Tarsem S. Sahota

In vivo study of a polymeric glucose-sensitive insulin delivery system using a rat model†

This study assesses the feasibility of an intraperitoneal (IP) implantable closed-loop insulin delivery device in rats, that delivers insulin via a glucose-sensitive material such that blood glucose (BG) levels are adjusted automatically to within normal tolerances. A gateway layer of this gel governs the output of insulin from an insulin reservoir device for IP implant. The performance of the system was compared over time in diabetic rats with a control system using oral glucose challenges and daily assessments of BG and body weight. The automated response of the active system was quantified using IP multiple dose injection (MDI) results in the same rat model. Successful control was found for the device containing active gel when assessed daily and when challenged with large glucose doses. This was not found when comparing an inactive gel analog as a control. The regimen was quantified by comparison with the informative MDI study. The device was well tolerated and might operate to further advantage when vascular omentum grows into the perforated front of the device. The successful device must have been outputting approximately 0.5 U/kg/h basal with 2 U/kg boosts in order to match the demand of the challenges. However, the device eventually exhausts and a refill mechanism needs to be devised in future models.

Covalent coupling of concanavalin A to a Carbopol 934P and 941P carrier in glucose-sensitive gels for delivery of insulin.

A novel glucose‐sensitive gel formulation, containing concanavalin A and specific polysaccharides, was stabilised via covalent coupling to two structurally different carbomers. The bonding was done to minimise leaching of gel components thereby preventing toxicity and preserving the working mechanism of the gel. Increased gel stability was introduced by covalently bonding amine groups present on the lysine residues of concanavalin A to carboxylic moieties on Carbopol 934P NF and 941P NF using carbodiimide chemistry. The introduction of dextran then produced a glucose‐sensitive formulation that transformed from gel to sol in the presence of free glucose. Rheological examination of glucose‐sensitive gels stabilised in this way and containing varying concentrations of glucose was conducted with a cone and plate viscometer used in continual rotation mode. A decrease in viscosity over the chosen glucose concentration range was exhibited by both carbomer‐stabilised formulations. The subsequent testing of such formulations in in‐vitro diffusion experiments revealed that the leaching of concanavalin A from the covalently coupled gels is restricted significantly with respect to non‐coupled formulations. In addition, insulin delivery in response to glucose in the physiologically relevant glucose concentration range has been demonstrated using the carbomer‐stabilised gels at 37°C. The performance of this self‐regulating drug delivery system has been improved in terms of increased gel stability with reduced component leaching.

A Covalently Stabilised Glucose Responsive Gel Formulation with a Carbopol ® Carrier

A novel glucose-responsive gel formulation was stabilised via covalent coupling to a carbomer resin. The gel formed between the plant lectin, concanavalin A and specific polysaccharides was stabilised to minimise leaching of gel components into the surroundings. This was required to prevent toxicity and to preserve the working mechanism of the formulation. Increased gel stability was introduced by covalently bonding amine groups present on the lysine residues of concanavalin A to carboxylic moieties on Carbopol ® 974P NF using carbodiimide chemistry. The introduction of dextran then produced a glucose-responsive formulation that transformed from gel to sol in the presence of free glucose. The rheological properties and in vitro component and insulin release of the carbomer-stabilised gel were evaluated. A decrease in viscosity over a chosen glucose concentration range was exhibited by the carbomer-based gel. The testing of such a formulation in in vitro diffusion experiments revealed that the leaching of concanavalin A from the covalently coupled gels was restricted significantly with respect to a non-coupled gel. Insulin delivery in response to glucose in the physiologically relevant glucose concentration range was demonstrated using the carbomer-stabilised gel at 37°C. The performance of this novel self-regulating drug delivery system has been improved in terms of increased gel stability with reduced component leaching.

Closed-loop delivery of insulin.

