OCULAR THERAPY

VI. OCULAR THERAPY

A. Corneal Drug and Gene Delivery The cornea is composed of epithelial, stromal, and endothelial layers that

provide a barrier to the penetration of most drugs used to treat ocular disease. As such, the ocular surface presents a challenge for chemotherapy. The epithelial surface is susceptible to bacterial, fungal, and viral infection and inflammation, as well as to mechanical injury. The stroma may be affected by granular or lattice corneal dystrophy, which are diseases that result in abnormal protein deposits in the stroma caused by autosomal dominant mutation. The corneal endothelium function to maintain corneal transparency and normal visual acuity. The endothelial cells cannot regen- erate, and therefore corneal transplants are the only way to correct endothe- lial damage that results in blindness. The ability of dendrimers to be targeted to specific cell types and to carry drugs or genes could allow for the treat- ment of these diseases and prevention of corneal graft rejection.

Hudde et al. (64) reported the efficiency of gene transfer to the corneal endothelium using PAMAM dendrimers in an ex vivo model. The dendri-

dase, in a ratio of 18 : 1 and incubated with a quarter of a full-thickness 6–10% of the endothelial cells were stained blue (64). The transfection of the

dendrimer-plasmid complex was also determined in human corneas but with from the dendrimer. The staining was seen only in the endothelium and

not in the epithelium or stroma in both rabbit and human corneas (64). In the same study, the introduction and expression of the gene, tumor necrosis factor receptor fusion protein (TNFR-Ig) was assessed in this ex vivo system. Tumor necrosis factor (TNF) is produced in high concentra- tions during transplant rejection and causes severe damage to the graft. TNFR-Ig could prevent the TNF response and therefore protect the graft. Rabbit corneas incubated with the dendrimer/plasmid secreted an active form of TNFR-Ig into the supernatant that inhibited TNF-mediated toxi- city in a bioassay (64).

Dendrimers 485 Corneas that will be used for transplantation are stored in a special

medium prior to surgery. This corneal storage medium could be modified to contain dendrimers complexed with genes expressing important proteins that could prevent graft rejection. Hudde et al. (64) demonstrated that the dendrimer/TNFR-Ig gene complex transfected into ex vivo corneas pro- duces an active protein that inhibits TNF-mediated cytotoxicity. Qin et al. (53) prolonged cardiac graft survival in mice by giving a dendrimer carrying

a plasmid coding for IL-10, an interleukin that enhances immunosuppres- sion and hence improves graft survival. Combinations of these strategies could prolong survival of the transplanted cornea. Dendrimer therapy could also correct hereditary diseases such as lattice corneal dystrophy by

gene) into the endothelial cells so a functioning protein could be expressed. While no ocular dendrimer drug delivery studies have been performed to date, the possibility of dendrimers being used carry drugs to corneal cells should be investigated.

B. Intraocular Drug and Gene Delivery Treatment of the inner ocular tissues and the posterior region of the eye has

posed a significant challenge to ophthalmologists. A major tissue to be targeted is the retina, the transparent tissue lining the back of the eye that is responsible for visual acuity, color discrimination, and peripheral vision. Diseases of the retina include retinitis pigmentosa and macular degenera- tion; both result in blindness due to the death of retinal cells. Drugs applied to the cornea usually do not reach the back of the eye. Therefore, direct injection of the drugs into the vitreous is required to target the retina. While this method provides adequate drug levels, it is not without significant risk of retinal detachment, endophthalmitis, vitreal hemorrhage, and/or cellular toxicity (65–67). The use of dendrimers to deliver drugs and gene therapy to this area of the eye has great potential.

Urtti et al. (68) investigated the efficacy of in vitro gene delivery in fetal primary human retinal pigment epithelial (RPE) cells using plasmids encod-

mids were complexed with either degraded sixth-generation PAMAM dendrimers, cationic lipids, or polyethylene imines (PEI; 25 or 750 kDa) to determine the level of protein expression and cellular toxicity in RPE cells. A degraded dendrimer has several branches removed by boiling in butanol, which does not alter the size and shape but does increase the spaces within the dendrimer and increases the flexibility of the molecule. Naked plasmid DNA was not as effective as complexed DNA in transfecting the RPE cells. When the luciferase plasmid was linked with the dendrimer or

486 Loutsch et al. PEI-25, the expression of luciferase was significantly higher than with the

other carriers (68). The dendrimer/plasmid and PEI-750/plasmid were also less toxic to the cells than the other vehicles. Although only a small number of RPE cells were transfected, a large amount of protein was expressed from the plasmid (68). This is especially significant with a secreted protein pro- duct; a few cells making the protein could secrete enough to counteract the deficiency in all the cells. This method of delivering genes works well in vitro, but additional investigations are needed before the in vivo impact can be determined.

VII. CONCLUSION The ability of dendrimers to carry drugs and genes may allow for the

advancement of drug and gene therapy, not only in ophthalmology, but in other areas of medicine. In the field of ocular drug and gene delivery, the controlled architecture and the nontoxic, nonimmunogenic nature of dendrimers raise the possibility of future replacement of currently proble- matic delivery systems. The creation of dendrimers that can target specific cells and carry a particular therapy will allow physicians to enhance treat- ment regimens.

ACKNOWLEDGMENTS The authors wish to acknowledge Jean Jacobs, Ph.D., Kristina Braud, and

Kathy Vu for assistance with this review. None of the authors have any financial or proprietary interest in any of the agents or devices in this review.

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