How Can You Increase Solubility: Strategies for Drug Formulation

The short answer: you can increase solubility by modifying the drug's physical form, adjusting formulation chemistry, and selecting the right delivery vehicle — with gelatin capsules playing a critical enabling role throughout. Poor aqueous solubility affects roughly 40% of marketed drugs and nearly 90% of compounds in discovery pipelines, making solubility enhancement one of the most commercially and scientifically significant challenges in pharmaceutical science today. This article walks through every major strategy with real data so you can evaluate which approach fits your compound, your manufacturing capabilities, and your timeline.

Solubility — formally defined as the maximum amount of a substance that dissolves in a given quantity of solvent at a specified temperature — directly determines bioavailability. A drug that cannot dissolve cannot be absorbed. The BCS (Biopharmaceutics Classification System) labels compounds with low solubility as Class II or Class IV, and formulators spend enormous effort converting these into clinically viable products. Understanding the physical chemistry underpinning each technique is just as important as knowing which technique exists.

Why Solubility Is the Dominant Formulation Challenge

Modern drug discovery has shifted heavily toward lipophilic, high-molecular-weight compounds that bind tightly to hydrophobic target sites. The trade-off is predictable: compounds that fit well into protein binding pockets tend to be poorly water-soluble. The mean logP (partition coefficient) of launched drugs has crept upward over the past three decades, and the pharmaceutical industry estimates that solubility problems add between $1–3 billion in development cost per failed candidate, not counting opportunity costs.

Aqueous solubility below 100 µg/mL is generally considered the threshold where active formulation strategies become necessary. Below 10 µg/mL, standard approaches like simple milling or pH adjustment are rarely sufficient, and technologies such as amorphous solid dispersions or lipid-based formulations in gelatin capsule shells become the primary options. Below 1 µg/mL, nanoparticle engineering or complexation chemistry is typically required.

The distribution above reflects estimates from WHO and FDA review data on marketed oral pharmaceuticals. Class II compounds — low solubility, high permeability — represent the largest segment and the area where most solubility-enhancement work is focused. These are also the compounds most commonly delivered in hard or soft gelatin capsule formulations following solubility improvement.

Particle Size Reduction: Micronization and Nanonization

Decreasing particle size increases surface area exposed to solvent, directly accelerating dissolution rate according to the Noyes-Whitney equation: dC/dt = DA(Cs − C)/h. Reducing diameter from 100 µm to 1 µm increases surface area by a factor of 100, and reducing it further to 100 nm increases surface area by 1,000-fold relative to the original.

Micronization (1–100 µm range)

Jet milling and ball milling can reduce API particles to the 1–10 µm range. This is sufficient for many BCS Class II compounds and is routinely used for drugs like griseofulvin, whose bioavailability increased by over 50% after micronization compared to the unmilled form. Micronized material is often filled directly into hard gelatin capsules or blended with excipients and then encapsulated, making capsule shell compatibility an important consideration.

Nanonization (100–1,000 nm range)

Below approximately 1 µm, the Ostwald-Freundlich equation predicts that particle curvature increases effective solubility — sometimes called the Kelvin effect. Nanoparticles in the 200–600 nm range can show 2–10× higher apparent solubility than the bulk crystalline material. Technologies include media milling (NanoCrystal® technology), high-pressure homogenization, and wet bead milling. Commercial examples include Rapamune (sirolimus), Emend (aprepitant), and Tricor (fenofibrate). Nanosuspensions are stabilized with polymers such as HPMC or PVP and surfactants, then filled into gelatin capsule shells or processed into tablets.

The chart above illustrates the nonlinear acceleration in dissolution rate as particle size decreases. Note the dramatic jump below 1 µm — this is where the Kelvin effect begins to contribute meaningfully alongside the surface area effect. The data are derived from aggregated experimental literature across BCS Class II model compounds including fenofibrate, griseofulvin, and itraconazole.

