The Photoactivated Depot (PAD): Light Triggered Control of Therapeutic Protein Solubility and Release

Many therapeutic proteins can benefit from controlling the timing and amount of their release. This is especially true for signaling molecules such as insulin, whose requirements vary continually throughout the day. Currently, the only way to provide this variable delivery is through a pump. Pumps, and their required cannulas/needles, introduce a wide range of problems, including cannula occlusion, infection, and biofouling. We have instead pursued the photoactivated depot or PAD approach, in which therapeutic proteins are released into the body through light activation of shallow, skin-based depots that are activated by small LED light sources ( Angew. Chem. 2013, 125(5), 1444-1449, Mol. Pharmaceutics 2016, 13(11), 3835-3841, J. Am. Chem. Soc. 2017, 139(49), 17861-17869, ACS Biomater Sci. Eng. 2021, 7(4), 1506-1514, and ACS Biomater Sci. Eng. 2024, 10(6), 3806-3812). By linking protein release to transcutaneous irradiation, we can control the amount and timing of therapeutic release by varying the amount and timing of irradiation. At the heart of this approach are PAD materials that contain three key elements: the therapeutic protein, a photocleavable (PC) group, and a solubility reducing moiety. This latter element is needed to allow the PAD material to stay at the site of injection, so that light can be effectively directed to it. The light causes the PC group to break its bond with the therapeutic protein, which can then diffuse into the capillary bed and be absorbed into systemic circulation. We have pursued four distinct methods of achieving solubility reduction prior to irradiation. The first approach is to use a highly insoluble polymer that is linked to the therapeutic protein via the PC group. This was the approach we used in our first attempt at making a PAD material and proved to be effective in both in vitro and in vivo settings. The main challenge with this first approach is that the polymer is left in the body after the protein is released, necessitating additional optimization to clear it, using biodegradation. In addition, it is very inefficient, with only a minority of the material being the therapeutic. In the second approach, we created polymers/oligomers out of the protein, using small light-cleaved links. The simplest of these, a trimer of proteins linked to a central core, is 90% therapeutic, and retains the preirradiation insolubility required of the PAD approach. In the third approach, we link charged groups to the protein to shift its iso-electric point, such that the material will be insoluble (and hence able to form a depot) at pH 7, but will release native, active protein after photolysis cleaves off the charged groups. Finally, in the fourth approach, we confer insolubility by attaching highly nonpolar groups to the therapeutic protein via a PC linkage. In this article, the challenges, strengths and weaknesses of each of these approaches will be described, and guidance will be given for the application of the PAD approach to other systems that can benefit from the controlled release that it enables.