Solid form development: Evaluating salt hits against free API to achieve an optimal target form
Andrew Blythe-Dickens
Senior Solid State Scientist
Jamie Allen-Marshall
Senior Solid State Scientist
Voiceover by Jamie Allen-Marshall
Introduction Listen Now
Solid form development is a crucial step in preparing an active pharmaceutical ingredient (API) for clinical and commercial readiness. The selection of an optimal solid form directly impacts solubility, bioavailability, and ultimately efficacy. In this article, we’ll step through an example of how multi-faceted solid form investigations can be utilised to identify an ideal target solid form.
In one project at Sterling, an API was presented for development, clinical manufacture, and ultimately, progression into larger-scale manufacturing. Based on the free API’s chemical structure, the Sterling team determined that it would have limited aqueous solubility, so the team started with salt screening to identify versions with improved solubility and bioavailability.
Given the low pKa of the protonation centres of the molecule, only strongly acidic counter-ions were likely to form stable salts. After initial qualitative solubility testing and further assessment of counter-ions, the team identified four hits with desirable physico-chemical and solubility characteristics to merit further investigation.
Keep reading to learn how salt screening, pre-formulation evaluation, and crystallisation development were used to achieve an optimal solid form in this project.
Key takeaways from this case study:
- A well-designed salt screen is critical to uncover viable candidates early.
- Stability and form change assessments (especially under humidity/temperature stress) are essential before scale-up.
- Polymorphism investigations must continue into development to anticipate any unexpected forms.
- Process simplification, such as adopting reactive salt crystallisation, can drive efficiency, reduce solvent burden, and improve yield.
- Solid form decisions deeply influence downstream manufacturability, product robustness, and regulatory risk.
4
suitable solvent
systems
15
counter-ions
assessed
12
salt hits
identified
4
hits considered
desirable
Salt screening and initial characterisation Listen Now
The four desirable salt candidates were characterised via X-ray powder diffraction (XRPD), differential scanning calorimetry (DSC), and reproducibility of crystallisation from multiple solvent mixtures. Counter-ion A and C salts demonstrated form consistency by XRPD and DSC across crystallisation in multiple solvent mixtures; however, their aqueous solubility was relatively low compared to counter-ion B and D salts.
Salts B and D, while demonstrating improved solubility, showed potential issues with solvate formation and evidence of form or version variability.
The free API, as expected, showed limited aqueous solubility, so any solubility across the salt forms was considered an improvement.
The remaining eight hits of the original 12 were discarded at this stage, as they demonstrated issues such as poor solubility, complex DSC thermal profiles, and tendencies towards solvation or thermal instability.
Pre-formulation and stability evaluation Listen Now
To support salt version selection, the four salt candidates were carried forward into a pre-formulation evaluation to identify those that were best suited to proceed based on solubility and accelerated stability assessments. The team produced additional quantities of each salt to aid in these evaluations.
Counter-ions B and C were successfully isolated as di-salts upon scale-up, as was counter-ion A. Counter-ion D proved to be unreliable and was removed from further investigations; on two separate occasions, the API to counter-ion ratio varied, and residual solvent content was significant.
As a result, three salt versions were assessed alongside the free API for solubility and form behaviour in various dissolution media at 37°C. In addition, the team evaluated form and chemical stability over three weeks of storage.
When evaluating solubility, the team noted several key observations:
- Solubility and pH characteristics of counter-ion A and B salts were similar across all dissolution media examined, greater than 30 mg/mL, though both showed reduced solubility in pH 4.5 acetate buffer and FeSSIF (pH ~5)
- The free API showed very low solubility overall, except in 0.1 molar hydrochloric acid and the acetate pH 4.5 buffer
- The solubility and pH characteristics of counter-ion C salt fell between the other two salts and the free API
The team also assessed solid form stability by XRPD. In these studies, XRPD patterns of the isolated solids for the free API and salts A and B matched, indicating disproportionation of the salts back to the free API. For salt C, XRPD patterns of the isolated solids matched the input salt, suggesting retained salt stability.
