Carboxyvinyl polymer having Newtonian viscosity

Abstract: The present invention provides carboxyvinyl polymer having Newtonian viscosity, ophthalmic preparations containing the polymer, a process for preparing the polymer, and a vehicle for ophthalmic preparations containing the polymer. By using the polymer in a vehicle, an ophthalmic preparation having a low viscosity and good permeability of a drug can be obtained.

Description: FIELD OF THE INVENTION This invention relates to a novel carboxyvinyl polymer having Newtonian viscosity, a process for preparing the same and ophthalmic preparations containing the same. DESCRIPTION OF THE PRIOR ART Carboxyvinyl polymer is a water soluble copolymer of acrylic acid partially crosslinked by various crosslinking agents such as polyallylsucrose. Various carboxyvinyl polymers having molecular weights ranging from one million to three millions and various viscosities have been applied in cosmetics, pharmaceutics, chemical engineering and others. Examples of such carboxyvinyl polymers are Carbomer 910 in the United States Pharmacopoeia, of which 1 % (W/V) aqueous solution has such a low viscosity ranging from 3000 to 7000 centipoises (hereinafter referred to as cP), and carboxyvinyl polymer in the Japanese Standards of Pharmaceutical Ingredients, of which 0.2 % (W/V) aqueous solution has such a high viscosity ranging from 1500 to 50000 cP. It is a feature of these carboxyvinyl polymers to have non-Newtonian viscosity having a high yield value. Namely, the utility of the carboxyvinyl polymer is that a very high viscosity can be obtained by adding a small amount thereof. Yield value means a value of shear stress when shear rate is zero in the case where the viscosity of a solution is measured by a rotating viscometer. In ophthalmic application, carboxyvinyl polymers contained in an ophthalmic preparation as vehicle for sustained release of a drug have been known (Japanese Unexamined Patent Publications 67021/1979, 110312/1979, 503201/1990). These prior arts disclosed that sustained release of a drug can be achieved by keeping a drug dissolved in an aqueous solution containing carboxyvinyl polymer, which becomes gel, on the cornea for a long period with the help of the high viscosity of the carboxyvinyl polymer and by gradual degradation of the gel. Therefore, the carboxyvinyl polymers of the above-described prior arts are limited to those having a viscosity of 1000 cP or more and they teach that such a high viscosity is necessary for sustained release effect of a drug. As mentioned above, the feature of the prior arts is to utilize the property of the carboxyvinyl polymer having non-Newtonian viscosity, the property being that the polymer provides high viscosity to ophthalmic preparations by addition of the same even in a small amount. Application of conventional carboxyvinyl polymer having non-Newtonian viscosity to ophthalmic preparations has an advantage of sustained release, and, on the other hand, a disadvantage of excessively high viscosity. Since foreign body sensation occurs when the viscosity of an ophthalmic preparation is high, it is desirable that viscosity of an ophthalmic preparation is as low as possible. Especially, when the viscosity is 10000 cP or more, the ophthalmic preparation becomes like an eye ointment, with the result that it is difficult for a patient to instill such ophthalmic preparation in constant volume from an eyedropper, and further, it is even difficult to instill by himself. Consequently, it is necessary for the patient to have the ophthalmic preparation applied to conjunctival sac by a medical doctor. Further, various problems exist in the process of preparing a highly viscous ophthalmic preparation having a viscosity of 1000 cP or more. For example, the preparation contains air bubbles which are difficult to remove, and it is impossible to sterile the preparation by filtration. It is, therefore, desirable that viscosity of an ophthalmic preparation is less than 1000 cP for convenience of application and in order to avoid the problems in production. An approach to solve these problems is to extremely reduce the amount of the carboxyvinyl polymer to be added in order to adjust the viscosity to be less than 1000 cP. Figure 1 shows relationships between concentration and viscosity of known carboxyvinyl polymers. While the problem of viscosity is solved by using this approach, these carboxyvinyl polymers can be added only in such an extremely low concentration, as is clear from Figure 1, that the expected effect by adding the polymer cannot be exerted. Carboxyvinyl polymer is expected to exert effect of enhancing permeability of a drug, which no prior arts have disclosed yet, and study on the novel effect of carboxyvinyl polymer is desired. However, with an extremely small amount of carboxyvinyl polymer to be