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The first metabolic step that azathioprine undergoes in the body is the conversion to 6-mercaptopurine ( 6-MP ; see Pharmacokinetics ) , which is itself an immunosuppressant prodrug .
 6-mercaptopurine
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 6-mercaptopurine