I bought LGND years ago when they transitioned to the royalty model which I like a lot. I sold too early trying to be clever and raise cash thinking it was too ahead of itself (foolish mistake).
I owned another (small canadian) drug royalty company that was bought out on the very cheap (At one point they had a royalty on Cambridge Antibody which was entailed to Humara royalty). I looked briefly at PDL but their royalties/deals didn't seem particularly appealing.
Royalty Pharma at one point was looking to go public but don't know if they decided wasn't worth the trouble and they probably don't need the cash now. Judging by the size of deals they've done I doubt they need the cash either.
If anyone knows of other (preferably investable) drug royalty companies/funds would be interested in the names.