South Korea’s Celltrion said Friday that it has filed five petitions for an inter partes review of Genentech’s US patents on its blockbuster breast cancer drug Herceptin, as it prepares to launch Herzuma, its biosimilar version of Herceptin, in the US.

Celltrion has filed applications with the US Patent and Trademark Office challenging five patents on Herceptin held by Roche-owned Genentech from the course of Feb. 20 to March 24, the company said.

The patents in question concern Herceptin’s administration methods as well as methods of treating breast cancer using a regimen that includes the breast cancer drug, according to the Korean drugmaker.

Celltrion is also planning to file additional IPR applications to challenge Herceptin’s main formulation patent next month, a Celltrion spokesperson told The Korea Herald.

“The most important formulation patent for Herceptin is due to expire in 2019. We’re planning to take action early on and challenge this key patent next month,” the spokesperson said.

Herceptin is one of Genentech’s top selling drugs, generating global sales of $6.5 billion in 2015.

Though Genentech’s main formulation patent on Herceptin is scheduled to expire in 2019, the US firm has been filing new patents to protect its exclusivity until 2030 to fend off biosimilar competition from firms like Mylan, Pfizer-owned Hospira, Amgen, Samsung Bioepis and Celltrion.

Biosimilars refer to cheaper, near-replicas of live cell biologic medications whose patents have expired.

Celltrion has already filed Herzuma for approval by the European Medicines Agency, making the submission in October 2016.

The firm is currently wrapping up final-stage clinical trials for Herzuma in the US and preparing to file the drug for approval by the US Food and Drug Agency, the firm said.

Meanwhile, Herzuma was already approved for sale in Korea in January 2014 for the treatment of metastatic breast cancer, early stage breast cancer and metastatic stomach cancer.

By Sohn Ji-young (jys@heraldcorp.com)