Cashing in on rising drug prices often unleashes an outcry from consumers and politicians. But a little-known private equity investor, Royalty Pharma, has built an unusual investment portfolio valued at $15 billion — it buys up the rights to royalties on future drug sales — while largely avoiding public controversy. By its own count, Royalty Pharma owns partial rights to seven of the 30 top-selling drugs in the United States, including giants like Humira, the arthritis treatment that is the...