In a recent decision, the Korean Patent Court found it inappropriate to apply the doctrine of (prosecution history) estoppel to the expression “prodrug ester,” which was deleted from the claims by the applicant in response to the Examiner’s rejection for indefiniteness. During prosecution, the applicant interpreted the Examiner’s rejection of the expression to be that as a rejection of a functional language encompassing any prodrug esters that could be converted to the parent compound in vivo and exhibit the pharmacological activity, and the applicant deleted the term to quickly resolve the Examiner’s rejection issued for the reason of employing a functional language without meaning to surrender all ester compounds falling under the concept of a prodrug ester (Patent Court Case No. 2020 Heo 5832 dated February 17, 2022; appeal pending).

▶ Background of the Case

A. The Subject Invention and Prosecution History

The claimed invention of the subject patent relates to novel compounds of general formula I, including dapagliflozin (), used as an oral agent for treating diabetes.

During the prosecution of the subject patent, the Examiner rejected claim 1, which was directed to a compound of general formula I, or a pharmaceutically acceptable salt, stereoisomer, or prodrug ester thereof for failure to meet the description requirement under Article 42(4)(ii) of the Korean Patent Act, stating that the term “prodrug ester” was an indefinite expression that did not specify a specific structure or component. In response thereto, the applicant deleted the term “prodrug ester” from claim 1, while submitting remarks indicating that the Examiner’s rejection was deemed resolved by the amendment.

Thereafter, the subject invention was allowed by the Examiner.

B. Scope Confirmation Trial

Company A, a generic competitor of the patentee, lodged a scope confirmation trial, seeking a declaratory judgement as to whether the accused party’s mode of practice fell within the scope of the patent claim. Company A’s mode of practice was dapagliflozin formate (), an ester prodrug of dapagliflozin, which could be changed to dapagliflozin (parent compound) in vivo and exhibit its pharmacological activity.

The issue of the case was whether Company A’s mode of practice was equivalent to the claimed invention of the subject patent. Specifically, the issue focused on whether the patentee intentionally excluded Company A’s mode of practice from the scope of the claimed invention during prosecution.

The Trial Board decided that Company A’s mode of practice was not equivalent to the subject invention and, thus, fell outside the scope of the subject patent because a person skilled in the art could not easily change dapagliflozin (subject invention) to dapagliflozin formate (Company A’s mode of practice), and dapagliflozin formate (an ester prodrug of dapagliflozin) was considered intentionally excluded during the prosecution of the subject patent.

Thereafter, the patentee filed an appeal before the Patent Court.

▶ The Patent Court Decision

By way of review, according to the Korean Supreme Court, a product that does not literally infringe upon the express terms of a patent claim may nonetheless be found to infringe the claim, if (i) an invention defined in a patent claim and an accused product are identical in terms of the technical idea or principle to solve a common problem (prong 1); (ii) an element recited in the patent claim and a corresponding element in the accused product to the claimed element achieve substantially the same effect (prong 2); (iii) interchangeability between the claim element and the corresponding element is obvious to a person having ordinary skill in the art (prong 3); and (iv) the corresponding element was not intentionally excluded from the scope of the claim during the prosecution of the patent (prong 4).

In the proceedings before the Patent Court, there was no dispute between the parties with regard to prongs 1 and 2 of the doctrine of equivalents. Instead, the parties were in dispute over prongs 3 and 4.

Concerning prong 3 whether it would be obvious to change dapagliflozin to dapagliflozin formate, the Patent Court found that a person skilled in the art could easily change dapagliflozin to dapagliflozin formate in consideration of the following circumstances: (i) screening of prodrugs with improved physical properties had routinely been carried out in the course of developing the formulation of drugs such as dapagliflozin; (ii) it was well known in the art to prepare a prodrug in an ester form by means of chemical modification of a hydroxy (-OH) group present in an active compound such as dapagliflozin; (iii) a formate was the simplest ester form that could be used and other prodrugs in the formate ester form had previously been employed in the art; and (iv) there was no technical difficulty in chemically synthesizing dapagliflozin formate.

As to prong 4 (whether dapagliflozin formate was intentionally excluded), the Patent Court held that it would be difficult to conclude that the patentee had intentionally excluded dapagliflozin formate from the scope of the subject invention during the prosecution in light of the following facts: (i) it was the KIPO’s practice at the time of the filing date of the subject patent not to allow employment of the term “prodrug” in a patent claim for the reason of indefiniteness; (ii) in response to the Examiner’s rejection that the expression “prodrug ester” was indefinite for failure to specify its structure or component, the patentee interpreted that the Examiner had rejected this expression as functional language that failed to specify the chemical structure of the compound, encompassing any prodrug esters that could be converted to the parent compound (dapagliflozin) in vivo and exhibit the pharmacological activity; and (iii) the patentee deleted the expression to quickly resolve the Examiner’s rejection against the functional language, but did not mean to surrender all ester compounds falling under the concept of prodrug esters.

As a result, the Patent Court held that dapagliflozin formate was equivalent to the subject invention and, thus, fell within the scope of the subject patent.

Company A’s appeal against the Patent Court decision is currently pending before the Supreme Court.