By Tamara Mathias

(Reuters) – Abarca, a small Puerto Rico-based pharmacy benefits manager, said on Tuesday Amgen Inc had agreed to give its health insurer clients an additional discount on its blockbuster rheumatoid arthritis drug Enbrel, if patients discontinue taking it after three months.

Pharmacy benefits managers like Abarca act as middlemen in the drug supply chain, and negotiate discounts on drugs on behalf of health insurers. Depending on the discounts drugmakers are willing to provide, pharmacy benefits managers make decisions about which drugs to include in coverage plans.

Abarca said its new deal with Amgen will help health insurers save costs when patients discontinue the drug because of side effects or a lack of effectiveness. It did not disclose the size of the additional discount.

Last year, Abarca, which currently serves 2.5 million members, signed a similar agreement with Amgen over its cholesterol drug Repatha and with Biogen Inc over coverage of its medicines to treat multiple sclerosis.

Abarca’s Chief Operating Officer Javier Gonzalez estimates Enbrel costs the health insurance plans the company represents between $4,000 and $5,000 per patient every month.

The drug has been on the U.S. market since 1998 and is Amgen’s best-selling product, bringing in revenue of $1.37 billion in the third quarter.

While the U.S. Food and Drug Administration has approved biosimilars to Enbrel, including one from Novartis AG, they have not yet been launched because of ongoing patent litigation.

(Reporting by Tamara Mathias in Bengaluru; Editing by Shailesh Kuber)