On each schedule, cohorts of 3-6 patients receive escalating doses of ABT-751 until the maximum tolerated dose (MTD) is determined. The MTD is defined as the dose preceding that at which at least 2 of 6 patients experience dose-limiting toxicity. Once the MTD is determined, up to 9 patients (a minimum of 3 patients age 11 and under and 3 patients age 12 to 18) are treated at the MTD.