An experimental drug added to chemotherapy slowed aggressive, late-stage breast cancers that resumed growing despite standard treatment with the drug Herceptin, U.S. researchers said on Saturday.

The drug, Tykerb, by GlaxoSmithKline Plc, also showed potential to help some patients with advanced kidney cancer, another study showed.

Glaxo hopes to begin selling the drug next year, but must await regulatory approval. Industry analysts predict it could become a multi-billion-dollar seller.

A Glaxo-sponsored study of 321 women showed breast cancer did not progress for an average of 37 weeks in patients treated with Tykerb and oral chemotherapy drug Xeloda, compared with 20 weeks for others who received only Xeloda.

The findings were so striking that researchers halted the study early.

All of the women were treated previously with Herceptin, a drug that fights the 20 percent to 25 percent of breast cancers that produce large amounts of a protein called HER-2. Herceptin works for a time, but the cancer eventually grows.

Tykerb, if approved for sale, could offer hope to women whose cancers return after they were successfully beaten back by Herceptin, said Dr. Charles Geyer, director of breast medical oncology at Allegheny General Hospital in Pittsburgh and the study’s lead author.

“The ability to control cancer growth a second time on this order of magnitude ... is really quite substantial for a patient,” Geyer said.

The findings were released at an annual meeting of the American Society of Clinical Oncology.

Xeloda, known generically as capecitabine, is sold by Roche AG, which also sells Herceptin along with Genentech Inc. Tykerb’s generic name is lapatinib.

Glaxo is testing if Tykerb works earlier in breast cancer. As a pill it could be a convenient alternative to Herceptin, which must be injected. Herceptin’s annual sales are about $1.8 billion.

Dr. Pamela Klein, Genentech vice president for oncology and hematology, said doctors would continue to prescribe Herceptin widely given its track record in extending survival.

“I think Herceptin is not going to be given up right now,” she said.

ADVANTAGE TYKERB?

Future research may give Tykerb an advantage over Herceptin for some but not all patients, said Dr. Julie Gralow, a breast cancer specialist at the University of Washington in Seattle.

An estimated 215,000 cases of breast cancer are expected to be diagnosed in the United States this year, and about 41,000 women are projected to die from the disease, according to the American Cancer Society.

Tykerb targets the HER2 protein as well as another known as EGFR that is involved in tumor growth.

Geyer said no Tykerb patients in the breast cancer study developed symptoms of congestive heart failure, a serious potential side effect from Herceptin. Women who took Tykerb were more likely to experience diarrhea and rashes than others who got only Xeloda.

In a separate study, Tykerb failed to improve overall survival in a trial of 416 patients with advanced kidney cancer. Benefits were seen, however, in 241 of those who had tumors that produced the highest EGFR amounts. Patients in that group who took Tykerb survived an average of 46 weeks, compared with 38 weeks for standard hormonal therapy.