Finally, as part of pre-formulation evaluations, the team evaluated thermal and hygroscopic behaviour for the free API and salts. In these evaluations, they determined:
- Solid form characteristics for salts B and C were reproducible, affording crystalline solids with simple thermal profiles
- Salt A demonstrated partial solvation and greater sensitivity to humidity
- After three weeks of storage at 40°C and 75 % relative humidity (RH), the free API and salts B and C remained unchanged, while salt A demonstrated hydration and appearance changes
- Following dynamic vapour sorption analysis, salts B and C were free-flowing powders upon isolation at 90% RH, while salt A showed significant water uptake
After these studies, salt A was deemed unsuitable for further progression due to its issues with form stability and hydration.
Polymorph screening and form selection Listen Now
After determining that salts B and C were the preferred versions to progress, the team proceeded with polymorphism investigations. These screenings would allow the team to assess the propensity of the two salt versions to polymorphism, as well as to characterise any new versions and evaluate their potential for further development based on stability, solubility, and particle habit.
In these investigations, the team applied several manipulations to the salt versions, including generation of amorphous API salt versions, equilibration with thermal modulation of the supplied API salt versions and amorphous versions, saturated solution cooling crystallisation, and anti-solvent driven crystallisation, all of which are common in polymorphism investigations.
Throughout these studies, salt B demonstrated considerable propensity to version change, with two polymorphic forms, multiple hydrates, evidence of solvates, and evidence of partial salt disproportionation in one solvent system.
Salt C predominantly afforded one polymorphic form, which was identified as pattern A. Even when amorphous salt C was equilibrated, it largely reverted to pattern A. Two additional patterns were isolated in specific solvents, but they were confirmed as distinct non-solvates. Upon thermal manipulation, pattern A was determined to be the thermodynamically stable form of salt C.
As a result of these investigations, salt C pattern A was selected as the leading solid form, as it demonstrated minimal polymorphic risk, no observed solvation or hydration, and improved solubility relative to the free API.
Crystallisation development and scale-up Listen Now
Salt C pattern A moved into crystallisation development in order to afford the target API version with controlled purity, acceptable particle habit, and desirable yield. The initial protocol was a two-step, high volume process, isolated from over 50 volumes of high boiling point solvent, which presented challenges for scale-up and drying.
Therefore, initial investigations focused on exploring aqueous recrystallisations and optimisation of solvent and water ratios. The team determined that a direct reactive salt formation-driven crystallisation, in which they combined the API and counter-ion during crystallisation, would be more efficient than generating a salt and recrystallizing. This launched an investigation into targeting a one-step isolation of the API salt from the free API.
During early scale-up, a new polymorphic form, termed pattern D, emerged. Like patterns B and C, pattern D was a metastable form that converted to pattern A upon thermal manipulation under DSC and thermogravimetric analysis (TGA).
Leveraging insights from polymorph screening and chemistry development, the team targeted aqueous-based recrystallisation systems to generate the target salt form, while enabling greater impurity control over non-aqueous solvents.
The team assessed a range of solvent/water mixtures, with a 9:1 ratio demonstrating good recovery over 80%. The initial process involved dissolution of the free API in over 30 volumes of alcohol and dissolution of the counter ion in over three volumes of water respective to the free API input. Further investigations revealed that the free API could be dissolved in considerably less alcohol, and the counter-ion in considerably less water, to improve recovery. The final optimised route achieved ~83% recovery on a 4-kilogram scale in GMP manufacture.
Conclusion Listen Now
In this case study, a 15-counter-ion salt screen yielded 12 mobile salt candidates, of which only four passed initial physico-chemical and solubility criteria. Pre-formulation evaluations eliminated one salt form, leaving salts A, B, and C, plus the free API, for further investigation. Salt A failed due to hydration and form instability, while salt B exhibited excessive polymorphism risk, leaving salt C pattern A as the optimal solid form. Crystallisation development refined a process that shifted from a high volume, multi-step pathway to an efficient one-step reactive crystallisation, culminating in strong recovery.
Salt screening
12 hits identified; 4 suitable candidates
Pre-formulation evaluation
Narrowed to 2 candidates
Polymorphism investigations
1 candidate demonstrated form change, 1 demonstrated form stability
Crystallisation development
Reduced to low-volume, one-step salt forming crystallisation with impurity control and desirable recovery
Solid state chemistry at Sterling
At Sterling, our experienced solid state team delivers extensive expertise across all elements of solid form selection and development. Through tailored discovery and evaluation, robust crystallisation development, and expert analysis, we are committed to helping customers identify and progress the ideal solid form of their API, ensuring optimal solubility and bioavailability. If you’re interested in learning how Sterling can support your solid form investigations, speak to an expert.
