Niraparib monotherapy is not as efficacious as irinotecan monotherapy in the HCT116 (MSI) or HT29 (MSS) xenograft models. A. and B. In vivo efficacy of niraparib monotherapy on the HCT116 (MSI) and HT29 (MSS) xenograft models. Niraparib was dosed at 50, 25, or 10 mpk p.o. q.d., for 5 days a week (2 days off) for 4 weeks. Relative tumor size (% versus Day 0) is shown with SEM in error bars. C. and D. The 5 day dosing regimen was well tolerated with less than an average of 10% body weight loss in all doses tested. Relative body weight (% versus Day 0) is shown. For each group, n = 7-10.