Patent Grant
9272038
The present invention relates to a gel for use in topical treatment of wounds and surface areas of the body. More specifically the invention provides a cavity filling gel formulation with low bioadhesive strength and cohesive integrity which carries at least one pharmaceutical or cosmetically active ingredient.
The invention primarily addresses the area of PVA gels subjected to reversible or partially reversible cross-linking, for use in wound care to deliver substances topically. The substances can be antibiotics or other drugs or anaesthetic or antiseptics.
Volumes of patent literature exist in this general field of gels for use in medical situations. Other attempts have been made to make a similar product unsuccessfully. For example CN1093897 relates to a surface anaesthesia film made using PVA and lidocaine in a distilled water and ethanol mix.
PVA-borate gel complexes are well known and their formation has been extensively studied in the prior art. The present invention is based upon making a gel with particular rheology and flow properties for use in wounds.
In particular there is a requirement for a gel delivery system which is semisolid, is mouldable when handled but acts like a viscous liquid when placed in a space where it has room to flow. The gel moves very slowly to fill a space that it is situated in. Also, the gel should be relatively non adhesive and can be removed intact without leaving remnants and without causing tissue trauma.
The advantages of such a delivery system for topical application of substances in a medical situation are clear and it could be assumed that if such a gel existed that it would be known in the field. No such gel based delivery systems appear to exist, with creams, ointments, powders and viscous gels presently used, being unable to fill lacerations completely without adding pressure and hurting the wound and/or requiring irrigation and some form of washing protocol during removal.
Much of the prior art in the area of polyol modulation of PVA borate gels appears to be in the area of ophthalmology rather than wound care.
The use of borate/polyol complexes to formulate ophthalmic compositions containing PVA is described in U.S. Pat. No. 5,505,953 (Chowhan). This patent teaches a method to overcome the incompatibility of PVA with borates by addition of monomeric polyols such as mannitol or sorbitol. This patent relates only to ophthalmic compositions and describes the use of PVA as a viscosity enhancer. The polyol addition stops borate and PVA coming out of solution and the complex has antimicrobial effects. The combination of ingredients described would not have the rheology and flow properties of a gel as required in the present application.
US Patent Application No. 20070059274 also relates to ophthalmic products and describes how the borate or PVA concentration may be manipulated in order to arrive at the appropriate viscosity of the composition upon gel activation (i.e., after administration to the eye). If a strongly gelling composition is desired, then the borate or polymer concentration may be increased. If a weaker gelling composition is desired, such as a partially gelling composition, then the borate or polymer concentration may be reduced. It describes that other factors may influence the gelling features of the compositions of the present invention, such as the nature and concentration of additional ingredients in the compositions, e.g., salts, preservatives, chelating agents and so on.
U.S. Pat. No. 6,444,199 entitled Solid borate-diol interaction products for use in wounds appears to be the closest single piece of prior art in the field of the invention and addresses the problem faced by the present inventors about how to fill a wound cavity effectively with a substance that can deliver an active ingredient.
This patent describes how, depending on the concentration of the polymer or polymers, the borate, and other additives, if any, the consistency of a gel can vary from somewhat viscous fluids to crisp amorphous solids. At selected concentrations of the components, the reaction products behave like self-restoring or healable solids that will flow at body temperatures. This property gives them possible value as wound-cavity fillers or wound putties. Other soluble and insoluble components can be added to impart desired properties, such as increased body-fluid absorption or fluid donation.
Although the principle use envisaged for the product of U.S. Pat. No. 6,444,199 is wound cavity-fillers, a number of other medical and non-medical markets are mentioned and these include drug delivery, prosthetics and pads (fillers or in situ-formed coatings), and the toy and possibly executive stress-reliever markets.
For use as a wound cavity-filler, U.S. Pat. No. 6,444,199 explains that the product must be firm enough to handle, yet flow at body temperature to meet wound shape in about 10 to 30 seconds. The product needs to be easy to handle and either absorb or donate moisture, or both. Materials used must be biocompatible, non-toxic, and non-cytotoxic. In this respect, samples of PVA (5% and 20%) with added borate exhibited no cytotoxicity when tested on L929 mouse fibroblasts. Importantly, this patent is not concerned with modulating the gel system to allow the inclusion of soluble active ingredients.
It is clear that there is a lot of prior art in the general field and that the problem faced by the present inventors is a known problem. However, at present a formulation having the properties of a gel as described herein for use in the wound care market including an active substance in a topical delivery product does not appear to have been disclosed or suggested before.
