Capsule Formulation: Hard and Soft Gelatin, HPMC Capsules

Capsules are solid dosage forms in which the drug is enclosed within a hard or soft shell. They offer several advantages over tablets: easier formulation for low-compressibility APIs, natural patient acceptability, the ability to mask unpleasant taste and odour, and flexibility in fill type (powders, pellets, liquids, semi-solids). Understanding capsule shell composition and fill formulation is essential for pharmaceutical scientists.

Hard gelatin capsules (HGCs) consist of two-piece shells — a body and a cap — made from gelatin, water, and optional colorants or opacifiers. Gelatin is derived from collagen hydrolysis of bovine or porcine origin. Shell moisture content is 13–16%; below 10%, shells become brittle; above 18%, they soften and distort. The gelatin concentration (28–30% w/w) determines shell strength and dissolution characteristics. Hard capsules are filled with powder blends, granules, pellets, tablets, or semi-solid melts.

Powder-filled hard capsules require good flow and consistent bulk density. Fill weights range from 100 mg to 1000 mg depending on capsule size (000 to 5, with 000 being largest). Key formulation issues include segregation of blend components, static charge causing adhesion, and hygroscopicity affecting shell integrity. Lubricants (magnesium stearate) are still required to prevent powder adhesion to filling machine components. Disintegrants accelerate powder dispersion after shell dissolution.

Pellet-filled capsules exploit multiparticulate advantages — dose flexibility, reduced food effect, improved absorption uniformity, and the ability to include incompatible APIs in separate pellet populations. Modified-release pellets are coated with Eudragit or ethylcellulose films and filled into capsules.

Soft gelatin capsules (SGCs, softgels) are one-piece, hermetically sealed shells containing liquid or semi-solid fill. The shell formulation includes gelatin, plasticiser (glycerol, sorbitol), water, and colourings. Plasticiser-to-gelatin ratio determines shell flexibility. Fill materials must be non-aqueous (to prevent shell dissolution): oils, PEG 400, propylene glycol solutions, or self-emulsifying drug delivery systems (SEDDS). SGCs dramatically enhance bioavailability of BCS Class II drugs by presenting them in solution or micro-emulsion form.

HPMC (hydroxypropyl methylcellulose) capsules are the leading vegetarian/vegan alternative to gelatin capsules. HPMC shells are derived from plant cellulose and are suitable for all dietary and religious requirements. Key differences from gelatin: HPMC has lower moisture content (5–7%), making it advantageous for hygroscopic APIs; gelling behaviour is thermally reversible (gels on heating, contrary to gelatin); dissolution is independent of storage humidity changes. HPMC capsules are compatible with liquid fills and are used extensively in nutraceuticals.

Pullulan capsules are another plant-based alternative with ultra-low oxygen permeability, beneficial for oxygen-sensitive APIs. Starch-based capsules exist but have inferior mechanical properties.

Capsule dissolution is influenced by shell cross-linking. Gelatin cross-linking occurs on exposure to aldehydes (formaldehyde from excipient degradation, reducing sugars via Maillard reaction) or high humidity/temperature, forming insoluble pellicles. This is a major stability concern causing dissolution failure — the two-tier dissolution test (with enzyme pepsin in Stage 2) accounts for pellicle formation.

Fill-shell compatibility must be assessed. Hygroscopic fills extract water from the shell, causing brittleness. Lipophilic fills are incompatible with aqueous shell systems. Migration of plasticiser between fill and shell can alter mechanical properties over shelf life. Regulatory requirements include dissolution testing, uniformity of fill weight, and leak testing for SGCs.