21993
Replacement of hydrogen with fluorine within three pairs of structurally similar small molecule inhibitors of heat shock protein 90 ( HSP90 ) resulted in differences in inhibition constants ( Ki ) in vitro as well as marked differences in rat intravenous pharmacokinetic profiles .
 Chemical: hydrogen; Chemical: fluorine
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: hydrogen; Chemical: fluorine