12129
Over the past several years , biochemical characteristics of the brain ( i . e . , tight junction protein complexes at brain barrier sites , expression of influx and efflux transporters ) have been shown to be directly involved in determining CNS permeation of therapeutic agents ; however , the vast majority of these studies have focused on understanding those mechanisms that prevent drugs from entering the CNS .
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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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