25568
However , until now , Ad has only been delivered directly to primary tumors because the therapeutic efficacy of systemic delivery is limited by the immune response of the host , short blood circulation times , and non - specific liver uptake of Ad .
 None
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
 Entities:  
Answer:
 None