Patients receive phenylbutyrate IV continuously on days 1-7 of weeks 1, 5, 7, 9, 11, 13, 15, 17, and 19. Patients also receive oral tretinoin three times daily on days 1-7 of weeks 3, 5, 7, 9, 11, 13, 15, 17, and 19. Treatment continues in the absence of disease progression or unacceptable toxicity.