added, such effect cannot be expected. Accordingly, development of novel carboxyvinyl polymer is desired. SUMMARY OF THE INVENTION In view of this, the inventors investigated carboxyvinyl polymer from which ophthalmic preparations having a low viscosity can be obtained even if the amount of the carboxyvinyl polymer to be added is not extremely reduced. As the result, the inventors found a carboxyvinyl polymer which has a low yield value and Newtonian viscosity. The inventors also found that by using the carboxyvinyl polymer we can obtain ophthalmic preparations having a viscosity of less than 1000 cP and simultaneously maintaining sufficient concentration to exert the effect by adding the polymer. First, the inventors studied a process for preparing carboxyvinyl polymer having Newtonian viscosity and found that the polymer can be prepared by shearing carboxyvinyl polymer having non-Newtonian viscosity with the use of an ultrasonic generating device. Next, the inventors studied the obtained carboxyvinyl polymer having Newtonian viscosity with respect to permeability of a drug by adding the polymer to ophthalmic preparations and found that the polymer enhances permeability of a drug. This invention relates to a novel carboxyvinyl polymer having Newtonian viscosity (hereinafter referred to as N-CVP), a process for preparing the same and ophthalmic preparations containing the same. According to a first aspect of the present invention, there is provided a carboxyvinyl polymer, which is a water-soluble partially crosslinked copolymer of acrylic acid, derived by ultrasonic or mechanical shearing of an aqueous solution thereof, said carboxyvinyl polymer having the following properties when dissolved in water; a) Newtonian behaviour in the sense that the relationship between shear stress and shear rate is approximately linear; and b) a yield value, viz. a value of shear stress when the shear rate is zero, of not more than 0.5 dyn/cm 2 . The carboxyvinyl polymer preferably has a molecular weight ranging from 20,000 to 500,000. According to a second aspect of the present invention, a process for preparing such carboxyvinyl polymer is provided, comprising shearing a carboxyvinyl polymer having non-Newtonian viscosity in an aqueous solution. In such process, the shearing may be carried out with an ultrasonic generating device, a high pressure homogenizer, or a high speed stirrer. According to a third aspect of the present invention, ophthalmic preparations and vehicle therefor are provided. The preparations contain a carboxyvinyl polymer in accordance with the invention. The concentration of polymer preferably ranges from 0.1% to 5% (W/V). The viscosity of the preparations is preferably less than 1000 cP. By using a carboxyvinyl polymer in accordance with the present invention, ophthalmic solution having low viscosity and good permeability of a drug can be provided. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph showing relationships between concentration and viscosity of various carboxyvinyl polymers commercialized by Wako Pure Chemical Industries Co. (The graph is reproduced from Wako's bochure.) The ordinate designates viscosity (cP) and the abscissa designates concentration (%). Figure 2 is a graph showing the relationship between the shear stress and the shear rate of 0.2 % aqueous solution of Hiviswako 105 (pH 7.0) measured with a rotating viscometer. The ordinate designates shear stress (dyn/cm 2 ) and the abscissa designates shear rate (1/s). Figure 3 is a graph showing the relationship between the shear stress and the shear rate of 0.2 % aqueous solution of N-CVP (pH 7.0) obtained in Example 1, measured with a rotating viscometer. The ordinate desgnates shear stress (dyn/cm 2 ) and the abscissa designates shear rate (1/s). Figure 4 is a graph showing concentration of fluorescein in aqueous humor of rabbit eyes measured at each measuring time after the preparation of the present invention and the control both containing fluorescein were instilled to rabbit eyes respectively. The ordinate designates concentration (ng/ml) and the abscissa designates time (hr). - ▪ - designates the concentration obtained with the preparation of the present invention and - □ - designates that obtained with the control. DETAILED DESCRIPTION OF THE INVENTION In the present invention, N-CVP stands for copolymer of acrylic acid crosslinked by: polyalkenylpolyether such as polyallylsucrose or polyallylpentaerythritol; bifunctional crosslining agent such as divinylglycol or divinylbenzene; non-hydrophilic diolefin crosslinking agent; etc.; and when viscosity of the aqueous solution of N-CVP is measured by using a rotating viscometer, N-CVP shows a low yield value not more than 0.5 dyn/cm 2 and almost a linear relationship between the shear stress and the shear rate. N-CVP of the present invention is characterized