HYPERSENSITIVITY

Field of the Invention

The present invention relates to methods for treating tooth hypersensitivity using certain particulate calcium carbonates.

Background and Prior Art

Tooth hypersensitivity is a common but painful condition affecting up to 20% of the adult population. Hypersensitive teeth may be sensitive to cold, heat, air or sugary foods. The incidence of tooth hypersensitivity increases with age. Tooth hypersensitivity is believed to be related to the general increase in exposed root surfaces of teeth as a result of periodontal disease, toothbrush abrasion, or cyclic loading fatigue of the thin enamel near the dento-enamel junction. When root surfaces are exposed, dentinal tubules are also exposed. Dentinal tubules are naturally present in the dentinal layer of the tooth and they provide for an osmotic flow between the inner pulp region of the tooth and the outer root surfaces.

The currently accepted theory for tooth hypersensitivity is the hydrodynamic theory, based on the belief that open exposed dentinal tubules allow fluid flow through the tubules. This flow excites the nerve endings in the dental pulp. Clinical replicas of sensitive teeth viewed in a scanning electron microscope (SEM) reveal varying numbers of open or partially occluded dentinal tubules.

There are two categories of therapy for the treatment of tooth hypersensitivity based upon two modes of action. The first category, nerve-depolarising agents, are pharmaceutical agents such as potassium nitrate, which function by interfering with neural transduction of the pain stimulus.

The second category, known as occluding agents, function by physically blocking the exposed ends of the dentinal tubules, thereby reducing dentinal fluid movement and reducing the irritation associated with the shear stress described by the hydrodynamic theory.

An example of an occluding agent is found in US 5,270,031 which describes a tubule occluding desensitizer comprising a polyacrylic acid such as Carbopol® polymeric materials. Another tubule occluding composition is disclosed in US 5,374,417 which discloses a potassium salt of a synthetic anionic polymer, such as a polycarboxylate. There is a continuing need for desensitizing agents which can provide treatment and relief of hypersensitivity whilst maintaining a pleasing "mouth-feel" for the product in which they are formulated.

The present inventors have found that this problem may be solved by the use of certain particulate calcium carbonates of specific particle size and shape.

Summary of the Invention

The present invention provides a composition containing particulate calcium carbonate composed of primary particles which are prismatic and which have an average size of 2 microns or less; for the treatment of hypersensitivity arising in natural human teeth by application of the composition thereto. The invention also provides a method of treating hypersensitivity arising in natural human teeth, the method comprising applying thereto an effective amount of a composition as described above. The invention also provides the use of particulate calcium carbonate composed of primary particles which are prismatic and which have an average size of 2 microns or less; for the treatment of hypersensitivity arising in natural human teeth. The prismatic calcium carbonate particles as defined above are effective as dentinal tubule occluding agents. Furthermore they can provide a smooth and pleasant mouth-feel when formulated into products such as dentifrices.

Additionally, by reacting with phosphates (such as those naturally found in saliva), the prismatic calcium carbonate particles as defined above may assist with strengthening or remineralising enamel or dentine, and may help to form plugs in tubules which further reduce sensitivity.

Prismatic calcium carbonates have been proposed in the literature as a dentifrice component. However, the dentifrices in question are designed for bridgework, dentures and other forms of artificial teeth which are made from polymers. There is no suggestion that prismatic calcium carbonates would be effective in the context now described, namely the treatment of hypersensitivity arising in the natural human tooth, and in particular the occlusion of dentinal tubules.

DETAILED DESCRIPTION OF THE INVENTION

Particulate calcium carbonate The composition for use in the invention comprises a particulate calcium carbonate composed of primary particles which have a specific shape and size, as defined above.

By "primary particles" is meant individual particles, defined as the smallest discrete particles that can be seen by electron microscopy analysis (such as, for example, individual crystals). For the purposes of the present invention, the primary particles of the particulate calcium carbonate are prismatic and have an average size of 2 microns or less. The primary particles may associate under certain conditions to form larger secondary structures such as aggregates or agglomerates.

For the purposes of the present invention, a suitable source of particulate calcium carbonate as defined above includes crystalline calcium carbonates in which the individual crystals have a prismatic morphology and an average size of 2 microns or less.

The term "crystalline" (in the context of particulate calcium carbonate) generally means a particulate calcium carbonate in which at least 50% by weight, preferably at least 75% by weight, more preferably at least 90% by weight, most preferably more than 95% by weight and ideally more than 99% by weight of the calcium carbonate particles are in the form of crystals.

