A 47-year-old woman has fatigue, pruritus and intermittent jaundice. Alanine aminotransferase is 680 U/L (5–40), IgG is 31 g/L (7–16), antinuclear and smooth-muscle antibodies are positive, and liver biopsy shows interface hepatitis with plasma cells and established cirrhosis. Viral hepatitis and Wilson disease have been excluded. Which one of the following is the most appropriate initial treatment?