by having a low yield value and almost a linear relationship between the shear stress and the shear rate, and not directly characterized by molecular weight, which can be estimated from the value of intrinsic viscosity. The molecular weight of the N-CVP is estimated to be approximately one-fifth to one-fiftieth of that of known carboxyvinyl polymer having non-Newtonian viscosity used as raw material, namely within the range of approximately 20,000 to 500,000. The ophthalmic preparation containing N-CVP can be applied to almost all of the therapeutic fields such as anti-viral, anti-glaucoma, anti-cataract, anti-inflammatory, anti-allergy or diagnosis. The application does not restricted by nature of drug substance, accordingly N-CVP can be applied to various drugs used in ophthalmology as well as conventional carboxyvinyl polymer having non-Newtonian viscosity. A process for preparing N-CVP of the present invention is to change viscosity of carboxyvinyl polymer having non-Newtonian viscosity to Newtonian viscosity by shearing the carboxyvinyl polymer in an aqueous solution with the use of an apparatus which has a function of shearing. Examples of the apparatus used are an ultrasonic generating device, a high pressure homogenizer such as Manton Gaulin or microfluidizer, or a high speed stirrer. Such carboxyvinyl polymers having non-Newtonian viscosity are commercialized under the trade names of Carbopol 910, 934, 934P, 940, 941, 976 by Goodrich Chemical Co., and also Hiviswako 103, 104, 105, 204, 304 by Wako Pure Chemical Industries Co. As will be described in detail in the article of the measurement of viscosity, while carboxyvinyl polymer having non-Newtonian viscosity showed a high yield value, which is a feature thereof, N-CVP showed a low yield value and the viscosity of the solution of the N-CVP is much lower than that of the carboxyvinyl polymer having non-Newtonian viscosity. An ophthalmic preparation which N-CVP is applied to contains few air bubbles and can be sterilized by filtration, by which ophthalmic preparations containing conventional carboxyvinyl polymer cannot be sterilized. Therefore, the ophthlmic preparation containing N-CVP is prepared much more easily than that containing conventionally known carboxyvinyl polymer. Since the viscosity of the ophthalmic preparation containing N-CVP is low, there exist merits that it is possible for a patient to instill the ophthalmic preparation by himself and also possible to instill in constant volume from an eyedropper. To examine the effect of N-CVP, the inventors tested the permeability of a drug by instilling the ophthalmic preparation of the present invention containing N-CVP, and a preparation which does not contain N-CVP as the control respectively to rabbit eyes. As will be described in detail detail in the article of permeability test, the area under the curve, which shows the concentration of a drug in tissues, of the ophthalmic preparation of the present invention was obviously larger than that of the control, leading to the conclusion that N-CVP of the present invention greatly enhances the permeability of a drug. In other words, N-CVP can increase a drug concentration in tissues by enhancing the permeability of a drug, thereby improving the permeability of a drug which is hardly absorbed in tissues, and decreasing a dosage and administration frequency. The ophthalmic preparation of the present invention can be prepared by known methods using N-CVP. For example, a drug and N-CVP are added to sterile purified water, and finally pH of the mixture is adjusted by using a pH adjusting agent such as sodium chloride or dilute hydrochloric acid, thereby giving the preparation. If necessary, a tonicity agent such as sodium chloride, potassium chloride or glycerin, a buffering agent such as sodium phosphate or potassium phosphate, a stabilizer such as disodium edetate, a preservative such as ethyl p-hydroxybenzoate, butyl p-hydroxybenzoate or sorbic acid, can be added to the preparation. The ophthalmic preparation of the present invention can also be prepared by shearing the solution of carboxyvinyl polymer having non-Newtonian viscosity, and then adding some kinds of additive without isolating resulting N-CVP. The pH range of the ophthalmic preparation of the present invention can be varied within a range applicable to ophthalmic preparations conventionally used. The preferable range is 3 to 8. The amount of N-CVP to be added is determined to be such that the viscosity of the ophthalmic preparation is less than 1000 cP, which slightly varies according to pH or the preparation process. The preferable concentration is 0.1 to 5 % (W/V). The more preferable range is 0.1 to 2 % (W/V). Examples of preparation of N-CVP and ophthalmic preparations thereof are shown below. 