For an inhaled medication to work as intended, the drug first has to reach the right place.
That sounds simple, but the respiratory tract presents a challenging delivery path. Particles released from an inhaler must travel through the mouth and upper airways before reaching the lungs. Their size, along with the design of the inhaler and the way the patient inhales, can influence where those particles ultimately deposit.
This makes inhaler particle size an important consideration during pharmaceutical development. Particle size characterization can help manufacturers better understand a formulation, evaluate consistency and develop products designed for effective respiratory drug delivery.
Why Inhaler Particle Size Matters
When a patient activates an inhaler, the goal is not simply to release medication into the mouth. The drug needs to travel far enough into the respiratory system to reach its intended site of action.
Particle size plays an important role in that journey.
Larger particles are more likely to impact and deposit in the mouth, throat or upper airways before reaching the lungs. Smaller particles can travel farther into the respiratory tract. However, smaller does not automatically mean better. Particles that are extremely small may remain suspended and eventually be exhaled rather than depositing.
Research on dry powder inhalers identifies an aerodynamic particle size range of approximately 1.5 to 5 µm as particularly relevant for lung deposition. Particles larger than approximately 5 µm have an increased likelihood of depositing in the oropharyngeal region, while particles below approximately 1 µm may have a greater chance of being exhaled.
The exact target can vary depending on the drug, device and intended region of the respiratory tract. This is why pharmaceutical developers are interested not only in an average particle size, but in the particle size distribution throughout the formulation.
It Is Not Simply a Case of “Smaller Is Better”
It can be tempting to assume that the smallest possible particles will provide the best inhaled medication. The science is more complicated.
For example, one study of patients with severe but stable asthma compared bronchodilator aerosols with mass median aerodynamic diameters of 1.4 µm and 5.5 µm. Researchers found no significant difference in lung distribution or bronchodilator effect between the two particle sizes.
Other research involving patients with mild to moderate asthma found that 3 µm and 6 µm albuterol aerosols produced a stronger bronchodilator response than 1.5 µm particles. These findings further illustrate why the smallest particle is not necessarily the most effective particle for every application.
The goal, then, is an appropriate particle size distribution for the particular drug and application.
Particle size also works together with other variables, including inhaler design, particle density, inspiratory flow rate, airway geometry, formulation characteristics and particle agglomeration.
The Special Challenge of Dry Powder Inhalers
Dry powder inhaler testing presents an interesting particle characterization challenge.
In many dry powder inhaler formulations, very small active pharmaceutical ingredient particles may be associated with larger carrier particles or exist within agglomerates. During inhalation, airflow through the device helps separate or de-agglomerate the material so smaller drug particles can be dispersed into the respiratory tract.
The performance of a dry powder inhaler therefore depends on more than the original size of the particles. Formulation characteristics, device design and the patient’s inspiratory flow can all affect the resulting aerosol.
Particle characterization can provide valuable information during formulation development by helping researchers understand the starting material, investigate agglomeration and compare particle size distributions among formulations or manufacturing conditions.
Aerodynamic Particle Size vs. Physical Particle Size
An important distinction in inhalation testing is the difference between aerodynamic particle size and physical particle size.
Aerodynamic Particle Size Distribution, commonly abbreviated APSD, describes particle behavior in air and incorporates characteristics such as particle size, shape and density. Those properties influence how particles travel through the respiratory tract and where they may deposit.
Cascade impaction is the most common industry-accepted approach for determining APSD. Laser diffraction can also provide valuable particle sizing information, but the two techniques answer different analytical questions.
That distinction is important. Laser diffraction does not, by itself, determine exactly where particles will deposit in the lungs. Instead, it characterizes particle size distribution based on the interaction of particles with laser light.
Using Laser Diffraction in Pharmaceutical Particle Characterization
Laser diffraction is widely used for particle size characterization because it can rapidly measure a broad range of particle sizes.
During analysis, particles pass through a laser beam and scatter light. The resulting scattering pattern is interpreted to calculate a particle size distribution based on equivalent spherical diameter.
For laser diffraction pharmaceutical applications, this information can help answer questions such as:
- Is the formulation producing the expected particle size distribution?
- Are different batches comparable?
- Has milling or another manufacturing process changed particle size?
- Is agglomeration affecting the observed distribution?
- How do candidate formulations compare?
- Are process changes shifting the distribution toward finer or coarser particles?
These questions can be particularly relevant to inhalation products because particle size is one of the factors that can influence subsequent aerosol behavior and drug delivery.
Where Laser Diffraction Fits Into Respiratory Drug Development
Laser diffraction is best considered one part of a broader characterization strategy.
Developers of inhaled pharmaceuticals may need to understand both the physical characteristics of the formulation and how the finished product behaves when aerosolized. APSD testing provides information about aerodynamic behavior, while laser diffraction provides information about particle size distribution.
Together with other analytical techniques, particle size data can support formulation development, method development, manufacturing investigations, quality control and troubleshooting.
For example, researchers may use particle size analysis to compare candidate formulations, investigate changes caused by milling or processing, establish repeatable analytical methods or determine why a material is behaving differently from previous batches.
What About Nasal Spray Testing?
Particle and droplet size are also important considerations for orally inhaled and nasal drug products.
As with inhalers, the size distribution produced by a nasal delivery system can influence how the formulation behaves after leaving the device. The appropriate nasal spray testing approach depends on the formulation, delivery system and analytical question.
Physical particle or droplet size, aerodynamic behavior and delivered dose provide different types of information and may require different analytical techniques. Identifying the question first helps determine which measurement approach will provide the most meaningful data
Selecting the Right Technique for the Question
There is no single particle sizing technique that answers every question about an inhaled pharmaceutical product.
Laser diffraction provides valuable particle size distribution information. Cascade impaction is commonly used when aerodynamic particle size distribution is required. Additional techniques may be appropriate when researchers need information about particle shape, agglomeration, surface characteristics or other material properties.
The most useful analytical program starts with understanding what needs to be learned about the material.
At Particle Technology Labs (PTL), our scientists work with pharmaceutical clients to understand the sample, intended application and analytical question before recommending an approach. This consultative process helps clients select techniques that provide meaningful information about their formulation.
For inhaled drug products, particle size is one piece of a complex delivery system, but it is an important one. Understanding particle size distribution can provide valuable insight as pharmaceutical developers work toward consistent formulations designed to deliver medication where it needs to go.
Have questions about laser diffraction or particle size analysis for an inhaled pharmaceutical product? Contact Particle Technology Labs to discuss your material and testing needs.
