Onyx Pharmaceuticals ( ONXX) said Wednesday that its cancer drug Nexavar extended progression-free survival in a mid-stage study of women with advanced breast cancer. The announcement helped push Onyx shares up 29% to $37 in pre-market trading. Nexavar, a pill, is currently marketed as a treatment for kidney and liver cancers. Onyx and its partner, the German drug firm Bayer, have been testing Nexavar in other types of cancer as well. The phase II study announced Wednesday evaluated treatment with Nexavar and the oral chemotherapy drug Xeloda, marketed by Roche, compared to Xeloda plus placebo. Women with advanced Her-2 negative breast cancer treated with the Nexavar-Xeloda combination demonstrated a lengthening of time before their cancer grew compared to woman treated with Xeloda alone. Onyx said the results were statistically significant although details about the study were being withheld so that they could be presented later at a medical conference. "Based on these encouraging data, Onyx and Bayer are evaluating various strategies for Nexavar in breast cancer," said Dr. Todd Yancey, vice president of clinical development at Onyx. Onyx and Bayer are currently conducting other phase II studies of Nexavar in breast cancer.