Insmed Stock Jumps 34% as Brinsupri Sales Drive Guidance Raise

A Launch Strong Enough to Move the Sector

Insmed's second-quarter report on August 6 produced one of the biggest single-day gains of the 2026 biotech earnings season. The Bridgewater, New Jersey drugmaker led a rally in the sector, while rare-disease peer Mirum Pharmaceuticals fell sharply on the same day.13 Reports put the size of the jump at slightly different levels. Insmed was up 28.3% to $127.04 in premarket trading,33 and some later accounts put the full-session gain at roughly 34%.15 Whatever the exact figure, the cause is clear: Brinsupri, Insmed's bronchiectasis drug, is selling well ahead of what the company and Wall Street expected.

The raw numbers are striking. Insmed reported total second-quarter revenue of $425.5 million. Brinsupri brought in $309.2 million of that, a 49% jump over the first quarter, and the older drug Arikayce added $116.3 million, up 8% from a year earlier.37 Total revenue beat analyst forecasts by about 8.4%.33 Brinsupri's quarterly sales have climbed steadily: $144.6 million in the fourth quarter of 2025,12 then $207.4 million in the first quarter of 2026,33 then $309.2 million.

Two months later, though, the shares have given back all of that gain and more. That gap between how well the drug is selling and how the stock is trading is now the main question for investors.

What Insmed Actually Changed

Insmed did more than beat estimates. It raised its forecasts at several levels. Full-year 2026 Brinsupri guidance went from "at least $1 billion" to a range of $1.25 billion to $1.40 billion. The company also raised its estimate of peak annual sales across its three lead programs to more than $14 billion. That breaks down as more than $7 billion for Brinsupri, more than $6 billion for the inhaled pulmonary hypertension drug TPIP, and more than $1 billion for Arikayce.37 Accounts of the call agree that the combined figure is 75% above the previous estimate of more than $8 billion.38 The TPIP estimate tripled from $2 billion, which management tied to Phase II data in two forms of pulmonary hypertension: PH-ILD, linked to interstitial lung disease, and PAH, pulmonary arterial hypertension.38

CEO Will Lewis described how quickly expectations have shifted. The company's early benchmark for success, based on other specialty respiratory launches, was $500 million to $700 million in Brinsupri's first full year. Guidance was then raised above $1 billion, and has now been raised again by another 25% to 40% after only three full quarters on the market.35

The details behind the headline figures point to steady demand rather than a one-off spike. Brinsupri added about 7,000 new patients in the quarter, above the company's own estimate of about 6,300. Management expects roughly 7,000 new starts in each remaining quarter of 2026.32 Insurers approve about 90% of prescriptions, most within a week.32 Gross-to-net guidance, which measures how much of the list price is lost to rebates and discounts, narrowed to the mid-to-high 20% range from the mid-20s to low-30s.32 The prescriber base reached 6,300 doctors, and 30% of them have now written Brinsupri for at least five patients, up from 20% at the end of the first quarter.38

One point stands out. Management said the backlog of patients who were "ready and waiting" for the drug at launch has been used up, so every new start this quarter reflects ongoing demand.38 That matters because early launch quarters often look strong simply because pent-up demand gets cleared. Insmed is saying that phase is over and growth is still accelerating.

The Profitability Story Is the Real Catalyst

For a company that has spent decades losing money, the margin numbers may matter more than the top line. Cost of product revenue fell to 16% of total revenue from 26% a year earlier, because Brinsupri is more profitable per sale than Arikayce.33 The net loss shrank to $13.2 million, or six cents a share, from $321.7 million in the same quarter last year.38 Insmed ended June with about $1.2 billion in cash and says it does not plan to raise more money before turning cash-flow positive in 2027.35

Our reading is that this is what drove the August rally. Biotech investors usually pay for pipeline hopes and worry about dilution, meaning new share sales that shrink existing holders' stakes. Insmed offered something rarer: a commercial drug that can pay for its own pipeline. The company plans to spend more in the second half on TPIP Phase III trials, direct-to-consumer advertising for Brinsupri and a commercial setup in Japan. It also expects to pay AstraZeneca $50 million in milestone payments.32 Even so, it is sticking to its 2027 cash-flow target. That combination of growth and self-funding is what institutional investors have been looking for.

The Mirum Contrast

The same day showed how quickly investors punish regulatory setbacks. Mirum also beat on sales. Second-quarter net product sales reached $176 million against a consensus of about $166 million, and it raised full-year guidance to $680 million to $700 million.28 The stock still fell about 24% after hours to $80.50.28 One trading-news account put the regular-session drop at about 15%, describing an "unexpected" FDA request for a Phase 3 study as the cause.25

Coverage differs on why Mirum fell. One analysis focused on the wider quarterly loss and reported earnings of minus 80 cents a share.21 Other sources report minus $1.06 a share against a forecast of minus 74 cents,28 and point to the delay of the filing for volixibat, Mirum's drug for primary sclerosing cholangitis (PSC), a chronic liver disease, to the first half of 2027.28 The analyst reaction suggests regulators were the bigger factor. Morgan Stanley, RBC, TD Cowen, Baird and H.C. Wainwright all cut their price targets on August 6, and one headline said Mirum sank after the FDA recommended a Phase 3 study.24 The lesson is consistent across both stocks: sales beats get rewarded only when nothing threatens the longer-term story.

Why the Rally Faded

The August surge did not last. One technical analysis dates the stock's peak to August 10 and says shares had fallen more than 15% from there by late September.6 On October 5, the stock fell almost 7% to about $103.61 after Insmed said CFO Sara Bonstein would step down on October 30 after nearly seven years. The company reaffirmed its full-year guidance at the same time.3 By October 8, shares were near $98.71, close to the bottom of a 52-week range of $90.39 to $212.75, with a market value of about $21.6 billion.1 One quote service had the stock down about 41% for the year as of early October.7

The company's news over this period was not negative. In September, the FDA granted priority review to Insmed's application for full approval of Arikayce for MAC lung disease, a chronic lung infection, with a decision due by January 28, 2027. Full approval would let doctors use the drug earlier in the disease, including in newly diagnosed patients.85 Lewis said the company is "well positioned to reach cash flow positivity next year."3 Investor Stanley Druckenmiller was reported to have built a position of nearly $300 million, including call options.34

So why the decline? Part of the answer appears to be the broader market. Coverage of the sector in September describes a risk-off mood driven by rising Treasury yields that weighed on healthcare and biotech stocks.22 Part of it may be company-specific. Losing a CFO just as a company approaches profitability raises questions even when guidance holds. Analysts also flagged caveats on the earnings call. Lewis said budget pressures make a traditional commercial launch less attractive in most European countries, and the $7 billion Brinsupri forecast assumes Medicare price negotiation under the Inflation Reduction Act takes effect in 2035.38 Barclays kept its overweight rating but cut its target to $207 from $237.1

The Bottom Line

Our view is that the August 6 rally was justified and the pullback since then reflects market mood and timing more than any problem with the drug. Insmed's quarterly numbers show a launch that, in management's words, is more than doubling the best specialty respiratory launches.38 Analysts' average price target is still around $198, roughly double the current share price.1

The next test is the third-quarter report, expected around October 29. Analysts expect revenue of about $464 million and a loss of about 62 cents a share.3 If Brinsupri again adds about 7,000 new patients and margins keep improving, the gap between the stock price and the business could start to close. If new-patient growth stalls, the August rally will look like the high point of the launch.