Diabetes mellitus is a major public health problem, which affects 387 million people worldwide. The traditional medical care for the Type 1 and advanced Type 2 diabetics requires continuous monitoring of blood glucose levels and subsequent insulin injections to maintain the normoglycemia. However, such self-administration is associated with pain and often inadequate glucose control. For the past few years, a chemical approach utilizing an insulin-loaded matrix with glucose-sensing elements and a relevant actuator, which could avoid those limitations and may prove more effective for closed-loop insulin release, has attracted a lot of attention. The matrix can undergo structural transformations-shrink, swell or dissociate, regulated by glucose concentration variances, resulting in a glucose-stimulated insulin release. The typical glucose-sensing moieties include phenylboronic acid (PBA), glucose-binding protein (GBP), and glucose oxidase (GOx). Despite these available sensing chemistries, the majority of existing synthetic closed-loop systems have only been studied in vitro, with relatively few showing applicability in vivo due to specific challenges for each glucose-sensing strategy. In order to exploite synthetic glucose-responsive insulin delivery systems, several challenges should be addressed, including 1) fast responsiveness; 2) ease of administration; and 3) excellent biocompatibility.
Current GOx-based glucose-responsive insulin delivery systems mainly utilize matrices consisting of pH-sensitive materials, which release insulin by either protonation or degradation due to enzymatic generation of gluconic acid. However, their effectiveness is limited by slow response upon blood glucose changes especially under a buffered physiological environment. Here we demonstrate the first enzyme-based glucose-responsive insulin delivery strategy utilizing sensitivity to hypoxia rather than pH variance. A local hypoxic microenvironment could be rapidly generated in PBS buffer solution due to the enzymatic consumption of oxygen, as evidenced by an oxygen-sensitive phosphorescent probe. Subsequently, the hydrophobic side-chains of HS-HA were reduced into hydrophilic chains under a bioreductive condition, resulting in disassembly of GRVs, subsequently releasing insulin. The obvious change in morphology could be observed by TEM 20 min post-incubation with PBS buffer containing 400 mg/dL glucose. The in vitro insulin release profile of GRVs indicated a remarkably faster release rate compared to pH-sensitive based glucose-responsive nanoparticles previously reported. In addition, the insulin release kinetics can be adjusted by varying the enzyme dose both in vitro and in vivo, further implying that the release of insulin undergoes a glucose-mediated and hypoxia-dependent process.
Furthermore, the GRVs were integrated into an HA-based MN-array patch for convenient, painless and continuous administration of insulin. The crosslinked HA matrix not only helped to improve mechanical strength and skin penetration capability, but also restricted the leak of GRVs to avoid burst release. Additionally, the framework of needle patches and vesicles both derived from HA, which is highly biocompatible. The GRV(E+I)-loaded MN exhibited excellent regulation of glucose into a normal range with fast responsiveness. Furthermore, besides the highly sensitive vesicles, the rapid uptake by the lymphatics through the transcutaneous administration may also contribute to the fast responsivity. The in vivo glucose tolerance test indicated that GRV-loaded MNs were not only rapidly responsive to glucose challenge, but that they could also efficiently minimize the risk of hypoglycemia. In addition, through a serial administration with MNs, it could precisely control glucose in a normal range for prolonged periods in a diabetic mouse model. Also considering that mice have reduced sensitivity to the human insulin used in this study, the real dose will be significantly lower if used in human clinical trial. This “smart insulin patch” with its novel trigger mechanism offers a clinical opportunity for closed-loop delivery of insulin in a fast glucose-responsive, pain-free, and safe manner. It will also guide the development of a useful drug delivery platform for treating other diseases using synthetic vesicles, which can be “smartly” activated and regulated by the change of physiological signals.
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