Two Strategies for the Same Problem

Poor aqueous solubility is one of the most common challenges in small molecule drug development. A molecule that does not dissolve adequately in the gastrointestinal fluid cannot be absorbed at a rate sufficient to achieve the intended therapeutic effect, regardless of how potent it is in a biochemical assay. Two of the most widely used solid form strategies for addressing this problem are pharmaceutical salts and co-crystals. Both involve creating a multicomponent solid that modifies the physical properties of the API, but they do so through different mechanisms, and each has a different regulatory footprint.

What Is a Pharmaceutical Salt?

A pharmaceutical salt is formed when an ionisable API reacts with a pharmaceutically acceptable acid or base to form an ionic compound. The positive and negative charges of the salt components are held together by electrostatic interactions. Salt formation is only possible when the API has an ionisable group with a suitable pKa, and where the difference in pKa between the API and the counterion is sufficient to drive proton transfer. As a general rule, a pKa difference of at least two units is required for reliable salt formation.

Salts are regulatory well-established. They are classified as the same active ingredient as the free form in most jurisdictions, which simplifies the regulatory pathway. The FDA has published guidance on the pharmaceutical salts framework, and the EMA’s ICH Q6A guideline addresses solid state characterisation requirements.

What Is a Pharmaceutical Co-Crystal?

A co-crystal is a multicomponent crystalline solid in which the API and a coformer are held together in the crystal lattice by non-covalent interactions, typically hydrogen bonds, rather than ionic bonds. Because no proton transfer occurs, co-crystal formation is not limited to ionisable molecules. This makes co-crystals particularly valuable for neutral or weakly ionisable APIs where salt formation is not feasible.

The regulatory status of co-crystals has evolved significantly over the past decade. The FDA’s 2018 guidance on co-crystals classifies them as drug products containing a drug substance, meaning they follow a standard NDA or ANDA regulatory pathway rather than being treated as new chemical entities. The EMA has taken a broadly similar position.

Salt vs. Co-Crystal: A Practical Comparison

Factor	Salt	Co-Crystal
Applicable molecules	Ionisable APIs (pKa difference > 2 units)	Ionisable and non-ionisable APIs
Interaction type	Ionic (electrostatic)	Non-ionic (hydrogen bonds, pi-pi stacking)
Regulatory classification	Same active ingredient as free form	Drug product containing drug substance (FDA)
IP potential	Well-established, many precedents	Growing body of co-crystal patents
Solubility improvement	Can be significant; dependent on counterion	Variable; coformer selection is critical
Physical stability	Generally robust if polymorph is stable	Can be hygroscopic; humidity sensitivity must be assessed
Coformer options	Pharmaceutically acceptable acids/bases	GRAS substances, excipients, or other APIs

Choosing the Right Approach for Your Molecule

Start with Ionisability

The first question is whether the API is ionisable. If it has a basic nitrogen or an acidic group with a pKa in the right range, salt screening is typically the first strategy to evaluate. If the molecule is neutral or weakly ionisable, co-crystal screening becomes the primary option alongside other solubility-enhancing technologies.

Consider the Stability Requirements

Some salts are hygroscopic or unstable at elevated humidity, which creates challenges during manufacturing and storage. Co-crystals can also show humidity sensitivity depending on the coformer. Understanding the stability profile of candidate forms under conditions relevant to your intended market, using ICH storage conditions as a minimum, is essential before committing to a development form.

Factor in the IP Landscape

If the free form and common salts of your API are already in the prior art, a novel co-crystal form with demonstrated physicochemical advantages may offer a patentable differentiation. A solid state research team with experience in the IP dimensions of form screening can help structure the work to generate data that supports a patent filing.

Ardena’s Approach to Salt and Co-Crystal Screening

Ardena’s solid state research team in Ghent conducts both salt and co-crystal screening as part of pre-formulation development programmes. The screening process uses a design-of-experiments approach to evaluate a broad range of counterions and coformers, with XRPD, DSC, and solution NMR used to characterise and confirm the nature of each candidate form.

Screening results are evaluated against the full development context, including target dose, intended manufacturing process, and the regulatory and IP strategy, so that the form recommendation reflects not just the best physical chemistry but the best outcome for the programme as a whole.

