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.

Fragility Is Built Into the Molecule

mRNA is not a robust molecule. It was never designed to be. In a living cell, mRNA is translated quickly and then degraded, making way for new instructions. A half-life of minutes is a feature of cellular biology, not a bug.

As a pharmaceutical drug substance, that biological instability is a serious development problem. Every formulation decision you make, every excipient you select, every storage condition you define, has to work against a molecule that nature built to fall apart.

The good news is that the tools to manage it are well established. The bad news is that getting them wrong is extremely easy, and the consequences, degraded mRNA, lost potency, failed stability studies, tend to surface at the worst possible moment.

The Three Degradation Pathways That Matter

Hydrolysis

The phosphodiester backbone of RNA is susceptible to hydrolysis. Water attacks the 2-prime hydroxyl group of the ribose ring and cleaves the backbone, producing shorter RNA fragments. This reaction is accelerated by heat and by metal ions, particularly magnesium, which are present in many buffer systems.

This is why mRNA formulations use metal chelators such as EDTA to sequester trace metal ions. It is also why formulation pH matters. Hydrolysis rate is lower at slightly acidic pH; alkaline conditions accelerate degradation. Most mRNA formulations target a pH between 6.5 and 7.5 for this reason.

Ribonuclease Degradation

RNases are ubiquitous. They are on surfaces, in reagents, on gloves, in the air. A single RNase molecule can degrade thousands of mRNA molecules in seconds. mRNA formulation and manufacturing must be conducted in an RNase-controlled environment with validated procedures for equipment decontamination and reagent preparation.

The LNP formulation itself provides significant protection against RNase degradation: encapsulated mRNA is inaccessible to the enzyme. But any free mRNA in the formulation, unencapsulated drug substance that has not been removed during formulation or buffer exchange, remains vulnerable.

Physical Degradation

mRNA is a large, negatively charged polymer. It is vulnerable to aggregation, fragmentation under shear, and adsorption to surfaces. Freeze-thaw cycling can cause mRNA fragmentation if the process is not controlled. Repeated agitation during shipping can similarly degrade both the mRNA and the LNP that carries it.

The Formulation Arsenal Against Degradation

Degradation Threat	Formulation Counter-Strategy	Critical Parameter
Hydrolytic backbone cleavage	Buffer at pH 6.5-7.5; EDTA as metal chelator; minimise free water activity	pH and buffer species; EDTA concentration
RNase contamination	RNase-controlled manufacturing environment; encapsulation within LNP; validated cleaning	Encapsulation efficiency; environmental monitoring for RNase activity
Thermal degradation	Frozen storage at minus 20 or minus 80 degrees C; validated cold chain; minimal freeze-thaw cycles	Storage temperature; freeze-thaw validation; shipping qualification
Shear-induced fragmentation	Controlled mixing during formulation; validated fill-finish parameters; avoid high-shear pumping	Mixing speed and duration; fill-finish pump type and speed
Oxidation	Purge headspace with nitrogen; antioxidants where compatible; limit light exposure	Nitrogen overlay; vial headspace composition

Measuring Stability: What the Tests Actually Tell You

mRNA integrity is most commonly assessed by gel electrophoresis or capillary electrophoresis. A single sharp band at the expected size indicates intact, full-length mRNA. Smearing or lower molecular weight bands indicate degradation. The test is sensitive but semi-quantitative; small amounts of degradation can be missed.

Potency by in vitro translation assay is the functional complement. Intact mRNA produces protein; degraded mRNA does not. A product that shows apparent integrity by gel but reduced potency by translation assay has a problem that the gel alone would not have detected.

Both tests are required for GMP release of mRNA drug products under current FDA and EMA expectations. ICH Q2(R2) on analytical validation applies to the translation assay, and method development must demonstrate adequate sensitivity to detect potency losses that are clinically relevant.

The Cold Chain Question That Every Sponsor Faces

Current approved mRNA LNP products require storage at minus 20 or minus 80 degrees Celsius. That cold chain dependency limits access in lower-resource settings and adds significant cost and complexity to global clinical supply.

Lyophilised mRNA LNP formulations offer a route to improved thermostability, with some programmes targeting ambient or refrigerated storage for the freeze-dried product. The cryoprotectant system, typically sucrose or trehalose, must protect both the mRNA and the LNP structure during lyophilisation and on reconstitution. This is an active area of formulation research, and the regulatory pathway for lyophilised mRNA LNP products is still maturing.

