Glucose-induced release of glycosylpoly(ethylene glycol) insulin bound to a soluble conjugate of concanavalin A

Abstract

Treatment of diabetes mellitus by insulin injections provides long-term control of the disease but lacks any feedback response to glucose concentration changes, which finally leads to a number of life-threatening conditions. The purpose of this study was to improve and optimize an implantable, concanavalin A (Con A) based, glucose-responsive insulin delivery system studied earlier [Jeong, S. Y., Kim, S. W., Holmberg, D. L., and McRea, J. C. (1985) J. Controlled Release 2, 143-152], which can be used for long-term diabetes treatment. To optimize the ''insulin component'' of the delivery system, we prepared PheB1 insulin amino group monosubstituted monoglucosylpoly(ethylene glycol) (G-PEG) insulin conjugates (PEG M-r 600 or 2000), which showed preserved bioactivity, significantly improved solubility and solution stability at neutral pH, and substantially suppressed hexamerization/ dimerization. To improve the delivery system further, we synthesized and characterized a conjugate of Con A and monomethoxypoly(ethylene glycol) (mPEG, M-r 5000) grafted hydrophilic poly(vinylpyrrolidone-co-acrylic acid) (PVPAA) with M-r of 250 000. The optimal conjugate contained around eight PEG chains and two to three Con A tetramers attached through the amide bonds to the PVPAA chain. The Con A sugar binding characteristics were preserved, and, more importantly, Con A solubility at pH 7.4 substantially increased. This also holds true for a complex formed by the Con A conjugate and G-PEG insulin, which is soluble and does not precipitate under the physiologically relevant conditions under which the complex formed by the Con A conjugate and glycosyl insulin immediately precipitates. Finally, no leakage of the Con A conjugate from a membrane device was detected. Preliminary in vitro release experiments with Con A conjugate and G-PEG insulin complex enclosed in the membrane device showed a pulsative, reversible release pattern for G-PEG insulin in response to glucose challenges of 50-500 mg/dL, demonstrating the feasibility of the release system for use in planned, chronic in vivo studies with diabetic (pancreatectomized) dogs.