Amorphous Solid Dispersions: Converting Crystal to Glass

The crystalline state is thermodynamically stable but poorly soluble. The amorphous state lacks long-range molecular order, resulting in higher internal energy and, consequently, higher apparent solubility — sometimes 10–1,600× above the crystalline equilibrium solubility, depending on the compound. The challenge is that amorphous materials tend to recrystallize over time, losing their solubility advantage.

Amorphous solid dispersions (ASDs) address this by dispersing the amorphous API molecularly within a polymer matrix — typically HPMC-AS (hypromellose acetate succinate), PVP-VA (polyvinylpyrrolidone-vinyl acetate), or HPMC. The polymer inhibits recrystallization by raising the glass transition temperature (Tg) of the system and forming hydrogen bonds with the drug. A rule of thumb is that the Tg of the final ASD should exceed 50°C above the intended storage temperature, meaning a Tg of at least 70–80°C for room-temperature storage.

Manufacturing methods include hot melt extrusion (HME) and spray drying. HME requires thermal stability of the drug (processing temperatures of 120–180°C are common), while spray drying is preferred for thermolabile compounds. The resulting solid dispersion powder is typically filled into hard gelatin capsules or compressed into tablets. Several blockbuster drugs rely on this platform: Kaletra (lopinavir/ritonavir) uses HME, while Zelboraf (vemurafenib) uses a microprecipitated bulk powder (MBP) technology.

Cyclodextrin Complexation: Molecular Encapsulation

Cyclodextrins (CDs) are cyclic oligosaccharides with a hydrophilic outer surface and a hydrophobic interior cavity. Drug molecules that fit into this cavity form inclusion complexes, with the hydrophilic shell dramatically improving apparent aqueous solubility. The cavity diameter of β-cyclodextrin is approximately 6.0–6.5 Å, which accommodates many aromatic drug molecules.

HP-β-CD (hydroxypropyl-β-cyclodextrin) is the most widely used pharmaceutical-grade cyclodextrin, approved by the FDA and EMA in oral, injectable, and nasal formulations. Complexation can increase solubility by 5–5,000 fold depending on the binding constant (Ka) and the compound's lipophilicity. Commercial examples include Sporanox oral solution (itraconazole/HP-β-CD) and Vfend IV (voriconazole/SBE-β-CD).

For solid oral dosage forms, the drug-CD complex powder is commonly filled into hard gelatin capsule shells, particularly when the complex is hygroscopic or when tabletability is poor. The gelatin capsule provides a physical barrier against moisture uptake, which is important because cyclodextrin complexes can decomplex if water activity rises during storage.

Note the log scale — HP-β-CD and SBE-β-CD dominate pharmaceutical use because of their superior solubilizing capacity and established regulatory acceptance. Methyl-β-CD is a potent solubilizer but has cytotoxicity concerns limiting its use in parenteral routes.

Lipid-Based Drug Delivery Systems and the Gelatin Capsule Connection

Lipid-based formulations (LBFs) exploit the body's natural fat digestion pathways to solubilize lipophilic drugs. Rather than forcing the drug to dissolve in aqueous medium before absorption, LBFs keep the drug dissolved in a lipid or surfactant matrix, presenting it to the intestinal epithelium in a form that is immediately available for uptake via micellar or vesicular transport.

The Lipid Formulation Classification System (LFCS) organizes these formulations into four types based on composition:

• Type I: Simple oils (triglycerides) — no surfactant. Simplest but requires digestion for drug release. Example: vitamin E in corn oil.
• Type II (SEDDS): Oil + surfactant (HLB < 12). Self-emulsifying drug delivery systems that form coarse emulsions on contact with water. Droplet size ~100–300 µm.
• Type III (SMEDDS/SNEDDS): Oil + surfactant + co-solvent. Form fine (100–250 nm) or nanoemulsions (<100 nm) spontaneously. Cyclosporine (Neoral) is the prototype SMEDDS.
• Type IV: Surfactant/co-solvent mixture without significant oil. Highly water-miscible, fast drug release, sensitive to dilution effects.

