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Chemical Purity at the EPO

14 October 2021

Purifying a compound or polymer is an important step during product development, with seemingly small changes potentially having a pronounced effect on product performance.  Yet chemical purity is often overlooked as a source of patentable inventions.  ‘Purity patents’ can, however, be used to extend the patent lifecycle for a product many years beyond expiry of the initial proof-of-concept patents.

The EPO has softened its stance towards the patentability of purity in recent years, but obtaining purity patents remains challenging.  Patent examiners are still adapting to the new, softer standards and purity claims typically face a challenging prior art landscape. Below are some practical tips on how to avoid the many pitfalls of patenting chemical purity at the EPO.

1. Obviousness, not novelty

The EPO assessed chemical purity under the novelty standard for many years because it was previously held that once a substance is disclosed, the skilled person could readily obtain any grade of purity they desired unless proven otherwise.  The EPO has now reversed this position.  This subtle change shifts the main patentability hurdle from novelty to inventive step – i.e., it is necessary to explain why enhancing purity was not an obvious step to take.  The key to overcoming this hurdle is in the detail, as explained below.

2. Be specific

The case that shifted the EPO stance on purity concerned an antihypertensive drug that is easier to absorb into the body in its amorphous form[i].  The compound was claimed as being at least 99.5% amorphous and containing less than 0.5% of the crystalline form when determined by HPLC analysis.  This illustrates an important tactic for patenting purity, namely specifying upper content limits on individual impurities instead of simply defining a threshold level of overall purity.  This detail should further distance a claim from the prior art and, when supported by comparative data, can show the Examiner that a technical advance is present.

3. Cast doubt on the prior art

Where possible, patent applications relying on purity should explain why routine purification steps are inadequate, e.g. because a specific impurity is difficult to remove by conventional measures.  Ideally, the application should contain experimental data to illustrate the failure of routine purification.  Making this extra effort during the drafting phase can provide powerful evidence to reply upon during prosecution and beyond[ii].

4. Define the purification process

Describing the purification steps that lead to the purity requirements of the claims is important to meet the fundamental requirement of the invention being sufficiently disclosed (known as ‘enablement’ at some patent offices).  However, rather than just including an experimental example, it is better to include a full written disclosure that could serve as the basis for a claim to the process.  Writing a claim to the purification process serves as a ‘Plan B’ in case the claims to the purified product are rejected for any reason

5. High purity starting materials are easier to patent

It is easier to obtain a European patent covering a pure starting material instead of a pure product.  This difference arises because the EPO presumes that the skilled person would use an impure starting material provided that this enables an adequate yield of product[iii]. It is therefore easier to persuade patent examiners that a high purity starting material is non-obvious.   This rationale has important implications in the context of compositions, as explained in section 7 below.

6. High purity high molecular weight compounds are easier to patent

It is also generally easier to patent high purity substances that have a high molecular weight, e.g. polymers.  The EPO tends to assume that high molecular weight compounds are harder to purify by conventional methods and that the skilled person would also be more likely to accept a lower grade of purity because of the difficulty in purification.[iv]  The same reasoning should apply to biological substances.

7. Compositions comprising a pure substance can be easier to patent

Section 5 above introduced the idea that claiming a compound in high purity is easier when the compound is a starting material for a chemical synthesis.  The EPO has also applied this reasoning to compositions containing high purity components because it considered the composition to be a product and the individual components to be starting materials used to produce the composition.  For example, a composition comprising a solvent (ethyl lactate) having a purity greater than 99% was considered to be new over a prior art composition containing such a solvent whose purity was not specified.[v]


Greg Stepney
Life Sciences & Chemistry

For more information on any of the topics raised above, please message Greg Stepney or your usual contact at Withers & Rogers. This publication is a general summary of the law. It should not replace legal advice tailored to your specific circumstances.

© Withers & Rogers LLP October 2021



[i] EPO Decision T 1085/13

[ii] EPO Decision T 728/98

[iii] EPO Decision T 786/00

[iv] EPO Decision T 142/06

[v] EPO Decision T 112/00