The term "crystal" means an essentially fully dense solid composed of atoms arranged in an orderly repetitive array bounded by plane surfaces which are the external expression of internal structure. Crystals may be identified and

characterised by standard techniques known to those skilled in the art such as X- ray diffraction. Crystalline forms of calcium carbonate are available naturally, or may be synthetically produced in three particular crystalline morphologies, calcite, aragonite, and less commonly found, vaterite. The vaterite form of calcium carbonate is metastable and irreversibly transforms into calcite and aragonite. There are many different polymorphs (crystal habits) for each of these crystalline forms. The calcite crystalline morphology is the most commonly used crystal form of calcium carbonate. Over 300 crystalline forms of calcite have been reported in the literature.

References to a "prismatic crystal morphology" (in the context of crystalline calcium carbonates) generally denote a crystalline calcium carbonate in which at least 50% by weight, preferably at least 75% by weight, more preferably at least 90% by weight, most preferably more than 95% by weight and ideally more than 99% by weight of the crystals are prismatic crystals. Prismatic crystals have a set of faces with parallel edges that are parallel to the vertical or "c"-axis direction. There can be three, four, six, eight or even twelve faces that can form a prism. For the purposes of the present invention, preferred prismatic crystals have a generally uniform, typically generally hexagonal cross-sectional shape. Crystal shape may be determined by standard techniques known to those skilled in the art such as scanning electron microscopy (SEM). SEM is an imaging and analysis technique based on the detection of electrons and X-rays that are emitted from a material when irradiated by a scanning electron beam. Imaging allows the user to distinguish between primary particles and aggregates or agglomerates.

Usually, crystals grow in three directions, length, width and height. Some crystals however, have one or two preferred growth directions. For the purposes of the present invention, preferred prismatic calcium carbonate crystals have a ratio between the length and the width of the crystal (the so-called aspect ratio) ranging from about 1 :1 up to about 3:1 (length:width). The aspect ratio of a particle (e.g. a crystal) may typically be obtained from SEM photographs by averaging the ratio between the length and width of the particle, measured on at least 10 particles for 1 photograph.

For the purposes of the present invention, preferred prismatic calcium carbonate crystals have an average size which generally ranges from about 0.1 to 2 microns, with a preferred size ranging from about 0.2 to about 1 .5, more preferably from about 0.3 to about 1 , most preferably from about 0.5 to 0.9 microns. Particle (e.g. crystal) size may be determined by standard techniques known to those skilled in the art such as sedimentation. Particle size values obtained from sedimentation techniques are normally expressed in terms of the equivalent spherical diameter (ESD), i.e. the diameter of a notional sphere having the same volume as the particle. The ESD value may be calculated based on the sedimentation rate of the particle in question as defined by Stokes' Law (Micromeritics SediGraph® 5100 Particle Size Analysis System Operator's Manual, V2.03, 1990). The

sedimentation rate is measured using a finely collimated beam of low energy X- rays which pass through the sample cell to a detector. For a sample population of particles, the distribution of particle mass at various points in the cell affects the number of X-ray pulses reaching the detector. This X-ray pulse count is used to derive the particle size distribution expressed as the percent mass at given particle diameters. The median value of the ESD which can be derived from this distribution (i.e. the particular ESD value on the cumulative mass distribution curve at which 50 percent mass of the population has a higher ESD and 50 percent mass of the population has a lower ESD) is usually quoted as the average particle size of the sample population. For the purposes of the present invention, preferred prismatic calcium carbonate crystals generally have a BET specific surface area higher than or equal to 0.1 m2/g, preferably higher than or equal to 1 m2/g, more preferably higher than or equal to 3 m2/g and most preferably higher than or equal to 4 m2/g. The calcium carbonate particles according to the invention generally have a BET specific surface area lower than or equal to 30 m2/g, preferably lower than or equal to 25 m2/g, more preferably lower than or equal to 20 m2/g, and most preferably lower than or equal to 15 m2/g. The BET specific surface area is usually measured according to the standard ISO 9277 which specifies the determination of the overall specific external and internal surface area of disperse or porous solids by measuring the amount of physically adsorbed gas according to the Brunauer, Emmett and Teller (BET) method.

Crystalline forms of calcium carbonate having prismatic crystal morphology (as described above) and suitable for use in the invention are available naturally, or may be produced by precipitation production technology. Typically, precipitated calcium carbonate is prepared by exposing calcium hydroxide slurry to a carbonation reaction. Control of the specific solution environment during the nucleation and growth of calcium carbonate governs the size and the shape of the crystals in the resulting precipitated calcium carbonate product.

