21870
However , particle aggregation / dissociation and / or major differences in the bio - distribution and targeting capability of drugs and imaging probes are main obstacles for the efficient , coordinated delivery of multiple agents , unless the different agents can be tightly bound and well - protected during their circulation in vivo .
 None
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 None