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said 35% of women with locally advanced or metastatic breast cancer responded to lapatinib as a first-line therapy, according to interim results of a study reported at the annual meeting of the American Society of Clinical Oncology.

The company also said it will push back its planned time frame for filing for U.S. approval for the treatment.

The data are the first to be reported on the use of lapatinib as a first-line therapy. The results were derived from an interim analysis planned at the start of the trial and have been confirmed through an independent review. In the patients who experienced a partial response, tumor size was reduced by at least 30%. An additional 35% (another 14 of the 40 patients) showed stable disease through 12 weeks of therapy.

Another study identified biomarkers from tissue and serum that may help in predicting drug response as part of a continuing effort to precisely direct lapatinib therapy to the patients who will get the most benefit, GlaxoSmithKline said. This study relied on data from two phase II trials (EGF 20002 and EGF 20008) on patients with late-stage disease who were administered lapatinib after multiple other treatment options had been exhausted.

Considering the data presented at ASCO, the company now plans to submit a new drug application for lapatinib with the Food and Drug Administration in late 2006 or early 2007. Previously, the company had considered an initial filing in late 2005 on the basis of the EGF 20002 and EGF 20008 studies.