Why the First Crystal Form Is Rarely the Right One

When a medicinal chemist synthesises a new active pharmaceutical ingredient for the first time, the crystal form that precipitates out of solution is a function of the solvent system, the temperature, and the rate of crystallisation used. It is rarely the thermodynamically stable form, and it is rarely the form with the best combination of solubility, stability, and processability for a drug product.

Solid state screening is the systematic process of identifying the range of crystal forms a molecule can adopt and selecting the one, or the combination of salt and form, that best supports the target product profile. Getting this right in pre-formulation is one of the highest-value decisions in early drug development. Getting it wrong after GMP manufacturing has started can require a regulatory amendment, a reformulation programme, and a significant delay to the clinical timeline.

What Solid State Screening Covers

Polymorph Screening

Many organic molecules can crystallise in more than one arrangement of molecules in the solid state. These different arrangements, known as polymorphs, have different physical properties including melting point, solubility, dissolution rate, chemical stability, and hygroscopicity. Two polymorphs of the same API can have dramatically different bioavailability profiles. A metastable form that gives excellent dissolution performance in early development may convert to a more stable but less soluble form under the conditions of manufacturing or storage.

Polymorph screening uses a range of crystallisation conditions, solvents, temperatures, and seeding experiments to map the landscape of forms available to a molecule. X-ray powder diffraction (XRPD) is the primary analytical tool for identifying and characterising each form. The ICH Q6A guidance on specifications for new drug substances requires the solid form to be defined and controlled, making this screening work a regulatory necessity as well as a scientific one.

Salt Screening

For ionisable molecules, forming a pharmaceutical salt is often the most effective way to improve aqueous solubility and chemical stability. Salt screening evaluates a range of pharmaceutically acceptable counterions to identify which salt forms are crystalline, physically stable, and give meaningful solubility improvements over the free form. The selection of a salt is also an intellectual property decision: a novel salt form with demonstrated advantages over existing forms may be patentable, extending the effective commercial life of the molecule.

Solvate and Hydrate Assessment

Some molecules preferentially incorporate solvent or water molecules into their crystal lattice, forming solvates or hydrates. These forms can be more stable than anhydrous forms under certain conditions, or they can be a liability if they dehydrate or desolvate during processing. Understanding which solvated forms exist, and under what conditions they are stable, is part of a complete solid state assessment.

Key Solid State Forms and Their Implications

Form Type	Definition	Key Implication for Development
Polymorph	Different crystal packing of the same molecule	Can affect solubility, stability, and bioavailability. Must be controlled in the GMP process.
Salt	Ionic form with a pharmaceutically acceptable counterion	Often improves solubility and stability. May be patentable.
Cocrystal	Non-ionic multicomponent crystal with a coformer	Useful for non-ionisable molecules. Can improve physical properties.
Solvate / Hydrate	Crystal lattice containing solvent or water	Can be stable or unstable depending on humidity. Must be assessed during stability studies.
Amorphous	Non-crystalline solid form	Higher energy state, typically higher solubility but lower stability. Requires stabilisation strategy.

The IP Dimension of Solid State Work

Beyond the formulation and regulatory arguments for thorough solid state screening, there is a commercial argument that is often underappreciated at the early development stage. A patent covering a novel salt form, polymorph, or cocrystal of a development-stage API can significantly extend the period of market exclusivity beyond the composition of matter patent. This is not a secondary consideration. For molecules in highly competitive therapeutic areas, the solid form patent strategy can be as commercially important as the clinical development programme.

Ardena’s solid state research team works closely with clients to ensure that screening work is conducted in a way that generates patentable data where the science supports it, and that the relationship between solid form, IP strategy, and regulatory filing is considered from the outset.

Ardena’s Solid State Research Capabilities in Ghent

Ardena’s solid state research team in Ghent conducts polymorph screening, salt selection, cocrystal screening, and hydrate/solvate assessment using a combination of high-throughput crystallisation and advanced characterisation techniques. XRPD, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and dynamic vapour sorption (DVS) are used to characterise each form and assess its stability under relevant conditions.

The solid state data generated in Ghent is directly connected to the drug product formulation work, meaning that form selection decisions are made with the full development context in mind, not in isolation. This integration is particularly valuable when the optimal form for stability conflicts with the optimal form for dissolution, and a scientifically informed compromise is required.