Priority patent applications for biotech and chemistry startups

The answer is often unsatisfactory… it depends on what you need the application to achieve.

In practice, there are usually two separate reasons for including data in an early patent filing.

The first, and the focus of this article, is to support a valid claim to priority. This is fundamentally about ensuring that the invention has genuinely been disclosed at the priority date.

The second is to support broader claim scope in the later-filed application. Experimental data added after the priority filing may sometimes strengthen inventive step arguments, support technical effects, or justify broader functional claims.

These two issues are often blurred together. However, they are legally distinct and getting them confused can be costly.

Importantly, while limitations in claim scope can sometimes be addressed by adding further experimental support in the later priority-claiming application (for instance into a PCT application filed 12 months after the priority application), problems with priority entitlement are usually much harder to repair retrospectively. If the invention was not properly disclosed in the original priority filing, the priority claim itself may fail entirely.

For startups operating in fast-moving technical fields, that can be catastrophic. A failed priority claim can mean the difference between a granted patent and a patent destroyed by your own publication, a competitor's filing, or a conference talk that occurred during what you assumed was a safely protected priority year.

What is a priority application and why (or when) does it matter?

A priority application aims to secure an early filing date for an invention for prior art purposes, while allowing an applicant to defer decision-making points and expensive patent filings in their chosen countries until much later (particularly when combined with the PCT system).

If a later patent application claims priority from that first filing, the later claims are treated as having been filed on the earlier priority date but only if they are properly disclosed in the priority application.

This matters because patentability is assessed against everything made available to the public before the relevant filing date. A valid priority claim can therefore protect the invention from:

• Scientific publications
• Competitor patent filings
• Conference disclosures
• Investor presentations
• The applicant's own publications (including press releases made after the priority date but before the filing date of the later application)

In rapidly developing fields such as biotechnology and chemistry, the difference of even a few months can be commercially decisive.

However, priority only applies to subject matter that was actually disclosed in the priority application. If the later claims extend beyond what was properly disclosed in the original filing, the priority claim may fail for those claims or claim features.

This is where the question of "data" becomes critically important for some technologies.

The legal test for priority at the EPO

The concept of priority is set out in the Paris Convention.

The Paris Convention makes no mention of data at all. Amongst other formal requirements, it states that a later application may claim priority from an earlier filing relating to the "same invention". But the interpretation of this and, therefore, the questions of whether a priority claim stands or falls, is down to the local patent office and, ultimately, the national courts.

For patent applications filed at the European Patent Office (EPO), the European Patent Convention does not define precisely what constitutes the "same invention", nor does it expressly state whether experimental data is required.

So why are we saying that you might need some data?

The modern EPO approach requires that the priority document disclose "the same invention" as the later claim. In practice, this means the skilled person must be able to derive the claimed subject matter "directly and unambiguously, using common general knowledge" from the priority application as a whole.

Importantly, this is effectively the same disclosure standard the EPO applies when assessing whether amendments add new matter. This is a notoriously strict test - essentially, if the aspect of the invention that you want to claim priority for is not almost written word for word in the priority application, then your priority claim can fail, since "the same invention" is not disclosed in the priority application.

This is an important reason why your priority applications should not be brief "quick and cheap" documents but fully developed applications that disclose all the necessary combinations of features that you may want to claim later.

But this strict disclosure test has another dimension that is often overlooked. The EPO's Boards of Appeal have made clear that the priority document must provide an enabling disclosure of the claimed invention - that is, a disclosure sufficient for the skilled person to actually carry out the invention. A purely textual disclosure of words describing an invention is not enough if essential features are missing or if the skilled person cannot, from the priority document as a whole, understand the invention as a real technical teaching.

This principle has real teeth.

The Boards have held that it would be a misuse of the priority system if applicants could jump ahead of competitors on the basis of mere expectations, omitting critical features of the invention altogether.^[1] Gaps in the disclosure relating to essential elements of the invention cannot be retrospectively filled by relying on knowledge acquired only after the priority filing.

So, priority is fundamentally a disclosure question: is your invention in the priority application or not? It ought to be that simple, but the critical point is that the EPO does not assess disclosure as a purely literal exercise. What is disclosed to the skilled person from the priority document is the technical teaching; the skilled person does not derive anything from purely hypothetical and aspirational statements.

The issue is not simply whether words describing the invention appear somewhere in the application. Rather, the question is whether the skilled person would understand the application as actually disclosing a real technical teaching.

This distinction becomes particularly important in biotechnology and chemistry.

