Powder inhalation and DPI formulation: complete guide to inhalation powders

What is a Dry Powder Inhaler (DPI)?

A Dry Powder Inhaler (DPI) is an inhalation device that delivers dry powder directly into the lungs without a propellant. The patient activates the dose by inhaling: the airflow through the device deagglomerates the powder and transports the active particles to the bronchioles and alveoli. DPI systems are widely used for asthma, COPD and lung infections (including inhaled antibiotics and vaccines).

The success of a DPI stands or falls with formulation and manufacturing process. Particle size, carrier particles, blend uniformity and moisture sensitivity together determine the Fine Particle Fraction (FPF) — the proportion of the dose that actually reaches the lungs.

The pharmacology of inhalation: why particle size determines everything

Only particles of 1–5 µm reach the deeper lung regions (bronchioles, alveoli). Larger particles (>10 µm) deposit in the oropharynx and are pharmacologically inactive. Particles <1 µm are exhaled. The aerodynamic particle size (MMAD, Mass Median Aerodynamic Diameter) is therefore the most critical quality parameter for inhalation formulations, measured via cascade impactor (NGI, Andersen).

DPI formulation strategy

Carrier-based formulation

The most widely used DPI formulation strategy combines fine API particles (1–5 µm) with coarse carrier particles of lactose (50–200 µm). The API adheres to the surface of the carrier and is released upon inhalation by turbulence within the device. Critical parameters:

Lactose quality: morphology, specific surface area and moisture content determine API-carrier interaction
Lactose fines fraction: small lactose particles (fines, <10 µm) compete with the API for carrier surface and improve the FPF
Mixing order and intensity: determine adhesion uniformity and avoid excessive deagglomeration during production

Carrier-free formulation

For highly potent APIs or when no suitable carrier is available, carrier-free formulations are used: spray-dried or lyophilised porous particles with low density and aerodynamically favourable morphology. Higher production complexity, but superior FPF (up to >60%).

Micronisation and particle production

API micronisation for DPI formulation requires particle size control at sub-5µm level. Common methods:

Jet milling (fluidised bed jet mill): air- or nitrogen-driven interparticle collisions, no thermal load, scalable to production volume
Spray drying: solution or suspension evaporated in a hot air stream, produces spherical particles with controllable size and morphology
Supercritical fluids (RESS, SAS): niche application for polymorph-sensitive APIs

Containment during micronisation of HPAPIs (OEB 4–5) is a critical engineering requirement: closed mills with integrated filter separation and negative pressure are standard for potent inhalation APIs.

Blend uniformity and mixing

DPI blends require extreme uniformity: Content Uniformity (CU) specifications of <5% RSD at dose level are common. This places high demands on:

Deagglomeration of micronised material before mixing (via sieving or sonication)
Mixing equipment (tumble blender, bin blender) and mixing parameters (rpm, fill degree, mixing time)
Electrostatic charge management (use of grounded equipment, anti-static liners)

Moisture sensitivity and packaging

Inhalation powders are extremely moisture-sensitive: even at 40–60% RH, lactose-based blends can sinter or API crystallisation can occur, dramatically reducing FPF. Production under controlled conditions (RH <30%, T 20–22°C) and packaging in moisture-resistant blister packs (aluminium-aluminium) or capsules with desiccant are standard GMP practice.

Regulatory requirements and quality control

DPI products are characterised via: Aerodynamic Particle Size Distribution (APSD) via NGI, Fine Particle Fraction (FPF), Delivered Dose Uniformity, MMAD and GSD. ICH Q4B Annex 9 and EMA/FDA inhalation guidelines describe test methods and acceptance criteria. Device-formulation combination testing is mandatory for registration.

Conclusion

DPI formulation is one of the most complex areas in pharmaceutical product development. Particle size, carrier interaction, micronisation process, blend uniformity and moisture control must all be optimised simultaneously for a robust product. The process steps require specialised equipment for both production and containment.

More about inhalation powders and DPI production equipment: hecht-technology.nl.