Synergy of topotecan in combination with vincristine for treatment of pediatric solid tumor xenografts

Topotecan and vincristine were evaluated alone or in combination against 13 independent xenografts and 1 vincristine-resistant derivative, representing childhood neuroblastoma ( n = 6), rhabdomyosarcoma ( n = 5), or brain tumors ( n = 3). Topotecan was given by i.v. bolus on a schedule found previously to be optimal. Drug was administered daily for 5 days on 2 consecutive weeks with cycles repeated every 21 days over a period of 8 weeks. Doses of topotecan ranged from 0.16 to 1.5 mg/kg to simulate clinically achievable topotecan lactone plasma systemic exposures. Vincristine was administered i.v. every 7 days at a fixed dose of 1 mg/kg. Given as a single agent, vincristine induced complete responses (CRs) in all mice bearing two rhabdomyosarcomas (Rh28 and Rh30) and some CRs in Rh12-bearing mice (57%) but relatively few CRs (