For the purposes of the present invention, a particularly preferred class of particulate calcium carbonate includes crystalline calcium carbonates in which the individual crystals have a prismatic morphology and an average size of 2 microns or less, and also in which at least 50% by weight, preferably at least 80% by weight of the crystals are stubby-prismatic shaped calcite crystals with a size averaging about 1 micron in length by about 0.5 to 0.75 micron in width, and an aspect ratio ranging from 1 :1 to 3:1 . Such materials are described for example in US 3,320,026. The crystals are characterised in terms of shape as "calcite crystals of stubby-prismatic, doubly terminated form in which each of the two termini consist of three faces of a rhombohedron, all six faces belonging to one unit calcite rhombohedron of 75 ^Q interfacial angle, the prismatic portion being characterised by six gently curved convex subhedral faces substantially parallel to the prismatic length and constituting a substantially vertical prism". The crystals are further characterised in terms of aspect ratio: the prismatic length is between about one and three times the prismatic cross-sectional width, averaging about 1 .5 times the prismatic cross-sectional width. The crystals are further

characterised in terms of dimensions: usually averaging about 1 micron in length by about 0.5 to 0.75 micron in width. US 3,320,026 describes a process for the preparation of such a calcite, in which carbon dioxide is introduced into an aqueous suspension of coarse calcium hydroxide (at least about 50 weight percent of the hydroxide consisting of particles coarser than 10 microns) while maintaining the temperature below about 20 ^Q C at least until calcite crystallisation is in progress. Carbon dioxide introduction is then continued, and the temperature may be permitted to rise, while calcite crystallisation continues to substantial completion.

A commercially available source of particulate calcium carbonate suitable for use in the invention is ViCALity® ALBAFIL® Precipitated Calcium Carbonate (PCC), ex Specialty Minerals Inc., Bethlehem, Pa. 18017.

Mixtures of any of the above described materials may also be used.

In order to treat hypersensitivity arising in natural human teeth, the particulate calcium carbonate as defined above (hereinafter termed "prismatic calcium carbonate") will generally be incorporated into an oral care composition. Examples of such oral care compositions include dentifrice, mouthwash, tooth powder, chewing gum, lozenge or other oral care product form suitable for topical administration to teeth. The relative amount of prismatic calcium carbonate to be incorporated into such a composition will depend to some extent on the type of composition used and its typical mode of use (for example typical product dosage and the amount of time it stays in contact with the affected site).

In dentifrices for use in the invention, for example, the amount of prismatic calcium carbonate may generally range from 1 to 50%, preferably from 10 to 40%, more preferably from 30 to 40%, by total weight prismatic calcium carbonate based on the total weight of the dentifrice.

A dentifrice represents a preferred type of oral care composition in the context of the present invention. The term "dentifrice" denotes a composition which is used to clean the surfaces of the oral cavity. The dentifrice is an oral composition that is not intentionally swallowed for purposes of systemic administration of therapeutic agents, but is retained in the oral cavity for a sufficient time to contact substantially all of the dental surfaces and/or mucosal tissues for purposes of oral activity. Preferably the dentifrice is suitable for application with a toothbrush and is rinsed off after use. Preferably the dentifrice is in the form of an extrudable semi-solid such as a cream, paste or gel (or mixture thereof).

A dentifrice for use in the invention will usually contain a liquid continuous phase in an amount of from 40 to 99% by weight based on the total weight of the dentifrice. Such a liquid continuous phase will typically comprise a mixture of water and polyhydric alcohol in various relative amounts, with the amount of water generally ranging from 10 to 45% by weight (based on the total weight of the dentifrice) and the amount of polyhydric alcohol generally ranging from 30 to 70% by weight (based on the total weight of the dentifrice). Typical polyhydric alcohols include humectants such as glycerol, sorbitol, polyethylene glycol, polypropylene glycol, propylene glycol, xylitol (and other edible polyhydric alcohols),

