South Korean biopharmaceutical company Celltrion (KOSDAQ: 068270) has filed five petitions for a review of Genentech's US patents on its blockbuster breast cancer drug Herceptin, it announced on Friday.

The news comes as Celltrion prepares to launch its biosimilar version of Herceptin, Herzuma, in the US.

Applications have been filed with the US Patent and Trademark Office, challenging five existing Herceptin patents held by Genentech, which is owned by Roche.

According to the Korean drugmaker, the patents take into account the administration methods used for Herceptin, as well as breast cancer treatment methods that include the drug.

A spokesperson from Celltrion told the Korea Herald that the company plans to file additional IPR applications to contest Herceptin's main formulation patent next month.

Herceptin is a best-selling drug for Genentech, generating revenues of USD6.5bn in 2015. Its main formulation patent is due to expire in 2019, but the US company is taking steps to protect its exclusivity until 2030 with new patents. This would keep biosimilar competition at bay.

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

Celltrion has filed the drug for approval by the EMA, and is awaiting final-stage clinical trial results before filing for approval with the FDA in the US.