This study will test the safety, tolerability, and pharmacokinetics of single doses of ISIS 333611 administered as 12-hour intrathecal infusions. Four dose levels (0.15, 0.5, 1.5 and 3 mg) will be evaluated sequentially. The volume of the infusion is 0.25 mL/12 hours. Each dose level will be studied in a cohort of 8 patients where 6 are randomized to active treatment with ISIS 333611 and 2 are randomized to placebo.