Modular approach for theranostic polymer conjugates with activatable fluorescence: Impact of linker design on the stimuli-induced release of doxorubicin.

The introduction of cleavable motifs by dynamic covalent chemistry is widely applied in the design of drug delivery systems (DDS) to introduce controlled release properties. Since the cleavable moieties can be triggered by various exogenous or endogenous stimuli, the choice of the linker has substantial implications on the performance of the DDS. Here, we present a pair of theranostic polymer conjugates (TPC) to study the influence of the cleavable bond on the cell-mediated drug release by a facile in vitro fluorescence assay. The TPC represent model DDS that consist of dendritic polyglycerol as polymeric carrier labeled with an indodicarbocyanine (IDCC) dye and the chemotherapeutic drug doxorubicin (Dox) conjugated through different cleavable linkers. Cleavage of the conjugate can be mediated by either acidic environment or protease activity. The spatial proximity of the IDCC dye and the fluorescent drug led to effective quenching of Dox fluorescence when bound to the carrier. The stimuli-induced linker cleavage was correlated with the recovery of fluorescence giving real-time information about the stimuli-dependent drug release. By tracking the fluorescence recovery in a cell-based high throughput microplate assay, we were able to obtain characteristic release profiles of Dox for different cell lines. Here, we found that the pH-cleavable linker was more suitable for drug delivery applications since the enzyme-sensitive system suffered premature release due to the presence of extracellular proteases. This had a pronounced effect on the treatment of a multidrug-resistant cell line where an intracellular drug release is crucial to overcome the resistance mechanisms. We want to highlight that the modular synthetic approach combined with the cell-based assay has potential to extend the common in vitro methods to evaluate DDS performance and suitability as the design can be easily employed for diverse carrier/linker systems as well as various cell lines.