Wound infection may be defined as the entry, growth, metabolic activity and resulting pathophysiological effects of a microorganism upon patient tissue. Infection has been shown to impair wound healing for both acute and chronic wounds. Staphylococcus aureus and coagulase-negative Staphylococci are the bacteria most frequently isolated from the deep tissue of chronic wounds with S. aureus reported to be present in up to 43% of infected leg ulcers and in up to 88% of non infected leg ulcers.
Bacterial wound infections are treated currently using topical antibiotics, such as mupirocin, fusidic acid and neomycin or systemic antibiotics, such as the β-lactams, macrolides and metronidazole, or a combination of both. However, bacterial resistance to all of these agents has been reported widely and numerous antibiotic-resistant bacteria now exist, including meticillin-resistant S. aureus (MRSA) which is the primary pathogen isolated from antibiotic resistant, nosocomial wound infections. The increasing resistance of wound infections to both systemic and topical antibiotics has made effective treatment more difficult and accordingly, interest has arisen in the development of new treatment regimens.
Photodynamic antimicrobial chemotherapy (PACT) is defined as a medical treatment by which a combination of a sensitising drug and visible light causes selective destruction of microbial cells through the generation of singlet oxygen.
It is an aim of the present invention to provide a PVA-borate gel system incorporating an active ingredient such as a local anaesthetic such as lidocaine hydrochloride or antibiotics or photosensitising compounds that form part of PACT and are able to photosensitise bacterial cells or other pathogens making them amenable to the photodynamic effect.
The incorporation of active ingredients changes the suitability and rheological properties of the gel for use in a wound cavity and it is therefore a further aim of the invention to make a gel as required which has flow properties suitable for filling a wound cavity.
It is a particular aim to incorporate an active ingredient or ingredients such as lidocaine hydrochloride or the drugs used in PACT into PVA-borate hydrogels and to modulate the rheological properties that allow flow into a wound space and subsequent removal without irrigation or washing.
It is a further aim of the invention to provide PVA-borate hydrogel formulations for use in treating wounds including acute lacerations, chronic wounds and large acute wounds such as burns, wounds arising from vascular irregularities and abnormal function, wounds arising from infection by viral, fungal and bacteriological organisms, wounds where the healing is impaired by viral, fungal and bacteriological organisms, wounds on epithelised and non-epithelised tissues, wounds on mucous membranes and topical skin disorders.
According to the present invention there is provided a gel formulation suitable for topical use and in filling a wound cavity and delivering an active ingredient thereto, the gel having a pH range of 4.5 to 8.5, but preferably having a pH range of 6.5 to 7.5, having low bioadhesive strength and cohesive integrity and being formed from a polymer having extensive hydroxyl functionalities such as PVA, a cross-linker able to form associative interactions between and within said polymer, preferably but not limited to a salt form of boron that produces borate ions in aqueous solution such as borax, at least one compound which has a beneficial effect as an active ingredient in the wound and at least one modulator, the modulator being a low molecular weight species that is capable of binding borate or PVA in aqueous solution through a mono-diol or di-diol formation and can reduce the pH of PVA-borate hydrogels.
Preferably the modulator is a low molecular weight compound possessing a plurality of functional groups able to bind to borate, the functional groups being preferably hydroxyl groups and the compound being preferably a sugar alcohol and most preferably the modulator being D-mannitol.
D-mannitol belongs to a group of chemicals described a sugar alcohols. Other sugar alcohols, whilst may not being as affective as D-mannitol, can also be used to produce the same affect as D-mannitol. Additionally, other low-molecular weight molecules containing two hydroxyl groups on neighbouring carbon atoms, in the cis-position, can also be used to bind borate and reduce the pH of PVA-borate hydrogels.
Other sugar alcohols and compounds possessing the characteristics and structural features of two hydroxyl groups, in the cis-position and attached to adjacent or neighbouring carbon atoms so as to allow presentation of two or more hydroxyl functionalities in a conformation recognisable as a cis conformation, which may be used in a gel formulation of the present invention include but is not necessarily limited to malititol, dulitol, D-sorbitol, xylitol, meso-erythritol and 1,2-propandiol and glycerol.
PVA can be obtained in various grades of hydrolysis and in various molecular weights. However, the reaction with borate is no different for the PVA variants, the only thing that would change is the quantity required to produce an equivalent gel. Whilst PVA is the best candidate polymer, other polymers with extensive hydroxyl functionalities could also be used.
The PVA could have a Molecular Weight of from 1,000 to 100,000. Typically the Molecular Weight used in practice of the invention would be from 15,000 to 50,000, more preferably 20,000 to 40,000.
Typically the gel has a degree of hydrolysis of between 55 to 100%. More typically this is from 65 to 100% and more preferably from 75 to 99%.
