TAS-102 is a novel oral nucleoside antitumor agent consisting of trifluridine 
and tipiracil hydrochloride at a molar ratio of 1:0.5. TAS-102 was approved in 
Japan in March 2014 for the treatment of patients with unresectable, advanced or 
recurrent colorectal cancer that is refractory to standard therapies. In the 
present study, enhancement of the therapeutic efficacy using a combination 
therapy of TAS-102 and irinotecan hydrochloride (CPT-11) was evaluated in a 
colorectal and gastric cancer xenograft-bearing nude mouse model. TAS-102 was 
orally administered twice a day from day 1 to 14, and CPT-11 was administered 
intravenously on days 1 and 8. The growth-inhibitory activity was evaluated 
based on the tumor volume and the growth-delay period, which was estimated based 
on the period required to reach a tumor volume that was five-times greater than 
the initial volume (RTV5). The tumor growth-inhibitory activity and the RTV5 of 
the group receiving TAS-102 with CPT-11 were significantly superior to those of 
both agents as monotherapy for mice with KM12C, KM12C/5-FU, DLD-1/5-FU, and SC-2 
xenografts (p<0.01). No significant decrease in body weight was observed. The 
present pre-clinical findings indicated that the combination of TAS-102 and 
CPT-11 is a promising treatment option for colorectal or gastric cancer, not 
only for chemo-naïve tumors, but also for recurrent tumors after 5-fluorouracil 
(5-FU)-based chemotherapy.