The Solubility Problem in Modern Drug Development
Estimates from published literature suggest that more than 70% of new chemical entities in development pipelines have poor aqueous solubility. Many of these fall into BCS Class II (low solubility, high permeability) or Class IV (low solubility, low permeability) under the Biopharmaceutics Classification System. For these molecules, getting the drug into solution in the gastrointestinal tract fast enough and in sufficient concentration to achieve meaningful absorption is the central formulation challenge.
Amorphous solid dispersions (ASDs) address this challenge by converting the crystalline API into an amorphous form and dispersing it within a polymer matrix. The result is a solid that dissolves faster and achieves higher apparent solubility in the gut than the crystalline starting material.
The Science Behind ASDs
Why Amorphous Forms Dissolve Faster
Crystalline solids require energy to break down the lattice structure before molecules can enter solution. Amorphous forms lack this ordered lattice, meaning the activation energy for dissolution is lower. An amorphous API can achieve supersaturation concentrations in the gut, providing a higher driving force for absorption, provided the supersaturated state is maintained long enough for absorption to occur.
The Role of the Polymer
Without stabilisation, amorphous APIs will tend to recrystallise over time, losing their bioavailability advantage. The polymer matrix in an ASD serves two functions: it provides a physical barrier that inhibits molecular mobility and recrystallisation, and it can interact with drug molecules through hydrogen bonding or other non-covalent interactions that further stabilise the amorphous state. Common polymers used in ASD manufacture include HPMC-AS, PVPVA, and Eudragit systems, each with different dissolution and stabilisation profiles.
Supersaturation and Precipitation Inhibition
In the gastrointestinal environment, a well-designed ASD generates a supersaturated drug solution. Maintaining that supersaturation requires the polymer to act as a precipitation inhibitor, preventing the drug from recrystallising in the gut fluid before it can be absorbed. The selection of polymer type and drug-to-polymer ratio is therefore critical not just for solid-state stability during storage but for the in vivo performance of the dosage form.
ASD Technology Platforms
| Technology | Principle | Best Suited For | Key Consideration |
| Spray drying | API and polymer dissolved in solvent, spray atomised and dried | High-throughput screening; scale-up to commercial | Solvent selection and residual solvent control |
| Hot melt extrusion (HME) | API and polymer melt-blended under heat and shear | Thermally stable APIs; solvent-free processing | API must be thermally stable at processing temperatures |
| Coprecipitation | Anti-solvent addition causes simultaneous precipitation | Research scale; proof of concept | Scale-up can be challenging |
| Electrospinning | High-voltage electric field creates nanofibre matrices | Very low dose, highly potent APIs | Complex scale-up; niche application |
Development Considerations for ASD Programmes
Drug Loading
Higher drug loading reduces the tablet size needed to deliver the target dose, which is important for patient acceptability. However, higher drug loading typically increases the risk of recrystallisation during storage. The optimal drug loading is a balance between these competing requirements, established through screening studies that evaluate physical stability at relevant storage conditions.
Stability Testing
An ASD formulation must demonstrate physical stability over its intended shelf life. Stability studies following ICH Q1 guidelines, combined with accelerated stress testing and XRPD monitoring to detect any crystalline conversion, form the basis of the stability data package. The glass transition temperature of the dispersion is a key parameter: a high Tg relative to storage temperature provides a greater kinetic barrier to recrystallisation.
Downstream Processing
The ASD intermediate, whether produced by spray drying or HME, needs to be converted into a final dosage form. The hygroscopicity, flowability, and compactibility of the ASD intermediate determine what downstream processing approach is feasible. Some ASDs require granulation before tabletting; others can be directly compressed.
Ardena’s ASD Capabilities
Ardena has dedicated ASD manufacturing capabilities at its Somerset, New Jersey facility and at the Pamplona (Idifarma) site in Spain. Both sites operate spray drying and hot melt extrusion platforms with clinical and commercial-scale capability. The development teams at these sites work in close coordination with Ardena’s solid state research group, ensuring that the polymer and drug loading decisions made in screening translate directly into the development programme.
For BCS Class II or IV molecules that have shown limited bioavailability in early animal studies, a conversation with Ardena’s formulation scientists about ASD feasibility is a worthwhile investment before committing to a clinical formulation strategy.
