For oral solid dose drugs, capsules offer certain advantages over tablets and there are several types of capsules available.
Oral solid dose (OSD) drugs can be taken in tablet or capsule form. Some drugs are only available in capsule or tablet form, while others can be supplied in both forms. There are various types of capsules with shells made of different materials. When choosing the type of capsule, the formulator should consider factors such as the barrier to water and oxygen, the reactivity of the shell and the material from which it is made.
Dry filled capsules include mainly hard gelatine and hard hydroxypropylmethylcellulose or hydroxypropylmethylcellulose (HPMC) capsules. Liquid-filled capsules include hard (gelatine or HPMC) and soft gelatine capsules.
Large sizes (000) are mainly used in veterinary practice. The fill weight increases with the size of the capsule and the bulk density of the filling material and ranges from 0.3-1.5 g/cc. For example, the smallest capsule may have a fill weight of 39 mg; the largest may weigh up to 1425 mg.
Enteric-Coated Empty Gelatin Capsules
The shell of hard gelatine capsules contains 13-16% water. Storage of hard gelatine capsules at very low humidity can cause them to become brittle. Gelatine capsules do not protect the hygroscopic material from atmospheric water vapour, as moisture can diffuse through the gelatine walls. If stored at high humidity levels, the capsules will become flimsy. In such cases, primary packaging materials such as aluminium strip packs, moisture-proof blister foils (e.g. Aclar) or bottles should be used.
Soft gelatine capsules (SGC) have a soft, spherical gelatine shell that is somewhat thicker than hard gelatine capsules. The gel is plasticised by the addition of glycerol, sorbitol or similar polyols. It may contain preservatives to prevent fungal growth and SGC preparation generally requires mass production methods.
Hard gelatine capsules. The gelatine used to make the most common capsules is obtained from the raw material gelatine by hydrolysis. Gelatine is a natural, safe, non-allergenic, clean and economical ingredient. Two-piece hard gelatine capsules are available in a variety of sizes; from the largest to the smallest, these sizes are 000, 00el, 00, 0xel, 0el+, 0el, 0, 1el, 1, 2, 3, 4, 5.
Fish gelatin capsules. Marinecaps are made from fish gelatine and do not pose a BSE or TSE risk. They are the first choice for filling marine supplements such as fish oil rich in EPA [eicosapentaenoic acid].
Starch capsules. Starch capsules are made from potato starch. Their solubility is pH independent and they are suitable for enteric coating. The moisture content of starch capsules ranges from 12-14% w/w, of which more than 30% is tightly bound.
Prululan capsules. These vegetarian capsules are made from tapioca, which is naturally fermented into branched chain starch. They provide a high barrier to oxygen.
Polyvinyl acetate (PVA) capsules. Capsules made from PVA can be used to fill insoluble drugs dissolved in polyethylene glycol (PEG) 400. PEG 400 is incompatible with other hard capsules when used as a single carrier. PVA has a low oxygen permeability and a high barrier to oxygen.
HPMC capsules. HPMC capsules are stable at low humidity levels, have a low moisture content (3-8%) and a low electrostatic charge. These natural capsules are available in sizes ranging from 00 to 4. HPMC capsules are suitable for highly reactive molecules (as they are not cross-linked). HPMC is more suitable for moisture sensitive products, hygroscopic products and low relative humidity applications than hard gelatine. HPMC is not of animal origin and does not pose a risk of contamination by organisms that cause Bovine Spongiform Encephalopathy (BSE) or Transmissible Spongiform Encephalopathy (TSE). Due to its vegetarian nature, HPMC capsules are widely used in OSD medicines as well as nutritional supplements, dietary supplements and herbal products.
Liquid-filled hard capsules (LFHC). Two-piece hard capsules made from gelatine or HPMC can be used to fill and seal non-aqueous liquids, pastes, suspensions, hot melts and other carriers with a melting point of up to 70 °C that flow easily.LFHC can also be filled with tablets, pills or other capsules as a combination filler.LFHC can be used for drugs that are sensitive to moisture. These can be a cost-effective alternative to some soft gelatine capsule products, and can also improve bioavailability and product stability.
Liquid encapsulation technology helps to overcome many of the problems associated with the use of soft gelatin capsules, including high cost, waste, cross contamination, migration of drugs into the capsule shell and low bioavailability. Liquid-filled and semi-solid capsules are inherently resistant to crushing and pulverisation and therefore provide a good basis for the development of abuse-resistant formulations. These capsules can also be enteric coated and HPMC hard capsules do not become brittle when they lose water.
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