Ardena’s mRNA Stability Expertise at Oss

Ardena’s formulation team at Oss develops and executes stability programmes for mRNA LNP products, including mRNA integrity and potency testing alongside physicochemical CQA monitoring. The site has minus 20 and minus 80 degree Celsius stability storage, and the team has experience designing stability protocols that satisfy both FDA and EMA expectations for mRNA drug substance and drug product.

Two Goals That Sound the Same but Are Not

Topical and transdermal are often used interchangeably. They should not be.

A topical formulation delivers drug to the skin or the tissue beneath it. The goal is local effect: treat a skin condition, reduce inflammation in a joint just below the surface, or act on target tissue within the accessible layers of the dermis. Systemic exposure is often undesirable and should be minimised.

A transdermal formulation uses the skin as a route of administration to the systemic circulation. The goal is to bypass the gastrointestinal tract, avoid first-pass metabolism, and deliver drug at a controlled rate into the bloodstream. Systemic exposure is exactly what you want.

The formulation strategies, the regulatory requirements, and the development challenges are substantially different for each. Getting them confused at the start of a programme creates problems that are expensive to fix later.

The Skin Barrier: The Common Challenge

Whether you want to keep the drug local or get it into the blood, you have to deal with the stratum corneum. This outermost layer of skin, roughly 10 to 20 micrometres of densely packed dead keratinocytes embedded in a lipid matrix, is one of the most effective biological barriers in the body. It evolved to keep the outside world out. That function works against topical and transdermal drug delivery alike.

Flux through the stratum corneum follows Fick’s law of diffusion: it is proportional to the concentration gradient, the diffusivity of the drug in the membrane, and the membrane area, and inversely proportional to the membrane thickness. Drugs with low molecular weight (below approximately 500 Daltons), intermediate lipophilicity (log P between 1 and 4), and low melting point tend to permeate best. Most pharmaceutically active molecules do not meet all three criteria, which is why formulation science is required to make topical and transdermal delivery work for most drugs.

Formulation Types and Their Clinical Applications

Formulation Type	Typical Clinical Use	Key Formulation Consideration
Creams (O/W or W/O emulsion)	Topical dermatology; anti-inflammatory; antifungal	Emulsion stability; preservative system; skin tolerability; drug partitioning into aqueous or oil phase
Ointments (oil-based or hydrocarbon)	Chronic skin conditions; wound healing; occlusive therapy	High oil content increases skin hydration and drug absorption; patient acceptability vs efficacy trade-off
Gels (hydrophilic polymer-based)	Topical NSAIDs; acne; hormonal gel formulations	Drug solubility in aqueous gel base; alcohol content and skin tolerability; spreading and drying on application
Patches (transdermal)	Systemic delivery of analgesics, hormones, antiemetics, smoking cessation agents	Membrane-controlled or matrix-controlled release; adhesive compatibility with drug; skin irritation from prolonged contact
Lotions (low-viscosity emulsion)	Large-body-surface-area application; hair loss; mild skin conditions	Ease of application; evaporation of vehicle after application; residue on skin or clothing
Foams (pressurised aerosol)	Scalp conditions; dermatology; rectal applications	Propellant selection; drug stability in pressurised container; foaming behaviour on skin

Permeation Enhancers: Helping the Drug Cross the Barrier

Chemical penetration enhancers work by temporarily disrupting the stratum corneum lipid structure, increasing the fluidity of the lipid bilayers and creating pathways for drug diffusion. Common classes include alcohols, fatty acids, terpenes, and surfactants. The challenge is selectivity: an enhancer powerful enough to meaningfully increase drug flux often causes skin irritation, limiting the concentration and contact time that patients will tolerate.

Formulation development for topical products with a permeation enhancer must balance in vitro flux data against skin tolerability assessment. An enhancer that doubles flux but causes erythema in 20% of patients is not a development success.

Regulatory and Bioequivalence Considerations

For generic topical products, demonstrating bioequivalence is more complex than for oral dosage forms. A simple pharmacokinetic study is often not appropriate because systemic exposure is low and local bioavailability in the skin is what matters clinically. The FDA’s product-specific guidance documents for generic topical products describe the expected bioequivalence study designs, which may include in vitro release testing (IVRT), tape stripping studies to measure drug concentration in the stratum corneum, or clinical endpoint studies.

