Many drug compounds can exist in more than one crystalline form, or ‘polymorph’. Different polymorphs can have different properties and identification of the optimal polymorph can be a key factor in the success of a drug. Polymorphs may therefore have high commercial value, but obtaining patents for polymorphs at the European Patent Office (EPO) can be challenging.
Below are 5 tips and best practices from our experience, to give you the best chance of success when patenting polymorphs at the EPO.
Tip: Include extensive characterisation of the polymorph in your application. Consider including a comparison with a previously known form or known synthesis in your application. If this isn’t possible, be aware that you may need to provide this during prosecution or in post-grant proceedings.
Further detail: Characterising data obtained by established techniques for identifying polymorphs (e.g. X‑ray diffraction data) should be used. Not all analytical techniques are suitable. For example, in T 885/92 a difference in IR peak listings was not sufficient to show that two forms prepared by similar methods were different.
Data may be required, even if an earlier disclosure is simply of an uncharacterised solid compound. Repetition of the earlier compound synthesis and characterisation may be needed to show it differs from the claimed polymorph. If repetition of an earlier compound synthesis proved to be unsuccessful, details substantiating the failed attempts should be provided (T 1416/21).
2. The EPO considers claims in the form “Crystalline Form X of compound Y” to lack clarity
Tip: Define your polymorph using characterising data and provide experimental details of the analytical techniques used in the application. Include complete analytical data (e.g. a full XRPD peak listing or DSC thermogram), but also include claims and embodiments in the description specifying key characterising information for the polymorph (e.g. a listing of characteristic XRPD peaks, onset/peak DSC endotherm temperatures).
Further detail: A polymorph should be defined by characterising data obtained using analytic techniques, for example, XRPD, single crystal X-ray diffraction or thermal methods such as thermogravimetric analysis (TGA), differential thermal analysis (DTA), differential scanning calorimetry (DSC) or melting point. In our experience, most polymorph applications granted by the EPO have the polymorph defined at least by characteristic XRPD peaks.
Key characterising information, such as 2Theta values for characteristic XRPD peaks or onset/peak endotherm temperatures from a DSC thermogram, should be disclosed in the original application. Without this, options for claim amendments may be limited, given the EPO’s strict approach to added matter. For example, a full listing of 2Theta values and relative intensities for XRPD peaks might not be considered to provide basis (support) for claiming just the 2Theta values of selected characteristic peaks (T 764/06). Ideally, the original application should include progressively narrowing lists of peaks (e.g. starting with the main characteristic peaks, then progressively adding others as options).
If priority is claimed, be careful to define XRPD peaks and error margins in the same way in both the earlier and priority‑claiming application. Any differences here could cause loss of priority and open the application up to patentability challenges in view of intervening prior art (T 1303/18).
3. An unexpected technical effect generally must be demonstrated for a polymorph to be considered inventive. The EPO views polymorph screening as a routine part of drug development
Tip: Be prepared to provide evidence of an unexpected technical effect. Wherever possible, include such evidence from the outset/when filing. If you may need to rely on post-filed data to demonstrate an effect, it is important to ensure that effect is at least “derivable” from the originally filed application. This means the effect should be conceptually comprised or encompassed by the technical teaching, and embodied by the same originally disclosed invention. For example, you could qualitatively foreshadow that the invention provides a specific unexpected improvement in a property by including statements of advantage explaining this, and then follow-up later with quantitative post-filed data backing this up.
Further detail: Inventive step for polymorphs often depends on whether the closest prior art is an amorphous or crystalline form. The bar is somewhat higher when the closest art is an amorphous form since arguments based on typical polymorph properties (e.g. solubility, hygroscopicity or stability) are likely to be rejected (see below).
However, an unpredictable combination of properties, such as an unpredictable combination of two or more of low hygroscopicity, good stability, solubility, permeability and bioavailability may be successful, regardless of whether the closest prior art is an amorphous form (T 94/11) or a crystalline form(T 643/12).
Read on for more details on different scenarios where the closest prior art is amorphous-form or crystalline form.
Closest prior art is an amorphous form
In landmark case T 777/08, inventive step was denied for a polymorph with improved filterability and drying compared to an amorphous form, because amorphous forms are generally more soluble, and less easy to isolate, purify and dry than crystalline forms.
