Joris DeeneA biosimilar is developed to be therapeutically equivalent to an existing reference drug. Does that equivalence mean, then, that the biosimilar infringes a patent that protects the formulation of that reference drug? No, ruled the presiding judge of the Dutch-speaking Commercial Court in Brussels in summary proceedings on January 8, 2026, in the case of Regeneron v. Celltrion. Bioequivalence under pharmaceutical law is not the same as technical equivalence under patent law, and whoever alleges patent infringement based on equivalence bears the burden of proof.
The facts
Regeneron developed the drug Eylea®, which contains the active ingredient aflibercept, for the treatment of eye conditions such as wet age-related macular degeneration. Aflibercept is a VEGF antagonist that binds to vascular endothelial growth factor, thereby inhibiting the pathological formation of new blood vessels. The basic patent for aflibercept had already expired; the supplementary protection certificate, which extended the protection, was set to expire on November 23, 2025.
Celltrion developed Eydenzelt® (code name CT-P42), a biosimilar of Eylea® for all of its therapeutic indications. The European Commission granted a marketing authorization on February 12, 2025, and the product has been on the list of reimbursable prescription drugs since October 1, 2025. Celltrion announced that it would enter the Belgian market as soon as the patent situation allowed.
To block the launch, Regeneron did not rely on the expired basic patent, but on a supplementary European patent, EP 2 364 691 (EP 691), for formulations of a VEGF antagonist suitable for intravitreal administration. Claim 1 of that patent describes a formulation containing specific excipients in specific concentrations: a buffer system, an osmotic agent, and a stabilizer. The parties agreed that the Eydenzelt® formulation literally incorporates some features but not three others, including the buffer system: EP 691 uses a sodium phosphate buffer, while Eydenzelt® uses a histidine buffer. Regeneron argued that this constituted an infringement by equivalence.
The decision
The presiding judge declared that he had jurisdiction, ruled that the claim was admissible but without merit, and ordered Regeneron to pay the costs.
With regard to jurisdiction, the presiding judge confirmed that the judge hearing the motion for preliminary relief has jurisdiction as soon as the plaintiff cites circumstances in the summons from which the urgency can be inferred; the use of the word “urgent” is not required for this purpose (Art. 584 Jud.C.). The chair of the Commercial Court in Brussels has subject-matter and territorial jurisdiction (Art. XI.337, § 1 CEL).
With regard to admissibility, the presiding judge rejected Celltrion’s defense that Regeneron lacked a standing interest. A party to the proceedings who claims to be the holder of a subjective right has the standing and interest required for admissibility, even if that right is disputed; the examination of the existence and scope of that right concerns the merits, not admissibility.
On the merits, the presiding judge recalled the framework for assessing preliminary injunctions: there must be a prima facie case justifying the provisional measure, without the court definitively determining the legal position of the parties (Art. 1039 of the Code of Civil Procedure). The patent holder must demonstrate an apparent infringement. To this end, the court first determines the scope of protection of the patent and then compares the essential characteristics of the invention with the product of the alleged infringer. Infringement exists only if those essential characteristics are present in that product, either literally or by equivalent.
The scope of protection of a European patent is determined by the claims, which are interpreted in light of the description and the drawings, taking into account any equivalent elements (Art. 69 EPC and the Protocol on Interpretation; Art. XI.28 WER). To assess infringement by equivalence, Belgian courts apply the function-way-result test: does the allegedly equivalent feature essentially perform the same function, in essentially the same way, and with essentially the same result?
The chair concluded that Regeneron had not demonstrated that equivalence. Three findings were decisive. First, the chair distinguished between bioequivalence under pharmaceutical law and technical equivalence under patent law. Bioequivalence does not preclude differences between the composition of the biosimilar and that of the reference drug with regard to the pharmaceutical form, the chemical form of the active ingredient, or the excipients. If one were to rule otherwise, every biosimilar would, by definition, constitute an infringement of every formulation patent of the reference drug. The presiding judge then noted that Regeneron bore the burden of proof and had limited itself to general considerations, without demonstrating that the histidine buffer and the sodium phosphate buffer perform the buffering function in essentially the same manner and with essentially the same result. On the contrary, Regeneron’s own documents showed that the choice of excipients affects the stability of the formulation and that the results are unpredictable in advance. Finally, Regeneron could not rely solely on a German ruling in which the equivalent effect of the buffer was not disputed, while the English court had held the exact opposite in parallel proceedings.
In the absence of evidence of reproduction with an equivalent buffer characteristic, the chair ruled that there was no prima facie infringement. Moreover, the balancing of interests also favored Celltrion: Regeneron’s damages (loss of market share, price declines) was quantifiable and recoverable, whereas Celltrion’s damages in the event of an unjustified ban were more difficult to estimate; furthermore, the public interest and cost savings for social security were also factors.
Legal analysis and interpretation
Bioequivalence is not technical equivalence
The crux of the ruling lies in the sharp distinction between two concepts of equivalence that, at first glance, appear related but function entirely differently in legal terms. Bioequivalence is a concept from pharmaceutical law: it refers to the therapeutic interchangeability of a biosimilar or generic drug with its reference product, and is compatible with differences in composition. Technical equivalence under patent law, on the other hand, assesses whether a differing technical feature performs the same function in the same manner and with the same result as the patented feature.