Ardena’s Topical Formulation Capabilities

Ardena’s formulation team has experience developing semi-solid and topical dosage forms for both branded and generic programmes. Development work includes formulation screening, in vitro permeation studies using Franz diffusion cells, and stability testing of semi-solid systems to ICH guidelines. For programmes requiring regulatory bioequivalence strategies, the team can advise on the appropriate study design based on the FDA and EMA product-specific expectations.

Why the Lung Is Worth the Complexity

The lungs offer a drug delivery surface area of roughly 70 to 100 square metres, a thin alveolar membrane, rich vasculature, and, for respiratory conditions, direct access to the target tissue. For drugs targeting the lung, inhalation delivers drug precisely where it is needed, at doses a fraction of what would be required orally. For systemic delivery, the pulmonary route bypasses first-pass metabolism and produces rapid onset of action.

The price of those advantages is formulation complexity. The aerosol must be generated in a particle size range that reaches the target region of the lung: too large and it deposits in the throat; too small and it is exhaled. The formulation must be stable in the device, the drug must not irritate the airways, and the device and the formulation must work together as an integrated system. None of that is straightforward.

The Three Primary Inhaled Delivery Systems

Dry Powder Inhalers (DPIs)

DPIs deliver a dry powder formulation that is aerosolised by the patient’s inspiratory effort. The powder is typically a blend of micronised API and a carrier (most commonly lactose), where the carrier particles are large enough to be retained in the throat while the drug particles detach and penetrate into the lung.

The critical particle size range for deep lung deposition is 1 to 5 micrometres. Achieving this requires micronisation of the API, usually by jet milling, to a consistent and controlled particle size distribution. The surface properties of the micronised particles, which affect their tendency to agglomerate and their detachment from the carrier under airflow, are as important as the particle size itself.

Metered Dose Inhalers (MDIs)

MDIs deliver a metered dose of drug dissolved or suspended in a liquefied propellant, typically HFA 134a or HFA 227ea following the phase-out of CFCs. When the canister is actuated, the propellant evaporates rapidly after the valve opens, atomising the drug into an aerosol.

Formulation challenges for MDIs include drug solubility or suspension stability in the propellant, co-solvents and surfactants to assist solubilisation, and protection of the formulation against valve and canister interactions over the product shelf life. Suspension MDIs require controlled particle size in the formulation to ensure dose uniformity and appropriate aerosol performance.

Nebulisers

Nebulisers convert a liquid drug solution or suspension into an aerosol using jet, ultrasonic, or vibrating mesh technology. They are predominantly used in hospital settings or for patients unable to use DPIs or MDIs, including infants and severely compromised patients.

The formulation requirements for nebuliser use are primarily those of a sterile or microbiologically controlled solution: appropriate pH, tonicity, and absence of irritants. The nebuliser device drives the aerosol characteristics rather than the formulation, which simplifies the formulation challenge but introduces a device variable that affects clinical performance.

Key Characterisation Parameters for Inhaled Products

Parameter	Why It Matters	Measurement Method
Aerodynamic particle size distribution (APSD)	Determines lung deposition pattern; particles must be 1-5 micrometres for deep lung delivery	Next Generation Impactor (NGI) or Andersen Cascade Impactor (ACI); required for all inhaled product registration
Fine Particle Fraction (FPF)	Proportion of dose in respirable size range; key efficacy indicator	Derived from APSD data; particles below 5 micrometres as fraction of total emitted dose
Mass median aerodynamic diameter (MMAD)	Central tendency of aerodynamic particle size distribution	Derived from APSD data; should be 1-3 micrometres for peripheral lung targeting
Emitted dose and dose uniformity	Total dose leaving the device; must be consistent across the life of the product	Dose uniformity testing by actuation throughout container life; USP/Ph. Eur. specifications
Moisture content (DPI)	Affects powder flowability, aerosolisation efficiency, and drug stability	Karl Fischer titration or gravimetric analysis; critical for DPI performance and stability

Regulatory Considerations for Inhaled Products

Inhaled drug products are subject to specific regulatory guidance that goes beyond standard pharmaceutical development requirements. In the US, the FDA’s guidance on MDIs and DPIs describes the CMC and bioequivalence study requirements in detail. In Europe, the EMA’s guidelines on oral inhalation and nasal drug products set similar expectations. A key feature of both sets of guidance is that the device and the formulation are assessed as an integrated combination product, not separately.