The amount of borate used in preparing the gel depends on the type of PVA used. Typically from 0.5 to 5% borate could be used. More typically 1.5 to 4% and most preferably from 2 to 3%
The amount of modulator added in the preparation of the gel depends on the PVA and borate and the choice of modulator. Typically from 0.1 to 5% could be used. More preferably from 0.5 to 2%. For example, mannitol is a better modulator than glycerol and therefore less mannitol would be required than glycerol.
The modulator is essential to the invention as it allows the gel to maintain stability when the active ingredient is added. Without the modulator the gel viscosity collapses or the gel contracts to form hard lumps or undergoes detrimental coacervation or is seen to exhibit a separation of the polymeric and solution phases into a two-phase system by a process known as demixing. The modulator acts to re-establish the properties that allow the gel formulation to be used in cavities to administer active ingredients.
Accordingly the invention relates to the use of a modulator in a PVA-borate gel incorporating an active ingredient to maintain pH and gel structure.
The active ingredient can be any active ingredient suitable for topical administration that exhibits a degree of solubility in a PVA borate gel. A list of potential medications that could be incorporated as active ingredient is included as annex 1. The active ingredient is typically a medicament. The active ingredient can alternatively be a cosmetic or hair removal product or a therapeutic ingredient such as an essential oil or tea tree oil. The active ingredient can be a chemical compound that can be used to sensitise bacterial, fungal, viral and eukaryotic cells as part of photodynamic therapy or PACT. The active ingredient can be a bioactive peptide, an oligonucleotide, or other material used in the cosmetic regeneration of the skin surface.
The gel of the invention can be mounted on or incorporate a support such as a mesh or gauze. This may be advantageous to cover a large surface area.
In one preferred embodiment the active ingredient is a salt form of lidocaine, most preferably lidocaine hydrochloride monohydrate. Alternatively the active ingredient can be another amide local anaesthetic such as prilocalne, bupivacaine etc. or indeed any active ingredient that produces a conjugate acid and is stable in the presence of borate ions.
In an alternative embodiment the active ingredient is at least one medicament used in PACT.
The present invention also provided the use of poly-functionalised sugar alcohols to modulate the formation of a PVA-borate gel system incorporating an active ingredient wherein the gel system has flow properties that make it suitable to fill a wound cavity.
The present invention realises the combined effects of polyol sugars and lidocaine hydrochloride on the physical properties of PVA-borate hydrogels and produces an optimised formulation that would be suited to topical delivery.
The invention is illustrated with reference to the following examples and the figures and tables wherein
a), 3(b) and 3(c) show the effect of D-mannitol on the physical appearance of lidocaine hydrochloride-loaded PVA-borate hydrogels.
a) shows the affect of D-mannitol and lidocaine HCL monohydrate on the pH of a hydrogel consisting of 10% w/w PVA and 2.5% w/w Borate.
a), 6(b) and 6(c) show the effects of D-mannitol and lidocaine HCl monohydrate concentration on the viscosity, hardness and compressibility (respectively) of a hydrogel containing 10% w/w PVA and 2.5% w/w borate.
The following description outlines the properties, interactions and limitations of PVA borate hydrogels. It also gives an insight into the properties of polyols (e.g. D-mannitol) and their interaction with borax and PVA-borate hydrogels.
Derivation of six candidate formulations and their production are explained below.
Derivation of Candidate Formulations
Equation (1) is a derivation of the Henderson-Hasselbalch equation. It is used to determine the pH (pHp) at which the free base (lidocaine) of a conjugate acid (lidocaine hydrochloride) begins to precipitate from solution. The pHp is dependent upon the solubility of the free base (So=0.015 Mol L−1=lidocaine), the amount of conjugate acid added to solution (S=lidocaine hydrochloride) and the pKa of the conjugate acid (pKa=7.9).
Table (1) above shows examples of pHp of lidocaine hydrochloride in aqueous solution and within the environment of PVA-Borate hydrogels. The determination of the pHp in the aqueous environment is determined using the equation (1), whilst the determination of the pHp within the gel environment is derived experimentally. Whilst there are differences between the pHp determined in aqueous and gel environments (due to electrostatic interactions within the gel), it is clear that lidocaine hydrochloride has greater solubility at lower pH values e.g. 4% lidocaine hydrochloride monohydrate is insoluble at pH values >7.26 within PVA-Borate hydrogels. Therefore, as PVA-Borate hydrogels generally have pH values within the basic range (>7.5), manipulation of the hydrogels would be required in order the solubilise 4% w/w lidocaine hydrochloride and achieve a formulation suited to purpose.
Formulation (6) was produced by keeping PVA and Borax constant and increasing D-mannitol concentration until the pH was sufficiently low to solubilise 4% w/w Lidocaine HCL monohydrate.
