A case examining the so-called “patent dance,” a regulation that allows biologic and biosimilar manufacturers to resolve patent disputes, was debated in the Supreme Court Wednesday.

Amgen and Sandoz delivered oral arguments to the justices following Amgen’s 2014 lawsuit against Sandoz, Novartis’ generics and biosimilars business unit. Amgen had originally sued Sandoz for patent infringement.

In 2015 Sandoz’s Zarxio became the first biosimilar approved by the FDA. It is a copycat version of Amgen’s cancer-treatment Neupogen. Along with patent infringement, Amgen said that Sandoz should be “enjoined,” or prohibited from selling and marketing Zarxio because it did not provide Amgen a copy of its biosimilar application.

See also: Infographic: the growing biosimilar market in the U.S.

The Biologics Price Competition and Innovation Act (BPCIA), a provision of the Affordable Care Act, created a pathway for manufacturers to bring biosimilars to market. As part of that process, the provision allows a company developing a biosimilar and the reference product drugmaker or sponsor — essentially, the drugmaker that markets the biologic version of the drug — to exchange information regarding a biosimilar application in an attempt to resolve patent disputes.

Importantly, the legislation states that biosimilar manufacturers shall provide notice to the biologic manufacturer at least 180 days before the biosimilar manufacturer begins marketing the drug. The Supreme Court this week heard arguments over whether the biologic sponsor should receive that notice before or after the biosimilar is approved and whether that notice is mandatory.

A letter from Sandoz to Amgen sparked the original lawsuit. Sandoz told Amgen in 2014, when the FDA accepted its application for Zarxio, that it would not participate in the patent dance and that the letter served as the start of its 180-notice. Amgen then filed a suit, alleging patent infringement. The Federal Circuit Court ruled last year that BPCIA’s 180-day provision is mandatory, and that effective notice can only be given after FDA approval — as a result, Sandoz petitioned the Supreme Court to hear the case.

See also: Merck educates doctors about biosimilars, long before it will sell one in the U.S.

Deanne Maynard, Sandoz’s lawyer, argued to the justices that Sandoz provided Amgen sufficient notice for Zarxio, saying that Congress “created a comprehensive and self-contained scheme for the early resolution of patient disputes, and that courts should apply that comprehensive scheme as written. They shouldn’t look elsewhere for consequences,” such as the Federal Circuit’s recent ruling.

She also contended that this ruling misread the original bill and “wrongly delays the marketing of every biosimilar.” Providing biologic sponsors with a 180-day delay is “nowhere in the statute,” she added, and requiring that delay would extend the patent length of the biologic drug over the 12 years it already has.

Seth Waxman, Amgen’s attorney, argued that the bill’s use of the word “shall,” in the provision implies that the patent dance is mandatory. “Congress did not create detailed procedures for resolving biosimilar disputes and repeatedly use the word ‘shall’ merely to have to have applicants…then disregard those mandates.”

See also: What do physicians think about biosimilars?

The Supreme Court is expected to issue a decision by the end of June, according to Big Molecule Watch.

The stakes are high for Amgen, as the drugmaker is facing a loss of revenue due to the arrival of biosimilar versions of Neupogen. The drugmaker reported that sales for Neupogen were down 31% to $101 million in the first three months of 2017, compared to the same period a year ago, citing “the impact of competition.” At its height in 2013, Neupogen brought in annual sales of $1.3 billion.