Inhaled Formulation Development at Ardena

Ardena’s formulation and analytical teams have experience developing inhaled drug products including DPI powder blends and nebuliser solutions. The team provides particle size characterisation, APSD measurement using cascade impaction, and stability testing designed to meet ICH and product-specific regulatory requirements. For programmes combining inhaled formulation with HPAPI handling requirements, the site capabilities at Pamplona are available for contained processing of potent inhaled APIs.

Excipients Are Not Passive

The word inactive ingredient implies that excipients do nothing. That is a convenient regulatory classification, not a pharmacological reality.

Excipients interact with APIs chemically. They compete for water. They affect the pH microenvironment within a tablet. They adsorb onto drug particles and change their surface properties. They can accelerate degradation pathways, inhibit dissolution, or modulate the release of a drug in ways that were not designed and were not tested. The excipient that looked innocuous in a pre-formulation compatibility screen can be the root cause of a stability failure three years into the programme.

Getting excipient selection right early is one of the highest-value investments in pharmaceutical formulation development. Changing an excipient late in development, when stability data has been generated and the CMC package is being assembled, is expensive and sometimes programme-threatening.

The Excipient Compatibility Screening Process

Before a formulation is finalised, the API should be assessed for compatibility with each candidate excipient under stressed conditions. The standard approach is to prepare binary mixtures of API and each excipient at a defined ratio, store them under accelerated conditions (typically 40 degrees Celsius at 75% relative humidity), and analyse them at defined timepoints for changes in appearance, moisture uptake, and chemical purity by HPLC.

A compatibility issue, visible as increased degradation in the API-excipient mixture relative to the API alone, is a signal to investigate further before including that excipient in the formulation. It is not always a reason to exclude it; sometimes the incompatibility is concentration-dependent, or is manageable with a different ratio or processing approach. But it needs to be understood.

Key Excipient Categories and Their Common Interaction Risks

Excipient Category	Common Examples	Known Interaction Risks with APIs
Fillers/diluents	Lactose, microcrystalline cellulose, mannitol, calcium phosphate	Lactose Maillard reaction with primary amines; calcium phosphate may affect absorption of some drugs
Binders	HPMC, PVP, povidone, starch	PVP peroxide content can oxidise sensitive APIs; HPMC may affect dissolution through viscosity in some formulations
Disintegrants	Croscarmellose sodium, sodium starch glycolate, crospovidone	Sodium starch glycolate has residual sodium content; cross-linking agents may affect hygroscopicity
Lubricants	Magnesium stearate, stearic acid, sodium stearyl fumarate	Magnesium stearate can form salts with acidic APIs; over-lubrication delays dissolution; hydrophobic film on granule surface
Coatings	HPMC, Opadry systems, Eudragit grades	Plasticiser migration into core; coating solvent residuals; Eudragit ionic interaction with drug under specific pH conditions
Preservatives	Benzyl alcohol, phenol, parabens	Benzyl alcohol contraindicated in neonates; parabens estrogenic activity concern; phenol incompatible with some biologics
Surfactants/solubilisers	SLS, polysorbate 80, Cremophor EL	SLS promotes degradation of some APIs via radical mechanism; polysorbate oxidation products can degrade susceptible drugs

Regulatory Considerations: Novel vs. Established Excipients

Excipients with an established history of use in approved drug products benefit from a well-understood safety and regulatory profile. Novel excipients, those not previously used or used at significantly higher levels than in approved products, require a more comprehensive safety justification in the regulatory dossier. The FDA’s inactive ingredients database is the primary reference for established excipient use in US-approved products, and the EMA’s list of excipients with established function serves an equivalent role in Europe.

For parenteral and ophthalmic products, excipient safety standards are higher than for oral products because the biological barriers that would otherwise limit systemic exposure are bypassed. An excipient that is acceptable in an oral tablet at a given concentration may not be acceptable in an injectable product at the same concentration.

The Cost of Getting It Wrong Late

Excipient changes after a stability programme has been initiated require an assessment of whether the change is significant enough to trigger new stability studies. In the worst case, a late excipient change resets the stability clock entirely, adding 12 or more months to the timeline before a regulatory filing can proceed.

Even a change that does not require new stability studies requires a change control, a regulatory assessment, and documentation that the change was justified and controlled. None of this is insurmountable, but all of it takes time that was not planned for.