The above formulations have varying physical and dosing properties, as shown in the table below (table (2). Whilst each constituent of the gel affects its properties, D-mannitol has the greatest influence. Therefore, as well as the six quoted formulations, other formulations can be produced by varying the concentration of D-mannitol in order to tailor the properties. For example, Formulation 2 can be produced with greater quantities of D-mannitol, which will produce a formulation with relatively greater adhesive properties and drug release kinetics, whilst having reduced hardness (less viscous) and pH.
Drug release describes the % lidocaine HCL released after six hours, at 37° C., from the formulation into sufficient quantity of receiver phase to achieve sink conditions. The methods of analysis are explained in more detail below.
Poly (vinyl alcohol) (PVA) is a water-soluble polymer that forms ion complexes with a range of ions and charged secondary diazo dyes, such as borate1, vanadate2, Congo Red3 and cupric ions4. Of these ion complexes, the PVA-borate polymer-ion complex is preferred, especially in terms of biomedical compatibility, to other crosslinked networks, such as those based on vanadate, for example, that display appreciable toxicity to living tissue. The mechanism of PVA-borate complex formation has been previously elucidated through 13C and 11B nuclear magnetic resonance5,6, with the underlying means of network formation believed to be a di-diol complexation formed between two adjacent diol groups from PVA on one hand and the borate ion on the other. The cross-link reaction is a two step procedure, whereby an initial mono-diol complexation produces a poly(electrolyte), which as a result of electrostatic repulsion, causes the expansion of individual polymer chains and produces a sterically favourable environment for the proceeding di-diol complexation reaction to occur8.
Both inter- and intra-molecular cross-links occur as a result of the di-diol complexation reaction. Once enough inter-molecular cross-links exist, the network takes on the form of a gel system. This gelation is dependent on the number of induced temporary cross-links formed, which is dependent on the concentration of borate ions and, to a lesser extent, the concentration of PVA9-11. Borate ions, which form the cross-links with PVA and induce the anionic electrostatic repulsion on the chains, are produced through the aqueous dissociation of sodium tetraborate decahydrate (borax) to give equimolar portions of boric acid, tetraborate ions and sodium ions11. The sodium cations attenuate the overall negative charge on the poly(electrolyte) and in so doing, allow for borate-PVA cross-links to reside closer together. This effectively increases the cross-link density21. The overall situation, as described by Lin et al. (2005), is that the conformation of the polymer chains results from a balance between their excluded volume, electrostatic repulsion between charged complexes bound to the polymeric chains and the shielding effect of free sodium ions associated with the charged complexes12.
PVA-borate hydrogels are of particular interest to topical drug delivery, given their unique and characteristic flow properties that have been studied extensively through dynamic viscoelastic measurements12-16. These properties have been attributed to the finite lifetime (tlife) of the thermo-reversible, borate mediated, cross-links. Therefore, viscoelastic properties can be explained in terms of the complex modulus (G): if the length of an observation is long (low frequency) the cross-links have sufficient time to dissociate (t>tlife); the system behaves like a viscous liquid (G″>G′). In contrast, if the length of an observation is short (high frequency) the cross-links have insufficient time to dissociate (t<tlife) and the system behaves like an elastic solid (G′>G″)12, 17, 18. Indeed, they begin to resemble the cross-links found in elastic, covalently bonded, hydrogels, such as glutaraldehyde-PVA hydrogels, which display no appreciable flow due to appreciable structural integrity maintained by cross-links with an infinite life-time12.
One potential use for PVA-borate hydrogels is inducing localised anaesthesia upon exposed topical surfaces, such as those uncovered during skin lacerations. At present, only unsuitable formulations such as EMLA® cream (prilocalne 2.5%: lidocaine 2.5% w/v), in-hospital prepared lidocaine-epinephrine-tetracaine gel (LET) or in-hospital prepared solution of tetracane (0.5%)-adreniline (1:20,000)-cocaine (11.8%) are used to provide local anesthesia for lacerated wounds {{93 Anderson, A. B. 1990; 92 Hegenbarth, M. A. 1990; 91 Singer, A. J. 2001;}}. Unlike PVA-borate hydrogels, these formulations offer poor residence in a wound and must be removed with a saline wash. However, as the above formulations have been shown to provide sufficient anaesthesia {{91 Singer, A. J. 2001; 93 Anderson, A. B. 1990; 92 Hegenbarth, M. A. 1990;}}, incorporation of a local anaesthetic into PVA-borate hydrogels provides a formulation that is physically suited to treatment of lacerated wounds (novel flow properties) while still having the potential to provide effective local anaesthesia.
