UPC Weekly brings you timely, thoughtful and joined-up insight into the evolving landscape of patent litigation at the Unified Patent Court.
2026 Week 45
The Munich Local Division (LD) has revoked Enanta Pharmaceuticals’s EP 4051265 B1, asserted against Pfizer in relation to Paxlovid™ and its active ingredient nirmatrelvir. The decision in Enanta v. Pfizer (UPC LD Munich, UPC_CFI_0768/2025 29 October 2026) turned principally on priority and intervening prior art.
The decision confirms that the EPO’s direct and unambiguous disclosure standard applies not only to an assessment of added matter under Article 123(2) EPC but also to substantive priority entitlement at the UPC and it gives that principle particular practical force in the context of generic chemical claims.
The existence of a parallel EPO opposition allows a direct comparison of the approaches taken. Just a month before the UPC judgment, the EPO Opposition Division had considered substantially the same priority problem and reached a strikingly similar preliminary conclusion, including an express finding on partial priority (following the EPO Enlarged Board decision G 1/15). The result also follows a parallel invalidity finding in the US.
The priority issue
The patent concerns peptidomimetic compounds for inhibiting coronavirus proteases. Like many chemical patents, it defines compounds using generic structural formulae, often referred to as ‘Markush’ formulae. These specify a common chemical structure but allow particular positions within it to be occupied by groups selected from a range of alternatives. A single generic formula can therefore encompass a very large number of individual compounds.
This gives rise to an important distinction between encompassing a compound and disclosing it. A particular compound may fall within a generic formula without being individually disclosed by it. Similarly, where several positions in the formula are each defined by lists of possible substituents, disclosure of those lists does not necessarily amount to disclosure of every combination obtainable by selecting members from them.
That distinction was central to the case. The relevant claims required a certain generic formula and a combination of particular substituent definitions. The Munich LD concluded that the claimed combination was not directly and unambiguously disclosed in the priority application, confirming in the headnotes of the decision that “[a] disclosure of a generic chemical structural formula in a document does not, in principle, disclose the individual compounds or more specific subclasses falling within that formula. A subject matter described in this way is generally very broad. There are numerous variants that may be covered by it. Individual compounds or more specific subclasses which are embraced by such a generic formula are only disclosed if they are directly and unambiguously derivable by a person skilled in the art from the complete teaching of the document.”
Once the priority claim failed, Pfizer’s intervening disclosure of the chemical structure of nirmatrelvir became available as an anticipatory disclosure, and their own co-pending patent application became relevant Article 54(3) prior art, with fatal consequences for novelty.
Partial priority
A finding that a claim as a whole is not entitled to priority does not necessarily mean that all subject matter falling within it has the same effective date. This concept is familiar at the EPO, where it is now well established that a generic claim can be conceptually divided for priority purposes. Subject matter within the claim which is directly and unambiguously disclosed in the priority application can enjoy the priority date, while the remaining subject matter has only the later filing date.
In this case, the Munich LD did not need to determine whether any part of the claims actually enjoyed partial priority. The alleged infringement, nirmatrelvir, fell within the non-priority-entitled subject matter exposed to Pfizer’s intervening disclosure. An amendment confined to any priority-entitled subject matter might therefore have preserved a valid claim, but not one covering the infringing product. Partial priority therefore could not save Enanta’s overall case.
Nevertheless, the Court chose to address the point expressly and indeed made it one of its three headnotes. It held that “[e]ven if a patent claim is entitled to partial priority and would, in that respect, be valid in relation to the state of the art while another part of its scope remains invalid, it cannot be regarded as partially valid and upheld without the patent proprietor filing an Application to amend the patent.”
The prominence given to a point which, on the facts, the Court did not actually need to determine suggests that the Munich LD considered the procedural consequence worthy of more general guidance. The distinction is between conceptually dividing a claim to determine the effective date of different subject matter and actually maintaining a narrower claim. The former does not achieve the latter.