The importance of exactly matching blood glucose levels in patients with diabetes mellitus with the dose and timing of insulin cannot be overemphasized. This is true for both type 1 and type 2 diabetes. The prevention of the serious consequences of diabetes should be a therapeutic goal because of the personal and national economic costs of treating the disease and its sequelae. Avoidance of sequelae cannot be achieved without so-called intensive treatment of the disease; however, this is a difficult and sometimes hazardous process. An automated and ’safe’ method of achieving intensive treatment (closed-loop insulin delivery) has been an important focus of research for many years since it was realized that the simple replacement of insulin saved the lives of patients with diabetes, but did not guarantee good health.Following the general successes of organ transplant, the obvious line of enquiry in the last three decades was to replace the pancreas with donor tissue; however, even when superseded by the simpler process of islet transplant, the benefits of this have not been widespread. This is because of the shortcomings and incompatibilities of immunosuppression with the functioning of the graft, among other problems. The two types of rejection that may occur in the presence of the autoimmune type of diabetes further endanger graft function; however, recently there have been advances that have partially solved this problem. Nevertheless, the lifetime risks of immunosuppression and the shortage of donor material still make transplant less attractive than first thought.Consequently, other approaches to closed-loop insulin delivery have been important. These include implantable pumps coupled with sensors and a range of devices incorporating glucose-sensitive materials that could be engineered into delivery systems to work by closed loop. Each of these has relative advantages and drawbacks, but some have the capacity to reach the market for clinical use. Of these, the MiniMed pump would appear to be in the lead, and is at present undergoing clinical trials that have given promising results but that have not yet entirely solved the closed-loop problem. Other designs may yet become important contenders in this interesting contest.

Diabetes Mellitus and Closed-Loop Insulin Delivery

Diabetes mellitus is presently one of the major health problems of the European and North American continents affecting at least 6% of the population. Of these, about 10% suffer from type 1, insulin-dependent disease. Diabetes, whether insulindependent or not, is a leading cause of death in developed countries (Cotran et al., 1994; McGee et al., 1992). In 1985 the World Health Organization (WHO, 1985) expert committee on diabetes defined this disease as a state of chronic hyperglycaemia, that is to say, the state of having an excessive concentration of glucose in the blood. The causes of hyperglycaemia are insulin deficiency or ineffectiveness, which leads to defective carbohydrate utilization and resultant aberrations in lipid and protein metabolism. The clinical manifestations of a patient presenting with this disease are thirst, hunger, fatigue and glucose in the urine. Depending on the stage and type, this may progress to stupor, coma and death. The symptoms may be less evident in type 2 patients in whom the secondary complications of diabetes may give rise to the presenting symptoms. The incidence of diabetes is rising sharply worldwide (Amos et al., 1997) but this is a disease that has affected man for millennia, the symptoms being described in the Ebers Papyrus of Egypt which date back to 1550 B.C. In about 200 AD., Aretaeus of Cappadocia named the disease diabetes, the Greek word meaning to flow through a siphon (Pickup and Williams, 1997). Aretaeus described diabetes as leading to a ’moist and cold wasting of the flesh and limbs into urine’. He described the disease as chronic in character, and slowly engendered, though the patient ’does not survive long when it is completely established, for the marasmus produced is rapid and death speedy’. In the 6th century, Hindu physicians recognized that the urine from the

In Vitro Iontophoretic Release of Lithium Chloride and Lidocaine Hydrochloride from Polymer Electrolytes

Ionically conducting polymers, frequently known as polymer electrolytes, are potential candidates as hosts for drugs to be delivered iontophoretically. The iontophoretic delivery of lithium or lidocaine from polymer electrolyte films through a cellophane membrane was examined using different delivery current regimes. Thin, mechanically strong, polymer electrolyte films were fabricated from poly(ethylene oxide) (PEO) with lithium chloride or lidocaine hydrochloride. Experiments showed that iontophoretic transport of both lithium chloride and lidocaine hydrochloride might be achieved from these PEO-based films. Cation transport number determinations give values for PEO-based films of about 0.4 for lithium chloride systems and 0.12 for lidocaine hydrochloride systems. The mechanism of transport from these PEO-based polymer electrolyte films allows the delivery of ionic salts such as lithium chloride and lidocaine hydrochloride to be controlled solely by current, thus providing a system that can deliver precise amounts of drug.

UV Cross-Linked Dextran Methacrylate-Concanavalin A Methacrylamide Gel Materials for Self-Regulated Insulin Delivery

In this study, the successful acrylic derivatization of dextran and concanavalin A (con A) to form dextran methacrylate and con A methacrylamide is shown. These derivatized acrylic monomers are then photopolymerized in the presence of a water soluble photoinitiator Irgacure® under various conditions to form covalently bonded glucose-responsive gel materials, which undergo a transformation to sol in the presence of free glucose. Rheological data have revealed that as the degree of substitution for dextran methacrylate is increased, a more elastic material is produced due to the increased covalent linkages. Some of these gel systems show negligible component loss in in vitro diffusion experiments used to simulate the behavior of the cross-linked gel, as would be used in a self-regulated insulin delivery device.

