盐型筛选还是共晶筛选？弱碱性API的决策框架

Originally published by Crystal Pharmatech. Republished with company attribution. Original technical perspectives, references and source links are retained.

A practical framework for deciding between salt screening, cocrystal screening, or both when developing a weakly basic API.

For a weakly basic or acidic API, salt screening is the default starting point when the pKa difference between the drug and the counterion acid is ≥2, sufficient to form a stable ionized species. Cocrystal screening becomes the preferred or complementary path when the API is non-ionizable, ΔpKa is insufficient for stable salt formation, or the IP landscape around known salts is constrained. The two screens are not mutually exclusive: many programs run both in parallel at early stage. ICH Q6A and the FDA’s 2018 guidance on cocrystals together define the regulatory framework within which both options are evaluated.1,2

Why the Decision Starts With pKa

Salt screening and cocrystal screening both aim to improve the physicochemical profile of a drug substance, but they work through different mechanisms and are governed by different structural requirements. Formulation considerations matter, and they shape the final selection. But the molecular properties of the API determine which tools are even available, so they are the right place to begin the evaluation.

For salt formation, the essential requirement is a sufficient difference in pKa between the API and the counterion. The commonly applied "salt rule," supported by Stahl and Wermuth, states that a ΔpKa of approximately 2 units or greater between the API and the counterion acid (for a basic drug) is generally required for stable salt formation in the solid state.3 Below this threshold, the proton transfer that defines a pharmaceutical salt is thermodynamically unfavorable, and the resulting solid may behave more like a mixture of the free base and the acid than a true salt — with corresponding instability and unpredictable dissolution behavior.

Cocrystal formation, by contrast, does not require ionization. A cocrystal is a crystalline solid composed of two or more distinct molecular components in a defined stoichiometric ratio, held together by non-covalent interactions such as hydrogen bonding, pi-pi stacking, or van der Waals forces.4 Because no proton transfer is required, cocrystals are accessible to non-ionizable APIs, to weakly basic compounds with insufficient ΔpKa for salt formation, and to neutral compounds that cannot form conventional salts at all.

The FDA’s 2018 guidance on cocrystals clarified their regulatory classification: cocrystals are treated as a distinct solid form category, separate from polymorphs, salts, and solvates, but subject to the same characterization and specification requirements that apply to other solid forms.2 This regulatory clarity has substantially reduced the development risk of choosing a cocrystal path, and it has expanded the range of programs for which cocrystal screening is a rational first-line option.

A 4-Question Triage for Choosing Between Salt and Cocrystal Screening

Before committing API to either a salt screen or a cocrystal screen, four questions should be answered. The responses together point toward the appropriate strategy:

1. Does the API have an ionizable group a pKa between 2 and 12 ? If yes (ΔpKa ≥2), salt screening is viable and should be the starting point. If no, cocrystal screening is likely a more productive path. Some weakly basic APIs can form stable salts with strong acid counterions even at borderline ΔpKa — this should be confirmed experimentally rather than ruled out on calculation alone.
2. What is the IP landscape around known salts? If the primary salt forms of the API are covered in existing patents and the freedom-to-operate window is narrow, cocrystals offer a meaningful differentiation opportunity. A cocrystal is a distinct crystalline entity that can support composition-of-matter claims separate from the free base and its salts.
3. How large a solubility improvement is needed, and is pH-dependence acceptable? Salts of weak bases can deliver order-of-magnitude solubility improvements in acidic media, but this advantage is pH-dependent; in the higher-pH intestinal environment the free base may reprecipitate. Cocrystals can generate supersaturation through a spring-and-parachute effect, dissolving rapidly before precipitating toward the stable form unless controlled. Because the advantage comes from the crystal structure rather than ionization, it is less pH-dependent.
4. What is the disproportionation risk tolerance? As discussed in the previous blog in this series, salts of weak bases carry a disproportionation risk when formulated with common excipients or exposed to elevated humidity. Because a cocrystal does not rely on a maintained proton-transfer equilibrium the way a salt does, it generally carries a lower disproportionation risk, though co-former dissociation remains a consideration. For programs expected to use standard tablet excipients or anticipating storage at elevated humidity, this factor may tip the balance toward a cocrystal approach.

Salt vs. Cocrystal: A Side-by-Side Comparison

What Each Screen Actually Looks Like in Practice

Salt screening proceeds through a structured sequence:

• Counterion selection. Identify pharmaceutically acceptable acids (for basic APIs) whose pKa values are sufficiently below the API’s pKa to drive stable proton transfer. Commonly evaluated counterions include hydrochloride, mesylate, maleate, fumarate, phosphate, and tartrate.
• Experimental screening. Evaluate each API-counterion combination across solvents, stoichiometries, and crystallization methods.
• Form confirmation. Use XRPD to confirm salt formation (a distinct diffraction pattern from both the free base and the acid) and DSC to characterize melting behavior and thermal stability.
• Property evaluation. Measure aqueous solubility and intrinsic dissolution rate for each confirmed salt, and assess disproportionation risk under humidity and in the presence of common excipients before selecting a counterion.

The FDA and ICH maintain references for acceptable counterions; counterion choice is guided by ΔpKa, pharmaceutical acceptability, and the intended route of administration.5

Cocrystal screening follows a parallel structure:

• Co-former selection. Identify GRAS or established pharmaceutical excipient molecules whose functional groups are complementary to those of the API for non-covalent crystal packing. Computational tools, including hydrogen bond propensity analysis and COSMO-based screening, are increasingly used to prioritize co-formers before experimental work.
• Experimental screening. Use liquid-assisted grinding, solution crystallization, and slurry methods across ~20–40 co-formers.7
• Form confirmation. Use XRPD to confirm cocrystal formation (a new diffraction pattern distinct from both components) and DSC to confirm a single melting event.
• Property evaluation. Quantify solubility and dissolution improvement relative to the parent crystal form, and assess co-former dissociation risk.

