Transscleral iontophoretic and intravitreal delivery of a macromolecule: Study of ocular distribution in vivo and postmortem with MRI

The distribution and clearance of macromolecules in ocular delivery are not well understood. It has been hypothesized that iontophoresis can enhance transscleral delivery of macromolecules. The objective of this study was to investigate the ocular distribution of a macromolecule after transscleral iontophoretic delivery and intravitreal injection in vivo using nuclear magnetic resonance imaging (MRI) and to compare these results. Experiments of constant current transscleral iontophoresis of 4 mA or intravitreal injection were performed on New Zealand white rabbits in vivo. Iontophoresis experiments were also performed on rabbits postmortem. Galbumin™ (Gd-labeled albumin) was the model permeant surrogate to clinical therapeutic agents. MRI was used to monitor the distribution of the molecule in the eye after ocular iontophoresis and intravitreal injection. In addition, the conjunctiva, sclera, choroid, and retina were extracted in the transscleral iontophoresis study to determine the amounts of Galbumin™ in these tissues using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The results show that iontophoresis enhanced the ocular delivery of Galbumin™. The macromolecule was mainly delivered into the conjunctiva and sclera in microgram quantities and then diffused towards the posterior section in the upper hemisphere of the eye in vivo. Both in vivo and postmortem studies show that the iontophoretic delivery of Galbumin™ into the vitreous was below the detection limit. In the intravitreal injection study, the diffusion coefficient of Galbumin™ in the vitreous humor was estimated to be close to that of free aqueous diffusion.