Ibrance is a small capsule that's been making a big difference during Pfizer Inc.'s (NYSE:PFE) earnings reports since it launched in 2015. Unfortunately, it didn't make a significant overall survival difference for a group of cancer patients during a clinical trial that wrapped up recently.

Ibrance sales are rising fast enough that it could become the company's best-selling drug in a couple of years. Do the latest long-term results spell big trouble for Ibrance and Pfizer's future?

Suprised looking person in front of a computer.

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A significant failure

Pfizer began the week with the sad announcement that adding Ibrance to a common chemotherapy, fulvestrant, failed to provide an overall survival benefit for a group of patients that already failed at least one hormonal therapy. This trial already hit its primary endpoint, an improvement in the time between beginning treatment and disease progression or death. Right now there's a whole lot of head-scratching over what this unusual situation means for a drug that was on pace to achieve $4 billion in sales this year.

This isn't the sort of mishap that forces regulators to revoke marketing privileges, but it does take the shine off of Ibrance at a particularly bad time. Last year, Eli Lilly (NYSE:LLY) launched a similar drug called Verzenio with an approval to treat the same populations of pre-treated patients that Ibrance is cleared to treat. More recently, Lilly's competing drug earned another approval to treat newly diagnosed patients as part of a combination that includes hormone-based treatments traditionally used in the first-line setting.

Verzenio isn't the only new competitor Ibrance has to compete with for new patients, either. Last March, Kisqali from Novartis (NYSE:NVS) earned a similar first-line approval. These drugs haven't been evaluated in head-to-head trials, so it's hard to say which is best. Without any overall survival benefit to point at though, Pfizer's negotiating position could drop a few notches during the next round of formulary discussions with America's increasingly muscular pharmacy benefits managers.

Reasons to stay calm

Overall survival is the gold standard of cancer therapy efficacy measurements, but we can't ignore Ibrance's proven ability to stop metastatic breast cancer from getting worse. Adding it to endocrine therapy reduced the risk of disease progression by 44% during the Paloma 2 trial.

Physician consulting with a cancer patient in a hospital bed.

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It's hard to believe a drug that extended median time to disease worsening from 14.5 months to 27.5 months in Paloma 2 can't give these patients a better chance at long-term survival. Pfizer's explanation that Paloma 3 may have failed on the secondary endpoint because it wasn't optimized to detect a significant difference in overall survival is a little hard to swallow considering there were 521 patients in the trial. That said, it's important to realize Paloma 3 enrolled advanced-stage patients, and median progression-free survival in the group treated with chemo alone was just 3.8 months.

Other contributors on the rise

Pfizer's a big, diverse company with a lot of moving pieces. Even if the recent overall survival miss stops Ibrance sales from growing by double digits, the therapy contributed just 7% of total sales in the most recent quarter. If its growth stalls, other drugs on the rise can probably keep the needle moving forward.

Pfizer's treatment for prostate cancer that has already spread, Xtandi, could get a huge boost in the second half of the year. In July, the FDA is expected to make an approval decision that could make the pills available for castration-resistant prostate cancer that hasn't spread yet.

The American Cancer Society estimates more than 164,000 men in the U.S. were newly diagnosed with prostate cancer last year. Most cases eventually stop responding to hormone deprivation treatment, and giving Xtandi to these patients before they go metastatic could make it a $5 billion per year drug.

Pfizer recently earned approval to sell its rheumatoid arthritis drug to people with ulcerative colitis. This is an inflammatory bowel disease that affects more than 900,000 Americans, many of which could probably use a new treatment option.

Investors can probably look forward to a pipeline that racks up quite a few more approvals in the years ahead. At the end of May, the company counted 96 programs in clinical development, 12 of which had applications under review. Plenty of potential growth drivers are one good reason, among several, that investors would do well to keep this stock in their portfolios for the long haul.

Cory Renauer has no position in any of the stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has a disclosure policy.