The hydrochloride salt of lidocaine is a good choice for administration because it has a quick onset of action and a long duration of action. However, direct incorporation into PVA-borate hydrogels must take its potential effects on network stability. Addition of lidocaine HCl to PVA-borate hydrogels will effectively increase the total free ion concentration within the gel and in so doing, will potentionally have a profound effect on the internal structure of such hydrogels21,22. The initial poly(electrolyte) formed between borate and PVA extenuates the complexation constant between further PVA and borate interactions by way of electrostatic repulsion. However, this effect can be attenuated by the shielding effect of free ions, such as those arising from inclusion of the salt form of a drug substance9. Therefore, free ions reverse any reductions in the complexation constant and if the additional ionic strength is sufficient, the system cross-links prodigiously and demixes into a two phase system that is of no practical use for topical delivery.
The complexation of borate by polyols has been studied extensively3. Penn et al (1997) showed that D-sorbitol and D-mannitol (ligands) forms monoesters (one ligand) and diesters (two ligands) with borate ions, with the diester complex being more energetically favourable. Additionally, no ligand was seen to contain more than one borate ion, which is a consequence of electrostatic forces preventing more than one borate ion attaching to each D-mannitol/D-sorbitol molecule. Furthermore, Penn et al (1997) showed that the diol carbons involved in the monoester/diester affected the stability of the resultant complex. Through modelling studies, it was shown that the 3,4-diol carbons give rise to the most favourable complex, because on formation, the free hydroxyl groups appear orientated toward the centre and so stabilise the complex29. D-mannitol, for example, has a strong affinity for borate ions and if added to PVA-borate hydrogels has been shown to cause the system to fluidize19. This comes about as a result of D-mannitol binding up free borate ions and removing borate ions from PVA molecules. The latter process causes the network to fluidize with excess borate ions being bound by D-mannitol and borate ions bound to PVA diol functionalities being sequestered by D-mannitol. Therefore, D-mannitol will potentionally enhance the solubility of Lidocaine hydrochloride (through a reduction in pH by binding of free borate ions—Lewis base) and prevent demixing brought about by the lidocaine anions (through reduction in available borate ions).
An aspect of this invention is that D-mannitol acts as a modulator of viscosity, a modulator of pH and as a means to prevent demixing, or also recognisable as a process of syneresis, of gel and continuous phases.
Materials and Methods
Materials
Poly(vinyl alcohol) (PVA) (98-99% hydrolyzed, MW=31,000-50,000), sodium tetraborate decahydrate (Borax), lidocaine hydrochloride, xylitol, meso-erythritol, maltitol, D-mannitol, D-sorbitol, dulcitol, 1,2-propandiol, glycerol, propan-2-ol and D-mannitol were purchased from Sigma-Aldrich Company Ltd. (Gillingham, Dorset, UK). All reagents and solvents were of appropriate laboratory standard and used without further purification.
Preparation and pH Measurements of PVA-Borate Hydrogels
PVA-borate hydrogels (10% w/w PVA and 2.5% w/w borax) were prepared by combining equal volumes of separate stock solutions (20% w/w PVA and 5% w/w borax) prepared in de-ionised water. A homogenous gel was formed upon heating the mixture to 80° C. for approximately two hours, with periodic stirring. Gels were stored for 48 hours at room temperature to allow complete gelation prior to evaluation. Excipients, such as lidocaine HCL and D-mannitol, were added to the sodium tetraborate solution prior to addition to the PVA solution.
The pH of gel formulations were recorded using a Sensorex® spear-tip electrode (S175CD, Sensys Ltd, Stevenage), which is designed to penetrate and measure the pH of semi-solid systems. Measurements of pH in borax solutions (5% w/v), following successive additions of D-mannitol (5.0×10−3 mole), were recorded using the same electrode apparatus.
Texture Analysis
Evaluation of the mechanical properties of PVA-borate hydrogels was performed using a TA-XT2 Texture Analyzer (Stable Micro Systems, Haslmere, UK) in texture profile analysis (TPA) mode. Formulations were placed in poly(propylene) containers (44 mm diameter×55 mm depth) (Sarstedt©, Wexford, Republic of Ireland) with any air bubbles in the gels removed upon standing at ambient temperature (>6.0 h) prior to investigation. The tubular probe (10.0 mm in diameter and 150.0 mm in length) was compressed twice into each sample to a depth of 15.0 mm at a rate of 10.0 mm s−1, with a 15.0 s delay between compressions. Four replicate measurements were made in each case at ambient temperature.
Hardness and compressibility, which have previously been used to define the mechanical properties of hydrogels were derived from the force-time plots produced by the TPA analysis30. Those skilled in the art of TPA, will be familiar with production of said force-time plots and derivation of hardness and compressibility data from aforementioned data. Hardness, the force required to achieve a given deformation, was determined by the force maximum of the first positive curve of the force-time plot30. Compressibility, the work required to deform the product during the first compression of the probe was determined by the area under the first positive curve of the force-time plot30.