Ardena’s Excipient Compatibility Expertise

Ardena’s pre-formulation team at Ghent conducts excipient compatibility screening as a standard element of the pre-formulation programme, using stressed storage conditions and HPLC stability-indicating methods. Findings are interpreted in the context of the formulation design and the intended regulatory strategy, ensuring that excipient decisions are made with full visibility of their downstream consequences.

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.

Why Patients Want Prefilled Syringes

Ask a patient which they would prefer for a monthly self-injection: a glass vial, a needle, and a syringe they have to assemble themselves, or a prefilled autoinjector they click and hold against their arm. The answer is obvious.

Prefilled syringes and autoinjectors have become the standard delivery format for biologics intended for self-administration. They reduce administration errors, improve patient adherence, and eliminate the multi-step preparation that creates both errors and anxiety. For many biologic drug products, the prefilled syringe is not a convenience upgrade; it is a commercial requirement.

The formulation challenge is that the syringe barrel is not a passive container. It is a component that interacts with the drug product. Getting that transition right takes careful development work.

The Key Compatibility Challenges

Silicone Oil

Glass syringes are siliconised to allow the plunger to slide smoothly during injection. The silicone oil droplets that migrate into the drug solution are a known source of protein aggregation for biologic drug products. High shear at the needle tip during injection can further fragment silicone droplets and increase the surface area available for protein adsorption.

The risk is managed by selecting syringe formats with optimised siliconisation (cross-linked silicone; baked-on silicone coatings) and by demonstrating in formulation development that the drug product is tolerant to the silicone levels expected in the chosen syringe. For highly aggregation-prone proteins, silicone-free syringe options including cyclic olefin polymer (COP) or cyclic olefin copolymer (COC) barrels may be required.

Tungsten Residues

Glass syringes are formed using tungsten pins to create the needle tip geometry. Residual tungsten at low microgram levels can remain in the syringe barrel after manufacture and can interact with some proteins to induce aggregation or precipitation. Tungsten compatibility must be assessed during development by incubating the drug product in the intended syringe format and monitoring for aggregation by DLS or SEC.

Extractables and Leachables

Every component that contacts the drug product, the syringe barrel, the rubber plunger stopper, the tip cap, and the needle, can contribute extractable compounds that migrate into the solution over the shelf life of the product. For prefilled syringes, the longer contact time and the absence of a separate packaging step mean the extractables profile is more complex than for a vial-stopper system.

The PQRI leachables guidance for parenteral and ophthalmic drug products and the ICH Q3E guideline on extractables and leachables provide the framework for extractables and leachables assessment. A complete E&L programme for a prefilled syringe system is a significant analytical undertaking that must be planned well in advance of the registration-stage stability studies.

Formulation Parameters That Affect PFS Performance

Parameter	Effect in PFS Format	Development Consideration
Viscosity	High viscosity formulations require greater injection force; affects autoinjector spring design and patient experience	Target viscosity below 20 cP for standard autoinjectors; reformulate or dilute if above 50 cP
pH and buffer	Plunger stopper rubber contains components that can affect pH; buffer capacity must maintain pH over shelf life	Confirm pH stability in the syringe over proposed shelf life; select low-extractables stoppers
Surfactant type and concentration	Surfactants protect against agitation-induced aggregation; must be compatible with syringe components	Polysorbate 20 or 80 standard; confirm compatibility with silicone and rubber components
Protein concentration	Higher concentrations increase aggregation risk from silicone and tungsten interactions	Assess aggregation at target concentration in the syringe format during compatibility studies
Preservatives	Multi-dose PFS may require preservative; single-dose typically preservative-free	Benzyl alcohol and phenol are common; confirm antimicrobial efficacy and compatibility with drug

Device Development: The Formulation-Device Interface

The autoinjector or pen injector selected for a self-injectable drug product must be compatible with the formulation parameters, particularly viscosity, fill volume, and injection speed. These are not independent design choices. A formulation with a viscosity of 40 cP will require a different spring force than one at 5 cP, which changes the injection duration and the force felt by the patient.

Device selection should happen in parallel with formulation development, not after it. The target product profile for the device and the formulation should be established together, with viscosity, fill volume, and container closure system selected as a package.

Ardena’s Injectable Formulation Capabilities

Ardena’s formulation and analytical teams at Ghent support injectable drug product development for both vial and prefilled syringe formats, including compatibility assessment, extractables and leachables planning, and GMP fill-finish of clinical batches. The team has experience with complex biologic formulations and the specific challenges that arise in the transition from vial to PFS.

