18211
Although a number of biosimilar proteins have been approved , especially in Europe , issues on substitutability , extrapolation to other disease indications , and selection of reference standards and comparators , remains to be standardized at a global level .
 None
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 None