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We identified potent , selective PDE2 inhibitors by optimizing residual PDE2 activity in a series of phosphodiesterase 4 ( PDE4 ) inhibitors , while minimizing PDE4 inhibitory activity .
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Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
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