Two Routes to the Same Goal

When a BCS Class II molecule needs bioavailability enhancement and simpler approaches such as particle size reduction or salt selection have not provided a sufficient solution, amorphous solid dispersions are often the next logical step. The two dominant manufacturing technologies for pharmaceutical ASDs are hot melt extrusion (HME) and spray drying, and both have a strong track record of producing commercially approved products.

The choice between them is not arbitrary. Each technology imposes different constraints on the API, the polymer, and the downstream processing, and the right choice depends on the physicochemical properties of the molecule, the intended commercial manufacturing scale, and the regulatory pathway. Making this decision early, at the pre-formulation or early formulation stage, avoids late-stage redevelopment.

How Each Technology Works

Hot Melt Extrusion (HME)

In HME, the API and polymer are blended as dry powders and fed into a heated extruder barrel where the polymer melts and the API dissolves or disperses within the polymer melt. The twin-screw extruder applies both heat and mechanical shear to the material, facilitating mixing and dispersion at the molecular level. The molten extrudate is cooled and then processed into the desired particle size, typically by milling or pelletisation, before being formulated into tablets or capsules.

HME is a continuous, solvent-free process, which is an advantage both for manufacturing efficiency and for regulatory simplicity around residual solvent control. The absence of organic solvents also makes it more straightforward to implement in a GMP environment from an occupational health and environmental perspective.

Spray Drying

In spray drying, the API and polymer are co-dissolved in a common organic solvent (or solvent mixture) and the solution is atomised into a heated drying chamber where the solvent evaporates rapidly, leaving behind solid particles of the API-polymer dispersion. The particle size of the spray-dried intermediate is controlled by atomisation conditions including feed rate, atomiser type, and inlet temperature.

Spray drying is a batch or semi-continuous process that requires solvent handling, solvent recovery, and residual solvent control to below ICH Q3C limits. However, it is applicable to a wider range of APIs and polymers than HME because it does not require the API to be thermally stable at the processing temperatures needed to melt the polymer.

Head-to-Head Comparison

Selection Factor	Hot Melt Extrusion	Spray Drying
API thermal stability	Required: API must be stable at polymer melt temperature (typically 100-200 degrees C)	Not required: API only exposed to moderate temperatures during solvent evaporation
API solubility in polymer	Good molecular mixing requires API to dissolve in or be miscible with polymer melt	API and polymer must share a common solvent; molecular solubility in polymer less critical
Polymer selection	Must melt at processable temperatures; HPMC-AS and PVPVA are less suited to HME due to high Tg	Broad polymer compatibility; HPMC-AS, PVPVA, PVP-K, Eudragit all spray-dried commercially
Solvent use	Solvent-free; no residual solvent concern	Requires solvent handling, recovery, and ICH Q3C compliance
Drug loading	Typically limited by melt viscosity and extruder torque; often 10-40% w/w	Broader range achievable; depends on API/polymer solubility in solvent
Scale-up	Highly scalable; continuous processing; well-established at commercial scale	Well-established at commercial scale; batch or semi-continuous
Regulatory precedent	Extensive; Kaletra (lopinavir/ritonavir), Noxafil (posaconazole)	Extensive; Zelboraf (vemurafenib), Incivek (telaprevir)
Best suited for	Thermally stable APIs; solvent-free preference; continuous manufacturing	Thermally labile APIs; broad polymer choice; rapid early-stage screening

Making the Decision in Practice

For molecules with good thermal stability and compatibility with HME-processable polymers, HME offers a cleaner manufacturing process with no solvent handling overhead and strong scale-up credentials. For thermally labile molecules, or programmes where early-stage feasibility screening across a broad polymer matrix is the priority, spray drying is typically the first technology evaluated.

In practice, the two technologies are often evaluated in parallel at the screening stage, and the selection is made based on the comparative performance data, manufacturability assessment, and the commercial manufacturing infrastructure available at the intended development and manufacturing partner.

Ardena’s ASD Manufacturing Capabilities

Ardena operates spray drying and hot melt extrusion capabilities at its Somerset, New Jersey facility and at Pamplona (Idifarma) in Spain. Both sites can support ASD development from early screening through to GMP clinical batch manufacture. The formulation development teams at these sites have extensive experience with the full range of ASD-relevant polymers and can advise on technology selection based on the API’s physicochemical profile and the programme’s development goals.

