In a suit filed in the state of Delaware, the Swiss-based Roche Holding is suing Pfizer to block the future marketing a biosimilar of its breast-cancer drug Herceptin in the US. The holding claims that the US drugs giant’s PF-05280014 infringes 40 patents held by its Genentech unit.

Roche is also seeking compensation for lost revenue if the Pfizer drug, which has not yet been approved by the US Food and Drug Administration (FDA), is launched before the Herceptin patents expire, starting in 2019.

Pfizer said it would comment on the lawsuit “at an appropriate time.” According to pharmaceutical journals, the US company itself has filed still-pending challenges to Genentech patents.

The suit is believed to be the first filed by Roche against a biosimilar version of Herceptin, which was worth an estimated $2.5 billion in US sales last year.

Pfizer’s drug, however, is not the first biosimilar of Herceptin, which uses the antibody trastuzumab to target the mechanism that makes certain types of breast cancer especially aggressive. The European Commission recently approved Ontruzant, a biosimilar of Herceptin manufactured by Samsung Bioepis.

Two other Roche medicines, including the cancer treatments Avastin and Rituxan, are also facing competition from biosimilars. Analysts have calculated that the Swiss holding could lose nearly $4 billion in sales by 2020 as cheaper versions hit the market.

Mylan and Biocon, which also have a biosimilar of Herceptin, have clinched a license deal with Roche that will kick in after their new product – set to be manufactured in India – is approved by the FDA, possibly in early December.

Europe is seen as being far ahead of the US in the use of biosimilars. In the seven years since Congress passed legislation regulating the drug copies, the FDA has approved only five biosimilars, compared to Europe’s 25, the news agency Bloomberg said.