Marking your line in the sand: Why sufficiency matters at the priority date

A recent decision by the European Patent Office has highlighted that, for therapeutic inventions, simply outlining a treatment plan without supporting data in a priority filing is not enough to meet sufficiency requirements and secure priority. Here, we explore why case T 0883/23 reinforces the fact that, while a priority filing can help you to ‘mark your line in the sand’, it won’t shield you from prior art unless the invention is sufficiently disclosed from the outset.

What is a priority filing?

A priority filing sets the earliest date for your invention and acts as a ‘line in the sand’. Any of your subsequent applications that are filed within 12 months can claim this earlier date, helping to shield your invention from prior art that is published after the priority date.

This is particularly useful for inventors who want to disclose the invention in the priority filing to investors, publish scientific research, or continue developing the invention post-filing, as it allows them to do so without making public disclosures that could affect the patentability of future filings.

To successfully use a priority filing, both the priority application and any subsequent filing that wishes to use the priority date must relate to the ‘same invention’. In patent law, this means that a skilled person must be able to directly and unambiguously derive the same claimed subject matter from the priority filing using common general knowledge.

This test, which was established under Article 87(1) of the European Patent Convention (EPC), ensures that the priority document contains a clear and complete technical teaching of the same invention.

It is also possible to claim partial priority, where only the claims matching the priority application may benefit from the earlier date, while others will not.

If a priority date is not validly claimed, documents published between the priority date and a later filing date can be used to challenge both the novelty and inventive step of the subsequently filed application.

Case in point: T 0883/23

During opposition proceedings before the European Patent Office (EPO), third parties often challenge a patent’s entitlement to priority. This is a strategic move because, if the priority claim fails, prior art published after the priority date but before the filing date of an opposed patent will then become citable and thus potentially undermine the inventive step of a patent.

A recent example of this is case law T 0883/23. This concerned EP 3337478, a patent relating to a combination therapy for metastatic pancreatic cancer in patients who had not received prior chemotherapy.

The claimed invention involved a four-drug regimen of liposomal irinotecan, oxaliplatin, leucovorin, and 5-fluorouracil. It also claimed specific doses, notably 60 mg/m2 of both liposomal irinotecan and oxaliplatin.

Opposition and appeal: The priority challenge

The patent was initially opposed by a global pharmaceutical company known for generic and biosimilar medicine, which argued that the claimed invention was not entitled to its priority date. The EPO Opposition Division rejected this, maintaining the priority claim.

In its appeal, the pharmaceutical company raised two key questions:

1. Was the claimed 60/60 dose combination entitled to priority?
2. If not, did the patent still involve an inventive step in light of newly citable prior art documents?

Question 1: Priority and sufficiency at the priority date

Whilst the priority document for EP 3337478 described a study protocol that listed the claimed dose as one of several options, it did not include the study’s results.

The EP 3337478 patent, however, did include the study’s results, demonstrating that the 60/60 dose (a key technical feature of the patent) was tolerable in humans, whereas a higher dose, listed as 80 mg/m² of liposomal irinotecan, was not.
 
Applying the ‘direct and unambiguous derivability’ test, the Board of Appeal concluded that the invention was not sufficiently disclosed in the priority document because essential subject matter was absent.

It reasoned that, under established case law, the tolerability of the dose was a “functional technical feature” of the claim, and that the tolerability of a treatment is also “a prerequisite for the therapeutic efficacy.”

As such, the priority document did not meet the requirements for sufficiency under patent law, because it was not enough for the document to simply suggest an idea (in this case, the dosage). The technical teaching had to be complete and plausible too.

Question 2: Inventive step

With the priority claim lost, the patent’s effective filing date shifted forwards, resulting in the patent being vulnerable to newly citable prior art documents. 

The Board of Appeal thus assessed whether the claimed dose regimen was obvious in light of these newly citable existing treatments.

Despite the introduction of new prior art, the Board still found the claimed 60/60 dose combination to be inventive. Whilst similar therapies existed, they used different doses or formulations, and the available data did not reliably predict tolerability in humans.
 
The specific combination cited in the patent, however, showed an unexpected balance of efficacy and tolerability. As such, a technical effect was not suggested by the prior art.

Will your line in the sand withstand the waves?

A priority filing is your opportunity to mark a clear line in the sand by securing the earliest possible date for your invention and shielding it from future disclosures.

As T 0883/23 illustrates, however, that line only holds if the invention is sufficiently disclosed at the priority date. For therapeutic inventions, particularly those repurposing drugs or concerning new drug combinations, this means more than just outlining a treatment concept. It requires credible, plausible evidence that the invention works, even if clinical data are not always necessary (in vitro data are acceptable).

Hypothetical examples or study protocols alone won’t meet the bar for sufficiency. The EPO expects enough detail to convince a skilled person that the invention is workable, without relying on post-filed data. If sufficiency isn’t met at the priority stage, the priority claim may be lost, thus exposing the application to prior art and weakening its protection.

By working with our patent attorneys at the earliest possible opportunity, however, you can ensure that your first filing is, not only timely, but also technically sound, thereby giving your invention the best chance of standing up to scrutiny and securing long-term rights.