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Coatings By Gamal Shazly Tablet Coatings Tablet coating is the application of a coating material to the exterior of a tablet with the intention of conferring benefits and properties to the dosage form over the uncoated variety. What is the rationale for coating a solid dosage form? Main coating processes Functional coatings What is the rationale for coating a solid dosage form? Coating of a solid dosage form is often designed to perform a specific function. For example; protection against moisture, taste masking pH or time controlled release. Tablets can be easily coated and a variety of products are available on the market. Generally, the coating process gives rise to; Increased bioavailability Improved patient acceptance Formulation stability The rationale for coating pharmaceutical dosage form such as a tablet categorised into three main headings: Therapy Technology Marketing can be What is the rationale for coating a solid dosage form? Therapy To minimise irritation of the oesophagus and stomach. Minimise inactivation in the stomach. Improve drug effectiveness. Improve patient compliance e.g. easier to swallow, masks unpleasant taste. What is the rationale for coating a solid dosage form? Technology Minimise dust formation and contamination with respect to tablets. Masks batch differences in the appearance of raw materials. Facilitates their handling on high speed automated filling and packaging equipment. Improves drug stability e.g. Protection of active ingredient from environment such as sunlight, moisture. What is the rationale for coating a solid dosage form? Marketing Aid sales appeal as improved appearance and acceptability with respect to gloss and colouration. Mask unpleasant taste. Improve product identity. Main coating processes 1.Film coating 2. Sugar coating 3. Press coating Sugar coating Traditionally sugar coatings formed the bulk of coated tablets but today film coatings are the more modern technology in tablet coating. Description of tablets: Smooth, rounded and polished to a high gloss. Process: Multistage process involving 6 separate operations. 1. 2. 3. 4. 5. 6. Seal tablet core Sub coating Smoothing Colouring Polishing Printing Multistage process 1. Sealing tablet core- application of a water impermeable polymer such as 2. Sub coating -by adding bulking agents such as calcium carbonate or talc in 3. Smoothing process 4. 5. 6. Shellac, cellulose acetate phthalate and polyvinyl acetate phthalate, which protects the core from moisture, increasing its shelf life. combination with sucrose solution. application of sucrose syrup. Colouring included. -remove rough layers formed in step 2 with the - for aesthetic purposes often titanium based pigments are Polishing - effectively polished to give characteristic shine, commonly using beeswax, carnauba wax. Printing -indelible ink for characterisation. Simplified representation of sugar coating process Process details Typically tablets are sugar coated by a panning technique. The simplest form would be a traditional sugar-coating pan with a supply of drying air (preferably of variable temperature and thermostatically controlled) and a fan-assisted extract to remove dust- and moisture-laden air. Methods of applying the coating syrup include manually using a ladle, and, automatic control. In modern equipment some form of automatic control is available for the application of coating syrups. Ideal characteristics of sugar-coated tablets 1-tablets must comply with finished product specifications and any appropriate compendial requirements. 2- Sugar-coated tablets should ideally be of a perfectly smooth rounded contour with even colour coverage. Most manufacturers take advantage of the aethetic appeal of a sugar-coated tablet and polish to a high gloss. 3-Any printing should be distinct, with no smudging or broken print. Coating faults These are usually associated with process defects, such as splitting of the coat on storage, caused by inadequate drying during the coating application. Film coating Approach to coating tablets, capsules, or pellets by polymeric material. surrounding them with a thin layer of Advantages Produce tablets in a single step process in relatively short period of time. Process enables functional coatings to be incorporated into the dosage form. Disadvantages There are environmental and safety implications of using organic solvents as well as their financial expense. Process: Single stage process, which involves spraying a coating solution containing the following; 1. Polymer 2. Solvent 3. Plasticizer 4. Colourant The solution is sprayed onto a rotating tablet bed followed by drying, which facilitates the removal of the solvent leaving behind the deposition of thin film of coating materials around each tablet. Polymers used in film coating Ideal characteristics of a film coating polymer Solubility The polymer should have good solubility in aqueous fluids to facilitate the dissolution of the active ingredient. However, for modified release the polymer system should have low water solubility Viscosity The polymers should have a low viscosity for a given concentration. This will permit the easy, trouble-free spraying of their solutions in industrial film coating equipment. Permeability the polymers should be efficient barriers against the permeability of water Vapour or other atmospheric gases. These properties vary widely between the individual polymers. Mechanical properties polymer chosen for a film coat formulation must : Be of adequate strength to withstand the impact and abrasion encountered in normal handling. Insufficient coating strength will be demonstrated by the development of cracks and other imperfections in the coating. comply with the relevant regulatory and pharmacopoeial requirements current in the intended marketing area. Examples of the polymers used; - Cellulose derivatives e.g. MC, HPC, HPMC - Methacrylate amino ester copolymers. Plasticizer used in film coating Plasticizers are generally added to film coating formulations to modify the physical properties of the polymer to make it more usable. One important property is their ability to decrease film brittleness Examples; Polyols - Polyethylene glycol 400 Organic esters - diethyl phthalate Oils/glycerides - fractional coconut oil Colourants used in film coating Examples; Iron oxide pigments, Titanium dioxide, and Aluminium lakes. Water insoluble pigments are more favourable than water soluble colours for the following reasons; Better chemically stability in light Optimised impermeability to water vapour Better opacity Better covering ability Solvents used in film coating Modern techniques now rely on water as a polymer solvent because of the significant drawbacks that readily became apparent with the use of organic solvents. The disadvantages of organic solvents for the process: 1. Environmental: the venting of untreated organic solvent vapor into the atmosphere is ecologically unacceptable, and efficient solvent vapor removal from gaseous effluent is expensive. 2. Safety: organic solvents provide explosion, fire and toxic hazards to plant operators. 