Screening of chemical linkers for development of pullulan bioconjugates for intravitreal ocular applications

The treatment of posterior segment disorders of the eye requires therapeutic strategies to achieve drug effects over prolonged times. Innovative colloidal delivery systems can be designed to deliver drugs to the retina and prolong their intravitreal permanence. In order to exploit pullulan (Pull) as polymeric drug carrier for intravitreal drug delivery, derivatives of hydrophobic model molecule rhodamine B (RhB) were conjugated to the pullulan backbone through linkers with different stability, namely ether (Et), hydrazone (Hy) or ester (Es) bond to obtain Pull-Et-RhB, Pull-Hy-RhB and Pull-Es-RhB, respectively. Dynamic light scattering and transmission electron microscopy analyses showed that the polymer conjugates self-assembled into 20-25 nm particles. Pull-Et-RhB was fairly stable at all tested pH values. At the vitreal pH of 7.4, 50% of RhB was released from Pull-Hy-RhB and Pull-Es-RhB in 11 and 6 days, respectively. At endosomal pH (5.5), 50% of RhB was released from Pull-Hy-RhB and Pull-Es-RhB in 4 and 1 days, respectively. Multiple particle tracking analyses in ex vivo porcine eye model showed that the diffusivity of the bioconjugates in the vitreous was about 10³ times lower than in water. Flow cytometry and confocal microscopy analyses showed that bioconjugates are remarkably taken up by the retinal RPE cells. In vivo studies showed that after intravitreal injection to mice, the bioconjugates localize in the ganglion cell layer and in the inner and outer plexiform layers. Pull-Hy-RhB particles were detected also inside the retinal blood vessels. These results demonstrate that pullulan with tailored linkers for drug conjugation is a promising vehicle for long-acting intravitreal injectables that are capable to permeate to the retina.