Republished from the official company website. Author and content source: Crystal Pharmatech.
Poor aqueous solubility is one of the leading causes of failure in oral drug development.
Originally published by Crystal Pharmatech. Republished with company attribution. Original technical perspectives, references and source links are retained.
Poor aqueous solubility is one of the leading causes of failure in oral drug development. Approximately 40% of marketed drugs and up to 70–90% of drug candidates in development exhibit poor aqueous solubility, placing many compounds in Biopharmaceutics Classification System (BCS) Class II or IV categories where dissolution rate or solubility limits oral bioavailability.
The core challenge is thermodynamic: crystalline drug molecules are held together by lattice energy, and the energy required to solvate these molecules in gastrointestinal fluids may exceed what the physiological environment can provide. Without intervention, a poorly soluble API may never reach therapeutic concentrations in systemic circulation.
Overcoming solubility-related bioavailability barriers requires a formulation strategy grounded in physical chemistry, material science, and biopharmaceutical principles. The optimal approach must be tailored to the API's physicochemical profile, target product profile (TPP), clinical stage, and scalability requirements.
A successful formulation is not defined by solubility alone—it must balance bioavailability, physical stability, and manufacturability to ensure a seamless transition from early development to clinical and commercial stages.
Molecule
We begin with a deep scientific understanding of your compound:
Material
We analyze the physical and chemical foundation of the formulation:
Medicine
We design for the final patient experience and industrial scale drug product:
In pharmaceutical development, solubility enhancement technologies are rooted in three principles:
A range of enabling technologies is available to improve exposure for poorly soluble APIs. The optimal choice is not universal; it depends on compound properties, dose, and the intended development path.
ASD disperses drug molecules in a polymer matrix at the molecular level, creating a high-energy amorphous state that eliminates the lattice energy barrier to dissolution. ASD is widely used to enhance solubility and oral bioavailability of poorly water-soluble drugs.
Spray Drying
Spray drying atomizes a drug-polymer solution into a heated chamber for rapid solvent evaporation, producing amorphous dispersion particles with controlled morphology. It is particularly useful for heat-sensitive compounds and early-stage formulation development.
Hot Melt Extrusion (HME)
HME processes drug-polymer mixtures above the glass transition temperature using mechanical shear. It is a solvent-free, continuous process with regulatory precedent, but requires careful assessment of API thermal stability.
Lipid-based systems, including self-microemulsifying drug delivery systems (SMEDDS), self-emulsifying drug delivery systems (SEDDS), and other self-emulsifying formulations, present drugs in a pre-dissolved state. They can be particularly effective for highly lipophilic compounds and may enhance absorption through intestinal solubilization and lymphatic transport pathways.
Nano-milling and micronization increase surface area to improve dissolution kinetics. These approaches are often effective for Developability Classification System (DCS) IIa compounds with good permeability but dissolution rate control.
Ionizable APIs may benefit from salt formation, while non-ionizable compounds may benefit from co-crystal strategies. These approaches modify solid-state properties while maintaining a crystalline structure, which can offer better physical stability than amorphous approaches.
The following decision layer helps development teams identify a practical starting point for formulation screening. In practice, early-stage teams often screen more than one technology in parallel to reduce risk and avoid platform bias.
| If your compound is... | Consider this approach first |
| BCS Class II with dissolution-limited absorption | ASD, nano-milling, or salt/co-crystal depending on API properties |
| Highly lipophilic with logP > 5 and lower dose | ASD, Lipid-based formulation |
| Thermally stable, low Tm and Tg, and suited for continuous processing | HME-based ASD |
| Heat-sensitive and soluble in volatile solvent systems | Spray-dried ASD |
| Ionizable with feasible counterion options | Salt formation |
| Crystalline and physically stable but slow dissolving | Micronization, nano-milling, ASD |
| Poor polymer miscibility | Mesoporous silica, lipid-based systems |
The table below summarizes practical attributes of common solubility enhancement approaches. It is intended as a guide for early decision-making and should be paired with compound-specific feasibility studies.
