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 lomustine taken by mouth at bedtime for 1 day (Day 4). After a rest period of 1 week, capecitabine and celecoxib will be taken by mouth twice a day (12 hours apart) for 14 days (Days 11-24). Each cycle of treatment on arm 2 will take 42 days.