The modification of insulin to enhance oral delivery systems
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While a number of PEGylated proteins have been studied for injectable applications and reviewers have used this data to speculate possible oral delivery improvements, a detailed investigation of PEGylated insulin for oral delivery and the development of an optimized pH-sensitive carrier for PEGylated insulin conjugates had yet to be accomplished. In order to proceed with oral delivery study, improvements in yield, with respect to previous PEGylation methods were necessary to enable the completion of high throughput drug delivery studies. Subsequently, a reaction scheme for the covalent attachment of PEG to insulin using nitrophenyl carbonate-PEG was developed. It was demonstrated that this reaction occurred at a 1:1 ratio and was site specific at the B29Lys position. A P(MAA-g-EG) hydrogel carrier was developed to optimize loading and release behavior for PEGylated insulin. It was demonstrated that the density and length of polymer grafts affected both loading and release behavior of PEGylated insulin. The best performing grafted polymers had a 3:1 methacrylic acid: ethylene glycol (MAA:EG) ratio and achieved loading efficiencies from 96% to nearly 100%. With respect to release, polymer particles containing fewer, but longer grafts shown to release faster than polymers with shorter grafts with the same MAA:EG ratio. Finally, the effects of PEGylation on intestinal absorption was investigated using an intestinal epithelial model as well as a rat model. It was demonstrated that PEGylated insulin in the presence of P(MAA-g-EG) microparticles did not significantly alter the tight junctions over unmodified insulin. However, the conjugate permeabilities across the membrane were reduced. The pharmacological availability (PA) was then verified by injecting the insulin conjugates subcutaneously in fasted Sprague-Dawley rats. It was determined that PEG 1000 insulin (1KPI) had a PA roughly equivalent to insulin, while it was reduced by 59% for 2KPI and by 81% for 5KPI. The effectiveness of utilizing PEGylated insulin as an oral drug delivery candidate was evaluated with a closed loop intestinal study, in which PEGylated insulin or insulin in solution was delivered directly to the jejunum. It was shown that 1KPI and insulin performed identically; with a pharmacological availability of 0.56%. 2KPI, however improved the pharmacological availability of insulin by 2.8 times. These results demonstrate that PEGylation holds promise for improving the oral delivery of proteins.