Physical Characterization of Polymer Electrolytes as Novel Iontophoretic Drug Delivery Devices

Polymer electrolytes are solid-like materials formed by dispersing a salt at the molecular level in a high molecular weight polymer such as poly(ethylene oxide) (PEO). They have been extensively studied for use in electrochemical applications such as batteries and display devices. This paper considers a novel application of polymer electrolytes as the basis of iontophoretic drug delivery systems. Polymer electrolyte films were cast from solutions of PEO and various drug salts using either water or an acetonitrile/ethanol mixture as the solvent. These films were characterized by variable-temperature polarizing microscopy (VTPM), differential scanning calorimetry (DSC), and alternating current (AC) impedance analysis. The films were around 100-micron thick and mechanically strong; the optical and thermal methods provided evidence that the polymer electrolytes had crystalline and amorphous phases, although some drugs may exist in films as nanodispersions. The amorphous phase is important as ions have greater mobility in this phase and therefore allow a current to be passed when the material is incorporated into a device such as one suitable for drug delivery by iontophoresis. The AC impedance analysis showed that the conductivity of the films varied between 10(-6) and 10(-3) S cm-1, depending on the salt, casting solvent, and temperature. Two drugs in particular were shown to be promising candidates in these systems: lidocaine hydrochloride and lithium chloride.

Effect of varying molecular weight of dextran on acrylic-derivatized dextran and concanavalin A glucose-responsive materials for closed-loop insulin delivery

Aim: Dextran methacrylate (dex-MA) and concanavalin A (con A)-methacrylamide were photopolymerized to produce covalently cross-linked glucose-sensitive gels for the basis of an implantable closed-loop insulin delivery device. Methods: The viscoelastic properties of these polymerized gels were tested rheologically in the non-destructive oscillatory mode within the linear viscoelastic range at glucose concentrations between 0 and 5% (w/w). Results: For each cross-linked gel, as the glucose concentration was raised, a decrease in storage modulus, loss modulus and complex viscosity (compared at 1 Hz) was observed, indicating that these materials were glucose responsive. The higher molecular weight acrylic-derivatized dextrans [degree of substitution (DS) 3 and 8%] produced higher complex viscosities across the glucose concentration range. Conclusions: These studies coupled with in vitro diffusion experiments show that dex-MA of 70 kDa and DS (3%) was the optimum mass average molar mass to produce gels that show reduced component leach, glucose responsiveness, and insulin transport useful as part of a self-regulating insulin delivery device.

Closed-loop glycaemic control using an implantable artificial pancreas in diabetic domestic pig (Sus scrofa domesticus).

The performance of a completely implantable peritoneal artificial pancreas (AP) has been demonstrated in principle in a live diabetic domestic pig. The device consists of a smart glucose-sensitive gel that forms a gateway to an insulin reservoir and is designed to both sense glucose and deliver insulin in the peritoneal cavity. It can be refilled with insulin via subcutaneous ports and surgery was developed to insert the AP. Diabetes was induced with streptozotocin (STZ), the device filled with insulin (Humulin(®) R U-500) in situ and the animal observed for several weeks, during which time there was normal access to food and water and several oral glucose challenges. Blood glucose (BG) levels were brought down from >30 mmol/L (540 mg/dL) to non-fasted values between 7 and 13 mmol/L (126-234 mg/dL) about five days after filling the device. Glucose challenge responses improved ultimately so that, starting at 10 mmol/L (180 mg/dL), the BG peak was 18 mmol/L (324 mg/dL) and fell to 7 mmol/L (126 mg/dL) after 30 min, contrasting with intravenous attempts. The reservoir solution was removed after 8 days of blood glucose levels during which they had been increasingly better controlled. A rapid return to diabetic BG levels (30 mmol/L) occurred only after a further 24 days implying some insulin had remained in the device after removal of the reservoir solution. Thus, the closed loop system appeared to have particular influence on the basal and bolus needs for the 8 days in which the reservoir solution was in place and substantial impact for a further 3 weeks. No additional insulin manual adjustment was given during this period.