28103
Among these , 5 , 5 - dioxo - 7 , 8 - dihydro - 6H - thiopyrano [ 3 , 2 - d ] pyrimidine derivative ( 18 ) showed potent PDE4B inhibitory activity ( IC50 = 25 nM ) .
 5 , 5 - dioxo - 7 , 8 - dihydro - 6H - thiopyrano [ 3 , 2 - d ] pyrimidine
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 5 , 5 - dioxo - 7 , 8 - dihydro - 6H - thiopyrano [ 3 , 2 - d ] pyrimidine