Activatable Photosensitizers in Photodynamic Therapy

Photodynamic therapy (PDT) harnesses light-activated compounds, known as photosensitizers, to generate cytotoxic reactive oxygen species—most notably singlet oxygen—at the site of disease. Traditional photosensitizers can suffer from poor selectivity and off-target toxicity because they are “always on” once administered. Activatable photosensitizers offer refined control by remaining inert until a specific molecular or environmental trigger induces their photochemical activity. Such triggers include enzymatic cleavage, changes in pH, redox potential or the presence of bioorthogonal reactants. By confining singlet oxygen generation to diseased tissue, activatable designs enhance therapeutic precision, reduce collateral damage and open avenues for image-guided PDT. Contemporary constructs range from small-molecule prodrug analogues and enzyme-responsive beacons to supramolecular cages and metabolic warheads. These systems not only improve safety profiles but also enable real-time monitoring of therapeutic response, paving the way for personalised and minimally invasive interventions across oncology, infectious disease and beyond.

Research from Nature Portfolio

Recent studies have introduced generalisable caging strategies that fine-tune singlet oxygen output in diverse photosensitizer scaffolds. One approach involves transient incorporation of electron-withdrawing moieties that fully quench photoreactivity until removed by bioresponsive or bioorthogonal stimuli, restoring both fluorescence and singlet oxygen production on demand. This method has been validated across multiple chromophoric classes and applied to targeted ablation of human cells as well as regulated oxidations in semi-synthetic workflows. Another platform exploits amino-substituted benzoselenadiazole derivatives that mimic endogenous metabolites; on light exposure, these compounds selectively accumulate in pathological cells and produce cytotoxic species only within those targets, minimising harm to healthy tissues in vivo. Together, these advances demonstrate broad applicability of activatable photosensitizers with modular triggers and versatile spectral properties.

Activatable Photosensitizers in Photodynamic Therapy publication trend

The graph below shows the total number of articles in activatable photosensitizers in photodynamic therapy across all publications each year (not limited to Nature Index journals).

Technical terms

Activatable photosensitizer: A photosensitive compound engineered to remain inactive until exposed to a specific biological or chemical stimulus, upon which it generates cytotoxic species under light.

Singlet oxygen: A highly reactive form of molecular oxygen that transfers energy to cellular targets, causing oxidative damage during photodynamic therapy.

Bioorthogonal stimulus: A chemical trigger that operates selectively in living systems without interfering with native biochemical processes, used to activate caged photosensitizers.

Prodrug: An inert precursor that undergoes enzymatic or chemical conversion in vivo to release an active therapeutic agent, here applied to photosensitizer activation.

Molecular beacon: A structured probe that undergoes conformational or cleavage-mediated changes to reveal fluorescence and/or therapeutic function in response to specific enzymes or nucleic acids.

References

1. Tuning singlet oxygen generation with caged organic photosensitizers. Nature Communications (2024).
2. Photoactivatable metabolic warheads enable precise and safe ablation of target cells in vivo. Nature Communications (2021).
3. The design of small-molecule prodrugs and activatable phototherapeutics for cancer therapy. Chemical Society Reviews (2023).
4. Enzyme-Responsive Double-Locked Photodynamic Molecular Beacon for Targeted Photodynamic Anticancer Therapy. Journal of the American Chemical Society (2023).
5. A Bioorthogonal Antidote Against the Photosensitivity after Photodynamic Therapy. Advanced Science (2023).