Comparison - Aqueous vs. Solvent based Ethylcellulose Films. H.S. Hall, K.D. Lillie & R.E. Pondell

Transcription

1 Comparison - Aqueous vs. Solvent based Ethylcellulose Films 80-2 by H.S. Hall, K.D. Lillie & R.E. Pondell Ethylcellulose has a long history of use in the pharmaceutical for coating pharmaceuticals in various forms, including tablets, granules and powders. Reasons for coating pharmaceuticals include taste masking, delayed or sustained release, ease of swallowing, and appearance. Ethylcellulose is used as the primary film former and as a modifier for waxes, glycerides, and other cellulosic resins. While hydroxypropyl cellulose (HPC) and hydroxypropyl methyl cellulose (HPMC) are water-soluble and can be applied as aqueous solutions, (1,2,3,4) commercial ethylcellulose is not water-soluble and to date has been applied form solution in organic solvents. Any preparation involving organic solvents has the potential of releasing solvent vapors into the atmosphere. This is being regulated more and more closely and may require expensive equipment to limit such emissions. Ingestible products prepared using organic solvents must be monitored for residual organic solvent in the product. For some foods tolerances for residual solvents have been established, all other food uses require a Food Additive Petition be filed. Residual solvents are also an area of increasing concern in the pharmaceutical field. Further, organic solvents are rapidly increasing in cost and for this reason it is desirable to limit their use. For encapsulated products the organic solvent may represent 20% to 50 % of the total cost. For all the above reasons it is desirable to formulate without the use of organic solvents when possible. To date this has not been the case for ethylcellulose, however recently as aqueous dispersion of ethylcellulose has become available under the tradename Aquacoat (5). This material, developed at Purdue University by Drs. Banker and Peck, offers the possibility of formulating without the use of organic solvents and this paper is a report of preliminary work using this aqueous ethylcellulose. The material is available under the name Aquacoat as a 30% dispersion (by weight) in water. Surfactants used are not specifically identified but are reported to be GRAS under FDA guidelines. Since an aqueous dispersion of ethylcellulose is clearly different from a solution in organic solvents, it is also clear that the aqueous form should not be used as a direct replacement for organic solutions. We must keep the differences and peculiarities of the aqueous form in mind if we are to realize maximum benefit of this material. While the viscosity of solutions is directly related to the molecular weight of the polymer, the viscosity of dispersions is nearly independent of molecular weight. (6). Since polymer chain length does not contribute to viscosity directly, dispersions can be prepared at higher solids content than 1

2 polymer solutions without excessive viscosity being encountered. For instance, while solutions of ethylcellulose to 10 cps are seldom used for coating at concentrations above 10% or 12% solids, the aqueous dispersion can be used at 25% to 30% solids. This usually results in a net savings in process time to apply the coating. The reason for this becomes clear when one looks at viscosity curves for solutions of ethylcellulose (7,8), see, Figure 1. Even low molecular weight ethylcellulose typically attains viscosities of 50 to 100 cps at concentrations of 10% to 12% solids by weight. The aqueous dispersion has a viscosity of about 50 cps at 30% solids. While plasticizing the latex can swell the particles and cause an increase in viscosity, it is apparent that the latex can be utilized at relatively high solids content. In solution form the substituted cellulose chains are unraveled and flexible. As the solvent is evaporated the chains knit together in a tangled mass forming a continuous film. Such films are brittle and require a plasticizer to impart useful properties for most applications. In latex form the particles are rigid and the polymer chains are not dispersed or highly flexible, thus they will not fuse into a continuous film unless plasticized. It is necessary that the plasticizer permeate, soften and swell the polymer making it soft enough to fuse into a continuous film when dried. FIGURE 1 2

