3749
To expand this limited chemical space and improve understanding of structural determinants of inverse agonist - receptor interaction , this study screened a purine compound library for lead structures using wild - type ( WT ) human P2Y ( 12 ) and 28 constitutively active mutants .
 Chemical: purine
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: purine