The results from the Phase 2, or midstage, study were presented here at the annual meeting of the American Association for Cancer Research. They are being closely watched on Wall Street, because palbociclib is considered a jewel in Pfizer’s product pipeline, with analysts predicting annual sales of billions of dollars. Amgen is entitled to an 8 percent royalty on sales of the drug.

As strong as the results were, it is possible they will be a bit of a letdown to some investors.

That is partly because they were not quite as good as interim results presented about halfway through the trial. At that point, the difference in median progression-free survival was 26.1 months for palbociclib versus 7.5 months for the control group.

The lack of a statistically significant survival benefit could also give investors pause.

Dr. Finn said, however, that a statistically significant survival benefit should not have been expected at this point because only 61 of the 165 patients in the trial had died. Also, patients can use other drugs after leaving the trial, which can dilute any effect of palbociclib.

Palbociclib slows the runaway proliferation of cancer cells by inhibiting the activity of two related enzymes involved in cell division — cyclin-dependent kinases 4 and 6.

While Pfizer is in the lead to bring this new class of drugs to market, Novartis has begun late-stage testing of its own CDK 4/6 inhibitor. Eli Lilly is at an earlier stage, with some results for its drug presented here. While breast cancer is the initial focus, the drugs are being tested for other cancers.