1003 Background: TN and BRCA1-deficient breast cancer (BC) cell lines exhibit enhanced sensitivity to DNA damaging agents. This study was designed to assess efficacy, safety and predictors of response to iniparib in combination with GC in early-stage TN and BRCA1/2 mutation-associated BC. Methods: This single-arm, phase II study (NCT00813956) enrolled pts with clinical stage I-IIIA (T ≥ 1cm by MRI) ER-negative (≤ 5%), PR-negative (≤ 5%), and HER2-negative or BRCA1/2 mutation-associated BC. Neoadjuvant G (1000 mg/m2; IV; D1, 8), C (AUC 2; IV; D1, 8), and iniparib (5.6 mg/kg; IV; D1, 4, 8, 11) were given every 21 days for 4 cycles, until the protocol was amended to increase the treatment duration to 6 cycles, with enrollment of 80 pts at multiple PrECOG institutions. The primary endpoint is pathologic complete response (pCR), defined as no invasive carcinoma in the breast and axilla. Pathologic response was centrally assessed by the residual cancer burden (RCB) index. Assuming 76/80 eligible and treated pts, the regimen would be deemed effective if the lower bound of a 90% exact binomial CI on the pCR rate exceeded 25%. Secondary endpoints are safety, MRI response, and breast conservation. Results: Among 80 eligible pts treated with 6 cycles, median age is 48 years, 19 pts have germline BRCA1/2 mutations (90% tested to date) and clinical stage is I (13%), IIA (36%), IIB (36%), IIIA (15%). Pathologic response data (ITT population) are detailed below. 69 pts completed treatment per protocol: 5 progressed, 5 discontinued due to an AE and 1 mutation carrier was lost to follow-up. Most common G3/4 adverse events are neutropenia (49%), elevated ALT/AST (14%), and anemia (10%). Conclusions: Preoperative GC plus iniparib is active in the treatment of early-stage TN and BRCA1/2 mutation-associated BC. Clinical trial information: NCT00813956. [Table: see text]