Evaluation of irinotecan in combination with 5-fluorouracil or etoposide in xenograft models of colon adenocarcinoma and rhabdomyosarcoma

Irinotecan [7-ethyl-10-(4-[1-piperidino]-1-piperidino)-carbonyloxy-camptothec in] administered i.v. in two courses, each course consisting of administration every day for 5 days [(dx5)2] on days 1-5 and 8-12, has demonstrated significant activity against advanced human tumor xenografts derived from colon adenocarcinomas and several childhood cancers. To build on this therapy, we have evaluated the combination of irinotecan given on this schedule with 5-fluorouracil given on days 1, 7, and 14 with or without leucovorin [(dx5)3 i.v.] against colon tumors, or combined with etoposide administered (dx5)2 i.v. either 2 h before or 2 h after irinotecan for treatment of colon tumors and rhabdomyosarcomas. A combination of 5-fluorouracil at 75% and irinotecan at 50% of their respective maximum tolerated doses when administered as single agents on this schedule gave acceptable toxicity. Against colon adenocarcinoma xenografts, 5-fluorouracil did not enhance the response rate compared with that obtained with the optimum dose of irinotecan given as a single agent. Against GC3/TK- xenografts, which lack thymidine kinase and cannot salvage thymidine to circumvent the inhibition of thymidylate synthase, the addition of leucovorin to the combination increased the complete response rate from 10 to >90%, whereas the response rates for the optimal doses of irinotecan or 5-fluorouracil, as single agents, were 30 and