Published May 19, 2026

What a polymorph screen should cover at each development stage, what outputs to expect, and how to know if yours is thorough enough.

Polymorph screening is the systematic experimental search for all crystalline forms of a drug substance — including polymorphs, hydrates, solvates, and salts — under a defined set of crystallization conditions. A well-designed IND-stage screen surveys 80–120 conditions across at least 6–8 solvent classes and typically identifies 3–6 distinct solid forms in the majority of small molecule APIs.¹ The right scope depends on program stage: a late-stage or IP-protection screen expands to 150–200+ conditions for comprehensive regulatory documentation and patent claims. ICH Q6A requires investigation of polymorphic forms where they are known or suspected to exist and may affect drug product performance.²

What Polymorph Screening Is — and What It Produces

A polymorph screen works by deliberately varying the crystallization environment to coax a drug molecule into different lattice arrangements. Solvents, temperatures, anti-solvent addition rates, humidity, and seeding all influence which form nucleates and grows. Solvent diversity — spanning different polarity classes, hydrogen-bonding capacity, and proton-donor/acceptor balance — drives more form discoveries per experiment than simply increasing the total number of experiments within a narrow chemical space.³

What comes out of a thorough screen is a portfolio of distinct solid forms, each characterized by XRPD, DSC, and TGA, along with a stability ranking identifying thermodynamically stable and metastable forms. The solubility difference between polymorphs can exceed four-fold for some compounds,⁴ and the gap between amorphous and crystalline forms is often considerably larger. Finding the right form at the right stage has direct consequences for bioavailability, processability, and shelf life.

Why Phase Determines What “Comprehensive” Means

Phase-appropriate polymorph screening is a well-established practice in pharmaceutical development. The scope and depth of the screen should match the program stage: a preclinical screen focuses on identifying the thermodynamically stable form with limited API; a late-stage screen needs to map the full solid form landscape to support regulatory filing and IP protection.^5,6

Manual screening assumes ~20 mg API per experiment; HTP/automated platforms typically use 1–10 mg per experiment.1 Published pharmaceutical screening programs report IND-to-late-stage condition sets typically ranging from 100 to 200 experiments, with solvent diversity weighted more heavily than raw condition count.1,6 By combining in silico prediction modeling with experimental screening, Crystal Pharmatech’s preclinical screens can reduce total API consumption to as little as 50–200 mg.

The business logic behind this progression is cost asymmetry. A polymorph discovered at preformulation costs a few weeks of additional characterization. The same polymorph discovered after process validation can require reformulation, process revalidation, and, in severe cases, product withdrawal. Ritonavir’s Form II emergence in 1998 — appearing two years after commercial launch — disrupted manufacturing at a cost estimated to exceed $250 million.7,8 Morissette et al. later demonstrated through high-throughput crystallization that five distinct forms of ritonavir could be identified when screening conditions were sufficiently diverse.⁹

What a Thorough Screen Actually Covers

A standard IND-stage polymorph screen covers:

• Solvent diversity: 6–8+ solvent classes spanning aprotic polar, protic, aromatic, halogenated, ester, ether, and mixed aqueous-organic systems, selected using statistical clustering to maximize physicochemical diversity.³

• Crystallization methods: slow evaporation, cooling crystallization, anti-solvent addition, solvent-mediated phase transformation (slurry conversion), and vapor diffusion.

• Temperature variation: ambient to 60°C crystallization and controlled cooling profiles to assess enantiotropic behavior.

• Humidity-stressed conditions: crystallization at elevated relative humidity to identify hydrate formation propensity.

• Competitive slurry maturation: suspension of multiple solid forms in solvent to reach thermodynamic equilibrium and identify the most stable form.

Slurry maturation is the most important element. In a suspension where two or more forms coexist with solvent, the less stable form dissolves and recrystallizes as the more stable form until equilibrium is reached. This approach identifies the thermodynamically stable polymorph more reliably than kinetically biased crystallization experiments alone.⁵ Screens that omit slurry maturation miss exactly the late-appearing, more stable forms that have derailed commercial programs.

For late-stage or IP-protection screens, scope expands to include milling-induced transformations, high-temperature crystallization, and process-relevant solvents to confirm that the selected form is stable under the actual conditions planned for scale-up.

Five Signs Your Program Needs a Polymorph Screen

1. Variable dissolution performance between batches. If two batches made by the same process show different dissolution curves, a form change is the most likely explanation. ICH Q6A notes that dissolution can serve as a surrogate measure of polymorphic form.2 Until the solid form landscape is characterized, there is no basis for controlling it.

   
   

2. Multiple thermal events on DSC. A drug substance with two or more endotherms on the same lot — particularly if they shift between lots — is exhibiting polymorphic behavior. Without a screen, the team cannot identify which form they have or predict when it will convert.

   
   

3. Milling or grinding changes physical appearance or dissolution behavior. Mechanical stress is one of the most common triggers for solid form transformation. If milling the API changes anything measurable, the crystalline form is not robust to processing and the landscape needs to be characterized.

   
   

4. The API is highly hygroscopic. Moisture uptake under ambient conditions is a strong predictor of hydrate formation. Hydrates can differ meaningfully from the anhydrous form in solubility and dissolution rate. A screen that omits humidity-stressed conditions will not detect them.

