GB2107185A - Insulin formulations containing surface-active agents - Google Patents

Abstract

Aqueous insulin solutions exhibiting improved hydraulic and non-corrosive properties comprise insulin dissolved in an aqueous solution of a polymeric compound comprising a polyether or other compound with non-cationic, hydrophilic properties. The solutions find particular application in external, mechanical and implanted devices for delivery of insulin.

Description

SPECIFICATION Insulin formulation and method of production thereof This invention relates to formulation of insulin solutions, particularly to the improvement of the hydraulic characteristics of such formulations.

The use of automatic and semi-automatic devices which have been developed recently for the controlled delivery of insulin, has proved superior to conventional therapy in reducing the metabolic and hormonal abnormalities of diabetes mellitus. If however these devices are to become available for general use in both external and implanted delivery systems then insulin solutions must be more stable, less corrosive and have improved flow and lubrication characteristics.

Gentle agitation of conventional formulations such as would occur in an implanted reservoir causes orderly aggregation ot insulin and formation of a hexamer gel. Although this is still biologically active its high viscosity makes it difficult to pump. At much higher shear rates such as occur in many pumps, insulin is denatured as a result of intramolecular collisions and by Impact with the containing walls. It aggregates in a disorderly manner and precipitates to form a biologically inactive sludge.

Free ions have been found to be necessary to get insulin into solution. In conventional preparations these have been derived from such substances as hydrochloric acid and sodium phosphate. Galvanic corrosion occurs where insulin solutions are in contact with dissimilar metals. This results in the precipitation of a sludge containing metal-insulin complexes. Furthermore the presence of free ions accelerates the problems of crevice corrosion, stress corrosion and hydrogen ion embrittlement of welds, all of which can lead to premature mechanical failure.

Insulin solution has lubrication properties similar to those of water and consequently is subject to film breakdown at low stresses. Not only will this increase the energy required to drive a device but it will also increase the rate at which wear occurs. To further conserve energy, which is important in implanted systems, it is desirable that insulin solutions should exhibit shear thinning and have low friction with respect to interconnecting and delivery tubes.

Many approaches to the problems associated with the conversion of insulin solutions into a satisfactory hydraulic fluids have been tried.

However none encompasses all of the problems and some improve one facet while worsening another. All have been aimed at the problems of stopping precipitation caused by high hydraulic shear. The addition of 70% glycerol to insulin has been described and is satisfactory in reducing the tendency of insulin to precipitate at high shear rates. However the resuitant solution is very viscous and this greatly increases the energy requirements of the delivery system. Problems associated with corrosion are unaffected. The addition of albumin, other proteins or plasma has been described. This marginally increases resistance to shear precipitation but does not improve any other characteristics.The addition of sodium bicarbonate, calcium ions and excess hydrogen ions are marginaily effective against shear aggregation but do not improve other characteristics and worsen corrosion problems.

The addition of non-ionic surfactants to insulin solution has been described and is satisfactory against shear aggregation but does not significantly improve any other characteristic.

According to the present invention there is provided insulin dissolved in an aqueous solution of a polymeric compound.

The polymeric compound may have a molecular weight between 5000 and 100,000. The polymeric compound preferably comprises an anionic or non-ionic compound, for example a polymeric non-cationic surfactant.

A particularly preferred polymeric compound comprises a polyether. Poly(ethylene oxide) derivatives such as polyoxethylene have been found to be particularly efficacious, for example polyetherphenyl4-sulphonic acid derivatives.

Alternative preferred polymers include gelatine, polyceline, many gums, Tween and Span surfactants and those marketed under the IC trade marks Manoxal and Dispersol.

Preferred polymeric compounds include hydrophilic terminated chains. Mono or polyhydroxyl terminal groupings may be employed. For example sugar molecules or derivatives thereof may be condensed to the polymer chain. Preferred sugars include glucose and dextrose. Sorbitol is especially preferred.

It has been found that termination with hydrophilic groups is essential if the molecular weight of the polymer compound exceeds 30,000.

Preferred polymers comprise unbranched or generally linear polyether chains. Preferred concentrations of polymers range from 1 part in 5000 (w/v) to any convenient higher value.

Solutions of insulin in accordance with this invention have many advantages. Such solutions are resistant to gel formation and shear aggregation. The lubricating properties of insulin solutions in accordance with this invention are superior to those of conventional insulin solutions.

The viscosity may be relatively decreased with increasing shear.

Insulin forms a clear aqueous solution in which the molecule is completely solvated and possesses an adsorbed layer of water. Crystalline insulin will not dissolve in pure water although eiectrophoresis of solutions of insulin suggests that a charged moeity is present. It is postulated that the charge arises on the insulin molecule by absorption of either hydroxonium or hydroxyl ions dependent on the pH.

It has also been found that dissolved insulin molecules aggregate when sufficient energy is dissipated in the solution to cause intermolecular impact or impact iwth the containing vessel.

Aggregation may be caused by dissipation of heat, hydraulic shear forces, electrical current or cationic surface energy. Prolonged exposure to a low energy source may cause an orderly aggregation of insulin molecules with no loss of biological activity. Input of high energy, even for a short time may cause disorderly aggregation resulting in precipitation of denatured and fragmented insulin together with occluded active insulin.

When a closed partially filled container is gently agitated in a water bath at 350C the contained insulin becomes uniformly cloudy after 20 days but there is no apparent increase in viscosity. After 60 days the solution becomes progressively more viscous until the insulin has set and ceases to flow at low shear stresses. Electron microscopy suggests that the basic unit of the gel is an insulin hexamer.