1. Preparation of N-CVP Example 1 Hiviswako 105 (1.0 g, Wako Pure Chemical Industries Co.) was dissolved in water (100 ml). The solution was treated for 9 minutes with a prove type ultrasonic generating device (oscillating frequency: 20 kHz) and the solution was lyophilized to give N-CVP (quantitative). By the same method as described above with the exception that the treating time was varied, N-CVP having various yield values was obtained from commercialized carboxyvinyl polymers. Example 2 Carbopol 976 (1.0 g, Goodrich Chemical Co.) was dissolved in water (100 ml). The solution was stirred for 30 minutes with a high speed stirrer (10000 revolutions per minute) and the solution was lyophilized to give N-CVP (quantitative). By the same method as described above with the exception that the stirring time was varied, N-CVP having various yield values was obtained from commercialized carboxyvinyl polymers. 2. Measurement of Viscosity To examine the nature of N-CVP obtained in Example 1, 0.2 % (W/V) aqueous solution of N-CVP was adjusted to pH 7 and the viscosity of the solution was measured with a rotating viscometer (Rotovisco CV20 type viscosity meter : HAAKE Co.) at 25 °C. For comparison, an aqueous solution of Hiviswako 105 (Wako Pure Chemical Industries Co.), as a typical example of conventional carboxyvinyl polymer, was prepared and the viscosity was measured in the same manner as the above. As shown in Fig. 2, Hiviswako 105 has a high yield value (yield value means the shear stress when the shear rate is 0) of 52 dyn/cm 2 and shows non-Newtonian viscosity, which is represented by a nonlinear relationship between the shear stress and the shear rate. As shown in Fig. 3, N-CVP obtained in Example 1 shows a low yield value of 0.02 dyn/cm 2 and a linear relationship between the shear stress and the shear rate, thereby indicating that the N-CVP has Newtonian viscosity. N-CVP of Example 1 also shows such an extremely low viscosity as 6 cP at a shear rate of 1.92 (l/second), as compared with Hiviswako 105 which shows 4700 cP. As shown above, when N-CVP is used instead of carboxyvinyl polymer having non-Newtonian viscosity, the high yield value, which is a feature of carboxyvinyl polymer having non-Newtonian viscosity, turns into low yield value and the viscosity of the solution is greatly lowered. 3. Measurement of Molecular Weight The relationship between intrinsic viscosity ([η] ) and molecular weight (M) is represented by the following formula (1): [η] = KM α wherein K and α stand for empirical constants respectively. The molecular weight of N-CVP is calculated by using the above formula. N-CVP obtained in Example 1 was dissolved in a 0.9 % aqueous solution of sodium chloride to give a concentration of 0.08 % (W/V). Then, the viscosity (η 1 ) of the solution was measured with a Ubbelohde type viscometer. The viscosity (η 2 ) of a 0.9 % sodium chloride aqueous solution, which was the solvent, was also measured in the same manner. By using these values measured, specific viscosity, which is represented by the following formula, was calculated. η 3 = (η 1 /η 2 ) - 1 Then, intrinsic viscosity, which equals reduced viscosity obtained by dividing the specific viscosity by concentration of solute, was obtained. For comparison, Hiviswako 105 was treated in the same manner as above and intrinsic viscosity thereof was obtained. The molecular weight of N-CVP which was calculated by using the intrinsic viscosity and the constants obtained with reference to dextran ( K=9. 00 × 10 -4 , α=0. 50, described in KOBUNSHIKAGAKU (high-molecular chemistry), 13 , 20, (1956)) was 118,000 and that of Hiviswako 105 was 1,600,000. These results show that the molecualr weight of N-CVP obtained by shearing according to the manner of Example 1 is approximately one-fourteenth of that of Hiviswako. 4. Preparation of Ophthalmic Preparation Preparing Example 1 Formulation 1-1 Carbopol 976 (Goodrich Chemical Co.) 1.3 g Fluorescein sodium salt 0.01 g Sodium chloride 0.9 g Sodium hydroxide q.s. Sterile purified water q.s. Total 100 ml Process for preparation: Carbopol 976 was dissolved in sterile purified water and the solution was adjusted to pH 4.3 with sodium hydroxide, then treated for 5 minutes with a prove type ultrasonic generating device (oscillating frequency: 20 kHz) to give a solution having Newtonian viscosity. Then, fluorescein sodium salt and sodium chloride were added to the solution. The viscosity of the solution was measured at 25 °C at a shear rate of 1.92 (1/second) by using Rotovisco CV 20 viscometer (Haake Co.) with the result that the viscosity was 260 cP. Hereinafter, viscosities were measured under the same conditions. Ophthalmic preparations having the following formulations can be obtained in the same manner as Formulation 1-1. Formulation 1-2 (viscosity: 20 cP) Carbopol 976 (Goodrich Chemical Co.) 0.1 g Fluorescein sodium salt 0.01 g Sodium chloride 