hydrogenated partially hydrolyzed polysaccharides and mixtures thereof. A dentifrice for use in the invention will also generally contain further ingredients to enhance performance and/or consumer acceptability. For example, the dentifrice will usually contain a binder or thickening agent in an amount of from 0.5 to 10% by weight based on the total weight of the dentifrice. Suitable binders or thickening agents include carboxyvinyl polymers (such as polyacrylic acids cross-linked with polyallyl sucrose or polyallyl pentaerythritol), hydroxyethyl cellulose, hydroxypropyl cellulose, water soluble salts of cellulose ethers (such as sodium carboxymethyl cellulose and sodium carboxymethyl hydroxyethyl cellulose), natural gums (such as carrageenan, gum karaya, guar gum, xanthan gum, gum arabic, and gum tragacanth), finely divided silicas, hectorites, colloidal magnesium aluminium silicates and mixtures thereof. The dentifrice will also usually contain a surfactant in an amount of from 0.2 to 5% by weight based on the total weight of the dentifrice. Suitable surfactants include anionic surfactants, such as the sodium, magnesium, ammonium or ethanolamine salts of C ₈ to C-I8 alkyl sulphates (for example sodium lauryl sulphate), C ₈ to C-| ₈ alkyl sulphosuccinates (for example dioctyl sodium sulphosuccinate), C ₈ to d ₈ alkyl sulphoacetates (such as sodium lauryl sulphoacetate), C ₈ to Ci ₈ alkyl sarcosinates (such as sodium lauryl sarcosinate), C ₈ to C-| ₈ alkyl phosphates (which can optionally comprise up to 10 ethylene oxide and/or propylene oxide units) and sulphated monoglycerides. Other suitable surfactants include nonionic surfactants, such as optionally polyethoxylated fatty acid sorbitan esters, ethoxylated fatty acids, esters of polyethylene glycol, ethoxylates of fatty acid monoglycerides and diglycerides, and ethylene oxide/propylene oxide block polymers. Other suitable surfactants include amphoteric surfactants, such as betaines or sulphobetaines. Mixtures of any of the above described materials may also be used. An additional advantage of the prismatic calcium carbonate of the invention is that it may provide a smooth and pleasant mouth-feel when incorporated into a dentifrice product form, such as described above. This is particularly

advantageous in the context of dental hypersensitivity treatment since it encourages regular and repeated product use by the consumer, which reinforces the anti-hypersensitivity benefit achieved.

A further advantage of the prismatic calcium carbonate of the invention is that it may play a dual role of desensitizer and gentle tooth whitener, when incorporated into a dentifrice product form, such as described above.

Accordingly, a preferred dentifrice for use in the invention demonstrates

satisfactory levels of cleaning, yet is not unduly abrasive and damaging to the teeth, when measured for example as described below.

Such a dentifrice for use in the invention may comprise abrasive materials selected from abrasive silicas, calcium carbonates (which are different to the prismatic calcium carbonate described above), dicalcium phosphate, tricalcium phosphate, calcined alumina, sodium and potassium metaphosphate, sodium and potassium pyrophosphates, sodium trimetaphosphate, sodium

hexametaphosphate, particulate hydroxyapatite and mixtures thereof. However, it is preferred that the total amount of these abrasive materials is less than 5%, more preferably less than 1 %, and most preferably less than 0.1 % by weight based on the total weight of the dentifrice.

The level of cleaning ability of a dentifrice for use in the invention may be measured by assessing its effect on accumulated pellicle, for example by using a method such as that described by Pickles et al. {International Dental Journal 55 (2005), pp. 197-202). This model uses enamel slabs cut from bovine central incisors, embedded in methacrylate resin. The enamel surfaces are smoothed by hand using an alumina paste on a glass block and lightly acid etched in order to facilitate stain accumulation and adherence. The enamel blocks are placed in an incubator set at a constant temperature of 50 °C and slowly rotated to alternate between immersion in a staining broth (consisting of tea, coffee and mucin) and air drying. The broth is changed daily and after five days the slabs are removed and washed with distilled water to remove any loose debris. The stained slabs are then brushed in a mechanical brushing machine with distilled water to remove any loosely adhered stain. They are then dried and the L* values (L* _st ained) from the CIE L ^* a ^* b ^* colour system are measured with a chroma meter in L ^* a ^* b ^* mode. To assess cleaning performance, the stained specimens are then mounted in the mechanical brushing machine and the required load applied to each brush. The test composition is dispersed in an aqueous diluent to form a slurry (typically 38.5g test composition and 61 .5g of distilled water) and the stained specimens brushed for a set number of brush strokes with 10ml of slurry. After brushing, the enamel slabs are rinsed with distilled water, dried and the L* values (LVushed) are re-measured. In the final stage, all traces of stain are removed from the enamel slabs using flour of pumice, on a soft cloth using a grinder/polisher. The enamel slabs are then rinsed with distilled water, dried and the L* values (L*p _Umice d) are measured and recorded. The percentage of the stain removed by the test composition compared to full removal by pumice (hereinafter referred to as the pellicle cleaning ratio or PCR) may be calculated using the following equation:

PCR = [(L*brushed)-(L*stained)/ (L*p _Um j _C ed)-(l-*stained)] X 100 A dentifrice for use in the invention preferably has a PCR value of at least 30%, more preferably at least 40%, most preferably at least 45%.

