Patients undergo collection of 2 skin biopsies with hair follicles at 4 and 8 hours or at 4 and 6 hours after the start of irinotecan hydrochloride or gemcitabine hydrochloride treatment on day 1 of course 1. Repeat biopsies will be taken at 4, 6, or 8 hours after the start of irinotecan hydrochloride or gemcitabine hydrochloride on day 1 of 2 successive courses.