For programs with limited API, parallel screening in 96-well plate formats can evaluate both salt and cocrystal options simultaneously, prioritizing the most promising hits for scale-up and characterization.

What the Regulatory Package Looks Like for Each Path

For salts, the characterization and specification requirements under ICH Q6A are well established.1 The solid form is characterized by XRPD (with reference diffractogram in the specification), DSC, TGA, and moisture uptake by dynamic vapor sorption (DVS). The counterion is identified and quantified. Stability data demonstrating physical and chemical stability of the salt under ICH conditions are expected, along with disproportionation data under stressed conditions and under the conditions of the intended formulation process.

For cocrystals, the FDA's 2018 guidance specifies that a cocrystal should be characterized as a distinct solid form with its own specification, separate from the free base and any salt forms.2 This includes XRPD, DSC, stoichiometry confirmation by elemental analysis or NMR, and demonstration that the cocrystal is a single-phase material. Stability data should demonstrate that the cocrystal does not dissociate into its components under the conditions of manufacture and storage.

In both cases, the solid form selection report — documenting the screening rationale, the forms identified, the developability evaluation, and the evidence for selecting the development candidate — should be complete before the formulation is designed. This is Module 3.2.S.3.1 of the Common Technical Document,8 and reviewers expect to see the selection justified with data.

How Crystal Pharmatech Can Help

Crystal Pharmatech is a solid-state-focused CRO/CDMO with deep specialization in salt and cocrystal screening and selection. Our approach begins with structure-based prediction: we analyze the API’s molecular structure and physicochemical properties to identify the most probable salt-forming and cocrystal-forming sites, then use that insight to select counterions and co-formers rationally rather than by brute force.

For cocrystal design, we apply COSMO-based in silico screening to prioritize co-formers computationally before committing API to the bench, narrowing a field of dozens of candidates to the most promising handful. This is paired with two complementary experimental capabilities: comprehensive manual screening for maximum solid form coverage and IP protection, and a material-sparing in silico-guided approach for early-stage programs where API is limited. The result is fewer experiments, shorter timelines, and lower material consumption without sacrificing thoroughness.

Our salt and cocrystal screening are fully integrated with solid form developability evaluation and characterization, and we are equipped to handle high-potency compounds (OEB 4 and 5) and all categories of controlled substances. Every program concludes with a solid form selection report structured to support Common Technical Document Module 3.2.S.3.1.

Previous in this series: Polymorph Screening: A Phase-Appropriate Strategy Guide for Small Molecules

Frequently Asked Questions

When should I run a cocrystal screen instead of a salt screen?

Run a cocrystal screen when the API is non-ionizable, when ΔpKa between the API and available counterions is below ~2 units, when the IP landscape around known salts is constrained, or when a pH-independent solubility improvement is preferred over the larger but pH-dependent gain from a salt.1,2 Many programs screen both options in parallel at early stage.

How do I choose between a salt and a cocrystal for a weakly basic API?

The primary filter is ΔpKa.3 If ΔpKa ≥2, begin with salt screening. If ΔpKa is borderline or insufficient, prioritize cocrystal screening. Secondary filters include IP strategy, required solubility lift, pH-dependence of the target application, and disproportionation risk tolerance under the intended formulation conditions.

What counterions are used in pharmaceutical salt screening?

The most commonly evaluated counterions for basic APIs include hydrochloride, mesylate, maleate, fumarate, phosphate, sulfate, tartrate, and citrate. Counterion selection is guided by ΔpKa, pharmaceutical acceptability, and the intended route of administration.5

How are cocrystals regulated compared to salts?

The FDA’s 2018 cocrystal guidance classifies cocrystals as a distinct regulatory category from both salts and polymorphs.2 They require their own characterization and specification package, including XRPD, DSC, stoichiometry confirmation, and stability data demonstrating that the cocrystal does not dissociate under manufacture and storage conditions. They do not require demonstration of proton transfer — a key difference from salts.

References

1. ICH Q6A. Specifications: Test Procedures and Acceptance Criteria for New Drug Substances and New Drug Products: Chemical Substances. International Conference on Harmonisation; 2000.
2. U.S. Food and Drug Administration. Guidance for Industry: Regulatory Classification of Pharmaceutical Co-Crystals. FDA; 2018.
3. Stahl PH, Wermuth CG, eds. Handbook of Pharmaceutical Salts: Properties, Selection, and Use. 2nd ed. Wiley-VCH; 2011.
4. Berry DJ, Steed JW. Pharmaceutical cocrystals, salts and multicomponent systems; intermolecular interactions and property based design. Adv Drug Deliv Rev. 2017;117:3–27.
5. Berge SM, Bighley LD, Monkhouse DC. Pharmaceutical salts. J Pharm Sci. 1977;66(1):1–19.
6. Morissette SL, Almarsson Ö, Peterson ML, et al. High-throughput crystallization: polymorphs, salts, co-crystals and solvates of pharmaceutical solids. Adv Drug Deliv Rev. 2004;56(3):275–300.
7. Cruz-Cabeza AJ. Acid-base crystalline complexes and the pKa rule. CrystEngComm. 2012;14(20):6362–6365.
8. ICH M4Q (R1). The Common Technical Document for the Registration of Pharmaceuticals for Human Use: Quality. International Conference on Harmonisation; 2002. Section 3.2.S.3.1 (Elucidation of Structure and Other Characteristics).

By the Crystal Pharmatech Solid-State Science and Marketing Teams

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