Biogen executives said Thursday that the launch of the company's highly anticipated drug for Alzheimer's disease isn't going as quickly or smoothly as they hoped, as many doctors won't prescribe the drug until more is known about its performance in clinical trials and whether healthcare insurers will ultimately cover it.

Called Aduhelm, the drug is the first in the United States approved to treat Alzheimer's itself, rather than just symptoms of the disease. Analysts predicted a drug like Aduhelm would be in high demand among patients and doctors, in turn putting it on a fast track to billions of dollars in sales.

But that hasn't been the case, at least so far. Controversy has engulfed Aduhelm, with critics pointing to its potentially budget-busting price tag, mixed track record in clinical testing, and the unusually close relationship Biogen forged with the Food and Drug Administration leading up to the drug's approval in June.

Those criticisms, according to Biogen CEO Michel Vounatsos, are overblown. And yet, the lingering uncertainties are starting to have an effect. While Biogen had forecasted "modest" revenue from Aduhelm this year, the actual amount coming in may end up being lower still.

"We are certainly revisiting our very low sales target that we had already for 2021," Vounatsos told Matthew Harrison, an analyst at Morgan Stanley, during a Thursday interview at the bank's annual healthcare conference.

Biogen's share price subsequently fell almost 7%, erasing $3 billion from the company's value.

Biogen had been preparing to sell a first-of-its-kind Alzheimer's drug well before Aduhelm was cleared for market. The company identified several hundred treatment centers that serve a high volume of patients, and, alongside its development partner Eisai, earmarked $600 million to spend on launch activities.

But even prepared, executives say they hadn't anticipated certain aspects of the launch being so difficult or time-consuming.

One challenge, according to Alisha Alaimo, president of Biogen's U.S. therapeutic operations, is that some doctors are waiting to make prescribing decisions until more detailed clinical data on Aduhelm becomes available.

In July, Axios reported that Biogen, which had said it planned to detail Aduhelm results in a peer-reviewed publication, withdrew a paper from the journal JAMA after being asked to make edits. Vounatsos said Thursday a manuscript of late-stage data for Aduhelm has been submitted and is currently under review, though he didn't provide more specifics.

Aside from additional data, Alaimo said the majority of treatment sites that Biogen engaged with are waiting to hear whether insurance companies will cover Aduhelm.

Some providers have already made up their mind; certain affiliates of Blue Cross Blue Shield, for example, chose not to cover because a "clinical benefit has not been established." Humana, meanwhile, hasn't finalized its policy, but it is currently covering Aduhelm for patients who meet criteria used in the clinical studies of the drug.

Overall, many of the country's largest insurers are holding back until the Centers for Medicare and Medicaid Services makes a call. The agency is in charge of Medicare, the government insurance program for people aged 65 and up, and therefore covers the vast majority of patients likely to receive Aduhelm.

In mid-July, the agency said it was beginning the process for a "National Coverage Determination," or NCD, which will establish a single national policy for Alzheimer's drugs that work like Aduhelm. An initial proposal is slated for January, with a final decision expected by April.

"Additional guidance from CMS on the coverage policy in the Medicare program is essential and will influence our final coverage determination," a Humana spokesperson wrote in an emailed statement.

In the meantime, the lack of an NCD has added to an already "higher-than-expected level of uncertainty with respect to reimbursement," Alaimo said. Biogen estimates that approximately 50 sites are infusing Aduhelm, representing a small portion of the 900 or so the company has been targeting. Biogen said in July that about a third of those sites had completed reviews of Aduhelm and adopted either a positive opinion or at least would not redo their reviews.

Alaimo claims the company is not aware of any instances in which Medicare Advantage or Medicare Administrative Contractors denied coverage when patients matched the clinical trial criteria and the required paperwork was available.

Getting on formulary lists is just one of the many steps Biogen must take before its drug reaches patients. Some tests used to determine whether Aduhelm is an appropriate treatment for patients are expensive and frequently not covered by insurance. Vounatsos called this problem a "bottleneck" that is "impacting the patient journey even more than what we anticipated."

Alaimo noted, too, the various "operational issues" that neurologists and other doctors must deal with before patients can be infused with Aduhelm.

"For most of these specialists, it is also the first time they are learning how to buy and bill a product, and get reimbursed for it," she said. "Building this infrastructure does take time, it takes resources and it takes a lot of planning."

Still, the Biogen team has confidence the launch will ramp up. Alaimo said the number of new sites — defined as having at least one patient through all the required steps to receive their first infusion — has been growing at an increased rate over the past several weeks.

Vounatsos said patient access, not revenue, will be the priority for the coming months. That stance "makes good sense," according to Jefferies analyst Michael Yee, though it also leaves Wall Street "very negative on the early launch" of the drug.

The consensus among Wall Street analysts is that Aduhelm will bring in roughly $60 million to $70 million this year, according to Yee.

"Although we are facing some near-term challenges, and everybody can see that, we continue to see a very high level of physician and patient interest, Vounatsos said. "And we continue to believe the mid- to long-term opportunity remains significant."