An experimental drug showed impressive effectiveness and safety in a midstage trial against a common form of advanced breast cancer, lifting Pfizer Inc shares nearly 2 percent.

The drug, called PD-0332991, delayed by more than 18 months the worsening of symptoms for postmenopausal women with the most common form of breast cancer. Data from the study of the drug were presented on Wednesday at the annual San Antonio Breast Cancer Symposium.

The favorable results prompted the drugmaker to plan largelate-stage trials of the drug next year. If approved, the medicine could fetch multi-billion dollar sales, industry analysts said.

The medicine, which blocks enzymes known as CDK4 and 6 kinases, was taken in combination with a standard drug called letrozole among women who were estrogen receptor positive — meaning tumors grow in response to estrogen — and HER2-negative, meaning that the HER2 protein is not causing the cancer. Such patients make up about 60 percent of breast cancer cases.

Patients had either locally advanced tumors or cancer that had spread to other parts of the body.

Those taking both drugs went an average of 26.1 months before tumors worsened. That compared with 7.5 months for those taking letrozole, but not PD-0332991. The 18.6 month difference was considered statistically significant.

“In a disease where a several-month improvement in progression-free survival is considered impressive, we view an 18.6-month improvement in PFS as remarkable,” said analyst Jon LeCroy of MKM Partners. He predicted Pfizer will introduce the medicine in 2017.

Letrozole is the chemical name of Femara, a Novartis drug that belongs to a class of treatments called aromatase inhibitors that block production of estrogen.

Side effects seen in patients taking the drug combination included anemia, fatigue and neutropenia — a decline in white blood cells called neutrophils that can put patients at higher risk of infection.

“If approved in a front-line breast cancer setting, this drug has the potential to generate $2 billion to $6 billion” in worldwide sales for these types of breast cancer patients, said Mark Schoenebaum, an analyst with ISI Group, of PD-0332991.

Schoenebaum, who had not previously predicted any revenue for the drug, said the medicine could provide considerable “upside” to Pfizer revenue if it is cleared by regulators.

He noted that researchers have not yet disclosed to what degree the Pfizer drug might have prolonged patient survival.

Pfizer had released some data from the trial in May, but released updated findings on Wednesday.