A practical guide to choosing the solid form of an active pharmaceutical ingredient — what each option changes, when each is available, and what it costs to decide late.
Almost every small-molecule drug reaches the patient as a solid. That solid can usually exist in more than one arrangement, and the arrangement is not cosmetic: it governs solubility, dissolution rate, chemical and physical stability, hygroscopicity, melting behaviour, particle habit, flow and compaction. Two samples identical by every chemical assay can behave differently in the body and on the production line.
The consequence is that the solid form sits underneath everything built afterwards. Once it is chosen, the formulation, the manufacturing process, the analytical methods, the stability programme and the regulatory dossier are constructed on top of it. Changing it later means repeating much of that work and explaining the change to a regulator.
The economics are therefore asymmetric. A form discovered during a screening is a scientific result, priced in analytical time. The same form appearing during scale-up, or after a batch has been on stability for a year, is a development crisis. This paper sets out what the available options actually do, so the decision can be made deliberately and early.
Polymorphs are distinct crystalline arrangements of the same molecule. No atoms are added or removed; only the packing and, often, the molecular conformation change. That is enough to move solubility, dissolution rate, melting point, mechanical behaviour and chemical stability — sometimes by margins large enough to decide whether a product is viable.
Polymorphism cannot be predicted reliably. It is not possible to know in advance how many forms a molecule can adopt, how to prepare them, or what the properties of forms still undiscovered will be. A screening is therefore an experimental search, and its value lies in its breadth: solvent set, concentration, cooling and evaporation regimes, anti-solvent addition, slurry maturation, thermal cycling and mechanochemical routes each access different regions of the crystallization landscape.
At a given temperature one polymorph is thermodynamically stable and the others are metastable. Metastable forms can be perfectly manufacturable — many marketed products use them — but only if the conditions that produce them are understood and controlled, and only if the conversion to the stable form is slow enough over the product's shelf life.
The failure mode to guard against is a process that produces a metastable form by accident, because it happens to remain in a kinetic window nobody mapped. Such a process works until something shifts: a different vessel, a slower cooling ramp, a trace of the stable form seeding the plant. Competitive slurry experiments, which allow forms to convert to the most stable one under defined conditions, are the standard way to establish which form is which.
Hydrates and solvates incorporate water or solvent molecules into the lattice, so they differ from the anhydrous form in composition as well as packing. They behave as distinct solid forms with their own stability windows, and they are strongly sensitive to humidity and to drying conditions. A hydrate can dehydrate during drying and rehydrate in the package; an anhydrous form can pick up water and convert. Because isolation and drying are where this most often happens, hydrate behaviour has to be established before the isolation train is fixed, not after.
An amorphous solid has no long-range order. It is typically the most soluble form available and the least stable, because it is thermodynamically driven to crystallize. For poorly soluble candidates that advantage can be decisive, which is why amorphous solid dispersions — the API molecularly dispersed in a polymer matrix — have become a standard enabling technology rather than a last resort.
The engineering problem is keeping the material amorphous. Polymer choice, drug loading, preparation route and storage conditions all bear on the crystallization risk, and the risk has to be measured rather than argued: accelerated and long-term studies under controlled temperature and humidity, with analytical methods sensitive enough to see the onset of crystallization well below the level at which a diffraction pattern shows a peak.
One further point is often missed. Amorphous material is also generated unintentionally, by milling, micronisation and other high-energy processing. Partial amorphous content introduced at the end of a process can change dissolution and stability without anyone having decided to create it.
Salt formation transfers a proton between the API and a counter-ion, producing an ionic crystal. Where it is available it is often the most effective single intervention: a well-chosen counter-ion can raise aqueous solubility by orders of magnitude, improve chemical stability, reduce hygroscopicity, raise the melting point and give a solid that filters, dries and compresses properly. The regulatory path is well established.
The constraint is chemical. The API must possess a suitable ionisable site, acidic or basic, and the difference in pKa between API and counter-ion must be large enough for a stable salt to form rather than a mixture that disproportionates. A substantial share of modern drug candidates — neutral, lipophilic molecules — does not meet that condition.
The counter-ion is not a detail to be settled by convenience. Two salts of the same molecule can differ enough that one is developable and the other is not. Selection should weigh three things: whether the pKa difference supports a stable salt, whether the counter-ion is pharmaceutically acceptable at the projected dose, and whether there is regulatory precedent for its use by the intended route.
A salt screening is therefore a comparison exercise. Candidates are prepared in parallel and measured side by side against the free form for solubility across physiological pH, dissolution rate, hygroscopicity, and physical and chemical stability. Hygroscopicity frequently decides between two otherwise comparable salts: a deliquescent salt is a manufacturing problem whatever its solubility.
Every salt is itself a crystalline solid and can exist in several polymorphs, hydrates and solvates. A salt selected without a form screening on the leading candidates simply relocates the polymorphism risk rather than removing it. A focused screening on the one or two leaders is normally the proportionate response.
