Targeted Delivery and Site-Specific Activation of beta-Cyclodextrin-Conjugated Photosensitizers for Photodynamic Therapy through a Supramolecular Bio-orthogonal Approach

Targeted delivery of photosensitizers using hydrophilic and tumor-directing carriers and site-specific activation of their photocytotoxicity are two common strategies to enhance the specificity of anticancer photodynamic therapy. We report herein a novel supramolecular bio-orthogonal approach to integrate these two functions. A beta-cyclodextrinsubstituted aza-boron-dipyrromethene-based photosensitizer was first complexed with a ferrocene- substituted black-hole quencher to inhibit its photosensitizing ability. Upon encountering the adamantane moieties that had been delivered to target cancer cells through specific binding of the conjugated peptide to the overexpressed epidermal growth factor receptor, the ferrocene-based guest species were displaced due to the stronger binding interactions between beta-cyclodextrin and adamantane, thereby restoring the photodynamic activity of the photosensitizer. Hence, this two-step process enabled targeted delivery and sitespecific activation of the photosensitizer, as demonstrated through a series of experiments in aqueous media, in a range of cancer cell lines and in tumor-bearing nude mice.