Atlanta ? Women with advanced breast cancer soon may have another treatment option: A novel experimental drug delayed the growth of tumors nearly twice as long as standard chemotherapy did in patients who had stopped responding to Herceptin, doctors reported Saturday.

The experimental drug, Tykerb, worked so well that an international study of it was stopped early, in March, and all participants were offered it.

In the study, women who received Tykerb plus the chemotherapy drug Xeloda had no growth of their tumors for an average of 8 1/2 months.

That compares to 4 1/2 months for those given only Xeloda, said Dr. Charles Geyer Jr. of Allegheny General Hospital in Pittsburgh.

He led the study and reported results Saturday at a meeting in Atlanta of the American Society of Clinical Oncology.

Tykerb’s manufacturer, British-based GlaxoSmithKline PLC, paid for the study and said it would expand global access to the drug under compassionate use provisions.

The company plans to seek approval to sell Tykerb in the United States and elsewhere later this year.

“This is huge,” said Dr. Roy Herbst, a cancer specialist at the University of Texas’ M.D. Anderson Cancer Center in Houston, who had no role in the study but has consulted for Glaxo in the past.

Herceptin and Tykerb are members of a new generation of cancer medicines that more precisely target tumors without killing lots of healthy cells. Herceptin has been an important option for many women with advanced breast cancer, but eventually it stops working.

Tykerb works in a similar yet completely novel way. Like Herceptin, it targets a protein called HER-2/neu, which is made in abnormally large quantities in roughly one-fourth of all breast cancers.

Herceptin blocks the protein on the cell’s surface; Tykerb does it inside the cell, and blocks a second abnormal protein, too.

The benefits seemed to come without serious side effects – at least in this study of 321 women, Geyer said. Diarrhea, mostly mild, and rash were more common in women taking Tykerb.

No patients developed heart failure, but four of the 160 on the drug combination had a modest decrease in pumping power of the main chamber of the heart – side effects that also have been seen with Herceptin.