Viscosity Analysis
The viscosity of PVA-borate hydrogels was determined at a constant shear force using the falling sphere technique and applying the Stoke's equation, as shown in Equation 2;
where η is the viscosity at constant shear force, ΔP is the difference in density between the sphere and the gel, g is the acceleration due to gravity (9.807 m s−1), a is the radius of the sphere and v is the experimentally derived velocity of the sphere through the test gel. Constant velocity was derived from the time required for the metallic sphere to travel a defined distance into the gel. To ensure measurements were taken upon attaining terminal velocity, the defined distance was measured from a point when the sphere has depressed 2.0 cm into the gel and was stopped 5.0 cm before the sphere has reached the bottom of the container. The viscosity was determined for each hydrogel and subsequently repeated for replicate samples (n=4).
Comparison of Polyol Affinity for Borate Ions
The following polyols were used in this investigation; maltitol, dulcitol, D-mannitol, D-sorbitol, xylitol, meso-erythritol, 1,2-propanediol, glycerol. Additionally, the affect of propan-2-ol was also investigated. The affinity of each polyol for borate was estimated using direct titrimetry and by analysis of gel characteristics. In the former method, incremental additions (1.0 ml) of a 0.5 M solution of polyol was added to 100.0 ml of 5% w/v Borax solution and the pH recorded. The reduction in free borate and subsequent percentage reduction per 5.0×10−3 moles of polyol were calculated using Equation 3;
10(pH-pKa)[H3BO3]0═[B(OH)4−]f (3)
where, [H3BO3]0 is the concentration of boric acid, [B(OH)4−]f is the concentration of free borate ions and pKa is the pKa of boric acid (derived from the initial pH of the 5% w/v borax solution). The equation assumes that boric acid concentration does not change with addition of the polyol, and that only monoborate ions occur at the concentration of borax used in the analysis (i.e. no borate aggregates exist). These assumptions are consistent with work of other researchers27,28.
Additionally, the effect of the polyols on the concentration of free borate ions within the hydrogel was estimated using changes in the pH of the gel after addition of a defined polyol. Standard hydrogels of 10% w/w PVA and 2.5% borax were produced containing each polyol at a concentration of 0.1 Molar. The pH of each gel was measured upon equilibration, with each measurement performed for four samples (n=4). Similarly, the changes in gel hardness arising from variations in free borate induced by polyol addition were determined using texture profile analysis, as described above. As before, four replicate measurements were made in each case at ambient temperature (n=4).
In Vitro Release Studies
One dimensional drug release was evaluated using cell culture inserts (Nunc®, No. 137508, Rochester, USA) constructed with an integral poly(carbonate) membrane of 8 μm pore size, as shown in
Release experiments commenced once gel-loaded inserts were placed in 100 ml of receiver phase, which was stirred at 250 rpm. A phosphate buffer (BP 1999, pH=6.8) was used as the receiver phase to mimic the slightly acidic environment of an infected/inflamed wound31. Sink conditions were maintained throughout the release experiment by ensuring that the total drug possible in the receiver phase never exceeded 10% of its solubility in this compartment. At defined time intervals, 5.0 ml of receiver phase was removed, replaced by fresh buffer and lidocaine concentration determined spectrophotometrically at 265 nm (Carey® 50 scan UV-Visible spectrophotometer). In vitro release studies were carried out at ambient temperature, 37° C. and 50° C. Once complete, the gel height and weight were measured again. Each release experiment was performed three times (n=3).
Data Treatment and Statistical Analysis
To elucidate the drug release mechanism as a function of temperature, the drug release data was fitted to the exponential equation described by Peppas and shown in Equation 425;
where Mt is the amount of drug released at time t, M∞ is total amount of drug in the system, Mt/M∞ is the fraction of dug released at time t, k is the kinetic constant that incorporates the properties of the polymeric system and the drug and n is the diffusional exponent of the drug release, used to characterise the drug transport mechanism. Therefore, calculation of the release exponent n allows for the determination of the mechanism of diffusion in the polymeric systems. Only the region of the first 60% of drug release with a stable release profile was used to determine n i.e. initial region of high flux was excluded from the determination of n.
The effects of D-mannitol and lidocaine hydrochloride on the hardness, compressibility and viscosity of PVA-borate hydrogels were evaluated using a two-way analysis of variance (ANOVA) with a 5×7 factorial design. Post-hoc analysis, namely Tukey's test, was employed for comparison of the means of the individual groups. Additionally, the effect of each polyol on hardness of standard hydrogels was evaluated using ANOVA with a 1×7 factorial design. With Tukey's test again being employed for post-hoc analysis. P<0.05 denoted significance for all statistical comparisons.
