Chapter 41. Inflammatory Bowel Disease
Which drug formulation of mesalamine is most effective as initial therapy for RJ’s UC confined to the rectum?
Answer a is correct. Ulcerative proctitis is a subset of UC in which inflammation is localized to the rectum. Mesalamine is a first-line agent for mild to moderate proctitis. The suppository formulation delivers mesalamine directly to the site of action and treats up to 20 cm of the rectal area, thus it is the preferred formulation for treatment of proctitis.
Answer b is incorrect. While the enema form of mesalamine would treat proctitis, it delivers the drug up to the splenic flexure. This would result in unnecessary coverage of this area of the GI tract and would be more appropriate for a patient with distal or left-sided disease.
Answer c is incorrect. While the tablet form of mesalamine would treat proctitis, it delivers the drug either throughout the small and large intestine or just throughout the colon, depending on the formulation. This would result in unnecessary coverage of this area of the GI tract and would be more appropriate for a patient with extensive disease.
Answer d is incorrect. Mesalamine is not available in the United States in an injectable formulation.
Three years later, RJ’s UC has advanced to moderately severe disease. After a course of steroids, his gastroenterologist wants to start him on azathioprine. Which enzyme should be tested to determine RJ’s ability to metabolize azathioprine?
b. Dihydrofolate reductase
c. Thiopurine methyltransferase
Answer c is correct. Thiopurine methyltransferase is an enzyme that is partially responsible for the metabolism of azathioprine. Genetic polymorphisms of this enzyme may result in reduced enzyme activity. This may predispose patients to higher blood levels of azathioprine and increase the potential for toxicity. Patients should have their thiopurine methyltransferase activity level measured prior to initiating therapy with azathioprine.
Answer a is incorrect. Rasburicase is not an enzyme, but rather a drug used in the treatment of tumor lysis syndrome.
Answer b is incorrect. Dihydrofolate reductase is an enzyme that is involved in the metabolism of methotrexate and does not affect azathioprine.
Answer d is incorrect. Hyaluronidase is a substance, which increases the permeability of connective tissue. It is often used in surgical procedures to increase drug absorption.