Sentence:  On Feb. 12 , 1959 , <WSD>purified</WSD> corticotropin ( ACTH Gel ) , 20 units daily intramuscularly , was started but had to be discontinued 3 weeks later because of excessive fluid retention . 
Meaning of the word marked with <WSD> tags:  remove impurities from, increase the concentration of, and separate through the process of distillation