UPDATE 1-GSK stops Tykerb only arm of late-stage trial

* Trial looks at Tykerb versus Roche's Herceptin

* Three other arms of late-stage study to continue

* GSK shares weaken (Adds details, background)

LONDON, Sept 9 (Reuters) - Patients in a late stage clinical trial will stop getting GlaxoSmithKline's Tykerb breast cancer drug alone after an independent committee decided the drug was likely to produce worse results that Roche's Herceptin.

Shares in GSK dropped around 0.5 percent after the news and were trading at 1,309 pence by 1133 GMT.

The British drugmaker said the other three arms of the trial -- which is designed to examine the performance of Tykerb, known generically as lapatinib, in combination with Herceptin or trastuzumab -- would continue as planned.

"The (independent data monitoring) committee has indicated that the lapatinib alone arm is unlikely to meet the pre-specified criteria to demonstrate non-inferiority to trastuzumab alone with respect to disease-free survival," GSK said in a statement on Friday.

Tykerb is marketed as Tyverb in Europe. Both it and Herceptin are designed for women with HER2-positive breast cancer, a particularly aggressive form of the disease, and are given alongside standard chemotherapy.

Patients in this trial, known as ALTTO, were originally randomised to receive either Tykerb alone, Herceptin alone, Herceptin followed by Tykerb, or a combination of the two drugs together.

GSK said patients assigned to the lapatinib alone arm of the trial would discontinue the drug and discuss treatment options with their doctors. (Reporting by Kate Kelland, editing by Paul Sandle)