extraction of metals from critical metalloproteins and inhibition of metalloenzymes. One can estimate that chelators capable of complexing metal ions with dissociation constants in the submicromolar range are thermodynamically capable of extracting metal ions from some metalloproteins and disrupting regular function. Such dissociation constants are easily attainable for multidentate chelators interacting with first-row d-block metal cations in relevant +1, + 2, and +3 oxidation states. To overcome this challenge of indiscriminate metal chelation, we have pursued a prodrug strategy for chelating agents in which the resulting "prochelator" has negligible metal binding affinity until a specific stimulus generates a favorable metal binding site. The prochelator strategy enables conditional metal chelation to occur preferentially in locations affected by disease- or therapy-associated stimuli, thereby minimizing off-target metal chelation. Our design of responsive prochelators encompasses three general approaches of activation: the "removal" approach operates by eliminating a masking group that blocks a potential metal chelation site to reveal the complete binding site under the desired conditions; the molecular "switch" approach involves a reversible conformational change between inactive and active forms of a chelator with differential metal binding affinity under specific conditions; and the "addition" approach adds a new ligand donor arm to the prochelator to constitute a complete metal chelation site. Adopting these approaches, we have created four categories of triggerable prochelators that respond to (1) reactive oxygen species, (2) light, (3) specific enzymes, and (4) biological regulatory events. This Account highlights progress from our group on building prochelators that showcase these four categories of responsive metal chelating agents for manipulating cellular metals. The creation and chemical understanding of such stimulus-responsive prochelators enables exciting applications for understanding the cell biology of metals and for developing therapies based on metal-dependent processes in a variety of diseases.
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