Amgen stock falls on weight loss drug’s bone density loss data

Shares of Amgen fell more than 7% Tuesday as analysts chewed over bone density loss data from an early-stage trial on its experimental weight loss injection, MariTide.

One analyst said the additional data suggests a new potential safety risk tied to the drug. But others said the share move was an overreaction, and that more data on a larger group of patients is needed.

Amgen did not immediately respond to a request for comment on the data 

The drug is a promising potential competitor in the weight loss drug market. It is designed to be taken monthly, rather than once a week like existing injections from Novo Nordisk and Eli Lilly, and promotes weight loss differently.

Wall Street is waiting for crucial phase two trial results on MariTide, which are set to be released before the end of the year. 

Analysts on Tuesday cited additional publicly available data from a phase one study showing that the highest dose of MariTide – 420 milligrams – was linked to roughly 4% loss of bone mineral density over 12 weeks. A decrease in bone mineral density refers to when bones lose calcium and other minerals, making them weaker and more likely to break. 

In a research note, Cantor Fitzgerald analyst Olivia Brayer called the data a “big unknown” and suggested it could be a potential risk associated with drugs like MariTide, which work by using so-called GIPR antagonism. Amgen’s injection works by blocking a gut hormone receptor called GIP but also activates another appetite-suppressing hormone called GLP-1. 

That’s unlike Eli Lilly’s obesity drug, Zepbound, which activates GIP and GLP-1. Wegovy activates GLP-1 but does not target GIP, which may also affect how the body breaks down sugar and fat.

“On one hand, patients could naturally lose bone mineral density during weight loss treatment,” Brayer wrote. 

But Brayer said, “on the other hand, this could be a non-starter because there seems to be a dose-dependent increase” in bone mineral density loss. That means patients appear to lose more bone mineral density the higher the dose they take. 

Meanwhile, Jefferies analyst Michael Yee wrote in a note that the additional MariTide data seems to be a “non-issue.” Yee acknowledged that people on the highest dose of the drug had declines in bone density, but said “the data is all over the place.” 

For example, he pointed to data on a lower dose of the drug showing that bone density actually increased by 1% before normalizing. Yee added that bone mineral density “changes” are a known side effect of weight loss drugs in the first one to three months of use because people lose significant weight quickly. 

Amgen is also aware of the “hypothetical concern” of bone mineral density loss, Yee said, citing the firm’s discussions with management.  

“While obviously not saying there is zero effect, we are saying we don’t think there is a concern, significant [bone mineral density] drop sustained over time, or clinical risk or concern,” Jefferies said. “Overall we don’t believe there is an issue and the effect is normalized over time.”

BMO analyst Evan Seigerman wrote in a note Tuesday that “We’d be cautious about making an overarching judgment on the safety profile of MariTide with this data.” 

He added that “we’d be more comfortable judging the safety profile from a larger cohort of patients.” There may not be a clear answer until Amgen releases full phase two trial data on the drug. 

“Our view on MariTide hasn’t changed with this and if anything we see the selling as overdone,” Seigerman wrote.