In its push to backstop sales of blockbuster Soliris, Alexion has touted its better-than-expected success at switching patients to follow-up drug Ultomiris. But if switching to the newer med is going so well, why are investors so wary?

Forty percent of Soliris patients treated for adult paroxysmal nocturnal hemoglobinuria (PNH) have switched over to the newer drug as of this week, after its launch late last year, Alexion leaders said in a second-quarter earnings call Wednesday.

That figure is higher than the company previously forecast, putting the two drugs on track for a 70% conversion target by mid-2020, CEO Ludwig Hantson said. That bodes well for Ultomiris, which has a matching PNH approval with Soliris and a recent FDA approval in atypical hemolytic uremic syndrome (aHUS), as well as a cheaper price tag. And, crucially, much more time before its patents expire.

But if Ultomiris is beating its milestones, why isn’t the company’s share price going up, too? That’s the answer SVB Leerink analyst Geoffrey Porges wanted to know during Alexion’s earnings call.

Alexion closed at $120.97 Wednesday, a 3.4% decrease from the same date three years ago. Even after Ultomiris’ launch in late 2018, the company’s stock price has stayed pretty much flat.

By Hantson’s telling, Alexion is in the process of switching "from old to new" with its drug portfolio, and because of that he expected investors—and thus share prices—would eventually catch up with what company leadership already sees.

"My story is we don’t change this company overnight—we are delivering on what we said we are going to do, and every quarter we are getting stronger," Hanston said. "As we continue to execute on our conversion, those questions are going to become less relevant."

But did that rosy analysis soothe analysts’ concerns? Not so much.

In a note to investors, Porges said he was "quite cautious" on Alexion’s upside as the drugmaker faces a potential Soliris biosim from Amgen in Europe. While Alexion execs offered little in the way of predictions for a related patent dispute, Porges said he found their comments "not particularly encouraging."

Analysts with Evercore ISI pointed to similar concerns with Alexion’s presentation despite the strong switching data and continued sales growth for Soliris and Ultomiris. In their view, Alexion’s reliance on expanding the aging Soliris, coupled with its thin pipeline going forward, don't bode well for diversifying its offerings.

"The Soliris/Ultomiris franchise is growing so big that diversification will remain an ongoing struggle," Evercore ISI analysts wrote in a note to investors.

Even recent Soliris approvals in neuromyelitis optica spectrum disorder (NMOSD)—a possible $1 billion indication—and generalized myasthenia gravis (GMG) have left analysts skeptical.

Soliris in NMOSD will challenge a much-cheaper alternative in Roche’s Rituxan, which lists at about a 90% discount to the Alexion med. Porges said payers would likely require proof of patient failure on Rituxan before reimbursing Soliris, but Soliris could corner the market in preventing patient relapse.

In GMG, Evercore ISI analysts said Soliris could be limited to treating ultra-severe patients—with 1,193 already on the drug—capping its upside in that indication. However, the analysts said GMG could still prove to be a $530 million indication on the high side.

Despite analysts’ concerns, Alexion turned in a steady quarter with $1.2 billion in sales, a 2% increase from the year before. Ultomiris posted $54 million in sales in its second full quarter since approval—nearly doubling its first-quarter results.

Meanwhile, Soliris cleared $980.8 million in second-quarter sales, a 9% increase from the previous year.