SNF 2 gel of improved stand-up and stability

A SnF.sub.2 gel composition exhibiting improved stand-up and stability to exposure to air containing moisture which composition contains SNF.sub.2, anhydrous glycerin, a polyethylene glycol having an average molecular weight of 1000 and xanthan gum.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a stannous fluoride dental gel having improved 
cosmetic stability to air and moisture. 
2. The Prior Art 
Stannous fluoride (SnF.sub.2) has been reported to be an effective agent 
for treating various oral conditions. Included in the dental benefits 
imparted by SnF.sub.2 is the reduction of dental caries. The anticaries 
benefit has been attributed to the fluoride ion component of the SnF.sub.2 
salt. 
SnF.sub.2 has also been indicated clinically to be efficacious in the 
reduction of dentinal hypersensitivity. This latter therapeutic effect is 
believed to be attributable, to a large degree, to the stannous ion 
(Sn.sup.2+) component of the salt. 
In order for Sn.sup.2+ to be efficacious it must be stable and freely 
available and not be in chemical combination with other ingredients. 
Stannous fluoride as a 0.4% by weight SnF.sub.2 preparation has most 
frequently been demonstrated to be the concentration of choice in the 
treatment of dental caries and dentine hypersensitivity. 
Due to the chemical instability of stannous fluoride in aqueous solutions, 
the fluoride salt is normally applied to the teeth as a nonaqueous gel 
wherein anhydrous glycerin is a carrier for the SnF.sub.2 salt. SnF.sub.2 
in the form of a solution in anhydrous glycerin is presently provided to 
the professional for use as a topical treatment in the dental office. The 
professional dilutes this formulation with an aqueous solution immediately 
prior to application to the teeth. 
While topical applications are frequently performed in the dental office 
there is also a need for follow-up daily application and use by the 
patient. Thus, "home-care" or "patient-care" availability is desirable. 
For this purpose a gel with the requisite physical properties to 
accommodate toothbrush application is the desired marketable form. 
Unfortunately, SnF.sub.2 dissolved in anhydrous glycerin, the form found 
most suitable for preserving the chemical stability of SnF.sub.2, does not 
lend itself to application with a toothbrush. For example, U.S. Pat. Nos. 
4,418,057 and 4,533,544 disclose topical preparations of 0.40% SnF.sub.2 
which are chemically stable when suspended in anhydrous glycerin thickened 
with hydroxyethylcellulose. A drawback to these gel preparations is that 
the gels have a semi-liquid consistency, and when dispensed on the 
bristles of a toothbrush, the get immediately sinks through the bristles 
and runs off the brush so that only a relatively small portion of the 
dispensed product is retained on the toothbrush. Consequently, SnF.sub.2 
suspended in hydroxyethylcellulose thickened anhydrous glycerin has found 
limited acceptability as a home-care product as the semi-fluid gel 
composition cannot be controllably retained on the surface of toothbrush 
bristles and then applied to teeth to reliably supply SnF.sub.2 for the 
treatment of dental caries or dentine hypersensitivity. 
To preclude the erratic, inconsistent applications of SnF.sub.2 encountered 
with the SnF.sub.2 semi-liquid gels of the prior art, there is disclosed 
in copending patent application Ser. No. 08/200,034, a SnF.sub.2 gel 
preparation formulated with a polyethylene glycol having an average 
molecular weight of 1000 ("polyethylene glycol 1000") that has improved 
"stand-up" properties, i.e., the gel has an extrudable consistency such 
that extruded ribbons of the gel, when dispensed onto the bristles of a 
toothbrush, will stand-up on the top surface of the bristles for a time 
sufficient to allow full application to the teeth, e.g., a time interval 
of at least 0.5-1.0 minute. 
A drawback to the SnF.sub.2 gel disclosed in Ser. No. 08/200,034 is 
encountered during storage or use, as the gel, due to the hygroscopic 
nature of the polyethylene glycol 1000 present to enhance stand-up, 
deliquesces upon exposure to air containing moisture which substantially 
destroys its stand-up property, degrading the gel to a semi-liquid 
consistency whereby the gel is unsuitable for home care usage. 
