Pulmonary Drug Delivery Formulation: Inhaler and Nasal Spray Design

Why the Respiratory Route Is Formulation's Hardest Delivery Problem

The lung and nasal cavity evolved to keep foreign particles out, which is precisely the barrier a formulation has to work with rather than around. Oral dosage forms have a comparatively forgiving absorption window spanning meters of gut and hours of transit time. Inhaled and nasal products have to hit a target measured in microns and hold it through manufacturing, storage, and patient use, with almost no room for drift before the dose simply lands in the wrong place.

The defenses a formulation has to work around are physical, chemical, and immunological, and they operate on different timescales throughout the respiratory tract.

• Mucociliary clearance — the ciliated epithelium lining the nose and conducting airways sweeps mucus and trapped particles toward the throat on a timescale of minutes, limiting the residence time available for absorption.
• Inertial impaction losses — oversized particles collide with the moist walls of the mouth, throat, and upper airway before ever reaching the target zone, and this single mechanism accounts for the majority of dose loss in a poorly engineered inhaler.
• Alveolar macrophage clearance — particles that do reach the deep lung can still be phagocytosed and cleared by resident immune cells, shortening the effective residence time of a deposited dose.
• Cough and bronchoconstriction reflexes — irritant formulations, extreme pH, or inappropriate tonicity can trigger a protective reflex that expels the dose before it has a chance to act.

Every platform covered in this article is, at its core, a different engineering answer to the same question: how do you get a therapeutic dose past these defenses and onto the right tissue, reliably, from a device a patient operates without supervision? Metered-dose inhalers were the first widely adopted answer, and their propellant chemistry still shapes how the entire category is regulated today.

Metered-Dose Inhalers: Propellant Chemistry and Suspension Design

A pressurized metered-dose inhaler, or pMDI, holds the drug either dissolved or suspended in a liquefied propellant inside a sealed canister, released through a precision metering valve that dispenses an identical volume with every actuation. The propellant itself is an active formulation ingredient, not an inert carrier, because its vapor pressure and polarity directly determine spray force, plume velocity, and the resulting particle size distribution. Chlorofluorocarbon propellants were phased out under the Montreal Protocol because of their ozone-depleting effect, and the industry moved to hydrofluoroalkane propellants such as HFA-134a and HFA-227, a transition that forced most legacy products through a full reformulation rather than a simple substitution.

Whether the drug is dissolved or suspended in that propellant sets the entire formulation strategy that follows.

• Solution MDIs — the drug dissolves fully in the propellant, usually with an ethanol cosolvent, producing a formulation less sensitive to particle size drift but more dependent on cosolvent and leak-rate control over shelf life.
• Suspension MDIs — micronized drug particles are dispersed in propellant with a surfactant such as oleic acid or lecithin, which prevents aggregation and lubricates the metering valve, but the suspension must remain physically stable against creaming and caking between shakes.
• Valve and actuator geometry — the metering chamber volume and the orifice diameter of the actuator nozzle jointly control the atomization energy applied to the formulation, and small dimensional changes here can shift the aerodynamic particle size distribution as much as a change in the formulation itself.
• Spacer and holding chamber compatibility — add-on spacer devices reduce oropharyngeal deposition by letting propellant evaporate and larger droplets settle out before inhalation, and formulations are frequently tested with and without a spacer to characterize both use cases.

Background on the propellant chemistry and valve engineering behind these devices is well documented in general references on the metered-dose inhaler. Suspension and solution MDIs both remain dominant for asthma and COPD therapy, but they share one structural limitation: patients must coordinate actuation with inhalation precisely, which is the constraint that drove development of the breath-actuated dry powder platform.

Dry Powder Inhalers: Carrier-Based Powder Engineering

A dry powder inhaler removes the propellant entirely and relies on the patient's own inspiratory airflow to disperse a powder formulation into a breathable aerosol. That design eliminates propellant chemistry and coordination problems, but it introduces a powder-engineering problem in their place: micronized drug particles in the 1 to 5 micron range are so cohesive that they will not flow or meter reproducibly on their own. Most DPI formulations solve this with ordered mixing, adhering the fine drug particles to the surface of much larger lactose monohydrate carrier particles through van der Waals and electrostatic forces, producing a blend that flows and doses like a coarse powder while still containing individually respirable drug particles.

Getting a DPI blend to detach and disperse efficiently on inhalation, rather than simply falling apart poorly or staying stubbornly stuck to the carrier, depends on several interacting variables.

