Potent therapeutic activity of irinotecan (CPT-11) and its schedule dependency in medulloblastoma xenografts in nude mice

The anti-tumor activity of irinotecan (CPT-II), a DNA-topoisomerase 1 inhibitor, was evaluated in 5 advanced stage subcutaneous medulloblastoma xenografts in nude mice, using different schedules of administration. With a 5-day schedule, the highest i.v. dose tested (40 mg kg^-1 day^-1) induced complete regressions in all xenografts but I, and delays in tumor growth always exceeded 30 days. Two xenografts, IGRM11 and IGRM33, were highly sensitive, and animals survived tumor-free beyond 120 days after treatment. CPT-11 clearly retained its anti-tumor activity at a lower dosage (27 mg kg^{- 1} day^-1). CPT-II was significantly more active than cyclophosphamide, thiotepa and etoposide against the 3 xenografts evaluated. To study the schedule dependency of its anti-tumor activity, CPT-II was given i.v. at the same total doses over the same period (33 days) using either a protracted or a sequential schedule in IGRM34-bearing mice. With a dose of 10 mg kg^-1 day^-1 given on days 0-4, days 7-11, days 21-25 and days 282-32 (total dose, 200 mg kg^-1), 3 of 6 animals were tumor free on day 378. The same total dose given with a sequential schedule, i.e., 20 mg kg^-1 day^-1 on days 0- 4 and days 28-32, failed to induce complete regression. The plasma pharmacokinetics of CPT-11 and SN-38 were studied in IGRM34-bearing animals after a single i.v. dose of 10 and 40 mg kg^-1. The plasma clearance rate of CPT-II was dose dependent. The ratio between the SN-38 and CPT-II area under the curve in plasma was 0.4-0.65, i.e., significantly higher than that observed in humans at the maximum tolerated dose (0.01-0.05). Conversely, this ratio was 10-fold lower in tumor than in plasma. Clinical development of irinotecan is warranted in pediatric malignancies.