One of the world’s largest biotechnology companies wants a new calling card. Long known for multiple sclerosis drugs, Biogen believes its next big franchise will be in Alzheimer’s disease. And key to its ambitions — and near-term growth — is a recently approved medicine called Leqembi.

Yet, Biogen doesn’t have the clearest line of sight on just how well this prized asset is performing. That’s because much of what the company knows about Leqembi’s launch comes from its development partner, Eisai, which is leading commercialization efforts in the U.S.

Speaking to reporters Tuesday, Biogen’s CEO Chris Viehbacher said there are around 2,000 U.S. patients receiving Leqembi and, to his knowledge, another 3,800 or so on a registry. But Biogen doesn’t have “precise numbers” or access to this registry. “We’re relying on others to provide the data,” he said.

Viehbacher noted how there are multiple registries, so the numbers could be higher. He clarified, too, that these are lists of patients “where there was a clear intent to treat by the physician.”

Earlier this month, Eisai said 8,000 U.S. patients were on a waiting list for Leqembi. The tally comes from the company’s “neurology account specialists,” who have been engaging with healthcare providers and facilities and gathering information about the number of patients awaiting Leqembi treatment.

Eisai and Biogen had aimed for 10,000 U.S. patients to be on their drug by the end of March, but walked back that goal as the launch proved slower and more challenging than anticipated. So far, Leqembi, billed by some on Wall Street as a blockbuster-to-be, has only generated around $10 million in revenue.

“What’s important is we are now seeing steady progress,” Viehbacher said, adding that “there’s plenty of demand” and “it really is a question of the systems being able to accommodate this new flow of patients.”

Biogen, now more confident in Leqembi’s launch, plans to increase the number of field representatives selling Leqembi by 30%. According to Eisai, weekly sales of the drug had reached about $1.5 million by Jan. 26.

To get Leqembi, a patient often needs to secure an appointment with a neurologist, which can be a difficult task in itself. The neurologist then runs tests to confirm the patient is at an early stage of Alzheimer’s and is positive for an abnormal protein tied to the disease. If eligible, the patient will be prescribed Leqembi, which is currently administered as a one-hour infusion every two weeks.

Viehbacher previously described Leqembi’s sales cycle as “lengthy,” taking two to three months from when patients request a neurologist appointment to when they actually receive the drug.

For Biogen, Leqembi’s success is vital to offsetting declines elsewhere in the business. The company recorded $2.4 billion in revenue and $2.95 in earnings per share for the final three months of 2023, both of which were below the average Wall Street forecast.

While Biogen expects “core pharmaceutical revenue” — which encompasses all its products plus its cut of Leqembi sales — to be about the same in 2024 versus 2023, total revenue is set to decline by a low- to mid-single-digit percentage. The company generated $9.8 billion in revenue last year.

Biogen’s share price fell more than 6% Tuesday morning, to trade below $230. Shares will be weighed down until the company shows “the [Leqembi] opportunity is real,” wrote Stifel analyst Paul Matteis, in a note to clients.

“While the gradual rollout has gone very much as expected, we’ve still been a bit surprised by the lack of urgency to treat [and the] time to ramp for even the top treatment centers in the U.S.,” Matteis wrote.