Results
The effect of D-mannitol on the physical appearance of lidocaine hydrochloride-loaded PVA-borate hydrogels is shown in
Table 3 shows the effect of each polyol on free borate concentration, pH and hardness of the test hydrogel (10% w/w PVA and 2.5% w/w borax).
The overwhelming trend is that affinity for borate is increased as the number of hydroxyl groups per molecule is increased. There is a clear increase in the reduction in free borate (as seen by the reduction in pH) and hardness as this structural feature is incremented. In fact, each sequential reduction in hydroxyl group number per molecule, that is, going from 6 to 5 to 4 and so on, resulted in significant increases in hardness of the hydrogel, as shown in
Additionally, hydrogels loaded with 0.1 Molar propan-2-ol and 1,2-propandiol, with one and two hydroxyl groups per molecule respectively, was shown to have no affect on the hardness of the hydrogel when compared to the hydrogel containing no polyol.
The effect of D-mannitol and lidocaine HCl incorporation on the pH of PVA-borate hydrogels is shown in
The effects of D-mannitol and lidocaine HCl incorporation on hardness, compressibility and viscosity are shown in
The in vitro release profiles are shown in
These results indicate a change in drug release mechanism where diffusion is the predominant mechanism when the temperature is low, changing steadily to one approaching zero-order kinetics as the temperature rises. Indeed, the profile at 50° C. shows evidence of the lidocaine release approaching linearity for the first 60% of the drug's release Vs time.
Discussion
PVA-borate polymeric matrices are of novel interest because they offer a unique set of characteristic flow properties that make them an effective delivery vehicle of local anaesthetics into cavernous wounds. Their low bioadhesive strength and cohesive integrity ensure that the system can be removed as an intact piece and without inflicting further trauma. The formulation and drug release of lidocaine HCl from polymeric matrixes has been studied extensively20,24. It has been shown by this work that without the incorporation of D-mannitol into the analyzed PVA-borate polymeric hydrogel, lidocaine hydrochloride compatibility would be poor i.e. either a precipitate is formed or the hydrogel demixes. An important finding of this work has been to show that D-mannitol is able to modify the cross-linking dynamics of these hydrogels and therefore alleviate ionic-induced network collapse or prevent precipitate formation induced by lidocaine HCL. Indeed, it was shown that an incorporation of up to and exceeding 4.0% w/v lidocaine HCl in a homogenous gel was possible. It should be remembered that lidocaine HCl dissolves in water to form an acidic solution, as shown below.
LIDH+Cl−→LIDH++Cl−
LIDH+⇄LID+H+pKa=7.9
The conjugate acid LIDH+ will dissociate slightly to form the relatively insoluble lidocaine base LID (solubility=0.015 mol L−1). The formation of the lidocaine base and any subsequent precipitation can be related to initial lidocaine HCl concentration and pKa by Equation 120;
where pHp is the pH above which the drug begins to precipitate from solution as the free base, pKa is the pKa of lidocaine hydrochloride (=7.9), S0 is the solubility of lidocaine base (0.015 mol L−1) and S is the concentration of lidocaine hydrochloride dissolved in solution. According to equation (1), homogenous solutions are favoured at low pH and dissolved drug is only found as the free base above pH 7.9. Therefore, given the inherent alkalinity of PVA-borate hydrogels (5% w/v Borax solution is a buffer at pH 9.3 at 20′C), producing a solution of lidocaine HCl at therapeutic concentrations in such gels is potentially unattainable.
The increased solubility of lidocaine hydrochloride in PVA-borate hydrogels, brought about by D-mannitol, as shown in
By binding up free borate ions, D-mannitol effectively drives the pH of the aqueous environment down, as confirmed in
This study has shown that the solubility of lidocaine HCl in PVA-borate systems decreased as the temperature was increased, as shown in
It is well described in the literature that D-mannitol has a greater affinity for borate ions relative to PVA, which means that D-mannitol will progressively compete and remove borate ions from PVA/borate hydrogels causing the system to fluidize19. The progressive reduction in the cross-link density of PVA-borate hydrogels by increasing D-mannitol concentration is evident from the texture and viscosity analysis. Physical characterisation showed that increasing D-mannitol in 0.5% increments up to a final concentration of 2.5% w/w for texture analysis and up to 2% w/w for viscosity determination produced significant reductions in both (
The investigation of drug release was carried out on a formulation judged to possess favourable flow characteristics for topical drug delivery. Over the three temperatures tested, similar patterns were observed, namely an initial region of high flux followed by a stable phase of release. The initial high flux can be attributed to an initial burst effect, known to occur over a short period of time, being unpredictable and often occurring in hydrogels as a result of higher surface concentrations brought about by migration of the drug to the surface26. Ongoing flux took about 60 minutes to reach a more stable profile. It was interesting to note that the duration of initial anomalous flux was temperature dependent, with higher temperatures producing a more stable profile after a shorter duration, indicating that this initial phase can not simply be explained in terms of a burst effect. Effects arising from system swelling and forming an equilibrium with the surrounding environment may be attributive. In any event, the large release of lidocaine at the beginning of the release profile could be construed as beneficial, because the initial high levels will offer a quick and effective local aesthetic effect to a target wound site.
