Pfizer breast-cancer fighter Ibrance posted a rare sales miss Tuesday, but it wasn't because new rivals are stealing its share.

The fast-growing blockbuster still boasts a 91% share of the CDK 4/6 class, CEO Ian Read told investors on Pfizer’s quarterly conference call, despite new competition from Novartis’ Kisqali and Eli Lilly’s Verzenio. The shortfall performance "has nothing to do with us losing market share," COO Albert Bourla added, noting that "our market share remains strong, remains stable and is actually a 10% increase."

So what made the therapy, which generated $1.02 billion for the quarter, miss Wall Street’s $1.06 billion expectations?

A slowing of the overall CDK 4/6 market in the U.S., Bourla said. In other words, Pfizer and its peers have already persuaded the willing doctors out there to prescribe the new class, and the less-willing physicians "need more time to be convinced."

"Right now, the low-hanging fruit has already been harvested and what remains is late adopters," he said.

That’s not to say drugmakers can’t win them over. It just may take extra work. Pfizer needs to keep at its DTC advertising and keep plugging the benefits of CDK therapy.

At the same time, Pfizer doesn’t plan to put all its eggs in that basket. "We are working with currently prescribing physicans to increase usage and to increase the scripts that they process," Bourla said. "Growth in the U.S., I think in the short term, will come from expanding the usage or the prescription habits of physicians already prescribing."

There’s a bigger growth opportunity out there, too, and that’s moving the drug earlier into treatment, the way Novartis recently did with struggling Kisqali. But the pharma giants could run into hesitant adopters in the front-line setting, too.

Conservative doctors tend to hold back on using new combinations until after further lines of chemo, even when data supports first-line use. "I’d like to tell you I think it’s going to be used a lot more given these data, but just given what I’ve seen in the past, it’s going to be interesting" to see how prescribing patterns play out, Kisqali trial investigator Dennis Slamon, M.D., Ph.D., said of the Novartis drug in June.

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