Oncothyreon Announces Presentation of Positive ONT-380 Data in HER2-Positive Breast Cancer at ASCO Annual Meeting and Updates Development Plan

SEATTLE, May 30, 2015 -- Oncothyreon Inc. (ONTY) today announced the presentation of positive data from the company's ongoing trials of ONT-380, an orally active, reversible and selective small-molecule HER2 inhibitor for the treatment of breast cancer, at the American Society of Clinical Oncology (ASCO) 2015 Annual Meeting. The first presentation updated data from the Phase 1b trial of ONT-380 in combination with Xeloda(R) (capecitabine)and Herceptin(R) (trastuzumab) in third line treatment of HER2-positive metastatic breast cancer. The data support Oncothyreon's plans to initiate a blinded, randomized, placebo-controlled Phase 2 trial in this indication. The second presentation focused on the role of ONT-380 in the treatment of HER2-positive breast cancer central nervous system (CNS) metastases.

The Phase 1b trial (ClinicalTrials.gov Identifier NCT02025192) of ONT-380 in combination with Xeloda(R) (capecitabine) and/or Herceptin(R) (trastuzumab) is a parallel dose escalation study in patients previously treated with Herceptin and Kadcyla(R) (ado-trastuzumab emtansine or TDM-1) for metastatic breast cancer. Data were presented for 32 patients, including seven in the ONT-380 plus Xeloda cohort, 13 in the ONT-380 plus Herceptin cohort and 12 in the ONT-380 plus Xeloda and Herceptin cohort. Twenty-seven of the patients were evaluable for best overall response using RECIST 1.1 criteria. Considering the three cohorts together, a complete response (CR) was seen in two patients, a partial response (PR) in 12 patients, stable disease (SD) in nine patients and progressive disease (PD) in four patients.

Patients included in the presentation on the role of ONT-380 in the treatment of CNS metastases were selected from the above trial and from an ongoing Phase 1b trial (ClinicalTrials.gov Identifier NCT01983501) of ONT-380 in combination with Kadcyla in patients who have been previously treated with Herceptin and a taxane for metastatic breast cancer. Patients were included if their presenting CNS lesions were evaluable for response using RECIST 1.1 criteria and they had either untreated, asymptomatic lesions having never received radiotherapy or surgery to the CNS (n=8) or new or progressive lesions following prior CNS therapy (n=14). Best CNS response was a CR in one patient, a PR in four patients and SD in nine patients. No patient had progressive disease as a CNS best response. One patient was not evaluable for response having undergone surgery for a symptomatic CNS lesion; pathologic examination of the resected specimen found no evidence of viable tumor. Two patients were not evaluated because of progressive disease outside of the CNS, while five patients in the series remain too early to evaluate.

ONT-380 has been well tolerated in both Phase 1b trials. The most common adverse events included diarrhea, nausea, constipation, fatigue, dyspepsia, headache and vomiting. Most adverse events were grade 1 or 2 in severity. Laboratory abnormalities included asymptomatic elevated liver function tests, which were more common in patients also receiving Kadcyla. No grade 3 diarrhea was seen in either trial at the recommended dose of ONT-380; anti-diarrheal prophylaxis was not a study requirement.

"The level of clinical activity seen in these trials, both systemically and in the CNS, in heavily pretreated patients is very encouraging," said Stacy Moulder, M.D., Associate Professor, Breast Medical Oncology, University of Texas MD Anderson Cancer Center and one of the authors of both presentations. "Moreover, ONT-380 has been well-tolerated in these trials, an important consideration in patients who may need prolonged therapy."

"Oncothyreon is currently planning a blinded, randomized placebo-controlled Phase 2 trial of ONT-380 in combination with Herceptin and Xeloda for the treatment of HER2-positive metastatic breast cancer in patients who have failed both Perjeta(R) (pertuzumab) and Kadcyla, which we expect to initiate late this year," said Diana Hausman, M.D., Chief Medical Officer of Oncothyreon. "The trial design includes endpoints focused on the prevention and treatment of CNS metastases. We have also now enrolled over 40 patients in our trial of ONT-380 in combination with Kadcyla and continue to evaluate this combination for the treatment of patients with CNS metastases."