The essential delivery vehicle for liquid and semisolid LBFs is the soft gelatin capsule (softgel). The soft gelatin capsule shell — composed of gelatin, plasticizer (glycerin or sorbitol), and water — provides a hermetic seal around the liquid fill, preventing leakage, oxidation, and moisture ingress. Cyclosporine (Neoral), saquinavir (Fortovase), and isotretinoin (Accutane) are all classic examples of high-value lipophilic drugs formulated in soft gelatin capsule dosage forms.

For semisolid fills — materials that are solid or paste at room temperature but melt at body temperature — hard gelatin capsules (two-piece) are also widely used via hot-fill or thermosoftening processes. This eliminates the need for the specialized rotary die machines required for softgel manufacturing, reducing capital cost significantly.

LBFs can boost bioavailability dramatically. Cyclosporine's oral bioavailability rose from approximately 30% (Sandimmune corn oil capsule) to nearly 60% (Neoral microemulsion softgel), with reduced food effect and improved dose linearity. This is a real-world demonstration of how selecting the right lipid vehicle and combining it with an appropriate gelatin capsule shell can solve a commercially critical bioavailability problem.

pH Manipulation and Salt Selection

For ionizable compounds, solubility depends strongly on pH via the Henderson-Hasselbalch relationship. A weak acid with a pKa of 4.5 will have solubility increase approximately 10-fold for every unit of pH increase above its pKa. A weak base shows the inverse: solubility increases as pH decreases below the pKa. This pH-solubility relationship is why most acidic drugs show higher gastric dissolution at low pH, while basic drugs dissolve readily in stomach acid but may precipitate in the neutral intestine.

Salt formation is the most widely used solubility-enhancement strategy for ionizable drugs — used in approximately 50% of marketed drug products. Common salt-forming counterions for acids include sodium, potassium, calcium, and meglumine; for bases, hydrochloride, sulfate, mesylate, and maleate are most common. Salt forms can show 10–1,000× higher intrinsic dissolution rates compared to the free acid or base.

However, salt selection must account for the "pH-solubility minimum" phenomenon (common ion effect) and potential conversion back to the free acid/base in GI fluids. Formulating the optimum salt form in a hard gelatin capsule with appropriate buffering excipients (e.g., citric acid or sodium bicarbonate) creates a favorable microenvironmental pH inside the capsule that preserves the solubility advantage through dissolution.

Cocrystal Engineering

Pharmaceutical cocrystals are multi-component crystals containing the API and one or more coformers (non-ionic molecules) held together by non-covalent interactions such as hydrogen bonds, π-stacking, or van der Waals forces. Unlike salt formation, cocrystallization does not require ionizable groups, making it applicable to a broader chemical space.

The first FDA-approved cocrystal product, Entresto (sacubitril-valsartan hemicalcium), was approved in 2015 and demonstrated that cocrystals can deliver not just solubility benefits but unique pharmacological properties from the stoichiometric ratio of two APIs in a single crystal lattice. In pure solubility enhancement context, cocrystals of drugs like carbamazepine, indomethacin, and quercetin have shown 2–20× solubility improvements over parent crystalline forms, with the added advantage of thermodynamic stability surpassing amorphous forms.

Cocrystal powders are physically stable, processable, and compatible with standard encapsulation into hard gelatin capsules. The regulatory pathway for cocrystals was clarified by the FDA in 2018 guidance, classifying them as drug substances rather than mixtures, which has accelerated their development. Approximately 30+ cocrystal candidates were in pharmaceutical development pipelines as of recent industry surveys.

Surfactants, Co-Solvents, and Polymer Solubilization

Surfactants reduce interfacial tension between drug particles and aqueous media, and above their critical micelle concentration (CMC), they form micelles that solubilize hydrophobic drug molecules in their interior. Solubility enhancement via micellar solubilization typically reaches 2–100× depending on the drug's logP and the surfactant's micellar partition coefficient.

