2.3. Capsules

2.3.1. Capsules are small containers of medication encased in an outer shell, which dissolves in the digestive tract. Once the shell dissolves, medication is absorbed into the bloodstream in a manner similar to tablets, although the process is usually faster when the medication is taken as a capsule. Depending on the capsule type, medication can be filled as a dry powder or liquid form. Fill weights vary from 39 mg to 1,425 mg, although the largest capsules are used mostly for veterinary purposes.^[13]

2.4. Powders

2.4.1. Although powders are typically processed into tablets and capsules, they are maintained in powder form as therapeutics, particularly in dry powder inhalers (DPIs). DPI formulations have historically been used primarily to treat respiratory diseases, such as asthma and chronic obstructive pulmonary disease (COPD), because the drug is delivered directly to the lungs; however, they are being increasingly explored for other indications owing to their avoidance of first-pass metabolism, improved compliance over injection delivery, and ability to bypass the blood–brain barrier. As with other OSD forms, the inhaler contains APIs and excipients formulated together for delivery by a device. To maintain suitable flowability characteristics of the powder, force control agents such as leucine, lecithin, and magnesium stearate are part of the formulation. DPI powders are processed by many of the same techniques as powders for tableting or incorporation into capsules such as high- or low-shear blending and spray-drying.^[17]

2.4.2. While amorphous solid dispersions (ASD) are common in the OSD industry, the purpose of creating an ASD is often to enhance solubility, wetting characteristics, dissolution, and bioavailability.^[18] While all the characteristics are also important for a DPI, the shape of the particle is the critical attribute for DPIs. DPI therapeutics must be spherical, with an aerodynamic size of 0.5–5 μm. Four features of the drug–device combination are important: dose metering, aerosolization, disaggregation, and adapter to direct aerosol. Each of these are directly dependent on the physical characteristics of the powder itself.^[19]