Not Every Solubility Problem Needs a Complex Solution

When a development team encounters a BCS Class II molecule with poor aqueous solubility, the instinct is often to reach for the most technically sophisticated tool available: an amorphous solid dispersion by hot melt extrusion or spray drying, or a lipid-based formulation system. These technologies are powerful, but they are also complex, expensive, and demanding in terms of development time and manufacturing infrastructure.

A nanosuspension, a stabilised colloidal dispersion of drug nanocrystals in an aqueous medium, offers a different proposition. It keeps the API in its crystalline form, which means no stability concerns around amorphous recrystallisation. It improves dissolution rate through surface area enhancement, without requiring a polymer matrix, a solvent system, or a high-shear processing step. For the right molecule, it is genuinely the simplest route to adequate oral bioavailability.

The Science of Dissolution Enhancement by Nanosuspension

The Noyes-Whitney equation tells us that dissolution rate is proportional to surface area. Reducing a drug particle from a D90 of 100 micrometres to a D90 of 200 nanometres increases the specific surface area by approximately a factor of 500, producing a proportional increase in intrinsic dissolution rate. For BCS Class II molecules where dissolution is the rate-limiting step in absorption, this translates into a meaningful increase in Cmax and AUC relative to a conventional micronised formulation.

Nanosuspensions can also achieve a modest increase in apparent solubility relative to the bulk crystalline form through the Ostwald-Freundlich effect, which predicts an increase in solubility for very small particles due to the increased surface free energy. This effect is generally modest for particles above 100 nanometres, and the primary driver of bioavailability improvement in pharmaceutical nanosuspensions is dissolution rate enhancement rather than solubility enhancement.

Nanosuspension vs. ASD: Choosing the Right Approach

Factor	Nanosuspension	Amorphous Solid Dispersion (ASD)
API solid state	Crystalline throughout	Amorphous; stability risk
Solubility enhancement mechanism	Dissolution rate (surface area); modest Ostwald-Freundlich effect	Supersaturation; higher apparent solubility; requires precipitation inhibitor
Magnitude of enhancement	Moderate; typically 2-10 fold improvement in AUC vs micronised	Can be substantial; 10-100 fold for low-solubility molecules
Development complexity	Lower; no polymer excipient system required	Higher; polymer selection, drug-polymer miscibility, downstream processing
Manufacturing equipment	Wet milling or media milling; established technology	Spray drying or HME; specialist equipment
Physical stability	High; crystalline API is thermodynamically stable	Lower; amorphous conversion risk requires careful packaging and storage
Best suited for	Moderate solubility gap; stable crystalline API; simple downstream processing	Severe solubility gap; molecule responds well to supersaturation strategy

Critical Formulation Considerations for Nanosuspensions

Stabiliser Selection

Nanosuspensions are thermodynamically unstable systems that will aggregate if not adequately stabilised. Stabilisers adsorb to the particle surface and prevent particle-particle contact by steric or electrostatic mechanisms. Common pharmaceutical stabilisers for nanosuspensions include HPMC, HPMC-AS, poloxamer 188, poloxamer 407, and polysorbate 80. The selection of stabiliser type and concentration is empirical and depends on the surface chemistry of the specific API.

Milling Parameters

For wet-milled nanosuspensions, the target particle size and size distribution are controlled by milling time, bead size and loading, mill speed, and temperature. Milling at elevated temperature can lead to polymorphic conversion in sensitive molecules, making temperature monitoring and control during milling important. The milling process must be developed and characterised to define the process parameters that reproducibly achieve the target particle size specification.

Downstream Conversion to Solid Dosage Form

Most clinical and commercial nanosuspension products are converted from the liquid nanosuspension intermediate into a solid dosage form, typically a tablet or capsule, by spray drying, fluid bed granulation, or lyophilisation. The conversion process must maintain the nanoparticle size distribution: if particles aggregate during drying or granulation, the bioavailability advantage is lost. Redispersibility testing of the final solid dosage form in simulated gastrointestinal fluids is an important part of the characterisation package.

Ardena’s Nanosuspension Development Capabilities

Ardena’s nanomedicine team at Oss has wet milling and media milling capability for nanosuspension development at both screening and scale-up. Particle size characterisation by DLS and laser diffraction, stabiliser screening, and downstream conversion studies are conducted as part of integrated nanosuspension development programmes.