A good reason for the LD to draw attention to this is in view of the UPC’s approach to considering the validity of a granted claim set. In UPC Weekly 2026 w29, we explained how the UPC, unlike the EPO, will consider the validity of the dependent claims even if the main claim is decided to be invalid, without the patentee needing to file amendments or auxiliary requests. But here, where part of a single claim is considered invalid, the LD is saying that it will not uphold the validity of the remainder of the claim unless the patentee removes the invalid part.
This appears consistent with EPO practice and the parallel opposition to the same patent provides a direct illustration of this.
The parallel EPO opposition
Pfizer has also opposed the same patent at the EPO. The Opposition Division issued its preliminary opinion on 28 September 2026, only a month before the UPC judgment.
Its provisional analysis of priority was notably similar to the conclusion of the Munich LD. Although the individual elements of the claimed combination could be found in the priority application, the OD considered that there was no disclosure directing the skilled person to their particular combination. The granted claim as a whole therefore did not enjoy priority.
Unlike the UPC, however, the OD went on actually to conduct the partial-priority analysis under G 1/15. It identified specific compounds E1, E3, E5, E7, E9, E11, E13, E15 and E17 disclosed in Table 5 of the priority application and provisionally concluded that claim 2 could enjoy partial priority to that extent.
That finding did not save the granted claim. The remainder of its scope lacked priority and was provisionally considered anticipated by Pfizer’s intervening disclosures. At this stage, as at the UPC, Enanta Pharmaceuticals is only requesting maintenance of the patent as granted and has not filed any auxiliary requests directed to potentially priority-entitled subject matter.
The two proceedings therefore arrive at the same practical point from slightly different directions. The UPC assumed the possibility of partial priority but held that it could not save the claim in the absence of an application to amend; the EPO provisionally found that particular subject matter did enjoy partial priority, but that finding could not save the broader granted claim.
The corresponding US proceedings
The European decisions also follow a recent US defeat for Enanta Pharmaceuticals. In June 2026, the Federal Circuit invalidated the corresponding US patent. There, the key factor was that the provisional application disclosed a C2-C12 alkyl substituent, whereas the later patent extended the range to C1-C12 to cover Paxlovid (which required C1). The Court held that the provisional did not provide written-description support for C1: the fact that Enanta Pharmaceuticals characterised C2 as a ‘drafting error’ could not supply subject matter which the provisional itself had not disclosed. Loss of the earlier date allowed Pfizer’s intervening disclosure of nirmatrelvir to anticipate the claims, as in Europe.
Commentators have also linked the decision to the so-called “blaze marks” approach from In re Ruschig, under which merely encompassing a later-claimed compound within a broad chemical disclosure is not necessarily enough: the original application must provide sufficient pointers towards the particular subject matter later claimed to demonstrate that the patentee had possession of that invention at the priority date. Although the legal tests differ, that has an obvious parallel with the UPC’s treatment of the generic chemical disclosure in the present case.
Practical implications
The Munich Local Division’s treatment of priority appears closely aligned with established EPO principles, an impression reinforced by the parallel EPO opposition against the same patent.
There are three points of particular practical relevance.
First, particularly for chemical genus claims, the fact that each individual element of a claimed combination can be found somewhere in the priority document is unlikely to be enough. The combination itself must be directly and unambiguously disclosed.
Second, partial priority may be crucially important where intervening prior art is relevant only to part of the scope of a generic claim. G 1/15 allows priority-entitled subject matter to retain its earlier effective date even where the broader claim cannot do so.
Finally, identifying that surviving subject matter is only part of the task. If the remainder of the claim is invalid, the proprietor needs to have put forward an allowable amended claim which captures the subject matter that survives.
The priority test at the EPO has long been a stringent one, following the ‘gold standard’ of direct and unambiguous disclosure.
This case may reflect a broader trend towards similarly close scrutiny of priority entitlement by the courts. While the legal tests differ between Europe and the US, both the Federal Circuit and the UPC focussed on what the earliest application actually disclosed, rather than what its broad teaching might encompass.
For patent applicants, particularly in fast-moving fields, the take-home message is that it cannot safely be assumed that gaps in an early priority filing can be plugged during the priority year, especially if there is a risk of intervening competitor disclosures.
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