3. Financial: the use of organic solvents necessitates the building of flame- and explosion-proof facilities. Ingredient cost is also comparatively high, and the associated costs of storage and quality control must also be taken in to consideration. 4. Solvent residues: for a given process the amount of residual organic solvent in the film must be investigated. With increasing regulatory pressure this will become an area for additional control in the future Process details The vast majority of film coated tablets are produced by a process which involves spraying of the coating material on to a bed of tablets. Accela Cota is one example of equipment used for film coating. Basic process requirements for film coating 1. Adequate atomizing the spray liquid for application to the tablet cores. 2. Adequate mixing and agitation of the tablet bed. 3. Sufficient heat input to provide the latent heat of evaporation of the solvent. This is particularly important with aqueous-based spraying 4-Good exhaust facilities to remove dust- and solvent-laden air. Ideal characteristics of film-coated tablets Film-coated tablets should display An even coverage of film and colour. No abiasion of tablet edges or crowns. Logos and break lines should be distinct and not filled in. The tablet must also be within specifications and any relevant compendial requirements. Coating faults These arise from two distinct causes: 1. Processing: for example, inadequate drying conditions will permit coating previously deposited on the tablet surface to stick against neighbouring tablets. When parted, this will reveal the original core surface underneath. 2. Formulation faults: film cracking or ‘bridging’ of break lines are examples of this type. After taking due account of the mechanical properties of the film, reformulation will almost certainly be successful in overcoming the problem Why is film coating favoured over sugar coating ? Film coating Sugar coating Tablet appearance Tablet appearance Rounded with high degree of polish Retains shape of original core Larger weight increase 30-50% due to Small weight increase of 2-3% due to coating material coating material Logo or ‘break lines’ are possible logo or ‘break lines’ possible Process Process Difficult to automated e.g. traditional coating pan Can be automated e.g. Accela Cota Considerable training operation required Easy training operation Multistage process Single stage process Not able to be used for controlled release Easily adaptable for controlled release apart from enteric coating. allows for functional coatings. coating pans fluid beds PRESS COATING involves the compaction of granular material around an already preformed Core using compression equipment similar to that used for the core itself,. press coating is used to separate chemically incompatible materials, one or more being placed in the core and the other(s) in the coating layer. However, there is still an interface contact left between the two layers. In cases where even this is important then the process of pre coating can be taken one stage further. It is possible to apply two press coatings to a tablet core using suitable equipment. This equipment produces press-coated tablets with perfect separation between active core and coating, as the two can be separated by an inert middle layer. Functional coatings Functional coatings are coatings, which perform a pharmaceutical function. These include; Controlled release coating Enteric coating Multiparticulates (Pellets’ or ‘Beads), find favor over conventional nondisintegrating tablets for controlled release use, owing to a number of factors: 1. Their small size (typically 0.7—2.00 mm) allows them to pass through the constricted pyloric sphincter and distribute themselves along the gastrointestinal tract. This tends to overcome the disadvantage that whole tablets have of a rather irregular passage through the gastrointestinal tract and consequent irregular absorption 2. Whole, non-disintegrating tablets can be liable to lodge in restrictions within the gastrointestinal tract, and this can lead to ulcerative damage to the gastric mucosa as the drug solution is leached out from the tablet. Because of their small size, this is not a problem with multiparticulates 3-Should an individual bead or pellet fail and release all of its contents at once the patient Would not be exposed to any undue risk. This is Certainly not the case if a non-disintegrating tablet failed, when the consequences would Potentially be serious Types of multiparticulate Extruded/spheroflized granulates produced in modified granulating equipment , with the drug granulation extruded through a mesh or other device under pressure to form small granulates which are subsequently spheronized. Non-pareilsThese are sucrose spheres which are coated with the drug plus an adhesive water-soluble polymer (Fig 28.4). After their formation and any necessary intermediate steps such as drying, they may be coated with the controlled-release coating. Mechanisms of drug release from multiparticulates 1. Diffusion On contact with the aqueous fluids of the gastrointestinal tract, water will enter the interior of the particle by diffusion. Dissolution of the drug will occur and drug solution will diffuse across the controlled-release coat to the exterior. 2. Erosion Some coatings can be designed to erode gradually with time, thereby releasing the drug contained within the pellet. 3. Osmosis In allowing water to enter, an osmotic pressure can be built up within the interior of the pellet. drug solution will be forced out of the pellet to the exterior . Enteric coating The technique involved in enteric coating is protection of the tablet core from disintegration in the acidic environment of the stomach by employing pH sensitive polymer, which swell or solubilize in response to an increase in pH to release the drug. The enteric polymers (CAP,PVAP, suitable acrylic derivative ) are capable of forming a direct film in a film-coating process. Sufficient weight of enteric polymer must be used to ensure an efficient enteric effect. This is normally two or three times that required for a simple film coating. Aims of Enteric protection: To mask taste or odour Protection of active ingredients, from the acidic environment of the stomach. Protection from local irritation of the stomach mucosa. Release of active ingredient in specific target area within gastrointestinal tract. The pH status of enteric coated polymers in the stomach The polymers used for enteric coatings remain unionise at low pH, and therefore remain insoluble. As the pH increases in the gastrointestinal tract the acidic functional groups are capable of ionisation, and the polymer swells or becomes soluble in the intestinal fluid. Thus, an enteric polymeric film coating allows the coated solid to pass intact through the stomach to the small intestine, where the drug is then released for absorption through the intestinal mucosa into the human body where it can exert its pharmacologic effects. The ideal properties of enteric coated material? Permeable to intestinal fluid Compatibility with coating solution and drug Formation of continuous film Nontoxic Cheap and ease of application Ability to be readily printed Resistance to gastric fluids