Glucose-responsive materials for self-regulated insulin delivery

Abstract

Achieving normal glycemic control in diabetic patients is nearly impossible given the pharmacokinetics after subcutaneous injection of commercial insulin. Glucose- regulated insulin delivery (GRID) would vastly improve glycemic control while reducing (i) the required frequency of subcutaneous injections and finger-stick glucose tests, (ii) the incidence of hypoglycemia and hyperglycemia, and (iii) the resulting nerve, kidney, retinal and cardiovascular complications associated with the disease. We have chosen to formulate a GRID system into a repeated dosage form capable of being administered through injection, inhalation, or oral routes to eliminate the need for surgery. In our particular system, dextran, a glucose-containing polymer, was crosslinked using the tetrafunctional glucose-binding protein, concanavalin A (Con A). Glucose then directly competed with the polymer for Con A binding sites, causing displacement of the polymer and disruption of the crosslinks. Through careful manipulation of dextran molecular weight (MW) and Con A/dextran crosslink ratio we have synthesized self-contained gels that erode at rates that depended directly on environmental glucose concentration. Since Con A binding affinity and glucose set-point (GSP) decreased from 25⁰C to 37⁰C, covalent mannosylation was required to increase binding affinity and minimize gel dissolution at glucose concentrations below 100 mg/dl. Hybrid gels constructed from varying weight ratios of mannosylated and unmodified dextrans further provided a convenient means for fine- tuning the GSP to closely match the physiological, glucose-dependent pancreatic -cell response.

(cont.) The rate of gel dissolution increased by as much as 150x from hypo- to hyper- glycemia, and decreased rapidly when glucose concentrations returned to hypoglycemic levels. The reversibility was a direct result of the high degree of crosslinking that restricted component dissolution to the gel surface. By conjugating insulin to mannosylated and non-mannosylated dextran and optimizing the crosslink ratio and dextran MW, we have synthesized the first self-contained GRID system, requiring no external membranes while preserving the precise glucose sensitivity achieved with the membrane-encapsulated soluble system first developed by Brownlee and Cerami in 1979. Further benefits of insulin-dextran conjugation included (i) purely glucose-sensitive release with no glucose-independent leakage, (ii) controlled dosing by varying non-conjugated dextran to insulin-dextran ratio, (iii) tunable glucose sensitivity based on dextran mannosylation, and (iv) reversible insulin release due to direct coupling with reversible glucose-responsive polymer dissolution. Furthermore, the insulin-dextran gels were active in vivo as demonstrated by their ability to control both fasting and mealtime-simulated blood glucose levels in streptozotocin-induced diabetic rats. Reverse microemulsions (RM's) were chosen as a means to formulate our GRID into a nanoparticle dispersion for convenient delivery. The phase behavior of biocompatible RM's has been determined in the presence of dissolved dextran, insulin, and Con A using a novel rapid screening technique developed in our laboratory ...