On September 4, 2026, Amgen Inc. (NASDAQ:AMGN) saw its market value fall by roughly $12 billion during after-hours trading. The stock declined by approximately 5% to $415. The cause behind the move was a drug that Amgen does not control. Novartis revealed that pelacarsen, an Lp(a)-lowering treatment, failed its Phase 3 cardiovascular-outcomes trial, Lp(a)HORIZON. Because Amgen’s late-stage candidate olpasiran rests on the same scientific foundation, traders quickly reduced Amgen’s share price on the negative carryover effect.
Why a Rival’s Failure Hit Amgen
In prior trials, Pelacarsen had reduced Lp(a) by approximately 80%. In the Lp(a)HORIZON study, Novartis reported that Pelacarsen substantially lowered Lp(a). Yet it failed to cut the composite of cardiovascular death, heart attack, stroke, and urgent revascularization. That composite endpoint was missed again. The failure now puts the whole idea that lowering Lp(a), a genetic cardiovascular risk factor, actually reduces events, into direct question.
Olpasiran rests on the same hypothesis, which is why odds got repriced as soon as Pelacarsen’s data came out.
The Bull’s Hope and Its Limit
Olpasiran, an siRNA from Amgen, cut Lp(a) by over 95% at specific doses during Phase 2, compared to roughly 80% reductions seen with pelacarsen in prior research. The bullish argument holds that this larger drop might enable olpasiran to produce better results for the heart. Biologically speaking, this is possible, though the Lp(a)HORIZON trial found no proof of a clinical threshold above an 80% reduction. The headline from Phase 3 did not show that a greater reduction in Lp(a) would have brought a heart benefit.
So it is fair to say the miss lowered olpasiran’s probability of success rather than pretending it is irrelevant. Eli Lilly’s lepodisiran is repeating the same deep-reduction test, which means the hypothesis will eventually be put to the test.
Why It’s 5%, Not 25%
MariTide in weight loss, Repatha in cholesterol, and Tezspire in asthma carry more value for Amgen than its Lp(a) program ever will, and that variety gives the stock a buffer, even though the decline eventually grew far beyond the initial 5% after-hours response. Since the start of 2026, positioning has remained steady. According to Insider Monkey data, 66 hedge funds owned AMGN during the second quarter of 2026, a slight rise from 65 in the first. The short interest stands at just 2.4% of the float, showing almost no betting against the stock in the marketplace.
What Happens Next
The question now is whether olpasiran can separate itself from the pelacarsen narrative. The two drugs lower Lp(a) by different amounts, and olpasiran’s Phase 2 data suggest it goes further. But the Lp(a)HORIZON trial showed no threshold above 80% that guaranteed a benefit. Eli Lilly’s lepodisiran will test the same hypothesis.
Amgen’s future rests on its own evidence. Before it releases its own headline figures, shareholders must judge between a tested pipeline and a theory that recently suffered a setback.
Key Facts Box
- Stock price: $415 after the drop
- Market value loss: Roughly $12 billion
- Date: September 4, 2026
- Trial: Lp(a)HORIZON, a cardiovascular-outcomes trial for pelacarsen
- Lp(a) reduction: pelacarsen roughly 80%; olpasiran more than 95% at certain doses
- Short interest: 2.4% of float
The stock decline was significant because the underlying idea matters more than the actual product. Amgen’s olpasiran is a distinct drug, yet it depends on the same presumption. Should that presumption hold up, the stock could bounce back. Should it fail to do so, the safety net provided by MariTide, Repatha, and Tezspire might prove insufficient. Trading activity was swift on September 4, and the next round of movement will hinge on Amgen’s own data.
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