Treatment will begin with 6-thioguanine taken by mouth 4 times a day (every 6 hours) for 3 days in a row (Days 1-3). This will be followed by temozolomide taken by mouth at bedtime for 5 days in a row (Days 4-8). After a rest period of 6 days, capecitabine and celecoxib will be taken by mouth twice a day (12 hours apart) for 14 days (Days 14-27). Each cycle of treatment on arm 1 will be 28 days.