degeneration] (Moshfeghi et al. 2006). Bevacizumab was given as an intravenous infusion of 5 mg/kg twice or thrice every 2 weeks to patients suffering from neovascular AMD. On an average, there was increase in the visual acuity by 14 letters, while there was decrease in retinal thickness by 112 mm after 24 weeks. Bevacizumab was well tolerated and there were no significant systemic adverse effects. Although the study proved that systemic delivery is effective, it was inconclusive as to whether there was a possibility for systemic side effects in patients suffering from illness or disease. Thus, this route may be of interest only if a targeted drug delivery system is developed that can deliver proteins or macromolecules specifically to ocular tissues and reduce the systemic distribution to other parts of the body such as liver, kidney, and heart. Singh et al. (2009) demonstrated that nanoparticles functionalized on their surface with peptide/protein ligands for integrin and transferrin receptors enhance nanoparticle delivery to the neovascular region of the choroid following intravenous administration, without enhancing nanoparticle uptake by various other organs.

Table 17.1 summarizes various routes of administration for protein drug delivery to the back of the eye.

17.3  Advantages and Challenges of Protein Delivery

Proteins and peptides have unique tertiary structures that fold into unique conformations that allow for the protein to have selective interactions with its target. Many proteins function as enzymes, transcription factors, and membrane receptors. Since proteins are biologically active molecules, they have been naturally designed to work at specific physiological conditions (pH, temperature, and salt conditions). In addition, the body has designed specific mechanisms to generate mature proteins and degrade them as necessary. The body has also developed a mechanism to recognize and destroy foreign macromolecules including proteins (the immune system). The biological function of a protein not only makes it a strong candidate to treat ocular diseases, but it is intrinsically flawed due to its instability, degradation by proteolytic enzymes, rapid excretion, and immunogenicity. Proteins are highly sensitive to their environment and therefore are extremely unstable outside of their optimal physiological conditions. Protein properties can change with a change in pH, temperature, and ion concentration of the solvent, and upon exposure to proteases, oxygen, or heavy metals. Freezing and thawing can also change the physical conditions of protein (Simpson 2010). In addition, proteins have poor shelf lives due to their instability and sensitivity to altered environments. Protein formulations are extremely sensitive to temperature. Generally, the storage temperature preferred is below 4°C; however, depending upon the nature of protein, the storage temperature may be as low as −70°C. For example, the accelerated stability testing of bevacizumab at 30°C showed changes in size exclusion chromatographs, indicating change in the molecular size of the protein; ion-exchange chromatography indicated changes in the ionic variants of