   
   

5. Candidate selection was made without solid-form data. If the form used in toxicology studies has not been characterized against alternative polymorphs, the team cannot guarantee that the same form will be delivered in clinical trials. Regulatory agencies expect polymorphism data at IND.²

From Screening Data to Form Selection

A polymorph screen produces more data than any single method can interpret alone. The path from screening output to a development-ready form involves three evaluation steps:

Stability ranking by slurry maturation. Forms that survive slurry competition are candidates for development. Forms that convert away are characterized for IP purposes and transformation risk.⁵

Developability profiling of lead forms. Aqueous solubility, intrinsic dissolution rate, hygroscopicity, and particle size distribution are measured for stable form candidates. A form with acceptable solubility but high hygroscopicity may require different storage or manufacturing conditions than a form with slightly lower solubility but superior physical stability.

Process-relevant stability testing. Before committing a form to manufacturing, its behavior under milling, granulation, and drying is confirmed. Where milling is part of the planned manufacturing process, confirming stability under mechanical stress is advisable, as milling-induced form changes have been documented for certain compounds. This testing requires small API quantities and should be completed before the manufacturing process is designed around the form.

The deliverable is a solid form selection report identifying the recommended development form, documenting the evidence for its thermodynamic stability, flagging transformation risks, and outlining the process conditions under which the form is stable. This report becomes the foundation for crystallization process development.

How Crystal Pharmatech Can Help

Crystal Pharmatech's polymorph screening programs are designed to match program stage — from material-sparing preclinical screens to comprehensive late-stage IP packages. By combining in silico prediction modeling with miniaturized experimental screening, our early feasibility programs can reduce API consumption to as little as 50–200 mg at the preclinical stage, preserving material for parallel developability studies.

Every screen is integrated with our solid-state characterization and crystallization process development teams, so the form identified in screening moves directly into developability evaluation and process design through our Mol2Med™ First-Time-Right approach — without handoffs or delays. Our platform has been applied to 2,000+ drug molecules, supporting 100+ early-phase programs and 19 commercialized products globally.

Frequently Asked Questions

How many conditions should a polymorph screen cover?

Solvent diversity matters more than raw condition count.³ A preclinical screen should cover at minimum 50–80 conditions across 4–6 solvent classes. An IND-stage screen should cover 80–120 conditions across 6–8 solvent classes, including humidity-stressed conditions and slurry maturation. A late-stage or IP-protection screen typically covers 150–200+ conditions. Published pharmaceutical screening programs report IND-to-late-stage condition sets typically ranging from 100 to 200 experiments.^1,6

How much API is needed for a polymorph screen?

High-throughput miniaturized screening has demonstrated that solid form identification can be achieved with 1–10 mg of API per experiment.1 For manual bench-scale screening (~20 mg per experiment), a preclinical screen requires approximately 1–2 g; an IND-stage screen approximately 1.5–2.5 g; and a late-stage screen approximately 3–4 g. By combining in silico prediction modeling with experimental screening, Crystal Pharmatech’s preclinical screens can reduce total API consumption to as little as 50–200 mg.

When in development should a polymorph screen be run?

An initial screen should be completed before the drug substance form is selected for toxicology studies — typically at candidate selection or lead optimization.⁵ A more comprehensive screen should be completed before IND filing, and a full IP-protection screen no later than Phase II to support patent claims and Common Technical Document documentation.²

What is slurry maturation and why does it matter?

Slurry maturation is a crystallization experiment in which multiple solid forms are suspended together in a solvent until thermodynamic equilibrium is reached. The more stable form grows at the expense of the less stable form. It is the most reliable method for identifying the thermodynamically stable polymorph5 and the one most likely to surface late-appearing stable forms like ritonavir's Form II.

References

1. 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.

2. ICH Q6A. Specifications: Test Procedures and Acceptance Criteria for New Drug Substances and New Drug Products: Chemical Substances. International Conference on Harmonisation; 2000. Decision Trees 4(1)–4(3).

3. Allesø M, van den Berg F, Cornett C, et al. Solvent diversity in polymorph screening. J Pharm Sci. 2008;97(6):2145–2159.

4. Bernstein J. Polymorphism in Molecular Crystals. 2nd ed. Oxford University Press; 2007.

5. Desikan S, Parsons RL Jr, Davis WP, et al. Identifying the stable polymorph early in the drug discovery–development process. Pharm Dev Technol. 2005;10(2):177–188.

6. Campeta AM, Chekal BP, Abramov YA, et al. Development of a targeted polymorph screening approach for a complex polymorphic and highly solvating API. J Pharm Sci. 2010;99(9):3874–3886.

7. Bauer J, Spanton S, Henry R, et al. Ritonavir: an extraordinary example of conformational polymorphism. Pharm Res. 2001;18(6):859–866.

8. Chemburkar SR, Bauer J, Deming K, et al. Dealing with the impact of ritonavir polymorphs on the late stages of bulk drug process development. Org Process Res Dev. 2000;4(5):413–417.

9. Morissette SL, Soukasene S, Levinson D, Cima MJ, Almarsson Ö. Elucidation of crystal form diversity of the HIV protease inhibitor ritonavir by high-throughput crystallization. Proc Natl Acad Sci USA. 2003;100(5):2180–2184.

10. Lee EH. A practical guide to pharmaceutical polymorph screening and selection. Asian J Pharm Sci. 2014;9(4):163–175.

By the Crystal Pharmatech Solid-State Science and Marketing Teams • May 19, 2026