Study of TG6002 (VV TK-RR-FCU1) in Combination With 5-FC in Patients With Advanced Gastro-intestinal Tumors.

This study will include two parts: * In the phase I part: safety will be assessed in consecutive cohorts of 3 to 6 patients at increasing doses of TG6002 in combination with oral flucytosine (5-FC) in patients with advanced gastro-intestinal (GI) tumors. * In the phase IIa part: evaluation of efficacy and further evaluation of safety of multiple administrations of TG6002 in combination with flucytosine (5-FC) in patients with colorectal cancer and liver metastases. In both parts, tumor response will be evaluated on local assessment using RECIST 1.1. All patients will be followed up until disease progression or death due to any cause or the date of data cut-off, whichever occurs first.