A co-crystal is a multicomponent crystal in which at least one component is the API and no proton transfer occurs. The components are held together by hydrogen bonding and other neutral interactions with a coformer, usually a pharmaceutically acceptable, non-toxic solid.
The practical significance is that co-crystallization places no acid–base requirement on the molecule. In principle any API can be co-crystallized, which makes it the primary solid-state route for the neutral molecules that salt formation cannot reach. The result is a genuinely new solid with its own solubility, dissolution rate, hygroscopicity, melting point and mechanical behaviour, while the API itself remains chemically unchanged.
Coformer selection is guided by hydrogen-bonding complementarity, acid–base considerations and precedent, and restricted to compounds acceptable in a medicinal product. Mechanochemistry — neat and liquid-assisted grinding — makes an efficient first pass, because it consumes very little API and reveals promising pairs quickly. Hits must then be reproduced by solution or slurry crystallization: that step is what separates a laboratory observation from a co-crystal that can be manufactured.
False positives are easy to generate, so confirmation requires several independent lines of evidence. Diffraction establishes that a new crystalline phase exists; thermal analysis and vibrational spectroscopy distinguish a co-crystal from a physical mixture or a eutectic; and the salt-versus-co-crystal question — whether the proton transferred — is settled definitively only by locating the hydrogen, which is why single crystal or neutron diffraction is sometimes warranted. The distinction carries regulatory and patent consequences, so it should be demonstrated rather than assumed.
Some co-crystals dissociate on contact with aqueous media or under compression, returning the API to its parent behaviour. Others are robust. Physical stability under temperature, humidity and mechanical stress is therefore part of the screening, not a later confirmation — and a co-crystal that dissociates is better identified before a formulation is built on it.
| Approach | Requires | Typically improves | Principal risk |
|---|---|---|---|
| New polymorph | Nothing beyond the molecule | Stability, processability, sometimes dissolution | Conversion to another form during process or storage |
| Hydrate / solvate | Nothing beyond the molecule | Sometimes stability or habit | Strong sensitivity to humidity and drying conditions |
| Salt | A suitable ionisable site and adequate pKa difference | Solubility, dissolution, stability, hygroscopicity | Disproportionation; the salt's own polymorphism |
| Co-crystal | A complementary coformer | Solubility, dissolution, processability | Dissociation in aqueous media or under compression |
| Amorphous dispersion | A compatible polymer and a scalable process | Apparent solubility and exposure, often markedly | Recrystallization on storage; loss of the advantage |
The approaches are not mutually exclusive. Running salt or co-crystal screening alongside an amorphous feasibility study allows the routes to be compared on the same molecule, under the same analytical conditions, rather than sequentially.
Start from the property that is failing. A candidate limited by exposure, by chemical instability, or by a solid that will not filter calls for different work. Screening everything in every direction consumes material that early programmes rarely have.
Rank on evidence, under identical conditions. Candidate forms should be compared in one programme: dissolution and solubility measured the same way, stability under the same stress, processability assessed on material prepared at comparable scale and purity. Differences between forms are only meaningful when the method is held constant.
Do not select on solubility alone. The right form balances solubility and dissolution against physical stability, chemical stability, hygroscopicity, manufacturability and intellectual property position, weighted by route of administration and target product profile. A form that cannot be processed is not a candidate whatever its dissolution profile.
Confirm the form can be made on purpose. A form that appeared once in a screening vial is a finding; a form with a reproducible preparation procedure and a defined operating window is a development option. Establishing the second from the first is a distinct piece of work, and it is where seeding strategy usually earns its place.
Every novel solid form — polymorph, hydrate, salt or co-crystal — is in principle patentable, and a form that has been discovered, characterized and shown to be reproducible can be claimed. Forms never looked for can be claimed by others. A thorough screening therefore has a commercial function alongside its technical one, and the documentation it produces is the evidence a filing rests on.
On the regulatory side, authorities expect a demonstrated understanding of the solid-state landscape: which forms exist, which one is in the product, how it is controlled, and what happens to it under stress. A screening record that includes the negative results — the conditions tried that produced nothing — is materially stronger than a report listing only what was found.
Useful solid-state work can begin as soon as a few hundred milligrams of reasonably pure material exist. Early screening is cheaper, consumes less API and, critically, arrives while the programme can still act on the result. The same study run after the process is fixed answers the same scientific question but costs far more to act on.
In practice the sequence that avoids most trouble is: characterize the starting material properly; screen for forms; if the molecule is ionisable, screen salts, and if it is not, screen co-crystals; select between the candidates on comparative data; then develop the crystallization that delivers the selected form reproducibly at scale.
PCL Research & Manufacturing is a CRO/CDMO specialised in API solid-state chemistry: screening, characterization, structure determination and process development, with a GMP analytical laboratory certified by AIFA and inspected by the FDA, authorized for HPAPI up to OEB5 and for controlled substances.
Medicina (BO), Italy · www.pcl-rm.com · businessdev@pcl-rm.com