3 Latex particles form a film by first forming a closely packed layer of particles surrounded by a film of water (9,10.11). The surface tension of the water film creates great pressure on the particles and the film of water shrinks. The particles must be deformable enough to fuse together under this pressure if a continuous film is to be formed. Hercules literature (8) lists many of the plasticizers, which have been found useful with ethylcellulose films in the past. The latex formulation places constraints on incorporation of the plasticizers, which limits the usefulness of some of these. The plasticizer should not be highly water-soluble since this would cause the plasticizer to remain in the water phase and not soften the latex particles. The plasticizer must also be efficient, that is markedly soften the latex particles so that they may be readily deformed into a continuous film upon drying. Table 1 lists plasticizers commonly used with ethylcellulose. Those marked with a + have been found useful with the latex form, those marked with a - have not been found useful to date, while those not marked have not been evaluated at this time. Table I PHTHALATE ESTERS PHOSPHATE ESTERS Cyclohexyl ethyl phthalate Santicizer 140 Cyclohexyl methyl phthalate Tricresyl Phosphate +Dibutyl phthalate Triethyl phosphate +Diethyl phthalate Tripehenyl phosphate Diisopropyl phthalate Dimethyl phthalate +Dioctyl phthalate OILS Lubricating oil MISCELLANEOUS ESTERS -Mineral oils, refined Acetyl tributyl citrate Caster oil Acetyl triethyl citrate -Corn oil Butyl stearate Cottonseed oil +Dibutyl sebacate -Durkex 500 Dibutyl tartrate Diisobutyl adipate OTHERS +Glycerol Monostearate Oleic acid Pegosperse 100 ml -Stearic acid Methoxyethyl oleate -PEG 4000 Butoxyethyl stearate -PEG Triethyl citrate Cetyl alcohol +Tributyl citrate Myristyl alcohol +Tributyrin (glycerol tributyrate) Stearyl alcohol 3

4 Because the latex has surfactants present, many of the plasticizers found useful with the latex can be incorporated simply by stirring them directly into the latex. If very high levels of plasticizers are to be added, more than 20%, it may be necessary to first form an emulsion of the plasticizer in water. The plasticizer/latex combination should be stirred gently for at least one-hour prior to use to permit the plasticizer to migrate into the latex particles. A typical formulation is given below for plasticized latex at 30% solids. Total Solids Aquacoat (30% solids) 333 g. 100 g. Dibutyl Sebacate 25 g. 25 g. Water 59 g. Our interest in this material is as a coating, which may provide useful properties without the use of organic solvents. One of the first trials involved in the coating of a proprietary drug for sustained release purposes. Samples of the drug, a granular product, were coated with several levels of the latex coating plasticized with diethyl phthalate (DEP). As expected, the release curves show a decreasing rate, Figure 2, of release as the coating level increases. The initial data indicates that the aqueous coating leaches at a faster rate than does an ethylcellulose film applied from methanol/methylene chloride however later work showed that the leach rate for coatings prepared with the latex are sensitive to the plasticizer chosen. The data in figure 2 are for a latex coating plasticized with diethylphthalate (DEP). Figure 3 is a comparison of the release rate for films plasticized with DEP and Tributryn. This data clearly shows that the Tributryn film releases more slowly than the DEP film. While further work is needed, it is believed that this reflects better film formation when DEP is used to plasticize the latex. FIGURE 2 FIGURE 3 4

5 Dr. S. Jan et. al of Purdue University has reported (12) that a film of Aquacoat plasticized with tributyl citrate (13) applied onto lactose tablets displayed enteric release, that is they did not disintegrate for one hour in simulated gastric fluid but did release when immersed in simulated intestinal fluid. It has been noted that when the tablets disintegrated in intestinal fluid the film did not dissolve, but did fragment and allow the tablets to disintegrate. Efforts to reproduce this work were not successful when placebo tablets containing Avicel were used, but tests with lactose tablets did confirm Jan's results. Since the coating does not dissolve upon disintegration, and since no ph functionality is expected from ethylcellulose films we are unable to adequately explain the release mechanism. The reason that the Avicel cores do not display this behavior seems to be that the rapid swelling associated with the Avicel ruptures the film. Additional light was shed on this phenomenon by some release data provided by another client. We had prepared coated particles using an Aquacoat/DEP composition, which was tested for sustained, released. In one release test the ph of the media was varied with time to simulate the ph progression in the gut. (Figure 4). An apparent increase in release rate was observed when the ph was raised form ph=2.5 to ph=4.5, but this increase was not sustained as the ph rose to 7.0. Release curves were then determined at three different ph's (Figure 5). Although the active is proprietary and cannot be identified here, its solubility does not account for a release maxima at ph's between ph=4.0 and ph=7.0. FIGURE 4 FIGURE 5 5