The region of stable flux (up to a maximum of 60% of drug released) was used to determine the mechanism of diffusion through by calculation of the release exponent, which were seen to approach 1.0 as the environmental temperature was increased. This temperature-induced alteration in the release mechanism can be explained on the basis of the concentration of free borate ions, which increase as the temperature is increased. The ensuing elevation in pH forces lidocaine base to form a precipitate once saturation is reached. This nascent precipitate acts as a drug reservoir that constantly replaces the soluble lidocaine that diffuses out of the system. Indeed, the hydrogel can be observed to turn opalescent in appearance when warmed. As a result of the reservoir effect, a constant diffusion gradient can be maintained effectively whilst the precipitate is present. This effect was most evident for the release at 50° C. (n=0.83), where the precipitate was still observed throughout the hydrogel even after 60% of the drug had been released. In contrast, cooler temperatures resulted in both diminutions in precipitate and the release exponent until, at ambient temperature (n=0.66), no precipitate was evident. As a consequence, the hydrogel at ambient temperature contains drug completely dissolved in a hydrophilic matrix. Therefore, it is not surprising that diffusion through this system approaches a square root of time relationship (n=0.5), as this relationship is often seen in polymer matrixes where the drug is completely dissolved in the system.
In conclusion, D-mannitol was shown to be an effective and requisite excipient for formulating lidocaine HCL into PVA-borate hydrogels. D-mannitol was shown to circumvent network constriction induced by ionic effects upon adding a hydrochloride salt of a drug substance. Formulation 6 (4% lidocaine, 2% D-mannitol, 10% PVA and 2.5% borax) was shown to offer an effective delivery system, which was characterised by an initial burst release and a drug release mechanism dependent on temperature, changing from a diffusion-controlled system to one with the properties of a reservoir system. The novel flow properties and innocuous adhesion of PVA-borate hydrogels mean they are suited for the drug delivery to exposed epithelial surfaces, such as lacerated wounds.
Gel Formulation including drugs for Photodynamic antimicrobial chemotherapy (PACT) which is defined as a medical treatment by which a combination of a sensitising drug and visible light causes selective destruction of microbial cells through the generation of singlet oxygen. Such a gel can be added to wounds to prevent and treat infection and infestation such as that caused by MRSA.
A typical formulation would be prepared as for lidocaine and include medicaments such as haematoporphyrin derivative, chlorins and bacteriochlorins, phthalocyanines and derivatives there off, benzoporphyrin derivatives, derivatives of 5-aminolaevulinic acid (ALA), purpurins, porphycenes, pheophorbides and verdins, psoralens and their derivatives, anthracycline compounds and their derivatives, phenothiazinium compounds such as methylene blue and toluidine blue, cyanines such as merocyanine 540, acridine dyes, derivatives of Nile blue, and rhodamines such as Rhodamine 123.
Examples of formulations containing a photosensitising compound as an active compound, PVA, cross-linker and modulator are shown below.
The gel may be used for the topical application of drug substances to the following areas of the body for the purposes of achieving any or all of drug absorption to the systemic circulation, localised drug absorption into the underlying tissue structures, such as epithelial layers, dermis and epidermis, or localised drug absorption into a cavity or space associated with the anatomical structures mentioned below.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0718435.1 | Sep 2007 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2008/050828 | 9/16/2008 | WO | 00 | 7/21/2010 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2009/037500 | 3/26/2009 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 5505953 | Chowhan | Apr 1996 | A |
| 6444199 | Renn | Sep 2002 | B1 |
| 20020122771 | Holland et al. | Sep 2002 | A1 |
| 20040166131 | Pinna et al. | Aug 2004 | A1 |
| 20050214376 | Faure et al. | Sep 2005 | A1 |
| 20070059274 | Asgharian et al. | Mar 2007 | A1 |
| Number | Date | Country |
|---|---|---|
| 1093897 | Oct 1994 | CN |
| Entry |
|---|
| International Search Report for International Application No. PCT/GB2008/050828, pp. 1-3. |
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| 20100286205 A1 | Nov 2010 | US |