Commonly used pharmaceutical surfactants include polysorbate 80 (Tween 80), sodium lauryl sulfate (SLS), poloxamers (Pluronics), and TPGS (D-α-tocopheryl polyethylene glycol 1000 succinate). TPGS is particularly interesting because it simultaneously inhibits P-gp efflux transporters, improving permeability alongside solubility.

Co-solvents like PEG 400, propylene glycol, and ethanol increase solubility through a log-linear relationship with co-solvent volume fraction. These are commonly used in liquid-fill gelatin capsule formulations. PEG 400 at 50% v/v can increase solubility of poorly soluble drugs by 10–1,000×, though in vivo dilution in GI fluids must be managed carefully to prevent precipitation.

Amphiphilic polymers such as HPMC, PVP, and Soluplus® can also act as polymeric solubilizers. Soluplus® (polyvinyl caprolactam-polyvinyl acetate-polyethylene glycol graft copolymer) is a dedicated solubilizing polymer developed for HME and spray drying with an extraordinarily broad compatibility profile. Its micellar CMC is around 7.6 mg/L in water, and it has demonstrated 4–200× solubility improvements for multiple BCS Class II model drugs in preclinical studies.

Hard vs. Soft Gelatin Capsule Shells: Choosing the Right Container

The choice of gelatin capsule type is not cosmetic — it is an integral formulation decision that interacts with solubility strategies in meaningful ways. Both hard and soft gelatin capsule shells are composed primarily of hydrolyzed collagen (gelatin), but their composition, manufacturing, fill compatibility, and dissolution behavior differ substantially.

Hard Gelatin Capsules (HGCs)

Hard two-piece gelatin capsules consist of a body and cap, typically containing 10–15% moisture at equilibrium. They accept solid fills (powders, granules, pellets, tablets) and semisolid or liquid fills when appropriately sealed. HGCs dissolve rapidly in gastric or intestinal fluid — typically within 5–10 minutes under standard USP dissolution conditions — making them excellent for immediate-release applications where fast dissolution is required. Thermosetting semisolid fills (e.g., PEG-based or glyceride-based matrices) in HGCs represent a growing segment of lipid-based formulations.

Soft Gelatin Capsules (Softgels)

Soft gelatin capsules are one-piece shells with higher plasticizer content (glycerin and/or sorbitol, 20–30% w/w on gelatin) that allows the shell to remain flexible. They require rotary die or plate-press manufacturing and are specifically designed for liquid or semisolid fills. Softgels are the preferred vehicle for Type II–IV LBFs, and their sealed nature protects oxygen-sensitive fills effectively. Shell moisture content is typically 6–10% at equilibrium, and water migration between shell and fill during storage requires careful management — fill formulations must be designed with water activity and shell-fill compatibility in mind.

HPMC (Vegetarian) Capsule Alternatives

Hydroxypropyl methylcellulose capsules are increasingly used as alternatives to gelatin capsules for vegetarian/vegan markets and for hygroscopic fills that interact with gelatin. HPMC capsule dissolution is somewhat slower than gelatin — particularly at low moisture — and they have lower reactivity with aldehydes, making them preferred for formulations containing PEG 400 or polysorbates that can generate trace peroxides. However, for most solubility-enhancement applications, the gelatin capsule remains the industry standard due to its established regulatory history, superior mechanical properties, and broader fill compatibility.

Polymorphism and Crystal Habit Control

Many drug substances can exist in multiple crystalline forms (polymorphs) with different packing arrangements and therefore different lattice energies and solubilities. Ritonavir's infamous Form II polymorph, which appeared in commercial softgels in 1998, had approximately 4× lower solubility than Form I and caused a major recall of Abbott's HIV drug Norvir — one of the most consequential polymorph failures in pharmaceutical history.

Selecting the highest-energy (and therefore highest-solubility) stable polymorph is one approach, though the most soluble polymorph may not be the most stable. Crystal habit modification — changing the external shape without altering internal structure — can also improve dissolution by exposing higher-surface-area crystal faces to solvent.