6 Another area in, which an aqueous dispersion of ethylcellulose shows promise is as a modifier in water based coatings using water-soluble polymers. As an example, hydroxypropyl methylcellulose (HPMC) is used as a film coating on tablets from aqueous solution. Because of viscosity solution concentrations of 10% or less are usually used. The aqueous ethylcellulose at 30% solids can be added to such solutions with the effect of increasing the solids content while reducing the viscosity. This translates into higher throughput per coating machine. (see Figure 6) In addition, coatings of HPMC, which contain ethylcellulose, are less sensitive to humidity while retaining rapid release properties. This can be particularly important for products distributed in tropical areas. Figure 7 shows percent weight gain for placebo tablets containing dicalcium phosphate and Avicel at 52% and 92 % relative humidity at room temperature. 6

7 FIGURE 7 An additional benefit of the ethylcellulose dispersion added to HPMC films is a reduced shrinkage of the HPMC film as water is evaporated. When monogrammed tablets are coated with aqueous HPMC a problem, which may arise, is obscuring of the monogram. This is often caused by shrinkage of the HPMC film in drying. As the film shrinks tension causes the film to be pulled up out of the monogram resulting in the film being stretched across the embossed monogram, obscuring it. With reduced shrinkage this problem is largely avoided. One area of concern with all latex formulations concerns the fusing of the film. As mentioned earlier, the water plays as important role in causing the particles to fuse into a continuous film. If the latex is applied onto highly soluble cores, or cores which absorb water very rapidly, the core may compete for the water, interfering with film formation. (9,14) In such cases a hydrophobic precoat may be required. We are currently pursuing such precoatings, which may be applied, from aqueous systems. Initial data indicate that the presence of a precoat between water sensitive cores and the latex coating markedly improve the integrity of the resulting film. We'd like to thank Dr. Dev Mehra of FMC as well as those clients who made data available but asked that their identity be protected for permission to use that data in this report. 7

8 1) Air Suspension Encapsulation of Moisture Sensitive Particles Using Aqueous Systems; Hall Hinkes; Proceeding ACS Symposium on Microencapsulation; Processes and Applications; August, ) Tablet Coating in an Aqueous System; Shin-Etsu Chemical (Biddle Sawyer), ) Methocel Cellulose Ethers - Aqueous Systems for Tablet Coating; Dow Chemical; Form ; ) Film Coating of Solid Dosage Forms - Aqueous Systems; Dr. G.J. Jackson, presented Academy of Pharmaceutical Sciences 15th Annual Eastern Regional Meeting; ) FMC Research Update; Aquacoat Application Bulletin; FMC Corporation; ) Latex Presentation, B.F. Goodrich Chemical Co. Whitlock, et. al.; B.F. Goodrich Chemical Co.; ) Ethocel Ethylcellulose Resins; Dow Chemical Co. Form , ) Ethylcellulose, Properties and Uses; Hercules, Inc.; 63675LP, ) Film Forming Characteristics of Emulsion Polymers; Zdanowski & Brown; 44th Mid-Year Meeting Proceedings of the Chemical Specialties Manufactures Association, May ) Use of Aqueous Synthetic Polymer Dispersions for Coating Pharmaceutical Dosage Forms; Lehman & Dreher; Drugs Made in Germany; Vol XVI, ) Focus - PVDC Coatings; Elschnig, et. al.; Paper, Film, & Foil Converter; October March ) Development of Enteric Coatings Using Ethylcellulose Latex; Jan, Banker, Peck, unpublished, August ) Technical Bulletin No. 31, Citroflex Plasticizers; Pfizer Chemicals Div.; ) B. F. Goodrich in-house publication on Latexes,