Formulation and Physical Properties of Soft Capsules
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11 Chap 11 (bc/d) 12/1/04 8:52 am Page 201 11 Formulation and physical properties of soft capsules Gabriele Reich Introduction aspects (reduced operator and environmental contamination with highly potent or cytotoxic compounds) and consumer preference (easy to Soft capsules are a single-unit solid dosage form, swallow). The most interesting advances have consisting of a liquid or semi-solid fill enveloped recently been made in the area of developing by a one-piece hermetically sealed elastic outer liquid and semi-solid formulations in a soft shell. They are formed, filled and sealed in one gelatin capsule to address particular bioperfor- continuous operation, preferably by the rotary mance issues, namely increased bioavailability die process. Depending on the polymer forming and decreased plasma variability by improved the shell, they can be subdivided into two solubility and absorption-enhancing techniques. categories, namely soft gelatin capsules or ‘soft- The proper design for a specific soft gelatin gels’ and non-gelatin soft capsules. The majority capsule formulation requires the appropriate of soft capsules are made from gelatin owing to selection of shell and fill composition and the its unique physical properties that make it an optimisation of the two to allow for the efficient ideal excipient for the rotary die process. Soft production of a chemically and physically stable capsules based on plant-derived and/or synthetic product with the desired biopharmaceutical non-gelatin alternatives have, however, been properties. patented and a few prototype products have recently entered the market. Formulation and physical properties of both soft capsule categories Composition of the capsule shell will be discussed. The shell of a soft gelatin capsule is composed of gelatin, a plasticizer or a combination of plasti- Soft gelatin capsules cizers and water. In addition, it may contain preservatives, colouring and opacifying agents, General aspects flavourings and sweeteners, possibly sugars to impart chewable characteristics to the shell, Originally developed in the 19th century to mask gastroresistant substances and in special cases unpleasant taste and odour of drug substances, even active compounds. soft gelatin capsules are used in many appli- The water serves as a solvent to make a molten cations, for pharmaceutical and health and nutri- gelatin mass with a pourable viscosity at 60–70°C. tion products, cosmetic applications and even The ratio by weight of water to dry gelatin (W/G) recreational products such as paint balls. can vary from 0.7 to 1.3, depending on the vis- In the pharmaceutical field soft gelatin cap- cosity of the gelatin being used. After capsule for- sules are increasingly being chosen for strategic mation, most of the water is removed by drying, reasons (line extension), technological issues (high leading to finished capsules with a moisture content uniformity of low-dose drugs), safety content of 4–10%. 201 11 Chap 11 (bc/d) 12/1/04 8:52 am Page 202 202 Chapter 11 • Formulation and physical properties of soft capsules Gelatin Considering these aspects, the technologically relevant gelatin parameters are gel strength, vis- 2 The gelatins used for pharmaceutical or health cosity at 60°C and 6–3% w/w concentration in and nutrition soft capsule products are described water, viscosity breakdown (the impact of tem- by the official pharmacopoeias such as USP perature and time on the degradation of gelatin), (United States Pharmacopoeia), PhEur (European melting point, setting point, setting time, parti- Pharmacopoeia) etc., or approved by local author- cle size and molecular weight distribution. A ities, with additional physicochemical specifi- perfect soft capsule gelatin should have the cations (Babel, 2000). The specifications and following specifications: controls for gelatins are discussed in Chapter 2. • Gel strength: 150–200 Bloom, depending on For soft capsule production, the pharma- the gelatin type; copoeial specifications generally represent the •Viscosity (60°C/6–2% w/w in water): 2.8–4.5 minimum requirements. Capsule manu- 3 mPa s, depending on the gelatin type; facturers’ specifications are more detailed and •Well-controlled degree of viscosity breakdown; stringent with respect to the performance- •Well-defined particle size to allow fast dissolu- related physicochemical properties of the tion and deaeration of the molten mass, even gelatins (Reich and Babel, 2001). This is due to at high gelatin concentrations; the fact that these parameters are critical for •A broad molecular weight distribution to economic soft capsule production by the rotary provide a fast setting and the fusion tempera- die process and for the quality of the final ture being well below the melting temperature product. Gelatin types and grades that are ade- of the plasticized wet film. quate for continuous commercial soft capsule production require the ability to set at a fast rate The main gelatin types and grades used for