The level of abrasivity of a dentifrice for use in the invention may be measured by assessing its Radioactive Dentine Abrasion Test (RDA) value. A standard test procedure for measuring these values follows the method for assessment of dentifrice abrasivity recommended by the American Dental Association (Journal of Dental Research 55(4) 563, 1976). In this procedure, extracted human teeth are irradiated with a neutron flux and subjected to a standard brushing regime. The radioactive phosphorus 32 removed from the dentin in the roots is used as the index of the abrasion of the composition tested. A reference slurry containing 10 g of calcium pyrophosphate in 50 cm ³ of 0.5% aqueous solution of sodium

carboxymethyl cellulose is also measured and the RDA of this mixture is arbitrarily taken as 100. In order to measure the RDA for the test composition, a slurry of 25g of the test composition in 40 cm ³ water is prepared and submitted to the same brushing regime.

A dentifrice for use in the invention preferably has a RDA value of no more than 100, more preferably no more than 80, most preferably no more than 60. A dentifrice for use in the invention preferably has a PCR:RDA ratio of at least 0.5, more preferably at least 0.6, most preferably at least 0.8.

Compositions for use in the invention (such as in particular dentifrices) may also contain further optional ingredients customary in the art such as fluoride ion sources, anticalculus agents, buffers, flavouring agents, sweetening agents, colouring agents, opacifying agents, preservatives, antisensitivity agents and antimicrobial agents.

The invention is further illustrated with reference to the following, non-limiting Example. EXAMPLE

A dentifrice formulation according to the invention (Example 1 ) was prepared. The ingredients are shown in Table 1 below.

All ingredients are expressed by weight percent of the total formulation, and as level of active ingredient.

Table 1

⁽ ' Precipitated calcium carbonate ex Speciality Minerals Inc., consisting of prismatic calcite crystals. The crystals have an average (median) particle size of 0.7 microns (equivalent spherical diameter). Tubule Flow Measurement

The formulation of Example 1 , a comparative formulation A (representing an existing commercially marketed dentifrice formulation for the treatment of sensitive teeth) and a 10% solution of oxalic acid (positive control) were tested for their ability to reduce hydrodynamic fluid flow in the measurement of hydraulic conductance.

All formulations were subjected to the standard hydraulic conductance procedure and the hydrodynamic flow measured.

Protocol

Sound caries free human molars were sectioned with a watering saw and dentin discs taken from between the crown and the pulp cavity. The discs were approximately 800 microns thick after sectioning. The discs were polished flat with 800 grit paper to a thickness of 500 microns and were finally polished with 2500 grit paper to give a flat polished dentin surface on both sides of the disc. The discs were then placed into 6% w/w citric acid and sonicated for five minutes.

The hydraulic conductance equipment was connected to a compressed air supply and the solvent chamber pressurised to 1 .0 PSI. A dentin disc was placed into the holding chamber and EBSS Earles balanced salt solution (ex Sigma-Aldrich) passed through the system. An air bubble was introduced into the capillary tube via the input port and allowed to proceed along the capillary tube for a few seconds before being timed from a defined start point. A record was then made of the start position of the bubble and its progression timed over five minutes and the distance travelled measured at one minute intervals.

The test formulation was applied to the dentin disc using a Benda brush for 10 seconds and then left to soak in the formulation for two minutes. The disk was then rinsed with deionised water obtained by reverse osmosis. A second air bubble was introduced into the capillary tube and, after a brief pause, the distance travelled again measured over five minutes with the distance travel recorded at one minute intervals.

Hydraulic conductance results

The flow rate was calculated from the displacement of liquid from the capillary tube over five minutes. The linearity of the flow rate was determined by calculating the r ² of distance over time. Each occlusion value is the result of ten separate determinations. The average percentage occlusion for each of the treatments is shown in Table 2 below.

Table 2

ORAL-B® Pro Expert Sensitive + Gentle Whitening

A statistical evaluation of the data sets was performed using one way ANOVA and Tukey means comparison. Data sets were first tested for normal distribution using Shapiro-Wilks normality test. All data sets had a probability factor greater than 0.05 indicating they were normally distributed. Conclusions

Using the standard hydraulic conductance method, it was found that the 10% oxalic acid solution (positive control) gave the greatest occlusion of all the treatments and resulted in 94.5% occlusion. Example 1 according to the invention gave significantly more tubule occlusion than Comparative Example A.