Shares of Amgen were recently down more than 9%, putting the biotechnology company on pace for its largest decrease since October 2000.

The decline came after Novartis announced a failure of its heart disease drug, pelacarsen, to reduce the risk of cardiovascular events compared with a placebo.

The failure, which sent U.S.-traded Novartis shares down 14%, also cast doubt on similar drugs, including Amgen's investigational treatment olpasiran.

Citi analyst Geoff Meacham wrote in a note that Amgen's drug faces the clearest risk besides Novartis as a result of the failure, adding that differences in its study design "offer differentiation rather than insulation from the pelacarsen result."

Novartis's pelacarsen was a big bet on the potential of lowering lipoprotein(a), or Lp(a), levels to reduce heart disease risk. Eli Lilly, which also has an Lp(a)-lowering drug in the works, is less likely to be affected because the treatment contributes relatively little to its valuation, Meacham added. Lilly's shares were down 1.9% Tuesday.

Amgen, whose decline was exacerbated by a BMO Capital Markets downgrade, was the worst performer in the S&P 500 and Nasdaq 100.

"We were thinking that the only way that Amgen would really be down on this readout is if it kind of blows up the Lp(a) thesis," said Barclays analyst Emily Field. "I think we're kind of at that place right now."