UPDATE: TESARO Issues Details on Niraparib Phase 1 Clinical Data, Recist Response Rate 75% Achieved at 300mg Dose, 50% Achieved Across All Dose Levels

TESARO, Inc.

, an oncology-focused biopharmaceutical company, today announced that final results from a Phase 1 trial of niraparib, an inhibitor of poly ADP-ribose polymerase (PARP), were presented this morning at the American Society of Clinical Oncology (ASCO) annual meeting in Chicago. At a dose of 300 milligrams once daily, which represents the dose and schedule that will be utilized in the Phase 3 studies, 75% (three of four patients) with platinum sensitive high grade serous ovarian cancer (HGSOC) achieved a RECIST response. Across all dose levels (30 milligrams to 400 milligrams daily), a RECIST response rate of 46% (6/13 patients) was observed in this population. A RECIST response rate of 50% (5/10 patients) was achieved in patients with platinum sensitive ovarian cancer and germline BRCA mutations. The median duration of response was 431 days for platinum sensitive germline BRCA patients and 444 days for platinum sensitive patients who were not germline BRCA mutation carriers. In addition, 50% (two of the four patients) with BRCA-positive breast cancer achieved a response. Of the 10 patients with prostate cancer who had evaluable circulating tumor cell count (CTC) levels, seven (70%) had a reduction of >30% in CTC count. The ovarian cancer patients enrolled in this study had received a median of six previous regimens of systemic therapy. The breast cancer patients enrolled in this study had received a median of five previous regimens of systemic therapy. Pharmacokinetic parameters were dose proportional and support once per day oral dosing of niraparib. Niraparib was generally well tolerated at 300 milligrams daily, with a low rate of grade 3/4 toxicities. Thrombocytopenia was identified as the dose-limiting toxicity at a daily dose level of 400 milligrams. The most frequently observed adverse events at the 300 milligram dose included grade 1/2 anemia, fatigue and nausea.    "We are very pleased with the RECIST response rates observed in both ovarian and breast cancer patients in this trial, particularly at the 300 milligrams once daily dose that will be evaluated in our Phase 3 program," said Dr. Mary Lynne Hedley, President of TESARO. "The results of this study provide further support for the design of our planned Phase 3 trials in ovarian and breast cancer and, in particular, the potential utility of platinum sensitivity and BRCA status as predictors of response to niraparib." Phase 1 Study Design This Phase 1 study was designed as a two part trial to determine the maximum tolerated dose and evaluate preliminary antitumor activity, safety/tolerability, pharmacokinetic and pharmacodynamic properties of niraparib. Part A enrolled 60 patients who received a single daily dose of 30 to 400 milligrams of niraparib. Following identification of the maximum tolerated dose, Part B of the study enrolled an additional 40 patients with platinum resistant high grade serous ovarian cancer or castration-resistant prostate cancer who received a single daily dose of 300 milligrams of niraparib. Preliminary results of this study were presented at the ASCO annual meeting in 2011.