Photo-activated drugs provide an opportunity to improve efficacy alongside reducing side-effects in the treatment of severe diseases, such as cancer. Herein, we describe a photoactivation decaging method of isobutylene-caged thiols through a UV-initiated thiol-ene reaction. The method was demonstrated with an isobutylene-caged cysteine, cyclic disulfide-peptide and thiol-containing drug, all of which were rapidly and efficiently released under mild UV irradiation in the presence of thiol sources and a photoinitiator. Importantly, we show that the activity of histone deacetylase inhibitor Largazole can be switched-off when stapled, but selectively switched-on when irradiated with non-phototoxic light in cancer cells.