The Case for Sustained Release at the Nanoscale

Many therapeutic molecules have pharmacokinetic profiles that are poorly suited to their intended use. A drug that clears from circulation within a few hours may need to be dosed multiple times daily to maintain therapeutic concentrations, creating peaks and troughs in exposure that lead to toxicity at Cmax and loss of efficacy at Ctrough. For chronic conditions requiring long-term treatment, the dosing burden affects patient adherence and quality of life.

Polymeric nanoparticles address this by encapsulating the drug within a biodegradable polymer matrix that releases its payload gradually as the polymer degrades or the drug diffuses through the matrix wall. Release profiles ranging from days to weeks can be engineered by selecting the polymer composition, molecular weight, and drug loading, providing a level of pharmacokinetic control that is difficult to achieve with conventional extended-release oral formulations for many drug classes.

PLGA: The Workhorse Polymer for Sustained Release Nanoparticles

Poly(lactic-co-glycolic acid) (PLGA) is the most widely used biodegradable polymer in pharmaceutical nanoparticle formulations. It is approved for use in parenteral drug products, has a well-characterised safety profile, and degrades in vivo by hydrolysis to lactic acid and glycolic acid, both naturally occurring metabolic products. The FDA’s guidance on biodegradable drug products provides the framework for the characterisation and safety data required for PLGA-based products.

The degradation rate of PLGA depends on the ratio of lactic to glycolic acid monomer units, the molecular weight of the polymer, and the end-group chemistry. High glycolic acid content and low molecular weight produce faster degradation; high lactic acid content and high molecular weight produce slower degradation. This tunability allows the release profile to be tailored across a wide range by selecting the appropriate PLGA grade.

Release Mechanisms in Polymeric Nanoparticles

Release Mechanism	How It Works	Polymer System	Typical Release Profile
Surface erosion	Polymer degrades from outside in; drug released as surface layers are removed	Polyanhydrides; surface-eroding PLGA formulations	Near zero-order release; sustained over weeks
Bulk erosion	Polymer absorbs water and degrades throughout the matrix simultaneously	PLGA (predominant mechanism); PLA	Biphasic: initial burst followed by sustained release as matrix erodes
Diffusion	Drug diffuses through the intact polymer matrix to the surface	Dense polymer cores; crosslinked hydrogels	Rate depends on drug size, matrix porosity, and diffusion path length
Swelling and diffusion	Polymer swells on water uptake; drug diffuses through swollen matrix	HPMC; polyacrylates; hydrophilic matrices	Relatively constant release rate; suitable for hydrophilic drugs

Formulation Variables That Control Release Rate

Polymer Molecular Weight and Composition

As discussed above, PLGA molecular weight and L:G ratio are the primary handles for controlling the bulk degradation rate and thus the drug release profile. A PLGA 50:50 (equal lactic and glycolic acid) degrades significantly faster than a PLGA 75:25 or PLGA 85:15. Within a given composition, higher molecular weight polymers degrade more slowly. Matching polymer selection to the target release duration is the starting point for any polymeric nanoparticle formulation design.

Drug Loading and Drug-Polymer Interactions

Higher drug loading can increase the initial burst release if the drug is present at the particle surface, and can affect matrix integrity and degradation rate if the drug-polymer interaction alters the physical properties of the matrix. Drug loading is typically optimised alongside release profile in early development to identify the combination that achieves the target exposure profile without compromising physical stability or manufacturability.

Particle Size

Smaller particles have a larger surface area to volume ratio, which generally increases the rate of surface-mediated drug release and can enhance the initial burst. For sustained release applications where a prolonged low-level release is desired, larger particles or core-shell architectures may better suppress the burst release component.

Analytical Characterisation of Polymeric Sustained Release Nanoparticles

In addition to the standard nanoparticle CQAs of particle size, PDI, and zeta potential, sustained release polymeric nanoparticle products require drug content assay, in vitro release testing, and polymer characterisation including molecular weight distribution by gel permeation chromatography (GPC). In vitro release testing for parenteral sustained release nanoparticles is technically challenging because standard dissolution apparatus is not designed for nanoparticle systems; sample and separate methods, membrane diffusion cells, and dialysis-based approaches are all used, and the choice of method affects the release profile observed.

Ardena’s Polymeric Nanoparticle Capabilities at Oss

Ardena’s nanomedicine team at Oss has formulation expertise in polymeric nanoparticle systems including PLGA, PLA, and other biodegradable polymer platforms. Development programmes include polymer selection, drug loading optimisation, in vitro release method development, and physicochemical characterisation using DLS, NTA, and GPC.