1. Carrier particle size and surface texture — smoother or rougher lactose surfaces change the adhesion force holding drug particles in place, directly setting how much of the dose detaches during inhalation.
2. Ternary agents — a small addition of fine lactose or magnesium stearate can occupy high-energy binding sites on the carrier surface, weakening drug-carrier adhesion and improving the fraction of drug that detaches and disperses.
3. Moisture control — lactose monohydrate is hygroscopic, and absorbed moisture forms liquid bridges between particles that increase cohesion, reduce flowability, and can trigger caking during storage, so DPI manufacturing typically runs in low-humidity suites.
4. Electrostatic charge — the micronization process itself can leave drug particles charged, and uncontrolled charge affects both blend uniformity during manufacturing and dispersion behavior at the point of use.
5. Device resistance — passive DPIs rely entirely on the patient's inhalation effort to generate dispersion energy, so the device's internal airflow resistance has to be matched to the formulation and to the target patient population's typical inspiratory flow rate.

The fraction of the metered dose that ends up in the respirable particle size range, known as the fine particle fraction, is the critical quality attribute that ties all of these variables together, and it is measured directly by cascade impactor testing. Further technical background on device and formulation design is available in general references on the dry-powder inhaler.

Each platform trades patient burden against formulation complexity in a different place, which is exactly why device and formulation selection has to happen together at the start of a program rather than being decided after the chemistry is fixed. Liquid formulations sidestep the powder-flow problem entirely, but they introduce a different set of constraints tied to water as a solvent.

Nebulizer Solutions and Nasal Sprays: Aqueous Formulation Constraints

Nebulizers and nasal sprays are both aqueous liquid platforms, but they are formulated to opposite droplet size targets and carry different tolerability constraints, which makes it easy to underestimate how different the two disciplines actually are. A nebulizer converts a solution or suspension into a fine respirable mist using jet, ultrasonic, or vibrating mesh energy, and the choice of device technology is itself a formulation decision, since ultrasonic nebulizers generate localized heating that can degrade heat-sensitive proteins while mesh nebulizers are generally gentler and more compatible with biologics.

Because both platforms put an aqueous formulation directly onto sensitive mucosal or bronchial tissue, tolerability constraints are stricter than for most other dosage forms.

• Tonicity — nebulized and nasal solutions are formulated close to physiological osmolality, since hypotonic or hypertonic solutions can trigger cough, bronchospasm, or nasal irritation on contact with sensitive tissue.
• pH matching — nasal formulations are typically held near the mildly acidic pH of nasal mucosa to avoid stinging and to support mucociliary function, while nebulizer solutions are buffered to minimize airway irritation.
• Preservative selection — benzalkonium chloride and similar preservatives are documented in the clinical literature to provoke bronchospasm when delivered directly to the lower airway, which is why many nebulizer products are packaged as preservative-free single-dose vials rather than multi-dose bottles.
• Viscosity and mucoadhesion — nasal sprays often include a viscosity modifier to slow drainage out of the nasal cavity and extend contact time with the absorptive mucosa before mucociliary clearance removes the dose.

Device selection compounds these constraints rather than sitting apart from them, since the same solution can perform very differently depending on how it is atomized. Whichever platform is chosen, though, the formulation is ultimately being judged against a single physical parameter that determines where in the respiratory tract the dose actually lands.

Particle Size and Aerodynamic Deposition: The Variable Behind Everything

Every design choice covered so far — propellant, carrier, device technology, droplet size — ultimately expresses itself through one measurable property: the aerodynamic particle size distribution of the delivered dose. Aerodynamic diameter describes how a particle actually moves through an airstream, accounting for its density and shape rather than just its physical dimension, which is why it is the parameter both formulators and regulators use to predict where a dose will deposit.

Deposition follows three physical mechanisms that dominate in different size ranges and different regions of the respiratory tract.

This distribution is measured directly with cascade impactor testing, using instruments such as the Andersen Cascade Impactor or the Next Generation Impactor, which physically separate an aerosol into size fractions across a series of graded stages so the fine particle fraction can be quantified rather than estimated. A formulation and device combination that cannot demonstrate a consistent, tight distribution around its target size range will show variable clinical performance regardless of how sound the underlying chemistry is. That target itself, however, depends entirely on where in the respiratory tract the drug is actually meant to act — a distinction that becomes central once nasal delivery is being used for something beyond local, topical effect.

Nasal Drug Delivery: Local Action Versus Systemic and Nose-to-Brain Strategy

Nasal formulation strategy splits along a fundamental line: is the nasal cavity the target tissue, or is it simply a convenient, needle-free route into systemic circulation or the central nervous system? Corticosteroid sprays for allergic rhinitis are formulated for local action, where the goal is prolonged mucosal contact and minimal systemic absorption. Peptide hormones such as calcitonin and triptans such as sumatriptan are formulated for the opposite goal — rapid, efficient absorption across the nasal mucosa into the bloodstream, offering a non-invasive alternative to injection for drugs that would otherwise be destroyed by oral first-pass metabolism.

Several formulation levers exist specifically to push absorption toward one of these two outcomes.

