16815
Especially , compound 57 exhibited the strongest binding affinity on the CB2 receptor ( CB2K ( i ) of 0 . 5 nM ) and the best selectivity over the CB1 receptor ( selectivity index of 2594 ) .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
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