Disclosure vs speculation: when words alone are not enough

In predictable technical fields, relatively little experimental support may be required for the skilled person to recognise that an invention has genuinely been disclosed. In engineering, where the laws of physics rule, a simple line drawing can be enough. It is often not even necessary to have physically made the invention.

Similarly, some biotechnology inventions are predictable once devised. For example, in nucleic acid nanotechnology, the behaviour of oligonucleotides and single-stranded DNA constructs is generally well understood. The skilled person already knows:

• How complementary sequences hybridise
• How ssDNA structures assemble
• How such molecules behave under established principles of nucleic acid chemistry

In this situation, disclosure of the relevant oligonucleotide sequences and structural arrangements may itself communicate a concrete technical teaching, even in the absence of experimental figures or laboratory data. If the skilled person made those constructs, they would inevitably act as predicted.

There is a caveat here. We need to be careful when relying on common general knowledge too heavily, as we may then be in danger of making the invention sound obvious. However, the core clever bit can still be inventive. The idea of putting X, Y and Z together can be entirely non-obvious, but the fact that we know how X, Y and Z work from our common general knowledge helps explain that the XYZ combination would work as expected.

But the position can be very different in more scientifically unpredictable areas.

Where an application merely proposes that a therapeutic agent "may" treat disease, or predicts that a complex engineered biological system will exhibit a particular phenotype, the EPO may ask whether the alleged invention was truly disclosed at all - or whether the application merely describes a hoped-for research outcome.

This is why the need for data depends heavily on the nature of the invention.

Do we need data to file a priority patent application? A chemistry example

Consider a startup developing a modified catalyst for a known class of chemical reactions.

If the catalyst scaffold is established, the mechanism of action is understood, and the structural modifications are relatively conservative, the skilled person may readily accept that the catalyst will function substantially as described. In such circumstances, a detailed structural disclosure combined with a mechanistic explanation may be sufficient to support priority entitlement even with limited experimental data.

Now consider a different chemistry invention, a broad new class of compounds, coupled with sweeping claims about dramatically improved selectivity or activity, but with no supporting examples or comparative results.

The absence of technical support becomes much more problematic. The EPO may regard the disclosure as speculative because the skilled person cannot reliably determine whether the alleged technical effect genuinely forms part of the disclosed invention.

Although on the one hand the literal wording may be present in the priority application to define the subject matter being claimed, the technical disclosure is absent. The skilled person does not derive from the priority document that the invention actually works.

Do we need data to file a priority patent application? A pharmaceutical example

Therapeutic inventions are often among the most demanding cases and are almost always where the stakes are highest.  They are also increasingly the battleground where priority disputes are won and lost.

This is because the EPO treats medical use claims (i.e. claims in the form of "compound X for use in the treatment of disease Y") differently from simple product or process claims. For a medical use claim, attaining the claimed therapeutic effect is considered a functional technical feature of the claim. It is not just a label or a statement of intended purpose; it is something the invention must actually achieve. Whether the therapeutic effect has been shown to be achievable is assessed as a question of sufficiency of disclosure.

What does this mean in practice?

The patent and, perhaps more critically for our purposes, the priority document must make it credible that the claimed therapeutic agent is actually suitable for the claimed therapeutic application. Mere verbal statements asserting therapeutic utility are not enough. The application must provide some information, for example experimental tests showing that the claimed compound has a direct effect on a metabolic mechanism specifically involved in the disease, or other evidence that goes beyond a bare assertion.

Clinical data are not always required. In vitro examples may be sufficient if, for the skilled person, they directly and unambiguously reflect the therapeutic application. A scientifically coherent mechanistic explanation grounded in established biology may also contribute to credibility. However, a priority application that contains nothing more than a statement that compound X "may be used for treating disease Y" without any supporting evidence risks being treated as a speculative wish rather than a genuine disclosure of a therapeutic invention.

The EPO's Boards of Appeal have confronted this directly in the context of priority. In one case involving a second medical use claim, the Board found that the priority document contained no experimental data or other evidence of any kind that went beyond a mere allegation that the claimed peptides were suitable for treating the listed disorders. The Board concluded that the priority document did not even provide an initial basis for believing that the peptides were suitable for treating those disorders and, therefore, did not disclose the invention in a manner sufficiently clear and complete for it to be carried out by the skilled person. Priority was denied.^[2]

This is a striking result. The words were in the priority document, the peptides were disclosed, the diseases were listed, but without any supporting evidence of therapeutic suitability - no animal model data, no cell assay results, no mechanistic rationale grounded in specific evidence - the priority document was treated as disclosing nothing more than a research hypothesis.