• Permeation enhancers — excipients such as chitosan and certain cyclodextrins transiently increase paracellular transport across the nasal epithelium, improving systemic bioavailability for peptides and other poorly permeable molecules.
• Mucoadhesive polymers — carbomers, hydroxypropyl methylcellulose, and similar polymers extend formulation residence time against mucociliary clearance, which typically turns over nasal secretions within roughly 15 to 20 minutes.
• Droplet size targeting — systemic and local nasal products are still formulated to deposit above the nasal cavity's lower size cutoff, avoiding the pulmonary or aspiration risk that comes with producing droplets fine enough to travel further down the airway.
• Olfactory-region targeting — nose-to-brain delivery programs specifically engineer spray geometry and viscosity to reach the olfactory epithelium high in the nasal cavity, which occupies a small fraction of total nasal surface area but provides a direct anatomical route to the brain via the olfactory and trigeminal nerves, bypassing the blood-brain barrier.

Nose-to-brain delivery remains an active pharmaceutical research area precisely because that anatomical shortcut is real, but reaching it reliably in a marketed product is a harder formulation problem than reaching the general nasal mucosa for local or systemic absorption. Whichever strategy a program pursues, the finished formulation still has to clear the same regulatory and testing gates that apply across every inhaled and nasal platform.

Regulatory Framework and In Vitro Testing for Inhaled and Nasal Products

Inhaled and nasal products are tested against a different set of standards than oral or parenteral dosage forms, because the properties that predict clinical performance — spray pattern, plume geometry, and aerodynamic particle size — have no equivalent in a dissolution or assay test. These methods exist specifically because release specifications built for other dosage forms simply do not capture whether an inhaled dose will actually reach its target tissue.

Regulatory scrutiny is compounded for these products because they are legally combination products — a drug and a device evaluated as one inseparable system. Method standards and general chapters for these procedures are maintained by bodies including the United States Pharmacopeia, while overall drug product oversight in the US sits with the FDA's Center for Drug Evaluation and Research. Leachables and extractables studies on propellant-contact and device-contact materials are typically run alongside these performance tests, since valve elastomers and device plastics can both contribute impurities and be degraded by the formulation itself over shelf life. Passing this testing framework in the lab is necessary but not sufficient — the same performance has to survive the transition from development batch to full-scale, validated commercial manufacturing.

Formulation, Device Combination, and Scale-Up Challenges

Scaling an inhaled or nasal product is unusually unforgiving compared with most other dosage forms, because the critical quality attribute is a physical particle size distribution rather than a chemical assay result, and physical properties are far more sensitive to equipment and process differences between a pilot line and a commercial line. A change that looks purely operational — a different homogenizer, a different fill speed, a different humidity setpoint — can move the fine particle fraction enough to affect a regulated performance claim without any ingredient in the formula changing at all.

• Micronization consistency — the jet-milling or spray-drying process that sets initial drug particle size must be tightly controlled and requalified at each scale, since small shifts here propagate directly into the finished aerodynamic particle size distribution.
• Moisture-controlled manufacturing — DPI blending and filling commonly run in low-humidity suites to prevent lactose hydrate transitions and powder caking during the manufacturing window itself.
• Valve and actuator technology transfer — MDI crimping and valve-fitting equipment must reproduce metering chamber tolerances exactly, since dimensional drift here changes both dose uniformity and atomization energy.
• Capsule and blister filling precision — DPI doses are often measured in micrograms, so filling equipment must hold weight uniformity within a narrow tolerance across an entire production run.
• Device-formulation combination testing — because these are combination products, technology transfer has to validate the drug, the device, and their interaction together, not the formulation in isolation.

These constraints are also why inhaled and nasal programs benefit from involving manufacturing and device expertise from the earliest formulation decisions rather than treating scale-up as a downstream handoff. Emulsion and topical semisolid platforms carry their own distinct scale-up profile, covered in our guide to parenteral formulation development, sterility, and tonicity, which shares much of the aseptic and tonicity-control discipline nasal and nebulizer liquids depend on. Excipient and carrier selection for solid dosage forms, including the lactose grades that show up again in DPI blends, is covered in our guide to excipient selection for oral solid dosage forms.

The decision framework across all four platforms reduces to three questions, asked in this order. Which deposition zone does the therapy actually need — nasal, tracheobronchial, or alveolar — and does the chosen device and formulation combination reliably hit that aerodynamic size range? Does the formulation survive the specific chemical or physical stress of its platform, whether that is propellant compatibility, powder moisture sensitivity, or aqueous tonicity and preservative tolerability? And does the manufacturing process reproduce the particle size distribution the clinical program was built on, batch after batch, at commercial scale? Teams that work through those questions early with a formulation consultant avoid the most expensive outcome in this category: a device-formulation combination that performs beautifully in vitro and inconsistently in the patient. That discipline sits within the broader pharmaceutical and healthcare formulation practice Global Formulation supports across dosage forms.