Solvates and hydrates are also relevant: anhydrous forms typically have higher solubility than hydrates (the hydrate has already satisfied hydrogen bonding), while certain solvates can show dramatically higher solubility than either. For example, the anhydrous form of theophylline is about 1.25× more soluble in water at 25°C than its monohydrate. These differences may seem modest, but for drugs at the borderline of therapeutic window, polymorph control is critical. Encapsulating the optimized polymorph in a hard gelatin capsule with low moisture activity helps prevent in-package solid-form conversion.

Supercritical Fluid Technology and Emerging Approaches

Supercritical CO₂ (scCO₂) has unique solvent properties tunable by pressure and temperature. RESS (Rapid Expansion of Supercritical Solutions) and SAS (Supercritical Anti-Solvent) processes can produce drug nanoparticles or amorphous co-precipitates with polymers at precisely controlled particle sizes and morphologies without residual organic solvents. For example, SAS processing of felodipine with HPMC produced amorphous particles with 8× higher dissolution rate compared to untreated drug.

Electrospinning produces nanofibrous polymer-drug composites with extremely high surface area and rapid dissolution. Drug-loaded nanofibers from polymers like PVP or HPMC-AS can show near-complete dissolution within 5 minutes — a dramatic improvement over crystalline API. The ultrafine fiber structure disintegrates rapidly in GI fluid, and the product can be collected and filled into gelatin capsules.

3D printing (additive manufacturing) is opening new possibilities for creating drug-loaded structures with customized geometry that maximizes surface-area-to-volume ratios. Tablet geometries with internal channels or lattice structures can provide controlled dissolution profiles unachievable with conventional compaction. These printed forms may be inserted into hard gelatin capsules or used as standalone dosage forms.

Mesoporous silica (e.g., Syloid® grades) can load amorphous drug into nanoscale pores (2–50 nm diameter), physically constraining molecules to prevent recrystallization while dramatically increasing surface area. Loading efficiencies of 20–40% w/w are typical, and dissolution can approach that of freely dissolved drug. The resulting drug-silica powder flows well and is fully compatible with filling into standard hard gelatin capsule shells.

Comparing Solubility Enhancement Strategies: A Practical Framework

No single strategy is universally superior. The optimal approach depends on the compound's physicochemical properties (logP, pKa, melting point, molecular weight), desired dose, development timeline, manufacturing capability, and regulatory strategy. The chart below benchmarks key approaches across five practical dimensions that matter most in pharmaceutical development.

Salt formation scores highest on development speed and stability — it is the first-line strategy for ionizable compounds and should always be evaluated before more complex technologies. For non-ionizable, highly lipophilic compounds (logP > 4, solubility < 10 µg/mL), amorphous solid dispersions or lipid-based formulations in gelatin capsule shells are typically the most effective paths, despite higher development complexity.

The combination of cyclodextrin complexation with lipid excipients, or amorphous ASD with surfactant-loaded gelatin capsule fill, is increasingly common for extreme low-solubility compounds in oncology and antifungal pipelines where achieving target plasma concentrations requires stacking multiple mechanisms.

Excipient Compatibility and Gelatin Capsule Stability Considerations

Formulating for maximum solubility is only useful if the product remains stable through its shelf life. Gelatin is a protein and is reactive with aldehydes — a well-known incompatibility that causes cross-linking of the gelatin capsule shell, leading to slow dissolution and potential in vivo failure. Sources of aldehydes include:

• Lipid peroxidation products (particularly from unsaturated fatty acids in LBFs)
• Polysorbate 80 oxidation
• PEG degradation at elevated temperatures
• Reducing sugars (Maillard-type reactions)
• Certain flavors and colorants

Antioxidants (BHA, BHT, vitamin E, rosemary extract) are routinely added to LBF fills at 0.01–0.1% to prevent lipid oxidation and protect the gelatin capsule shell. Nitrogen blanketing during manufacturing and packaging with desiccants and oxygen scavengers further protect the finished product. USP <711> requires dissolution testing that accounts for potential cross-linking using enzymes or the two-stage dissolution method specified in FDA guidance on gelatin cross-linking.