Liquefaction of the gel of Ser. No. 08/200,034 has been observed when the 
product is in contact to air containing moisture for a period of time as 
short as one minute. Liquefaction becomes a serious problem when the cap 
to the orifice of the tube or pump in which the gel is packaged is allowed 
to remain open by the user causing a cosmetically unacceptable 
liquefaction of the gel ingredients with the result that the gel product 
will drip out of the tube or pump prior to use. 
SUMMARY OF THE INVENTION 
In accordance with the present invention there is provided a chemically and 
cosmetically stable stannous fluoride gel that has stand-up properties 
suitable for tooth brush applications and does not undergo liquefaction 
when exposed to air containing moisture wherein the composition is 
comprised of SnF.sub.2, anhydrous glycerin, a polyethylene glycol 
thickener having an average molecular weight of 1000 and a small amount of 
xanthan gum effective to inhibit liquefaction of the gel. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The SnF.sub.2 gel compositions of the present invention are generally 
comprised of about 0.30 to about 1.0% by weight SnF.sub.2 and preferably 
about 0.35 to about 0.90% by weight; about 87 to about 99% by weight 
anhydrous glycerin and preferably about 90 to about 95% by weight and 
about 3.0 to about 10.0% by weight of a polyethylene glycol thickening 
agent having an average molecular weight of 1000 and preferably about 5.0 
to about 8.0% by weight and about 0.01 to about 0.5% of a xanthan gum and 
preferably about 0.2 to about 0.4% by weight. The polyethylene glycol used 
in the practice of the present invention is a nonionic polymer of ethylene 
oxide having an average molecular weight of 1000 and the general formula 
EQU HOCH.sub.2 (CH.sub.2 OCH.sub.2).sub.n CH.sub.2 OH 
wherein n represents the average number of oxyethylene groups, such 
polyethylene glycol being designated hereinafter as polyethylene glycol 
1000, the number 1000 representing the average molecular weight. 
It is essential to the practice of the present invention to use xanthan gum 
to inhibit liquefaction of the SnF.sub.2 gel. Xanthan gum is known in the 
art and has been proposed for use in dentifrice compositions in U.S. Pat. 
No. 4,401,648. Xanthan gum is a fermentation product prepared by action of 
the bacteria of the genus Xanthomonas upon carbohydrates. Four species of 
Xanthomonas, namely, X. campetris, S. phaseoli, X. malvocearum and X. 
carotae are reported in the literature to be the most efficient gum 
producers. Although the exact structure is not determined, it is generally 
accepted to be a heteropolysaccharide with a molecular weight of several 
million and contains D-glucose, D-mannose, and D-glucoronic acid in molar 
ratio of 2.8:3:2.0. The molecule contains 4.7% acetyl and about 3% 
pyruvate. The proposed chemical structure configuration can be found in 
McNeely and Kang, Industrial gums, ed. R. L. Whistler, Ch. XXI, 2nd 
Edition, New York, 1973. The procedure for growing, isolating and 
purifying the xanthan gum is found in Manufacturing Chemist, May 1960, 
pages 206-208 (including mention at page 208 of potential use of gums 
therein described for formulating toothpastes). 
Xanthan gum is incorporated in the SnF.sub.2 gel composition of the present 
invention in an amount of about 0.01 to 0.5% by weight and preferably 
about 0.2 to about 0.4% by weight. It has been found necessary to use 
xanthan gum as a liquefaction inhibitor in order to provide a stable, 
semi-solid, extrudable dentifrice which undergoes substantially no 
liquefaction upon exposure to air containing moisture for extended time 
periods. As will hereinafter be demonstrated, other gums commonly used in 
as dentifrice formulations such as hydroxyethyl cellulose, iota 
carrageenan, carboxymethyl cellulose and Carbopol, a crosslinked acrylic 
acid copolymer, have been found to be substantially inoperative in 
preventing liquefaction of the SnF.sub.2 gel formulated using a 
polyethylene glycol 1000 thickener upon exposure to air containing 
moisture with the result that a cosmetically unacceptable product is 
produced which lacks the rheological properties required for an a stable 
dentifrice. 
Also included in the compositions of the present invention is an effective 
flavoring amount of a flavor compatible and stable with the stannous 
fluoride salt. The flavor ingredient constitutes about 0.05 to about 1% by 
weight and preferably about 0.1 to about 0.9% by weight of the gel 
composition. Suitable flavoring constituents are flavoring oils, e.g. oils 
of spearmint, peppermint, wintergreen, clove, methyl salicylate and 
menthol. 
