Why Particle Size Matters for Oral Drug Absorption
The Noyes-Whitney equation, a cornerstone of pharmaceutical dissolution science, tells us that the rate of dissolution of a solid is proportional to its surface area. Reducing the particle size of a poorly soluble API increases the surface area exposed to the dissolution medium, which increases the dissolution rate, and for molecules where dissolution is the rate-limiting step in absorption, this translates directly into improved oral bioavailability.
Particle size reduction is one of the most widely used and well-understood strategies for addressing solubility challenges in oral drug products. It does not require a change in solid form, a new polymer system, or a significant change in formulation architecture. For many BCS Class II molecules, it can provide all the bioavailability improvement needed with a relatively straightforward manufacturing process.
The Three Primary Particle Size Reduction Technologies
Jet Milling (Micronisation)
Jet milling uses high-pressure streams of gas, typically nitrogen or compressed air, to accelerate drug particles to high velocities and drive them into collisions with each other and with the walls of the milling chamber. The result is a dry, crystalline powder with a particle size typically in the range of 1 to 20 micrometres.
Jet milling is well-established, scalable, and produces a product that is relatively easy to characterise and handle. It is the particle size reduction method of choice for potent compounds where containment during processing is critical, because the dry milling process can be conducted in a closed, contained system. However, it has limitations: it cannot reliably produce particles below about 1 micrometre, and it can cause polymorphic conversion or surface amorphisation if the API is thermally or mechanically sensitive.
Wet Milling (Media Milling)
Wet milling suspends the API in an aqueous medium with appropriate stabilisers and uses grinding media, typically ceramic or polymeric beads, to reduce particle size through attrition and impact. Wet milling can achieve smaller and more uniform particle sizes than jet milling, typically in the 200 nanometre to 5 micrometre range.
The stabiliser system used in wet milling is critical. The stabiliser prevents particle aggregation during milling and in the final suspension. Common stabilisers include HPMC, poloxamer, and polysorbate 20 or 80. The choice of stabiliser affects not only the physical stability of the suspension but also the dissolution performance of the final product.
Bottom-Up Precipitation (Nanosuspension by Anti-Solvent)
Rather than breaking down larger particles, bottom-up approaches start with a molecular solution of the API and induce controlled precipitation of nanoparticles by mixing with an anti-solvent. The particle size and distribution are controlled by mixing conditions, temperature, and the stabiliser system. Bottom-up approaches can achieve very small and uniform particle sizes but are more complex to scale and control than top-down methods.
Technology Comparison for Particle Size Reduction
| Factor | Jet Milling | Wet Milling | Bottom-Up Precipitation |
| Achievable particle size | 1-20 micrometres | 200 nm – 5 micrometres | 50-500 nm |
| Solid state of product | Crystalline (risk of surface amorphisation) | Crystalline | Variable; can be amorphous |
| Process type | Dry | Wet suspension | Wet suspension |
| Containment for HPAPIs | Excellent; closed system | Moderate | Moderate |
| Scale-up complexity | Established, well-understood | Established for pharmaceutical use | More complex; sensitive to mixing conditions |
| Regulatory precedent | Extensive | Extensive (Rapamune, Tricor precedents) | Growing |
| Key limitation | Cannot achieve sub-micron reliably | Stabiliser selection critical | Scale-up control challenging |
Particle size ranges and characteristics above are representative of typical pharmaceutical processes and are drawn from published literature. Programme-specific outcomes depend on API properties and process conditions.
Choosing the Right Approach for Your Molecule
The selection of particle size reduction technology depends on several factors: the target particle size needed to achieve the desired dissolution enhancement, the physical and chemical stability of the API under milling conditions, the intended final dosage form, and the manufacturing scale at which the process will ultimately run.
For potent or cytotoxic compounds, the ability to conduct the entire process in a contained, dry environment makes jet milling the preferred first choice. For molecules where sub-micron particles are needed to achieve meaningful bioavailability improvement, wet milling or a combined approach may be required. A feasibility study comparing dissolution performance across particle size ranges is the most reliable way to establish the target specification and select the technology.
Ardena’s Particle Size Reduction Capabilities
Ardena’s Pamplona (Idifarma) facility has dedicated particle size reduction capabilities including jet milling for API micronisation, with containment suitable for high-potency compounds. The analytical teams at Ardena use laser diffraction and dynamic light scattering for particle size characterisation and dissolution testing in biorelevant media to assess the in vitro performance impact of particle size reduction.
Particle size reduction studies at Ardena are conducted alongside the broader formulation programme, ensuring that decisions about milling approach and target particle size are informed by dissolution data and downstream processability assessment rather than treated as a standalone analytical exercise.
