The purpose of this study is to learn if adding an experimental drug, veliparib (also known as ABT-888), to a usual treatment is better than the usual treatment alone for people with locally advanced rectal cancer. To be better than the usual treatment, the addition of the experimental drug to the usual treatment needs to shrink more of the cancer than the usual treatment alone.
Another purpose of this study is to test the good and bad effects of veliparib when added to the usual treatment. Veliparib may shrink the cancer but it can also cause side effects. Veliparib is not FDA-approved for treating cancer but it is being used in other clinical trials.