STUDIES OF THE EFFICACY AND PHARMACOLOGY OF IRINOTECAN AGAINST HUMAN COLON-TUMOR XENOGRAFT MODELS

Irinotecan, administered i.v. on days 1-5 and 8-12 [(dx5)2 i.v.] has d emonstrated significant activity against advanced human tumor xenograf ts, To explore the feasibility of prolonged oral administration of iri notecan, we compared the efficacy of oral and i.v. irinotecan on the ( dx5)2 schedule. We also evaluated oral therapy for 12 consecutive week s [(dx5)12] at 25 and 50 mg/kg and two consecutive 5-day courses repea ted every 21 days for up to four cycles (r(dx5)2]4) at 50 and 75 md/kg /dose in a series of human colon carcinoma xenograft lines, In additio n, we evaluated the effect of a sensitive (HC1) and resistant (ELC2) h uman colon adenocarcinoma xenograft on irinotecan and SN-38 lactone di sposition after administration of irinotecan 10 mg/kg i.v. and 10 and 25 mg/kg p.o. Irinotecan i.v. at 40 mg/kg and oral at 50 and 75 mg/kg on the (dx5)2 schedule had similar activity against the panel of adult colon adenocarcinoma xenografts, Irinotecan given p.o. also demonstra ted significant activity against a topotecan-resistant derivative, VRC 5/TOPO, Oral administration of 75 mg/kg r(dx5)2]4 and 50 mg/kg (dx5)12 achieved complete response in five of seven xenograft lines evaluated , After i.v. administration, mice bearing HC1 xenografts had 43% great er SN-38 lactone systemic exposure compared to those with ELC2 xenogra fts and non-tumor-bearing mice, After oral (10 mg/kg) administration, there was a 5-fold higher molar formation of SN-38 lactone compared to i.v. (10 mg/kg) administration in tumor and non-tumor-bearing mice, S N-38 systemic exposure associated with the lowest oral dose (25 mg/kg) achieving complete response for HC1 was 942.6 ng/ml.h. These results emphasize the importance of pharmacokinetic studies as part of tumor r esponse studies in xenograft models.