The chair’s reasoning—that equating the two concepts would mean that every biosimilar, by definition, infringes every formulation patent—is convincing and touches on the economic rationale of the biosimilar system. If the opposite position were accepted, a formulation patent would de facto extend the protection of the expired substance patent—not by monopolizing the active ingredient, but by encompassing every therapeutically equivalent formulation. This conflicts with the delimitation of the scope of protection as defined by the claims (Art. XI.28 WER): that scope extends only to formulations containing the specific excipients and concentrations listed in the claim, not to every stable aflibercept formulation for intravitreal administration.
The equivalence test and the divergence among national courts
The ruling aptly illustrates that, in practice, the function-way-result test primarily stumbles on the third prong: the result. The fact that a histidine buffer and a sodium phosphate buffer both maintain the pH—the same function, in a comparable manner—was not disputed. The decisive factor was that Regeneron failed to demonstrate that both buffers lead to essentially the same result, since the choice of buffer also affects the stability of the formulation and that stability is the result of a specific combination of excipients.
It is particularly instructive to see how the chairperson handles the differing rulings by German and English courts regarding the same patent. The Belgian equivalence test is assessed in essentially the same way as in neighboring countries, but the outcome depends on the specific arguments presented by the parties. In the German proceedings, the parties did not dispute the equivalent effect of the buffer, so that issue was not central there; in the English proceedings and in the present case, however, that effect was disputed. This explains why a seemingly favorable foreign judgment yields little benefit for the patent holder when the factual discussion elsewhere takes a different course. This is consistent with Belgian interim relief case law, which does not accord a presumption to a foreign decision concerning another national aspect of a European patent, but merely examines whether it is prima facie relevant to the case at hand.
The Burden of Proof as a Pivotal Issue in Preliminary Injunction Proceedings
The ruling confirms that the burden of proof regarding the alleged infringement rests entirely with the patent holder, and that general technical considerations are insufficient. The finding that Regeneron limited itself to general observations, while its own expert reports underscored the unpredictability of formulation changes, illustrates how a patent holder can shoot itself in the foot. In summary proceedings, where the judge assesses a prima facie case within the bounds of reasonableness without ruling on the merits of the case, such an internal contradiction carries significant weight. The fact that the presiding judge explicitly left the urgency requirement unaddressed—because the infringement requirement had not been met—confirms the logical sequence of the test: without a prima facie case, there is no reason to further examine the urgency.
Specifically, what does this mean?
For holders of formulation patents in the pharmaceutical industry. A formulation patent does not constitute a disguised extension of an expired substance patent. Anyone who, after the expiration of the basic patent and the supplementary protection certificate, wishes to block market access for a biosimilar on the basis of a formulation patent must substantiate equivalence technically and concretely, characteristic by characteristic. General references to bioequivalence or to the fact that the competing product was approved as a biosimilar are not sufficient. Build your case around targeted expert evidence that demonstrates not only the function and mode of action but also the result of the substitute characteristic, and ensure that your own documents do not undermine that position.
For manufacturers of biosimilars and generics. The ruling provides a useful defense template. It is advisable to deliberately deviate from the patented excipients and concentrations during development and to document those deviations technically, so that it can be demonstrated that the result—particularly the stability—is not necessarily the same. Acting diligently also helps: timely notification to the patent holder, as provided for in the regulations governing the supplementary protection certificate, strengthens one’s position in the balancing of interests. Inaction on the part of the patent holder—who was aware of the situation for months but only responded belatedly—may work to the detriment of the patent holder.
For parties to patent summary proceedings. The order in which the court examines these issues is strategically relevant: the court first examines the alleged infringement and only then the urgency. A defendant who can seriously dispute the infringement does not even need to refute the urgency. Foreign rulings concerning other national divisions of the same European patent are useful but not decisive; what matters is whether the factual dispute in the Belgian case is the same as that in other jurisdictions.
Frequently asked questions (FAQ)
Does a biosimilar automatically infringe on a patent for the original drug?
No. The fact that a biosimilar is therapeutically equivalent (bioequivalent) to the reference drug does not mean that it infringes a patent protecting the formulation of that reference drug. Bioequivalence allows for differences in composition, whereas patent infringement requires that the essential technical features be reproduced, either literally or by equivalence.
What is the difference between bioequivalence and technical equivalence?
Bioequivalence is a concept in pharmaceutical law that refers to the therapeutic interchangeability of two drugs. Technical equivalence is a concept in patent law and is assessed using the function-way-result test: a differing feature is considered equivalent only if it essentially performs the same function, in essentially the same manner, and with essentially the same result as the patented feature.
Who must prove patent infringement in summary proceedings?
The patent holder bears the burden of proof. The patent holder must establish a prima facie case of infringement; general technical considerations are not sufficient. If the patent holder fails to do so, the claim will be dismissed for lack of a prima facie case, without the court needing to examine the urgency of the matter.
Conclusion
This ruling confirms that a formulation patent cannot extend the protection of an expired substance patent through the doctrine of equivalents. Bioequivalence under pharmaceutical law and technical equivalence under patent law are two different things, and a patent holder who invokes infringement by equivalence must demonstrate this concretely, feature by feature—with evidence that the result is also the same, not just the function. It is not sufficient to point to the therapeutic equivalence of the competing product.
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