That said, there are instances where typical polymorph properties can give rise to inventive step. Greater stability with respect to epimerisation compared to an amorphous form, resulting in improved biological activity, was successful in T 1422/12. The Board held that general knowledge of crystalline stability could not automatically be applied to epimerisation of the claimed tetracycline antibiotic. Therefore it’s important to drill into the details when dealing with polymorph properties, to see if these are somehow different to the typically expected improvements (e.g. is it a nuanced and unexpected type of stability, as here).
However, in T 1894/15, the same board denied an inventive step for improved stability of crystalline tigecycline with respect to epimerisation compared to the amorphous form. This decision was based on different prior art to T 1422/12 (above). This prior art demonstrated that tetracyclines tend to epimerise in the presence of water, and that crystalline forms of tetracyclines are less hygroscopic than the corresponding amorphous forms. Thus, in seeking a form of tigecycline with improved stability to epimerisation, the Board held that it would be obvious to consider a crystalline form of tigecycline. This highlights that the EPO will routinely reject applications to typical polymorph property improvements, even if there are nuances, where there is teaching in the art incentivising research into these.
Closest prior art is a crystalline form
Perhaps counterintuitively, demonstrating inventive step is generally easier when the closest prior art is another crystalline form.
Effects that have been successful include greater stability or solubility compared to the known crystalline form. Unlike when moving from an amorphous form to a crystalline form, an existing crystalline form does not automatically provide a reasonable expectation that the new crystalline form would be more stable or have improved solubility, let alone a pointer to these improvements (T 1555/12, T 1684/16, T 325/16, T 1667/15).
Therefore, if a crystalline form is known but an improved crystalline form has been developed, this improved form may have a good chance of success at the EPO.
Post-filed data and the impact of G 2/21
A topic that has been of much discussion lately is the EPO Enlarged Board of Appeal’s decision on the use of post‑filed data to support inventive step in G 2/21 (read more here).
G 2/21 was applied to polymorphs in T 1994/22. The patentee sought to rely on post-filed data to demonstrate that a claimed crystalline form of selexipag had improved photostability compared to an alternative polymorph. The application as filed made no mention of photostability of the claimed polymorph, or polymorphic forms at all. The Board acknowledged that, in general, the purported technical effect need only be “conceptually comprised by the broadest technical teaching of the application as filed” (as set out in T 0116/18). However, they did not agree with the patentee’s assertation that statements in the application as filed referring to a novel crystal of selexipag and a pharmaceutical product of "high quality for which constant effect can always be shown and a form which is handled easily industrially" were enough to demonstrate that improved photostability was derivable from the originally filed application.
4. If the polymorph was particularly difficult to obtain or if non-standard techniques were required, this may be used to argue the polymorph is inventive, because there is an argument it would not have been found through routine screening methods
Tip: If non-standard techniques have been used, or the polymorph was particularly difficult to obtain, consider claiming the polymorph itself as well as the non-standard process.
Further detail: In T 1825/21, an inventive step was acknowledged for a claim to a crystalline form of ceritinib free base. The closest prior art was an amorphous HCl salt of ceritinib. The board agreed that, in seeking to provide a form of ceritinib with improved stability and ease of drying, the skilled person would seek a crystalline form (i.e. provision of a crystalline form alone would be obvious). However, the appellant demonstrated that attempts to prepare crystalline ceritinib HCl salt had been unsuccessful. In fact, such attempts had only resulted in amorphous precipitates having chloride levels inconsistent with a stoichiometric salt. An inventive step was acknowledged on the grounds that the skilled person would have difficulties to obtain a crystalline form of ceritinib free base.
In joined cases T 1065/18 and T 1079/18, the proprietor argued that differential scanning calorimetry (DSC) represented a non-routine screening method. Inventive step arguments also relied on improved solubility of the claimed crystalline form versus a known polymorphic form. However, the Board considered that, when faced with the problem of seeking a polymorph with improved solubility, the skilled person would know, based on their common general knowledge, to evaluate the thermodynamics of any phase transition of the solid form using DSC. The Board therefore concluded that the DSC screening method of obtaining the claimed polymorph was obvious.
5. A complete and reproducible method for obtaining the polymorph should be provided
Tip: Ensure that the application describes a complete method of preparing the polymorph. If the method involves seeding, include the method used for preparing seed crystals.
Further detail: A polymorph application can fail due to insufficiency (lack of enablement) if it doesn’t disclose a complete and reproducible method for obtaining the polymorph. The method should include reproducible means of obtaining any required seed crystals or intermediate solvates (T 2370/19).
For more information, please contact Anna Chamberlain, James Snaith, Jennifer Harris, or your usual Kilburn & Strode advisor.