The Solubility Wall
Oral bioavailability depends on two things: how much drug dissolves in the gastrointestinal tract and how much of the dissolved drug crosses the intestinal wall into the bloodstream.
For BCS Class I drugs, both happen efficiently and bioavailability is high. For BCS Class II drugs, the drug dissolves poorly and absorption is limited by how fast and how much drug enters solution. For BCS Class III, the drug dissolves readily but crosses the intestinal membrane slowly. For Class IV, both are problems.
The majority of pharmaceutical development programmes now involve BCS Class II or IV molecules. That is not a coincidence. Simple, soluble molecules were developed first. The pipeline that remains is disproportionately full of hard problems.
Matching the Strategy to the Molecule
There is no universal bioavailability enhancement approach. The right strategy depends on the answer to a specific set of questions about the molecule.
- Is poor solubility the rate-limiting step, or is permeability also limiting?
- What is the dose? A molecule dosed at 1 mg faces a different solubility challenge than one dosed at 500 mg.
- Is the molecule thermally stable? That determines whether hot melt extrusion is an option.
- Is it ionisable? That opens the door to salt and co-crystal strategies.
- What is the log P? That informs whether lipid-based formulation is feasible.
- What is the target patient population and their ability to swallow different dosage forms?
Answering these questions rigorously in pre-formulation, before any formulation strategy is committed to, is how good programmes avoid expensive strategy changes halfway through development.
The Enhancement Strategy Landscape
| Strategy | Mechanism | Best Suited For | Key Limitation |
| Micronisation (jet milling) | Increases surface area; faster dissolution rate | Moderate solubility gap; crystalline API; doses above 50 mg | Cannot reliably produce particles below 1 micrometre; no solubility improvement |
| Nanosuspension (wet milling) | Marked surface area increase; modest Ostwald-Freundlich solubility improvement | BCS Class II with moderate solubility gap; API that is compatible with aqueous milling | Downstream processing needed; recrystallisation risk if poorly stabilised |
| Salt or co-crystal formation | Improved intrinsic solubility of salt/co-crystal form; faster dissolution | Ionisable APIs; moderate to severe solubility limitation | Disproportionation risk in GI tract; not applicable to non-ionisable molecules |
| Amorphous solid dispersion (ASD) | Converts crystalline API to amorphous; supersaturation in GI fluid | Severe solubility limitation; BCS Class II/IV; permeability not limiting | Physical stability risk; development and manufacturing complexity |
| Lipid-based formulation (SEDDS/SMEDDS) | Self-emulsification in GI tract; drug presented in dissolved form in lipid droplets | Highly lipophilic APIs (log P above 3); dose flexibility; fill-in-capsule acceptable | High lipid content; fill weight limits dose; not suitable for hydrophilic drugs |
| Cyclodextrin complexation | Inclusion complex improves apparent solubility; solid complex for oral delivery | Moderate log P range; fit of molecule in cyclodextrin cavity required | High cyclodextrin to drug ratio increases tablet size at higher doses |
| Prodrug approach | Chemical modification improves solubility or permeability; converted to active form in vivo | When other physical approaches are insufficient; hydrolysable ester prodrugs most common | Regulatory classification as new molecular entity; full development programme required |
The Dose-Solubility Ratio: A Practical Triage Tool
The dose number (D0 = dose / (solubility x volume)), introduced in the BCS classification framework, is a quick way to estimate whether solubility is likely to limit absorption in a typical patient. A dose number above 1 suggests that the drug cannot fully dissolve in the available GI fluid volume at the intended dose, even at maximum solubility, and that bioavailability enhancement is likely to be needed. This calculation, using even a rough solubility estimate, often provides enough information to rule in or rule out certain strategies before any formulation work begins.
Why Enhancement Strategy Selection Belongs in Pre-Formulation
Every enhancement strategy involves trade-offs. ASDs are powerful but introduce physical stability risk. Lipid formulations are effective for lipophilic drugs but are not manufacturable for highly potent APIs without specialised containment. Nanosuspensions require controlled downstream processing. Salt strategies do not work for non-ionisable molecules.
Understanding these trade-offs before the formulation programme starts, not after the first strategy fails, is the difference between a programme that reaches GMP on schedule and one that does not.
Ardena’s Multi-Site Bioavailability Enhancement Capabilities
Ardena’s formulation expertise spans the full enhancement landscape. Salt and co-crystal screening at Ghent, ASD development by spray drying and HME at Somerset and Pamplona, nanosuspension at Oss, and lipid-based formulation across the network. The pre-formulation team recommends the most appropriate strategy based on the molecule’s properties and the programme’s goals, with no bias towards a particular technology platform.
