Recently, Kim & Chang successfully obtained from the Korean courts a pair of decisions agreeing that the scope of a pharmaceutical compound patent can include prodrug forms of the compound, both during the regular term of the patent (Supreme Court Decision 2022Hu10210, February 2, 2023) and even during a Patent Term Extension (“PTE”) (Seoul Central District Court Decision 2022Kahap21672, March 16, 2023 (granting preliminary injunction against generic prodrugs found to be infringing)).

A prodrug is a form of a drug compound that is metabolized in the body to produce the active ingredient of a drug that exerts a pharmacological effect in the body. Prodrugs are commonly used to develop drug products in cases where the active ingredient itself may be difficult to formulate as a product directly due to side effects, safety issues, or absorbency problems. The Supreme Court and Seoul Central District Court decisions cited above are the first cases in Korea to rule that prodrugs of a pharmaceutical compound are within the scope of a patent on the compound itself.
 

In this case, AstraZeneca had a patent on the compound “dapagliflozin,” which is the active ingredient of a type II diabetes drug. Dong-A ST filed a scope confirmation action at the Intellectual Property Trial and Appeal Board (the “IPTAB”) seeking a determination that its generic product (a prodrug form of dapagliflozin (i.e., dapagliflozin formate)) was outside the scope of AstraZeneca’s compound patent. The IPTAB agreed with Dong-A ST, but on appeal, the Intellectual Property High Court (the “IP High Court,” formerly the Patent Court) ruled that Dong-A ST’s prodrug was within the scope of equivalents of AstraZeneca’s patent despite the differences in composition from AstraZeneca’s patent claim. The Supreme Court rejected Dong-A ST’s further appeal, and ruled that the IP High Court’s decision was reasonable.

The Supreme Court based its decision that dapagliflozin formate (i.e., Dong-A ST’s prodrug) was within the scope of equivalents of AstraZeneca’s patent on the following:
 

In this case, AstraZeneca filed for a preliminary injunction against Dong-A ST’s prodrugs for patent infringement during the PTE of its patent, not the original term. Under the Korean Patent Act, the scope of a patent during a PTE is limited compared to the original term, as the scope is essentially narrowed to cover the approved product that was the basis for the PTE. Dong-A ST tried to argue that this meant that AstraZeneca’s patent should be strictly limited to the literal scope of the claims during PTE (i.e., not entitled to any equivalents at all). However, the Seoul Central District Court rejected Dong-A ST’s argument and granted AstraZeneca a preliminary injunction, finding that (i) even during the PTE, the patent was entitled to a certain scope of equivalents, and (ii) the active ingredient in Dong-A ST’s product was also dapagliflozin, so Dong-A ST’s product was clearly within the “scope of the extended patent” as set forth in the Patent Act.
 

The court decisions introduced above have been positively received by many legal commentators in Korea as improving the effective protection of pharmaceutical compound patents, by not overly limiting the practical scope of the patent at issue in a way that would categorically remove prodrugs from coverage but simply applying general rules of patent equivalents. These decisions also made the important clarification that ease of substitution under the doctrine of equivalents must be evaluated at the time of the determination of infringement, not the priority date of the patent, thereby making the enforcement of drug patents against potential follow-on products significantly more practical.