Hydraulically the gel behaves as a thixotropic substance exhibiting shear thinning with recovery at rest.

It is postulated that during agitation charge steal gives the molecules different electostatic potentials so that they cling together to form a loose gel lattice. At the stage of hexamer formation, while the solution is still fluid, the hexamers do not precipitate so that the different velocity, a function of surface charge, is greater than the sedimentation velocity.

Addition of polymer in accordance with this invention prevents formation of the hexameric gel.

This may be attributed to protection of the insulin molecules within the large solvation shell of the polymer, reducing the likelihood of charged species being removed from the insulin.

Agitation of insulin solution at 3000 reciprocations per minute for 1 8 hours caused the solution to become cloudy. A precipitate formed on standing. Agitation of insulin solution containing one part in 5000 (w/v) of poly(ethylene oxide) with sorbitol-terminated chains for 100 days did not cause precipitation.

Conventional commercial insulin solution contains buffers, for example of hydrochloric acid and sodium phosphate. The presence of ionic species renders such insulin solutions sufficiently conductive for corrosion to occur of metals in contact with the solution. Insulin has been found to form complexes with metal ions e.g. zinc. A zinc(insulin) complex exhibits delayed physiological potency.

Solutions in accordance with this invention may be prepared by electrolysis.

Deionised water having a resistivity of 750,000 ohms.cm~t was placed in an electrolytic cell having platinum electrodes and provided with a stirrer. A polymer having the characteristics of an anionic surfactant was added to a concentration of 1/5000 w/v. The resistivity of the resulting solution was 630,000 ohm.cm~1. Crystalline insulin was added and allowed to settle onto the anode. A voltage of 5000 V was then applied to the electrodes. The insulin was caused to dissolve upon gentle stirring. Passage of alternating current was found to remove the need for stirring.

Solutions in accordance with this invention may also be prepared using buffers. The polymer may be added, for example at a concentration of 1/5000 (w/v) and the buffers may be subsequently dialysed from the solution, leaving a solution of insulin and polymer.

Solutions of insulin prepared by either of these methods have a low conductivity. Thus a specimen solution showed no formation of metal complexes after 30 days in a corrosion cell comprising copper and zinc electrodes.

Solutions in accordance with the invention exhibit improved lubricative properties. Use of a polymer at a concentration of 1% (w/v) increases the lubricity of insulin solution by a factor of 20.

Filamentary polymers having fiexible chains e.g.

polyoxethylene, form solutions in accordance with the invention which exhibit shear thinning. It requires less energy to pump such solutions and the longevity of the pump may also be enhanced.

Solutions of insulin including 1 part in 100,000 (w/v) of a polymer such as either polyoxethylene, polyceline or gelatine may be manipulated with reduced frictional losses. In a simple experiment a one metre length of tubing was connected to a constant head of solution. Solutions in accordance with the invention could be projected for distances 50% greater than pure insulin solutions. The reduced friction of solutions in accordance with the invention is particularly important in use of implanted devices. The Tween and Span surfactant polymers have been found to be particularly preferred.

Use of Manoxal or Dispersol is preferred for preferential reduction of metallic corrosion.

Poly(ethylene oxide) polymers could contain more than one polyether chain. The polyether chain could also be interrupted with substituent groups.

Claims (16)

1. An insulin solution comprising insulin disolved in a aqueous solution of a polymeric compound.

2. An insulin solution as claimed in claim 1, wherein the polymeric compound has a molecular weight of 5000 to 100,000.

3. An insulin solution as claimed in any preceding claim, wherein the polymeric compound comprises an anionic or non-ionic compound.

4. An insulin solution as claimed in claim 3, wherein the polymeric compound is a surfactant.

5. An insulin solution as claimed in any preceding claim, wherein the polymeric compound is a polyether.

6. An insulin solution as claimed in claim 5, wherein the polyether is a derivative of poly(ethylene oxide).

7. An insulin solution as claimed in any preceding claim, wherein the polymeric compound is selected from Manoxal, Dispersol, Tween or Span.

8. An insulin solution as claimed in any preceding claim, where the polymeric compound includes a chain terminated with a hydrophilic group.

9. An insulin solution as claimed in claim 8, wherein the chain is terminated with a group incorporating one or more hydroxyl groups.

10. An insulin solution as claimed in claim 9, wherein the group is derived from a sugar molecule.

1 An insulin solution as claimed in claim 10, wherein the sugar molecule is selected from sorbitol, glucose and dextrose.

12. An insulin solution as claimed in any preceding claim, wherein the polymeric compound comprises substantially linear chains.

13. An insulin solution as claimed in any preceding claim, wherein the polymeric compound is present at a concentration equal to or greater than one part in 5000 (w/v).

14. A method of preparing insulin solution in accordance with any preceding claim comprising electrolysis of a mixture of the constituents of the solution.

1 5. A method of preparing insulin solution in accordance with any of claims 1 to 13, comprising dissolution of the constituents in the presence of buffer compounds followed by removal of the buffer compounds by dialysis.

16. A method of modifying the hydraulic or corrosive properties of aqueous insulin solution by addition of a polymeric substance.

1 7. An insulin solution according to claim 1 substantially as hereinbefore described.

1 8. A method of preparing insulin solution in accordance with claim 1 substantially as hereinbefore described.