Consider how this plays out for a typical startup.

Suppose a biotech company identifies a biological target and files a priority application stating that an antibody against that target may be useful for treating autoimmune disease. If the application contains no binding data, no functional assay data, no disease model evidence, and little mechanistic rationale, the disclosure may be regarded as speculative.

By contrast, the position may be considerably stronger where the application contains:

• Receptor binding data
• Cell assay results
• Biomarker correlations
• At least a scientifically coherent mechanistic explanation grounded in established biology

The issue is not simply whether "data" is present. The question is whether the application genuinely discloses the invention as a credible technical teaching — one where the skilled person can believe, based on what is in the document, that the therapeutic effect is achievable. For medical use claims, this is not an optional extra; it is part of what makes the invention the invention.

The hidden role of common general knowledge

Much of the apparent inconsistency in EPO practice can be explained by the role of common general knowledge.

The EPO asks what the skilled person would understand from the application, considering what was already known in the field at the priority date.

In predictable technical areas, established scientific principles may themselves provide much of the technical credibility needed for disclosure. The suitability of a therapeutic agent may even derive its credibility from the prior art, for instance where the mechanism of action is well-established and the skilled person can readily extrapolate to the claimed use.

In more uncertain or unpredictable fields, however, common general knowledge may not be enough to bridge the gap between a theoretical proposal and a genuinely disclosed invention.

This is why two applications with similarly limited data can receive very different treatment depending on the underlying science.

Do we need data to file a priority patent application? Practical guidance for startups

From a practical perspective, startups should generally include as much technical support as reasonably possible before filing a priority application. The trick is balancing this with the need to secure an early filing date for prior art purposes, or to allow you to speak more freely about your technology.

The type and amount of support required will depend heavily on the predictability of the technology.

Useful supporting material may include:

• Proof-of-concept data
• Mechanistic explanations
• Comparative examples
• Pathway analyses
• Modelling
• Prototype characterisation
• Binding studies‍
• In vitro experimental results

For therapeutic inventions in particular, remember that the therapeutic effect is treated as a functional feature of your medical use claim. Your priority application needs to credibly disclose that the therapeutic effect is achievable, not just state that it is intended. This may require at least some experimental evidence showing that your compound has a relevant biological activity, even if full clinical data are not yet available.

The objective is not to conclusively prove every aspect of the invention at the priority date. Rather, the goal is to ensure that the skilled person would recognise the application as disclosing a genuine and credible technical teaching, rather than merely a future research plan.

So, do we need data to file a priority patent application?

The question of whether a priority application "needs data" has no universal answer.

At the EPO, the real issue is whether the invention has genuinely been disclosed such that the skilled person can derive it directly and unambiguously from the priority application using common general knowledge. The priority document must provide an enabling disclosure, one that allows the skilled person to carry it out, not merely a textual placeholder for work that has not yet been done.

We also need to keep in mind the strict disclosure test and the requirement for almost the same "word-for-word" language in the priority application as the later priority-claiming application.

In predictable technical fields, relatively little experimental support may sometimes be sufficient.

In more unpredictable areas, particularly therapeutics and complex biology, substantially more technical support may be needed before the invention is recognised as truly disclosed. For medical use claims, this means the priority document must make it credible that the therapeutic effect is achievable; a bare assertion of therapeutic utility is not enough.

For startups, understanding this distinction is critical. Problems relating to claim breadth can sometimes be addressed later with additional data. Problems with priority entitlement often cannot.

Key takeaways

File as early as possible whilst aligning with your business plan. Filing early means there is less prior art that can be cited against you.

Ensure that your priority application discloses the invention:

• Include credible technical rationale based on the common general knowledge - why it makes sense that it should work now that you have thought of it.
• Include some data to bridge the gap between the common general knowledge and the claimed invention where needed - particularly for therapeutic inventions, where you need to credibly show that the therapeutic effect is achievable.
• File a full and detailed priority application so that the text literally discloses the invention in as much detail as possible.
• Remember: the priority document must provide an enabling disclosure. Words describing a hoped-for outcome are not a substitute for evidence that the invention works.

^[1] EPO Case Law of the Boards of Appeal, 11th Edition, 2025, section II.D.4.6; T 81/87 (OJ 1990, 250).

^[2] EPO Case Law of the Boards of Appeal, 11th Edition, 2025, section II.D.4.6; T 411/19.