In an effort to protect its blockbuster Soliris, Alexion has employed a double strategy of nabbing additional approvals and positioning follow-up drug Ultomiris to switch over sales. Alexion added another indication this week that could rake in $1 billion for its portfolio—but there’s a catch.

The FDA approved Soliris on Friday as a first-of-its-kind therapy to treat adult anti-aquaporin-4 antibody-positive patients with neuromyelitis optica spectrum disorder (NMOSD), an ultra-rare autoimmune disease that is thought to affect between 4,000 and 8,000 Americans, Alexion said. Alongside Ultomiris, which is set to launch phase 3 trials in NMOSD in the third quarter, the new indication could rake in $1 billion in sales for Alexion by 2028, according to SVB Leerink analyst Geoffrey Porges.

However, despite Alexion’s hopes that NMOSD will add significantly to Soliris’ $3.6 billion in global sales in 2018, Porges said investors shouldn't get overly excited. The drug’s chances at gaining ground in previously untreated patients are slim in the face of Roche’s far-cheaper Rituxan, he said: Soliris has a first-year cost of around $700,000 compared to Rituxan’s $60,000, presenting a clear price gap to payers despite Soliris’ promising data in stopping relapses.

"Despite the label, we don’t think Soliris will capture significant use in front line treatment given Rituxan’s favorable cost-benefit profile," Porges said in a Friday note to investors. "We expect most insurance payers to require evidence of Rituxan failure before reimbursing for Soliris in this disease, but do expect relatively open access to Soliris for relapsed patients in late line treatment."

However, there is still a market for post-Rituxan treatment, and Soliris performed admirably in phase 3 trials at stopping patient relapses over placebo. In September, Alexion showed what it called a "remarkable" reduction in relapses, with trial data showing the drug reduced relapse risk by 94.2% compared with placebo. At the 48-week mark, 97.9% of Soliris patients were relapse-free.

Those results could go a long way to capturing the one-third of U.S. patients who suffer NMOSD relapses, Porges said, and position Soliris well to switch patients over to Ultomiris as the older drug’s 2027 patent expiration approaches.

When all is said and done—assuming Ultomiris receives an FDA nod in NMOSD of its own, as well as indications for generalized myasthenia gravis and atypical hemolytic uremic syndrome (aHUS)—Alexion’s C5 inhibitor portfolio could be sitting pretty into the 2020s, with the newer drug already bearing an approval for adult paroxysmal nocturnal hemoglobinuria (PNH). Alexion also has promising PNH switching data in hand for Soliris and Ultomiris, and 22% of PNH patients on taking Soliris had already moved over to the newer drug as of April.

Helping matters even more is a field mostly clear of competitors in NMOSD, but Porges said competitors in the IL-6 and CD-19 inhibitor classes are angling for a piece of the pie. However, those drugs’ weak showing so far in preventing relapses could limit their ability to stand against Soliris.

"In our opinion, Soliris is ahead of its competitors by at least a year in entering the NMOSD market, and with its better efficacy data in treating relapsed patients, will capture dominant share in the relapsed NMOSD market," Porges wrote.