Background: This NCTN multi-arm randomized phase II modular clinical trial platform utilizes TNT with parallel EAs in LARC. EAs are not intended for direct comparison, but rather to test a variety of hypotheses in a consistent high-risk pt population with correlative biomarkers. Primary endpoint (EP) and available secondary EPs from the first EA using veliparib (a PARPi) are reported. NCT02921256. Methods: Stage II/III pts with LARC (with any ONE of the following: distal location [cT3-4 ≤5cm from anal verge, any N]; bulky [any cT4 or tumor within 3mm of mesorectal fascia]; high risk for metastatic disease [cN2]; or not a sphincter-sparing surgery [SSS] candidate) were randomized to neoadjuvant FOLFOX (x 4mo) → chemoRT (cape with 50.4Gy +/- veliparib 400mg PO BID) → surgery 8-12 wks later. Primary EP: 4 point reduction in Neoadjuvant Rectal Cancer (NAR) score with a one-sided α=0.10 and 80% power. NAR compared by linear model controlling for stratification and possibly other factors. Secondary EPs: OS, DFS, toxicity, pCR, cCR, therapy completion, negative surgical margins, and SSS. Binary EPs compared by Fisher’s exact test. Reported p-values are two-sided. Results: From 10/2016 - 2/2018, 178 pts were randomized (88 control, 90 veliparib). Baseline characteristics were balanced except for candidate for SSS at entry (39% control, 61% veliparib). 140 pts were evaluable for NAR (72 control, 68 veliparib). Mean NAR was 12.6 control (95% CI: 9.8–15.3) vs 13.7 for veliparib (CI: 10.2–17.2). Controlling for stratification (p=0.69) or stratification and candidate for SSS (p=0.78), NAR difference was not significant. pCR=21.6% vs 33.8% (p=0.14); cCR=28.2% vs 33.3% (p=0.60); and SSS=52.5% vs 59.3% (p=0.43). Most common grade 3/4 AEs were diarrhea and cytopenias. The EA had two deaths (cardiac arrest [FOLFOX]; enterocolitis [chemoRT]). Conclusions: Veliparib added to chemoRT as part of TNT was safe and without unexpected short-term toxicities but failed to improve the NAR score. Support: U10CA180868, -180822; UG1-189867; U24-196067; AbbVie. Clinical trial information: NCT02921256