Patients receive carboxylesterase-expressing allogeneic neural stem cells intracranially over 1.5-4.5 hours on days 1 and 15 (day 1 only for patients at dose level 1) and irinotecan hydrochloride intravenously (IV) over 90 minutes on days 3 and 17. Courses repeat every 28 days in the absence of disease progression or unacceptable toxicity.