the to ribbons of defined thickness and reproducible manufacture of soft capsules are listed in Table microstructure and to produce films with a 11.1 together with their physicochemical specifi- mechanical strength and elasticity sufficient to cations. survive all the manipulations on the encapsula- The proper choice of the gelatin type and tion machine, i.e. to allow the wet films to be grade is related to technological issues, consumer easily removed from the drums, to stretch during preference and pricing. For pharmaceutical or filling, to be sealed at temperatures below the health and nutrition products, medium bloom melting point of the film and to be dried quickly limed bone (LB) gelatins, or blends of limed bone under ambient conditions to a specified capsule and pigskin (LB/PS) or limed bone, pigskin and strength. Moreover, the dissolution character- limed hide gelatin (LB/LH/PS) are commonly istics of the resulting capsules have to fulfil the used, with a certain preference for LB gelatin in pharmacopoeial requirements. the United States and for blended gelatins in Table 11.1 Physicochemical properties of pharmacopoeial-grade soft capsule gelatins Gelatin Raw material Type Bloom (g) Viscosity (mPa s) 2 2 (10°C; 6–3% w/w) (60°C; 6–3% w/w) 160 LB (= limed bone) Bovine/porcine bone B 155–185 3.4–4.2 160 LH (= limed hide) Bovine hide B 150–170 3.5–4.2 160 LB/LH Blend of bovine/porcine B 150–170 3.5–4.2 bone and bovine hide 200 AB (= acid bone) Bovine bone A 180–210 2.7–3.2 200 PS (= pigskin) Pigskin A 190–210 2.5–3.1 160 PS/LB/LH Blend of pigskin, bovine/ A/B 145–175 2.7–3.3 porcine bone and bovine hide 11 Chap 11 (bc/d) 12/1/04 8:52 am Page 203 Soft gelatin capsules 203 Europe. Low-viscosity, high-bloom gelatins such conventional soft capsule production, have been as a 200 Bloom pigskin (PS) or acid bone (AB) used for a small number of products. Acceptable gelatin are often used for the encapsulation of soft capsules can be produced by adjusting the hygroscopic formulations and/or water-sensitive formulation and process parameters, such as the drugs, where standard gelatin formulations have production speed in accordance to the reduced to be modified to contain less water and dry setting rates, the mechanical properties and the faster, thus improving the product stability drying characteristics of this gelatin type. during capsule manufacturing. Mixtures of low The use of plant-derived genetically engineered (Ͻ100 Bloom) and medium Bloom (Ͼ150 Bloom) gelatins for soft capsule production is not practi- gelatins have been proposed for the formulation cable. This is mainly due to technological issues, of chewable soft capsules (Overholt, 2001) to supply problems, high costs and, for pharma- achieve the desired mouthfeel and solubility of ceutical products, the regulatory issues. Only the shells, a low stickiness for improved machin- small amounts of gelatins, with gelling and ability and sufficient integrity for stable fill setting properties and mechanical characteristics encapsulation. different to mammalian gelatins, are available at In addition to the pharmacopoeial grade a multiple of the price of conventional gelatins. gelatin types listed in Table 11.1, succinylated pigskin gelatin (Bloom: 190–210 g; viscosity: Plasticizers 3.3–4.1 mPa s) is often used for products with reactive fill ingredients, such as aldehydes, to The formation of a soft capsule requires the use of prevent cross-linking of the shell. Gelatins a non-volatile plasticizer in addition to water to derived from poultry, fish or other sources have guarantee the mechanical stability, i.e. the elas- recently been proposed in the patent literature as ticity of the capsule shells during and after the alternatives to gelatin of bovine and porcine drying process. The additional plasticizer has to origin. Poultry and fish gelatins have recently counterbalance the stresses that are induced in been approved by PhEur. the shrinking capsule shells, as the plasticizing From a technological point of view, poultry effect of water in the shells decreases upon drying. gelatin is a potential alternative to the con- Practically, only a few plasticizers are in use, ventional soft capsule gelatins derived from namely polyalcohols, which are approved by the bovine and porcine origin, since its physico- official pharmacopoeias or by local regulatory chemical properties are comparable to those of authorities. Glycerol (85% and 98% w/w), special pigskin gelatins. In practice, it has not gained grades of non-crystallising aqueous sorbitol and high commercial interest yet because its avail- sorbitol/sorbitan solutions and combinations of ability is limited. these are the most used. In addition, propylene The use of cold- or warm-water fish gelatins for glycol and low molecular weight polyethylene soft capsule production is limited by the fact that glycol (PEG 200) have been used. their gelling, setting and drying properties are The type and concentration of plasticizer(s) in more or less different to those of mammalian the shell is related to the composition of the fill, gelatins.