0.9 g Sodium hydroxide q.s. Sterile purified water q.s. Total 100 ml Preparing Example 2 Formulation 2-1 (viscosity: 500 cP) Hiviswako 105 (Wako Pure Chemical Industries Co.) 1.5 g Betamethasone phosphate 0.1 g Sodium chloride 0.9 g Ethyl p-oxybenzoate 0.008 g Butyl p-oxybenzoate 0.004 g Sodium hydroxide q.s. Sterile purified water q.s. Total 100 ml Process for preparation: Hiviswako 105 was dissolved in sterile purified water and the solution was stirred for 10 minutes with a high speed stirrer (1000 revolutions per minute). Betamethasone phosphate, sodium chloride, ethyl p-oxybenzoate and butyl p-oxybenzoate were added to the solution, and then pH of the solution was adjusted to 6.0 by adding sodium hydroxide. Preparing Example 3 Process for preparation: N-CVP obtained in Example 1 was dissolved in sterile purified water. Then, Pirenoxine, sodium chloride, ethyl p-oxybenzoate and butyl p-oxybenzoate were added to the solution. After that, pH of the solution was adjusted to 6.0 by adding sodium hydroxide. By using the same method as in Preparing Example 3, an ophthalmic preparation having the following formulation was obtained. Formulation 3-2 (viscosity: 5 cP) N-CVP 0.2 g Pilocarpine hydrochloride 1.0 g Sodium chloride 0.6 g Sorbic acid 0.1 g Sodium hydrogen phosphate q.s. Sodium hydroxide q.s. Sterile purified water q.s. Total 100 ml Permeability Test To examine the effect of N-CVP, permeability of a drug was investigated by instilling an ophthalmic preparation of the invention to rabbits. The ophthalmic preparation of the formulation 1-1, which is a typical example of this invention, was instilled to rabbit eyes ( 1 group 5 rabbits) and the concentration of fluorescein in aqueous humor was measured after 1, 2, 3, 4, 6 and 8 hours with a fluorescence spectrophotometer. As the control, an ophthalmic preparation similar to the formulation 1-1 except that carboxyvinyl polymer was not contained was used. The concentration of fluorescein in aqueous humor at each measuring time is shown in Fig. 4. The area under the curve (AUC), which was calculated from the measured value, is shown in Table 1. AUC (ng.hr/ml) Ophthalmic preparation of the present invention (Formulation 1-1) 282.9 47.6 Control 54.4 5.2 As shown in Fig. 4 and Table 1, the permeability of the ophthalmic preparation which contains N-CVP is 5 times better than that of the control, which does not contain N-CVP. This result proves that N-CVP of the present invention enhances drug permeability.

Claims: A carboxyvinyl polymer, which is a water-soluble partially crosslinked copolymer of acrylic acid, derived by ultrasonic or mechanical shearing of an aqueous solution thereof, said carboxyvinyl polymer having the following properties when dissolved in water; a) Newtonian behaviour in the sense that the relationship between shear stress and shear rate is approximately linear; and b) a yield value, viz. a value of shear stress when the shear rate is zero, of not more than 0.5 dyn/cm 2 . A carboxyvinyl polymer as claimed in claim 1, characterised in that said carboxyvinyl polymer has a molecule weight ranging from 20,000 to 500,000. A process for preparing a carboxyvinyl polymer as claimed in claim 1 or 2, comprising shearing a carboxyvinyl polymer having non-Newtonian viscosity in an aqueous solution. A process for preparing a carboxyvinyl polymer as claimed in claim 3, characterised in that the shearing is carried out with an ultrasonic generating device. A process for preparing a carboxyvinyl polymer as claimed in claim 3, characterised in that the shearing is carried out with a high pressure homogenizer. A process for preparing a carboxyvinyl polymer as claimed in claim 3, characterised in that the shearing is carried out with a high speed stirrer. A vehicle for ophthalmic preparations, characterised in that said vehicle contains a carboxyvinyl polymer as claimed in claim 1 or 2. A vehicle for ophthalmic preparations as claimed in claim 7, characterised in that the concentration of said carboxyvinyl polymer ranges from 0.1% to 5% (W/V). A vehicle for ophthalmic preparations as claimed in claim 7 or 8, characterised in that the viscosity thereof is less than 1000 cP. An ophthalmic preparation comprising a carboxyvinyl polymer as claimed in claim 1 or 2, and a drug, said preparation having a drug permeability enhancing effect. An ophthalmic preparation as claimed in claim 10, characterised in that the concentration of said carboxyvinyl polymer ranges from 0.1 to 5% (W/V). An ophthalmic preparation as claimed in claim 10 or 11, characterised in that the viscosity thereof is less than 1000 cP.

Inventors: KAWASHIMA YOICHI, KUWANO MITSUAKI, KAWASHIMA, YOICHI, KUWANO, MITSUAKI
Applicants: SANTEN PHARMA CO LTD, SANTEN PHARMACEUTICAL CO., LTD.
Application year: 1992
Main classification: C08J3
IPC codes: C08L33, A61K47, C08J3, A61K9