What goes around comes around. It doesn’t seem that long ago that the Dublin-based drug firm Elan Corporation plc was fighting off the hostile advances of Royalty Pharma. It was July 2013, in fact, when Elan agreed to be acquired by Perrigo (NYSE: PRGO) for $8.6 billion.

To refresh the memory, Elan was a biotech company whose portfolio included royalties from Tysabri®, a multiple sclerosis treatment, and a neuropsychiatric pipeline with near-term value creation potential.  Early February 2013, Elan sold half of its rights to Tysabri to Biogen Idec, now known as Biogen Inc. (NASDAQ: BIIB), for $3.25 billion.

At the time, the terms called for $3.25 billion upfront cash plus 12% of global net sales for the first 12 months, then 18% for net sales up to $2.0 billion and 25% for sales in excess of $2.0 billion.

In February 2017, Perrigo divested its rights to the royalty stream from Tysabri’s global net sales. The buyer, RPI Finance Trust, an affiliate of Royalty Pharma, spent $2.2 billion in cash at closing. Perrigo is also eligible to receive up to $650 million in potential milestone payments.

In a recent 10-Q filing, Perrigo said it decided to sell the asset once its cash flows were determined to be less than its carrying value. Perrigo used a relief from royalty method and determined that the Tysabri asset was impaired by $2.6 billion.

Perrigo will assign to Royalty Pharma certain information and audit rights under Perrigo’s existing agreement with Biogen.

Royalty Pharma’s current portfolio includes royalty interests in more than 40 approved products including AbbVie‘s (NYSE: ABBV) Humira, AbbVie and Johnson & Johnson‘s (NYSE: JNJ) Imbruvica, Biogen’s Tecfidera, Vertex Pharmaceuticals‘ (NASDAQ: VRTX) Kalydeco and Orkambi, J&J’s Remicade, Merck‘s (NYSE: MRK) Januvia, Gilead Sciences‘ (NASDAQ: GILD) Atripla and Truvada, Pfizer‘s (NYSE: PFE) Lyrica, and Astellas Pharma (T: 4503) and Pfizer’s Xtandi.