Moisture management is equally critical. The gelatin capsule shell equilibrates its water content with ambient humidity. For hygroscopic fills — common in cyclodextrin complexes, amorphous powders, and salt forms — moisture migration from fill to shell can deform the shell, while migration from shell to fill can accelerate recrystallization of amorphous drug. Packaging in HDPE bottles with desiccant or in PVC/PVDC blister packs with appropriate barrier properties is standard practice for solubility-enhanced gelatin capsule products.

Regulatory Considerations for Solubility-Enhanced Formulations

Regulators treat solubility-enhanced formulations with additional scrutiny compared to conventional solid oral dosage forms, because performance depends on the interplay between API solid state, excipient matrix, and capsule shell behavior. Key regulatory touchpoints include:

• In vitro–in vivo correlation (IVIVC): FDA guidance encourages Level A IVIVC for complex formulations, which requires a mechanistic understanding of how dissolution in biorelevant media predicts plasma profiles. This is particularly important for LBF-gelatin capsule combinations where in vivo performance depends on gastric lipase activity, bile concentration, and GI motility.
• Biowaivers: BCS-based biowaivers (FDA 2017 guidance, EMA 2010 guideline) allow dissolution testing to replace in vivo bioequivalence studies for some Class I and III drugs. Class II drugs rarely qualify without extensive justification.
• Amorphous solid dispersions — ICH Q8/Q11: The physical form of the API at the time of manufacture and during storage must be controlled and characterized. DSC, XRPD, solid-state NMR, and PLM are all typically required for amorphous characterization submissions.
• Gelatin source and BSE/TSE compliance: All gelatin capsule manufacturers must comply with EMA Note for Guidance on minimizing risk of transmissible spongiform encephalopathies. Porcine gelatin and bone-sourced bovine gelatin have different risk profiles. This is documented in the Drug Master File (DMF) held by the capsule supplier.

The FDA's BCS classification and dissolution guidance documents (most recently updated in 2017 and 2021) strongly influence formulation strategy selection. Biopharmaceutics Modeling and Simulation (BM&S), including GastroPlus® and PK-Sim® platforms, is increasingly used in regulatory submissions to justify the formulation approach and predict human performance from in vitro data. Combining these computational tools with biorelevant dissolution testing (FaSSIF, FeSSIF, FaSSGF media) in early development reduces the risk of late-stage failures.

A Decision Tree for Selecting the Right Strategy

Given the breadth of options available, formulators benefit from a structured decision framework. The following logic tree covers the most common scenarios:

1. Is the compound ionizable? → Yes: Evaluate salt forms first (fastest, most cost-effective). Screen at minimum 5–8 counterions. If salt is unstable or insufficient, proceed to step 2.
2. Is logP > 3 and solubility < 100 µg/mL? → Yes: Evaluate LBF in soft or hard gelatin capsule. Run solubilization capacity screen (shake-flask method with 10+ lipid vehicles). If loading is insufficient (<10 mg/mL), proceed to step 3.
3. Is melting point < 180°C and compound thermally stable? → Yes: Consider HME for ASD. If melting point > 200°C or compound degrades, use spray drying instead.
4. Is the compound a good cavity-fitter for cyclodextrins? (MW 200–800, logP 1–5, aromatic/aliphatic cyclic structure) → Yes: Phase solubility diagram with HP-β-CD. Ka > 500 M⁻¹ indicates strong complexation potential.
5. For all of the above: The final dosage form is almost always a hard or soft gelatin capsule for speed to clinical and formulation flexibility.

The most important takeaway: do not attempt to solve all solubility problems with a single technology. The best formulations combine two or three mechanisms — for example, nanosizing an amorphous particle within a lipid vehicle, then delivering in a gelatin capsule that controls moisture exposure. The synergy between these approaches often produces bioavailability improvements far exceeding what any single strategy achieves alone.