| Approach | Best For | Key Advantage | Key Limitation |
| ASD (Spray Drying) | BCS II/IV; heat-sensitive | More polymer selection, less API use at early development stage,versatile | Solvent selection and consumption |
| ASD (HME) | BCS II; thermally stable APIs with low Tm and Tg | Continuous, solvent-free process | Thermal degradation risk |
| Lipid-based | Lipophilic (logP > 5) | Pre-dissolved; possible lymphatic absorption | High excipient load, low concentration |
| Nano-milling | DCS IIa crystalline compounds | minimal food effect, crystalline API with better stability | Ostwald ripening risk, high cost, redispersity challenge |
| Salt / Co-crystal | Ionizable APIs | Crystalline stability maintained | Not universal |
| KinetiSol® | Heat-sensitive ASD candidates | Fast processing; minimal degradation | Limited CDMO availability, no commercial product |
| Mesoporous Silica | Poor polymer miscibility | Non-polymer approach | Lower drug loading |
A systematic workflow ensures efficient use of limited API and supports data-driven formulation decisions throughout development.
The API profile drives formulation strategy. Key parameters include aqueous solubility, pH-solubility profile, logP/logD, pKa, Tm/Tg, TGA/DSC thermal behavior, glass transition potential, polymorphism, and crystal form options.
Target dose strongly influences technology selection. High-dose compounds may require solid oral dosage strategies such as ASD tablets, while lower-dose compounds can sometimes be effectively delivered through lipid-based systems.
Regulatory agencies expect science-based formulation justification, robust solid-state characterization, stability data, impurity control, and appropriate documentation for enabling formulations.
Early-stage programs benefit from rapid, material-sparing feasibility screening. Later-stage programs require deeper characterization, long-term stability data, scalable processes, and regulatory-grade technical reports.
When selecting a CRO or CDMO partner for bioavailability enhancement, evaluate whether the partner can provide breadth, not just a single preferred technology.
Crystal Pharmatech offers a comprehensive suite of bioavailability enhancement technologies, including ASD (spray drying and HME), lipid-based formulations (SMEDDS), and advanced characterization. The First-Time-Right strategy ensures that the optimal technology is selected based on the compound profile—not platform availability.
Our approach includes:
Improving bioavailability of poorly soluble drugs requires a strategic approach that considers the full spectrum of enabling technologies. No single approach works for every compound. The best choice depends on the API's physicochemical properties, target product profile, dose, clinical stage, and scalability requirements.
By combining scientific understanding, parallel feasibility screening, and stage-appropriate development, drug development teams can overcome solubility barriers earlier and reduce downstream risk.
Crystal Pharmatech helps sponsors identify the right formulation path early, generate decision-ready data, and advance poorly soluble molecules toward clinical development with confidence.
Amorphous solid dispersion is a formulation technique where drug molecules are dispersed at the molecular level within a polymer carrier. This eliminates the crystal structure and can increase the apparent solubility and maintain higher drug concentrations in the body.
Modern drug discovery often favors larger and more lipophilic molecules that bind strongly to biological targets but dissolve poorly in water-based gastrointestinal fluids.
BCS Class II drugs have low solubility but high permeability. Once dissolved, they can cross intestinal membranes, so dissolution is often the rate-limiting step for absorption.
A feasibility study evaluates polymer miscibility, drug loading, physical stability, and the ability to achieve and maintain supersaturation in biorelevant dissolution media.
ASD is often favored when a solid oral dosage form is preferred, the API has moderate lipophilicity, or the target dose is higher. Lipid-based systems may be preferred for highly lipophilic, lower-dose compounds, or when ASD does not work.
Initial feasibility screening can often begin with approximately 1-2 g of API for film casting and 40-50 g for lab-scale spray drying , depending on the study design and the technologies being evaluated.