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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.

The biochemical and pharmacological properties of bioactive peptides and proteins can be altered by conjugation with polymers. This report describes site-specific attachment of insulin to activated carboxyl groups of carboxymethyl dextran (CMD, MW=51000) through the GlyA1 insulin amino group. On average, three or four insulin molecules were grafted to a CMD linear chain. Coupled insulin molecules were properly folded, and the bioactivity of conjugated insulin in the blood glucose depression assay was 9.6 IU/mg, which was only 2.6 times less than that for native insulin. The cell growth study indicated that the CMD-insulin conjugate was as mitogenic as insulin on vascular smooth muscle cells, whereas the starting CMD polymer was not. The insulin receptor binding constant of the conjugate (3.6 x 10[9] M[-1]) compared well with that of native insulin (7.6 x 10[9] M[-1]), indicating that the CMD chain does not present any major constraints to binding. Plasma clearance of CMD-insulin obeyed a two-compartment pharmacokinetic (PK) model with a CMD-insulin conjugate plasma elimination half-life of 114.1 min, which was significantly longer than that of soluble Zn-insulin (12.4 min). In contrast, pharmacodynamic (PD) profiles (blood glucose lowering effects) after intravenous (iv) administration of the conjugate or insulin in rats were not different. Subcutaneous (sc) administration of the conjugate resulted in a significantly prolonged plasma profile with a noncompartmental PK parameter mean residence time (MRT) of 103.5 min which was significantly longer than that of soluble Zn-insulin (40.5 min). This was reflected in the protracted PD effect of sc administered conjugate with time needed to reach minimum glucose concentration Tnadir of 95.7 min, which was significantly longer than that of insulin (62 min). We conclude that the conjugation of insulin to CMD leads to a bioactive conjugate with a delayed sc PD profile showing prolonged response, resembling intermediate acting insulin preparations.

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