We have previously described the photoactivated depot (PAD) approach for the light-stimulated release of therapeutic proteins such as insulin. The aim of this method is to release insulin from a shallow dermal depot in response to blood glucose information, using transcutaneous irradiation. Our first approach utilized a photocleavable group that linked insulin to an insoluble but injectable polymer bead. The bead conferred insolubility, ensuring that the injected material stayed at the site of injection, until light cleaved the link, and allowed insulin to be absorbed systemically. While this proved to be effective, the use of a polymer to ensure insolubility introduces two major design problems: (1) low concentration of insulin, as a majority of the material is composed of polymer, and (2) upon release of the insulin, the polymer has to be cleared from the system. To address these two problems, in this work, we have pursued "hydrophobic tags", photocleavable small nonpolar molecules that confer insolubility to the modified insulin prior to irradiation without the bulk or need for biodegradation required of polymers. We developed a combined solid- and solution-phase synthetic approach that allowed us to incorporate a range of small nonpolar moieties, including peptides, into the final depot materials. The resulting materials are >90% w/w insulin and have sharply decreased solubilities relative to unmodified insulin (≤1000 × lower). We demonstrated that they can be milled into low micron-sized particles that can be readily injected through a 31G needle. These suspensions can be prepared at an effective concentration of 20 mM insulin, a concentration at which 120 μL contains 7 days of insulin for a typical adult. Finally, upon photolysis, the insoluble particles release soluble, native insulin in a predictable fashion. These combined properties make these new modified insulins nearly ideal as candidates for PAD materials.
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