The SnF.sub.2 dental gel of this invention may be prepared by suspending 
SnF.sub.2, predissolved in anhydrous glycerin, flavor, polyethylene glycol 
1000 and the xanthan gum in anhydrous glycerin heated to a temperature of 
35.degree. to 140.degree. C. by mixing in any suitable mixer, such as a 
Lightening mixer for about 30 minutes until a homogenous solution is 
formed. A substantially rigid, non-fluid gel product is obtained upon 
cooling. The final product may be packaged in any suitable container 
compatible with SnF.sub.2 gels such as plastic or laminate tubes or 
bottles. 
The SnF.sub.2 gel product of the present invention undergoes virtually no 
liquefaction when exposed to air containing moisture for extended time 
periods and has an extrudable consistency and upon being extruded as a 
ribbon onto the bristles of a toothbrush, the ribbon remains in a stand-up 
position on the toothbrush without substantially sinking through the 
bristles for at least two minutes. 
The following example is further illustrative of the present invention, but 
it is understood that the invention is not limited thereto. All amounts 
and proportions referred to herein and the appended claims are by weight.

EXAMPLE 
A SnF.sub.2 gel of the present invention was prepared with the following 
ingredients: 
______________________________________ 
Ingredient Concentration (wt %) 
______________________________________ 
Glycerin 92.800 
Polyethylene Glycol 1000 
6.00 
SnF.sub.2 0.4000 
Xanthan Gum 0.300 
Flavor 0.5000 
______________________________________ 
The glycerin, flavor, xanthan gum and polyethylene glycol 1000 were 
premixed at 100.degree. C. for 30 minutes to form a homogenous solution. 
The solution was then mixed with SnF.sub.2 predissolved in glycerin for 30 
minutes at a speed of 800 revolutions/min. with a Lightning mixer. When 
cool, the resultant gel was of extrudable consistency, cosmetically 
attractive and was tubed in plastic laminate tubes. When a ribbon of the 
gel was extruded onto the bristles of a toothbrush, the ribbon remained 
firm and did not significantly sink through the bristles for at least 2 
minutes. 
To determine the stability of the gel to moisture, 10 grams of the gel were 
placed in a pre-weighed, two ounce ointment jar to which was added 0.4 
milliliters water. The jar was sealed with a screw-on cap and stored for 4 
days at 23.degree. C. After the 4 day storage period, the jar was opened 
and inverted for one minute to allow any liquid to drain off. Thereafter 
the jar was reweighed to determine the loss in grams, if any, of the gel, 
such loss in grams being representative of the degree of liquefaction of 
the gel. The results of the liquefaction study are recorded in the Table 
below. 
For purposes of comparison, the procedure of the example was repeated 
except gums other than xanthan gum were substituted for xanthan in 
preparing the gels. The liquefaction results obtained with these 
comparative gums are also recorded in the Table below. 
TABLE 
__________________________________________________________________________ 
Wt. % 
PEG 1000 
Glycerin 
SnF.sub.2 
Flavor 
Weight Loss 
% Gel 
Dental Gel 
Gum Gum (Wt. %) 
(Wt. %) 
(Wt. %) 
(Wt. %) 
(grams) 
Loss 
__________________________________________________________________________ 
A Xanthan 
0.3 6 92.80 0.4 0.5 0.00 0.00 
B HEC* 0.31 6 92.80 0.4 0.5 2.80 28.0 
C HEC 0.41 6 92.80 0.4 0.5 1.68 16.8 
D CMC** 0.5 6 92.80 0.4 0.5 1.96 19.6 
E Carbopol 
0.25 6 92.80 0.4 0.5 2.52 25.2 
F None 0 7 92.10 0.4 0.5 5.33 53.3 
__________________________________________________________________________ 
*HEC -- Hydroxyethylcellulose 
**CMC -- Carboxymethylcellulose 
The % gel loss results recorded in the Table above indicate that only the 
composition of the Example, Dental Gel A containing xanthan gum exhibited 
acceptable stability wherein no gel loss occurred during the 4 day 
exposure to moisture, whereas the comparative compositions which contained 
a gum other than xanthan, encountered gel losses of 16.8-53.3%.