Celltrion and Samsung Bioepis target Merck’s Keytruda patents in South Korea

South Korean biosimilar developers challenge formulation patents that could delay competition for Merck’s blockbuster cancer drug until 2032.

South Korean biosimilar developers Celltrion and Samsung Bioepis have challenged patents covering the formulation of Merck’s blockbuster cancer drug Keytruda, in proceedings that could affect when competing products enter the South Korean market.

According to an exclusive report by The Asia Business Daily, the companies are seeking to overcome patent barriers that could extend protection beyond the expiration of Keytruda’s underlying substance patent.

The report suggests that a successful challenge could potentially bring biosimilar market entry forward from 2032 to 2028. However, the projected launch date is conditional on the outcome of the patent proceedings and other regulatory and commercial factors.

Formulation patents under challenge

Keytruda, whose active ingredient is pembrolizumab, is an immunotherapy used to treat multiple types of cancer. Developed by Merck, the drug generated approximately $31.7 billion in global sales in 2025, making it one of the pharmaceutical industry’s most commercially significant products.

South Korea’s underlying substance patent is reportedly due to expire in June 2028. However, a separate patent covering the drug’s formulation is reported to remain in force until March 2032. This distinction is central to the dispute. The expiration of a substance patent does not necessarily mean that competing products can immediately enter the market if other relevant patents remain enforceable.

Formulation patents can protect aspects of a medicine’s composition and other characteristics. Depending on the scope of the claims, they can therefore create additional obstacles for biosimilar developers seeking to commercialize competing products.

Celltrion and Samsung Bioepis are developing their own pembrolizumab biosimilars, designated CT-P51 and SB27, respectively. Both companies submitted applications for South Korean marketing approval in August 2026, adding regulatory progress to their efforts to prepare for market entry.

The precise patent numbers, claims, and legal grounds involved in the reported proceedings should be confirmed against records from South Korea’s Intellectual Property Trial and Appeal Board (IPTAB).

Potential to accelerate biosimilar competition

The possibility of bringing market entry forward by four years highlights the commercial importance of the challenges. If the relevant patent barriers are removed or otherwise overcome, an earlier launch could give Celltrion and Samsung Bioepis more time to establish their products in the South Korean market. It could also affect their commercialization strategies and competitive positioning as developers seek to capture a share of Keytruda’s substantial sales.

The timing of entry may be particularly important where multiple biosimilar developers are preparing competing products. Securing regulatory approval, resolving patent obstacles and establishing commercial supply arrangements will all influence which companies are best positioned to enter the market.

Nevertheless, filing a patent challenge does not establish that the patent is invalid or that a biosimilar can be launched without infringing other enforceable rights. The outcome will depend on the relevant claims, the evidence presented and the decisions reached by the competent authority.

Even if the challenges succeed, a 2028 launch is not guaranteed. Marketing approval, manufacturing readiness and any remaining patent restrictions could also affect the timetable.

A wider battle over Keytruda’s patent protection

The proceedings reflect the broader competition surrounding Keytruda as pharmaceutical companies prepare for the eventual loss of exclusivity associated with the drug’s core patent protection.

For biosimilar developers, challenging secondary patents can be an important part of preparing for market entry. For originator companies, maintaining enforceable protection over additional aspects of a medicine can help preserve commercial opportunities beyond the expiration of an underlying substance patent.

The outcome of the South Korean proceedings could therefore have implications for the launch strategies of both companies, although any wider effects will depend on the specific patents and markets concerned.

The next significant developments will be confirmation of the patents and claims under challenge, the legal grounds advanced by the petitioners and any substantive decisions issued by IPTAB.

Until those details are established through primary records, the reported 2028 launch date should be treated as a potential outcome rather than a confirmed timetable.

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